Crystalline polymers crystal growth

For many polymers, crystal growth can take place either from the melt or from dilute solution to yield single crystals. Crystal formations in polymers were studied intensively almost from the time of recognition of their existence in macromolecules. As a result, certain basic principles were established (1) The melt crystallization process is a first-order phase transition (see Section 1.4.5). (2) Crystallization from a molten polymer follows the general mathematical formulation for the kinetics of a phase change.Equilibrium conditions, however, are seldom if ever attained and complete crystallinity is not reached. 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

We note here that all the information presently available on high molecular weight polymer crystal structures is compatible with the bundle model. While very nearly all crystalline polymer polymorphs involve all-parallel chain arrangements, even the only known exception, namely y-iPP [104,105], where chains oriented at 80° to each other coexist, is characterized by bilayers of parallel chains with opposite orientation. This structure is thus easily compatible with crystallization mechanisms involving deposition of bundles of 5-10 antiparallel stems on the growing crystal surface. Also the preferred growth... 

Fig. 31 Structural formation model for the initial stage of polymer crystallization [19], N G nucleation and growth of oriented domains, SD spinodal decomposition into oriented and unoriented domains, Tb, Ts, and Tx bimodal, spinodal, and crystallization temperatures, respectively I isotropic, N smectic, and C crystalline...

Besides the multiplicity of defects that can be envisaged, there is also a wide range of solid phases within which such defects can reside. The differences between an alloy, a metallic sulfide, a crystalline fluoride, a silicate glass, or an amorphous polymer are significant. Moreover, developments in crystal growth and the production of nanoparticles have changed the perspective of earlier studies, which were usually made on polycrystalline solids, sometimes with uncertain degrees of impurity present. 

Crystallization is an inherently time-dependent process the nucleation and growth of crystalline structures, the degree of crystallinity, the phase structure and quality of crystal lamellae, and their connectedness strongly influence the mechanical properties of semi-crystalline polymers. It is for this... 

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