Quasi-static and dynamic mechanical behavior of transparent graft-interpenetrating polymer networks (graft-IPNs)

Mechanical characterization of transparent graft Interpenetrating Polymer Networks or graft-IPNs based on polyurethane and acrylic copolymer are reported under both quasi-static and dynamic loading conditions. This study builds on an earlier work detailing the synthesis of the graft-IPNs [1]. Optically transparent graft-IPNs were synthesized using elastomeric polyurethane (PU) phase and stiff acrylate-base copolymer (CoP) phase. The ratios of CoP:PU were varied from 90:10 up to 60:40 by weight. Two grades of graft-IPNs were synthesized by using 650 g mol−1 and 1400 g mol−1 poly (tetramethylene ether) glycol (PTMG) during synthesis to study the molecular weight effect. Quasi-static tensile and fracture tests as well as dynamic fracture tests were performed. The dynamic fracture tests were carried out at high strain rate using a modified-Hopkinson pressure bar in conjunction with an optical technique called Digital Gradient Sensing (DGS) and ultrahigh-speed photography. Quasi-static tests indicate significant enhancements in crack initiation toughness and change in failure mode (brittle to ductile) when the constituents were varied. Enhancements in crack inititation toughness were also observed under dynamic loading. These results were further compared with those for commercial poly (methyl methacrylate) (PMMA) and polycarbonate (PC) sheet stock. Further improvement in fracture properties were observed under quasi-static loading conditions with increase in molecular weight of PTMG. The relatively high values of fracture toughness obtained for graft-IPNs is attributed to the crosslinks generated between the CoP and PU networks.