A spin-label study of blends of immiscible polymers
The work presented in this thesis is concerned mainly with the study of the structure and composition of the interfacial region (interphase) in a blend of two immiscible polymers with particular reference to poly(methyl methacrylate) (PMMA) and poly(2-ethyl hexyl methacrylate) (PEHMA). In the initial part of this study PMMA was labelled at chain-end positions with a stable nitroxide free radical by modification of the termination step in the group transfer polymerisation (GTP) of MMA. Electron Spin Resonance (ESR) spectra of this end-labelled PMMA sample both in the pure state and in a 1:1 (w/w) freeze-dried blend with unlabelled PEHMA were subsequently recorded. Analysis of these spectra indicates that the mobility of a proportion of the labelled, glassy PMMA chain ends is greatly enhanced in the presence of the rubbery PEHMA component. Upon heating a sample of the metastable, freeze-dried blend to temperatures above the Tg of the high Tg component (PMMA), an increase is observed in the proportion of labelled PMMA chain ends which are plasticized. No evidence could be found to suggest that this behaviour is a direct result of the preferential solvation of the nitroxide labelled PMMA chain end by the PEHMA. These observations are wholly consistent with theoretical predictions, based on thermodynamic considerations, that chain ends are preferentially located, at the expense of internal chain segments, in the interphase region in blends of immiscible polymers. Parallel experiments, to those described above, were performed on a sample of PEHMA which was also labelled at chain ends with a nitroxide radical. Complementary evidence, to support the earlier observations and conclusions on the blend containing end-labelled PMMA, was obtained from the ESR spectra of the labelled PEHMA sample. The behaviour of the ESR spectra recorded in both systems suggests that the free volume within such a blend accumulates in the interphase region, as a direct result of the preference for chain ends to be located there.