Elasto-Viscous and Stress-Optical Properties of Commercial Polymerized Methyl Methacrylate as a Function of Temperature

Abstract

The elasto‐viscous and stress‐optical properties of commercial methyl methacrylate polymer have been measured. Between 66°C and 107°C Young's modulus drops from approximately 400,000 pounds per square inch to roughly 200 pounds per square inch and the material behaves like rubber. At 93°C the viscosity is approximately 10¹² poise. This drops to 10⁹ at 177°C. While the curve is not strictly linear with 1/T, an activation energy of 30,000 cal./mole can be deduced for this change. Below 93°C three rate constants are necessary to describe the delayed elastic process, but two suffice between 93°C and 135°C. At 149°C one such constant is enough and above this an instantaneous elastic and a viscous flow is sufficient. The change in rate constants with temperature gives rise to elastic activation energies of 9000 to 11,000 cal./mole. Methyl methacrylate is optically negative but has a stress optical sensitivity about that of glass. The stress optical coefficient varies markedly with temperature, showing a sharp maximum at 93.3°C. The stress‐optical coefficient is directly proportional to the average relaxation constant. X‐ray diffraction patterns show four rings corresponding to spacings of 2.19, 3.07, 6.7, and 14.7A. Some slight evidences of crystallinity are shown by diffraction patterns in fibers stretched at 93°C. Fibers stretched at 149°C show a lesser amount of order in agreement with the birefringence studies. The second‐order transition point occurs at 71.1°C.