Shape memory polymers behaviour

Biodegradable shape memory polymers are candidates for the next promising generation of implant materials. The fact that these materials belong to a polymer system allows the adjustment of certain properties in a wide range, e.g. mechanical properties and degradation behaviour. Today, such materials can be synthesized in a kilogram scale. 

Shape memory materials (SMMs) comprise a class of smart materials able to change their shape following application of an external stimulus. The first materials studied with shape memory behaviour were developed in the middle of the twentieth century. Since then, research into and demand for these materials have both increased, especially in recent years, due to their unique properties, versatility and growing industrial demands. Figure 7.1 shows the total number of scientific publications indexed on ISI Web of Knowledge and the number of research articles about shape memory polymers (SMPs) in the last 30 years in particular, the increase during the last six years is significant. 

Recent research on the recovery behaviour of oriented polymers (so-called shape memory polymers) where the initial plastic deformation can be reversed, either fiilly or partially, by heating to temperatures higher than that of the initial deformation, has thrown more light on the nature of the internal stress. It is clear that this stress cannot be simply regarded as directly akin to a rubber-like stress because its behaviour with regard to temperature and strain rate does not correspond quantitatively with that expected for a rubber. 

The phenomenon of shape memory effect in SMPs is brought about by large changes in elastic modulus, E, above and below the transition temperature. Figure 1.3 shows a typical modulus behaviour of SMPs with temperature. At a temperature above the transition, the polymer enters a rubbery elastic state, and hence the elastic modulus of the polymer is much reduced. Consequently the polymer can be easily deformed by application of an external force (Bar-Cohen, 1999 Liu et ah, 2007). If the material is allowed to cool below its transition temperature, under reasonable strain, its temporary deformation becomes hxed. At this stage, the polymer lacks its rubbery elasticity and displays a high modulus. This state is called the glassy state. This deformation can be recovered when the polymer is heated above the transition (Hu, 2007). 

Other industrial applications include the fabrication of two-part epoxy resins (similar to those commonly found in household maintenance stores) [95-97], These were synthesized using triglycerides and diamines. These resins are often used as adhesives these have also been studied using soybean oil, which provided beneficial properties in terms of fast curing, thermal stability and ease of removal (peel strength) [98], A blend of divinylbenzene/styrene/tung oil mix gave a polyurethane-based material which behaved like a smart polymer with shape memory behaviour [66]. 

Gianoncelh, A., Brisotto, M., Bontempi, E., and Ricco, T. (2013) One-way and two-way shape memory behaviour of semi-crystalline networks based on sol-gel cross-linked poly(e-caprolactone). Polymer, 54 (16), 4253-4265. 

Smola Anna, Dobrzynski Piotr, Cristea Mariana, et al. Bioresorbable terpolymers based on L-lactide, glycoUde and trimethylene carbonate with shape memory behaviour. Polym. Chem. 5 no. 7 (2014) 2442-2452. 

Structural concepts for tissue-compatible and biodegradable polymers, thermoplastic elastomers, and thermosets with shape memory capabilities will be introduced. Their thermal and mechanical properties and degradation behaviour will be explained. An important precondition for the shape memory effect of polymers is elasticity. An elastic polymeric material consists of flexible segments, so-called network chains, which are connected via netpoints or junctions. The permanent shape of such a polymer is determined by the netpoints. The network chains take a coil-like conformation in unloaded condition. If the polymer is stretched, the network chains become extended... 

ISI publications referring to polymers with shape memory behaviour. (Source ISI Web of Knowledge.)... 

As mentioned above, PLA is a thermoplastic polymer with shape memory behaviour. This polymer can be used as co-monomer with the aim of improving its properties. For instance, copolymers from lactic acid with gly-coUc add (i.e., poly(lactic-co-glycolic add) (PLGA)) (Meng et al, 2009) or -caprolactone (to form poly(I lactide-co-e-caprolactone) (PCLA))(Lu dfl/.,2008) present better shape memory behaviour than the PLA homopolymer. Other copolymers like poly(ethylene oxide) (PEO) with poly(ethylene terephthalate) (PET) have been studied (Luo et /., 2000). In this copolymer PEO is the switching phase with transition temperature between 40°C and 50°C and PET is the fixity phase. 

Blending is a good technique to obtain new polymeric materials and to enhance polymer properties such as thermal behaviour, mechanical properties, etc. Moreover, blending is an easy process to implement at the industrial level (Meng and Hu, 2009). For these reasons it is interesting to develop SMP blends. There are different methods to obtain polymer blends with shape memory behaviour that can be classified in two groups miscible and immiscible blends. 

Lin, J. R., Chen, L. W. (1998), Study on shape-memory behaviours of polyether-based polyurethanes. Part I Influence of the hard-segment content, J. Appl Polym. Sci., 69, 1563-74. 

The most frequently quoted example to illustrate this behaviour is the children s toy Silly Putty , which is a poly(dimethyl siloxane) polymer. Pulled rapidly it shows brittle fracture like any solid but if pulled slowly it flows as a liquid. The relaxation time for this material is 1 s. After t = 5t the stress will have fallen to 0.7% of its initial value so the material will have effectively forgotten its original shape. That is, one could describe it as having a memory of around 5 s (about that of a mackerel ). Many other materials in common use have relaxation times within an order of magnitude or so of 1 s. Examples are thickened detergents, personal care products and latex paints. This is of course no coincidence, and this timescale is frequently deliberately chosen by formulation adjustments. The reason is that it is in the middle of our,... 

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