Properties shape recovery

Mather [5] prepared elastomeric materials having excellent shape recovery properties by polymerizing cyclooctene using the dihydroimidazolylidene-modified Grubbs catalyst, (I), and then crosslinking the intermediate with dicumyl peroxide. 

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... 

Li et al. reported that immiscible high-density polyethylene (HDPE)/ poly(ethylene terephthalate) (PET) blends, prepared by means of melt extrusion with ethylene-butyl acrylate-glycidyl methacrylate (EBAGMA) terpoly-mer as a reactive compatibilizer, can exhibit shape memory effects [32]. They observed that the compatibilized blends showed improved shape memory effects along with better mechanical properties as compared to the simple binary blends. In the blend, HDPE acts as a reversible phase, and the response temperature in the shape recovery process is determined by of HDPE. The shape-recovery ratio of the 90/10/5 HDPE/PET/EBAGMA blend reached nearly 100%. Similar behavior was observed for immiscible HDPE/ nylon 6 blends [33]. The addition of maleated polyethylene-octene copolymer (POE-g-MAH) increases compatibility and phase-interfacial adhesion between HDPE and nylon 6, and shape memory property was improved. The shape recovery rate of HDPE/nylon 6/POE-g-MAH (80/20/10) blend is 96.5% when the stretch ratio is 75%. 

Dong, Z., U. E. Klotz, C. Leinenbach, A. Bergamini, C. Czaderski and M. MotavaUi (2009). A novel Ee-Mn-Si shape memory alloy with improved shape recovery properties by VCprecipitation.A vfl ce E g >Jee > gMflfer flZsll(l-2) pp.40-44. 

It is noted that while the majority of constitutive modeling focuses on thermally induced dual-shape memory behavior, triple-shape and multishape SMPs have been developed recently and they call for constimtive modeling [1]. In addition, the effect of programming temperature and strain rate on the constimtive behavior also needs modeling [2]. Furthermore, some recent smdies have found that while the shape recovery ratio can be 100%, other mechanical properties such as recovery stress or modulus become smaller and smaller as the thermomechanical cycles increase, which has been explained by the shape memory effect in the microscopic scale [24]. Obviously, these new findings also call for constitutive modeling. 

It has been found that Mesua ferrea L. seed oil-based thermoplastic hyperbranched polyurethane (HBPU) of the monoglyceride of the oil, PCL (M = 3000 g moT ), 2,4/2,6-toluene diisocyanate and glycerol with 30% hard segment (NCO/OH = 0.96), exhibit thermoresponsive shape memory properties. The shape recovery (88,91 and 95%) and shape retention (70, 75 and 80%) are also found to be different at different temperatures (50, 60 and 70°C respectively). Bisphenol-A-based epoxy resin modified... 

Shape memory (SM) properties are typically quantified by the shape fixity (ffj) and shape recovery ratios (/ ,). means the extent of fixing of the externally applied deformation in the temporary shape. Its value is 100% when the applied deformation, introduced above is fully kept below in the temporary... 

Numerous polymers have been proposed as shape-memory polymers (SMPs), and many of them are based on polyurethanes. This is because of the intrinsic versatility of segmented copolyurethane systems. By suitable choice of diisocyanate and macrodiol, a wide variation in properties may be obtained, allowing the possibility of tuning the shape-memory response to suit different applications. Usually they are phase-segregated materials. For example, a dispersed rigid phase (usually based on the diisocyanate) provides physical crosslinks, while the macrodiol provides a soft amorphous phase with low glass transition that provides the trigger temperature for shape recovery [63]. 

Shape-memory properties can be quantified in cyclic, stimuli-specific mechanical tests [23,40]. Each cycle consists of the SMPC and the recovery of the original, permanent shape. From the data obtained, the shape fixity ratio (Rf) and the shape recovery ratio (/ r) can be determined (see, e.g., [40-42] and Chapter Characterization Methods for Shape-Memory Polymers in this volume). Rf describes the ability of the switching segment to fix a mechanical deformation, e.g., an elongation to applied during SMCP resulting in the temporary shape. Rr quantifies the ability of the material to memorize its permanent shape. Different test protocols have been developed. They differ in SMCP, which can be performed under constant strain or constant stress conditions (see Chapter Characterization Methods for Shape-Memory Polymers in this volume). The recovery process under stress-free condition enables the determination of the switching temperature Tsw for thermally-induced SMP. 

A reduction of the required energy could be reached by the incorporation of conductive fillers such as heat conductive ceramics, carbon black and carbon nanotubes [103-105] as these materials allowed a better heat distribution between the heat source and the shape-memory devices. At the same time the incorporation of particles influenced the mechanical properties increased stiffness and recoverable strain levels could be reached by the incorporation of microscale particles [106, 107], while the usage of nanoscale particles enhanced stiffness and recoverable strain levels even more [108, 109]. When nanoscale particles are used to improve the photothermal effect and to enhance the mechanical properties, the molecular structure of the particles has to be considered. An inconsistent behavior in mechanical properties was observed by the reinforcement of polyesterurethanes with carbon nanotubes or carbon black or silicon carbide of similar size [3, 110]. While carbon black reinforced materials showed limited Ri around 25-30%, in carbon-nanotube reinforced polymers shape-recovery stresses increased and R s of almost 100% could be determined [110]. A synergism between the anisotropic carbon nanotubes and the crystallizing polyurethane switching segments was proposed as a possible... 

In 2007, Rezanejad and Kokabi developed a low density polyethylene (LDPE)/ nanoclay composite with shape memory properties [72], Although all SMPs do not necessarily rely on a two-phase structure such as that of melt processed LDPE, extrapolation from this work can be made for a wide range of SMP systems. The team used organically modified Cloisite 15A nanoclay as filler and demonstrated a 300% increase in both E and E" upon addition of up to 8% nanoclay. Although the neat polymer system had nearly 100% shape recovery, the 8% loaded sample recovered nearly 80% at pre-strains of both 50 and 100%. Recovery stress, however, was dramatically increased from 1 MPa for the neat LDPE to about 3 MPa at 50% pre-strain and above 3 MPa at 100% pre-strain. 

---