PROTON SPIN-DIFFUSION STUDIES OF POLYMER BLENDS HAVING MODEST MONOMER SIZE .1. POLYSTYRENE POLY(XYLYLENE ETHER), A MISCIBLE BLEND

Proton spin diffusion experiments, utilizing multiple-pulse methods, have been conducted on three samples of a 50/50 blend of polystyrene (PS) and poly(xylylene ether) (PXE) as well as on the PS and PXE homopolymers. The PS/PXE blend is considered miscible on a molecular scale; this was verified by demonstrating the strength of the early-time spin diffusion between the PS and PXE protons. An attempt was made to estimate the average number of dissimilar monomers surrounding a repeat unit of a given type, and for this discussion a model of a simple cubic lattice was adopted. This exercise involved several other assumptions and was semiquantitative. It was judged that, allowing for chain continuity, the number of dissimilar nearest neighbors surrounding any given repeat unit is close to or greater than that dictated by the relative abundances of the respective monomer units. A second motivation for this study is that it provides a base line for other spin diffusion studies on glassy, phase-separated blends of similar monomer size. In the general case, spin diffusion data from such blends involves three kinds of spin equilibration, namely, homopolymer, intraphase heteropolymer (assuming phases of mixed composition), and interphase heteropolymer, where only the latter process is considered interesting. The PS/PXE data and the homopolymer data relate to the appropriate modeling of the first two kinds of equilibration. Homopolymer equilibration is seen to be better than 99 % complete after 1 ms of spin diffusion, while intraphase heteropolymer equilibration has an initial fast component followed by a rather long tail, extending past 10 ms of spin diffusion, owing mostly to the statistics of mixing or the fluctuations in average composition over small volumes.