Synthetic thermally reversible gel systems. VI

Abstract

AbstractForming and conditioning thermally reversible aqueous gels of polyacrylyglycinamide (PAG) at various temperatures has little effect on either the melting point (Tm) of the gels or the heat of crosslinking (ΔHc) except at temperatures where partial hydrolysis can occur. This is added evidence that unlike with gelatin, crystallite formation does not play a role in gel formation. For unfractioned PAG, the linear relationship between the logarithm of molecular weight and 1/Tm predicted and observed for gelatin gels, does not hold. With mixed gelatin‐PAG gels, a gelatin/PAG ratio of ≥4 completely inhibits the formation of a PAG gel network. At lower gelatin/PAG ratios, the PAG network forms, and if gelatin is considered as an inert diluent, normal values for the melting points and ΔHc for PAG gels are observed. At a gelatin/PAG ratio of 4, the presence of PAG reduces the ΔHc for the gelatin gel by inhibiting the formation of as large or as ordered crystallite crosslinks. To reconcile the problem of aggregation preceding gelation one can assume that M̄w of an aggregate is a linear function of C2. If this is done, the same relationship which normally relates C with Tm is obtained. The equilibrium swelling of PAG films in water at 25°C is markedly molecular weight‐de‐pendent and can vary from below 5 to about 40 wt‐% polymer at equilibrium. It has been found that long‐term dark storage of dry samples of PAG under ambient temperature conditions results in pronounced decreases in the intrinsic viscosities of their aqueous solutions. It is speculated that this results from weak links, perhaps peroxy, in the polymer backbone. The possible relationship of this phenomenon to the slow stage of the viscosity deterioration of aqueous polyacrylamide solutions is pointed out. The higher viscosity of low DP PAG in 2M NaCNS compared to H2O and the larger percentage increse of [η] with increasing temperature in the latter, verify the greater solvent power of 2M aqueous thiocyanate for PAG. At a concentration level of 3%, aqueous PAG solutions are almost Newtonian whereas at higher concentrations (5%), the viscosity decreases appreciably with increasing rates of shear. The copolymerization of AG with isopropylacrylamide has been studied and the somewhat unusual results discussed. Copolymers containing an AG mole fraction greater than 0.40 do not exhibit a cloud point up to 100°C. If the isopropylacrylamide mole fraction approaches 0.60, the solutions do not gel down to 0°C. This ability to prepare copolymers over a narrow composition range that neither gel or undergo phase separation in the temperature range 0–100°C is probably related to the random distribution of monomer units in the copolymer backbone.