Entropy Theory of Polymer Glass-Formation Revisited. II. Influence of Mass, Pressure, and Size of Side Groups and Similarities to Equilibrium Polymerization

Published

February 8, 2006

Author(s)

J Dudowicz, Karl Freed, Jack F. Douglas

Abstract

Recent measurements reveal striking trends in the variation of fragility (the strength of the temperature dependence of the fluid viscosity) of glass-forming polymer liquids with monomer structure and pressure. The elucidation of the physical origin of these trends in the fragility of glass-forming liquids remains a vital issue that is relevant to numerous technological applications. We address this problem by developing an improved entropy theory of glass-formation that accounts for monomer shape, size, and flexibility and that is combined with the Adam-Gibbs model for dynamics of cooled liquids.This generalization, discussed in a companion paper, describes both equilibrium and dynamic properties in the vicinity as well as far above the glass transition. The present paper focuses on the calculation of the four characteristic temperatures of glass-formation (TK,Tg,TI,TA) whose ratios are commonly considered as model independent measures of fragility.The characteristic temperatures and their ratios are calculated as a function of polymer mass M and are found to correlate with direct calculations of the fragility. The fragility is estimated differently above and below a crossover temperature TI that separates two regimes with distinct temperature dependence for the configurational entropy sc. Calculations for variations with T and M of the specific volume and isothermal compressibility (which are experimentally more accessible than sc provide additional perspectives on glass-formation. The new entropy theory is also used to explain the influence of pressure and the side group size on fragility and on glass-formation, as well as the origin of the pressure analog of the VFTH equation for the structural relaxation time that has been first discovered empirically.

Citation

Journal of Chemical Physics

Volume

124

Pub Type

Journals

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Keywords

chain rigidity, fragility, free volume, glass-formation, isothermal compressiblity, Lindemann criterion, monomer structure, shear modulus

Citation

Dudowicz, J. , Freed, K. and Douglas, J. (2006), Entropy Theory of Polymer Glass-Formation Revisited. II. Influence of Mass, Pressure, and Size of Side Groups and Similarities to Equilibrium Polymerization, Journal of Chemical Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=852487 (Accessed April 17, 2024)