Plasticization and antiplasticization of polymer melts diluted by low molar mass species

TL;DR

This paper uses the generalized entropy theory to analyze how low molar mass additives can plasticize or antiplasticize polymer melts, affecting their glass transition temperature and fragility, with implications for material design.

Contribution

It introduces a theoretical framework combining the Adam-Gibbs model and lattice-based configurational entropy calculations to predict the effects of molecular additives on polymer glass formation.

Findings

Antiplasticizers increase toughness and decrease T_g when small, strongly attractive diluents are used.

Plasticizers lower T_g and soften the polymer by weakly attractive, small additives.

Decreased fragility can occur with both plasticization and antiplasticization, depending on additive properties.

Abstract

An analysis of glass formation for polymer melts that are diluted by structured molecular additives is derived by using the generalized entropy theory, which involves a combination of the Adam-Gibbs model and the direct computation of the configurational entropy based on a lattice model of polymer melts that includes monomer structural effects. Antiplasticization is accompanied by a "toughening" of the glass mixture relative to the pure polymer, and this effect is found to occur when the diluents are small species with strongly attractive interactions with the polymer matrix. Plasticization leads to a decreased glass transition temperature T_g and a "softening" of the fragile host polymer in the glass state. Plasticization is prompted by small additives with weakly attractive interactions with the polymer matrix. The shifts in T_g of polystyrene diluted by fully flexible short oligomers are evaluated from the computations, along with the relative changes in the isothermal compressibility at T_g to characterize the extent to which the additives act as antiplasticizers or plasticizers. The theory predicts that a decreased fragility can accompany both antiplasticization and plasticization of the glass by molecular additives. The general reduction in the T_g and fragility of polymers by these molecular additives is rationalized by analyzing the influence of the diluent's properties (cohesive energy, chain length, and stiffness) on glass formation in diluted polymer melts. The description of glass formation at fixed temperature that is induced upon change the fluid composition directly implies the Angell equation for the structural relaxation time as function of the polymer concentration, and the computed "zero mobility concentration" scales linearly with the inverse polymerization index N.