Shape memory polyurethanes (SMPUs) have attracted much attention from academic and industrial researchers due to strong potential in biomedical and consumer applications. Some of the limiting factors of these materials are low recovery stress (RS) and shape recovery (SR). Fundamental studies have focused on the improvement of RS and SR values using primarily two approaches. The first utilizes the nanocomposite route by which a few weight percentages of nanofillers are added to SMPU in order to increase the modulus and consequently to obtain enhancement in recovery stress. Although successful in the case of SMPU with amorphous soft segments, the nanofillers caused reduction in crystallinity of crystalline soft segment leading to deterioration of shape memory properties of SMPUs. In the second approach, chemical additives are added which either chemically bond with SMPU chains or form a separate phase and offer much stronger modulus than the soft and hard segments of SMPU. This second approach was followed in the current study.
Polybenzoxazine (PB-a) was incorporated into a thermoplastic polyurethane (PU) formulation, anticipating that it would play a similar role to hard segment and improve the shape memory properties. It was found that benzoxazine monomer formed miscible blends with the prepolymer derived from 4,4'-methylenebis (phenyl isocyanate) (MDI) and poly (tetramethylene) glycol (PTMG) with average molecular weight of 650 g/mol. This allowed chain extension of prepolymer using 1,4-butanediol (BD) as in the synthesis of regular polyurethanes. The benzoxazine was later polymerized into polybenzoxazine (PB-a) by thermal curing at 180 °C in 3 hrs.
The results of this study showed that both RS and SR increased with the addition of benzoxazine. A specimen with 17 wt. % benzoxazine produced the best RS and SR values with 13 MPa and 93%, respectively compared to RS of 6.8 MPa and SR of 72% for polyurethane. The deformation conditions were also found to exert significant influence on RS and SR values. Both stretching rate and stretching temperature increased the RS values. However, higher heating rates caused a reduction of the values of RS. The stress relaxation experiments were carried out to establish a correlation between the deformation conditions and the values of RS. It was found that specimens with 9 wt. % and 17 wt. % benzoxazine experienced high degrees of stress relaxation. Consequently, the RS values of these specimens, although higher than polyurethanes, were somewhat compromised. Furthermore, an investigation on surface morphology revealed that the specimens had different levels of hard and soft segment phase separation.