Crystals melting process

Samuels also made DSC measurements on a large number of samples of polypropylene resins that were isothermally crystallized in the temperature range of 130-160°C. Two endotherms were also observed for these samples, and when the endotherms were extrapolated to the equilibrium melting point, the high-temperature melting point was predicted to be 220°C, while the low-temperature melting point was predicted to be 185°C. The dual endotherm behavior of the isothermally crystallized polymer seemed to be a manifestation of the same crystal melting process as that observed for restrained fibers. 

Fig.1 Illustration of the crystallization/melting process of globular small molecules and long polymeric molecules. The latent entropy of the latter transition (AStr) consists of two major contributions the configurational term and the volume-dependent term (ASv) (see text)...

In the late 1980s, new fully aromatic polyester fibers were iatroduced for use ia composites and stmctural materials (18,19). In general, these materials are thermotropic Hquid crystal polymers that are melt-processible to give fibers with tensile properties and temperature resistance considerably higher than conventional polyester textile fibers. Vectran (Hoechst-Celanese and Kuraray) is a thermotropic Hquid crystal aromatic copolyester fiber composed of -hydroxyben2oic acid [99-96-7] and 6-hydroxy-2-naphthoic acid. Other fully aromatic polyester fiber composites have been iatroduced under various tradenames (19). 

As with the polysulphones, the deactivated aromatic nature of the polymer leads to a high degree of oxidative stability, with an indicated UL Temperature Index in excess of 250°C for PEEKK. The only other melt-processable polymers in the same league are poly(phenylene sulphides) and certain liquid crystal polyesters (see Chapter 25). 

These apparent restrictions in size and length of simulation time of the fully quantum-mechanical methods or molecular-dynamics methods with continuous degrees of freedom in real space are the basic reason why the direct simulation of lattice models of the Ising type or of solid-on-solid type is still the most popular technique to simulate crystal growth processes. Consequently, a substantial part of this article will deal with scientific problems on those time and length scales which are simultaneously accessible by the experimental STM methods on one hand and by Monte Carlo lattice simulations on the other hand. Even these methods, however, are too microscopic to incorporate the boundary conditions from the laboratory set-up into the models in a reahstic way. Therefore one uses phenomenological models of the phase-field or sharp-interface type, and finally even finite-element methods, to treat the diffusion transport and hydrodynamic convections which control a reahstic crystal growth process from the melt on an industrial scale. 

The polymer crystallization depends sensitively on the temperature Tc at which it occurs, more precisely on the degree of undercooling A T=Tm-Tc below the melting temperature Tm. Since we have to estimate Tm, at least roughly, of our lamellar crystal model, we first study the melting process of a lamella during the temperature increase at a constant rate. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

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... 

According to Hoffman s crystallization theory, a drop in the heat of fusion corresponds to an exponential decrease in nucleation and crystal growth rates [63]. Implicitly, the rate of crystallization is severely retarded by the presence of 3HV comonomer [64, 69, 72]. These low crystallization rates can hamper the melt processing of these copolymers since they necessitate longer processing cycle times. 

PROABD MSC [melt static crystallization] A process for purifying /r-xylene by crystallization. Used in conjunction with MSTDP. Piloted in France from 1994 to 1996 and proposed for installation in India in 1997 and in Bulgaria in 1998. 

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