Theoretical and experimental investigations were performed on multiphase polymers;
especially rubber toughened (RT) poly(methyl methacrylate) (PMMA) to explore
cavitation in the rubbery phase.
The main objectives of this project:
i. To identify experimental methods to effectively detect rubber particle cavitation.
ii. To relate intrinsic toughness with rubber properties (e.g. rubber type, particle
morphology, rubber content, particle size).
iii. To study the relationships between different pre-treatments, and control the onset of
cavitation.
Thermal contraction measurements, dynamic mechanical analysis, creep and fracture
tests were the techniques adopted. Results from those different methods were examined,
compared, and related to a specifically devised mathematical model. They were found
consistent.
Thermal contraction measurement presents valuable information about the progress of
cavitation after pre-strain. It shows extensive rubber cavitation at low longitudinal strain
(about 2 - 3%), which is sufficient to produce permanent damage, not recoverable by
annealing.
Dynamic mechanical procedure estimates the resistance of the soft phase to cavitation in
response to mechanically and thermally generated stresses. It can be used to detect
distributions of stress and strain within the soft phase after cavitation. The dynamic
mechanical tests, supported by electron microscopy, provide further insight into the
cavitation mechanism. It is suggested that a complete failure of the rubber will allow
any internal stresses to relax, and the rubber glass transition temperature (Tg) to become
independent of the tensile stress on the specimen. If the particles remain intact, the loss
peak will shift to lower temperature with increasing triaxial tension as the rubber free
volume increases in response to a growing dilatational volume strain. To any inbetween state, regarding rubber phase partial failure, will correspond a loss peak in the
temperature range defined by Tg of the stretched rubber and the one of the relaxed
rubber (upper limit).
A major advantage is that thermal contraction measurements and dynamic mechanical
tests provide an observation method for the onset of cavitation as a separate process,
without the complications that arise when shear yielding or multiple crazing occur at the
same time.
Analysis based on the energy-balance model suggested multiple cavitation as a possible
mechanism for complex particle morphology (e.g. salami or hard-soft-hard core-shell).
These results are consistent with experimental data.Ph