Phase separation induced by crystallization in blends of polycaprolactone and polystyrene: an investigation by etching and electron microscopy

Abstract

Blends of ϵ-polycaprolactone (PCL) and low molecular weight atactic polystyrene (PS) are among those which show an upper critical solution temperature, and where one component (in this case PCL) is crystallizable, leading to several possible competitive interactions between blend decomposition and crystallization. Scanning and transmission electron microscopy have been used to obtain finer morphological information relating to two compositions where this phenomenon has already been followed under the optical microscope. This has been made possible by the development of several etchants which allow inspection of the internal bulk morphology with good resolution. The system 40% PCL–60% PS decomposes binodally to a majority phase with roughly equal proportions of PCL and PS surrounding blobs of a minority phase composed almost entirely of PS. The large spherulites of PCL grow around and engulf the PS-rich blobs. The system 60% PCL–40% PS crystallizes from a homogeneous melt. In this case, as the low molecular weight PS is able to diffuse outward from the growing spherulite, the composition of the remaining melt changes to one which can decompose binodally. TEM shows the PCL lamellae to be separated in bundles, as is found in other blend systems. The PS-rich phase is observed to segregate either at the spherulitic growth front, where it forms small droplets of the order of 1μm in size, or outside the spherulites, where it coalesces into larger droplets, which in bulk specimens ooze to the surface giving a featureless layer largely concealing the spherulitic morphology. Spinodal decomposition has also been observed in melt which has been rapidly quenched in liquid nitrogen and allowed to warm up to room temperature.