Crystallization and Melting

PTT is a semiciystalline polymer widi a DSC peak melting point of 228 °C (Eigure 11.5). The equilibrium melting points, 7 m°, obtained from die 

Hoffman-Week plots, are 238 [48, 49], 244 [50] and 248 [51, 52] °C. Since the 7]n° values of semiciystalline polymers are usually 15-25 °C higher than their DSC Tin values, and also because the lower-melting PBT has a 7] ° of 245 °C [53], it is not unreasonable to assume that 248 °C might be a more appropriate r ° for PTT. 

There are two theories to explain the behavior of double endothermic peaks. One is the melt-recrystallization mechanism. The peak at lower temperature was from the melting of the initial crystal lamellae formed during the isothermal crystallization. When the 

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Valov P M and Leiman V I 1997 Size effects in the melting and crystallization temperatures of copper chloride nanocrystals in glass JETP Lett. 66 510... 

Figure 2 DTA curves of melting and crystallization of PE at 200 MPa. Numbers in the figure correspond to the experimental conditions described in the text. (From Ref. 52.)...

In recent years, there have been increasing reports on the study of the melting and crystallization behaviors of PE under high pressure by addition of other components. Nakafuku et al. [144-147] have reported that some high melting temperature diluents such as 1-, 2-, 4-, 5-tetrachlorobenzene I-, 3-, 5-tribromobenzene hexamethylbenzene and tetracontane affect the melting and crystallization processes, the phase transition, and... 

A.R. Ubbelohde, Melting and Crystal Structure, Clarendon Press, Oxford, 1965. 

Figure 1. Schematic diagram of the cold metal crucible system used for synthesis, zone melting and crystal pulling crystal growth (from ref. 5).

In this description the temperature field has been taken to be linear in the coordinate y and to be independent of the shape of the melt/crystal interface. This is a good assumption for systems with equal thermal conductivities in melt and crystal and negligible convective heat transport and latent heat release. Extensions of the model that include determination of the temperature field are discussed in the original analysis of Mullins and Sekerka (17) and in other papers (18,19). 

Table 1. Dimensionless values of parameters in the Solutal Model for two cases studied here. The systems I and II are representative of the thermophysical properties of the succinonitrile-acetone systems with differing values of the dif-fusivity ratio Rm, temperature gradient G and capillary parameter F. System III corresponds to parameters for a Pb-Sb alloy with equal diffusivities in melt and crystal...

Here, we show two cases for impurity segregation between melt and crystal as it grows in time. Note that an initial purification occurs in both cases but the distribution coefficient for the case on the right is such that the amount of impurity actually incorporated into the crystal, ki Cq. 

Examples. 2D SAXS/WAXS experiments on highly anisotropic polymer materials during melting and crystallization can be used to visualize and understand the evolution of nanostructure [56,57], Transformations of biopolymers in solution, e.g., virus crystallization can be studied in situ [58], It is possible to study solidification mechanisms of spider silk [59], or the self-assembly of micelles on a time-scale of milliseconds [60],... 

Free-Energy Barrier for Melting and Crystallization of a Single-Homopolymer Model... 

Fig. 16 Height of the equilibrium free-energy barrier for melting and crystallization vs. the chain length in single-chain systems. The circles are the simulation results, and the solid line is calculated from Eq. 20 with fitting parameter a = 15 p [127]... 

Fig-1 Schematic illustration of the crystallization and melting processes of polymers. The crystallization process corresponds to processes of disentanglement and chain sliding diffusion. The melting process is the reverse of the crystallization process. Between equilibrium melt and ideal crystal, there exists metastable melt and crystal. Cross marks indicate entanglement... 

Batclt-melting and crystallization where Qk is a symmetric mxm projector matrix such that... 

ISO 11357-3 1999 Plastics - Differential scanning calorimetry (DSC) - Part 3 Determination of temperature and enthalpy of melting and crystallization ISO 11357-7 2002 Plastics - Differential scanning calorimetry (DSC) - Part 7 Determination of crystallization kinetics... 

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