Isomorphous polymer pairs

Interesting blends of compatible polymers can be prepared by the cocrystallization of isomorphic polymer pairs (Allegra and Bassi, 1969). When two different types of crystallizable polymers contain monomer units of approximately the same shape and volume, and their chains are able to adopt a similar chain conformation, isomorphism is possible. In such a case, each mer can fit equally well in the crystal lattice, so that a mixed crystal forms. The isomorphous mers can exist in the same molecule, as in a copolymer of vinyl fluoride and vinylidene fluoride, or in different molecules (the case of interest here). 

Polymer pairs that co-crystalfize and form mixed crystals. These blends are generally composed of polymers with similar subunits that can substitute for each other in the same unit cells this is generally called isomorphous replacement. This table is probably incomplete even though co-crystallization is expected to be rare. 

Figure B3.3.11. The classical ring polymer isomorphism, forA = 2 atoms, using/ = 5 beads. The wavy lines represent quantum spring bonds between different imaginary-time representations of the same atom. The dashed lines represent real pair-potential interactions, each diminished by a factor P, between the atoms, linking corresponding imaginary times.

Many combinations of diacids—diamines and amino acids are recognized as isomorphic pairs (184), for example, adipic acid and terephthalic acid or 6-aminohexanoic acid and 4-aminocyclohexylacetic acid. In the type AABB copolymers the effect is dependent on the structure of the other comonomer forming the polyamide that is, adipic and terephthalic acids form an isomorphic pair with any of the linear, aliphatic C-6—C-12 diamines but not with -xylylenediamine (185). It is also possible to form nonrandom combinations of two polymers, eg, physical mixtures or blends (Fig. 10), block copolymers, and strictly alternating (187—188) or sequentially ordered copolymers (189), which show a variation in properties with composition differing from those of the random copolymer. Such combinations require care in their preparation and processing to maintain their nonrandom structure, because transamidation introduces significant randomization in a short time above the melting point. 

A trivial case of macromolecular isomorphism involves the mixing of species differing only in an isotope, for example, as isotactic polypropylene and isotactic polydeuteropropylene (Natta et ai, 1958). More interesting examples can be realized by melting together such polymers as poly(vinyl fluoride) and poly(vinylidene fluoride) (Natta et a/., 1971) or poly(isopropyl vinyl ether) and poly(sec-butyl vinyl ether) (Allegra and Bassi, 1969) that form isomorphic pairs at all relative compositions. 

This chapter is intended to provide a list of known pairs of isomorphous monomerie units in synthetic crystalline polymer systems we shall regard as isomorphism the statistieal eopresenee of different rmits within the crystal-Htes. Additional information will be also given, namely ... 

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