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{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{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{eks20,

title = {Structural Properties of Bound Layer in Polymer-Nanoparticle Composites},

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

url = {http://fstarr.wescreates.wesleyan.edu/publications/eks20.pdf},

doi = {10.1021/acs.macromol.0c01465},

year  = {2020},

date = {2020-01-01},

journal = {Macromolecules},

volume = {53},

number = {18},

pages = {7845-7850},

keywords = {Nanocomposites, Polymers},

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{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{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{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{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{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{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{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{sdg03,

title = {Origin of particle clustering in a simulated polymer nanocomposite and its impact on rheology},

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

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

doi = {10.1063/1.1580099},

issn = {0021-9606},

year  = {2003},

date = {2003-07-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {119},

number = {3},

pages = {1777-1788},

abstract = {Many nanoparticles have short-range interactions relative to their size, and these interactions tend to be ``patchy'' since the interatomic spacing is comparable to the nanoparticle size. For a dispersion of such particles, it is not a priori obvious what mechanism will control the clustering of the nanoparticles, and how the clustering will be affected by tuning various control parameters. To gain insight into these questions, we perform molecular dynamics simulations of polyhedral nanoparticles in a dense bead-spring polymer melt under both quiescent and steady shear conditions. We explore the mechanism that controls nanoparticle clustering and find that the crossover from dispersed to clustered states is consistent with the predictions for equilibrium particle association or equilibrium polymerization, and that the crossover does not appear to match the expectations for first-order phase separation typical for binary mixtures in the region of the phase diagram where we can equilibrate the system. At the same time, we cannot rule out the possibility of phase separation at a lower temperature. Utilizing the existing framework for dynamic clustering transitions offers the possibility of more rationally controlling the dispersion and properties of nanocomposite materials. Finally, we examine how nanocomposite rheology depends on the state of equilibrium clustering. We find that the shear viscosity for dispersed configurations is larger than that for clustered configurations, in contrast to expectations based on macroscopic colloidal dispersions. We explain this result by the alteration of the polymer matrix properties in the vicinity of the nanoparticles. We also show that shear tends to disperse clustered nanoparticle configurations in our system, an effect particularly important for processing. (C) 2003 American Institute of Physics.},

keywords = {Nanocomposites, Polymers, Self Assembly},

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}

}