Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

TL;DR

This study investigates how chain length affects the local segmental dynamics, fragility, and thermodynamic scaling in PMMA, revealing a unified scaling exponent and dynamic crossovers linked to thermodynamic properties.

Contribution

It introduces a thermodynamic scaling approach for segmental relaxation in PMMA, connecting dynamics with thermodynamics and identifying multiple dynamic crossovers.

Findings

Fragility and glass transition temperature vary with molecular weight.

A single scaling exponent g describes relaxation time and distribution.

Dynamic crossovers occur as functions of temperature and pressure.

Abstract

Local segmental relaxation properties of poly(methylmethacrylate) (PMMA) of varying molecular weight are measured by dielectric spectroscopy, and analyzed in combination the equation of state obtained from PVT measurements. The usual variation of glass transition temperature and fragility with molecular weight are observed. We also find, in accord with the general properties of glass-forming materials, that a single molecular weight dependent scaling exponent, g, is sufficient to define the mean segmental relaxation time, tau_alpha, and its distribution. This exponent can be connected to the Gruneisen parameter and related thermodynamic quantities, thus demonstrating the interrelationship between dynamics and thermodynamics in PMMA. Changes in the relaxation properties ("dynamic crossover") are observed as a function of both temperature and pressure, with tau_alpha serving as the control parameter for the crossover. At longer tau_alpha another change in the dynamics is apparent, associated with a decoupling of the local segmental process from ionic conductivity.