Shape memory behavior

FIGURE 5.79 Illustration of shape memory behavior of a commercial shape memory polymer Veriflex (CRG Industries), (a) Coupon of Veriflex having a rectangular memory shape. When heated above its transition temperature, it becomes elastic and can be manipulated into different shapes such as (b) and (c) and then cooled to maintain the new shape in a rigid state. When reheated above its transition temperature, it returns to its memorized shape. 

Liu C, Chun BS, Mather PT, Zheng L, Haley EH, Coughlin EB. 2002. Chemically cross linked polycyclooctene synthesis, characterization, and shape memory behavior. 

Fig. 21 (a) Stress-strain curves for polymer P3-4 and control P3-5 shown in Fig. 20. (b) Proposed molecular mechanism for shape-memory behavior. Adapted with permission from [72]. Copyright 2009 American Chemical Society... 

H.-J. Radusch, I. Kolesov, U. Gohs, G. Heinrich, Multiple shape-Memory behavior of polyethylene/polycyclooctene blends cross-linked by electron irradiation. Macromol. Mater. Eng. 297, 1225-1234 (2012)... 

I. Kolesov, O. Dolynchuk, S. Borieck, H.-J. Radusch, Morphology-controlled multiple one-and two-way shape-memory behavior of cross-linked polyethylene/poly( -caprolactone) blends. Polym. Adv. Technol. doi 10.1002/pat.3338... 

Li et al. synthesized a PMMA-PEG semi-IPN by radical polymerization and cross-linking of PMMA in the presence of linear PEG, which exhibits two independent shape memory effects at two transition temperatures, the of the PEG crystal and the Tg of the semi-IPN [39]. In the IPN, a single Tg appeared due to the miscibility of the amorphous phase of the two polymers. Based on a reversible order-disorder transition of the crystals below and above the of PEG, and the large difference in storage modulus below and above the Tg of the semi-IPN, the polymer has a recovery ratio of 91 and 99%, respectively For the shape-memory behavior at the of PEG crystals, the fixing phase was the PMMA network and the reversible phase was PEG crystals. For the shape memory behavior at the Tg of the semi-IPNs, the fixing phase was the chemical cross-linked point, while the reversible phase was the PMMA-PEG complex phase. 

Chemically cross-linked blends exhibiting shape memory effects were studied aiming at the effective cross-linking and improved shape memory behavior as well as development of a new SMP. 

Kolesov and Radusch prepared peroxide cross-linked binary and ternary blend SMPs from high density polyethylene and two ethylene-l-octene copolymers with medium and high degrees of branching [47]. The blends were prepared by a melt mixing and subsequently are cross-linked with 2 wt% of liquid peroxide 2,5-dimethyl-2,5-di-(tertbutylperoxy)-hexane at 190°C. The blends showed multiple shape memory behavior that appeared only at consequent stepwise application of convenient programming strains and temperatures. Obviously, that is caused by multiple melting behavior of these blends with many poorly separated peaks. 

Zhu, G., Xu, S., Wang, J., and Zhang, L. 2006. Shape memory behavior of radia-tion-crosslinked PCL/PMVS blends. Radiation Physics and Chemistry 75 443-448. 

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. 

Zhu et al. studied a biocompatible shape memory polymer blend based on poly(e-caprolactone) (PCL) and polymethylvinylsiloxane (PVMS). Pure PCL was subject to scission rather than cross-linking under irradiahon. In the presence of a small amoimt of PVMS (<20 wt%), both polymers are miscible in the amorphous phase and the radiation cross-linking of PCL is enhanced. Mechanical properties were improved, and a strong shape memory behavior was achieved. Above the melhng point of PCL, the blend exhibited a rubber-like state and could be deformed. The switch temperature was the melting temperature of PCL. With 5 to 15 wt% of PVMS and under 100 kGy y-irradiation, the deformation fixation ratio and the deformation recovery ratio were 100%. 

Fig. 14 Access to shape-memory materials from photocross-linked metallo-supramolecular polymers. (a) Formation of shape-memory materials using light as a stimulus (a) UV light is absorbed by the metal-ligand complexes and is converted to localized heat, which disrupts the metal complexation (i>) the material can then be deformed (c) removal of the light while the material is deformed allows the metal-ligand complexes to re-form and to lock-in the temporary shape id) additional exposure to UV light allows a return to the permanent shape, (b) Images demonstrating the shape-memory behavior. Reprinted with permission from [274]. Copyright 2011 American Chemical Society...

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. 

Based on this understanding, a mechanism based constitutive model incorporating the nonlinear structural relaxation model into the continuum finite-deformation thermoviscoelastic framework was developed as follows. The aim of this effort was to estabUsh a quantitative understanding of the shape memory behavior of the thermally responsive thermoset SMP programmed at temperamres below Tg. To simplify the formulation, several basic assumptions were made in this study ... 

Nguyen, T.D. (2013) Modeling shape-memory behavior of polymers. Polymer Reviews, 53, 130-152. 

Zhu, G.M., Xu, Q.Y., Liang, G.Z., and Zhou, H.F. (2005) Shape-memory behaviors of sensitizing radiation-crosslinked polycaprolactone with polyfunctional poly(ester acrylate). / Appl Polym. Sci., 95 (3), 634-639. 

Yang, J., Liu, E, Yang, L., and Li, S. (2010) Hydrolytic and enzymatic degradation of poly(trimethylene carbonate-co-D,L-lactide) random copolymers with shape memory behavior. Eur. Polym. J., 46 (4), 783-791. 

Zini, E. and Scandola, M. (2007) Shape memory behavior of novel (L-lactide-glycolide-trimethylene carbonate) terpolymers. Biomacromolecules, 8 (11), 3661-3667. 

Ham] Hamers, A.A.H., Wayman, C.M., Shape Memory Behavior in Fe-Mn-Co Alloys , Scr. Metall. Mater., 25, 2723-2728 (1991) (Meehan. Prop., Experimental, 13)... 

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