Melt —> crystal transition

FIGURE 9 Schematic comparison of glass transition and melt-crystal transition. 

As suggested in Sect. 3.6.1, TMDSC, detailed in Sect. 4.4, is a new and effective tool to analyze the nature of the melting/crystallization transition [34]. Figure 3.88 shows with quasi-isothermal measurements that melting/crystallization is largely irreversible for well-crystallized, extended-chain poly(oxyethylene) molecules of 5,000 molar mass. Such results are expected from Fig. 3.76. In the quasi-isothermal... 

HNOC-CeH4-CONH-] . Similar to the case of quenched PET (see Fig. 2.43), the glass transition is followed by an exothermic peak. This peak corresponds to an amorphous melt crystal transition. However, several crystal forms exist for this polymer, and crystal crystal transitions take place as the temperature increases. These crystal forms have not been identified, but were clearly seen by wide-angle X-ray diffraction (Menczel et al. 1996). So if the three existing crystal forms are designated with A, B, and C, then the lowest-temperature exotherm (the cold crystallization exotherm) corresponds to the melt crystal form A transition, followed by melting of crystal form A. After the melting of crystal form A is completed, the sample crystallizes into crystal form B. Then crystal form B melts, and the melt crystallizes into crystal form C, and finally crystal form C will melt. Some of these transitions can be avoided... 

If this sample contains also folded-chain crystals (reasons for their appearance during orientational crystallization were stated before), under isometric conditions they undergo melting at a higher temperature (at point 1 with respect to the oriented melt with transition to line A2) than under the conditions of free heating (point 1 with transition in the isotropic melt to line At). 

Thermal transitions can be studied by DSC. The crystallization transition is usually sharp with a good baseline. The melting transition is more complex and often not a single transition (Fig. 3.19)48 as it depends on the thermal history of the sample and the structural changes that can take place upon heating. In warming, solid-state transitions can take place in the unit cell, the lamellae can thicken, and secondary crystallization can also take place. The heats of crystallization and... 

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

Contents Scope of the Review. Experimental Methods. Glass Transitions. Melting. Crystallization. 

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

Below T0 the material is in the glassy state. Compared with the crystal the glass shows a larger specific volume and heat content, but both quantities have a smaller temperature coefficient than in the melt (< ). The transition from melt to glass is often called a transition of the second order (2, 3) since it is not accompanied by finite changes of volume and enthalpy, but only by changes of their temperature coefficients. 

No general rules about the entropy of transitions, as were found for liquid and plastic crystal transitions, can be set up for condis crystals. Two typical examples may illustrate this point. Polytetrafluoroethylene has a relatively small room-temperature transition-entropy on its change to the condis state and a larger transition entropy for final melting. Polyethylene has, in contrast, a higher condis crystal transition entropy than melting entropy (see Sect. 5.3.2). 

Convection in Melt Growth. Convection in the melt is pervasive in all terrestrial melt growth systems. Sources for flows include buoyancy-driven convection caused by the solute and temperature dependence of the density surface tension gradients along melt-fluid menisci forced convection introduced by the motion of solid surfaces, such as crucible and crystal rotation in the CZ and FZ systems and the motion of the melt induced by the solidification of material. These flows are important causes of the convection of heat and species and can have a dominant influence on the temperature field in the system and on solute incorporation into the crystal. Moreover, flow transitions from steady laminar, to time-periodic, chaotic, and turbulent motions cause temporal nonuniformities at the growth interface. These fluctuations in temperature and concentration can cause the melt-crystal interface to melt and resolidify and can lead to solute striations (25) and to the formation of microdefects, which will be described later. 

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