The three successive stages of creep of PMMA between 55 degrees C and 90 degrees C

After cooling from 140 degrees C, the physical aging of poly(methyl methacrylate), PMMA, has been studied at 55, 70, 84, and 90 degrees C by means of temporary microcreep tests. The systematic analysis of recoveries reveals that at 90 degrees C, creep follows successively three different stages. After stress application, creep starts by a logarithmic function of time, while recovery follows exactly the same kinetics. An Andrade creep begins after the logarithmic creep: It is in fact the beginning of the Kohlrausch-Williams-Watts creep with an exponent of 0.36. This creep is fully recoverable, but the duration of this recovery is similar to 3 times the creep duration. The third stage of creep is superimposed on the Andrade creep and follows a power law in t(0.8), but it is not recoverable in the usual duration of experiments. By comparison with the recoverable creep of PMMA studied by Plazek above T-g, which can be fitted by a KWW law with an exponent 0.8, this third creep is interpreted as the beginning of the rubbery deformation.