Glass rubber kinetic

H. Levine and L. Slade, Thermomechanical properties of small-carbohydrate-water glasses and rubbers kinetically meta.stable systems at sub-zero temperatures. J. Chem. Soc. Faraday Trans. I 4.-2619-2633 (1988). 

In a like fashion, the activities and final concentrations at which sorption kinetics change from anomalous to Case II (uptake linear with time) were estimated for several penetrants in PVC (4). Figure 8, for example, shows that this kinetic transition occurs at a penetrant activity of about 0.9 in the TCE/PVC system. It appears that Case II kinetics are observed only when the final uptake is at least equal to Cg i.e., when the polymer/penetrant system undergoes the glass-rubber transition during the sorption process. 

The glass-rubber-liquid transitions bear a direct relationship to the molecular structure of the polymer itself. In the glassy state the kinetic energy of the individual segments of the coiled and intertwined polymer chain is not sufficient to permit a specific segment to escape from the... 

The view that the glass-rubber transition is truly thermodynamic in nature, and not just a kinetic anomaly, is not universally accepted, though a strong case for a thermodynamic origin can be made (Meares, 1965). 

A most important conclusion can be drawn immediately and it concerns the nature of the main part, the glass-rubber transition. As we find a systematic shift of the time range of the transition with temperature, it is obvious that we are dealing here w ith a purely kinetical phenomenon rather than with a structural transition like the melting process or a solid-solid phase change. Curves demonstrate that whether a sample reacts like a glass or a rubber is just a question of time. Temperature enters only indirectly, in that it determines the characteristic time which separates glassy from rubbery behavior. 

It is found for some diffusion limited reactions that the temperature dependence of the rate of the reaction follows a WLF type of equation rather than an Arrhenius equation in the region of the glass transition. This is also true for rheological properties such as shear moduli. Plots which linearise the data are of the form of the logarithm of reaction rate against the reciprocal of terms similar to T — Tg, rather than the Arrhenius form, 1 /T. It is only for reactions limited by diffusion, such as the translational motion of a reactant in a glass/rubber. Reactions which are hmitedbysome other non diffusion based step are known as reaction limited and will obey Arrhenius kinetics. 

This relative importance of relaxation and diffusion has been quantified with the Deborah number, De [119,130-132], De is defined as the ratio of a characteristic relaxation time A. to a characteristic diffusion time 0 (0 = L2/D, where D is the diffusion coefficient over the characteristic length L) De = X/Q. Thus rubbers will have values of De less than 1 and glasses will have values of De greater than 1. If the value of De is either much greater or much less than 1, swelling kinetics can usually be correlated by Fick s law with the appropriate initial and boundary conditions. Such transport is variously referred to as diffusion-controlled, Fickian, or case I sorption. In the case of rubbery polymers well above Tg (De < c 1), substantial swelling may occur and... 

The applications of TGA are extensive and diverse and include oxy-salt decompositions, natural and synthetic polymer characterization, metal oxidation and corrosion analysis, compositional analysis of coals, polymers, and rubbers, study of glass materials, foodstuffs, catalytic materials, biological materials, and a wide range of chemical processing phenomena. It has been used very successfully to study the kinetics of chemical processes however, there is much controversy surrounding this application, particularly in terms of relating TGA data to reaction kinetics models. 

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