Nanodomain formation in a liquid polymer blend: The initial stages of phase separation

Abstract

The morphology of nanodomain structures in binary polymer blends of a random copolymer and a homopolymer is determined using electronic excitation transport (EET) studies. The experimental system employed is a copolymer, 6.5% atactic poly(methyl methacrylate-co-2-vinyl naphthalene) [P(MMA-2VN)], in atactic poly(vinyl acetate) (PVAc). The naphthalene groups serve as chromophores in the EET experiments. The mixtures are prepared such that initially the P(MMA-2VN) chains are randomly distributed in the PVAc matrix. The nanodomains are formed while low-concentration mixtures of the P(MMA-2VN) in PVAc are held at constant temperature in the melt state (T≳Tg), above the temperature at which phase separation occurs. In the melt the chains diffuse, and P(MMA-2VN) chains aggregate until the temperature is quenched below Tg. The structures of the resulting domains are examined with time-resolved fluorescence depolarization measurements, and the data are analyzed using an analytical theory to model EET among interacting polymer chains. The agreement between theory and data is very good. The results of the analysis indicate that the nanodomains correspond to aggregates with a characteristic size equal to the radius of gyration of the copolymer, Rg. The number of P(MMA-2VN) chains in aggregates prepared under different conditions is determined.