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 (110.3em0exh) near the glass transition temperature for thin films with thickness less than 260.3em0exnm. In the constant-rate mode including a temporary stop at a temperature Tₐ, the memory of the aging at Tₐ 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+ΔT) when ΔT≈-200.3em0exK. 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 ΔT≈-200.3em0exK. As ΔT 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 ΔT in the negative region of ΔT. As the film thickness decreases from 5140.3em0exnm0.3em0exto0.3em0ex260.3em0exnm, the ΔT 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 ΔT in thin-film geometry than in the bulk state.