Glass-transition temperatures and rheological behavior of methyl methacrylate-styrene random copolymers

Abstract

Poly(methyl methacrylate-ran-styrene) copolymers were synthesized under monomer-starved conditions by emulsion copolymerization. The glass-transition temperatures (T'gs) of the copolymers were measured by differential scanning calorimetry (DSC) and torsional braid analysis (TBA). The results showed that the methyl methacrylate-styrene random copolymers produced an asymmetric Tg versus composition curve, which could not even be interpreted by the Johnston equation with different contributions of dyads to the Tg of the copolymer considered. A new sequence distribution equation concerning different contributions of triads was introduced to predict the copolymer's Tg. The new equation fit the experimental data exactly. Also, the Tg determined by TBA (TgTBA) was higher than the one determined by DSC (TgDSC) and the difference was not constant. The rheological behavior of the copolymers was also studied. TgTBA - TgDSC increased with increasing flow index of the melt of the copolymer, and the reason was interpreted. © 2003 Wiley Periodicals, Inc.