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

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 (sigma-epsilon) curve for the RTPMMA begins to deviate from linearity at (epsilon)~1 -5%, and reaches a load maximum at (epsilon)6-7%. Visible whitening begins at (epsilon)~ l-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 (epsilon) > 2°/o 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 Tg. Lower pro-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 (epsilon)= 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 rubberʹs 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.