Aging phenomena in poly(methyl methacrylate) thin films: memory and rejuvenation effects.
The aging dynamics in thin films of poly(methyl methacrylate) (PMMA) have been investigated through dielectric measurements for different types of aging processes. The dielectric constant was found to decrease with increasing aging time at an aging temperature in many cases. An increase in the dielectric constant was also observed in the long-time region (>or=11 h) near the glass transition temperature for thin films with thickness less than 26 nm . In the constant-rate mode including a temporary stop at a temperature T(a) , the memory of the aging at T(a) was found to be kept and then to be recalled during the subsequent heating process. In the negative-temperature cycling process, a strong rejuvenation effect has been observed after a temperature shift from the initial temperature T1 to the second temperature T2 (= T1 +DeltaT) when DeltaT approximately -20 K . Furthermore, a full memory effect has also been observed for the temperature shift from T2 to T1 . This suggests that the aging at T1 is totally independent of that at T2 for DeltaT approximately -20 K. As /DeltaT/ decreases, the independence of the aging between the two temperatures was found to be weakened-i.e., the effective time, which is a measure of the contribution of the aging at T1 to that at T2 , is a decreasing function of /DeltaT/ in the negative region of DeltaT . As the film thickness decreases from 514 nm to 26 nm, the /DeltaT/ dependence of the effective time was found to become much stronger. The contribution of the aging at T2 to that at T1 disappears more rapidly with increasing /DeltaT/ in thin-film geometry than in the bulk state.
Fukao
K
K
Department of Polymer Science, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan. fukao@kit.ac.jp
Sakamoto
A
A