Near-Substrate Gradients in Chain Relaxation and Viscosity in a Model Low-Molecular Weight Polymer

T Rahman and DS Simmons, MACROMOLECULES, 54, 5935-5949 (2021).

DOI: 10.1021/acs.macromol.0c02888

Polymers in the nanoscale vicinity of interfaces can exhibit large alterations in dynamics and glass formation behavior. These changes are accompanied by alterations in rheological response, yet the precise nature of gradients in viscosity and whole-chain relaxation near interfaces is an open question. Here, we employ molecular dynamics simulations of a low-molecular weight glass-forming polymer between crystalline walls to probe this relationship. Results indicate that viscosity and whole-chain relaxation time gradients for this system obey the same qualitative phenomenology as do segmental relaxation time gradients, indicating that they emanate from the same underlying physics. At a quantitative level, however, our simulation and theoretical results indicate that unlike in small molecules, polymer viscous and whole-chain relaxation interfacial gradients should generally be expected to be weaker than underlying segmental relaxation time gradients-a consequence in large part of the typically weaker bulk temperature dependence of low-temperature viscosity and whole-chain relaxation than segmental dynamics in many polymers. Shifts in viscosity are shown to emerge from underlying alterations in the polymer's complex modulus near interfaces, with a high-frequency glassy plateau emerging at higher temperature and surviving to lower frequency near the walls. These results have implications for the rheological response of diverse nanostructured polymers including thin films, filled rubber, ionomers, and semicrystalline polymers, and they highlight the need for a generalization of the Rouse model to account for dynamical gradients.

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