Glass transition and dynamics of semiflexible polymer brushes

Huang, Jianhua; Sun, Dandan; Lu, Rong-Xing

doi:10.1039/d1cp00089f

Cited by 9 publications

(10 citation statements)

References 55 publications

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“…It has long been recognized from simulations of non-network polymers that some molecular parameters are particularly implicated in changes in the dynamics of GF polymer materials, such as polymer topology ( e.g. , ring and star polymers, polymer brushes, and polymers with side groups), − the addition of nanoparticles or additives, − changes in chain stiffness, − and the cohesive interaction strength of polymer molecules. − The present work extends these earlier studies on glass formation in polymer materials to describe cross-linked network polymers. To some degree, such network polymers can be considered to be just high molecular mass polymers, but the network topology of polymer material can evidently lead to new effects.…”

Section: Introductionmentioning

confidence: 72%

Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

et al. 2022

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To develop structure–property relationships for cross-linked thermosetting polymers, it is crucial to better understand key factors that control their segmental dynamics and macroscopic properties. Here, we employ a coarse-grained (CG) polymer model to systematically explore the combined effect of varying the cohesive energy (ε) and cross-link density (c) on the segmental relaxation time and mechanical properties for a model cross-linked glass-forming thermoset material. We find that increasing c increases both the glass transition temperature T g and fragility of glass formation, while the fragility decreases with an increase in ε. These competing effects of ε and c on fragility are practically important since fragility determines the overall temperature width of the glass formation over which the non-Arrhenius temperature dependence is observed. Our simulation results show that the basic mechanical properties (i.e., bulk and shear moduli) of cross-linked thermosets are mainly influenced by ε. More interestingly, the macroscopic mechanical properties are found to be strongly correlated with the Debye–Waller parameter ⟨u 2⟩, a measure of material “stiffness” at a molecular level. In particular, the distribution of local molecular stiffness, 1/⟨u 2⟩, exhibits a nearly universal Gaussian distribution at a fixed reduced temperature T/T g. Our work reveals the key and competitive roles of cohesive energy and cross-link density in controlling the segmental dynamics, large scale, and local mechanical properties of cross-linked thermosets, providing an understanding that should be useful in the molecular design of these materials.

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Section: Introductionmentioning

confidence: 72%

Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

et al. 2022

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“…Standard molecular dynamics (MD) simulations were performed using our previously validated coarse-grained model for PnBA networks (semiflexible chains − where each bead represents a single nBA monomer of size σ*) with explicit cross-links as the polymer matrix in which dilute spherical penetrants (diameter d ) are dissolved (Figure b). ,,,, The network mesh size is computed as the average distance between cross-link points and the penetrant diffusion constant extracted from the long-time limit of its mean square displacement; see, e.g., Figure S4, and the SI for other technical details of the simulation. − …”

Section: Methodsmentioning

confidence: 99%

How Segmental Dynamics and Mesh Confinement Determine the Selective Diffusivity of Molecules in Cross-Linked Dense Polymer Networks

et al. 2023

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The diffusion of molecules ("penetrants") of variable size, shape, and chemistry through dense cross-linked polymer networks is a fundamental scientific problem broadly relevant in materials, polymer, physical, and biological chemistry. Relevant applications include separation membranes, barrier materials, drug delivery, and nanofiltration. A major open question is the relationship between transport, thermodynamic state, and penetrant and polymer chemical structure. Here we combine experiment, simulation, and theory to unravel these competing effects on penetrant transport in rubbery and supercooled polymer permanent networks over a wide range of cross-link densities, size ratios, and temperatures. The crucial importance of the coupling of local penetrant hopping to polymer structural relaxation and the secondary importance of mesh confinement effects are established. Network cross-links strongly slow down nm-scale polymer relaxation, which greatly retards the activated penetrant diffusion. The demonstrated good agreement between experiment, simulation, and theory provides strong support for the size ratio (penetrant diameter to the polymer Kuhn length) as a key variable and the usefulness of coarse-grained simulation and theoretical models that average over Angstrom scale structure. The developed theory provides an understanding of the physical processes underlying the behaviors observed in experiment and simulation and suggests new strategies for enhancing selective polymer membrane design.

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Section: Simulation and Experimental Methodsmentioning

confidence: 99%

“…We note that our use of a semiflexible chain model − leads to a modest temperature dependence of C ∞ , which is known to affect studies of the α-relaxation due to small local chain extension upon cooling . Although such a model has been extensively used in the simulation literature to study the alpha relaxation process of model glass-forming polymer melts, − , Hsu and Kremer have recently proposed alternative potentials to mitigate this temperature dependence. We do not take this step, as we observe only minor (a maximum of less than 4%) increases of the Kuhn length in un-cross-linked melts over the entire temperature range we consider, which becomes increasingly negligible as the fraction of cross-links increases (see Figure S2 and related discussion in the SI).…”

Section: Simulation and Experimental Methodsmentioning

confidence: 99%

Structural Relaxation and Vitrification in Dense Cross-Linked Polymer Networks: Simulation, Theory, and Experiment

et al. 2022

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We present a coordinated experimental, simulation, and theoretical study of how polymer network permanent cross-links impact the segmental relaxation time over a wide range of temperatures and different criteria for defining the glass transition temperature, T g. The simulations adopt a coarse-grained model calibrated to represent the specific polymer chemistry of interest. The elastically collective nonlinear Langevin equation (ECNLE) theory of activated segmental relaxation is extended to explicitly treat chain semiflexibility and network cross-linkers, with the latter modeled as locally pinned or vibrating sites. Our key findings include the following: (i) tight cross-linking leads to very large increases of the segmental relaxation time and elevation of T g, which grows roughly linearly with cross-link fraction beyond a low threshold, (ii) a remarkably good (but not perfect) collapse of Angell plots of the alpha relaxation time for all cross-link densities studied based on using the cross-link fraction dependent dynamic T g, which applies for very different dynamic vitrification time scale criteria, and (iii) construction of a microscopic understanding of the experimental and simulation observations based on the central idea of ECNLE theory that activated structural relaxation involves cross-link fraction dependent coupled local cage and nonlocal collective elastic barriers. Overall, excellent agreement between experiment, theory, and simulation is found. We suggest that our study of how quenched chemical cross-links strongly modify the alpha relaxation is more generally valuable as a distinct probe of the basic physics of glassy polymer dynamics and as a flexible tool to manipulate small-molecule diffusion in membrane applications.

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Glass transition and dynamics of semiflexible polymer brushes

Abstract: The glass transition and dynamics of densely grafted semiflexible polymer brushes are studied by molecular dynamics simulation. The glass transition temperature (Tg) increases with the polymer rigidity. The local glass...

Cited by 9 publications

References 55 publications

Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

How Segmental Dynamics and Mesh Confinement Determine the Selective Diffusivity of Molecules in Cross-Linked Dense Polymer Networks

Structural Relaxation and Vitrification in Dense Cross-Linked Polymer Networks: Simulation, Theory, and Experiment

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