Detection of cavitation in rubber phase of rubber toughened poly (methyl methacrylate) using thermal contraction measurements

Abstract

Measurements of mechanical properties and thermal expansion–contraction behaviour have been carried out on plain PMMA and on a transparent grade of rubber toughened (RT) PMMA. Uniaxial tensile tests at 23°C show that the stress–strain (σ–ε) curve for the RTPMMA begins to deviate from linearity at ε ≈ 1·5%, and reaches a load maximum at ε ≈ 6·7%. Visible whitening begins at ε ≈ 1·5%, and intensifies with increasing strain, but annealing for 3 h at 115°C restores the material to its initial transparent condition. Thermal expansion and contraction tests show that pre-straining to ε ≥ 2% produces a marked decrease in the coefficient of thermal expansion (CTE) of RTPMMA over the temperature range -20 to +50°C, where the rubber is above its T _g. Lower pre-strains have little effect on expansion–contraction behaviour. Annealing after pre-straining leaves the CTE unchanged at its reduced value if the pre-strain is above 3%. However, it causes an increase in CTE towards its initial value in specimens pre-strained to ε= 2%. This work provides clear evidence for cavitation in the rubber particles. It also shows that annealing after pre-straining does not completely restore the rubber's resistance to cavitation. Creep tests at 23°C on pre-strained specimens, both before and after annealing, demonstrate that rubber particle cavitation promotes a substantial acceleration in strain rates.