Dynamics ofαandβprocesses in thin polymer films: Poly(vinyl acetate) and poly(methyl methacrylate)

Abstract

Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) have been investigated by dielectric relaxation spectroscopy in the frequency range from 0.1Hz to 1MHz at temperatures from 263K to 423K. The α-process, the key process of glass transition, is observed for thin films of PVAc and PMMA as a dielectric loss peak at a temperature Tα in temperature domain with a fixed frequency. For PMMA the β-process is also observed at a temperature T β . For PVAc, Tα decreases gradually with decreasing thickness, and the thickness dependence of Tα is almost independent of the molecular weight (Mw ≤2.4×10 5 ). For PMMA, Tα remains almost constant as thickness decreases down to a critical thickness dc, at which point it begins to decrease with decreasing thickness. Contrastingly, T β decreases gradually as thickness decreases to dc, and below dc it decreases drastically. For both PVAc and PMMA, the broadening of the distribution of the relaxation times in thinner films is observed and this broadening is more pronounced for the α-process than for the β-process. It is also observed that the relaxation strength is depressed as the thickness decreases for both the polymers.

Comparison of mechanical and dielectric relaxation processes in laser-deposited poly(methyl methacrylate) films

“…In addition, it is remarkable that the peak maximum of the dielectric loss from bulk and film differ by about 15 K. The difference could be originated in the different polymer chain length but also the reduced dimensionality of the film may play an important role [3]. Interestingly, the peak temperature for 5.4 kHz, deduced from the results reported in [3] on spin-coated PMMA films with a thickness of 900 nm, agrees well with the present result. The merging process of the α-peak and the β-relaxation due to the frequency change is more clearly seen for the bulk material.…”

“…The first maximum in the mechanical loss indicates a maximum of internal friction at around 412 K. As mentioned earlier, two microscopic processes are known in PMMA bulk material, that are responsible for this loss peak, the α-relaxation due to viscous flow and the β-process that is linked to the rotation of side groups. In the kHz frequency range, the relaxation time for both processes is of the same order of magnitude [1,3]. As a consequence, also in thin films only one rather broad maximum is visible at 5.4 kHz around 415 K.…”

“…At high temperatures/frequencies, only one process is observable, known as the a-process (sometimes also α β-process) [11]. Though great efforts have been made to illuminate the molecular mechanisms in the merging region (sometimes also called the splitting region), the details of this phenomenon are far from being understood [1]- [3], [11]- [13].…”

“…Paddle wings and electrodes (2) form capacitors to drive and detect the oscillation. The paddle is heated by radiation from two coils (3). the temperature range from room temperature up to 530 K. Both experiments are performed at a constant frequency of 5.4 kHz.…”

“…To improve relevant physical properties, and also for fundamental reasons, a better understanding of the glass transition, molecular dynamics and the underlying microscopic relaxation processes in liquid and glassy PMMA films is necessary. Experimental data about the dynamic processes in bulk materials in most cases are gained from dielectric spectroscopy that allows a broad range of frequencies and temperatures [1]- [3]. In addition, there have been various other experimental approaches such as photon correlation spectroscopy [4,5], multi-dimensional nuclear magnetic resonance [6,7] and mechanical spectroscopy [8]- [10], which are usually applied at constant temperature below and above the thermodynamic glass transition T g .…”