Side‐chain crystallinity. IV. Mechanical properties and transition temperatures of copolymers of methyl methacrylate with higher n‐alkyl acrylates and N‐n‐alkylacrylamides

Abstract

AbstractMechanical properties were correlated with glass transition temperatures for a series of random copolymers of methyl methacrylate with comonomers selected from the higher n‐alkyl acrylates and N‐n‐alkylacrylamides. The plasticizing comonomers were the n‐butyl, 2‐ethylhexyl, n‐octadecyl, and oleyl acrylates, and the N‐n‐butyl‐, N‐n‐octyl‐, N‐n‐octadecyl‐, and N‐oleylacrylamides. The complete range of compositions was investigated. However, the bulk of the data was obtained on compositions in the glassy region below the onset of the vitreous transition. In this region it was found that the decrease in tensile and flexural moduli and strengths with increase in internal plasticizer for all of the systems was directly proportional to the decrease in Tg. It was concluded that the additive contribution to the free volume made by each side‐chain methylene group was alone responsible for the magnitude of the rate of change of properties. However, polar contributions of the amide group to stiffening the main chain exceeded those of the ester, so that the amides were less efficient plasticizers. An empirical equation was derived which described, with fair accuracy, the decrease in the mechanical parameters with composition for the amorphous copolymers. It was reasonably successful in predicting properties even into the composition range where the ambient testing temperature corresponded to or exceeded the transition temperature. In this transition region an accelerated decrease in the magnitude of the physical properties was observed. All samples exhibited brittle fracture except those tested in the transition region. Here the strain was largely irrecoverable flow. Side‐chain crystallinity did not interfere significantly with the mechanical properties because moduli and strengths had already decayed to small values near the compositions where crystallinity commenced. Non‐random copolymers of vinyl stearate and methyl methacrylate showed no internal plasticization, apparently because of macrophase aggregation.