Polyvinylidene fluoride, crystal structur

Other halogenated copolymers, in which an isomorphous replacement is observed, were studied by Leshchenko, Karpov, and Kargin (19, 20) and later by Natta, Allegra, and Bassi (21). The former authors show that copolymers obtained from vinylidene fluoride and tetrafluoro ethylene are crystalline and show a tendency to altemance between the two types of monomer units (19). The same authors studied the vinylidene fluoride/hexafluoro propylene and the vinylidene fluoride/ trifluorochloro ethylene copolymer systems. In the former case crystallinity is observed up to contents of about 10% of the second component, the crystal structure being that of polyvinylidene fluoride. In the latter case crystallinity disappears for vinylidene fluoride contents in the order of 16%, the observed structure being that of polytrifluorochloro ethylene (20). 

The vinyl fluoride/vinylidene fluoride and the vinyl fluoride/tetra-fluoro ethylene copolymer systems were also studied (21). In the first case isomorphism is observed in the whole range of compositions, while the distribution of the two types of units is random. The crystal structure is that of polyvinyl fluoride, which is virtually identical with one of the three known crystalline forms of polyvinylidene fluoride, and characterized by a planar zig-zag chain conformation. High degrees of crystallinities in the whole range of compositions are also observed in the second case. However, the crystal structure of the two pure homopolymers is not the same hence we are in the presence of isodimorphism. In any case, for vinyl fluoride contents ranging between 0 and 75 mole-% the structure observed is essentially that of polytetrafluoro ethylene in the crystalline... 

In addition to the general steric requirements reported in the introductory section for macromolecular isomorphism, if chains differ in chemical structure, they must also show some degree of compatibility to intimate mixing and not too much different crystallization kinetics. The first condition is strictly similar to the one that applies to liquid mixtures. As a well known example, liquids without reciprocal affinity in general cannot form a unique phase. Attempts to obtain mixed crystals from polyethylene and polyvinyl or polyvinylidene fluoride has been unsuccessful hitherto, in spite of the similarity in shape and size of their chains. In view of the above somewhat strict requirements, it is not surprising that relatively few examples of this type of isomorphism have been reported. 

Similarly, by melting together polyvinyl and polyvinylidene fluoride at all relative compositions a unique crystalline phase is observed, which is identical with the structure of crystalline polyvinylfluoride (49) and also with the structure of one of the crystalline forms of polyvinylidene fluoride (21). Since the lattice constants of these two forms are quite close, no variation is observed in the X-ray spacings of the solid mixtures throughout the whole range of compositions. The existence of a true co-crystallization is shown by the melting point/composition curve, which shows no minimum. 

Shah D, Maiti P, Gunn E, Schmidt DF, Jiang DD, Batt CA, et al. Dramatic enhancements in toughness of polyvinylidene fluoride nanocomposites via nanoclay-directed crystal structure and morphology. Adv Mater August 2004 16 1173-7. 

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