Synthetic thermally reversible gel systems. III

Polyacrylylglycinamide (PAG) and its thermally reversible aqueous gels have been investigated and comparisons made with gelatin. For unfractionated PAG homopolymers in 2M NaCNS at 25°C, the Mark-Houwink-Sakurada equation is η] = 1.16 × 10^?3 M?_n^0.52. The Huggins k′ value is found to be about 0.9 and the Flory-Huggins polymer-solvent interaction parameter, 0.49. A theoretically calculated value of K_θ is very close to the experimental one and it is for this reason and the observed μ and M?_n exponent values that 2M NaCNS at 25°C approaches being a θ solvent for PAG. A thermodynamic approach based on equilibrium swelling and modulus measurements indicates that a single thermally reversible crosslink in a gelatin gel involves numerous peptide backbone units, whereas in PAG gels a crosslink perhaps involves only one residue from each of two chains. These results complement the very high exothermic heat of gelation crosslinking, δH_c, for gelatin compared to PAG. ΔH_c has been measured on PAG samples of different DP and found to be independent of chain length. Similar measurements on acrylylglycinamide copolymers and terpolymers containing basic and acidic groups produce no change in δH_c so that coulombic forces do not appear to be significant. Data are also included relating to the density of PAG, its glass transition and thermal decomposition temperature, the tensile modulus of equilibrium-swelled PAG films in water and the chain-transfer constant of methanol with the acrylylglycinamide free radical.