Homogeneous separation melt crystallization

Fig. 5.10 DSC measurements on individual samples of polymorphs R, Y, OP and ON of 5-XII (see caption for Fig. 5.9 for definition of terms), recorded at 10 °C min , each exhibiting homogeneous melting. Inset DSC trace of a mixture of the four modifications, recorded at 0.5 °C min, showing better separated melting endotherms and exotherms resulting from crystallization from supercooled melts, (from Yu et al. 2000, with permission.)...

In this chapter we consider the separation of species contained in a homogeneous phase, such as a liquid or gas. The separation is based on exploiting a fundamental difference that exists between the species. Section 4.0 gives some overall guidelines. Methods that exploit differences in vapor pressures are evaporation, in Section 4.1 and distillation, in Section 4.2. Methods that exploit differences in freezing temperature and solubility are freeze concentration. Section 4.3, melt crystallization. Section 4.4 and zone refining. Section 4.5. Methods exploiting solubility are solution crystallization. Section 4.6 precipitation. Section 4.7 absorption, Section 4.8, and desorption. Section 4.9. Solvent extraction. Section 4.10, exploits differences in partition coefficient. 

An excellent example of the role played by liquid-liquid phase separation in the ensuing crystallization is found in blends with syndiotactic poly(styrene).(77) Measurements of the glass temperature in mixtures with poly(2,6-dimethyl-l,4-diphenylene oxide) (PPO) indicate that the components are miscible in all proportions in the melt. However, mixtures of syndiotactic poly(styrene) with poly(vinyl methyl ether) represent partially miscible blends. When the poly(vinyl methyl ether) content exceeds 20% by weight, the melt separates into two liquid phases, one rich in syndiotactic poly(styrene), the other in poly(vinyl methyl ether). Thus, the two blends have a common crystallizing component. However, in one the crystallization takes place from a homogeneous melt in the other from one that is phase separated. The different melt structures profoundly affect the crystaflization kinetics. This can be seen when a comparison is made between the crystallization kinetics of syndiotactic poly(styrene) from a homogeneous or phase separated melt. 

System in which the two components form a continuous series of solid solutions. In all the preceding examples the individual components (A or B or A By) form separate crystals when solidifying from the melt. There are, however, a number of examples of the separation of a homogeneous solid solution of A and B (or A and A By, etc.). 

Polymer crystallization is usually divided into two separate processes primary nucleation and crystal growth [1]. The primary nucleation typically occurs in three-dimensional (3D) homogeneous disordered phases such as the melt or solution. The elementary process involved is a molecular transformation from a random-coil to a compact chain-folded crystallite induced by the changes in ambient temperature, pH, etc. Many uncertainties (the presence of various contaminations) and experimental difficulties have long hindered quantitative investigation of the primary nucleation. However, there are many works in the literature on the early events of crystallization by var-... 

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

There has been considerable interest recently in an alternative type of ABA triblock structure, where the end blocks could form crystalline domains, by crystallization, rather than amorphous domains by phase separation. It was felt that, since such a crystallization process need not depend on the incompatibility between the blocks, it should be possible to have a homogeneous melt, which should exhibit a much lower viscosity, and hence much easier processing, than the heterogeneous media of the conventional triblock copolymers. Furthermore, thermoplastic... 

Homogeneous melt, Todt < Tc > Tg. In diblock copolymers exhibiting homogeneous melts, microphase separation is driven by crystallization if Tg of the amorphous block is lower than Tc of the crystallizable block. This generally results in a lamellar morphology where crystalline lamellae are sandwiched by the amorphous block layers and spherulite formation can be observed depending on the composition [6-10]. 

Due to light scattering, crystalline polymers mostly yield turbid films.Their blends with other polymers are always demixed because polymers are not able to form mixed crystals. Consequently, crystallizable polymers only yield homogeneous blends above their melting point. As soon as crystallization sets in, the components will separate. 

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