Mechanical properties and fractography of block copolymers based on NR and MDI-based polyurethanes

Five series of block copolymers were synthesized from hydroxyl-terminated liquid natural rubber (HTNR) and polyurethane (PU) oligomers, from various diols and diphenyl methane-4,4'-diisocyanate (MDI). They were characterized by mechanical testing and fracture studies (SEM analysis). The block copolymer characteristics were assessed on the basis of the composition and the type of extender diols. Mechanical properties were found to be strongly dependent on the copolymer composition in all the series. Tensile properties were found to improve with the hard segment content. At low hard segment content samples resemble flexible elastomers whereas at high hard segment content they behave as rigid plastics. Where bisphenol A (BPA) is used as the extender diol sample rigidity was higher compared to the samples with aliphatic diols which is attributed to the presence of aromatic ring system in the former samples. Fracture mechanism was found to vary from ductile fracture to rigid and brittle fracture as the hard segment content increased. Fractography also shows the presence of some beads disposed on the sample surface which could be the uncombined polyurethane homopolymer fractions.