The yield and thermoelastic properties of oriented poly(methyl methacrylate)

Oriented sheets of poly(methyl methacrylate) were prepared by drawing to different draw ratios at a range of temperatures near the glass transition temperature. The degree of molecular orientation in these materials was characterized by measuring birefringence, and specimens prepared for measurements of tensile yield stress and shrinkage stress at elevated temperature. The shrinkage stress measurements were carried out by holding specimens at constant length above the glass transition temperature and determining the resulting stress as a function of time. The peak value of stress was found to be a function of birefringence only, independent of draw ratio or draw temperature. The function was linear for a substantial range of birefringence. A further series of specimens was used to measure the tensile yield stress (in the orientation direction) as a function of temperature and strain rate. At each testing temperature it was found that the yield stress correlated uniquely with the birefringence. These results for shrinkage stress and yield stress can both be interpreted through the concept of a frozen-in molecular network, and a semi-quantitative relationship proposed which relates to the anisotropy of the yield stress to the frozen-in network stress. This provides a molecular explanation for the Bauschinger effect observed in oriented polymers.