Glassy solidification

The question of whether the glassy solidification is a purely kinetic process or may be considered as a thermodynamic transition has been frequently discussed4,10. Gibbs and DiMarzio106 10 have assumed that there will be a second-order transition at a temperature, T2, at which the configurational entropy of the system becomes zero. Fewer conformations are available to the macromolecules at lower temperatures and as a result the molecular motion at T2 is slower. 

Two processes overlap in the curing of thermosets gelation and glassy solidification (Figure 36-8). If the reaction is carried out above the glass transition temperature, Tog, then the prepolymer liquid first converts to a gel. The gel consists of cross-linked chains of the polymerized, but not yet cross-linked residual prepolymer. The remaining prepolymer also cross-links with... 

It is impossible to date to derive from first principles which alloy compositions are prone to easy glass formation. It has been found empirically that multi-component alloys with components of significantly different ionic radii are suitable candidates in principle. Examples of Zr-based systems that show bulk glassy solidification behavior are listed in Table 3.1-38. 

Fig. 7 Reduced vapor pressure of TL as a function of the volume fraction of PS measured at 20°C with either HS-GC closed circles liquid solutions, open circles glassy mixtures), or at 30°C, by means of the flow-through method [32] (squares). According to [32], the crossed squares refer to compositions inside the range of glassy solidification no information is given for the open squares. Lines are guides for the eye...

A molten metal alloy would normally be expected to crystallize into one or several phases. To form an amorphous, ie, glassy metal alloy from the Hquid state means that the crystallization step must be avoided during solidification. This can be understood by considering a time—temperature—transformation (TTT) diagram (Eig. 2). Nucleating phases require an iacubation time to assemble atoms through a statistical process iato the correct crystal stmcture... 

In spite of this dominance of heat flow, the solidification speed of pure metals still obeys eqn. (6.15), and depends on temperature as shown in Fig. 6.6. But measurements of v(T) are almost impossible for metals. When the undercooling at the interface is big enough to measure easily (T, -T 1°C) then the velocity of the interface is so large (as much as 1 m s 0 that one does not have enough time to measure its temperature. However, as we shall see in a later case study, the kinetics of eqn. (6.15) have allowed the development of a whole new range of glassy metals with new and exciting properties. 

Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat and other properties to change dramatically. 

Fig. 7-15 Solidification during processing of glassy amorphous and crystalline TPs.

The denser packing of the molecular chains results in stronger interactions between segments and their near neighbours by means of van der Waals forces which are responsible for the rigidity of glassy polymers just as in the solidification of high polymers [56]. The important features of van der Waals forces are [57, 111] ... 

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