The time-dependent bulk response of Poly (Methyl Methacrylate)

The dynamic bulk compliance of Poly (Methyl Methacrylate) (PMMA) has been d etermined at atmospheric pressure over a wide range of temperatures and fre quencies, following a new examination of the bulk response of Poly (Vinyl A cetate) (PVAc) at different pressures. The overall bulk behavior of the cur rent measurements of PVAc agrees fairly well with those reported by Deng an d Knauss (1997) at atmospheric pressure, but a discrepancy in primarily the frequency response of the storage compliance is accredited to a difference in the moisture contents of the specimens. In PMMA, strong evidence of eff ects of physical aging on the volumetric behavior is observed and a careful prescription of thermal history was thus necessary to obtain consistent re sults. The master curves for the bulk compliance are generated by means of the time-temperature superposition principle. A comparison of the bulk and shear response shows that the extent of the transition ranges for the two m aterial functions are comparable. Further comparison of the shift factors f or bulk and shear responses supports the idea that different molecular mech anisms contribute to shear and bulk deformations. Another key observation r egards the time-temperature superposition principle when applied to bulk co mpliance data of PMMA. The shifting procedure appears to lead to consistent results for temperatures below 92 degrees, however its validity may have t o be questioned for higher temperatures. This failure is tentatively attrib uted to the presence of primary (alpha) and secondary (beta) relaxation mec hanisms with the latter possibly playing a dominant role in controlling the bulk deformations in the lower temperature range.