Surface enrichment in polymer blends involving hydrogen bonding

A phenolic polysiloxane, poly(4-ethenylphenolmethylsiloxane) (PEPS), that contains the phenolic hydroxyl group as a hydrogen-bond donor has been synthesized via hydrosilylation followed by hydrolysis. It was blended with a number of hydrogen-bond acceptors of different strengths, including poly(4-vinylpyridine) (PVPy) (strong), poly(vinylpyrrolidione) (PVPr) (strong), poly(dimethylacrylamide) (PDMA) (moderate), and poly(styrene-co-acrylonitrile) (PSAN) (weak). All blends were miscible in the bulk, as indicated by a single DSC T_g, and were shown to be homogeneous by optical microscopy. XPS measurements demonstrated that all PEPS blends had surface enrichment in PEPS, which has a lower surface energy. AFM images showed that the surface morphology in PVPy/PEPS, PVPr/PEPS, and PDMA/ PEPS blends did not differ from that in the bulk, whereas the PSAN/PEPS blend exhibited a different surface structure that consisted of PSAN covered with a thin layer of PEPS. Depth profiling by SIMS was in agreement with the XPS analysis. The results indicated that (1) surface enrichment in hydrogen-bonding polymer blends is governed by the interplay between the difference in the surface energy of the constituents and the bulk thermodynamics and (2) hydrogen-bonding interactions reduce surface enrichment.