Shape memory properties recovery

Cross-linked PUs with shape memory properties were prepared by Galia, Meier et al. using linear polyols synthesized by ADMET [140]. In this work, ADMET of a 10-undecenoic acid-derived a,co-diene containing a hydroxyl group was performed in the presence of 0.1 mol% of C4. 10-Undecenol was used as chain stopper, and the mixture of oligomers and diols (from 10-undecenol SM) obtained was cross-linked with MDI. The PUs obtained displayed outstanding values of strain fixity and recovery. 

Hydrogels are another class of polymers with shape memory properties. They are cross-linked polymers with a hydrophilic portion that has high affinity for water and a hydrophobic part that can be controlled by temperature variations. The cross-linked part is responsible for setting the permanent shape at an elevated temperature, whilst the hydrophobic part assumes a secondary shape at a specific or critical temperature. Heating above these temperatures completes the recovery (Liu et al., 2007). 

CNTs have been successfully used to aid in the remote recovery of stored energy in a shape-memory thermoplastic polyurethane (TPU) matrix (Figure 2.3). Pristine TPU exhibits relatively modest shape-memory properties. However, by adding MWNTs, the shape-memory properties are greatly... 

Figure 2.3 Shape-memory properties of MWNT-TPU composites, (a) Stretched (800%) 1 wt% MWNT-TPU composite ribbon, tied into a loose knot and heated at 55 °C. The knot closes on strain recovery, (b) Strain recovery and curling of the 1 wt% MWNT-TPU composite ribbon upon IR irradiation within 5 s. (c) Comparison of the stress recovery before (left) and after (right) remote actuation by IR irradiation. Neat TPU (M) bends and does not recover. In contrast, the 1 wt% MWNT-TPU composite (PCN) contracts on exposure to IR irradiation (arrow indicates moving direction), (d) Electrically stimulated stress recovery of a 16.7 wt% MWNT-TPU composite. Reprinted by permission from Macmillan Publishers Ltd. ...

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%. 

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 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. 

In this section, the sterilization and biocompatibility of SMPs are discussed jointly. All proposed SMP medical devices evenmally have to be validated with a designated sterilization method before they can be used clinically. The method of sterilization can influence the biocompatibility and performance of a device [104, 105], Subsequently, sterilization can also alter the thermomechanical properties of the polymer, which directly influence shape-memory properties such as shape storage (fixity) and recovery [106]. Currently, there are three types of sterilization methods including heat, radiation, and chemical techniques. 

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. 

Shape memory properties of SMPU are adjustable by controlhng the SSLs and HSCs. In the T -type SMPU, as the SSL increases, the of the soft segment will increase. Consequently, the T increases and the shape fixity will be improved. However, as the HSC increases, the X will decrease. The T and shape fixity will also decrease. The shape recovery is usually higher in the SMPU with higher HSCs. In the T -type SMPU, the T will move to a lower temperature range as the SSL increases or the HSC decreases. Hence, the T, decreases with the increase of SSL and the decrease of HSC. 

SS) and the hard phase e (HS). The shape fixing and shape recovery are strongly related to (SS) and (HS). The distribution of viscoelastic deformation between soft and hard phases is dependent on the two-phase morphology such as phase separation, phase composition, domain sizes, domain concentration and coimectivity, etc. Thus the shape memory properties ate defined by the two-phase morphological stmcture of the segmented polyurethanes. 

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