Publications

@article{mds21,

title = {Explaining the Sensitivity of Polymer Segmental Relaxation to Additive Size Based on the Localization Model},

author = {Thomas Q. McKenzie-Smith and Jack F. Douglas and Francis W. Starr},

url = {https://fstarr.wescreates.wesleyan.edu/publications/mds21.pdf},

doi = {10.1103/PhysRevLett.127.277802},

year  = {2021},

date = {2021-12-30},

journal = {Phys. Rev. Lett.},

volume = {127},

pages = {277802},

publisher = {American Physical Society},

keywords = {Glass Formation, Nanocomposites, Nanotechnology},

pubstate = {published},

tppubtype = {article}

}

@article{zsd21,

title = {Activation free energy gradient controls interfacial mobility gradient in thin polymer films},

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

url = {https://fstarr.wescreates.wesleyan.edu/publications/zsd21.pdf},

doi = {10.1063/5.0064866},

year  = {2021},

date = {2021-11-07},

journal = {The Journal of Chemical Physics},

volume = {155},

number = {17},

pages = {174901},

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

pubstate = {published},

tppubtype = {article}

}

@article{esk21,

title = {Detecting bound polymer layers in attractive polymeranoparticle hybrids},

author = {Hamed Emamy and Francis W. Starr and Sanat K. Kumar},

url = {https://https://fstarr.wescreates.wesleyan.edu/publications/esk21.pdf},

doi = {10.1039/D1NR02395K},

year  = {2021},

date = {2021-07-01},

journal = {Nanoscale},

volume = {13},

pages = {12910-12915},

publisher = {The Royal Society of Chemistry},

keywords = {Glass Formation, Nanocomposites, Nanotechnology},

pubstate = {published},

tppubtype = {article}

}

@article{zdcs21,

title = {Structure and Dynamics of Star Polymer Films from Coarse-Grained Molecular Simulations},

author = {Wengang Zhang and Jack F. Douglas and Alexandros Chremos and Francis W. Starr},

url = {https://https://fstarr.wescreates.wesleyan.edu/publications/zdcs21.pdf},

doi = {10.1021/acs.macromol.1c00504},

year  = {2021},

date = {2021-06-04},

journal = {Macromolecules},

volume = {54},

number = {12},

pages = {5344-5353},

keywords = {Glass Formation, Nanotechnology, Thin Films},

pubstate = {published},

tppubtype = {article}

}

@article{leds21,

title = {Effects of Chain Length on the Structure and Dynamics of Semidilute Nanoparticle-Polymer Composites},

author = {Ari Y. Liu and Hamed Emamy and Jack F. Douglas and Francis W. Starr},

url = {http://fstarr.wescreates.wesleyan.edu/publications/leds21.pdf},

doi = {10.1021/acs.macromol.0c02500},

year  = {2021},

date = {2021-03-23},

journal = {Macromolecules},

volume = {54},

number = {7},

pages = {3041-3051},

keywords = {Glass Formation, Nanocomposites, Nanotechnology},

pubstate = {published},

tppubtype = {article}

}

@article{gplsd20,

title = {Predictive relation for the α-relaxation time of a coarse-grained polymer melt under steady shear},

author = {Andrea Giuntoli and Francesco Puosi and Dino Leporini and Francis W Starr and Jack F Douglas},

url = {http://fstarr.wescreates.wesleyan.edu/publications/gplsd20.pdf},

doi = {10.1126/sciadv.aaz0777},

year  = {2020},

date = {2020-04-24},

journal = {Science Advances},

volume = {6},

number = {17},

publisher = {American Association for the Advancement of Science},

abstract = {We examine the influence of steady shear on structural relaxation in a simulated coarse-grained unentangled polymer melt over a wide range of temperature and shear rates. Shear is found to progressively suppress the $alpha$-relaxation process observed in the intermediate scattering function, leading ultimately to a purely inertially dominated $alpha$-relaxation at high shear rates, a trend similar to increasing temperature. On the basis of a scaling argument emphasizing dynamic heterogeneity in cooled liquids and its alteration under material deformation, we deduce and validate a parameter-free scaling relation for both the structural relaxation time 

$tau_alpha$ from the intermediate scattering function and the textquotedblleftstretching exponenttextquotedblright 

$beta$ quantifying the extent of dynamic heterogeneity over the entire range of temperatures and shear rates that we can simulate.},

keywords = {Glass Formation, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{fecds20,

title = {Dynamic heterogeneity and collective motion in star polymer melts},

author = {Jinpeng Fan and Hamed Emamy and Alexandros Chremos and Jack F Douglas and Francis W Starr},

url = {https://fstarr.wescreates.wesleyan.edu/publications/fecds20.pdf},

doi = {10.1063/1.5135731},

year  = {2020},

date = {2020-01-01},

journal = {The Journal of Chemical Physics},

volume = {152},

number = {5},

pages = {054904},

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

pubstate = {published},

tppubtype = {article}

}

@article{zsd20,

title = {Reconciling computational and experimental trends in the temperature dependence of the interfacial mobility of polymer films},

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

url = {https://fstarr.wescreates.wesleyan.edu/publications/zsd20.pdf},

doi = {10.1063/1.5144262},

year  = {2020},

date = {2020-01-01},

journal = {The Journal of Chemical Physics},

volume = {152},

number = {12},

pages = {124703},

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

pubstate = {published},

tppubtype = {article}

}

@article{szds20,

title = {How Does Monomer Structure Affect the Interfacial Dynamics of Supported Ultrathin Polymer Films?},

author = {Amber N Storey and Wengang Zhang and Jack F Douglas and Francis W Starr},

url = {http://fstarr.wescreates.wesleyan.edu/publications/szds20.pdf},

doi = {10.1021/acs.macromol.0c01413},

year  = {2020},

date = {2020-01-01},

journal = {Macromolecules},

volume = {53},

number = {21},

pages = {9654-9664},

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

pubstate = {published},

tppubtype = {article}

}

@article{gsp19,

title = {State variables for glasses: The case of amorphous ice},

author = {Nicolas Giovambattista and Francis W Starr and Peter H Poole},

url = {http://fstarr.web.wesleyan.edu/publications/gsp19.pdf},

doi = {10.1063/1.5092586},

year  = {2019},

date = {2019-06-10},

journal = {The Journal of Chemical Physics},

volume = {150},

number = {22},

pages = {224502},

abstract = {Glasses are out-of-equilibrium systems whose state cannot be uniquely defined by the usual set of equilibrium state variables. Here, we seek to identify an expanded set of variables that uniquely define the state of a glass. The potential energy landscape (PEL) formalism is a useful approach within statistical mechanics to describe supercooled liquids and glasses. We use the PEL formalism and computer simulations to study the transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA). We employ the ST2 water model, which exhibits an abrupt first-order-like phase transition from LDA to HDA, similar to that observed in experiments. We prepare a number of distinct samples of both LDA and HDA that have completely different preparation histories. We then study the evolution of these LDA and HDA samples during compression and decompression at temperatures sufficiently low that annealing is absent and also during heating. We find that the evolution of each glass sample, during compression/decompression or heating, is uniquely determined by six macroscopic properties of the initial glass sample. These six quantities consist of three conventional thermodynamic state variables, the number of molecules N, the system volume V, and the temperature T, as well as three properties of the PEL, the inherent structure (IS) energy EIS, the IS pressure PIS, and the average curvature of the PEL at the IS ?IS. In other words, (N,V,T,EIS,PIS,?IS) are state variables that define the glass state in the case of amorphous ice. An interpretation of our results in terms of the PEL formalism is provided. Since the behavior of water in the glassy state is more complex than for most substances, our results suggest that these six state variables may be applicable to amorphous solids in general and that there may be situations in which fewer than six variables would be sufficient to define the state of a glass.},

keywords = {Glass Formation, Water},

pubstate = {published},

tppubtype = {article}

}

@article{zepvsd19,

title = {The interfacial zone in thin polymer films and around nanoparticles in polymer nanocomposites},

author = {Wengang Zhang and Hamed Emamy and Beatriz A Pazmiño Betancourt and Fernando Vargas-Lara and Francis W Starr and Jack F Douglas},

url = {http://fstarr.web.wesleyan.edu/publications/zepvsd19.pdf},

doi = {10.1063/1.5119269},

year  = {2019},

date = {2019-01-01},

journal = {The Journal of Chemical Physics},

volume = {151},

number = {12},

pages = {124705},

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

pubstate = {published},

tppubtype = {article}

}

@article{zsd19,

title = {Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and Its Relation to Relaxation},

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

url = {http://fstarr.web.wesleyan.edu/publications/zsd19.pdf},

doi = {10.1021/acs.jpcb.9b04155},

year  = {2019},

date = {2019-01-01},

journal = {The Journal of Physical Chemistry B},

volume = {123},

number = {27},

pages = {5935-5941},

note = {PMID: 31192601},

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

pubstate = {published},

tppubtype = {article}

}

@article{eks18,

title = {Diminishing Interfacial Effects with Decreasing Nanoparticle Size in Polymer-Nanoparticle Composites},

author = {Hamed Emamy and Sanat K Kumar and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/eks18.pdf},

doi = {10.1103/PhysRevLett.121.207801},

year  = {2018},

date = {2018-11-01},

journal = {Phys. Rev. Lett.},

volume = {121},

pages = {207801},

publisher = {American Physical Society},

keywords = {Glass Formation, Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{zds18,

title = {Why we need to look beyond the glass transition temperature to characterize the dynamics of thin supported polymer films},

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

url = {http://fstarr.web.wesleyan.edu/publications/zds18.pdf},

doi = {10.1073/pnas.1722024115},

issn = {0027-8424},

year  = {2018},

date = {2018-05-14},

journal = {Proceedings of the National Academy of Sciences},

publisher = {National Academy of Sciences},

abstract = {The disparate results for Tg shifts of polymer thin films raise the question of whether Tg provides a good metric to characterize changes in the overall dynamics of these films. Our work demonstrates how different techniques to measure Tg are influenced by the relaxation gradient across the film. We find that different measurement methods can provide contradictory Tg estimates, depending on their sensitivity to dynamics near the substrate, providing a possible explanation for prior contradictory results. We take advantage of these differences by combining Tg estimates to predict Tg near the substrate. Our findings should be useful for polymer thin film applications, including microelectronic devices, lithium battery technology, and the development of biomedical devices.There is significant variation in the reported magnitude and even the sign of Tg shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate Tg in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that Tg based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer–substrate interaction strength ε. In contrast, we find that Tg based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but Tg increases when ε exceeds the polymer–polymer interaction strength. We show that these qualitatively different trends in Tg reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of Tg in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of Tg. Our results emphasize the limitations of using Tg to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of Tg can be combined to predict local changes in Tg near the substrate, providing a simple method to infer information about the mobility gradient.},

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

pubstate = {published},

tppubtype = {article}

}

@article{psd18,

title = {String-like collective motion in the α- and β-relaxation of a coarse-grained polymer melt},

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

url = {http://fstarr.web.wesleyan.edu/publications/psd18.pdf},

doi = {10.1063/1.5009442},

year  = {2018},

date = {2018-01-01},

journal = {The Journal of Chemical Physics},

volume = {148},

number = {10},

pages = {104508},

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

pubstate = {published},

tppubtype = {article}

}

@article{zds17-2,

title = {Effects of a “bound” substrate layer on the dynamics of supported polymer films},

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

url = {http://fstarr.web.wesleyan.edu/publications/zds17-2.pdf},

doi = {10.1063/1.4994064},

year  = {2017},

date = {2017-07-25},

journal = {The Journal of Chemical Physics},

volume = {147},

pages = {044901},

abstract = {It is widely appreciated that an attractive polymer-substrate interaction can slow relaxation in thin supported polymer films and polymer nanocomposites. Recent measurements and simulations on nancomposites have indicated that this slowing of polymer dynamics occurs more strongly near a highly attractive particle surface where a “bound” layer having a much lower mobility can form, strongly influencing the thermodynamics and dynamics of the film. Here we use molecular simulations to show that a bound interfacial layer having a very similar nature arises in thin supported polymer films when the polymer-polymer attraction is stronger than the polymer-polymer interaction strength. This bound polymer layer effectively insulates the remainder of the film from the strong interfacial interactions, and the resulting thermodynamically determined Tg is relatively insensitive to the polymer-substrate interaction strength when it exceeds that of the polymer-polymer interactions. The presence of this layer gives rise to an additional relaxation process in the self-intermediate scattering function that is not observed in the bulk material and leads to a slowing down of the average relaxation time of the film as a whole. On the other hand, the average relaxation time of the film outside the bound layer does not grow in proportion to the strength of the substrate attraction due to the weak coupling of the substrate relaxation to the relaxation in the interior of the film. At large substrate attraction, the bound layer effectively “cloaks” the substrate, reducing the effect of the polymer-surface interaction on Tg.},

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

pubstate = {published},

tppubtype = {article}

}

@article{gsp17,

title = {Influence of sample preparation on the transformation of low-density to high-density amorphous ice: An explanation based on the potential energy landscape},

author = {Nicolas Giovambattista and Francis W. Starr and Peter H. Poole},

url = {http://fstarr.web.wesleyan.edu/publications/gsp17.pdf},

doi = {10.1063/1.499356},

year  = {2017},

date = {2017-07-25},

journal = {The Journal of Chemical Physics},

volume = {147},

number = {4},

pages = {044501},

abstract = {Experiments and computer simulations of the transformations of amorphous ices display different behaviors depending on sample preparation methods and on the rates of change of temperature and pressure to which samples are subjected. In addition to these factors, simulation results also depend strongly on the chosen water model. Using computer simulations of the ST2 water model, we study how the sharpness of the compression-induced transition from low-density amorphous ice (LDA) to high-density amorphous ice (HDA) is influenced by the preparation of LDA. By studying LDA samples prepared using widely different procedures, we find that the sharpness of the LDA-to-HDA transformation is correlated with the depth of the initial LDA sample in the potential energy landscape (PEL), as characterized by the inherent structure energy. Our results show that the complex phenomenology of the amorphous ices reported in experiments and computer simulations can be understood and predicted in a unified way from knowledge of the PEL of the system.},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{zds17,

title = {Dynamical heterogeneity in a vapor-deposited polymer glass},

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

url = {http://fstarr.web.wesleyan.edu/publications/zds17.pdf},

doi = {10.1063/1.4976542},

year  = {2017},

date = {2017-01-01},

journal = {The Journal of Chemical Physics},

volume = {146},

number = {20},

pages = {203310},

abstract = {Recently, there has been great interest in ?ultrastable? glasses formed via vapor deposition, both because of emerging engineering applications of these materials (e.g., active layers in light-emitting diodes and photovoltaics) and, theoretically, as materials for probing the equilibrium properties of glassy materials below their glass transition, based on the conjecture that these materials are equivalent to glassy materials aged over astronomical time scales. We use molecular dynamics simulations to examine the properties of ultrastable vapor-deposited and ordinary polymer glasses. Based on the difference in the energy of the deposited and ordinary films, we estimate the effective cooling rate for the vapor deposited films to be 1 to 3 orders of magnitude larger than that of the ordinary film, depending on the deposition temperature. Similarly, we find an increase in the average segmental relaxation time of the vapor-deposited film compared to the ordinary glass. On the other hand, the normal mode spectrum is essentially identical for the vapor-deposited and the ordinary glass film, suggesting that the high-frequency dynamics should be similar. In short, the segmental relaxation dynamics of the polymer vapor-deposited glass are consistent with those of an ordinary polymer glass with a somewhat slower effective cooling rate. Of course, one would expect a larger effect on dynamics approaching the experimental glass transition, where the cooling rates are much slower than accessible in simulation. To more precisely probe the relationship between the dynamics of these glasses, we examine dynamical heterogeneity within the film. Due to the substantial mobility gradient in the glassy films, we find that it is crucial to distinguish the dynamics of the middle part of the film from those of the entire film. Considering the film as a whole, the average dynamical heterogeneity is dominated by the mobility gradient, and as a consequence the heterogeneity is nearly indistinguishable between the ordinary and vapor deposited glass films. In contrast, in the middle part of the film, where there is almost no mobility gradient, we find the dynamical heterogeneity within the deposited film is somewhat larger than that of the ordinary film at the same temperature. We further show that the scale of the interfacial region grows on cooling in the equilibrium film, but this trend reverses in the glass state. We attribute this reversal in part to a shrinking ratio of the relaxation time in the middle of the film to that of the interfacial layer in the non-equilibrium state. The dynamics in this mobile interfacial layer for the ordinary and deposited film are nearly the same, suggesting that the interfacial region is always in a near-equilibrium state. These results emphasize the importance of distinguishing between interfacial and internal relaxation processes in this emerging class of materials.},

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

pubstate = {published},

tppubtype = {article}

}

@article{sdmk16,

title = {Bound Layers "Cloak" Nanoparticles in Strongly Interacting Polymer Nanocomposites},

author = {Starr, Francis W. and Douglas, Jack F. and Meng, Dong and Kumar, Sanat K. },

url = {http://fstarr.web.wesleyan.edu/publications/sdmk16.pdf},

doi = {10.1021/acsnano.6b05683},

year  = {2016},

date = {2016-01-01},

journal = {ACS Nano},

volume = {10},

pages = {10960 - 10965},

abstract = {Polymer-nanoparticle (NP) interfacial interactions are expected to strongly influence the properties of nanocomposites, but surprisingly, experiments often report small or no changes in the glass transition temperature, Tg. To understand this paradoxical situation, we simulate nanocomposites over a broad range of polymer-NP interaction strengths ε. When ε is stronger than the polymer-polymer interaction, a distinct relaxation that is slower than the main α-relaxation emerges, arising from an adsorbed "bound" polymer layer near the NP surface. This bound layer "cloaks" the NPs, so that the dynamics of the matrix polymer are largely unaffected. Consequently, Tg defined from the temperature dependence of the routinely measured thermodynamics or the polymer matrix relaxation is nearly independent of ε, in accord with many experiments. Apparently, quasi-thermodynamic measurements do not reliably reflect dynamical changes in the bound layer, which alter the overall composite dynamics. These findings clarify the relation between quasi-thermodynamic Tg measurements and nanocomposite dynamics, and should also apply to thin polymer films.},

keywords = {Glass Formation, Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{gssp16,

title = {Potential energy landscape of the apparent first-order phase transition between low-density and high-density amorphous ice},

author = {Giovambattista, Nicolas and Sciortino, Francesco and Starr, Francis W. and Poole, Peter H.},

url = {http://fstarr.web.wesleyan.edu/publications/gssp16.pdf},

doi = {http://dx.doi.org/10.1063/1.4968047},

year  = {2016},

date = {2016-01-01},

journal = {The Journal of Chemical Physics},

volume = {145},

number = {22},

pages = {224501},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{hpds15,

title = {A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films},

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

url = {http://fstarr.web.wesleyan.edu/publications/hpds15.pdf},

doi = {10.1063/1.4922481},

issn = {0021-9606},

year  = {2015},

date = {2015-06-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {142},

number = {23},

pages = {234907},

abstract = {Changes in the dynamics of supported polymer films in comparison to bulk materials involve a complex convolution of effects, such as substrate interactions, roughness, and compliance, in addition to film thickness. We consider molecular dynamics simulations of substrate-supported, coarse-grained polymer films where these parameters are tuned separately to determine how each of these variables influence the molecular dynamics of thin polymer films. We find that all these variables significantly influence the film dynamics, leading to a seemingly intractable degree of complexity in describing these changes. However, by considering how these constraining variables influence string-like collective motion within the film, we show that all our observations can be understood in a unified and quantitative way. More specifically, the string model for glass-forming liquids implies that the changes in the structural relaxation of these films are governed by the changes in the average length of string-like cooperative motions and this model is confirmed under all conditions considered in our simulations. Ultimately, these changes are parameterized in terms of just the activation enthalpy and entropy for molecular organization, which have predictable dependences on substrate properties and film thickness, offering a promising approach for the rational design of film properties. (C) 2015 AIP Publishing LLC.},

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

pubstate = {published},

tppubtype = {article}

}

@article{phsd15,

title = {Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials},

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

url = {http://fstarr.web.wesleyan.edu/publications/phsd15.pdf},

doi = {10.1073/pnas.1418654112},

issn = {0027-8424},

year  = {2015},

date = {2015-03-01},

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

volume = {112},

number = {10},

pages = {2966-2971},

abstract = {The study of glass formation is largely framed by semiempirical models that emphasize the importance of progressively growing cooperative motion accompanying the drop in fluid configurational entropy, emergent elasticity, or the vanishing of accessible free volume available for molecular motion in cooled liquids. We investigate the extent to which these descriptions are related through computations on a model coarse-grained polymer melt, with and without nanoparticle additives, and for supported polymer films with smooth or rough surfaces, allowing for substantial variation of the glass transition temperature and the fragility of glass formation. We find quantitative relations between emergent elasticity, the average local volume accessible for particle motion, and the growth of collective motion in cooled liquids. Surprisingly, we find that each of these models of glass formation can equally well describe the relaxation data for all of the systems that we simulate. In this way, we uncover some unity in our understanding of glass-forming materials from perspectives formerly considered as distinct.},

keywords = {Glass Formation, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{hds14,

title = {Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films},

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

url = {http://fstarr.web.wesleyan.edu/publications/hds14.pdf},

doi = {10.1038/ncomms5163},

issn = {2041-1723},

year  = {2014},

date = {2014-06-01},

journal = {NATURE COMMUNICATIONS},

volume = {5},

pages = {4163},

abstract = {Thin polymer films are ubiquitous in manufacturing and medical applications, and there has been intense interest in how film thickness and substrate interactions influence film dynamics. It is appreciated that a polymer-air interfacial layer with enhanced mobility plays an important role in the observed changes and recent studies suggest that the length scale x of this interfacial layer is related to film relaxation. In the context of the Adam-Gibbs and random first-order transition models of glass formation, these results provide indirect evidence for a relation between xi and the scale of collective molecular motion. Here we report direct evidence for a proportionality between xi and the average length L of string-like particle displacements in simulations of polymer films supported on substrates with variable interaction strength and rigidity. This relation explicitly links xi to the theoretical scale of cooperatively rearranging regions, offering a promising route to experimentally determine this scale of cooperative motion.},

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

pubstate = {published},

tppubtype = {article}

}

@article{pds14,

title = {String model for the dynamics of glass-forming liquids},

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

url = {http://fstarr.web.wesleyan.edu/publications/pds14.pdf},

doi = {10.1063/1.4878502},

issn = {0021-9606},

year  = {2014},

date = {2014-05-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {140},

number = {20},

pages = {204509},

abstract = {We test the applicability of a living polymerization theory to describe cooperative string-like particle rearrangement clusters (strings) observed in simulations of a coarse-grained polymer melt. The theory quantitatively describes the interrelation between the average string length L, configurational entropy S-conf, and the order parameter for string assembly Phi without free parameters. Combining this theory with the Adam-Gibbs model allows us to predict the relaxation time tau in a lower temperature T range than accessible by current simulations. In particular, the combined theories suggest a return to Arrhenius behavior near T-g and a low T residual entropy, thus avoiding a Kauzmann ``entropy crisis.'' (C) 2014 AIP Publishing LLC.},

keywords = {Dynamic Heterogeneity, Glass Formation},

pubstate = {published},

tppubtype = {article}

}

@article{csg14,

title = {Heating-induced glass-glass and glass-liquid transformations in computer simulations of water},

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

url = {http://fstarr.web.wesleyan.edu/publications/csg14.pdf},

doi = {10.1063/1.4868028},

issn = {0021-9606},

year  = {2014},

date = {2014-03-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {140},

number = {11},

pages = {114504},

abstract = {Water exists in at least two families of glassy states, broadly categorized as the low-density (LDA) and high-density amorphous ice (HDA). Remarkably, LDA and HDA can be reversibly interconverted via appropriate thermodynamic paths, such as isothermal compression and isobaric heating, exhibiting first-order-like phase transitions. We perform out-of-equilibrium molecular dynamics simulations of glassy water using the ST2 model to study the evolution of LDA and HDA upon isobaric heating. Depending on pressure, glass-to-glass, glass-to-crystal, glass-to-vapor, as well as glass-to-liquid transformations are found. Specifically, heating LDA results in the following transformations, with increasing heating pressures: (i) LDA-to-vapor (sublimation), (ii) LDA-to-liquid (glass transition), (iii) LDA-to-HDA-to-liquid, (iv) LDA-to-HDA-to-liquid-to-crystal, and (v) LDA-to- HDA-to-crystal. Similarly, heating HDA results in the following transformations, with decreasing heating pressures: (a) HDA-to-crystal, (b) HDA-to-liquid-to-crystal, (c) HDA-to-liquid (glass transition), (d) HDA-to-LDA-to-liquid, and (e) HDA-to-LDA-to-vapor. A more complex sequence may be possible using lower heating rates. For each of these transformations, we determine the corresponding transformation temperature as function of pressure, and provide a P-T ``phase diagram'' for glassy water based on isobaric heating. Our results for isobaric heating dovetail with the LDA-HDA transformations reported for ST2 glassy water based on isothermal compression/decompression processes [ Chiu et al., J. Chem. Phys. 139, 184504 (2013)]. The resulting phase diagram is consistent with the liquid-liquid phase transition hypothesis. At the same time, the glass phase diagram is sensitive to sample preparation, such as heating or compression rates. Interestingly, at least for the rates explored, our results suggest that the LDA-to-liquid (HDA-to-liquid) and LDA-to-HDA (HDA-to-LDA) transformation lines on heating are related, both being associated with the limit of kinetic stability of LDA (HDA). (C) 2014 AIP Publishing LLC.},

keywords = {Glass Formation, Polyamorphism, Water},

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{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{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{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{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{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{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{bps06,

title = {Fractional Stokes-Einstein and Debye-Stokes-Einstein relations in a network-forming liquid},

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

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

doi = {10.1103/PhysRevLett.97.055901},

issn = {0031-9007},

year  = {2006},

date = {2006-08-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {97},

number = {5},

pages = {055901},

abstract = {We study the breakdown of the Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) relations for translational and rotational motion in a prototypical model of a network-forming liquid, the ST2 model of water. We find that the emergence of fractional SE and DSE relations at low temperature is ubiquitous in this system, with exponents that vary little over a range of distinct physical regimes. We also show that the same fractional SE relation is obeyed by both mobile and immobile dynamical heterogeneities of the liquid.},

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

pubstate = {published},

tppubtype = {article}

}

@article{mgss06,

title = {Relation between rotational and translational dynamic heterogeneities in water},

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

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

doi = {10.1103/PhysRevLett.96.057803},

issn = {0031-9007},

year  = {2006},

date = {2006-02-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {96},

number = {5},

pages = {057803},

abstract = {We use molecular dynamics simulations to probe the rotational dynamics of the extended simple point charge model of water for a range of temperatures down to 200 K, 6 K above the mode coupling temperature. We find that rotational dynamics is spatially heterogeneous; i.e., there are clusters of molecules that rotate significantly more than the average for a given time interval, and we study the size and the temporal behavior of these clusters. We find that the position of a rotational heterogeneity is strongly correlated with the position of a translational heterogeneity, and that the fraction of molecules belonging to both kinds of heterogeneities increases with decreasing temperature. We further find that although the two types of heterogeneities are not identical, they are related to the same physical picture.},

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

pubstate = {published},

tppubtype = {article}

}

@article{gbss05,

title = {Clusters of mobile molecules in supercooled water},

author = {Giovambattista, Nicolas and Buldyrev, Sergey V. and Stanley, H. Eugene and Starr, Francis W.},

url = {http://fstarr.web.wesleyan.edu/publications/gbss-pre05.pdf},

doi = {10.1103/PhysRevE.72.011202},

issn = {1539-3755},

year  = {2005},

date = {2005-07-01},

journal = {PHYSICAL REVIEW E},

volume = {72},

number = {1, 1},

pages = {011202},

abstract = {We study the spatially heterogeneous dynamics in water via molecular dynamics simulations using the extended simple point charge potential. We identify clusters formed by mobile molecules and study their properties. We find that these clusters grow in size and become more compact as temperature decreases. We analyze the probability density function of cluster size, and we study the cluster correlation length. We find that clusters appear to be characterized by a fractal dimension consistent with that of lattice animals. We relate the cluster size and correlation length to the configurational entropy, S-conf. We find that these quantities depend weakly on 1/S-conf. In particular, the linearity found between the cluster mass n(*) and 1/S-conf suggests that n(*) may be interpreted as the mass of the cooperatively rearranging regions that form the basis of the Adam-Gibbs approach to the dynamics of supercooled liquids. We study the motion of molecules within a cluster, and find that each molecule preferentially follows a neighboring molecule in the same cluster. Based on this finding we hypothesize that stringlike cooperative motion may be a general mechanism for molecular rearrangement of complex, as well as simple liquids. By mapping each equilibrium configuration onto its corresponding local potential energy minimum or inherent structure (IS), we are able to compare the mobile molecule clusters in the equilibrium system with the molecules forming the clusters identified in the transitions between IS. We find that (i) mobile molecule clusters obtained by comparing different system configurations and (ii) clusters obtained by comparing the corresponding IS are completely different for short time scales, but are the same on the longer time scales of diffusive motion.},

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

pubstate = {published},

tppubtype = {article}

}

@article{lssg03,

title = {Spatially heterogeneous dynamics investigated via a time-dependent four-point density correlation function},

author = {Lacevic, Naida and Starr, Francis W. and Schrøder, Thomas B. and Glotzer, Sharon C.},

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

doi = {10.1063/1.1605094},

issn = {0021-9606},

year  = {2003},

date = {2003-10-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {119},

number = {14},

pages = {7372-7387},

abstract = {Relaxation in supercooled liquids above their glass transition and below the onset temperature of ``slow'' dynamics involves the correlated motion of neighboring particles. This correlated motion results in the appearance of spatially heterogeneous dynamics or ``dynamical heterogeneity.'' Traditional two-point time-dependent density correlation functions, while providing information about the transient ``caging'' of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity. Here, we study a four-point, time-dependent density correlation function g(4)(r,t) and corresponding ``structure factor'' S(4)(q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity. We study g(4)(r,t) and S(4)(q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above. We find that the correlations between particles measured by g(4)(r,t) and S(4)(q,t) become increasingly pronounced on cooling. The corresponding dynamical correlation length xi(4)(t) extracted from the small-q behavior of S(4)(q,t) provides an estimate of the range of correlated particle motion. We find that xi(4)(t) has a maximum as a function of time t, and that the value of the maximum of xi(4)(t) increases steadily from less than one particle diameter to a value exceeding nine particle diameters in the temperature range approaching the mode coupling temperature from above. At the maximum, xi(4)(t) and the alpha relaxation time tau(alpha) are related by a power law. We also examine the individual contributions to g(4)(r,t), S(4)(q,t), and xi(4)(t), as well as the corresponding order parameter Q(t) and generalized susceptibility chi(4)(t), arising from the self and distinct contributions to Q(t). These contributions elucidate key differences between domains of localized and delocalized particles.(C) 2003 American Institute of Physics.},

keywords = {Dynamic Heterogeneity, Glass Formation},

pubstate = {published},

tppubtype = {article}

}

@article{sas03,

title = {Prediction of entropy and dynamic properties of water below the homogeneous nucleation temperature},

author = {Starr, Francis W. and Angell, C. Austen and Stanley, H. Eugene},

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

doi = {10.1016/S0378-4371(03)00012-8},

issn = {0378-4371},

year  = {2003},

date = {2003-05-01},

journal = {PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS},

volume = {323},

pages = {51-66},

abstract = {The behavior of thermodynamic and dynamic properties of liquid water at atmospheric pressure in the temperature range between the lower limit of supercooling (T-H approximate to 235 K) and the onset of the glassy state at T-g has been the focus of much research, and many questions remain about the properties of water in this region. Since direct measurements on water in this temperature range remain largely infeasible, we use existing experimental measurements of the entropy, specific heat, and enthalpy outside this range to construct a possible form of the entropy in the ``difficult-to-probe'' region. Assuming that the entropy is well-defined in extreme metastable states, and that there is no intervening discontinuity at atmospheric pressure, we estimate the excess entropy S-ex of the liquid over the crystal within relatively narrow limits. We find that our approximate form for S-ex shows atypical behavior when compared with other liquids: using a thermodynamic categorization of ``strong'' and ``fragile'' liquids, water appears to be fragile on initial cooling below the melting temperature, and strong in the temperature region near the glass transition. This thermodynamic construction can be used, with appropriate reservations, to estimate the behavior of the dynamic properties of water by means of the Adam-Gibbs equation-which relates configurational entropy S-conf to dynamic behavior. Although the Adam-Gibbs equation uses S-conf rather than S-ex as the control variable, the relation has been used successfully in a number of experimental studies with S-conf replaced by S-ex. This is likely a result of a proportionality between S-conf and S-ex, which we confirm for simulations of a model of water. Hence by using the constructed values of S-ex, together with experimental data in the range where S-ex is known, we estimate the temperature dependence of viscosity and diffusivity approaching the glass transition. Like the entropy plots, Arrhenius plots of viscosity or diffusion show an inflection, implying a crossover from fragile to strong liquid character below T-H. The dynamics results also imply T-g approximate to 160 K, which is considerably above the expected value of 136 K from older experiments, but consistent with other recent evidence based on hyperquenched glass properties. We discuss the possibility of experimentally verifying our predictions, and briefly discuss other liquids that also may follow a strong-to-fragile pattern. (C) 2003 Elsevier Science B.V. All rights reserved.},

keywords = {Glass Formation, Polyamorphism, Water},

pubstate = {published},

tppubtype = {article}

}

@article{gbss03,

title = {Connection between Adam-Gibbs theory and spatially heterogeneous dynamics},

author = {Giovambattista, Nicolas and Buldyrev, Sergey V. and Starr, Francis W. and Stanley, H. Eugene},

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

doi = {10.1103/PhysRevLett.90.085506},

issn = {0031-9007},

year  = {2003},

date = {2003-02-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {90},

number = {8},

pages = {085506},

abstract = {We investigate the spatially heterogeneous dynamics in the extended simple point charge model of water using molecular dynamics simulations. We relate the average mass n(*) of mobile particle clusters to the diffusion constant and the configurational entropy. Hence, n(*) can be interpreted as the mass of the ``cooperatively rearranging regions'' that form the basis of the Adam-Gibbs theory of the dynamics of supercooled liquids. We also examine the time and temperature dependence of these transient clusters.},

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

pubstate = {published},

tppubtype = {article}

}

@article{ISI:000178166900022,

title = {What do we learn from the local geometry of glass-forming liquids?},

author = {Starr, Francis W. and Sastry, S and Douglas, Jack F. and Glotzer, Sharon C.},

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

doi = {10.1103/PhysRevLett.89.125501},

issn = {0031-9007},

year  = {2002},

date = {2002-09-01},

journal = {PHYSICAL REVIEW LETTERS},

volume = {89},

number = {12},

pages = {125501},

abstract = {We examine the local geometry of a simulated glass-forming polymer melt. Using the Voronoi construction, we find that the distributions of Voronoi volume P(u(v)) and asphericity P(a) appear to be universal properties of dense liquids, supporting the use of packing approaches to understand liquid properties. We also calculate the average free volume [u(f)] along a path of constant density and find that [u(f)] extrapolates to zero at the same temperature T-0 that the extrapolated relaxation time diverges. We relate [u(f)] to the Debye-Waller factor, which is measurable by neutron scattering.},

keywords = {Glass Formation},

pubstate = {published},

tppubtype = {article}

}

@article{ISI:000175728100037,

title = {Molecular dynamics simulation of a polymer melt with a nanoscopic particle},

author = {Starr, Francis W. and Schrøder, Thomas B. and Glotzer, Sharon C.},

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

doi = {10.1021/ma010626p},

issn = {0024-9297},

year  = {2002},

date = {2002-05-01},

journal = {MACROMOLECULES},

volume = {35},

number = {11},

pages = {4481-4492},

abstract = {We perform molecular dynamics simulations of a bead-spring polymer melt surrounding a nanoscopic particle. We explore the effect of the polymer/nanoparticle interactions, surface-to-volume ratio, and boundary conditions on both the structure and dynamics of the polymer melt. We find that the chains near the nanoparticle surface are elongated and flattened and that this effect is independent of the interaction for the range of interactions we study. We show that the glass transition temperature T-g of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and nanoparticle. A gradual change of the polymer dynamics approaching the nanoparticle surface causes the change in the glass transition. The magnitude of the shift is exaggerated by increasing fraction of surface monomers in the system. These behaviors support a ``many-layer''-based interpretation of the dynamics. Our findings appear applicable to systems in which surface interactions dominate, including both traditional and nanofilled polymer melts, as well as systems with markedly different geometries, such as ultrathin polymer films. In particular, we show how our results might be compared with those obtained from experimental studies of ``bound'' polymer.},

keywords = {Glass Formation, Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{ssg01,

title = {Effects of a nanoscopic filler on the structure and dynamics of a simulated polymer melt and the relationship to ultrathin films},

author = {Starr, Francis W. and Schrøder, Thomas B. and Glotzer, Sharon C.},

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

doi = {10.1103/PhysRevE.64.021802},

issn = {1063-651X},

year  = {2001},

date = {2001-08-01},

journal = {PHYSICAL REVIEW E},

volume = {64},

number = {2, 1},

pages = {021802},

abstract = {We perform molecular dynamics simulations of an idealized polymer melt surrounding a nanoscopic filler particle. We show that the glass transition temperature T-g of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and filler. A gradual change of the polymer dynamics approaching the filler surface causes the change in the glass transition. We also find that polymers close to the surface tend to be elongated and flattened. Our findings show a strong similarity to those obtained for ultrathin polymer films.},

keywords = {Glass Formation, Nanocomposites, Polymers},

pubstate = {published},

tppubtype = {article}

}

@article{sslss-nature,

title = {Configurational entropy and diffusivity of supercooled water},

author = {Scala, Antonio and Starr, Francis W. and La Nave, Emilia and Sciortino, Francesco and Stanley, H. Eugene},

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

issn = {0028-0836},

year  = {2000},

date = {2000-07-01},

journal = {NATURE},

volume = {406},

number = {6792},

pages = {166-169},

abstract = {As a liquid approaches the glass transition, its properties are dominated by local potential minima(1,2) in its energy landscape. The liquid experiences localized vibrations in the basins of attraction surrounding the minima, and rearranges via relatively infrequent inter-basin jumps(3). As a result, the liquid dynamics at low temperature are related to the system's exploration of its own configuration space. The `thermodynamic approach' to the glass transition considers the reduction in configuration space(4-8) explored as the system cools, and predicts that the configurational entropy(5,9,10) (a measure of the number of local potential energy minima sampled by the liquid) is related to the diffusion constant. Here we report a stringent test of the thermodynamic approach for liquid water (a convenient system to study because of an anomalous pressure dependence in the diffusion constant). We calculate the configurational entropy at points spanning a large region of the temperature-density plane, using a model(11) that reproduces the dynamical anomalies of liquid water. We find that the thermodynamic approach can be used to understand the characteristic dynamic anomalies, and that the diffusive dynamics are governed by the configurational entropy. Our results indicate that the thermodynamic approach might be extended to predict the dynamical behaviour of supercooled liquids in general.},

keywords = {Glass Formation, Water},

pubstate = {published},

tppubtype = {article}

}

@article{sss99,

title = {Dynamics of simulated water under pressure},

author = {Starr, Francis W. and Sciortino, Francesco and Stanley, H. Eugene},

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

doi = {10.1103/PhysRevE.60.6757},

issn = {1063-651X},

year  = {1999},

date = {1999-12-01},

journal = {PHYSICAL REVIEW E},

volume = {60},

number = {6, A},

pages = {6757-6768},

abstract = {We present molecular dynamics simulations of the extended simple-point-charge model of water to probe the dynamic properties at temperatures from 350 K down to 190 K and pressures from 2.5 GPa (25 kbar) down to -300 MPa (-3 kbar). We compare our results with those obtained experimentally, both of which show a diffusivity maximum as a function of pressure. We find that our simulation results are consistent with the predictions of the mode-coupling theory for the dynamics of weakly supercooled liquids-strongly supporting the hypothesis that the apparent divergences of dynamic properties observed experimentally may be independent of a possible thermodynamic singularity at low temperature. The dramatic change in water's dynamic and structural properties as a function of pressure allows us to confirm the predictions of MCT over a much broader range of the von Schweidler exponent values than has been studied for simple atomic liquids. We also show how structural changes are reflected in the wave-vector dependence of dynamic properties of the liquid along a path of nearly constant diffusivity. For temperatures below the crossover temperature of MCT (where the predictions of MCT are expected to fail), we find tentative evidence for a crossover of the temperature dependence of the diffusivity from power-law to Arrhenius behavior, with an activation energy typical of a strong liquid. [S1063-651X(99)11712-4].},

keywords = {Glass Formation, Water},

pubstate = {published},

tppubtype = {article}

}