Relaxation Processes in the Glassy State

Relaxation processes in amorphous polymers below the glass transition involve local 

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In 1983 he joined the editorial board of Polymer, of which he thrar became main editor, and is also on the editorial boards of Journal of Applied Polymer Science, Polymer Contents, Polymers for Advanced Technologies, Korean Polymer Journal, and Trends in Polymer Science. He was awarded a D.Sc. from the Urriversity of Edinbtrrgh for Research Work on the hydrodynamics of polymCT solutiorrs, studies of relaxation processes in the glassy state of polymers, and on physical characterization of polymers. His crrrrent interests are centered on phase eqtrihbria in polymer blends, polymer hqtrid crystals, ion conduction in polymCT electrolytes, physical aging, and hqtrid crystalhne celltrlose/polymer blends and composites. 

As the relaxation processes in the glassy state and glass transition region are non-exponential and nonlinear, the theories must take account of the thermal history of glass formation and the asymmetry of the relaxations, which depend on how the system departs from equilibrium. 

The apparent activation energy Atfi for a relaxation process can be estimated using dynamic mechanical data collected at different frequencies. For a localized molecular relaxation process in the glassy state, Heijboer has proposed the relationship... 

The master plots of Fig. 5 show a remarkable relation between stmctural and JG relaxation both above and below Tg, demonstrating the interdependency of the two processes. Eq. (1) provides a rationale for the quantitative relation above Tg. As far as concerns below Tg, the master curve is possible since the activation energy Ejo of JG relaxation scales with P linearly with Tg(P). The activation energy of the intermolecular secondary process in the glassy state is scaling with the glass transition temperature, that is characteristic of the primary relaxation of the supercooled liquid. 

DSC responds to Tg with a sigmoidal step in Cp, which has various shapes depending on the comparison of molecular relaxation kinetics with experimental timescales. It actually always depends on heating rate, but a static Tg is frequently defined by extrapolating Tg at various rates to zero rate—not actually a correct procedure as it varies with log (rate). Secondary processes in the glassy state are not observed. Tm and crystallisation are strong processes in DSC. The heats of fusion and hence crystallinity can be accurately evaluated, but care must be taken in over-quantifying peak shapes and temperatures because of ill-defined instrumental relaxation times. 

First to observe and classify different dielectric relaxtion processes present in scLCPs were Zentel and coworkers/ who studied unoriented samples of several different polyacrylate-based scLCPs over a wide range of temperature, covering the isotropic, mesomorphic, and solid states (Fig. 4.23). They observed up to three high-frequency relaxations (namely, the and y-processes) in the glassy state, and... 

In the preparation and processing of ionomers, plasticizers may be added to reduce viscosity at elevated temperatures and to permit easier processing. These plasticizers have an effect, as well, on the mechanical properties, both in the rubbery state and in the glassy state these effects depend on the composition of the ionomer, the polar or nonpolar nature of the plasticizer and on the concentration. Many studies have been carried out on plasticized ionomers and on the influence of plasticizer on viscoelastic and relaxation behavior and a review of this subject has been given 119]. However, there is still relatively little information on effects of plasticizer type and concentration on specific mechanical properties of ionomers in the glassy state or solid state. 

Molecular Motion in amorphous atactic polystyrene (PS) is more complicated and a number of relaxation processes, a through 5 have been detected by various techniques as reviewed recently by Sillescu74). Of course, motions above and below the glass transition temperature Tg have to be treated separately, as well as chain and side group mobility, respectively. Motion well above Tg as well as phenyl motion in the glassy state, involving rapid 180° jumps around their axes to the backbone has been discussed in detail in Ref.17). Here we will concentrate on chain mobility in the vicinity of the glass transition. 

Below Tg, in the glassy state the main dynamic process is the secondary relaxation or the )0-process, also called Johari-Goldstein relaxation [116]. Again, this process has been well known for many years in polymer physics [111], and its features have been estabhshed from studies using relaxation techniques. This relaxation occurs independently of the existence of side groups in the polymer. It has traditionally been attributed to local relaxation of flexible parts (e.g. side groups) and, in main chain polymers, to twisting or crankshaft motion in the main chain [116]. Two well-estabhshed features characterize the secondary relaxation. 

Of the diluents known to affect the dynamic relaxation behavior of polymers in the glassy state, water has so far received the greatest attention. Many polymers, which in the dry state are lacking any secondary relaxation process at temperatures from 77 to 273 K, e.g. poly(methyl methacrylate)135, polymethacrylamide136, cellulose and its derivatives137, collagen138, polysulfones139, poly(2,6-dimethylphenylene oxide)139, and others,... 

Using Eq. (43) with a suitable distribution function, time constants of the p-process can be extracted from experimental susceptibility spectra in the glassy state (T < Tg). However, above Tg, where both a- and p-process are present, the spectral shape analysis becomes more involved. Taking into account that also fast (ps) relaxational and vibrational dynamics are present (cf. Section IV.B), the correlation function of a type B glass former near Tg is a three-step function, reflecting the dynamics occurring on different widely separated time scales. This is schematically shown in Fig. 34. 

The stimulated-echo technique is well suited not only to investigate the primary relaxation of glass forming liquids above Tg, but also the secondary relaxation in the glassy state. In 2H NMR studies on the -process, it is advisable to measure the correlation function rather than Fsin(tm p)- In the former experiment, the... 

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