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

Poly(methyl methacrylate‐ ran ‐styrene) copolymers were synthesized under monomer‐starved conditions by emulsion copolymerization. The glass‐transition temperatures ( T g 's) 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 T g versus composition curve, which could not even be interpreted by the Johnston equation with different contributions of dyads to the T g of the copolymer considered. A new sequence distribution equation concerning different contributions of triads was introduced to predict the copolymer's T g . The new equation fit the experimental data exactly. Also, the T g determined by TBA ( T g TBA ) was higher than the one determined by DSC ( T g DSC ) and the difference was not constant. The rheological behavior of the copolymers was also studied. T g TBA − T g DSC increased with increasing flow index of the melt of the copolymer, and the reason was interpreted. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2891–2896, 2003