Thermodynamic and Conformational Properties of Styrene–Methyl Methacrylate Block Copolymers in Dilute Solution. III. Viscometric Behavior in Relation to Incompatibility of Parent Homopolymers

Abstract

Viscometric behavior of nearly equimolar block copolymers of the poly(methyl methacrylate)–polystyrene–poly(methacrylate) type are studied in various solvents. The behavior is discussed in relation to the incompatibility of the parent homopolymers in each solvent. A correlation is found between the excluded-volume interactions in the block copolymer chain and the incompatibility of the homopolymer pair. A high compatibility is observed for the pair in cyclohexanol, and this reinforces our previous argument that this solvent may provide a genuine θ condition for the block copolymer, wherein random-flight statistics prevails. In certain common solvents, the homopolymer pair is fairly compatible, and the viscometric behavior of the block copolymer shows no anomalies: the so-called phenomenon of intrachain phase separation is unlikely to be taking place in such solvents. In θ solvents for either one of the homopolymers, they become less compatible. Nevertheless in a θ solvent for polystyrene the block copolymer shows no anomalies. On the other hand, in θ solvents for poly(methyl methacrylate), significant anomalies are observed. The anomalies must be due to the triblock architecture of the block copolymer chains: the two poly(methyl methacrylate) side-chains undergo intrachain association, thereby restricting allowable conformations of the central polystyrene chain.