Thermodynamic and Conformational Properties of Styrene–Methyl Methacrylate Block Copolymers in Dilute Solution. IV. Behavior of Diblock and Triblock Copolymers in Selective Solvents

Abstract

Solution properties of nearly equimolar AB-diblock and BAB-triblock copolymers, wherein A is polystyrene (PST) and B poly(methyl methacrylate) (PMMA), were examined in various selective solvents. In either type of selective solvents the diblock copolymers usually underwent intermolecular association and formed micelles, in which domains of PST- and PMMA-subchains were presumably segragated (intramolecular phase separation). The stability and size of such micelles depended on their molecular weight and composition as well as on the nature of solvents. The behavior of the triblock copolymers appeared to be more critically influenced by the solubility of the PMMA than that of the PST. In selective solvents having preferential solvency towards PMMA rather than PST, their behavior was more or less similar to that of the diblock copolymers whilst in selective solvents preferential to PST, they were usually unstable and liable to precipitate. In the transition region between the stable solution and precipitation states, the individual triblock copolymer chains often showed conformational anomalies without forming micelles, as shown by the intrinsic viscosity anomaly in p-xylene at 30°C. The possibility of intramolecular association of two PMMA subchains within a single triblock copolymer molecule was suggested as a hypothesis. However the hydrodynamic properties alone cannot provide decisive evidence for this possibility.