Thermal analysis of the structure of segmented polyurethane elastomers

Polyurethanes were prepared from 4,4'-methylenebis (phenyl isocyanate) (MDI), 1,4-butanediol (BD), and poly(tetrahydrofurane) polyether polyol (PTHF) by melt polymerization. The -OH functional group ratio of polyol/total diol was kept constant at 0.4, while the ratio of the isocyanate and hydroxyl groups (NCO/OH) changed between 0.940 and 1.150. The thermal analysis of the polymers by DSC and DMTA measurements indicated several transitions. The three glass transition temperatures observed were assigned to the relaxation of the aliphatic -CH2- groups of the polyol, and to that of the soft and hard segments, respectively. The glass transition temperature of the soft and hard phase changed with the NCO/OH ratio indicating changes in phase structure and composition confirmed also by the maximum in the number of relaxing soft segments. Changes in the relatively small number of end-groups result in considerable modification of mechanical properties. Strength is determined by molecular mass and interactions, while stiffness depends mainly on phase structure. Surprisingly enough, -OH excess yields stiffer polymers, since the interaction of the -OH groups results in a decrease in the amount of the soft phase. A unique correlation was found between tensile modulus and the number of relaxing soft segments.