Poly phase separation

The phenomena we discuss, phase separation and osmotic pressure, are developed with particular attention to their applications in polymer characterization. Phase separation can be used to fractionate poly disperse polymer specimens into samples in which the molecular weight distribution is more narrow. Osmostic pressure experiments can be used to provide absolute values for the number average molecular weight of a polymer. Alternative methods for both fractionation and molecular weight determination exist, but the methods discussed in this chapter occupy a place of prominence among the alternatives, both historically and in contemporary practice. 

If the poorer solvent is added incrementally to a system which is poly-disperse with respect to molecular weight, the phase separation affects molecules of larger n, while shorter chains are more uniformly distributed. These ideas constitute the basis for one method of polymer fractionation. We shall develop this topic in more detail in the next section. 

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

The in situ process is simpler because it requires less material handling (35) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80—100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubiUty to a higher molecular weight. If the reaction medium contains a protective coUoid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective coUoid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective coUoid serves to assist particle formation and stabUizes the final particles against coalescence. Some examples of protective coUoids are poly(vinyl alcohol), gum arabic, and hydroxyethjlceUulose. 

Blends of poly(vinyl chloride) (PVC) and a-methylstyrene—acrylonitrile copolymers (a-MSAN) exhibit a miscibiUty window that stems from an LCST-type phase diagram. Figure 3 shows how the phase-separation temperature of 50% PVC blends varies with the AN content of the copolymer (96). This behavior can be described by an appropriate equation-of-state theory and interaction energy of the form given by equation 9. 

Because of the aqueous solubiUty of polyelectrolyte precursor polymers, another method of polymer blend formation is possible. The precursor polymer is co-dissolved with a water-soluble matrix polymer, and films of the blend are cast. With heating, the fully conjugated conducting polymer is generated to form the composite film. This technique has been used for poly(arylene vinylenes) with a variety of water-soluble matrix polymers, including polyacrjiamide, poly(ethylene oxide), polyvinylpyrroHdinone, methylceUulose, and hydroxypropylceUulose (139—141). These blends generally exhibit phase-separated morphologies. 

Phase-separated polymers, 215 Phase separation, 217-222 Phase transfer catalysts, 288, 563-564 Phase-transfer-catalyzed alkaline hydrolysis of nylon-4,6, 570 of nylon-6,6, 569-570 PHB. See Poly(3-hydroxybutanoic acid) (PHB)... 

Tian et al. [56] have studied poly(G-caprolactone)-silica and Sengupta et al. [57] have investigated nylon 66-silica hybrid systems and have observed that the phase separation started when Si/H20 mole ratio is increased above 2 and the resultant hybrid films become opaque. Gao [11] has reported similar observations on sol-gel-derived ionomeric polyethylene-silica system. A wide range of literatures is not available on this topic of mbber-silica hybrid nanocomposites, though Bandyopadhyay et al. [34,35] have reported the hybrid formation with different TEOS/H2O mole ratios from ACM and ENR and also demonstrated detailed structure-property correlation in these systems. The hybrids have been prepared with 1 1, 1 2, 1 4, 1 6, 1 8, and 1 10 TEOS/H2O mole ratios. Figure 3.14 shows the morphology of the ACM-silica hybrid composites prepared from different TEOS/H2O mole ratios. 

Poly(VPGVG) (Fig. 6) has been smdied most thoroughly and it was shown that it exhibits an inverse phase transition. The biopolymer undergoes phase separation from solution upon increasing temperature, resulting in a p-spiral structure and simultaneous release of water molecules associated with the polymer chain (Fig. 7). 

This type of DNA condensation can be classified only formally as a product of DNA/poly-mer interactions, since no binding between these two components has been observed. Polymers that cause DNA condensation serve in this case as phase separation agents and concentrate DNA in the aqueous phase in high concentration. The presence of a certain amount of salt is required to overcome phosphate repulsion. 

Hollow and porous polymer capsules of micrometer size have been fabricated by using emulsion polymerization or through interfacial polymerization strategies [79,83-84, 88-90], Micron-size, hollow cross-linked polymer capsules were prepared by suspension polymerization of emulsion droplets with polystyrene dissolved in an aqueous solution of poly(vinyl alcohol) [88], while latex capsules with a multihollow structure were processed by seeded emulsion polymerization [89], Ceramic hollow capsules have also been prepared by emulsion/phase-separation procedures [14,91-96] For example, hollow silica capsules with diameters of 1-100 micrometers were obtained by interfacial reactions conducted in oil/water emulsions [91],... 

The two matrices in these cements are of a different nature an ionomer salt hydrogel and polyHEMA. For thermodynamic reasons, they do not interpenetrate but phase-separate as they are formed. In order to prevent phase separation, another version of resin glass polyalkenoate cement has been formulated by Mitra (1989). This is marketed as VitraBond, which we term a class II material. In these materials poly(acrylic acid), PAA, is replaced by modified PAAs. In these modified PAAs a small fraction of the pendant -COOH groups are converted to unsaturated groups by condensation reaction with a methacrylate containing a reactive terminal group. These methacrylates can be represented by the formula ... 

We have recently initiated our investigation of blends by examining the compatibility between our modified polymer sample 4 and poly(methyl methacrylate). Mixtures with a composition of between 10% and 30% of sample 4 yield compatible blends which are transparent under a polarized light microscope, and are characterized by a single Tg. Mixtures richer than 60% of 4 undergo complete phase separation. 

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