Water-soluble high molar mass polymer

Cryogels from Water-Soluble High Molar Mass Polymers. 201... 

This review summarizes the recent achievements in preparation of various supermacroporous polymer cryogels via UV-induced crosslinking in partly frozen systems. The method is equally effective for the formation of cryogels from both water-soluble high molar mass linear polymers and vinyl monomers. Special attention is paid to some novel materials based on biodegradable and/or stimuli-respmisive polymers and their application in some emerging fields, as well as the fabrication of nanocomposites with intriguing properties. 

Kalb et al. [456] compared the two polymerization types. First VF was polymerized with dibenzoyl peroxide at 85 °C for 10 h under 3 x lO Pa. The conversion to polymer was 52%. Substitution of the initiator by AIBN allows the pressure to be reduced to 0-7 x 10 Pa and the temperature to 70 C. After 19 h a 90% conversion to high-molar-mass polymer was obtained. When oil-soluble initiators were used, polymerization took place on top of the water surface. The polymer found was described as webs [456]. As these webs grew, they collapsed and floated on the water. 

Although low-molar-mass aliphatic polyesters and unsaturated polyesters can be synthesized without added catalyst (see Sections 2.4.1.1.1 and 2.4.2.1), the presence of a catalyst is generally required for the preparation of high-molar-mass polyesters. Strong acids are very efficient polyesterification catalysts but also catalyze a number of side reactions at elevated temperature (>160°C), leading to polymers of inferior quality. Acid catalysts are, therefore, not much used. An exception is the bulk synthesis of hyperbranched polyesters reported in Section 2.4.5.1, which is carried out at moderate temperature (140°C) under vacuum in the presence of p-toluene sulfonic acid catalyst. The use of strongly acidic oil-soluble catalysts has also been reported for the low-temperature synthesis of polyester oligomers in water-in-oil emulsions.216... 

The copolymerization of a rue thy latcd-/ -cy c 1 odextri n 1 1 host-guest compound of styrene with various molar ratios of sodium 4-(acrylamido)-phenyldiazosulfonate carried out in water with free radical initiator is described [40]. Depending on the amount of sodium 4-(aciylamido)-phenyldiazosulfonate incorporated in the copolymer, water- or DMF-soluble copolymers of high molar mass were obtained. Irradiation of the copolymers with UV light in solution resulted in rapid decomposition of the azo chromophore. Irradiation of the polymers as films led to crosslinking and thus to insolubility. 

Solutions of this high molar mass, water-soluble polymer have very unusual properties. 

The polymers used in waterborne coatings can be either soluble or dispersed in water (or a combination of water and a cosolvent). The problem of formulations containing soluble polymers is that the presence of polar/hydrophilic groups in the polymer makes the final films more sensitive to humidity. For this reason, a better solution is to formulate hydrophobic polymers that are stabilized in water by internal or external surfactants forming emulsions. Emulsified polymers have high molar masses but because they are dispersed in a particulate form, the viscosity of the media is not sensitive to their molar masses. Therefore, the physical properties are expected to be less dependent on the cure reactions. 

The synthesis of low-molar-mass products is usually carried out in bulk. High-molar-mass products for coatings are produced in organic solvents and those for injection molding in water-soluble solvents. The polymers are precipitated in water from these solvents. 

Carefully dried polyacids (e.g., by freeze-drying) dissolve extraordinarily well in water, even with high molar masses. After rigorous drying the solvation rate decreases. Other solvents for these polyacids are dioxane, dimethylformamide, and lower alcohols nonsolvents are acetone, ether, hydrocarbons, and the monomers. The solubility of poly(acrylic acid) increases with temperature, while the solubility of poly(methacrylic acid) decreases [445]. The solubility of the salts of the polyacids depends in a complex way on the pH value and the counterions. Alkali and ammonium salts are water soluble. Polyvalent cations form in water-swellable gels. The viscosity of aqueous solutions increases with the amount of polymer, to a constant value. Due to this experimental fact, it is not easy to calculate molar masses from the intrinsic viscosities [446]. 

Usually plastic materials are not water soluble and even if they are soluble to some extent, the polymer chains have a high molar mass and thus they cannot be transported directly through the cell membranes into microbial cells to be biochemically converted there (Figure 10.2). 

Microemulsions are thermodynamically stable oil-in-water emulsions, in which the droplet size (10-100 nm) is smaller than that in conventional emulsions (maaoemulsions 1-100 pm) and miniemulsions (50-500 nm). To prepare a stable miaoemul-sion, large amounts of anionic or cationic surfactants (more than 10-20% in the formulation or at similar levels as the monomer) and short-chain alcohols (e.g., n-pentanol) or other cosurfactants are employed. Upon polymerization of the microemulsion with a suitable initiator (either water soluble or oil soluble), polymer latexes with particle sizes in the 10-50 nm range and very high molar masses (>10 gmoT ) can be synthesized. 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

Published observations indicate that at room temperature water-soluble cellu-losics form mesophases at a critical volume fraction of polymer generally ranging from 0.3 to 0.5 for high molecular mass samples. For a given polymer and solvent, the critical volume fraction decreases with increasing molar mass, but increases with temperature. Highly polar and acidic solvents favor liquid crystal formation. 

Practical methods can be used to ensure that the required particle morphology is obtained. A high Tg or molar mass for the hrst-formed polymer forces the second-formed polymer to reside on its surface due to kinetic effects. The use of an inherently more water-soluble second polymer as compared to the first will also contribute to obtaining the desired morphology. Inverted core-shell latexes are most readily obtained when a more hydrophobic monomer for the second-formed polymer is polymerized in the presence of a less hydrophobic polymer latex. An alternative is to render the second monomer coUmdaUy unstable ufien polymerized, forcing it to reside in the core of the first formed polymer. 

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