Publications – Starr Lab

Publications by year, excluding conference proceedings.

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:

2020

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:

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:

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:

2019

Kennedy, Kiley E; Shafique, Neha; Douglas, Jack F; Starr, Francis W

Cooperative dynamics in a model DPPC membrane arise from membrane layer interactions Journal Article

In: Emergent Materials, vol. 2, no. 1, pp. 1-10, 2019.

Abstract | BibTeX | Tags: Biophysics, Dynamic Heterogeneity, Membranes | Links:

@article{ksds19,

title = {Cooperative dynamics in a model DPPC membrane arise from membrane layer interactions},

author = {Kiley E Kennedy and Neha Shafique and Jack F Douglas and Francis W Starr},

url = {http://fstarr.web.wesleyan.edu/publications/ksds19.pdf},

doi = {10.1007/s42247-018-0020-2},

year  = {2019},

date = {2019-03-01},

journal = {Emergent Materials},

volume = {2},

number = {1},

pages = {1-10},

abstract = {The dynamics of model membranes can be highly heterogeneous, especially in more ordered dense phases. To better understand the origins of this heterogeneity, as well as the degree to which monolayer systems mimic the dynamical properties of bilayer membranes, we use molecular simulations to contrast the dynamical behavior of a single-component dipalmitoylphosphatidylcholine (DPPC) lipid monolayer with that of a DPPC bilayer. DPPC is prevalent in both biological monolayers and bilayers, and we utilize the widely studied MARTINI model to describe the molecular interactions. As expected, our simulations confirm that the lateral structure of the monolayer and bilayer is nearly indistinguishable in both low- and high-density phases. Dynamically, the monolayer and bilayer both exhibit a drop in mobility for dense phases, but we find that there are substantial differences in the amplitude of these changes, as well as the nature of molecular displacements for these systems. Specifically, the monolayer exhibits no apparent cooperativity of the dynamics, while the bilayer shows substantial spatial and temporal heterogeneity in the dynamics. Consequently, the dynamical heterogeneity and cooperativity observed in the bilayer membrane case arises in part due to interlayer interactions. We indeed find a substantial interdigitation of the membrane leaflets which appears to impede molecular rearrangement. On the other hand, the monolayer, like the bilayer, does exhibit complex non-Brownian molecular displacements at intermediate time scales. For the monolayer system, the single particle motion can be well characterized by fractional Brownian motion, rather than being a consequence of strong correlations in the molecular motion previously observed in bilayer membranes. The significant differences in the dynamics of dense monolayers and bilayers suggest that care must be taken when making inferences about membrane dynamics on the basis of monolayer studies.},

keywords = {Biophysics, Dynamic Heterogeneity, Membranes},

pubstate = {published},

tppubtype = {article}

}

Zhang, Wengang; Emamy, Hamed; Betancourt, Beatriz A Pazmiño; Vargas-Lara, Fernando; Starr, Francis W; Douglas, Jack F

The interfacial zone in thin polymer films and around nanoparticles in polymer nanocomposites Journal Article

In: The Journal of Chemical Physics, vol. 151, no. 12, pp. 124705, 2019.

BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Nanocomposites, Nanotechnology | Links:

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:

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.

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:

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}

}

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

Shafique, Neha; Kennedy, Kiley E.; Douglas, Jack F.; Starr, Francis W.

Quantifying the Heterogeneous Dynamics of a Simulated DPPC Membrane Journal Article

In: JOURNAL OF PHYSICAL CHEMISTRY B, vol. 120, no. 23, pp. 5172–5182, 2016, (PMID: 27223339).

Abstract | BibTeX | Tags: Biophysics, Dynamic Heterogeneity, Membranes | Links:

@article{skds16,

title = {Quantifying the Heterogeneous Dynamics of a Simulated DPPC Membrane},

author = {Neha Shafique and Kiley E. Kennedy and Jack F. Douglas and Francis W. Starr},

url = {http://fstarr.web.wesleyan.edu/publications/skds16-links.pdf},

doi = {10.1021/acs.jpcb.6b02982},

year  = {2016},

date = {2016-05-25},

journal = {JOURNAL OF PHYSICAL CHEMISTRY B},

volume = {120},

number = {23},

pages = {5172–5182},

abstract = {Heterogeneity of dynamics plays a vital role in membrane function, but the methods for quantifying this heterogeneity are still being developed. Here we examine membrane dynamical heterogeneity via molecular simulations of a single-component dipalmitoylphosphatidylcholine (DPPC) lipid bilayer using the MARTINI force field. We draw upon well-established analysis methods developed in the study of glass-forming fluids and find significant changes in lipid dynamics between the fluid (Lα), and gel (Lβ) phases. In particular, we distinguish two mobility groups in the more ordered Lβ phase: (i) lipids that are transiently trapped by their neighbors and (ii) lipids with displacements on the scale of the intermolecular spacing. These distinct mobility groups spatially segregate, forming dynamic clusters that have characteristic time (1–2 μs) and length (1–10 nm) scales comparable to those of proteins and other biomolecules. We suggest that these dynamic clusters could couple to biomolecules within the membrane and thus may play a role in many membrane functions. In the equilibrium membrane, lipid molecules dynamically exchange between the mobility groups, and the resulting clusters are not associated with a thermodynamic phase separation. Dynamical clusters having similar characteristics arise in many other condensed phase materials, placing membranes in a broad class of materials with strong intermolecular interactions.},

note = {PMID: 27223339},

keywords = {Biophysics, Dynamic Heterogeneity, Membranes},

pubstate = {published},

tppubtype = {article}

}

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}

}

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:

Pazmiño Betancourt, Beatriz A.; Douglas, Jack F.; Starr, Francis W.

String model for the dynamics of glass-forming liquids Journal Article

In: JOURNAL OF CHEMICAL PHYSICS, vol. 140, no. 20, pp. 204509, 2014, ISSN: 0021-9606.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation | Links:

Starr, Francis W.; Hartmann, Benedikt; Douglas, Jack F.

Dynamical clustering and a mechanism for raft-like structures in a model lipid membrane Journal Article

In: SOFT MATTER, vol. 10, no. 17, pp. 3036-3047, 2014, ISSN: 1744-683X.

Abstract | BibTeX | Tags: Biophysics, Dynamic Heterogeneity, Membranes | Links:

@article{shd14,

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

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

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

doi = {10.1039/c3sm53187b},

issn = {1744-683X},

year  = {2014},

date = {2014-01-01},

journal = {SOFT MATTER},

volume = {10},

number = {17},

pages = {3036-3047},

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

keywords = {Biophysics, Dynamic Heterogeneity, Membranes},

pubstate = {published},

tppubtype = {article}

}

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}

}

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

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:

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:

2007

Mazza, Marco G.; Giovambattista, Nicolas; Stanley, H. Eugene; Starr, Francis W.

Connection of translational and rotational dynamical heterogeneities with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in water Journal Article

In: PHYSICAL REVIEW E, vol. 76, no. 3, pp. 031203, 2007, ISSN: 1539-3755.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Water | Links:

@article{mgss-pre07,

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

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

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

doi = {10.1103/PhysRevE.76.031203},

issn = {1539-3755},

year  = {2007},

date = {2007-09-01},

journal = {PHYSICAL REVIEW E},

volume = {76},

number = {3},

pages = {031203},

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

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

pubstate = {published},

tppubtype = {article}

}

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

2006

Becker, Stephen R.; Poole, Peter H.; Starr, Francis W.

Fractional Stokes-Einstein and Debye-Stokes-Einstein relations in a network-forming liquid Journal Article

In: PHYSICAL REVIEW LETTERS, vol. 97, no. 5, pp. 055901, 2006, ISSN: 0031-9007.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Water | Links:

Mazza, MG; Giovambattista, Nicolas; Starr, Francis W.; Stanley, H. Eugene

Relation between rotational and translational dynamic heterogeneities in water Journal Article

In: PHYSICAL REVIEW LETTERS, vol. 96, no. 5, pp. 057803, 2006, ISSN: 0031-9007.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Water | Links:

2005

Giovambattista, Nicolas; Buldyrev, Sergey V.; Stanley, H. Eugene; Starr, Francis W.

Clusters of mobile molecules in supercooled water Journal Article

In: PHYSICAL REVIEW E, vol. 72, no. 1, 1, pp. 011202, 2005, ISSN: 1539-3755.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Water | Links:

@article{gbss05,

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

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

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

doi = {10.1103/PhysRevE.72.011202},

issn = {1539-3755},

year  = {2005},

date = {2005-07-01},

journal = {PHYSICAL REVIEW E},

volume = {72},

number = {1, 1},

pages = {011202},

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

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

pubstate = {published},

tppubtype = {article}

}

2003

Lacevic, Naida; Starr, Francis W.; Schrøder, Thomas B.; Glotzer, Sharon C.

Spatially heterogeneous dynamics investigated via a time-dependent four-point density correlation function Journal Article

In: JOURNAL OF CHEMICAL PHYSICS, vol. 119, no. 14, pp. 7372-7387, 2003, ISSN: 0021-9606.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation | Links:

@article{lssg03,

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

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

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

doi = {10.1063/1.1605094},

issn = {0021-9606},

year  = {2003},

date = {2003-10-01},

journal = {JOURNAL OF CHEMICAL PHYSICS},

volume = {119},

number = {14},

pages = {7372-7387},

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

keywords = {Dynamic Heterogeneity, Glass Formation},

pubstate = {published},

tppubtype = {article}

}

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

Giovambattista, Nicolas; Buldyrev, Sergey V.; Starr, Francis W.; Stanley, H. Eugene

Connection between Adam-Gibbs theory and spatially heterogeneous dynamics Journal Article

In: PHYSICAL REVIEW LETTERS, vol. 90, no. 8, pp. 085506, 2003, ISSN: 0031-9007.

Abstract | BibTeX | Tags: Dynamic Heterogeneity, Glass Formation, Water | Links: