Thermal transition behavior of polybutadiene containing polyurethanes

The thermal transition behavior of a series of hydroxy terminated polybutadiene (HTPBD) containing segmented polyurethanes has been studied by differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA). Four transition regions are observed; the soft segment T_g, at ?74°C, two hard segment transitions T₁, at 40°C and T₂ at 103°C and a softening region by TMA at 180°C, presumed to arise from the dissociation of allophonate bonding, The low T_g, only 7°C higher than the T_g of free HTPBD, indicates nearly complete phase segregation despite the amorphous nature of the hard segment structure. The dependence of T₁, on hard segment length and thermal cycling suggests that it represents domains consisting primarily of shorter hard segments units. Factors contributing to the rather low mechanical properties of HTPBD polyurethanes are also discussed.