Shape memory polymers molecular mechanism

FIGURE 19.4 Molecular mechanism and macroscopic effect of a shape-memory polymer, (a) Schematic representation of the thermally induced shape-memory effect of a polymer network with (b) Shape recovery of a stent with T = 52° in water at 37°C. The stent gradually changed from its... 

Shape memory polymers basically have two structural features—that is, the cross-links that determine the permanent shape and the reversible segments acting as a switching phase. Molecular mechanisms describing thermally triggered shape memory process are shown in Figure 4.1. In the figure, is the thermal transition temperature of the reversible phase... 

Molecular mechanism of thermally triggered shape memory effect of polymers. T, 5 = thermal transition temperature related to the switching phase. (Adapted from Lendlein, A., and Kelch, S. 2002. Shape-memory polymers. Angewandte Chemie, International Edition 41 2034-2057. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.)... 

Molecular mechanism of shape memory polymers (Hu and Chen, 2010). 

Recently, Lendlein et al. created a series of responsive shape memory polymers which are mechanically tough, biocompatible, and biodegradable, applicable to a number of biomedical apphcations [300,301,305]. Such shape memory polymers are achieved by copolymerizing precursors with different thermal characteristics such as the melting transition temperature. These shape memory polymers can be deformed into a temporary compressed state and they can recover the permanent shape only with the aid of an external stimulus such as temperature. This type of ape memory polymer mainly consists of two components (i) molecular switches—precmsors that can imdergo stimuh responsive deformation and can fix the formed tempo-... 

Chemical oxidants also allowed switchable hydrogel formation by reversible competitive hosting of low molecular mass ferrocene units within cyclodextrins. Here, cyclodextrin complexes the otherwise-associating alkyl side chains, which reduces the viscosity of this well-formulated mixture. However, the alkyl groups start to associate under gel formation when reduced ferrocenecarboxyUc acid is added. Then, ferrocene interacts preferentially with the cyclodextrin. This state can be reversed upon chemical oxidation due to the unfavorable inclusion complexation of ferrocenium units with cyclodextrin [254]. Based on similar mechanisms, a redox-dependent shape memory polymer has been generated [340] and also the adhesion of different gels can be switched [341]. 

See also Smart polymers applications of, 22 355 biodegradable networks of, 22 364 cyclic and thermomechanical characterization of, 22 358—362 defined, 22 355-356 examples of, 22 362-364 molecular mechanism underlying, 22 356-358, 359t Shape-memory rings, 22 351 Shape-memory springs, in virtual two-way SMA devices, 22 346-347 Shape-memory stents, 22 352 Shape, of fiber polymers, 77 174-175. 

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... 

FIGURE 5.2.3 Classification of soft shape-memory materials from the viewpoint of nanoaivhitectonics. (a-c) Structures and (d) molecular mechanism, (a) Chemically cross-linked polymer network, (b) supramolecular network with clay nanosheets [29], and (c) inorganic/polymer composite network system, and their shape-memory profiles [30]. (d) The nanoscale molecular mechanism for one-way and two-way SME of a cross-linked semicrystalline polymer system. 

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... 

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