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). 

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. 

The mechanical behavior of isomorphic macromolecular systems would be expected to be quite different from the behavior observed in bicomponent or biconstituent systems. Indeed, isomorphic systems would be expected to behave in many respects like crystalline homopolymers, except that such properties as 7 and lattice spacings may be dependent on composition. Because of the single-phase situation, the glass-rubber transition and related properties may be expected to behave as if a random copolymer 

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The hterature contains extensive reports on investigations of the equiUbrium-phase behavior for an enormous number of polymer—polymer pairs (1,97). The number of blends known to be miscible has grown so rapidly since the mid-1980s that it is more instmctive to attempt to understand these observations in terms of the molecular stmctures of the components rather than to catalog them. 

Dispersive Interactions. For pairs of nonpolar polymers, the intermolecular forces are primarily of the dispersive type, and in such cases the energy of interaction between unlike segments is expected to be closely approximated by the geometric mean of the energies of interaction between the two like pairs (98). In this case, the Flory-Huggins interaction energy between this polymer pair can be expressed in terms of the solubiUty parameters 5 of the pure components. 

Polymer alloys are generally named polymer blends within the polymer community. In a recent overview of such blends, Robeson (1994) points out that the primary reason for the surge of academic and industrial interest in polymer blends is directly related to their potential for meeting end-use requirements . He points out that, in general, miscible polymer pairs confer better properties, mechanical ones in particular, than do phase-separated pairs. For instance, the first commercial... 

Most polymer pairs are thermodynamically incompatible, in the sense that their free energy of mixing is positive. This does not mean that there is absolutely no interdiffusion at all at the interface between them adjacent to the interface limited interdiffusion occurs, which can be seen as an increasing of the low surface entropy implied by a smooth surface [30-33]. This nanoscale roughening of an interface can increase the adhesion between the polymers. 

In a fundamental sense, the miscibility, adhesion, interfacial energies, and morphology developed are all thermodynamically interrelated in a complex way to the interaction forces between the polymers. Miscibility of a polymer blend containing two polymers depends on the mutual solubility of the polymeric components. The blend is termed compatible when the solubility parameter of the two components are close to each other and show a single-phase transition temperature. However, most polymer pairs tend to be immiscible due to differences in their viscoelastic properties, surface-tensions, and intermolecular interactions. According to the terminology, the polymer pairs are incompatible and show separate glass transitions. For many purposes, miscibility in polymer blends is neither required nor de-... 

The advances in polymer blending and alloying technology have occurred through three routes (1) similar-rheology polymer pairs, (2) miscible polymers such as polyphenylene oxide and polystyrene, or (3) interpenetrating polymer networks (IPNs). All these systems were limited to specific polymer combinations that have an inherent physical affinity for each other. However with... 

Fig. 39.—Plots of c/c against c from the data of Masson and Melville for the following solvent-polymer pairs curve 1, polyacrylonitrile in dimethylformamide at 13.5° C curves 2 and 4, poly-(vinyl acetate) s in benzene at 20°C curve 3, polyacenaphthylene in benzene at 25°C curve 5, polyvinylxylene in benzene at 24°C curve 6, poly-(methyl methacrylate) in benzene at 16°C. All curves have been calculated from Eq. (13). Units correspond to those in Fig. 38. (Fox, Flory, and Bueche. )...

Table 2 Selected data of helical polymers (or of enantiomeric polymer pairs) displaying both chiral and achiral modifications (adapted from [13])... 

TABLE 2 Quantitative Considerations of Selected Polymer Pairs... 

Polymer Pair, AB 3 -1 2 10 T a Mdiblock crit Mblend Mcrit... 

Integral equations can also be used to treat nonuniform fluids, such as fluids at surfaces. One starts with a binary mixture of spheres and polymers and takes the limit as the spheres become infinitely dilute and infinitely large [92-94]. The sphere polymer pair correlation function is then simply related to the density profile of the fluid. 

Generally, a negative X12 interaction parameter means that the polymer pair is miscible, and by using the values shown in Table 20.4, we obtained negative X12 values of ca. —3.5 x 10-4 to —6.7 x 10 4 for the P(HB60-ET40)/PET system... 

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

Flory-Huggins Approach. One explanation of blend behavior lies in the thermodynamics of the preceding section, where instead of a polymer-solvent mixture, we now have a polymer-polymer mixture. In these instances, the heat of mixing for polymer pairs (labeled 1 and 2) tends to be endothermic and can be approximated using the solubility parameter. The interaction parameter for a polymer-polymer mixture, Xi2, can be approximated by... 

The thermodynamics of the complexation of a typical polymer pair consisting of proton-donating polyacid and proton-accepting poly(ethylene oxide) (PEO) has been studied since the late 1970s [9,10]. For complexes formed by cooperative hydrogen bonding between a pair of polyacid and polybase with stoichio-... 

All polyimide alloys were solution blended at a 50/50 weight ratio by codissolving the polymer pairs in a common solvent, such as methylene chloride (MeCl2) or a mixture of MeCl2 and hexafluoroisopropanol (HFIP), co-precipi-tating in methanol and then drying overnight under vacuum at 100 °C. 

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