When the melt of a crystalline polymer is cooled to a temperature between the glass transition and the equilibrium melting point, the thermodynamic requirement for crystallization is fulfilled.
In a crystallizable miscible blend, however, the presence of an amorphous component, either thermoplastic or thermosetting, can either increase or decrease the tendency to crystallize depending on the effect of the composition of the blend on its glass transition and on the equilibrium melting point of the crystallizable component and also on the curing extent and conditions in case of thermosetting amorphous component. The type of segregation of the amorphous component, influenced by parameters such as crystallization conditions, chain microstructure, molecular weight, blend composition, and curing extent, determines to a large extent the crystalline morphology of a crystallizable binary blend. Separate crystallization, concurrent crystallization, or cocrystallization can occur in a blend of two crystallizable components. The spherulite growth of the crystallizable component in miscible blends is influenced by the type and molecular weight of the amorphous component, the former affecting the intermolecular interactions between both components and the latter the diffusion of the amorphous component. The blend composition, the crystallization conditions, the degree of miscibility and the mobility of both blend components, and the nucleation activity of the amorphous component are important factors with respect to the crystallization kinetics. The melting behavior of crystallizable miscible blends often reveals multiple DSC endotherms, which can be ascribed to recrystallization, secondary crystallization, or liquid-liquid phase separation. Complex crystallization behavior develops in miscible blends containing a crystallizable thermoplastic and a curable thermosetting component. That depends on the temperature and time of curing the thermosetting and also on whether crystallization is initiated before, during, or after the curing process.
For the discussion of the crystallization and melting behavior in immiscible polymer blends, a division into three main classes is proposed.
In blends with a crystallizable matrix and an amorphous dispersed phase, both the nucleation behavior and the spherulite growth rate of the matrix can be affected. Nucleation of the matrix always remains heterogeneous; however, the amount of nuclei can be altered due to migration of heterogeneous nuclei during melt-mixing. Blending can also influence the spherulite growth rate of the matrix. During their growth, the spherulites can have to reject, occlude, or deform the dispersed droplets. In general, the major influence of blending is a change in the spherulite size and semicrystalline morphology of the matrix.
A completely different behavior is reported for blends in which the crystallizable phase is dispersed. Fractionated crystallization of the dispersed droplets, associated with different degrees of undercooling and types of nuclei, is the rule. The most important reason is a lack of primary heterogeneous nuclei within each crystallizable droplet. An important consequence of fractionated crystallization may be a drastic reduction in the degree of crystallinity.
When two crystallizable components are blended, a more complex behavior due to the influence of both phases on each other is expected. In general, the discussion for matrix crystallization and droplet crystallization can be combined. However, crystallization of one of the phases can sometimes directly induce crystallization in the second phase. As a consequence, the discussion of blends of this type has been subdivided with respect to the physical state of the second phase during crystallization. The special case of “coincident crystallization,” in which the two phases crystallize at the same time, is discussed. Finally, the effect of compatibilization of crystalline/crystalline polymer blends is briefly reviewed.
A new section has been added, introduced to deal with crystallization phenomena in immiscible polymer blends containing nanoparticles. Recent reports, although few, discuss the effect of nanoparticles on crystallization and melting in immiscible polymer blends.