Blended polymers miscibility studies, thermodynamics

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

The information available on aqueous polymer blends is qualitative in nature because of the lack of a suitable theory to interpret the experimental observations. Mixed gels can be comprised of an interpenetrating network, a coupled network (as discussed above), or a phase-separated network [2]. The latter is the most common as the blends have a tendency to form two phases during gelation. In such cases the miscibility and thermodynamic stability have to be empirically investigated and proper conditions for miscible blends identified. This involves a phase diagram study as is described in [3]. 

PVDF is among the few semicrystalline polymers that exhibit thermodynamic compatibility with other polymers,80 in particular with acrylic or methacrylic resins.81 The morphology, properties, and performance of these blends depend on the structure and composition of the additive polymer, as well as on the particular PVDF resin. These aspects have been studied and are reported in some detail in Reference 82. For example, polyethyl acrylate is miscible with polyvinylidene fluoride, but polyisopropyl acrylate and homologues are not. Strong dipolar interactions are important to achieve miscibility with PVDF, as suggested by the observation that polyvinyl fluoride is incompatible with polyvinylidene fluoride.83... 

The review contains an introduction to the theory of polymer mixtures, the ways in which they can be made, how they can be studied, and how one can obtain thermodynamic data relating to them. It discusses how miscible systems having specific interactions differ from those without, evidence for the specific interactions, and how the interactions may affect the properties of blends. Finally it discusses to what extent the most widely used theories of polymer miscibility are able to deal with systems which show specific interactions. 

As stated earlier this method is the commonest method of preparing homogeneous blends in academic studies. It is however not without its pitfalls. Two phase blends can be formed by the evaporation of solvent from solutions of polymers which are themselves thermodynamically miscible. 

There have been several miscible high temperature polymer pairs that have been identified in both the patent and open literature. The term thermodynamics invariably brings to mind miscibility. The aim of this section is to discuss the thermodynamic features of polymer blends and highhght studies that have focused on a determination of the features that lead to miscibility. 

The miscibility behaviour of polymer solutions and blends have been studied extensively, and the experimental data and thermodynamic models generated for copolymer solutions and for polymer blends have made it possible to fine-tune and optimize the present polymer production plants and blend processing technologies. 

Miscibility in polymer blends has been studied by both theoreticians and experimentalists. The number of polymer blend systems that have been found to be thermodynamically miscible has increased in the past 20 years. Systems have also been found to exhibit the upper or lower critical solution temperatures. So complete miscibility is found only in limited temperature and composition ranges. A large number of polymer pairs form two-phase blends. This is consistent with the small entropy of mixing that can be expected of high polymers. These blends are characterized by opacity, distinct glass transition temperatures, and deteriorated mechanical properties. Some two-phase blends have been made into composites with improved mechanical properties. Often, incompatibility is the general rule, and miscibility or even partial miscibility is the exception. 

Miscibility in polymer blends is controlled by thermodynamic factors such as the polymer-polymer interaction parameter [8,9], the combinatorial entropy [10,11], polymer-solvent interactions [12,13] and the "free volume effect [14,15] in addition to kinetic factors such as the blending protocol, including the evaporation rate of the solvent and the drying conditions of the samples. If the blends appear to be miscible under the given preparation conditions, as is the ca.se for the blends dcscibcd here, it is important to investigate the reversibility of phase separation since the apparent one-phase state may be only metastable. To obtain reliable information about miscibility in these blends, the miscibility behavior was studied in the presence and absence of solvents under conditions which included a reversibility of pha.se separation. An equilibrium phase boundary was then obtained for the binary blend systems by extrapolating to zero solvent concentration. 

Compatibilized blends with addition of nanoparticles can become an alternative for conventional compatibilized blends containing block copolymers. Addition of oragnoclays to polymer blend affects multiple features thermodynamic phase behavior of the blend, the kinetics of phase separation and also the morphology formed in the two-phase region. Hemmati et al. proved that incorporation of organoclay evidently enhances the miscibility of PE and ethylene-vinyl acetate [EVA] phases in the amorphous regions of nanocomposites. In addition, the studies revealed that nanofiller influences the diffusion of polymer chains, which contributes to... 

Miscible Blends. Both Components Amorphous. Certainly one of the most commercially important and publicized examples of a miscible polymer blend system is that based on polystyrene and poly(phenylene oxide), which is sold under the trade name Noryl by General Electric. Many fundamental studies of this system have been published, many of which were devoted to proving that these two components are miscible in a thermodynamic sense (see chapter 5 of Ref. 10 by MacKnight, Karasz, and Fried). Commercial interest in this system involves both... 

Finally, it has been found that the deuterium staining of individual molecules, commonly used in condensed matter studies, in case of polymers can lead to serious consequences in bulk and surface thermodynamics. This was shown in this work by the phase separation of isotopic blends (Sect. 2.2.2), isotope swapping effect in blend miscibility (Sect. 2.2.3) and surface segregation (Sect. 3.1.2.5) as well as by the specific scaling law (Eq. 61) which governs the polymer brush conformation (Sect. 4.2.1). 

A solid-state C NMR study of the intermolecular hydrogen bonding formation in a blend of phenolic resin and poly(hydroxyl ether) of bisphenol A was reported by Wu et at) " The presence of a single glass transition temperature for all of the blend samples with different composition as disclosed by the DSC measurements demonstrates that the blends are thermodynamically miscible. The solid state NMR parameters, including chemical shift, efficiency of cross-polarization and Ti, confirm the presence of more free OH groups when one of the polymers is the minor component. 

These early studies demonstrated that excimer fluorescence is a useful addition to the battery of experimental tools available to study solid state polymer blends. However, the longer range goal of explaining the significance of the absolute value of R was not realized because there was insufficient companion information about the thermodynamics of the blends. The PS/PVME blend does not suffer from this limitation, and thus provides an excellent system for characterization of the photophysies under conditions for which miscibility or immiscibility are firmly established. In this section we examine results for PS/PVME as well as more recent work on dilute blends containing P2VN that are believed to be miscible. 

Polymer miscibihty has been the subject of numerous studies. Pressure is a thermodynamic parameter that can affect the phase behavior of polymer mixture and that can be used to enhance the miscibility of polymers. This properly may have an important apphcation in controlhng microstructure. The phase behavior of blends and block copolymers under pressure has recently received significant attention 1-3). 

Owing to low values of the combinatorial entropy mixing, miscibility in polymer-polymer systems requires the existence of strong specific interactions between the components, such as hydrogen bonding [Olabisi et al., 1979 Sole, 1982 Walsh and Rostami, 1985 Utracki, 1989]. The thermodynamic characterization of the interactions in miscible polymer blends has been the subject of extensive studies [Deshpande et al., 1974 Olabisi, 1975 Mandal et al., 1989 Lezeano et al., 1992, 1995, 1996 Farooque and Deshpande, 1992 Juana et al., 1994]. 

The question of polymer blend miscibility derives out of the nineteenth century development of thermodynamics and studies in the same periodic of binary mixtures of low molecular weight liquids. From a thermodynamic viewpoint, Gibbs (23) formulated the stability of multiphase systems in terms of the quantity G defined by (in modem notation (24))... 

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