Characterization of Polyurethane Networks

Linear polymers are usually characterized in solution. However, if enough crosslinking has occurred, such as in the vulcanization of elastomers, the polymer will swell but cannot dissolve in typical solvents. Then, it is impossible to derive molar mass (molecular weight) information from solution behavior. However, the average molar mass of the chains in the polymer network can be determined from swelling (1,2). If a polymer network attains an equilibrium degree of swelling, one can relate the number of "effective" chains per unit volume (υ*) or the average molar mass between crosslinks (M c ) to the degree of swelling (q i ), if certain assumptions are made; namely, a constant polymer/solvent interaction parameter (χ) and a reference degree of swelling in the unperturbed state (q o ) equal to unity for polymerization in bulk. However, we have calculated that M c derived from swelling measurements did not agree with values determined analytically by labelling crosslinks with C 14