Shape memory recovery

Until the present time, most SMPUs have been prepared from linear PUs, which have physically cross-linked segments. However, these linear SMPUs cannot endure repeated changes in shape memory. In fact, some studies have found that the shape retention and shape recovery of SMPUs decrease dramatically after the first cycle [200]. To improve cyclic shape memory retention and shape memory recovery, SMPUs have therefore been chemically cross-Knked by excess MDI or by glycerin [203]. 

Both, Ce-TZP [211] and Ce-Y-TZP [212] exhibit martensitic transformation-associated shape memory. For example, the 8Ce-0.50Y-TZP exhibits a complete shape memory recovery under a recoverable strain of 1.2% at a relatively high... 

The flexibility of the SMP composite material is important for folding the structure into the spacecraft for transfer to space the folding temperature is highly dependent on both the resin and fibre properties. Once the structure is packed into its folded position, it is constrained in that position until cooled to approximately 15°C, or lower below Tg at which point the SMP composite structure will remain locked or frozen in the packed position umestrained until it is again heated above the Tg. When the SMP structure is heated, internal strain energy will cause it to return to its initial cured shape. The speed and the accuracy of the shape return are a function of the shape memory recovery force of the composite. 

Fig. 2. The shape-memory process, where Tis temperature, (a) The cycle where the parent phase undergoes a self-accommodating martensite transformation on cooling to the 24 variants of martensite. No macroscopic shape change occurs. The variants coalesce under stress to a single martensite variant, resulting in deformation. Then, upon heating, they revert back to the original austenite crystallographic orientation, and reverse transformation, undergoing complete recovery to complete the cycle, (b) Shape deformation. Strain recovery is typically ca 7%.

Shape-memory polymers (SMPs) are a class of smart materials with the ability to change shape on demand in response to an environmental stimuli [322-325]. So far, the most commonly investigated SMPs are temperature-induced SMPs, whose shape-recovery behavior is triggered by thermal stimuli. Such SMPs have one shape at certain temperature and are converted to another shape at a different temperature (Fig. 22). Temperature-responsive SMPs usually require the combination... 

Fig. 22 (a-f) Time series photographs showing recovery of shape-memory tube from start to finish of the process total time 10 s, at 50 °C. The tube was made of PCL-dimethacrylate polymer network that had been programmed to form a flat helix. Reprinted from [323] with permission... 

Strain hardening effect, 20 224 Straining efficiency, 77 340 Strain rate, 73 473 Strain recovery rate (Rr), in testing shape-memory polymers, 22 361 Strain sensors, 77 150, 151-152 Strain tensor, for noncentrosymmetry pont group crystals, 77 93-94 Strain versus time curve factors affecting, 73 473 material and microstructure effect on, 73 473-474... 

Sieves - [SHAPE-MEMORY ALLOYS] (Vol 21) -formineral processing [MINERALS RECOVERY AND PROCESSING] (Vol 16) -role m particle si2e measurement [SIZE MEASUREMENT OF PARTICLES] (Vol 22)... 

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. 

Better dispersion of MWNTs in the polymer matrix caused by the formation of the chemical bonds leads to uniform stress distribution and enhanced shape memory (23). Jana et al. prepared nanocomposites of PU and MWNTs via in-situ polymerization and conventional method (105). PU nanocomposites obtained via an in-situ method with PCL-g-MWNTs showed better shape recovery, compared to conventional nanocomposites. 

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