Study On The Shape Recovery Force Of Shape Memory Polyurethane And Its Affecting Factors

Thermal sensitive shape memory polyurethane(SMPU) has wide applications in biomedical engineering, textile and package because of its sound shape memory effect, mechanical properties, biocompatibility and biodegradability. Shape recovery ratio(Rr) and recovery force(Fr) are two important parameters to determine SMPU's applications. Though extensive researches have been conducted on the regulation of shape recovery ratio by deformation ratio, deformation temperature, or recovery temperature, few attentions have been paid on the factors affecting recovery force, regardless of the affecting mechanism. Constructing an apparatus to quantitatively and dynamically detect Fr and exploring the factors to affect Fr are valuable to guide the design of SMPU and SMPU-based devices and the programming process of the devices.We have ever designed and fabricated a cantilever-based apparatus for detecting Fr and its feasibility has been confirmed. However, its accuracy still remains to improve. In this study, the calculation model of the apparatus was optimized in order to improve the accuracy. By using the modified apparatus, the effect of such factors as deformation mode, recovery temperature, deformation ratio on Fr were investigated. The main works and conclusions are listed as follows.(1) A modified cantilever-based tensometer was built by replacing the previous point-contact mode with a face-contact mode. The procedure and specification to perform Fr detection was established. After calibrated, the effective detection range is 0-5N with the error within 2% and the accuracy grade is 1.0. The apparatus is shown to have good reproductivity.(2) SMPU-based plates(Tg = 40.7oC) were fabricated by using press molding. The tensile recovery stress(?r) was detected by using the tensometer. It is found that ?r increased with increasing tensile ratio to a maximum ?r value, after which ?r decreased with increasing tensile ratio. The maximum ?r was seen by a tensile ratio of 120%, which is around the tensile elastic limit of SMPU plates.(3) The effect of bending deformation(180o, 150 o, 120o) on Fr was investigated. It is found that Fr increased with increasing bending ratio and the maximum Fr was seen by 180 o.(4) The effect of recovery temperature(>=40oC(Tg)) on ?r was investigated. It is found that ?r decreased yet the recovery rate increased with increasing recovery temperature. The maximum ?r was seen near Tg.(5) The tensile shape memory process of SMPU-based plates was theoretically analyzed. The relationship between tensile recovery stress ?r and tensile ratio ? was established: , where A0 and l0 are the initial area and length of the plate, K is the Boltzmann constant, T is the recovery temperature, and N is the total chain number. According to this formula, within the tensile elastic limit of SMPU plates, the tensile recovery stress ?r increases with the increase of tensile ratio, which is consistent with the experiment results.(6)The bending shape memory process of SMPU-based plates was theoretically analyzed. The relationship between bending recovery force Fr and bending ratio ? was obtained:, where R and r are the outer and inner diameter of bent plates, respectively, L and b are the total length and width of plates, ?s is the yield limit stress of plates. According to the formula, the bending recovery force Fr increases with increasing bending ratio, which is consistent with the experimental results.