Shear data in the temperature range from −145 °C to the flow zone are presented for the poly(n-alkyl methacrylate)s from methyl (C = 1) to lauryl (C = 12). Three qualitatively different glass transitions are observed in the shear curves at 10 rad/s:  (i) the conventional α process in the C < 5 members, (ii) the high temperature a process in the C > 5 members, and (iii) an additional polyethylene-like glass transition, αPE, in the C ≥ 3 members. All three processes depend systematically on side chain length. Two alternative empirical pictures for the coexistence of two glass transitions are discussed:  (a) a static nanophase separation between main chains and side chains and (b) a dynamic heterogeneity with two different time and length scales.

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...

The main transition of amorphous polymers is analyzed with respect to a fine structure by means of new experimental dynamic shear, dielectric, and heat capacity data for the following polymers:  poly(n-alkyl methacrylate)s with alkyl = methyl, ethyl, propyl, butyl, and hexyl, polystyrene, poly(vinyl acetate), a series of weakly vulcanized natural rubbers, a series of butyl rubbers with different carbon black content, polyisobutylene, and bromobutyl rubber. The components of the fine structure are assumed to be a proper glass transition at short times, followed by a confined flow zone, and, at large times, a hindering zone caused by entanglements at large times. Two lengths are assumed to correspond to the first and third components, respectively, the characteristic length to the proper glass transition and the entanglement spacing to the hindering zone. The confined flow will be described by a dispersion law (general scaling) across the main transition. The characteristic length of the glass transition for the poly(n-alkyl methacrylate)sonly of order 1 nm as determined by calorimetryis confirmed by backscaling from the entanglement spacing by means of a Rouse dispersion law for shear. The fate of the Rouse modes below the αβ splitting of the glass transition is discussed for the other amorphous polymers. Finally, a speculative molecular picture of the different modes in the main transition is described. The new element is a low-viscosity longitudinal motion of individual chain parts in the confined flow zone. A simple rheological model for the confined flow is also presented.

A slight shoulder in the shear relaxation spectrum (H) at the relaxation time for the main maximum of the retardation spectrum (L) was detected for three amorphous polymers:  polystyrene, poly(vinyl acetate), and a slightly vulcanized natural rubber. This finding is discussed in terms of a distinctive manifestation of different molecular modes across the glass (main) transition.

No abstract