Publications

@article{shd14,

title = {Dynamical clustering and a mechanism for raft-like structures in a model lipid membrane},

author = {Starr, Francis W. and Hartmann, Benedikt and Douglas, Jack F.},

url = {http://fstarr.web.wesleyan.edu/publications/shd14.pdf},

doi = {10.1039/c3sm53187b},

issn = {1744-683X},

year  = {2014},

date = {2014-01-01},

journal = {SOFT MATTER},

volume = {10},

number = {17},

pages = {3036-3047},

abstract = {We use molecular dynamics simulations to examine the dynamical heterogeneity of a model single-component lipid membrane using a coarse-grained representation of lipid molecules. This model qualitatively reproduces the known phase transitions between disordered, ordered, and gel membrane phases, and the phase transitions are accompanied by significant changes in the nature of the lipid dynamics. In particular, lipid diffusion in the liquid-ordered phase is hindered by the transient trapping of molecules by their neighbors, similar to the dynamics of a liquid approaching its glass transition. This transient molecular caging gives rise to two distinct mobility groups within a single-component membrane: lipids that are transiently trapped, and lipids with displacements on the scale of the intermolecular spacing. Most significantly, lipids within these distinct mobility states spatially segregate, creating transient ``islands'' of enhanced mobility having a size and time scale compatible with lipid ``rafts,'' dynamical structures thought to be important for cell membrane function. Although the dynamic lipid clusters that we observe do not themselves correspond to rafts (which are more complex, multicomponent structures), we hypothesize that such rafts may develop from the same universal mechanism, explaining why raft-like regions should arise, regardless of lipid structural or compositional details. These clusters are strikingly similar to the dynamical clusters found in glass-forming fluids, and distinct from phase-separation clusters. We also show that mobile lipid clusters can be dissected into smaller clusters of cooperatively rearranging molecules. The geometry of these clusters can be understood in the context of branched equilibrium polymers, related to percolation theory. We discuss how these dynamical structures relate to a range observations on the dynamics of lipid membranes.},

keywords = {Biophysics, Dynamic Heterogeneity, Membranes},

pubstate = {published},

tppubtype = {article}

}

@article{csg13,

title = {Pressure-induced transformations in computer simulations of glassy water},

author = {Chiu, Janet and Starr, Francis W. and Giovambattista, Nicolas},

url = {http://fstarr.web.wesleyan.edu/publications/csg13.pdf},

doi = {10.1063/1.4829276},

issn = {0021-9606},

year  = {2013},

date = {2013-11-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {139},

number = {18},

pages = {184504},

abstract = {Glassy water occurs in at least two broad categories: low-density amorphous (LDA) and high-density amorphous (HDA) solid water. We perform out-of-equilibrium molecular dynamics simulations to study the transformations of glassy water using the ST2 model. Specifically, we study the known (i) compression-induced LDA-to-HDA, (ii) decompression-induced HDA-to-LDA, and (iii) compression-induced hexagonal ice-to-HDA transformations. We study each transformation for a broad range of compression/decompression temperatures, enabling us to construct a ``P-T phase diagram'' for glassy water. The resulting phase diagram shows the same qualitative features reported from experiments. While many simulations have probed the liquid-state phase behavior, comparatively little work has examined the transitions of glassy water. We examine how the glass transformations relate to the (first-order) liquid-liquid phase transition previously reported for this model. Specifically, our results support the hypothesis that the liquid-liquid spinodal lines, between a low-density and high-density liquid, are extensions of the LDA-HDA transformation lines in the limit of slow compression. Extending decompression runs to negative pressures, we locate the sublimation lines for both LDA and hyperquenched glassy water (HGW), and find that HGW is relatively more stable to the vapor. Additionally, we observe spontaneous crystallization of HDA at high pressure to ice VII. Experiments have also seen crystallization of HDA, but to ice XII. Finally, we contrast the structure of LDA and HDA for the ST2 model with experiments. We find that while the radial distribution functions (RDFs) of LDA are similar to those observed in experiments, considerable differences exist between the HDA RDFs of ST2 water and experiment. The differences in HDA structure, as well as the formation of ice VII (a tetrahedral crystal), are a consequence of ST2 overemphasizing the tetrahedral character of water. (C) 2013 AIP Publishing LLC.},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{sds13,

title = {The relationship of dynamical heterogeneity to the Adam-Gibbs and random first-order transition theories of glass formation},

author = {Starr, Francis W. and Douglas, Jack F. and Sastry, Srikanth},

url = {http://fstarr.web.wesleyan.edu/publications/sds13.pdf},

doi = {10.1063/1.4790138},

issn = {0021-9606},

year  = {2013},

date = {2013-03-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {138},

number = {12},

pages = {12A541},

abstract = {We carefully examine common measures of dynamical heterogeneity for a model polymer melt and test how these scales compare with those hypothesized by the Adam and Gibbs (AG) and random first-order transition (RFOT) theories of relaxation in glass-forming liquids. To this end, we first analyze clusters of highly mobile particles, the string-like collective motion of these mobile particles, and clusters of relative low mobility. We show that the time scale of the high-mobility clusters and strings is associated with a diffusive time scale, while the low-mobility particles' time scale relates to a structural relaxation time. The difference of the characteristic times for the high-and low-mobility particles naturally explains the well-known decoupling of diffusion and structural relaxation time scales. Despite the inherent difference of dynamics between high-and low-mobility particles, we find a high degree of similarity in the geometrical structure of these particle clusters. In particular, we show that the fractal dimensions of these clusters are consistent with those of swollen branched polymers or branched polymers with screened excluded-volume interactions, corresponding to lattice animals and percolation clusters, respectively. In contrast, the fractal dimension of the strings crosses over from that of self-avoiding walks for small strings, to simple random walks for longer, more strongly interacting, strings, corresponding to flexible polymers with screened excluded-volume interactions. We examine the appropriateness of identifying the size scales of either mobile particle clusters or strings with the size of cooperatively rearranging regions (CRR) in the AG and RFOT theories. We find that the string size appears to be the most consistent measure of CRR for both the AG and RFOT models. Identifying strings or clusters with the ``mosaic'' length of the RFOT model relaxes the conventional assumption that the ``entropic droplets'' are compact. We also confirm the validity of the entropy formulation of the AG theory, constraining the exponent values of the RFOT theory. This constraint, together with the analysis of size scales, enables us to estimate the characteristic exponents of RFOT. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790138]},

keywords = {Dynamic Heterogeneity, Glass Formation, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{pds13,

title = {Fragility and cooperative motion in a glass-forming polymer-nanoparticle composite},

author = {Pazmiño Betancourt, Beatriz A. and Douglas, Jack F. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/pds13.pdf},

doi = {10.1039/c2sm26800k},

issn = {1744-683X},

year  = {2013},

date = {2013-01-01},

journal = {SOFT MATTER},

volume = {9},

number = {1},

pages = {241-254},

abstract = {Polymer-nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperature T-g due to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperature T - termed the fragility m of glass-formation - is comparatively poorly understood. In the present work, we calculate both T-g and m of a model polymer nanocomposite by molecular dynamics simulations. We systematically consider how T-g and m vary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and for representative attractive and repulsive polymer-NP interactions. We find large positive and negative changes in T-g and m that can be interpreted in terms of the Adam-Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. These results provide a molecular perspective of fragility changes due to the addition of NPs and for the physical origin of fragility more generally. We also contrast the behavior along isobaric and isochoric approaches to T-g, since these differing paths can be important to compare with experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and have fundamental significance for understanding the properties glass-forming materials more broadly.},

keywords = {Dynamic Heterogeneity, Glass Formation, Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{hds12,

title = {Local variation of fragility and glass transition temperature of ultra-thin supported polymer films},

author = {Hanakata, Paul Z. and Douglas, Jack F. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/hds12.pdf},

doi = {10.1063/1.4772402},

issn = {0021-9606},

year  = {2012},

date = {2012-12-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {137},

number = {24},

pages = {244901},

abstract = {Despite extensive efforts, a definitive picture of the glass transition of ultra-thin polymer films has yet to emerge. The effect of film thickness h on the glass transition temperature T-g has been widely examined, but this characterization does not account for the fragility of glass-formation, which quantifies how rapidly relaxation times vary with temperature T. Accordingly, we simulate supported polymer films of a bead-spring model and determine both T-g and fragility, both as a function of h and film depth. We contrast changes in the relaxation dynamics with density rho and demonstrate the limitations of the commonly invoked free-volume layer model. As opposed to bulk polymer materials, we find that the fragility and T-g do not generally vary proportionately. Consequently, the determination of the fragility profile-both locally and for the film as a whole-is essential for the characterization of changes in film dynamics with confinement. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772402]},

keywords = {Dynamic Heterogeneity, Glass Formation, Polymers, Thin Films},

pubstate = {published},

tppubtype = {article}

}

@article{cvtsg12,

title = {Internal Structure of Nanoparticle Dimers Linked by DNA},

author = {Chi, Cheng and Vargas-Lara, Fernando and Tkachenko, Alexei V. and Starr, Francis W. and Gang, Oleg},

url = {http://fstarr.web.wesleyan.edu/publications/cvtsg12.pdf},

doi = {10.1021/nn301528h},

issn = {1936-0851},

year  = {2012},

date = {2012-08-01},

journal = {ACS NANO},

volume = {6},

number = {8},

pages = {6793-6802},

abstract = {We construct nanoparticle dimers linked by DNA. These dimers are basic units in a possible multiscale, hierarchical assembly and serve as a model system to understand DNA-mediated interactions, especially in the nontrivial regime when the nanoparticle and DNA are comparable in their sizes. We examine the structure of nanoparticle dimers in detail by a combination of scattering experiments and molecular simulations. We find that, for a given DNA length, the interparticle separation within the dimer is controlled primarily by the number of linking DNA. We summarize our findings in a simple model that captures the Interplay of the number of DNA bridges, their length, the particle's curvature, and the excluded volume effects. We demonstrate the applicability of the model to our results, without any free parameters. As a consequence, the increase of dimer separation with increasing temperature can be understood as a result of changing the number of connecting DNA.},

keywords = {Biophysics, DNA},

pubstate = {published},

tppubtype = {article}

}

@article{glls12b,

title = {Interplay of the Glass Transition and the Liquid-Liquid Phase Transition 

 in Water},

author = {Nicolas Giovambattista and Thomas Loerting and Boris R Lukanov and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/glls12.pdf},

doi = {10.1038/srep00390},

issn = {2045-2322},

year  = {2012},

date = {2012-05-01},

journal = {SCIENTIFIC REPORTS},

volume = {2},

pages = {390},

abstract = {Water has multiple glassy states, often called amorphous ices. 

 Low-density (LDA) and high-density (HDA) amorphous ice are separated by 

 a dramatic, first-order like phase transition. It has been argued that 

 the LDA-HDA transformation connects to a first-order liquid-liquid phase 

 transition (LLPT) above the glass transition temperature T-g. Direct 

 experimental evidence of the LLPT is challenging to obtain, since the 

 LLPT occurs at conditions where water rapidly crystallizes. In this 

 work, we explore the implications of a LLPT on the pressure dependence 

 of T-g(P) for LDA and HDA by performing computer simulations of two 

 water models - one with a LLPT, and one without. In the absence of a 

 LLPT, T-g(P) for all glasses nearly coincide. When there is a LLPT, 

 different glasses exhibit dramatically different T-g(P) which are 

 directly linked with the LLPT. Available experimental data for T-g(P) 

 are only consistent with the scenario including a LLPT.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{glls12,

title = {Interplay of the Glass Transition and the Liquid-Liquid Phase Transition in Water},

author = {Giovambattista, Nicolas and Loerting, Thomas and Lukanov, Boris R. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/glls12.pdf},

doi = {10.1038/srep00390},

issn = {2045-2322},

year  = {2012},

date = {2012-05-01},

journal = {SCIENTIFIC REPORTS},

volume = {2},

pages = {390},

abstract = {Water has multiple glassy states, often called amorphous ices. Low-density (LDA) and high-density (HDA) amorphous ice are separated by a dramatic, first-order like phase transition. It has been argued that the LDA-HDA transformation connects to a first-order liquid-liquid phase transition (LLPT) above the glass transition temperature T-g. Direct experimental evidence of the LLPT is challenging to obtain, since the LLPT occurs at conditions where water rapidly crystallizes. In this work, we explore the implications of a LLPT on the pressure dependence of T-g(P) for LDA and HDA by performing computer simulations of two water models - one with a LLPT, and one without. In the absence of a LLPT, T-g(P) for all glasses nearly coincide. When there is a LLPT, different glasses exhibit dramatically different T-g(P) which are directly linked with the LLPT. Available experimental data for T-g(P) are only consistent with the scenario including a LLPT.},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{cks12b,

title = {Localization transition of instantaneous normal modes and liquid 

 diffusion},

author = {Vasile Iulian Clapa and Tsampikos Kottos and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/cks12.pdf},

doi = {10.1063/1.3701564},

issn = {0021-9606},

year  = {2012},

date = {2012-04-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {136},

number = {14},

pages = {144504},

abstract = {We analyze the structure of instantaneous normal modes (INM) associated 

 with the Hessian matrix of a liquid. Utilizing a scaling theory 

 developed in the framework of Anderson localization studies, we 

 unambiguously identify the crossover point in the INM spectrum between 

 extended and localized modes. We establish a relation between the 

 unstable, delocalized INMs and the liquid diffusion coefficient that 

 appears valid over a large temperature range, covering both Arrhenius 

 and non-Arrhenius regimes of temperature dependence. These results 

 suggest a possible route to theoretically relate dynamics to 

 thermodynamical properties of the liquid via the tomography of the INMs. 

 (C) 2012 American Institute of Physics. 

 [http://dx.doi.org/10.1063/1.3701564]},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{cks12,

title = {Localization transition of instantaneous normal modes and liquid diffusion},

author = {Clapa, Vasile Iulian and Kottos, Tsampikos and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/cks12.pdf},

doi = {10.1063/1.3701564},

issn = {0021-9606},

year  = {2012},

date = {2012-04-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {136},

number = {14},

pages = {144504},

abstract = {We analyze the structure of instantaneous normal modes (INM) associated with the Hessian matrix of a liquid. Utilizing a scaling theory developed in the framework of Anderson localization studies, we unambiguously identify the crossover point in the INM spectrum between extended and localized modes. We establish a relation between the unstable, delocalized INMs and the liquid diffusion coefficient that appears valid over a large temperature range, covering both Arrhenius and non-Arrhenius regimes of temperature dependence. These results suggest a possible route to theoretically relate dynamics to thermodynamical properties of the liquid via the tomography of the INMs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3701564]},

keywords = {Glass Formation},

pubstate = {published},

tppubtype = {article}

}

@article{pbss11b,

title = {Dynamical Behavior Near a Liquid-Liquid Phase Transition in Simulations 

 of Supercooled Water},

author = {Peter H Poole and Stephen R Becker and Francesco Sciortino and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/pbss11.pdf},

doi = {10.1021/jp204889m},

issn = {1520-6106},

year  = {2011},

date = {2011-12-01},

journal = {JOURNAL OF PHYSICAL CHEMISTRY B},

volume = {115},

number = {48},

pages = {14176-14183},

abstract = {We examine the behavior of the diffusion coefficient of the ST2 model of 

 water over a broad region of the phase diagram via molecular dynamics 

 simulations. The ST2 model has an accessible liquid-liquid transition 

 between low-density and high-density phases, making the model an ideal 

 candidate to explore the impacts of the liquid-liquid transition on 

 dynamics. We locate characteristic dynamical loci in the phase diagram 

 and compare them with the previously investigated thermodynamic loci. 

 The low-density liquid phase shows a crossover from non-Arrhenius to 

 Arrhenius behavior, signaling the onset of a crossover from 

 fragile-to-strong behavior. We explain this crossover in terms of the 

 asymptotic approach of the low-density liquid to a random tetrahedral 

 network and show that the temperature dependence of the diffusion 

 coefficient over a wide temperature range can be simply related to the 

 concentration of defects in the network. Our findings thus confirm that 

 the low-density phase of ST2 water is a well-defined metastable liquid.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{pbss11,

title = {Dynamical Behavior Near a Liquid-Liquid Phase Transition in Simulations of Supercooled Water},

author = {Poole, Peter H. and Becker, Stephen R. and Sciortino, Francesco and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/pbss11.pdf},

doi = {10.1021/jp204889m},

issn = {1520-6106},

year  = {2011},

date = {2011-12-01},

journal = {JOURNAL OF PHYSICAL CHEMISTRY B},

volume = {115},

number = {48},

pages = {14176-14183},

abstract = {We examine the behavior of the diffusion coefficient of the ST2 model of water over a broad region of the phase diagram via molecular dynamics simulations. The ST2 model has an accessible liquid-liquid transition between low-density and high-density phases, making the model an ideal candidate to explore the impacts of the liquid-liquid transition on dynamics. We locate characteristic dynamical loci in the phase diagram and compare them with the previously investigated thermodynamic loci. The low-density liquid phase shows a crossover from non-Arrhenius to Arrhenius behavior, signaling the onset of a crossover from fragile-to-strong behavior. We explain this crossover in terms of the asymptotic approach of the low-density liquid to a random tetrahedral network and show that the temperature dependence of the diffusion coefficient over a wide temperature range can be simply related to the concentration of defects in the network. Our findings thus confirm that the low-density phase of ST2 water is a well-defined metastable liquid.},

keywords = {Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{pvs11b,

title = {Stability of DNA-linked nanoparticle crystals: Effect of number of 

 strands, core size, and rigidity of strand attachment},

author = {Olivia Padovan-Merhar and Fernando Vargas Lara and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/pvs11.pdf},

doi = {10.1063/1.3596745},

issn = {0021-9606},

year  = {2011},

date = {2011-06-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {134},

number = {24},

pages = {244701},

abstract = {Three-dimensional ordered lattices of nanoparticles (NPs) linked by DNA 

 have potential applications in novel devices and materials, but most 

 experimental attempts to form crystals result in amorphous packing. Here 

 we use a coarse-grained computational model to address three factors 

 that impact the stability of bcc and fcc crystals formed by DNA-linked 

 NPs : (i) the number of attached strands to the NP surface, (ii) the 

 size of the NP core, and (iii) the rigidity of the strand attachment. We 

 find that allowing mobility in the attachment of DNA strands to the core 

 NP can very slightly increase or decrease melting temperature T(M). 

 Larger changes to T(M) result from increasing the number of strands, 

 which increases T(M), or by increasing the core NP diameter, which 

 decreases T(M). Both results are consistent with experimental findings. 

 Moreover, we show that the behavior of T(M) can be quantitatively 

 described by the model introduced previously [F. Vargas Lara and F. W. 

 Starr, Soft Matter, 7, 2085 (2011)]. (C) 2011 American Institute of 

 Physics. [doi: 10.1063/1.3596745]},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{pvs11,

title = {Stability of DNA-linked nanoparticle crystals: Effect of number of strands, core size, and rigidity of strand attachment},

author = {Padovan-Merhar, Olivia and Vargas Lara, Fernando and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/pvs11.pdf},

doi = {10.1063/1.3596745},

issn = {0021-9606},

year  = {2011},

date = {2011-06-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {134},

number = {24},

pages = {244701},

abstract = {Three-dimensional ordered lattices of nanoparticles (NPs) linked by DNA have potential applications in novel devices and materials, but most experimental attempts to form crystals result in amorphous packing. Here we use a coarse-grained computational model to address three factors that impact the stability of bcc and fcc crystals formed by DNA-linked NPs : (i) the number of attached strands to the NP surface, (ii) the size of the NP core, and (iii) the rigidity of the strand attachment. We find that allowing mobility in the attachment of DNA strands to the core NP can very slightly increase or decrease melting temperature T(M). Larger changes to T(M) result from increasing the number of strands, which increases T(M), or by increasing the core NP diameter, which decreases T(M). Both results are consistent with experimental findings. Moreover, we show that the behavior of T(M) can be quantitatively described by the model introduced previously [F. Vargas Lara and F. W. Starr, Soft Matter, 7, 2085 (2011)]. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3596745]},

keywords = {Biophysics, DNA, Nanotechnology, Self Assembly},

pubstate = {published},

tppubtype = {article}

}

@article{sd11b,

title = {Modifying Fragility and Collective Motion in Polymer Melts with 

 Nanoparticles},

author = {Francis W Starr and Jack F Douglas},

url = {http://fstarr.web.wesleyan.edu/publications/sd11.pdf},

doi = {10.1103/PhysRevLett.106.115702},

issn = {0031-9007},

year  = {2011},

date = {2011-03-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {106},

number = {11},

pages = {115702},

abstract = {We investigate the impact of nanoparticles (NP) on the fragility and 

 cooperative stringlike motion in a model glass-forming polymer melt by 

 molecular dynamics simulation. The NP cause significant changes to both 

 the fragility and the average length of stringlike motion, where the 

 effect depends on the NP-polymer interaction and NP concentration. We 

 interpret these changes via the Adam-Gibbs (AG) theory, assuming the 

 strings can be directly identified with the abstract ``cooperatively 

 rearranging regions'' of AG. Our findings indicate that fragility is 

 primarily a measure of the temperature dependence of the cooperativity 

 of molecular motion.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{sd11,

title = {Modifying Fragility and Collective Motion in Polymer Melts with Nanoparticles},

author = {Starr, Francis W. and Douglas, Jack F.},

url = {http://fstarr.web.wesleyan.edu/publications/sd11.pdf},

doi = {10.1103/PhysRevLett.106.115702},

issn = {0031-9007},

year  = {2011},

date = {2011-03-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {106},

number = {11},

pages = {115702},

abstract = {We investigate the impact of nanoparticles (NP) on the fragility and cooperative stringlike motion in a model glass-forming polymer melt by molecular dynamics simulation. The NP cause significant changes to both the fragility and the average length of stringlike motion, where the effect depends on the NP-polymer interaction and NP concentration. We interpret these changes via the Adam-Gibbs (AG) theory, assuming the strings can be directly identified with the abstract ``cooperatively rearranging regions'' of AG. Our findings indicate that fragility is primarily a measure of the temperature dependence of the cooperativity of molecular motion.},

keywords = {Dynamic Heterogeneity, Glass Formation, Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{vs11d,

title = {Stability of DNA-linked nanoparticle crystals I: Effect of linker 

 sequence and length},

author = {Fernando Vargas Lara and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/vs11.pdf},

doi = {10.1039/c0sm00989j},

issn = {1744-683X},

year  = {2011},

date = {2011-01-01},

journal = {SOFT MATTER},

volume = {7},

number = {5},

pages = {2085-2093},

abstract = {The creation of three-dimensional, crystalline-ordered nanoparticle (NP) 

 structures linked by DNA has proved experimentally challenging. Here we 

 aim to systematically study parameters that influence the relative 

 thermodynamic and kinetic stability of such crystals. To avoid 

 experimental bottlenecks and directly control molecular-scale 

 parameters, we carry out molecular dynamics simulations of a 

 coarse-grained model in which short DNA strands (6 to 12 bp) are 

 tethered to a NP core. We examine the influence of the number of bases 

 per strand L, number of linking bases l and the number of spacer bases s 

 on the stability of crystal states. We also consider the effect of using 

 a single linking NP type versus a binary linking system. We explicitly 

 compute the free energy, entropy, and melting point T(M) for BCC and FCC 

 lattices. We show that binary systems are preferable for generating BCC 

 lattices, while a single NP type generates the most stable FCC crystals. 

 We propose a simple model for short DNA strands that can account for 

 T(M) of all our data. The model also indicates that the heat of fusion 

 between crystal and amorphous phases grows linearly with l, providing a 

 route to maximize the relative crystal stability.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{vs11,

title = {Stability of DNA-linked nanoparticle crystals I: Effect of linker sequence and length},

author = {Vargas Lara, Fernando and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/vs11.pdf},

doi = {10.1039/c0sm00989j},

issn = {1744-683X},

year  = {2011},

date = {2011-01-01},

journal = {SOFT MATTER},

volume = {7},

number = {5},

pages = {2085-2093},

abstract = {The creation of three-dimensional, crystalline-ordered nanoparticle (NP) structures linked by DNA has proved experimentally challenging. Here we aim to systematically study parameters that influence the relative thermodynamic and kinetic stability of such crystals. To avoid experimental bottlenecks and directly control molecular-scale parameters, we carry out molecular dynamics simulations of a coarse-grained model in which short DNA strands (6 to 12 bp) are tethered to a NP core. We examine the influence of the number of bases per strand L, number of linking bases l and the number of spacer bases s on the stability of crystal states. We also consider the effect of using a single linking NP type versus a binary linking system. We explicitly compute the free energy, entropy, and melting point T(M) for BCC and FCC lattices. We show that binary systems are preferable for generating BCC lattices, while a single NP type generates the most stable FCC crystals. We propose a simple model for short DNA strands that can account for T(M) of all our data. The model also indicates that the heat of fusion between crystal and amorphous phases grows linearly with l, providing a route to maximize the relative crystal stability.},

keywords = {Biophysics, DNA, Nanotechnology, Self Assembly},

pubstate = {published},

tppubtype = {article}

}

@article{hss10c,

title = {Theoretical Description of a DNA-Linked Nanoparticle Self-Assembly},

author = {Chia Wei Hsu and Francesco Sciortino and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/hss10.pdf},

doi = {10.1103/PhysRevLett.105.055502},

issn = {0031-9007},

year  = {2010},

date = {2010-07-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {105},

number = {5},

pages = {055502},

abstract = {Nanoparticles tethered with DNA strands are promising building blocks 

 for bottom-up nanotechnology, and a theoretical understanding is 

 important for future development. Here we build on approaches developed 

 in polymer physics to provide theoretical descriptions for the 

 equilibrium clustering and dynamics, as well as the self-assembly 

 kinetics of DNA-linked nanoparticles. Striking agreement is observed 

 between the theory and molecular modeling of DNA-tethered nanoparticles.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{brno10b,

title = {Current issues in research on structure-property relationships in 

 polymer nanocomposites},

author = {Josef Jancar and Jack F Douglas and Francis W Starr and Sanat K Kumar and Philippe Cassagnau and Alan J Lesser and Sanford S Sternstein and Markus J Buehler},

url = {http://fstarr.web.wesleyan.edu/publications/brno10.pdf},

doi = {10.1016/j.polymer.2010.04.074},

issn = {0032-3861},

year  = {2010},

date = {2010-07-01},

journal = {POLYMER},

volume = {51},

number = {15},

pages = {3321-3343},

abstract = {The understanding of the basic physical relationships between nano-scale 

 structural variables and the macroscale properties of polymer 

 nanocomposites remains in its infancy. The primary objective of this 

 article is to ascertain the state of the art regarding the understanding 

 and prediction of the macroscale properties of polymers reinforced with 

 nanometer-sized solid inclusions over a wide temperature range. We 

 emphasize that the addition of nanoparticles with large specific surface 

 area to polymer matrices leads to amplification of a number of rather 

 distinct molecular processes resulting from interactions between chains 

 and solid surfaces. This results in a ``non-classical'' response of 

 these systems to mechanical and electro-optical excitations when 

 measured on the macroscale. For example, nanoparticles are expected to 

 be particularly effective at modifying the intrinsic nano-scale dynamic 

 heterogeneity of polymeric glass-formation and, correspondingly, recent 

 simulations indicate that both the strength of particle interaction with 

 the polymer matrix and the particle concentration can substantially 

 influence the dynamic fragility of polymer glass-formation, a measure of 

 the strength of the temperature dependence of the viscosity or 

 structural relaxation time. Another basic characteristic of 

 nanoparticles in polymer matrices is the tendency for the particles to 

 associate into extended structures that can dominate the rheological, 

 viscoelastic and mechanical properties of the nanocomposite so that 

 thermodynamic factors that effect nanoparticle dispersion can be 

 crucially important. Opportunities to exploit knowledge gained from 

 understanding biomechanics of hierarchical biological protein materials 

 and potential applications in materials design and nanotechnology are 

 among future research challenges. Research on nanocomposites formed from 

 block copolymers and nanoparticles offers huge promise in molecular 

 electronics and photovoltaics. The surface functionalization of 

 nanoparticles by the grafting of polymer brushes is expected to play 

 important role in the designing of novel organic/inorganic nanocomposite 

 materials. The formation of bulk heterojunctions at the nanometer scale 

 leads to efficient dissociation of the charge pairs generated under 

 sunlight. Based on the presentations and discussion, we make 

 recommendations for future work in this area by the physics, chemistry, 

 and engineering communities. (C) 2010 Elsevier Ltd. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{hss10,

title = {Theoretical Description of a DNA-Linked Nanoparticle Self-Assembly},

author = {Hsu, Chia Wei and Sciortino, Francesco and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/hss10.pdf},

doi = {10.1103/PhysRevLett.105.055502},

issn = {0031-9007},

year  = {2010},

date = {2010-07-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {105},

number = {5},

pages = {055502},

abstract = {Nanoparticles tethered with DNA strands are promising building blocks for bottom-up nanotechnology, and a theoretical understanding is important for future development. Here we build on approaches developed in polymer physics to provide theoretical descriptions for the equilibrium clustering and dynamics, as well as the self-assembly kinetics of DNA-linked nanoparticles. Striking agreement is observed between the theory and molecular modeling of DNA-tethered nanoparticles.},

keywords = {Biophysics, DNA, Self Assembly},

pubstate = {published},

tppubtype = {article}

}

@article{brno10,

title = {Current issues in research on structure-property relationships in polymer nanocomposites},

author = {Jancar, Josef and Douglas, Jack F. and Starr, Francis W. and Kumar, Sanat K. and Cassagnau, Philippe and Lesser, Alan J. and Sternstein, Sanford S. and Buehler, Markus J.},

url = {http://fstarr.web.wesleyan.edu/publications/brno10.pdf},

doi = {10.1016/j.polymer.2010.04.074},

issn = {0032-3861},

year  = {2010},

date = {2010-07-01},

journal = {POLYMER},

volume = {51},

number = {15},

pages = {3321-3343},

abstract = {The understanding of the basic physical relationships between nano-scale structural variables and the macroscale properties of polymer nanocomposites remains in its infancy. The primary objective of this article is to ascertain the state of the art regarding the understanding and prediction of the macroscale properties of polymers reinforced with nanometer-sized solid inclusions over a wide temperature range. We emphasize that the addition of nanoparticles with large specific surface area to polymer matrices leads to amplification of a number of rather distinct molecular processes resulting from interactions between chains and solid surfaces. This results in a ``non-classical'' response of these systems to mechanical and electro-optical excitations when measured on the macroscale. For example, nanoparticles are expected to be particularly effective at modifying the intrinsic nano-scale dynamic heterogeneity of polymeric glass-formation and, correspondingly, recent simulations indicate that both the strength of particle interaction with the polymer matrix and the particle concentration can substantially influence the dynamic fragility of polymer glass-formation, a measure of the strength of the temperature dependence of the viscosity or structural relaxation time. Another basic characteristic of nanoparticles in polymer matrices is the tendency for the particles to associate into extended structures that can dominate the rheological, viscoelastic and mechanical properties of the nanocomposite so that thermodynamic factors that effect nanoparticle dispersion can be crucially important. Opportunities to exploit knowledge gained from understanding biomechanics of hierarchical biological protein materials and potential applications in materials design and nanotechnology are among future research challenges. Research on nanocomposites formed from block copolymers and nanoparticles offers huge promise in molecular electronics and photovoltaics. The surface functionalization of nanoparticles by the grafting of polymer brushes is expected to play important role in the designing of novel organic/inorganic nanocomposite materials. The formation of bulk heterojunctions at the nanometer scale leads to efficient dissociation of the charge pairs generated under sunlight. Based on the presentations and discussion, we make recommendations for future work in this area by the physics, chemistry, and engineering communities. (C) 2010 Elsevier Ltd. All rights reserved.},

keywords = {Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{kds10b,

title = {Morphology and Transport Properties of Two-Dimensional Sheet Polymers},

author = {Scott T Knauert and Jack F Douglas and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/kds10+cover.pdf},

doi = {10.1021/ma902081m},

issn = {0024-9297},

year  = {2010},

date = {2010-04-01},

journal = {MACROMOLECULES},

volume = {43},

number = {7},

pages = {3438-3445},

abstract = {Whereas there has been extensive theoretical and experimental 

 investigation of the properties of linear polymer chains in solution, 

 there has been far less work on sheet-like polymers having 2D 

 connectivity and 3D crumpled or collapsed shapes caused by thermal 

 fluctuations, attractive self-interactions, or both. Sheet-like polymers 

 arise in a variety of contexts ranging from self-assembled biological 

 membranes (e.g., the spectrin network of red blood cells, microtubules, 

 etc.) to nanocomposite additives to polymers (carbon nanotubes, 

 graphene, and clay sheets) and polymerized monolayers. We investigate 

 the equilibrium properties of this broad class of polymers using a 

 simple model of a sheet polymer with a locally square symmetry of the 

 connecting beads. We quantify the sheet morphology and the dilute-limit 

 hydrodynamic solution properties as a function of molecular mass and 

 sheet stiffness. First, we reproduce the qualitative findings of 

 previous work indicating that variable sheet stiffness results in a wide 

 variety of morphologies, including flat, crumpled or collapsed 

 spherical, cylindrical or tubular, and folded sheets that serve to 

 characterize our particular 2D polymer model. Transport properties are 

 of significant interest in characterizing polymeric materials, and we 

 provide the first numerical computations of these properties for sheet 

 polymers. Specifically, we calculate the intrinsic viscosity and 

 hydrodynamic radius of these sheet morphologies using a novel 

 path-integration technique and find good agreement of our numerical 

 results with previous theoretical scaling predictions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{kds10,

title = {Morphology and Transport Properties of Two-Dimensional Sheet Polymers},

author = {Knauert, Scott T. and Douglas, Jack F. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/kds10+cover.pdf},

doi = {10.1021/ma902081m},

issn = {0024-9297},

year  = {2010},

date = {2010-04-01},

journal = {MACROMOLECULES},

volume = {43},

number = {7},

pages = {3438-3445},

abstract = {Whereas there has been extensive theoretical and experimental investigation of the properties of linear polymer chains in solution, there has been far less work on sheet-like polymers having 2D connectivity and 3D crumpled or collapsed shapes caused by thermal fluctuations, attractive self-interactions, or both. Sheet-like polymers arise in a variety of contexts ranging from self-assembled biological membranes (e.g., the spectrin network of red blood cells, microtubules, etc.) to nanocomposite additives to polymers (carbon nanotubes, graphene, and clay sheets) and polymerized monolayers. We investigate the equilibrium properties of this broad class of polymers using a simple model of a sheet polymer with a locally square symmetry of the connecting beads. We quantify the sheet morphology and the dilute-limit hydrodynamic solution properties as a function of molecular mass and sheet stiffness. First, we reproduce the qualitative findings of previous work indicating that variable sheet stiffness results in a wide variety of morphologies, including flat, crumpled or collapsed spherical, cylindrical or tubular, and folded sheets that serve to characterize our particular 2D polymer model. Transport properties are of significant interest in characterizing polymeric materials, and we provide the first numerical computations of these properties for sheet polymers. Specifically, we calculate the intrinsic viscosity and hydrodynamic radius of these sheet morphologies using a novel path-integration technique and find good agreement of our numerical results with previous theoretical scaling predictions.},

keywords = {Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{rs10b,

title = {Rapid Transport of Water via a Carbon Nanotube Syringe},

author = {Jose L Rivera and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/rs10.pdf},

doi = {10.1021/jp906527c},

issn = {1932-7447},

year  = {2010},

date = {2010-03-01},

journal = {JOURNAL OF PHYSICAL CHEMISTRY C},

volume = {114},

number = {9},

pages = {3737-3742},

abstract = {The controlled flow of water molecules at the nanoscale is an initial 

 step to many fluidic processes ill nanotechnology. Here we show how thin 

 films of water call be drawn through a nanosyringe built from a carbon 

 nanotube membrane and a ``plunger''. By increasing the speed of 

 withdrawal of the plunger, we call obtain Molecular transport through 

 the membrane at flux rates exceeding 1()25 molecules cm(-2) s(-1). Above 

 I threshold speed around 0.25 nm/ns (25 cm/s), molecules cannot fill the 

 chamber created by the plunger motion as fast as the chamber expands, 

 and the resulting flux rate drops. By considering hydrophobic or 

 hydrophilic Plungers, we unexpectedly find that the nature of the 

 water-plunger interactions does not affect the flux rate or the 

 threshold plunger speed. While the water structure near the plunger 

 Surface differs significantly For different plunger interactions, the 

 failure of the film away From the plunger surface is responsible for 

 loss of transport. As I result, the surface interactions play a limited 

 role in controlling the flux.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{dhss10b,

title = {Valency Dependence of Polymorphism and Polyamorphism in 

 DNA-Functionalized Nanoparticles},

author = {Wei Dai and Chia Wei Hsu and Francesco Sciortino and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/dhss.pdf},

doi = {10.1021/la903031p},

issn = {0743-7463},

year  = {2010},

date = {2010-03-01},

journal = {LANGMUIR},

volume = {26},

number = {5},

pages = {3601-3608},

abstract = {Nanoparticles (NP) functionalized with single-stranded DNA (ssDNA) offer 

 a route to custom-designed, self-assembled nanomaterials with 

 potentially unusual properties, The bonding, selectivity of DNA 

 guarantees one-to-one binding to form double-stranded DNA (dsDNA), and 

 an appropriate base sequence results in head-to-tail binding linking NP 

 into networks. We explore the phase behavior and structure of a model 

 for NP functionalized with between 3 and 6 short ssDNA through 

 simulations of a coarse-grained molecular model, allowing us to examine 

 both the role of the number of attached strands (valency) and their 

 relative orientations. The NP assemble into networks where the number of 

 NP links is controlled by the number of attached strands, The large 

 length scale of the DNA links relative to the core NP size opens the 

 possibility for the formation of interpenetrating networks that give 

 rise 10 multiple thermodynamically distinct states. We find that the 

 3-functionalized NP have only a single phase transition between a dilute 

 solution of NPs and an assembled network state. 4-Functionalized NP 

 (with tetrahedral symmetry) exhibit four amorphous phases, or 

 polyamorphism, each higher density phase consisting of an additional 

 interpenetrating network. The two investigated geometries of 

 5-functionalized NP both exhibit two phase transitions and three 

 amorphouos phases. Like the 4-functionalized NP, the highest density 

 phase consists of interpenetrating networks, demonstrating that regular 

 symmetry is not a prerequisite for interpenetration to produce 

 thermodynamically distinct phases. The width of theh coexistence regions 

 for all phase transitions increase with increasing functionality. 

 Finally, for 6-functionalized NP with octahedral symmetry. the 

 possibility of observing disordered phases with significantly bonded 

 particles is preempted by the formation of ordered crystal phases, 

 Interestingly, the extreme softness of the potential combined with the 

 directional interaction allows for the formation of (at least) six 

 distinct crystalline structures (ie., polymorphism) consisting of up to 

 six interpenetrating simple cubic lattices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{rs10,

title = {Rapid Transport of Water via a Carbon Nanotube Syringe},

author = {Rivera, Jose L. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/rs10.pdf},

doi = {10.1021/jp906527c},

issn = {1932-7447},

year  = {2010},

date = {2010-03-01},

journal = {JOURNAL OF PHYSICAL CHEMISTRY C},

volume = {114},

number = {9},

pages = {3737-3742},

abstract = {The controlled flow of water molecules at the nanoscale is an initial step to many fluidic processes ill nanotechnology. Here we show how thin films of water call be drawn through a nanosyringe built from a carbon nanotube membrane and a ``plunger''. By increasing the speed of withdrawal of the plunger, we call obtain Molecular transport through the membrane at flux rates exceeding 1()25 molecules cm(-2) s(-1). Above I threshold speed around 0.25 nm/ns (25 cm/s), molecules cannot fill the chamber created by the plunger motion as fast as the chamber expands, and the resulting flux rate drops. By considering hydrophobic or hydrophilic Plungers, we unexpectedly find that the nature of the water-plunger interactions does not affect the flux rate or the threshold plunger speed. While the water structure near the plunger Surface differs significantly For different plunger interactions, the failure of the film away From the plunger surface is responsible for loss of transport. As I result, the surface interactions play a limited role in controlling the flux.},

keywords = {Nanotechnology, Water},

pubstate = {published},

tppubtype = {article}

}

@article{dhss10,

title = {Valency Dependence of Polymorphism and Polyamorphism in DNA-Functionalized Nanoparticles},

author = {Dai, Wei and Hsu, Chia Wei and Sciortino, Francesco and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/dhss.pdf},

doi = {10.1021/la903031p},

issn = {0743-7463},

year  = {2010},

date = {2010-03-01},

journal = {LANGMUIR},

volume = {26},

number = {5},

pages = {3601-3608},

abstract = {Nanoparticles (NP) functionalized with single-stranded DNA (ssDNA) offer a route to custom-designed, self-assembled nanomaterials with potentially unusual properties, The bonding, selectivity of DNA guarantees one-to-one binding to form double-stranded DNA (dsDNA), and an appropriate base sequence results in head-to-tail binding linking NP into networks. We explore the phase behavior and structure of a model for NP functionalized with between 3 and 6 short ssDNA through simulations of a coarse-grained molecular model, allowing us to examine both the role of the number of attached strands (valency) and their relative orientations. The NP assemble into networks where the number of NP links is controlled by the number of attached strands, The large length scale of the DNA links relative to the core NP size opens the possibility for the formation of interpenetrating networks that give rise 10 multiple thermodynamically distinct states. We find that the 3-functionalized NP have only a single phase transition between a dilute solution of NPs and an assembled network state. 4-Functionalized NP (with tetrahedral symmetry) exhibit four amorphous phases, or polyamorphism, each higher density phase consisting of an additional interpenetrating network. The two investigated geometries of 5-functionalized NP both exhibit two phase transitions and three amorphouos phases. Like the 4-functionalized NP, the highest density phase consists of interpenetrating networks, demonstrating that regular symmetry is not a prerequisite for interpenetration to produce thermodynamically distinct phases. The width of theh coexistence regions for all phase transitions increase with increasing functionality. Finally, for 6-functionalized NP with octahedral symmetry. the possibility of observing disordered phases with significantly bonded particles is preempted by the formation of ordered crystal phases, Interestingly, the extreme softness of the potential combined with the directional interaction allows for the formation of (at least) six distinct crystalline structures (ie., polymorphism) consisting of up to six interpenetrating simple cubic lattices.},

keywords = {Biophysics, DNA, Nanotechnology, Polyamorphism},

pubstate = {published},

tppubtype = {article}

}

@article{dks10b,

title = {Universal two-step crystallization of DNA-functionalized nanoparticles},

author = {Wei Dai and Sanat K Kumar and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/dks10.pdf},

doi = {10.1039/c0sm00484g},

issn = {1744-683X},

year  = {2010},

date = {2010-01-01},

journal = {SOFT MATTER},

volume = {6},

number = {24},

pages = {6130-6135},

abstract = {We examine the crystallization dynamics of nanoparticles reversibly 

 tethered by DNA hybridization. We show that the crystallization happens 

 readily only in a narrow temperature ``slot'', and always proceeds via 

 a two-step process, mediated by a highly-connected amorphous 

 intermediate. For lower temperature quenches, the dynamics of unzipping 

 strands in the amorphous state is sufficiently slow that crystallization 

 is kinetically hindered. This accounts for the well-documented 

 difficulty of forming crystals in these systems. The strong parallel to 

 the crystallization behavior of proteins and colloids suggests that 

 these disparate systems crystallize in an apparently universal manner.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{dks10,

title = {Universal two-step crystallization of DNA-functionalized nanoparticles},

author = {Dai, Wei and Kumar, Sanat K. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/dks10.pdf},

doi = {10.1039/c0sm00484g},

issn = {1744-683X},

year  = {2010},

date = {2010-01-01},

journal = {SOFT MATTER},

volume = {6},

number = {24},

pages = {6130-6135},

abstract = {We examine the crystallization dynamics of nanoparticles reversibly tethered by DNA hybridization. We show that the crystallization happens readily only in a narrow temperature ``slot'', and always proceeds via a two-step process, mediated by a highly-connected amorphous intermediate. For lower temperature quenches, the dynamics of unzipping strands in the amorphous state is sufficiently slow that crystallization is kinetically hindered. This accounts for the well-documented difficulty of forming crystals in these systems. The strong parallel to the crystallization behavior of proteins and colloids suggests that these disparate systems crystallize in an apparently universal manner.},

keywords = {Biophysics, DNA, Nanotechnology, Self Assembly},

pubstate = {published},

tppubtype = {article}

}

@article{xmysbsb,

title = {Appearance of a fractional Stokes-Einstein relation in water and a 

 structural interpretation of its onset},

author = {Limei Xu and Francesco Mallamace and Zhenyu Yan and Francis W Starr and Sergey V Buldyrev and Eugene H Stanley},

url = {http://fstarr.web.wesleyan.edu/publications/xmysbs.pdf},

doi = {10.1038/NPHYS1328},

issn = {1745-2473},

year  = {2009},

date = {2009-08-01},

journal = {NATURE PHYSICS},

volume = {5},

number = {8},

pages = {565-569},

abstract = {The Stokes-Einstein relation has long been regarded as one of the 

 hallmarks of transport in liquids. It predicts that the self-diffusion 

 constant D is proportional to (tau/T)(-1), where tau is the structural 

 relaxation time and T is the temperature. Here, we present experimental 

 data on water confirming that, below a crossover temperature T-x 

 approximate to 290 K, the Stokes-Einstein relation is replaced by a 

 `fractional' Stokes-Einstein relation D similar to (tau/T)(-zeta) with 

 zeta approximate to 3/5 (refs 1-6). We interpret the microscopic origin 

 of this crossover by analysing the OH-stretch region of the Fourier 

 transform infrared spectrum over a temperature range from 350 down to 

 200 K. Simultaneous with the onset of fractional Stokes-Einstein 

 behaviour, we find that water begins to develop a local structure 

 similar to that of low-density amorphous solid H2O. These data lead to 

 an interpretation that the fractional Stokes-Einstein relation in water 

 arises from a specific change in the local water structure. Computer 

 simulations of two molecular models further support this interpretation.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{xmysbs,

title = {Appearance of a fractional Stokes-Einstein relation in water and a structural interpretation of its onset},

author = {Xu, Limei and Mallamace, Francesco and Yan, Zhenyu and Starr, Francis W. and Buldyrev, Sergey V. and Stanley, H. Eugene},

url = {http://fstarr.web.wesleyan.edu/publications/xmysbs.pdf},

doi = {10.1038/NPHYS1328},

issn = {1745-2473},

year  = {2009},

date = {2009-08-01},

journal = {NATURE PHYSICS},

volume = {5},

number = {8},

pages = {565-569},

abstract = {The Stokes-Einstein relation has long been regarded as one of the hallmarks of transport in liquids. It predicts that the self-diffusion constant D is proportional to (tau/T)(-1), where tau is the structural relaxation time and T is the temperature. Here, we present experimental data on water confirming that, below a crossover temperature T-x approximate to 290 K, the Stokes-Einstein relation is replaced by a `fractional' Stokes-Einstein relation D similar to (tau/T)(-zeta) with zeta approximate to 3/5 (refs 1-6). We interpret the microscopic origin of this crossover by analysing the OH-stretch region of the Fourier transform infrared spectrum over a temperature range from 350 down to 200 K. Simultaneous with the onset of fractional Stokes-Einstein behaviour, we find that water begins to develop a local structure similar to that of low-density amorphous solid H2O. These data lead to an interpretation that the fractional Stokes-Einstein relation in water arises from a specific change in the local water structure. Computer simulations of two molecular models further support this interpretation.},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{hs09b,

title = {Interpenetration as a mechanism for liquid-liquid phase transitions},

author = {Chia Wei Hsu and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/hs.pdf},

doi = {10.1103/PhysRevE.79.041502},

issn = {1539-3755},

year  = {2009},

date = {2009-04-01},

journal = {PHYSICAL REVIEW E},

volume = {79},

number = {4, 1},

pages = {041502},

abstract = {We study simple lattice systems to demonstrate the influence of 

 interpenetrating bond networks on phase behavior. We promote 

 interpenetration by using a Hamiltonian with a weakly repulsive 

 interaction with nearest neighbors and an attractive interaction with 

 second-nearest neighbors. In this way, bond networks will form between 

 second-nearest neighbors, allowing for two (locally) distinct networks 

 to form. We obtain the phase behavior from analytic solution in the 

 mean-field approximation and exact solution on the Bethe lattice. We 

 compare these results with exact numerical results for the phase 

 behavior from grand canonical Monte Carlo simulations on square, cubic, 

 and tetrahedral lattices. All results show that these simple systems 

 exhibit rich phase diagrams with two fluid-fluid critical points and 

 three thermodynamically distinct phases. We also consider including 

 third-nearest-neighbor interactions, which give rise to a phase diagram 

 with four critical points and five thermodynamically distinct phases. 

 Thus the interpenetration mechanism provides a simple route to generate 

 multiple liquid phases in single-component systems, such as hypothesized 

 in water and observed in several model and experimental systems. 

 Additionally, interpenetration of many such networks appears plausible 

 in a recently considered material made from nanoparticles functionalized 

 by single-strands of DNA.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{hs09,

title = {Interpenetration as a mechanism for liquid-liquid phase transitions},

author = {Hsu, Chia Wei and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/hs.pdf},

doi = {10.1103/PhysRevE.79.041502},

issn = {1539-3755},

year  = {2009},

date = {2009-04-01},

journal = {PHYSICAL REVIEW E},

volume = {79},

number = {4, 1},

pages = {041502},

abstract = {We study simple lattice systems to demonstrate the influence of interpenetrating bond networks on phase behavior. We promote interpenetration by using a Hamiltonian with a weakly repulsive interaction with nearest neighbors and an attractive interaction with second-nearest neighbors. In this way, bond networks will form between second-nearest neighbors, allowing for two (locally) distinct networks to form. We obtain the phase behavior from analytic solution in the mean-field approximation and exact solution on the Bethe lattice. We compare these results with exact numerical results for the phase behavior from grand canonical Monte Carlo simulations on square, cubic, and tetrahedral lattices. All results show that these simple systems exhibit rich phase diagrams with two fluid-fluid critical points and three thermodynamically distinct phases. We also consider including third-nearest-neighbor interactions, which give rise to a phase diagram with four critical points and five thermodynamically distinct phases. Thus the interpenetration mechanism provides a simple route to generate multiple liquid phases in single-component systems, such as hypothesized in water and observed in several model and experimental systems. Additionally, interpenetration of many such networks appears plausible in a recently considered material made from nanoparticles functionalized by single-strands of DNA.},

keywords = {Polyamorphism},

pubstate = {published},

tppubtype = {article}

}

@article{hlss08b,

title = {Hierarchies of networked phases induced by multiple liquid-liquid 

 critical points},

author = {Chia Wei Hsu and Julio Largo and Francesco Sciortino and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/hlss.pdf},

doi = {10.1073/pnas.0804854105},

issn = {0027-8424},

year  = {2008},

date = {2008-09-01},

journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF 

 AMERICA},

volume = {105},

number = {37},

pages = {13711-13715},

abstract = {Nanoparticles and colloids functionalized by four single strands of DNA 

 can be thought of as designed analogs to tetrahedral network-forming 

 atoms and molecules, with a difference that the attached DNA strands 

 allow for control of the length scale of bonding relative to the core 

 size. We explore the behavior of an experimentally realized model for 

 nanoparticles functionalized by four single strands of DNA (a tetramer), 

 and show that this single-component model exhibits a rich phase diagram 

 with at least three critical points and four thermodynamically distinct 

 amorphous phases. We demonstrate that the additional critical points are 

 part of the Ising universality class, like the ordinary liquid-gas 

 critical point. The dense phases consist of a hierarchy of 

 interpenetrating networks, reminiscent of a woven cloth. Thus, bonding 

 specificity of DNA provides an effective route to generate new 

 nano-networked materials with polyamorphic behavior. The concept of 

 network interpenetration helps to explain the generation of multiple 

 liquid phases in sing le-component systems, suggested to occur in some 

 atomic and molecular network-forming fluids, including water and silica.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{hlss08,

title = {Hierarchies of networked phases induced by multiple liquid-liquid critical points},

author = {Hsu, Chia Wei and Largo, Julio and Sciortino, Francesco and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/hlss.pdf},

doi = {10.1073/pnas.0804854105},

issn = {0027-8424},

year  = {2008},

date = {2008-09-01},

journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA},

volume = {105},

number = {37},

pages = {13711-13715},

abstract = {Nanoparticles and colloids functionalized by four single strands of DNA can be thought of as designed analogs to tetrahedral network-forming atoms and molecules, with a difference that the attached DNA strands allow for control of the length scale of bonding relative to the core size. We explore the behavior of an experimentally realized model for nanoparticles functionalized by four single strands of DNA (a tetramer), and show that this single-component model exhibits a rich phase diagram with at least three critical points and four thermodynamically distinct amorphous phases. We demonstrate that the additional critical points are part of the Ising universality class, like the ordinary liquid-gas critical point. The dense phases consist of a hierarchy of interpenetrating networks, reminiscent of a woven cloth. Thus, bonding specificity of DNA provides an effective route to generate new nano-networked materials with polyamorphic behavior. The concept of network interpenetration helps to explain the generation of multiple liquid phases in sing le-component systems, suggested to occur in some atomic and molecular network-forming fluids, including water and silica.},

keywords = {Biophysics, DNA, Polyamorphism, Self Assembly, Water},

pubstate = {published},

tppubtype = {article}

}

@article{rds08b,

title = {Model for reversible nanoparticle assembly in a polymer matrix},

author = {Andrew J Rahedi and Jack F Douglas and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/rds.pdf},

doi = {10.1063/1.2815809},

issn = {0021-9606},

year  = {2008},

date = {2008-01-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {128},

number = {2},

pages = {024902},

abstract = {The clustering of nanoparticles (NPs) in solutions and polymer melts 

 depends sensitively on the strength and directionality of the NP 

 interactions involved, as well as the molecular geometry and 

 interactions of the dispersing fluids. Since clustering can strongly 

 influence the properties of polymer-NP materials, we aim to better 

 elucidate the mechanism of reversible self-assembly of highly symmetric 

 NPs into clusters under equilibrium conditions. Our results are based on 

 molecular dynamics simulations of icosahedral NP with a long-ranged 

 interaction intended to mimic the polymer-mediated interactions of a 

 polymer-melt matrix. To distinguish effects of polymer-mediated 

 interactions from bare NP interactions, we compare the NP assembly in 

 our coarse-grained model to the case where the NP interactions are 

 purely short ranged. For the ``control'' case of NPs with short-ranged 

 interactions and no polymer matrix, we find that the particles exhibit 

 ordinary phase separation. By incorporating physically plausible 

 long-ranged interactions, we suppress phase separation and qualitatively 

 reproduce the thermally reversible cluster formation found previously in 

 computations for NPs with short-ranged interactions in an explicit 

 polymer-melt matrix. We further characterize the assembly process by 

 evaluating the cluster properties and the location of the self-assembly 

 transition. Our findings are consistent with a theoretical model for 

 equilibrium clustering when the particle association is subject to a 

 constraint. In particular, the density dependence of the average cluster 

 mass exhibits a linear concentration dependence, in contrast to the 

 square root dependence found in freely associating systems. The 

 coarse-grained model we use to simulate NP in a polymer matrix shares 

 many features of potentials used to model colloidal systems. The model 

 should be practically valuable for exploring factors that control the 

 dispersion of NP in polymer matrices where explicit simulation of the 

 polymer matrix is too time consuming. (c) 2008 American Institute of 

 Physics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{rds08,

title = {Model for reversible nanoparticle assembly in a polymer matrix},

author = {Rahedi, Andrew J. and Douglas, Jack F. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/rds.pdf},

doi = {10.1063/1.2815809},

issn = {0021-9606},

year  = {2008},

date = {2008-01-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {128},

number = {2},

pages = {024902},

abstract = {The clustering of nanoparticles (NPs) in solutions and polymer melts depends sensitively on the strength and directionality of the NP interactions involved, as well as the molecular geometry and interactions of the dispersing fluids. Since clustering can strongly influence the properties of polymer-NP materials, we aim to better elucidate the mechanism of reversible self-assembly of highly symmetric NPs into clusters under equilibrium conditions. Our results are based on molecular dynamics simulations of icosahedral NP with a long-ranged interaction intended to mimic the polymer-mediated interactions of a polymer-melt matrix. To distinguish effects of polymer-mediated interactions from bare NP interactions, we compare the NP assembly in our coarse-grained model to the case where the NP interactions are purely short ranged. For the ``control'' case of NPs with short-ranged interactions and no polymer matrix, we find that the particles exhibit ordinary phase separation. By incorporating physically plausible long-ranged interactions, we suppress phase separation and qualitatively reproduce the thermally reversible cluster formation found previously in computations for NPs with short-ranged interactions in an explicit polymer-melt matrix. We further characterize the assembly process by evaluating the cluster properties and the location of the self-assembly transition. Our findings are consistent with a theoretical model for equilibrium clustering when the particle association is subject to a constraint. In particular, the density dependence of the average cluster mass exhibits a linear concentration dependence, in contrast to the square root dependence found in freely associating systems. The coarse-grained model we use to simulate NP in a polymer matrix shares many features of potentials used to model colloidal systems. The model should be practically valuable for exploring factors that control the dispersion of NP in polymer matrices where explicit simulation of the polymer matrix is too time consuming. (c) 2008 American Institute of Physics.},

keywords = {Nanocomposites, Polymers, Self Assembly},

pubstate = {published},

tppubtype = {article}

}

@article{rrs07b,

title = {Interaction of water with cap-ended defective and nondefective small 

 carbon nanotubes},

author = {Jose L Rivera and Jose L Rico and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/rrs.pdf},

doi = {10.1021/jp075989r},

issn = {1932-7447},

year  = {2007},

date = {2007-12-01},

journal = {JOURNAL OF PHYSICAL CHEMISTRY C},

volume = {111},

number = {51},

pages = {18899-18905},

abstract = {We present a theoretical study of the structure, local curvature angles, 

 and reactivity of cap-ended (7,0), defective and nondefective carbon 

 nanotubes. We find that the most reactive sites are the atoms that form 

 part of the caps even when the Stone-Wales defect is present. Each 

 carbon in the carbon nanotube is located at the top of a pyramidal 

 structure with three walls of 5-, 6-, or 7-carbon rings. Among the 

 carbons making up the caps, the most reactive sites are the top 

 pyramidal atoms between two 5-carbon rings and one 6-carbon ring and 

 each 5-carbon ring has attached another 5-carbon ring. The least 

 reactive sites are the top pyramidal atoms between three 6-carbon rings. 

 The. activity of each pyramidal structure is strongly correlated to its 

 local curvature angle. The dissociation of one water molecule on the 

 surface of the carbon nanotubes confirms the location of the most active 

 site. The dissociation of water produces a hydroxyl group and a hydrogen 

 atom united each to two adjacent carbon atoms. The dissociation process 

 of water on carbon nanotubes is energetically favorable starting from 

 the isolated molecules.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{rrs07,

title = {Interaction of water with cap-ended defective and nondefective small carbon nanotubes},

author = {Rivera, Jose L. and Rico, Jose L. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/rrs.pdf},

doi = {10.1021/jp075989r},

issn = {1932-7447},

year  = {2007},

date = {2007-12-01},

journal = {JOURNAL OF PHYSICAL CHEMISTRY C},

volume = {111},

number = {51},

pages = {18899-18905},

abstract = {We present a theoretical study of the structure, local curvature angles, and reactivity of cap-ended (7,0), defective and nondefective carbon nanotubes. We find that the most reactive sites are the atoms that form part of the caps even when the Stone-Wales defect is present. Each carbon in the carbon nanotube is located at the top of a pyramidal structure with three walls of 5-, 6-, or 7-carbon rings. Among the carbons making up the caps, the most reactive sites are the top pyramidal atoms between two 5-carbon rings and one 6-carbon ring and each 5-carbon ring has attached another 5-carbon ring. The least reactive sites are the top pyramidal atoms between three 6-carbon rings. The. activity of each pyramidal structure is strongly correlated to its local curvature angle. The dissociation of one water molecule on the surface of the carbon nanotubes confirms the location of the most active site. The dissociation of water produces a hydroxyl group and a hydrogen atom united each to two adjacent carbon atoms. The dissociation process of water on carbon nanotubes is energetically favorable starting from the isolated molecules.},

keywords = {Nanotechnology, Water},

pubstate = {published},

tppubtype = {article}

}

@article{mgss-pre07b,

title = {Connection of translational and rotational dynamical heterogeneities 

 with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye 

 relations in water},

author = {Marco G Mazza and Nicolas Giovambattista and Eugene H Stanley and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/mgss-pre07.pdf},

doi = {10.1103/PhysRevE.76.031203},

issn = {1539-3755},

year  = {2007},

date = {2007-09-01},

journal = {PHYSICAL REVIEW E},

volume = {76},

number = {3},

pages = {031203},

abstract = {We study the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations, D(t)=k(B)T/6 pi eta R and D(r)=k(B)T/8 pi eta R(3), where 

 D(t) and D(r) are the translational and rotational diffusivity, 

 respectively, T is the temperature, eta the viscosity, k(B) the 

 Boltzmann constant, and R the ``molecular'' radius. Our results are 

 based on molecular dynamics simulations of the extended simple point 

 charge model of water. We find that both the SE and SED relations break 

 down at low temperature. To explore the relationship between these 

 breakdowns and dynamical heterogeneities (DHs), we also calculate the SE 

 and SED relations for subsets of the 7% ``fastest'' and 7% 

 ``slowest'' molecules. We find that the SE and SED relations break 

 down in both subsets, and that the breakdowns occur on all scales of 

 mobility. Thus these breakdowns appear to be generalized phenomena, in 

 contrast with a view where only the most mobile molecules are the origin 

 of the breakdown of the SE and SED relations, embedded in an inactive 

 background where these relations hold. At low temperature, the SE and 

 SED relations in both subsets of molecules are replaced with 

 ``fractional'' SE and SED relations, D(t)similar to(tau/T)(-xi)(t) and 

 D(r)similar to(tau/T)(-xi)(r), where xi(t)approximate to 0.84(< 1) and 

 xi(r)approximate to 0.75(< 1). We also find that there is a decoupling 

 between rotational and translational motion, and that this decoupling 

 occurs in both the fastest and slowest subsets of molecules. Further, we 

 find that, the decoupling increases upon cooling, but that the 

 probability of a molecule being classified as both translationally and 

 rotationally fastest also increases. To study the effect of time scale 

 for SE and SED breakdown and decoupling, we introduce a time-dependent 

 version of the SE and SED relations, and a time-dependent function that 

 measures the extent of decoupling. Our results suggest that both the 

 decoupling and SE and SED breakdowns originate at a time scale 

 corresponding to the end of the cage regime, when diffusion starts. This 

 is also the time scale when the DHs are more relevant. Our work also 

 demonstrates that selecting DHs on the basis of translational or 

 rotational motion more strongly biases the calculation of diffusion 

 constants than other dynamical properties such as relaxation times.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{mgss-pre07,

title = {Connection of translational and rotational dynamical heterogeneities with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in water},

author = {Mazza, Marco G. and Giovambattista, Nicolas and Stanley, H. Eugene and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/mgss-pre07.pdf},

doi = {10.1103/PhysRevE.76.031203},

issn = {1539-3755},

year  = {2007},

date = {2007-09-01},

journal = {PHYSICAL REVIEW E},

volume = {76},

number = {3},

pages = {031203},

abstract = {We study the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations, D(t)=k(B)T/6 pi eta R and D(r)=k(B)T/8 pi eta R(3), where D(t) and D(r) are the translational and rotational diffusivity, respectively, T is the temperature, eta the viscosity, k(B) the Boltzmann constant, and R the ``molecular'' radius. Our results are based on molecular dynamics simulations of the extended simple point charge model of water. We find that both the SE and SED relations break down at low temperature. To explore the relationship between these breakdowns and dynamical heterogeneities (DHs), we also calculate the SE and SED relations for subsets of the 7% ``fastest'' and 7% ``slowest'' molecules. We find that the SE and SED relations break down in both subsets, and that the breakdowns occur on all scales of mobility. Thus these breakdowns appear to be generalized phenomena, in contrast with a view where only the most mobile molecules are the origin of the breakdown of the SE and SED relations, embedded in an inactive background where these relations hold. At low temperature, the SE and SED relations in both subsets of molecules are replaced with ``fractional'' SE and SED relations, D(t)similar to(tau/T)(-xi)(t) and D(r)similar to(tau/T)(-xi)(r), where xi(t)approximate to 0.84(< 1) and xi(r)approximate to 0.75(< 1). We also find that there is a decoupling between rotational and translational motion, and that this decoupling occurs in both the fastest and slowest subsets of molecules. Further, we find that, the decoupling increases upon cooling, but that the probability of a molecule being classified as both translationally and rotationally fastest also increases. To study the effect of time scale for SE and SED breakdown and decoupling, we introduce a time-dependent version of the SE and SED relations, and a time-dependent function that measures the extent of decoupling. Our results suggest that both the decoupling and SE and SED breakdowns originate at a time scale corresponding to the end of the cage regime, when diffusion starts. This is also the time scale when the DHs are more relevant. Our work also demonstrates that selecting DHs on the basis of translational or rotational motion more strongly biases the calculation of diffusion constants than other dynamical properties such as relaxation times.},

keywords = {Dynamic Heterogeneity, Glass Formation, Water},

pubstate = {published},

tppubtype = {article}

}

@article{kds07b,

title = {The effect of nanoparticle shape on polymer-nanocomposite rheology and 

 tensile strength},

author = {Scott T Knauert and Jack F Douglas and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/kds.pdf},

doi = {10.1002/polb.21176},

issn = {0887-6266},

year  = {2007},

date = {2007-07-01},

journal = {JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS},

volume = {45},

number = {14},

pages = {1882-1897},

abstract = {Nanoparticles can influence the properties of polymer materials by a 

 variety of mechanisms. With fullerene, carbon nanotube, and clay or 

 graphene sheet nanocomposites in mind, we investigate how particle shape 

 influences the melt shear viscosity eta and the tensile strength tau, 

 which we determine via molecular dynamics simulations. Our simulations 

 of compact (icosahedral), tube or rod-like, and sheet-like model 

 nanoparticles, all at a volume fraction phi approximate to 0.05, 

 indicate an order of magnitude increase in the viscosity 17 relative to 

 the pure melt. This finding evidently can not be explained by continuum 

 hydrodynamics and we provide evidence that the eta increase in our model 

 nanocomposites has its origin in chain bridging between the 

 nanoparticles. We find that this increase is the largest for the 

 rod-like nanoparticles and least for the sheet-like nanoparticles. 

 Curiously, the enhancements of 17 and tau exhibit opposite trends with 

 increasing chain length N and with particle shape anisotropy. Evidently, 

 the concept of bridging chains alone cannot account for the increase in 

 tau and we suggest that the deformability or flexibility of the sheet 

 nanoparticles contributes to nanocomposite strength and toughness by 

 reducing the relative value of the Poisson ratio of the composite. The 

 molecular dynamics simulations in the present work focus on the 

 reference case where the modification of the melt structure associated 

 with glass-formation and entanglement interactions should not be an 

 issue. Since many applications require good particle dispersion, we also 

 focus on the case where the polymer-particle interactions favor 

 nanoparticle dispersion. Our simulations point to a substantial 

 contribution of nanoparticle shape to both mechanical and processing 

 properties of polymer nanocomposites. (c) 2007 Wiley Periodicals, Inc.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{kds07,

title = {The effect of nanoparticle shape on polymer-nanocomposite rheology and tensile strength},

author = {Knauert, Scott T. and Douglas, Jack F. and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/kds.pdf},

doi = {10.1002/polb.21176},

issn = {0887-6266},

year  = {2007},

date = {2007-07-01},

journal = {JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS},

volume = {45},

number = {14},

pages = {1882-1897},

abstract = {Nanoparticles can influence the properties of polymer materials by a variety of mechanisms. With fullerene, carbon nanotube, and clay or graphene sheet nanocomposites in mind, we investigate how particle shape influences the melt shear viscosity eta and the tensile strength tau, which we determine via molecular dynamics simulations. Our simulations of compact (icosahedral), tube or rod-like, and sheet-like model nanoparticles, all at a volume fraction phi approximate to 0.05, indicate an order of magnitude increase in the viscosity 17 relative to the pure melt. This finding evidently can not be explained by continuum hydrodynamics and we provide evidence that the eta increase in our model nanocomposites has its origin in chain bridging between the nanoparticles. We find that this increase is the largest for the rod-like nanoparticles and least for the sheet-like nanoparticles. Curiously, the enhancements of 17 and tau exhibit opposite trends with increasing chain length N and with particle shape anisotropy. Evidently, the concept of bridging chains alone cannot account for the increase in tau and we suggest that the deformability or flexibility of the sheet nanoparticles contributes to nanocomposite strength and toughness by reducing the relative value of the Poisson ratio of the composite. The molecular dynamics simulations in the present work focus on the reference case where the modification of the melt structure associated with glass-formation and entanglement interactions should not be an issue. Since many applications require good particle dispersion, we also focus on the case where the polymer-particle interactions favor nanoparticle dispersion. Our simulations point to a substantial contribution of nanoparticle shape to both mechanical and processing properties of polymer nanocomposites. (c) 2007 Wiley Periodicals, Inc.},

keywords = {Nanocomposites, Nanotechnology, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{kbbpssb,

title = {Relation between the Widom line and the breakdown of the Stokes-Einstein 

 relation in supercooled water},

author = {Pradeep Kumar and Sergey V Buldyrev and Stephen R Becker and Peter H Poole and Francis W Starr and Eugene H Stanley},

url = {http://fstarr.web.wesleyan.edu/publications/kbbpss.pdf},

doi = {10.1073/pnas.0702608104},

issn = {0027-8424},

year  = {2007},

date = {2007-06-01},

journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF 

 AMERICA},

volume = {104},

number = {23},

pages = {9575-9579},

abstract = {Supercooled water exhibits a breakdown of the Stokes-Einstein relation 

 between the diffusion constant D and the alpha relaxation time 

 $tau(alpha)$. For water simulated with two different potentials, TIP5P and 

 ST2, we find that the temperature of the decoupling of diffusion and 

 alpha relaxation correlates with the temperature of the maximum in 

 specific heat that corresponds to crossing the Widom line T(W)(P). 

 Specifically, we find that our results for D tau(alpha)/T collapse onto 

 a single ``master curve'' if temperature is replaced by T - T(W)(P). 

 We further find that the size of the mobile molecule clusters (dynamical 

 heterogeneities) increases sharply near T(W)w(P). Moreover, our 

 calculations of mobile particle cluster size < n(t*)>(w) for different 

 pressures, where t* is the time for which the mobile particle cluster 

 size is largest, also collapse onto a single master curve if T is, 

 replaced by T - T(W)(P). The crossover to a more locally structured low 

 density liquid (LDL) as T -> T(W)(P) appears to be well correlated both 

 with the breakdown of the Stokes-Einstein relation and with the growth 

 of dynamic heterogeneities. Our results are consistent with the 

 possibility that the breakdown of the SE relation in supercooled water 

 at low pressures is associated with the hypothesized liquid-liquid phase 

 transition.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{kbbpss,

title = {Relation between the Widom line and the breakdown of the Stokes-Einstein relation in supercooled water},

author = {Kumar, Pradeep and Buldyrev, Sergey V. and Becker, Stephen R. and Poole, Peter H. and Starr, Francis W. and Stanley, H. Eugene},

url = {http://fstarr.web.wesleyan.edu/publications/kbbpss.pdf},

doi = {10.1073/pnas.0702608104},

issn = {0027-8424},

year  = {2007},

date = {2007-06-01},

journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA},

volume = {104},

number = {23},

pages = {9575-9579},

abstract = {Supercooled water exhibits a breakdown of the Stokes-Einstein relation between the diffusion constant D and the alpha relaxation time $tau(alpha)$. For water simulated with two different potentials, TIP5P and ST2, we find that the temperature of the decoupling of diffusion and alpha relaxation correlates with the temperature of the maximum in specific heat that corresponds to crossing the Widom line T(W)(P). Specifically, we find that our results for D tau(alpha)/T collapse onto a single ``master curve'' if temperature is replaced by T - T(W)(P). We further find that the size of the mobile molecule clusters (dynamical heterogeneities) increases sharply near T(W)w(P). Moreover, our calculations of mobile particle cluster size < n(t*)>(w) for different pressures, where t* is the time for which the mobile particle cluster size is largest, also collapse onto a single master curve if T is, replaced by T - T(W)(P). The crossover to a more locally structured low density liquid (LDL) as T -> T(W)(P) appears to be well correlated both with the breakdown of the Stokes-Einstein relation and with the growth of dynamic heterogeneities. Our results are consistent with the possibility that the breakdown of the SE relation in supercooled water at low pressures is associated with the hypothesized liquid-liquid phase transition.},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{lssl,

title = {Self-assembling DNA dendrimers: A numerical study},

author = {Julio Largo and Francis W Starr and Francesco Sciortino},

url = {http://fstarr.web.wesleyan.edu/publications/lss.pdf},

doi = {10.1021/la063036z},

issn = {0743-7463},

year  = {2007},

date = {2007-05-01},

journal = {LANGMUIR},

volume = {23},

number = {11},

pages = {5896-5905},

abstract = {DNA is increasingly used as a specific linker to template nanostructured 

 materials. We present a molecular dynamics simulation study of a simple 

 DNA-dendrimer model designed to capture the basic characteristics of the 

 biological interactions, where selectivity and strong cooperativity play 

 an important role. Exploring a large set of densities and temperatures, 

 we follow the progressive formation of a percolating large-scale network 

 whose connectivity can be described by random percolation theory. We 

 identify the relative regions of network formation and kinetic arrest 

 versus phase separation and show that the location of the two-phase 

 region can be interpreted in the same framework as reduced valency 

 models. This correspondence provides guidelines for designing stable, 

 equilibrium self-assembled low-density networks. Finally, we demonstrate 

 a relation between bonding and dynamics, by showing that the temperature 

 dependence of the diffusion constant is controlled by the number of 

 fully unbonded dendrimers.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{lss,

title = {Self-assembling DNA dendrimers: A numerical study},

author = {Largo, Julio and Starr, Francis W. and Sciortino, Francesco},

url = {http://fstarr.web.wesleyan.edu/publications/lss.pdf},

doi = {10.1021/la063036z},

issn = {0743-7463},

year  = {2007},

date = {2007-05-01},

journal = {LANGMUIR},

volume = {23},

number = {11},

pages = {5896-5905},

abstract = {DNA is increasingly used as a specific linker to template nanostructured materials. We present a molecular dynamics simulation study of a simple DNA-dendrimer model designed to capture the basic characteristics of the biological interactions, where selectivity and strong cooperativity play an important role. Exploring a large set of densities and temperatures, we follow the progressive formation of a percolating large-scale network whose connectivity can be described by random percolation theory. We identify the relative regions of network formation and kinetic arrest versus phase separation and show that the location of the two-phase region can be interpreted in the same framework as reduced valency models. This correspondence provides guidelines for designing stable, equilibrium self-assembled low-density networks. Finally, we demonstrate a relation between bonding and dynamics, by showing that the temperature dependence of the diffusion constant is controlled by the number of fully unbonded dendrimers.},

keywords = {Biophysics, DNA, Nanotechnology, Self Assembly},

pubstate = {published},

tppubtype = {article}

}

@article{ksbsb,

title = {Effect of water-wall interaction potential on the properties of 

 nanoconfined water},

author = {Pradeep Kumar and Francis W Starr and Sergey V Buldyrev and Eugene H Stanley},

url = {http://fstarr.web.wesleyan.edu/publications/ksbs.pdf},

doi = {10.1103/PhysRevE.75.011202},

issn = {1539-3755},

year  = {2007},

date = {2007-01-01},

journal = {PHYSICAL REVIEW E},

volume = {75},

number = {1, 1},

pages = {011202},

abstract = {Much of the understanding of bulk liquids has progressed through study 

 of the limiting case in which molecules interact via purely repulsive 

 forces, such as a hard-core or ``repulsive ramp'' potential. In the 

 same spirit, we report progress on the understanding of confined water 

 by examining the behavior of waterlike molecules interacting with planar 

 walls via purely repulsive forces and compare our results with those 

 obtained for Lennard-Jones (LJ) interactions between the molecules and 

 the walls. Specifically, we perform molecular dynamics simulations of 

 512 waterlike molecules interacting via the TIP5P potential and confined 

 between two smooth planar walls that are separated by 1.1 nm. At this 

 separation, there are either two or three molecular layers of water, 

 depending on density. We study two different forms of repulsive 

 confinement, when the water-wall interaction potential is either (i) 

 1/r(9) or (ii) a WCA-like repulsive potential. We find that the 

 thermodynamic, dynamic, and structural properties of the liquid in 

 purely repulsive confinements qualitatively match those for a system 

 with a pure LJ attraction to the wall. In previous studies that include 

 attractions, freezing into monolayer or trilayer ice was seen for this 

 wall separation. Using the same separation as these previous studies, we 

 find that the crystal state is not stable with 1/r(9) repulsive walls 

 but is stable with WCA-like repulsive confinement. However, by carefully 

 adjusting the separation of the plates with 1/r(9) repulsive 

 interactions so that the effective space available to the molecules is 

 the same as that for LJ confinement, we find that the same crystal 

 phases are stable. This result emphasizes the importance of comparing 

 systems only using the same effective confinement, which may differ from 

 the geometric separation of the confining surfaces.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}