Thermodynamic scaling of dynamics in polymer melts: Predictions from the generalized entropy theory

Abstract

Many glass-forming fluids exhibit a remarkable thermodynamic scaling in which dynamic properties, such as the viscosity, the relaxation time, and the diffusion constant, can be described under different thermodynamic conditions in terms of a unique scaling function of the ratio rho^gamma/T, where rho is the density, T is the temperature, and gamma is a material dependent constant. Given the successes of the generalized entropy theory in elucidating the influence of molecular details on the universal properties of glass-forming polymers, this theory is extended here to investigate the thermodynamic scaling in polymer melts. The predictions of theory are in accord with the appearance of thermodynamic scaling for pressures not in excess of about 50 MPa. (The failure at higher pressures arises due to inherent limitations of a lattice model.) In line with arguments relating the magnitude of gamma to the steepness of the repulsive part of the intermolecular potential, the abrupt, square-well nature of the lattice model interactions lead, as expected, to much larger values of the scaling exponent. Nevertheless, the theory is employed to study how individual molecular parameters affect the scaling exponent in order to extract a molecular understanding of the information content contained in the exponent. The chain rigidity, cohesive energy, chain length, and the side group length are all found to significantly affect the magnitude of the scaling exponent, and the computed trends agree well with available experiments. The variations of gamma with these molecular parameters are explained by establishing a correlation between the computed molecular dependence of the scaling exponent and the fragility. Thus, the efficiency of packing the polymers is established as the universal physical mechanism determining both the fragility and the scaling exponent gamma.

11 pages, 12 figures, to be published in J. Chem. Phys

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Cited by in corpus (10)

• Polymer Glass Formation: Role of Activation Free Energy, Configurational Entropy, and Collective Motion
• Influence of Cohesive Energy and Chain Stiffness on Polymer Glass Formation
• Scaling of the dynamics of flexible Lennard-Jones chains
• Equation of State and Entropy Theory Approach to Thermodynamic Scaling in Polymeric Glass-Forming Liquids
• Generalized Entropy Theory of Glass Formation in Polymer Melts with Specific Interactions
• Generalized entropy theory of glass-formation in fully flexible polymer melts
• Estimating the density-scaling exponent of a monatomic liquid from its pair potential
• Polymer Glass-Formation in Variable Dimension
• Thermodynamic-Dynamic Interrelations in Glass-Forming Polymer Fluids
• Combined Description of Pressure-Volume-Temperature and Dielectric Relaxation of Several Polymeric and Low-Molecular-Weight Organic Glass-Formers using 'SL-TS2' Mean-Field Approach