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 10 12 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.