The creep behavior of ideally atactic and commercial polymethylmethacrylate

Summary

The shear creep behavior of polymethylmethacrylate, PMMA, samples has been studied in the neighborhood of and above their glass temperatures. One of the materials studied was “ideally” atactic with equal numbers of random isotactic and syndiotactic placements, while the other was a commercial or “conventional” PMMA which was about 76% syndiotactic. The glass temperatures,T _g , were found to be 106 and 117 °C respectively. Evacuation above the glass temperature for several weeks was necessary before reproducible creep compliance,J (t), curves could be obtained. It is believed that absorbed water plasticized the polar materials and its removal led to the shifting of theJ (t) curves to longer times. For both materials apparently successful temperature reduction was found to be possible within the temperature range of our investigations, i.e. up to 200 °C. Retardation spectra were calculated from the reduced curves and are compared. The temperature dependences, as described by the time scale shift factors,a _T , were similar when allowance is made for the different glass temperature. Botha _T curves could not be fitted to theWilliams, Landel, andFerry, WLF, free volume expression. These are the first examples of such a deviation for amorphous high polymers. It is proposed that the primary softening dispersion has two distinctly different groups of viscoelastic mechanisms contributing to it. On this basis the primary dispersion was decomposed into the two contributions. Both of the resulting temperature dependences were satisfactorily fitted to the WLF equation. Differences in the retardation spectra are noted. The glassy compliance of the commercial PMMA appears to be about twice that of the atactic PMMA.