Theta temperature Copolymers

Polymers in Solution. Polyacrylamide is soluble in water at all concentrations, temperatures, and pH values. An extrapolated theta temperature in water is approximately —40° C (17). Insoluble gel fractions are sometimes obtained owing to cross-link formation between chains or to the formation of imide groups along the polymer chains (18). In very dilute solution, polyacrylamide exists as unassociated coils which can have an eUipsoidal or beanlike stmcture (19). Large aggregates of polymer chains have been observed in hydrolyzed polyacrylamides (20) and in copolymers containing a small amount of hydrophobic groups (21). 

As will be discussed in Chapter 6, which deals with the physical properties of the poly(alkylene oxide)s, only poly(ethylene oxide) is generally considered to be water-soluble however, ethylene oxide polymers are water-soluble only within a specific temperature range. There are, in theory, upper and lower consolute temperatures, and for poly(ethylene oxide) in water, the "Flory" or "theta" temperature is 108°C. There are, also theoretically, consolute temperatures for other lower poly(alkylene oxide)s in water, though these temperatures may be outside the range of normally liquid water. As a result, a hydrophobe can be achieved by forming a random copolymer of ethylene oxide with another epoxide within a temperature and composition range, this copolymer block can serve as the hydrophobic block. Such a unit is sometimes termed a heteric block, and the block copolymer may consist of blocks of the heteric block and of poly(ethylene oxide). 

Styrene-methyl methacrylate diblock copolymers have also been studied in a selective solvent, i.e. a solvent which is thermodynamically better for one or other block. For these experiments, p-xylene was used as the solvent since it had been previously reported as a theta solvent for polymethyl methacrylate at drca 40 °C. Separate SANS measurements were made on a copolymer with a deuterated styrene block dissolved in deuterated xylene and in hydrogenous xylene at tmperatures of 30 °C and 40 °C. Deutero xylene has the same scattering length density as deuterostyrene, consequently the scattered intensity is solely attributable to the methyl methacrylate block when the copolymer is dissolved in this solvent. At the two temperatures used conventional Zimm plots were obtained, however the methyl methacrylate block molecular weight obtained at 30 °C was twice that at 40 °C indicating the formation of a bimolecular multimer at 30 °C. From the radii of gyration obtained at both temperatures it appears that the methyl methacrylate block in the multimer... 

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesized [11] and used for emulsion polymerization of styrene. These molecules are ideal since the polystyrene block is compatible with the polystyrene formed and thus it forms the best anchor chain. The PEG chain (the stabilizing chain) is strongly hydrated with water molecules and it extends into the aqueous phase forming the steric layer necessary for stabilization. However, the PEG chain can become dehydrated at high temperature (due to the breaking of hydrogen bonds) thus reducing the effective steric stabilization. Thus the emulsion polymerization should be carried out at temperatures well below the theta (0) temperature of PEG. 

IRA Irani, C.A. and Cozewith, C., Lower critical solution temperature behavior of ethylene-propylene copolymers in multicomponent solvents, J. Appl. Polym. Sci., 31,1879, 1986. 86KUE Kuecuekyavruz, Z. and Kuecuekyavruz, S., Theta-hehaviour of poly(p-tert-butylstyrene)-b-poly(dimethylsiloxane)-h-poly(p-tert-hutylstyrene), M A rowo/. Chem., 187, 2469, 1986. 86RAE Raetzsch, M.T., Kehlen, H., Browarzik, D., and Schirutschke, M., Cloud-point curve for the system copoly(ethylene-vinyl acetate) + methyl acetate. Measurement and prediction by continuous thermodynamics, J. Macromol. Sci.-Chem. A, 23, 1349, 1986. 

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