The Time-Dependent Bulk Response of Poly (Methyl Methacrylate)

The dynamic bulk compliance of Poly (Methyl Methacrylate) (PMMA) hasbeen determined at atmospheric pressure over a wide range oftemperatures and frequencies, following a new examination of the bulkresponse of Poly (Vinyl Acetate) (PVAc) at different pressures. Theoverall bulk behavior of the current measurements of PVAc agrees fairlywell with those reported by Deng and Knauss (1997) at atmosphericpressure, but a discrepancy in primarily the frequency response of thestorage compliance is accredited to a difference in the moisturecontents of the specimens. In PMMA, strong evidence of effects ofphysical aging on the volumetric behavior is observed and a carefulprescription of thermal history was thus necessary to obtain consistentresults. The master curves for the bulk compliance are generated bymeans of the time-temperature superposition principle. A comparison ofthe bulk and shear response shows that the extent of the transitionranges for the two material functions are comparable. Further comparisonof the shift factors for bulk and shear responses supports the idea thatdifferent molecular mechanisms contribute to shear and bulkdeformations. Another key observation regards the time-temperaturesuperposition principle when applied to bulk compliance data of PMMA.The shifting procedure appears to lead to consistent results for temperatures below 92°, however its validity may have to be questionedfor higher temperatures. This failure is tentatively attributed to thepresence of primary (α) and secondary (β) relaxation mechanisms with the latter possibly playing a dominant role in controlling the bulk deformations in the lower temperature range.