Shape memory properties chains

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. 

Diisocyanate is often used in the chain extension reactions of biopolymers such as PEA, PCL, and their copolymers [97-101]. The combination of hard segment and soft segment may confer the resulting polyurethanes with shape-memory property [100,101]. Polyurethane is an important type of elastomeric polymer for biomedical applications [1,9,11]. Chain extension or cross-linking by diisocyanate can be adapted to many -OH-terminated or H-containing polymers or prepolymers [102]. The convenience of the urethane chemistry has made it into a very popular way of polymer chain extension method in biomaterial designs. 

Ahn et al. reported [62] preparation of a smetic hquid crystalline elastomer with shape memory properties. Shape memory polymeric materials can recover their equilibrium, permanent shapes from nonequilibrium, temporary shapes as a result of external stimuli, like heat or light. Such materials have apphcation in medical practice. Main-chain polynorbomenes were linked with three different side-chains, cholesterol, poly(ethylene glycol), and butylacrylate. 

BD. They reported an improvement in shape memory properties as a result of introduction aromatic structure into the main chain. Yang and co-workers [122] compared the mechanical, dynamic mechanical and shape memory properties of PU block coPolymers with planar shape hard segment (1,6-diphenyl diisocyanate (PDI)) and bent shape hard segment (MDI). The PDl-based PU showed superior properties compared with MDI-based PU (Table 2.11) as a result of better interaction among hard segments due to the planar shape of PDI. 

Gebarowska, K., Kasperczyk, J., Dobrzynski, P., Scandola, M., Zini, E., Li, S., 2011. NMR analysis of the chain microstmcture of biodegradable terpolymers with shape memory properties. European Polymer Journal 47, 1315—1327. 

There is a broad range of studies on the SMP composites. These materials are based on a polymeric matrix with fillers or nanofiUers. Shape memory PLLA matrix composites with ceramic fillers, hydroxyapatite (Yu et al., 2009 Zhou et al., 2007) and j3-tricalcium (Zheng et al., 2008), have been reported with improved properties with respect to the PLLA homopolymer. In this case, PLLA chains are the switching phase while the fixity phase is constituted by the interaction between the particles and the polymer. Meng (2009) has achieved enhanced shape memory properties of PLLA using fillers of the biopolymer chitosan. In the following... 

Table 4.7 Shape memory properties of T -SIVIPUs with urethane chains as soft segments...

For polymer materials to show shape memory properties, it is necessary for them to be chemically (crosslinking agent) or physically (entanglement of polymer chains, ionic bonds, etc.) crosslinked [2]. Here, cross-linking indicates formation of networks by fixing relative positions of polymer chains through chemical or physical bonds. Shape memory of polymeric materials can be classified as follows. 

This method utilizes pH, chelate formation, oxidation-reduction reaction, or light to deform polymers isothermally and reversibly (see Fig. 5). Unlike the methods described thus far, the main characteristic of this method is reversible shape changes. For example, poly(acrylic acid) fibers that have the shape memory property with changes in pH possess dissociated charges. These charges repel each other and the polymer chains are stretched. On the other hand, in a low pH range, there are no charges and the polymer chains shrink. Another example is partially phosphated poly(vinyl alcohol) film, which responds to chelate formation. If Cu " is included, the film shrinks by formation of crosslinks. 

Accordingly, in the shape memory fimction that appears upon external thermal, chemical and physical stimuli, all deformations are based on changes in the stereoscopic positions of die polymer main chains. Hence, unless the relative position of die main chain is not fixed, the polymer will not show shape memory properties. 

All fillers were dried overnight under vacuum at 120°C to eliminate any absorbed moisture. BD and PCL were also dried overnight under vacuum at 45°C. A detailed account of the method used to prepare organoclay/SMPU nanocomposite can be found elsewhere [21, 22]. In short, prepolymer was synthesized at 80°C for 2 hours. Chain extension of prepolymer with BD was carried out in Brabender Plasticorder (model EPL 7752) at 80°C for 4 minutes with 9.8x10" mol/L of tin catalyst. Organoclay was then added and allowed to react for additional 11 minutes to produce nanocomposites. Clay content was maintained at 1 and 3 wt%. Eor other composites, chain extension step was carried out at 110°C for 2 minutes before addition of CNF, CB or SiC. Filler particles were mixed for an additional 5 minutes at 140°C. The total filler content was maintained at 1, 3 and 5 wt%. The composites were compression molded at 220°C to produce specimens with thickness of 0.5 mm for testing of mechanical, thermal, and shape memory properties. 

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]. 

Kolesov, I. S., and Radusch, H. J. 2008. Multiple shape-memory behavior and thermal-mechanical properties of peroxide cross-Unked blends of Unear and short-chain branched polyethylenes. eXPRESS Polymer Letters 2 461—473. 

Shape memory PU can be classified into two catergories amorphous and crystalline. The compositions and properties of different types of amorphous PU reported in recent literature are summarised [116-120] in Table 2.11. Wang and Yuen [121] synthesised a series of thermoplastic PU using aromatic chain extenders such as... 

Hydrogels with hydrophobic, crystallizable side chains formed by copolymerization of acrylic acid and stearyl acrylate crosslinked with methylenebisacrylamide (BIS) showed a strong temperature dependence in their mechanical properties [91-93]. Such shape-memory gels having Tians = 7m displayed a reversible order-disorder transition associated with the interactions between the alkyl side chains. While behaving like hard plastic below 25 °C, softening above 50 °C enabled the materials... 

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