Isomorphic Polymer Blends

Interpenetrating polymer networks Isomorphic polymer blends... 

A simultaneous (or concurrent) crystallization can only occur when the crystallization temperature ranges overlap and if the crystallizability of both blend components is similar. Cocrystallization is only possible when the components are isomorphic or miscible in the amorphous as well as in the crystalline phase. In both cases mixed crystals can result, but in the case of concurrent crystallization no changes in crystal strucmre may be induced. Cocrystallization requires chemical compatibihty, close matching of the chain conformations, lattice symmetry and comparable lattice dimensions [Olabisi et al., 1979]. Some examples of miscible polymer blends with two crystalline components are given in Table 3.3 together with the type of crystalhzation. 

Additional polymer blends comprising PAEK s offering property combinations of potential utility include PSF [Robeson and Harris, 1986 Harris and Robeson, 1989] structurally different poly(aryl ketones) [Harris and Robeson, 1986], PAr [Robeson and Harris, 1992], poly(amide-imide) PAI [Harris and Gavula, 1992], PPS [Robeson, 1987], and other PI [Harris et al., 1992]. Mixtures of structurally different PAEK s were noted to be isomorphic within specific limits of ether/ketone ratios [Harris and Robeson, 1987]. Blends of polybenzimidazole, PBI and several commercial PI (Ultem 1000 and Matrimid 5218) have been studied in depth at the University of Massachusetts and found to be miscible. FTIR studies [Guerra et al., 1988 Kim et al., 1993], NMR studies [Grobelny et al., 1990], thermal, dielectric, and mechanical... 

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). 

Molten polyethylenes of different type chain stmctures usually are immiscible (see Chap. 2, Thermodynamics of Polymer Blends ). Upon crystallization the spher-ulites of one PE (having higher Tm) are encapsulated by those of the other PEs. Co-crystallization of two PEs into a single-type isomorphic cell is rare (Utracki 1989a). However, due to low interfacial tension coefficient, the phase coarsening is slow. 

Figure4.9 Comparison of melting point versus composition relationships for random copolymers and polymer blends exhibiting isomorphic behavior...

Crystallization of Blends The first polymer blend was made from two polymeric mbbers in 1846, but polymer blend technology and a scientific understanding of the underlying principles controlling the compatibility (or lack of) in polymer mixtures (alloys as they have been named recently) has taken place only in the latter part of the current century. Many blends are non-crystalline but our interest in this document is focused on the kinetics of phase transformations of binary and ternary systems that receives more attention annually. Some of these systems can be very complicated, often comprised of multiple phases that m involve homopolymers, copolymers, mesophases and the like. Polymorphism and even isomorphism may occur... 

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. 

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. 

In contrast to the case of two species crystallizing independently of one another it is also possible for co-crystallization to take place. The occurrence of isomorphic blends between two polymer components is not common. There are just a... 

Crystallization of blended polymers leads to their phase separation, because the formation of isomorphic crystals occurs very rarely. Blends of poly(ethylene terephthalate) and poly(butylene terephthalate) remain miscible in the amorphous phase after crystallization of both components. Unlimited mutual solubility of polymers is exceptional. It can be achieved under certain conditions, for example, in blends of poly(vinyl chloride) and butadiene-nitrile rubber or poly(vinyl acetate) and cellulose nitrate. 

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