Melt viscosity characteristics of methacrylate–styrene copolymers

The activation energies of flow E_A of methacrylate–styrene copolymers containing n-butyl, n-hexyl, n-heptyl, n-octyl, n-decyl, n-dodecyl, n-tridecyl, n-octadecyl, and cyclohexyl methacrylate have been investigated as a function of molecular weight, composition, and methacrylate monomer. Below a critical pendent group molar volume per chain unit (120 ± 10 ml Le Bas units), E_A was found to increase with molar volume; and above this value, a decrease in E_A was observed, reflecting a decrease in copolymer density. Copolymers with pendent group molar volumes per average chain unit of between 96 and 140 ml (Le Bas units) were found to exhibit sufficiently high E_A values to render them suitable for use in thermoplastic and photothermoplastic devices with superior development and erasure rates, at temperatures which enabled the attainment of the development and erasure viscosities with a low expenditure of heat energy. Methacrylate–styrene copolymers with long-chain ester methacrylates (viz., n-decyl and n-dodecyl methacrylate) were found to exhibit critical molecular weights M_c below 3000; and M_c was found to decrease with increasing methacrylate tail length and methacrylate concentration. These M_c values correspond to critical chain lengths Z_c below 45. Similar Z_c values have been previously reported for acrylonitrile–methyl methacrylate copolymers³⁰ and ethylene–propylene copolymers.²⁸