Phase separation, polymer solubility

Emulsion phase separation. Water-soluble drugs are fabricated in the form of microcapsules by this method. An aqueous phase containing dissolved drug and an organic phase containing polymer are emulsified. Then polymer is phase separated using the techniques such as temperature change, addition of salts, etc. A nonsolvent then is used to harden the microspheres. 

TABLE 3 Range of diameter of soluble complexes as determined by quasi-elastic light scattering upon variation of pH from pHc (incipient association) to pH (phase separation). Polymer acronyms see the title of Table 1. (Reprinted with permission from Ref. 15. Copyright 1993 American Chemical Society.)... 

Phase separation of macromolecules is generally a slow process. This sometimes allows application of a nonsolvent mobile phase for polymer under study for the fast separations, which must be concluded before the onset of phase separation. Generally, solubility of polymers markedly depends on all their molecular characteristics. This enables very efficient separations of macromolecules employing phase separation and re-dissolution processes in special procedures of coupled methods of polymer HPLC (see section 11.8.4). On the other hand, due to the complexity of the phase separation phenomena the resulting retention volumes may simultaneously depend on several molecular characteristics of separated macromolecules. This complicates interpretation of the separation results. Both precipitation and re-dissolution of most polymers may be even affected by presence of seemingly inactive surface of the column packing. Therefore, careful control of the phase separation phenomena is recommended in coupled methods of polymer HPLC. 

Solution Properties. Typically, if a polymer is soluble ia a solvent, it is soluble ia all proportions. As solvent evaporates from the solution, no phase separation or precipitation occurs. The solution viscosity iacreases continually until a coherent film is formed. The film is held together by molecular entanglements and secondary bonding forces. The solubiUty of the acrylate polymers is affected by the nature of the side group. Polymers that contain short side chaias are relatively polar and are soluble ia polar solvents such as ketones, esters, or ether alcohols. As the side chaia iacreases ia length the polymers are less polar and dissolve ia relatively nonpolar solvents, such as aromatic or aUphatic hydrocarbons. 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

Copolymerizations of benzvalene with norhornene have been used to prepare block copolymers that are more stable and more soluble than the polybenzvalene (32). Upon conversion to (CH), some phase separation of nonconverted polynorhornene occurs. Other copolymerizations of acetylene with a variety of monomers and carrier polymers have been employed in the preparation of soluble polyacetylenes. Direct copolymeriza tion of acetylene with other monomers (33—39), and various techniques for grafting polyacetylene side chains onto solubilized carrier polymers (40—43), have been studied. In most cases, the resulting copolymers exhibit poorer electrical properties as solubiUty increases. 

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. 

The effects of increasing the concentration of initiator (i.e., increased conversion, decreased M , and broader PDi) and of reducing the reaction temperature (i.e., decreased conversion, increased M , and narrower PDi) for the polymerizations in ambient-temperature ionic liquids are the same as observed in conventional solvents. May et al. have reported similar results and in addition used NMR to investigate the stereochemistry of the PMMA produced in [BMIM][PFgj. They found that the stereochemistry was almost identical to that for PMMA produced by free radical polymerization in conventional solvents [43]. The homopolymerization and copolymerization of several other monomers were also reported. Similarly to the findings of Noda and Watanabe, the polymer was in many cases not soluble in the ionic liquid and thus phase-separated [43, 44]. 

Microspheres and microcapsules of lactide/glycolide polymers have received the most attention in recent years. Generally, three microencapsulation methods have been employed to afford controlled release formulations suitable for parenteral injection (1) solvent evaporation, (2) phase separation, and (3) fluidized bed coating. Each of these processes requires lactide/glycolide polymer soluble in an organic solvent. 

Phase separation microencapsulation procedures are suitable for entrapping water-soluble agents in lactide/glycolide excipients. Generally, the phase separation process involves coacervation of the polymer from an organic solvent by addition of a nonsolvent such as silicone oil. This process has proven useful for microencapsulation of water-soluble peptides and macromolecules (48). 

Lubricants can be classified as internal or external lubricants. Internal lubricants should be partially miscible with the polymer at processing temperatures (i.e., behave similar to a plasticiser), but phase separate at ordinary temperatures. Whereas plasticisers are completely miscible with the bulk polymer, lubricants have a limited solubility. 

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