Liquid-glass transition

To provide a rational framework in terms of which the student can become familiar with these concepts, we shall organize our discussion of the crystal-liquid transition in terms of thermodynamic, kinetic, and structural perspectives. Likewise, we shall discuss the glass-liquid transition in terms of thermodynamic and mechanistic principles. Every now and then, however, to impart a little flavor of the real world, we shall make reference to such complications as the prior history of the sample, which can also play a role in the solid behavior of a polymer. 

The so-called Boson peak is visible as a hump in the reduced DOS, g(E)IE (Fig. 9.39b), and is a measure of structural disorder, i.e., any deviation from the symmetry of the perfectly ordered crystal will lead to an excess vibrational contribution with respect to Debye behavior. The reduced DOS appears to be temperature-independent at low temperatures, becomes less pronounced with increasing temperature, and disappears at the glass-liquid transition. Thus, the significant part of modes constituting the Boson peak is clearly nonlocalized on FC. Instead, they represent the delocalized collective motions of the glasses with a correlation length of more than 20 A. 

This condition may be regarded as an additional criterion of the glass-liquid transition. According to this definition, the greater the free-volume for segmental motion the smaller the cooperative unit necessary for such motion and for the maintenance of isomobility at Tg. 

The solution of the precursors of the thermosetting polymer (mixture of monomers or oligomers with or without initiators, catalysts and different additives) is usually a liquid at room temperature e.g., unsaturated polyester styrene, some epoxy anhydride and epoxy amine formulations, cya-nate esters, one-stage phenolics, etc. Cooling any of these solutions below room temperature leads to a glass. The temperature at which the glass-liquid transition of the initial formulation takes place is denoted as Tg0. Some other particular formulations, such as two-stage phenolics (novo-lac-hexa mixtures), some epoxy-amine systems, etc., exhibit a Tg0 above room temperature. 

The best evidence so far for the glassy nature of HDA was provided (1) by measurements of the dielectric relaxation time under pressure at 140 K [206, 251], (2) by the direct vitrification of a pressurized liquid water emulsion to HDA [252], and (3) by a high-pressure study of the glass >liquid transition using differential thermal analysis (DTA) [253], We note here that these studies probe structurally relaxed HDA (eHDA) rather than unrelaxed HDA. It is possible that structurally relaxed HDA behaves glass like, whereas structurally uHDA shows a distinct behavior. Thus, more studies are needed in the future, which directly compare structurally relaxed and unrelaxed HDA. 

Some other interesting findings come from the study of glass-liquid transition and crystallization behavior of water trapped in loops of methemoglobin chains [53]. 

In the physics of glasses, the clarification of the nature and the description of the glass-liquid transition are of particular interest. In polyclusters, the restoration of ergodicity begins with the melting of boundaries. The thermodynamics of this transition is considered in Sect. 6.8. 

The low-energy excitations described above contribute essentially to the reversible relaxation processes [6.32], to the internal friction [6.33], and to the specific heat. The expressions for the excitation contributions ii)—iii) to the specific heat are given in [6.29, 30], where it is also shown that cooperative rearrangements iii) contribute greatly to the melting of cluster boundaries which is treated as the glass-liquid transition in polyclusters (Sect. 6.8). 

For glassy polymer systems without crystallites or other cross-links, a glass-liquid transition occurs. For systems with permanent, i.e., covalent, cross-links, the elastic modulus keeps decreasing with increasing temperature until a plateau value is reached. 

The phase behavior of ionic liquids can be complicated. Some are crystalline at low temperatures and show a sharp transition from crystal to liquid state (a true melting point) as the temperature is raised, but others exist as a glass at low temperatures and convert to a liquid at the glass-liquid transition temperature, denoted by a small change in heat capacity. Still others are glasses at very low temperatures, transform to crystals as the temperature is raised, and finally become liquid at a still higher temperature. See Reference 3 for a discussion of the types of phase behavior. 

---