Chapter 4 - Metastable States in Phase Transitions of Polymers

Abstract

This chapter focuses on polymer metastable states and phase transitions. The classical understanding of metastable states is introduced in terms of the supercooling of the isotropic melt and crystallization, as well as the superheating of the crystal and melting in the bulk of semicrystalline homopolymers. A general review is provided summarizing the development of polymer crystallization theories advanced so far and current issues remaining in this area. New definition of nucleation barrier, consisting of both enthalpic and entropic components, is proposed. The observation of lower melting temperatures for the metastable polymer crystals, compared with the corresponding equilibrium crystals, leads to the recognition of the phase size dependence of crystal metastability. The concept of morphologically metastable states is introduced to illustrate the different stages of the morphological evolution observed in phase-separated polymer blends. These morphologies are on the kinetic pathway to the two-layer equilibrium, phase-separated morphology. Polymers provide many examples of metastable states beyond the atomic length scale. The thermodynamic principles governing binary mixtures, such as liquid polymer blends, are very similar to small molecule and polymer solutions. The equilibrium phase morphology of ultimate stability is the same for a polymer blend independent of the phase separation mechanism. The morphologies on the path toward this ultimate morphological stability are determined by the phase separation mechanism. It is commonly understood that the droplet morphology indicates nucleation-limited phase separation, while a bicontinuous morphology indicates spontaneous decomposition.