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

Blends of epsilon-polycaprolactone (PCL) and low molecular weight atactic p olystyrene (PS) are among those which show an upper critical solution tempe rature, and where one component (in this case PCL) is crystallizable, leadi ng to several possible competitive interactions between blend decomposition and crystallization. Scanning and transmission electron microscopy have be en used to obtain finer morphological information relating to two compositi ons where this phenomenon has already been followed under the optical micro scope. This has been made possible by the development of several etchants w hich allow inspection of the internal bulk morphology with good resolution. The system 40% PCL-60% PS decomposes binodally to a majority phase with ro ughly equal proportions of PCL and PS surrounding blobs of a minority phase composed almost entirely of PS. The large spherulites of PCL grow around a nd 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 d iffuse outward from the growing spherulite, the composition of the remainin g melt changes to one which can decompose binodally. TEM shows the PCL lame llae 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 1 mu m in size, or outside the spherulites, where it coalesces into larger droplets, which in bulk spe cimens ooze to the surface giving a featureless layer largely concealing th e 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. (C) 2000 Elsevier Science Ltd. All rights reserved .