Effect of polyols and diisocyanates on thermo-mechanical and morphological properties of polyurethanes

Polyurethanes based on two polyols PPG (polypropylene glycol) and HTPB (hydroxy terminated polybutadiene) and diisocyanates TDI (toluene diisocyanate) and MDI (methylene diphenyl diisocyanate) were synthesised using three different concentrations of the crosslinker 1,1,1-trimethylolpropane (TMP) at NCO : OH equivalent ratio 1.2 and 1.3. The mechanical, thermal and morphological properties were investigated. HTPB/TDI system showed highest tensile strength whereas PPG/MDI system exhibited highest elongation at break. In differential scanning calorimetry (DSC) thermograms transition for soft segment was observed to be considerably higher in PPG based polyurethanes (PUs). The dielectric constant was also found to be higher in PPG based PUs. The tan δmax was found to lie in the range 0.7–1.0. From swelling behaviour the solubility parameter for PPG and HTPB based PUs was found to be 9.7 and 8.6 (cal/cm3)1/2, respectively. The molecular weight between crosslinks were determined by using the Flory–Rehner equation.