Enthalpy relaxation, glass-forming

Sipp A. and Richet P. (2002) Equivalence of volume, enthalpy and viscosity relaxation kinetics in glass-forming silicate liquids. /. Non-Cryst. Solids 298, 202-212. 

The glass transition temperature, Tg, of polymer glasses could be identified as that of the first order hole phase transition by satisfying the criterion consisted of Eqs. (1) and (2), in which fx has been added under the operational definition concluding that the stable glasses could not be formed easily without the generation of "ordered p>art / hole" pairs during the enthalpy relaxation at temperatures below Tg ) ... 

The glass formed at the fastest cooling rate q falls away from the liquid line at the highest temperature since the structural relaxation for this case is least able to follow the temperature. Its final enthalpy H(,q", T2) and fictive temperature Tf(T2, q") are the highest of the three glasses. The other glasses have lower enthalpies and lower fictive temperatures at T2, as follows from equation 38. 

The short time mode corresponds to the glass transition. In polymers like polystyrene, a narrow distribution is observed. Ihe width of the distribution reflects the width of the distribution of the order parameter it is increased after mechanical orientation by addition of a dopant or additive, or under special glass forming conditions (hydrostatic pressure or rheomolding). The distributed relaxation times obey a compensation law, they are reduced to a single time at the compensation temperature T. The departure of from the glass transition is related to the kinetic aspect of the transition. Thermodynamic models are based on the linear relationship between the activation enthalpy and the activation entropy. 

In chapter 7, several aspects of conductivity and dielectric relaxation were discussed. Various other properties such as shear modulus, viscosity, refractive index, volume, enthalpy etc. also exhibit relaxational behaviour particularly in the glass transition region. In this chapter, few further aspects of relaxation are discussed. Relaxation of generalized stress or perturbation whether electrical, mechanical or any other form is typically non-exponential in nature. The associated property is a function of time. A variety of empirical functions, (/) t), have been used to describe the relaxation. Some of them have already been discussed in chapters 6 and 7. The most widely used function is the Kohlraush-Williams-Watts (KWW) function (Kohlraush, 1847 Williams and Watts, 1970 Williams et al., 1971). It is more commonly referred to as the stretched exponential function. The decay or relaxation of the quantity is given by,... 

Polymers in the glassy sutc are seldom in thermodynamical equilibrium. By cooling an amorphous polymer down from above its glass transition temperature, a thermodynamically nonequUibrium state is formed, which, usually very slowly, relaxes toward equilibrium. This kind of relaxation process is referred to as volume-enthalpy or thermodynamical relaxation because it involves a decrease of the specific volume and enthalpy. The volume change can be measured dUatometrically, the enthalpy change calw-imetrically. Vblume relaxation is described quantitatively by the Kovacs [11] equatioo as... 

By simply quenching bom melt, the o form (crystal form II) is formed, which is not ferroelectric. In this polymer three dielectric transitions may be delected, the p transition at about —(KTC. Ok o. transition near —10 and the a, transition near 1S0 C The p transition exhibits an Arrhenius-like relaxation behavior with an activatioo enthalpy of SO kJAnol it is attributed to local motions in the amorphous as well as in the crystalline state. The a. tiansitioo has a WLF-type relaxatkm beluvior it is attributed to the glass Iransitioa of the amorphous part The a, tiansitioo is attributed to motioos in the crystalline phase, but most probably not within the lamellae but in the partially... 

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