Anisochemical homopolymer/diblock copolymer thin film blends

We examined the effect of the addition of homopolymers tetramethylbisphenol A polycarbonate, TMPC, and poly(norbornene-methyl-d(3)-carboxylate), NBMC, on the interlamellar spacing and the phase stability of symmetric poly(sty rene-b-methyl methacrylate) diblock copolymer thin films on silicon substra tes. The films were of thickness h, L < h < 5L, where L is the interlamella r spacing of the microphase separated PS-b-PMMA diblock copolymer. The homo polymers considered had degrees of polymerization, N-TMPC and N-NBMC, compa rable to one-half of the degree of polymerization of the diblock. In the TM PC/diblock blend, for low TMPC homopolymer concentrations, phi(TMPC) < 0.2, the homopolymer chains were localized in the middle of the PS micro-ordere d domains, and the interlamellar spacing increased as L/(1 - phi(TMPC)). Fo r phi(TMPC) > 0.2 the morphology of the diblock copolymer changed to accomm odate higher volume fractions of TMPC. This behavior is contrasted with ear lier observations in the PS-b-PMMA/PS homopolymer system where this diblock accommodated considerably higher PS homopolymer volume fractions while mai ntaining a lamellar phase. In the NBMC/diblock system the interlamellar spa cing increased as L/(1 - phi(NBMC)) for homopolymer concentrations up to ph i(NBMC) approximate to 0.05. For phi(NBMC) > 0.05, the NBMC formed a pure l ayer on the substrate, with the diblock maintaining its microphase separate d structure on this layer. These results are discussed in light of mean fie ld theory and in terms of the effect of the interfacial constraints on the phase behavior.