Glassy solidification transition

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... 

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. 

Many physical properties change in the vicinity of Tg (it is really a region), the most prominent being the dynamic ones. A transition between two dynamic states occurs in the amorphous phase—between the "glassy" state, wherein the mobility of chain fragments is frozen (frozen "free volume") and the elastic state wherein the chain mobility increases upon the rise of temperature (the free volume increases and viscosity decreases, respectively). (Free volume is defined as the difference between the volume of the liquid phase and the extrapolated value at absolute zero temperature.) It is customary to define the transition (TJ when a fraction of frozen free volume of 2.5% appears, and stays constant at lower temperatures. There is no "real" solidification, however, but a frozen liquid. The rate of the measurement (or the fre-... 

Transition from liquid to glass and crystallization are both phenomena responsible for polymer solidification at the end of a processing operation. Since they are kinetic phenomena, they lead to an out-of-equilibrium thermodynamic state glassy polymers present an excess of unstable conformations and free volume semi-crystalline polymers are not totally crystallized, their melting point being largely lower (usually some dozens of degrees) than the equilibrium value. 

The glass transition generally occurs over a relatively narrow temperature span and is similar to the solidification of a liquid to a glassy state it is not a phased transition. Not only do hardness and brittleness undergo rapid changes in this temperature region, but other properties, such as the coefficient of thermal expansion and specific heat, also change rapidly. This phenomenon has been called second-order transition, rubber transition, and rubbery transition. The word transformation has also been used instead of... 

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