On the use of DSC to assess radiation damage in isotactic poly(methyl methacrylate)

Abstract

Stereoregular poly(methyl methacylate) (PMMA) was precipitated from solution and subsequently heat-treated to different crystallinities and/or crystal distributions. Thereafter, the resulting wafers wrer exposed either in air or in static vacuum to high-energy λ-radiation via a Cs-137 source. Using DSC the melting temperatures, heats of fusion, and the crystalline fractions were determined for each irradiated sample. By evaluating either the depression of the melting point or the change in the heat of fusion with dose, a relationship was found between the thermodynamic property, on the one hand, and the chemical plus physical changes, i.e., G(-units), on the other. Present results indicate that G(-units) ≈16 and 10, when evaluated from heat of fusion and cryoscopic measurements, vs. 15, 6, or 1.7 obtained by X-ray diffraction, NMR, or gas analysis, respectively. No superheating effects were noted in the melting point measurements for heating rates up to 50°C/min. The heat of fusion measurements, on the other hand, evidenced a positive correlation, thereby suggesting a G(-units) dependence on heating rate. However, in neither case was reorganization due to premelt recrystallization observed. With these newer methods the partition of radiation-induced effects into chemical and physical changes should be possible for polymers which are not labile enough to undergo premelt recrystallization.