Publications by year, excluding conference proceedings.
Complete CV Google Scholar Page
2021
Zhang, Wengang; Starr, Francis W.; Douglas, Jack F.
Activation free energy gradient controls interfacial mobility gradient in thin polymer films Journal Article
In: The Journal of Chemical Physics, vol. 155, no. 17, pp. 174901, 2021.
BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Nanotechnology, Thin Films | Links:
@article{zsd21,
title = {Activation free energy gradient controls interfacial mobility gradient in thin polymer films},
author = {Wengang Zhang and Francis W. Starr and Jack F. Douglas},
url = {https://fstarr.wescreates.wesleyan.edu/publications/zsd21.pdf},
doi = {10.1063/5.0064866},
year = {2021},
date = {2021-11-07},
journal = {The Journal of Chemical Physics},
volume = {155},
number = {17},
pages = {174901},
keywords = {Dynamic Heterogeneity, Glass Formation, Nanotechnology, Thin Films},
pubstate = {published},
tppubtype = {article}
}
Zhang, Wengang; Douglas, Jack F.; Chremos, Alexandros; Starr, Francis W.
Structure and Dynamics of Star Polymer Films from Coarse-Grained Molecular Simulations Journal Article
In: Macromolecules, vol. 54, no. 12, pp. 5344-5353, 2021.
BibTeX | Tags: Glass Formation, Nanotechnology, Thin Films | Links:
@article{zdcs21,
title = {Structure and Dynamics of Star Polymer Films from Coarse-Grained Molecular Simulations},
author = {Wengang Zhang and Jack F. Douglas and Alexandros Chremos and Francis W. Starr},
url = {https://https://fstarr.wescreates.wesleyan.edu/publications/zdcs21.pdf},
doi = {10.1021/acs.macromol.1c00504},
year = {2021},
date = {2021-06-04},
journal = {Macromolecules},
volume = {54},
number = {12},
pages = {5344-5353},
keywords = {Glass Formation, Nanotechnology, Thin Films},
pubstate = {published},
tppubtype = {article}
}
2020
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}
}
2018
Zhang, Wengang; Douglas, Jack F.; Starr, Francis W.
Why we need to look beyond the glass transition temperature to characterize the dynamics of thin supported polymer films Journal Article
In: Proceedings of the National Academy of Sciences, 2018, ISSN: 0027-8424.
Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Thin Films | Links:
@article{zds18,
title = {Why we need to look beyond the glass transition temperature to characterize the dynamics of thin supported polymer films},
author = { Wengang Zhang and Jack F. Douglas and Francis W. Starr},
url = {http://fstarr.web.wesleyan.edu/publications/zds18.pdf},
doi = {10.1073/pnas.1722024115},
issn = {0027-8424},
year = {2018},
date = {2018-05-14},
journal = {Proceedings of the National Academy of Sciences},
publisher = {National Academy of Sciences},
abstract = {The disparate results for Tg shifts of polymer thin films raise the question of whether Tg provides a good metric to characterize changes in the overall dynamics of these films. Our work demonstrates how different techniques to measure Tg are influenced by the relaxation gradient across the film. We find that different measurement methods can provide contradictory Tg estimates, depending on their sensitivity to dynamics near the substrate, providing a possible explanation for prior contradictory results. We take advantage of these differences by combining Tg estimates to predict Tg near the substrate. Our findings should be useful for polymer thin film applications, including microelectronic devices, lithium battery technology, and the development of biomedical devices.There is significant variation in the reported magnitude and even the sign of Tg shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate Tg in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that Tg based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer–substrate interaction strength ε. In contrast, we find that Tg based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but Tg increases when ε exceeds the polymer–polymer interaction strength. We show that these qualitatively different trends in Tg reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of Tg in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of Tg. Our results emphasize the limitations of using Tg to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of Tg can be combined to predict local changes in Tg near the substrate, providing a simple method to infer information about the mobility gradient.},
keywords = {Dynamic Heterogeneity, Glass Formation, Thin Films},
pubstate = {published},
tppubtype = {article}
}
The disparate results for Tg shifts of polymer thin films raise the question of whether Tg provides a good metric to characterize changes in the overall dynamics of these films. Our work demonstrates how different techniques to measure Tg are influenced by the relaxation gradient across the film. We find that different measurement methods can provide contradictory Tg estimates, depending on their sensitivity to dynamics near the substrate, providing a possible explanation for prior contradictory results. We take advantage of these differences by combining Tg estimates to predict Tg near the substrate. Our findings should be useful for polymer thin film applications, including microelectronic devices, lithium battery technology, and the development of biomedical devices.There is significant variation in the reported magnitude and even the sign of Tg shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate Tg in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that Tg based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer–substrate interaction strength ε. In contrast, we find that Tg based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but Tg increases when ε exceeds the polymer–polymer interaction strength. We show that these qualitatively different trends in Tg reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of Tg in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of Tg. Our results emphasize the limitations of using Tg to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of Tg can be combined to predict local changes in Tg near the substrate, providing a simple method to infer information about the mobility gradient.
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.
Rivera, Jose L.; Villanueva-Mejia, Francisco; Navarro-Santos, Pedro; Starr, Francis W.
Desalination by dragging water using a low-energy nano-mechanical device of porous graphene Journal Article
In: RSC Adv., vol. 7, pp. 53729-53739, 2017.
Abstract | BibTeX | Tags: Nanotechnology, Thin Films, Water | Links:
@article{rvns17,
title = {Desalination by dragging water using a low-energy nano-mechanical device of porous graphene},
author = { Jose L. Rivera and Francisco Villanueva-Mejia and Pedro Navarro-Santos and Francis W. Starr},
url = {http://fstarr.web.wesleyan.edu/publications/rvns17.pdf},
doi = {10.1039/C7RA09847B},
year = {2017},
date = {2017-01-01},
journal = {RSC Adv.},
volume = {7},
pages = {53729-53739},
publisher = {The Royal Society of Chemistry},
abstract = {We propose a nano-structured suction system based on graphene sheets for water desalination processes. The desalination system modeled in this work is an alternative process to the commonly employed but energy intensive reverse osmosis process. The nano-structured system generates drag forces, which pull water molecules from the saline solution into a chamber. Our molecular simulations consist of two rigid walls of graphene: one wall with 5 A pores permeable to water molecules forms the membrane, while the other wall acts as a plunger to induce and control the transfer of desalinated water molecules, which accumulate in a chamber between the two walls. Prior to the desalination process, the chamber is saturated with one monolayer of water molecules. The desalination occurs when the plunger moves to create unsaturated space inside the chamber. At plunger speeds up to 10 cm s-1, the system desalinates saltwater films in the open part of the membrane. At higher plunger speeds, the desalination chamber expands faster than molecules can fill the chamber, resulting in cavitation and poor desalination. At plunger speeds of 0.5 cm s-1, the desalination occurs via a quasi-equilibrium process, which minimizes the energy necessary to drive desalination. Our findings suggest that the desalination process requires less energy than reverse osmosis methods at plunger speeds up to 0.15 cm s-1 (for the chosen pore density). The filling profile of desalinated water molecules inside the chamber occurs via three distinct regimes: the first two regimes correspond to the formation of one and then two monolayers adsorbed to the chamber's walls. The third regime corresponds to the filling of molecules between the adsorbed layers, which approaches a density close to the density of bulk liquid water. Including flexibility in the graphene sheets does not affect the energy consumption for desalination processes occurring after the formation of the second monolayer, but flexible membranes require a slightly larger pore diameter (7 A) than rigid membranes.},
keywords = {Nanotechnology, Thin Films, Water},
pubstate = {published},
tppubtype = {article}
}
We propose a nano-structured suction system based on graphene sheets for water desalination processes. The desalination system modeled in this work is an alternative process to the commonly employed but energy intensive reverse osmosis process. The nano-structured system generates drag forces, which pull water molecules from the saline solution into a chamber. Our molecular simulations consist of two rigid walls of graphene: one wall with 5 A pores permeable to water molecules forms the membrane, while the other wall acts as a plunger to induce and control the transfer of desalinated water molecules, which accumulate in a chamber between the two walls. Prior to the desalination process, the chamber is saturated with one monolayer of water molecules. The desalination occurs when the plunger moves to create unsaturated space inside the chamber. At plunger speeds up to 10 cm s-1, the system desalinates saltwater films in the open part of the membrane. At higher plunger speeds, the desalination chamber expands faster than molecules can fill the chamber, resulting in cavitation and poor desalination. At plunger speeds of 0.5 cm s-1, the desalination occurs via a quasi-equilibrium process, which minimizes the energy necessary to drive desalination. Our findings suggest that the desalination process requires less energy than reverse osmosis methods at plunger speeds up to 0.15 cm s-1 (for the chosen pore density). The filling profile of desalinated water molecules inside the chamber occurs via three distinct regimes: the first two regimes correspond to the formation of one and then two monolayers adsorbed to the chamber's walls. The third regime corresponds to the filling of molecules between the adsorbed layers, which approaches a density close to the density of bulk liquid water. Including flexibility in the graphene sheets does not affect the energy consumption for desalination processes occurring after the formation of the second monolayer, but flexible membranes require a slightly larger pore diameter (7 A) than rigid membranes.
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.
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.; 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]