Shape recovery temperature

As compared to metallic compounds used as shape memory materials, shape memory polymers have low density, high shape recoverability, easy processability, and low cost. Since the discovery by Mitsubishi in 1988, polyurethane SMPs have attracted a great deal of attention due to their unique properties, such as a wide range of shape recovery temperatures (— 30°C to 70°C) and excellent biocompatibility, besides the usual advantages of plastics. A series of shape memory polyurethanes (SPMUs), prepared from polycaprolactone diols (PCL), 1,4-butanediol (BDO) (chain extender), and 4,4 -diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) have recently been introduced [200—202]. 

SMPUs are basically block copolymers of soft segments, which are polyols, and hard segments built from diisocyantes and chain extenders (see Figure 4.31). Depending on the types and compositions of soft and hard segments, and preparation procedures, the structure-property relationships of SMPUs are extremely diverse and easily controlled, and hence shape recovery temperature can be set at any... 

Beloshenko et al. [31] measured the electrical resistance of epoxy composites containing either thermoexpanded graphite alone or thermoexpanded graphite and kaolin under uniaxial compression and over the shape recovery temperature range. The data obtained revealed that shape recovery was accompanied by a jumplike increase in electrical resistance, which was related to a change in sample length, and that the addition of kaolin gave rise to a decrease in electrical resistance and an increase in the value of the electrical resistance jump. 

The impact of the deformation temperature on the shape recovery temperature is more obvious in the PUPyBD053 sample. Figure 7.16 presents the strain recovery curves of PUPyBD053 at different deformation temperatures. For the sample deformed at 40°C, shape recovery is observed to begin at about 32°C, and its shape recovery temperature is about 50°C. When the deformation temperature is raised to above 80°C, the shape recovery decreases. For example, for a sample deformed at 125°C, only 30% shape recovery is possible before the condition temperature is raised to above 110°C. Although it is possible to fix the strain after deformation at 110°C, most of the deformation cannot be recovered at temperatures lower than the deformation temperature. 

The recovery forces of shape memory alloys are several tens kg/mm whereas those of polymers are approximately 1 kg/mm. The shape recovery ratio of the alloys is 7% at the maximum compared to those of pol5miers and gels, at an amazingly high 400-500%. The shape recovery temperatures of shape memory alloys can vary by as much as or more than 100°C by several percent variations in composition. This is in contrast to nearly constant recovery ratio in polymers and gels, which depends on the type of materials used. The price of typical Ni-Ti alloys is several hundred thousand yen/kg, which is much more expensive than the several thousand yen/kg that shape memory polymers and gels cost. Alloy density is 6.5 whereas polymer and gel density is 1. 

Among shape memory materials, shape memory alloys and bimetals are well known. Compared to these metallic compounds, SMP have a lower density, high shape recoverability, easy processability, and lower cost. Similar to conventional thermoplastics, SMP can be easily molded by the common methods, such as injection, extrusion, compression, and casting. In addition, their shape recovery temperature can be set at any value in the range room temperature 50 C, which allows a wide variety of applications. Also, SMP can be colored if desired because they are transparent. However, since the retractive force of the polymer is based on the small entropy elasticity, the SMP applications differ from those of metallic alloys. The basic differences between shape memory polymers and alloys are listed in Table 1. 

When crosslinks are introduced by an allophanate reaction, a quantitative control of their density is not feasible since the reaction occurs reversibly between free isocyanate groups and main-chain urethane groups, and hence the transition temperature is not closely controlled. Thus, when a reproducible close control of the shape recovery temperature is desired, multifunctional polyols or isocyanates can be used to provide TPUs with well controlled crosslink density. 

SMP should be composed of two phases, i.e., a reversible and a fixed phase. Typically, the transition temperature of the reversible phase becomes the shape recovery temperature and hence its range and sensitivity are of practical significance for their processability. Therefore, any thermoplastic material. 

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