Aging phenomena in PMMA thin films – memory and rejuvenation effects

Aging dynamics in thin films of poly(methyl methacrylate) (PMMA) is investigated by using dielectric measurements for different types of aging processes. The dielectric constant decreases with increasing aging time at a given aging temperature in the present measurements, except in the long time region (≥11h) near the glass transition temperature for the real part of the dynamic dielectric constant in thin films with thickness less than about 26nm. In the constant rate mode of temperature change with a temporary stop at a temperature Ta, the memory of the aging at Ta can be kept within the polymer system and then can be recalled during the subsequent heating process. In the negative temperature cycling process, the strong rejuvenation effect has been observed after the temperature shift from the initial temperature T1 to the second temperature T2(= T1 + ∆T) when ∆T ≈ −20K. Furthermore, the full memory effect has also been observed at the temperature shift from T2 to T1. For ∆T ≈ −20K, the aging at T1 is totally independent of the aging at T2. As |∆T | decreases, the independence of the aging between the two temperatures is weaken, i.e., the effective time, which is a measure of the contribution of the aging at T1 to that at T2, increases with decreasing |∆T | in the negative region of ∆T. As the film thickness decreases from 514nm to 26nm, the |∆T | dependence of the effective time becomes much stronger. The contribution of the aging at T2 to that at T1 disappears more rapidly with increasing |∆T | in thin film geometry than in the bulk state.