Dielectric Spectroscopy in the αβ Splitting Region of Glass Transition in Poly(ethyl methacrylate) and Poly(n-butyl methacrylate):  Different Evaluation Methods and Experimental Conditions

Abstract

Stability of the dielectric parameters in the R splitting region against methodical variations is demonstrated. The Williams product ansatz for correlation functions and an additive ansatz for dielectric functions are compared for poly(ethyl methacrylate). Both evaluation methods give a diminishing R intensity with increasing temperature, and a separate trace for the R relaxation different from both the secondary relaxation below and the high-temperature a process above the crossover. Furthermore, measurements on different poly(n-butyl methacrylate) samples and evaluation results of different experimentalists are compared. The intensity onset of the cooperative R relaxation, starting from zero at a characteristic temperature T on in the crossover region, and the parallel course of R and relaxations below Ton, separated by about one frequency decade in an Arrhenius diagram, remain stable against measurement and evaluation variations. A. IntroductionThe dynamic glass transition (main transition or, conventionally, R relaxation) in low-molecular-weight glass formers, network glasses, and amorphous polymeric materials is usually accompanied by different secondary relaxations ( , γ relaxations and so on). 1,2 Whereas the glass transition at low temperatures is assumed to be caused by the cooperative motion of many particles, the secondary relaxations are of more localized molecular origin (for instance, Johari-Goldstein or process). 3,4 This process is distinct from the fast relaxation process in the picosecond time scale as described by the mode-coupling theory, 5 and distinct from the scattering effects at the boson peak. 6 The temperature dependence of characteristic relaxation times τ ) 1/ω max for the different processes can be visualized in an Arrhenius or activation diagram (log τ as a function of 1/T, ω max is the frequency of the maximum dynamic loss susceptibility, ′′(ω) here). The main transition is characterized by a curved trace, in particular for fragile glass formers, 7 whereas the traces of the secondary relaxations usually are straight lines.With increasing temperature and frequency the main transition and the trace will usually approach each other. 1 This crossover region, 8 called the R splitting region here, is of great importance for understanding the development of the glass transition below the crossover and the relation between both relaxation processes. To distinguish the qualitatively different relaxation processes of the main transition above and below the crossover we call the low-temperature part R and the high-temperature part a. 9 The details of the R splitting region were investigated in poly(n-alkyl methacrylate)s by means of broadband dielectric spectroscopy. 10 Supposing additivity of the R and compliances, an intensity onset of the cooperative R process, starting from zero at a characteristic onset temperature T on , was observed in all examples. For poly(n-butyl methacrylate) the traces of the developing R relaxation and the relaxation are parallel in the crossover region, separate...

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

“…Because of the amorphous structure in PMMA and the randomly arranged molecular coils, there are at any time enough side groups that fulfil this condition and hence the strength of the β-relaxation in the dielectric loss exceeds that of the α-relaxation by far [2,3]. Especially at low temperatures, where the viscosity is high and translational diffusion is constricted, the β-peak dominates the dielectric loss spectrum [13]. With increasing temperature around the thermodynamic glass transition, viscous flow sets in.…”

“…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].…”

Molecular dynamics in polymeric systems

“…The increase of the side group length shifts the splitting region to lower temperatures and frequencies and thus the onset of the α process in PnBMA is around 50°C [74]. As the temperature decreases the a process bifurcates in both α and β processes in a parallel mode in the activation plot, with the α process located one decade below the straight line of the β process -scenario B in Fig.…”

“…Among the poly(n-alkyl methacrylate)s, the most studied ones are PMMA (poly-(methyl methacrylate)) [64,[68][69][70][71][72], PEMA (poly(ethyl methacrylate)) [64,[66][67][69][70][71][72][73][74] and PnBMA (poly(n-butyl methacrylate)) [64,[69][70][72][73][74], mainly by DRS and NMR. The first studies by dielectric relaxation spectroscopy of poly(n-alkyl methacrylate)s (revised in ref.…”

A contribution to the study of the molecular mobility in polymeric materials by Thermal and Dielectric Analysis

“…The deconvolution of the relaxation processes is carried out by the fitting of the experimental data ( ) ( ) '' ε ω to a sum of as many HN functions as existing relaxation processes (additive rule) (Schröter, et al, 1998;Kahle, et al, 1997;Bergman, et al, 1998;Alvarez, et al, 2005). However, sometimes the characterization of the relaxation processes is complicated because the processes are strongly overlapped.…”