Glassy state relaxation processes

Isotropic glasses are characterized by relaxation processes, which are directly related to the nonequilibrium nature of the glassy state. Similar processes should also occur in liquid crystalline glasses. Enthalpy relaxation was observed for samples, which had been annealed below the glass transition temperature for several hours, proving the non-equilibrium nature of smectic and nematic glasses. 

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,... 

The inplane shear stress-strain tests reported here have been well demonstrated to be a reliable test for matrix-dominated properties in composites 141). For the selected mechanical properties that were monitored, their sensitivity to the thermal history was well demonstrated. In particular, the embrittlement process during the sub-Tg annealing or physical aging has been clearly observed. This decrease in molecular mobility, which gives rise to an increase in relaxation time and hence a decrease in toughness, can be rationalized as a decrease in free volume in an approach towards the equilibrium glassy state. 

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. 

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