Isomorphic blends

An unusual feature of PAEK blends is that they can be both miscible (as evidenced by a single intermediate Tg) and isomorphic (as evidenced by a single melting point). Hence PEEK can be blended with a wide range of PAEK to produce miscible and isomorphic blends - although it does not appear to be miscible with PEKK [28]. This behaviour probably results from the crystallographic equivalence of ether and ketone in the PEEK crystal structure. Miscibility and isomorphism can be used to tailor the crystallisation and melting behaviour of PAEK. 

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... 

Blends of these two higher polyolefins with each other cocrystallized to form isomorphous blends, a relatively rare and interesting morphological phenomenon [9,126],... 

For instance, also the homopolymers PVF and PVDF have been described to crystallize in separate crystals in their blends [99] (Though constituted by isomorphous monomeric units which can cocrystallize in the copolymers in the whole range of composition, as seen in Sect. 4.1). Moreover, at least for the studied conditions, the polymorphic behavior of PVDF is not altered by the presence of PVF [99]. 

The monosulfides of the alkaline earth metals crystallize in the rock salt (MgS, CaS, SrS, BaS) and zinc blende (BeS) structures. BaS is insoluble in water, while the other monosulfides are sparingly soluble but hydrolyzed on warming (except MgS that is completely hydrolyzed). The monoselenides are isomorphous to the sulfides. The monotellurides CaTe, SrTe, BaTe adopt the rock salt stmcture, while BeTe has the zinc blende and MgTe the wurtzite structure. Alkaline earth polysulfides may be prepared by boiling a solution or suspension of the metal hydroxide with sulfur, e.g.,... 

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. 

In (a) and (c) there would be no great difference between the characters of the A-S and B—S bonds in a particular compound, while in (b) the B and S atoms form a covalent complex which may be finite or infinite in one, two, or three dimensions. By analogy with oxides we should describe (a) and (c) as complex sulphides and (b) as thio-salts. Compounds of type (c) are not found in oxy-compounds, and moreover the criterion for isomorphous replacement is different from that applicable to complex oxides because of the more ionic character of the bonding in the latter. In ionic compounds the possibility of isomorphous replacement depends largely on ionic radius, and the chemical properties of a particular ion are of minor importance. So we find the following ions replacing one another in oxide structures Fe, Mg , Mn , Zn, in positions of octahedral coordination, while Na" " more often replaces Ca (which has approximately the same size) than K , to which it is more closely related chemically. In sulphides, on the other hand, the criterion is the formation of the same number of directed bonds, and we find atoms such as Cu, Fe, Mo, Sn, Ag, and Hg replacing Zn in zinc-blende and closely related structures. 

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. 

COPO copolymer blend with EPCO terpolymer - isomorphic... 

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... 

Cadmium commonly occurs in isomorphic form in zinc minerals such as zinc blende (ZnS) with Cd contents of 0.1-0.5%, and galmei (ZnCOa) with Cd contents up to 5% (Stoeppler, 1991). 

First, a series of incompatible systems is discussed, including blends of different elastomers, two-component fibers and films, blends having paperlike characteristics, two-component membranes having highly ordered structures, and wood. Next, some aspects of the flow behavior of blends are considered, with emphasis on the effects of flow on morphology. Finally, the behavior of compatible blends, including isomorphic composition, is described. 

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). 

A, b=5.14 A, and c=12.87 A. Further X-ray intensity peaks of the two fibers also coincided at 2 = 20.3° and 2 = 23.0°, indieating that the ciystal structure of the blend fiber was very close to that of poly(p-phenylene terephthalamide). Son and Kim[131] ascribed it to an isomorphous replacement between terephthalic acid and adipic acid units as schematically depicted in Figure 10[133]. 

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. 

The PE immiscibility refers to the molten as well as to the solid state. Note that the small changes in molecular stmcture lead to difference in T, what affects a sequence of crystallizations of different blend components. Thus, in most cases the spherulites of one PE (having higher are encapsulated by those of the other PE. Co-crystallization of two PEs into single-type isomorphic cells rarely has been observed (Utracki 1989a, 1991). 

Generally, it is difficult to judge if a PO blend is miscible or not. Because of small difference between the refractive indices (Rl), the melt turbidity is often absent during or after the phase separation. When crystallized, co-crystals may form under specific set of the thermodynamic and kinetic conditions, e.g., co-crystallization is possible when the components are isomorphic or miscible in the amorphous and crystalline phase (Olabisi et al. 1979). 

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. 

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. 

Interpenetrating polymer networks Isomorphic polymer blends... 

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