The photoelastic behaviour of swollen networks of polymethacrylic acid

The visoelastic photoelastic behaviour of cross-linked polymethacrylic acid, swollen to equilibrium in variously concentrated aqueous NaCl solutions (c_x = 0.05-1.5 M), was studied. In the rubber-like region, stress and birefringence relaxation at various elongations (λ) can be separated into the deformation- and time-dependent parts. Both the extrapolated equilibrium modulus (G_r) and the interaction parameter (χ) strongly depend on the degree of swelling of the gel. One of the most interesting findings is that the exponent of the volume fraction of the polymer (ν₂) in the dependence of the modulus G_e on ν₂ is much higher than 1/3 as predicted by the theory of Gaussian networks, if the concentration of the crosslinking agent c is low. The same is true for the Mooney-Rivlin constant (C₁) even if the ratio $\text{C}{}_2\text{C}{}_1$ approaches zero at lower ν₂. The values of the stress-optical coefficient (C_e) increase with both increasing swelling and decreasing content of the cross-linking agent in the gel; the most pronounced increase in C_e is connected with the largest decrease in G_e. The same dependence of C_e on the modulus in the dry state (G_d), irrespective of the concentration of the cross-linking agent and c_s, suggests the conclusive role played by the macroform contribution to birefringence. The effects found are probably due to the disappearance of the secondary structure of PMA chains with increasing swelling and concentration of the cross-linking agent.