Publications by year, excluding conference proceedings.
Complete CV Google Scholar Page
2022
Zhang, Wengang; Starr, Francis W.; Beers, Kathryn L.; Douglas, Jack F.
Reactive Molecular Dynamics Simulations of the Depolymerization of Polyethylene Using Graphene-Oxide-Supported Platinum Nanoparticles Journal Article
In: The Journal of Physical Chemistry A, vol. 126, no. 20, pp. 3167-3173, 2022, (PMID: 35533406).
BibTeX | Tags: Nanotechnology, Polymers | Links:
@article{zsbd22,
title = {Reactive Molecular Dynamics Simulations of the Depolymerization of Polyethylene Using Graphene-Oxide-Supported Platinum Nanoparticles},
author = {Wengang Zhang and Francis W. Starr and Kathryn L. Beers and Jack F. Douglas},
url = {https://fstarr.wescreates.wesleyan.edu/publications/zsbd22.pdf},
doi = {10.1021/acs.jpca.2c01167},
year = {2022},
date = {2022-05-26},
urldate = {2022-01-01},
journal = {The Journal of Physical Chemistry A},
volume = {126},
number = {20},
pages = {3167-3173},
note = {PMID: 35533406},
keywords = {Nanotechnology, Polymers},
pubstate = {published},
tppubtype = {article}
}
2020
Giuntoli, Andrea; Puosi, Francesco; Leporini, Dino; Starr, Francis W; Douglas, Jack F
Predictive relation for the α-relaxation time of a coarse-grained polymer melt under steady shear Journal Article
In: Science Advances, vol. 6, no. 17, 2020.
Abstract | BibTeX | Tags: Glass Formation, Polymers | Links:
@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}
}
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.
$tau_alpha$ 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.
Fan, Jinpeng; Emamy, Hamed; Chremos, Alexandros; Douglas, Jack F; Starr, Francis W
Dynamic heterogeneity and collective motion in star polymer melts Journal Article
In: The Journal of Chemical Physics, vol. 152, no. 5, pp. 054904, 2020.
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers | Links:
@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}
}
Zhang, Wengang; Starr, Francis W; Douglas, Jack F
Reconciling computational and experimental trends in the temperature dependence of the interfacial mobility of polymer films Journal Article
In: The Journal of Chemical Physics, vol. 152, no. 12, pp. 124703, 2020.
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers | Links:
@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}
}
Storey, Amber N; Zhang, Wengang; Douglas, Jack F; Starr, Francis W
How Does Monomer Structure Affect the Interfacial Dynamics of Supported Ultrathin Polymer Films? Journal Article
In: Macromolecules, vol. 53, no. 21, pp. 9654-9664, 2020.
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers, Thin Films | Links:
@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}
}
Emamy, Hamed; Kumar, Sanat K; Starr, Francis W
Structural Properties of Bound Layer in Polymer-Nanoparticle Composites Journal Article
In: Macromolecules, vol. 53, no. 18, pp. 7845-7850, 2020.
BibTeX | Tags: Nanocomposites, Polymers | Links:
@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}
}
2019
Zhang, Wengang; Starr, Francis W; Douglas, Jack F
Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and Its Relation to Relaxation Journal Article
In: The Journal of Physical Chemistry B, vol. 123, no. 27, pp. 5935-5941, 2019, (PMID: 31192601).
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers | Links:
@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}
}
2018
Emamy, Hamed; Kumar, Sanat K; Starr, Francis W
Diminishing Interfacial Effects with Decreasing Nanoparticle Size in Polymer-Nanoparticle Composites Journal Article
In: Phys. Rev. Lett., vol. 121, pp. 207801, 2018.
BibTeX | Tags: Glass Formation, Nanocomposites, Polymers | Links:
@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}
}
Betancourt, Beatriz A. Pazmino; Starr, Francis W.; Douglas, Jack F.
String-like collective motion in the α- and β-relaxation of a coarse-grained polymer melt Journal Article
In: The Journal of Chemical Physics, vol. 148, no. 10, pp. 104508, 2018.
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers | Links:
@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}
}
2017
Zhang, Wengang; Douglas, Jack F.; Starr, Francis W.
Effects of a “bound” substrate layer on the dynamics of supported polymer films Journal Article
In: The Journal of Chemical Physics, vol. 147, pp. 044901, 2017.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers, Thin Films | Links:
@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}
}
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.
Zhang, Wengang; Douglas, Jack F.; Starr, Francis W.
Dynamical heterogeneity in a vapor-deposited polymer glass Journal Article
In: The Journal of Chemical Physics, vol. 146, no. 20, pp. 203310, 2017.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers | Links:
@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}
}
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.
2016
Starr, Francis W.; Douglas, Jack F.; Meng, Dong; Kumar, Sanat K.
Bound Layers "Cloak" Nanoparticles in Strongly Interacting Polymer Nanocomposites Journal Article
In: ACS Nano, vol. 10, pp. 10960 - 10965, 2016.
Abstract | BibTeX | Tags: Glass Formation, Nanocomposites, Polymers | Links:
@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}
}
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.
2015
Hanakata, Paul Z.; Pazmiño Betancourt, Beatriz A.; Douglas, Jack F.; Starr, Francis W.
A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films Journal Article
In: JOURNAL OF CHEMICAL PHYSICS, vol. 142, no. 23, pp. 234907, 2015, ISSN: 0021-9606.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers, Thin Films | Links:
@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}
}
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.
Pazmiño Betancourt, Beatriz A.; Hanakata, Paul Z.; Starr, Francis W.; Douglas, Jack F.
Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials Journal Article
In: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, vol. 112, no. 10, pp. 2966-2971, 2015, ISSN: 0027-8424.
Abstract | BibTeX | Tags: Glass Formation, Polymers | Links:
@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}
}
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.
2014
Hanakata, Paul Z.; Douglas, Jack F.; Starr, Francis W.
Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films Journal Article
In: NATURE COMMUNICATIONS, vol. 5, pp. 4163, 2014, ISSN: 2041-1723.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers, Thin Films | Links:
@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}
}
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.
2013
Starr, Francis W.; Douglas, Jack F.; Sastry, Srikanth
The relationship of dynamical heterogeneity to the Adam-Gibbs and random first-order transition theories of glass formation Journal Article
In: JOURNAL OF CHEMICAL PHYSICS, vol. 138, no. 12, pp. 12A541, 2013, ISSN: 0021-9606.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers | Links:
@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}
}
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]
Pazmiño Betancourt, Beatriz A.; Douglas, Jack F.; Starr, Francis W.
Fragility and cooperative motion in a glass-forming polymer-nanoparticle composite Journal Article
In: SOFT MATTER, vol. 9, no. 1, pp. 241-254, 2013, ISSN: 1744-683X.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Nanocomposites, Polymers | Links:
@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}
}
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.
Starr, Francis W.; Hanakata, Paul Z.; Pazmiño Betancourt, Beatrice A.; Sastry, Srikanth; Douglas, Jack F.
Fragility and Cooperative Motion in Polymer Glass Formation Book Section
In: Greer, A. L.; Kelton, K. F.; Sastry, S. (Ed.): Fragility of glass forming liquids, pp. 337-361, Hindustan, New Delhi, India, 2013.
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Nanocomposites, Polymers, Thin Films
@incollection{sdspb14,
title = {Fragility and Cooperative Motion in Polymer Glass Formation},
author = {Starr, Francis W. and Hanakata, Paul Z. and Pazmiño Betancourt, Beatrice A. and Sastry, Srikanth and Douglas, Jack F.},
editor = {Greer, A. L. and Kelton, K. F. and Sastry, S.},
year = {2013},
date = {2013-01-01},
booktitle = {Fragility of glass forming liquids},
pages = {337-361},
publisher = {Hindustan},
address = {New Delhi, India},
keywords = {Dynamic Heterogeneity, Glass Formation, Nanocomposites, Polymers, Thin Films},
pubstate = {published},
tppubtype = {incollection}
}
2012
Hanakata, Paul Z.; Douglas, Jack F.; Starr, Francis W.
Local variation of fragility and glass transition temperature of ultra-thin supported polymer films Journal Article
In: JOURNAL OF CHEMICAL PHYSICS, vol. 137, no. 24, pp. 244901, 2012, ISSN: 0021-9606.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Polymers, Thin Films | Links:
@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}
}
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]
2011
Starr, Francis W.; Douglas, Jack F.
Modifying Fragility and Collective Motion in Polymer Melts with Nanoparticles Journal Article
In: PHYSICAL REVIEW LETTERS, vol. 106, no. 11, pp. 115702, 2011, ISSN: 0031-9007.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Nanocomposites, Polymers | Links:
@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}
}
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.
2010
Jancar, Josef; Douglas, Jack F.; Starr, Francis W.; Kumar, Sanat K.; Cassagnau, Philippe; Lesser, Alan J.; Sternstein, Sanford S.; Buehler, Markus J.
Current issues in research on structure-property relationships in polymer nanocomposites Journal Article
In: POLYMER, vol. 51, no. 15, pp. 3321-3343, 2010, ISSN: 0032-3861.
Abstract | BibTeX | Tags: Nanocomposites, Polymers | Links:
@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}
}
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.
Knauert, Scott T.; Douglas, Jack F.; Starr, Francis W.
Morphology and Transport Properties of Two-Dimensional Sheet Polymers Journal Article
In: MACROMOLECULES, vol. 43, no. 7, pp. 3438-3445, 2010, ISSN: 0024-9297.
Abstract | BibTeX | Tags: Polymers | Links:
@article{kds10,
title = {Morphology and Transport Properties of Two-Dimensional Sheet Polymers},
author = {Knauert, Scott T. and Douglas, Jack F. and Starr, Francis W.},
url = {http://fstarr.web.wesleyan.edu/publications/kds10+cover.pdf},
doi = {10.1021/ma902081m},
issn = {0024-9297},
year = {2010},
date = {2010-04-01},
journal = {MACROMOLECULES},
volume = {43},
number = {7},
pages = {3438-3445},
abstract = {Whereas there has been extensive theoretical and experimental investigation of the properties of linear polymer chains in solution, there has been far less work on sheet-like polymers having 2D connectivity and 3D crumpled or collapsed shapes caused by thermal fluctuations, attractive self-interactions, or both. Sheet-like polymers arise in a variety of contexts ranging from self-assembled biological membranes (e.g., the spectrin network of red blood cells, microtubules, etc.) to nanocomposite additives to polymers (carbon nanotubes, graphene, and clay sheets) and polymerized monolayers. We investigate the equilibrium properties of this broad class of polymers using a simple model of a sheet polymer with a locally square symmetry of the connecting beads. We quantify the sheet morphology and the dilute-limit hydrodynamic solution properties as a function of molecular mass and sheet stiffness. First, we reproduce the qualitative findings of previous work indicating that variable sheet stiffness results in a wide variety of morphologies, including flat, crumpled or collapsed spherical, cylindrical or tubular, and folded sheets that serve to characterize our particular 2D polymer model. Transport properties are of significant interest in characterizing polymeric materials, and we provide the first numerical computations of these properties for sheet polymers. Specifically, we calculate the intrinsic viscosity and hydrodynamic radius of these sheet morphologies using a novel path-integration technique and find good agreement of our numerical results with previous theoretical scaling predictions.},
keywords = {Polymers},
pubstate = {published},
tppubtype = {article}
}
Whereas there has been extensive theoretical and experimental investigation of the properties of linear polymer chains in solution, there has been far less work on sheet-like polymers having 2D connectivity and 3D crumpled or collapsed shapes caused by thermal fluctuations, attractive self-interactions, or both. Sheet-like polymers arise in a variety of contexts ranging from self-assembled biological membranes (e.g., the spectrin network of red blood cells, microtubules, etc.) to nanocomposite additives to polymers (carbon nanotubes, graphene, and clay sheets) and polymerized monolayers. We investigate the equilibrium properties of this broad class of polymers using a simple model of a sheet polymer with a locally square symmetry of the connecting beads. We quantify the sheet morphology and the dilute-limit hydrodynamic solution properties as a function of molecular mass and sheet stiffness. First, we reproduce the qualitative findings of previous work indicating that variable sheet stiffness results in a wide variety of morphologies, including flat, crumpled or collapsed spherical, cylindrical or tubular, and folded sheets that serve to characterize our particular 2D polymer model. Transport properties are of significant interest in characterizing polymeric materials, and we provide the first numerical computations of these properties for sheet polymers. Specifically, we calculate the intrinsic viscosity and hydrodynamic radius of these sheet morphologies using a novel path-integration technique and find good agreement of our numerical results with previous theoretical scaling predictions.
2008
Rahedi, Andrew J.; Douglas, Jack F.; Starr, Francis W.
Model for reversible nanoparticle assembly in a polymer matrix Journal Article
In: JOURNAL OF CHEMICAL PHYSICS, vol. 128, no. 2, pp. 024902, 2008, ISSN: 0021-9606.
Abstract | BibTeX | Tags: Nanocomposites, Polymers, Self Assembly | Links:
@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}
}
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.
2007
Knauert, Scott T.; Douglas, Jack F.; Starr, Francis W.
The effect of nanoparticle shape on polymer-nanocomposite rheology and tensile strength Journal Article
In: JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS, vol. 45, no. 14, pp. 1882-1897, 2007, ISSN: 0887-6266.
Abstract | BibTeX | Tags: Nanocomposites, Nanotechnology, Polymers | Links:
@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}
}
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.
2003
Starr, Francis W.; Douglas, Jack F.; Glotzer, Sharon C.
Origin of particle clustering in a simulated polymer nanocomposite and its impact on rheology Journal Article
In: JOURNAL OF CHEMICAL PHYSICS, vol. 119, no. 3, pp. 1777-1788, 2003, ISSN: 0021-9606.
Abstract | BibTeX | Tags: Nanocomposites, Polymers, Self Assembly | Links:
@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}
}
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.
2002
Starr, Francis W.; Schrøder, Thomas B.; Glotzer, Sharon C.
Molecular dynamics simulation of a polymer melt with a nanoscopic particle Journal Article
In: MACROMOLECULES, vol. 35, no. 11, pp. 4481-4492, 2002, ISSN: 0024-9297.
Abstract | BibTeX | Tags: Glass Formation, Nanocomposites, Polymers | Links:
@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}
}
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.
2001
Starr, Francis W.; Schrøder, Thomas B.; Glotzer, Sharon C.
Effects of a nanoscopic filler on the structure and dynamics of a simulated polymer melt and the relationship to ultrathin films Journal Article
In: PHYSICAL REVIEW E, vol. 64, no. 2, 1, pp. 021802, 2001, ISSN: 1063-651X.
Abstract | BibTeX | Tags: Glass Formation, Nanocomposites, Polymers | Links:
@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}
}
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.