Miscible polymers polymer pairs

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. 

In those miscible, high polymer pairs showing no specific interaction the polymers are usually very similar chemically. Other polymers which are not similar chemically are nevertheless miscible due to specific interactions between dissimilar groups in... 

Olabisi (79) evaluated the miscibility of polymer pairs by gas chromatr raphy with mixed poly(e-caprolactone)-poly(vinyl chloride) stationa phases. Some interaction parameters for the pure and mixed stationary phases are given in Table 10. 

The most basic question when considering a polymer blend concerns the thermodynamic miscibility. Many polymer pairs are now known to be miscible or partially miscible, and many have become commercially Important. Considerable attention has been focussed on the origins of miscibility and binary polymer/polymer phase diagrams. In the latter case, it has usually been observed that high molar mass polymer pairs showing partial miscibility usually exhibit phase diagrams with lower critical solution temperatures (LCST). 

Eisenberg and coworkers have employed acid-base interactions to improve the miscibility of a number of polymer-polymer pairs. Miscible blends were prepared using acid-base interactions, e.g., with SPS (acid derivative) and poly (ethylacrylate-co-4-vinylpyrldine) (91), sulfonated polyisoprene and poly (styrene-co-4-vinylpyridine) (92), and using ion-dipole interactions, e.g., poly (styrene-co-llthium methacrylate) and poly (ethylene oxide) (93). Similarly, Weiss et al. (94) prepared miscible blends of SPS(acid) and amino-terminated poly (alkylene oxide). In addition to miscibility improvements, the interactions between two functionalized polymers offers the possibility for achieving unique molecular architecture with a polymer blend. Sen and Weiss describe the preparation of graft-copolymers by transition metal complexation of two functionalized polymers in another chapter. 

Krause [25] and Helfand [26] have given theories, based on lattice models, for estimating block copolymer miscibilities including the type of interface developed between the microphases in these systems. Coleman, et al. [27] have assessed the validity of solubility parameters in estimating the miscibility of polymer pairs. 

The fact that only a limited amount of miscible biodegradable polymer pairs are available constrains preparation of biodegradable polymeric gradient materials with high quality and... 

Much of the work that has been done up to this point on high temperature polymer blends is the definition of miscible blend polymer pairs and an understanding of the features that lead to that miscibility. The development of miscible blends often leads to the ability to tailor the properties, including the Tg of mixtures. Such a tailoring is an alternative to the development of entirely new polymeric materials with the desired property profile. One of the advantages of the blend approach is that it is generally faster and less expensive than the synthesis and scale-up of an entirely new polymer. The downside of the blend approach is that it is difficult to define miscible pairs and miscibility is often the situation that is not observed with polymer mixtures. 

One of the major factors affecting the structure and properties of polymer blends is their miscibility. Most polymer pairs exhibit pronounced immiscibility, and only certain pairs are thermodynamically miscible. Generally, polymer blends are at least two-phase non-homogeneous systems. 

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... 

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... 

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. 

Without doubt the simple mixing of two or more polymers is the easiest, most direct and the cheapest way of producing new materials. However, the properties of such mixtures are very dependent on the miscibility of the components. Of the few polymer pairs which are thermodynamically miscible, some examples are ... 

By rough count, more than 50 miscible systems have been reported in the literature, and from intensive studies of these systems a more optimistic view of the potential for miscibility is beginning to develop. Most of these miscible polymer pairs have chemical structures that are capable of forming strong specific interactions such as donor-acceptor complexes and hydrogen bonds. While they have not all been completely studied, those that have show behavior which can only result from the presence of exothermic or negative heats of mixing. 

At one time very few polymer pairs were believed to be miscible but over the last twenty years a host of new miscible pairs have been identified As a relatively new field, the phenomenon of miscibility and the new materials derived from it will certainly have many as yet undiscovered applications. 

Polymers were thought to be usually immiscible due to their low combinatorial entropy of mixing. Any small unfavourable heat of mixing, positive AH, would thus preclude miscibility. However, many pairs of polymers are now known to show specific interactions such as hydrogen bonds which result in a favourable heat of mixing. It is part of the intentions of this review to stress the importance of these specific interactions and to show that a consideration of these interactions is essential to an understanding of the phenomena and theory of polymer miscibility. 

At T3 the polymer pair are miscible in all proportions and 5 (AGn,)/8 is positive over all compositions. At T, compositions between and can phase separate to reduce the overall energy to give two phases at composition and where the values... 

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