Temperature-responsive polymers shape-memory polymer

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... 

Numerous polymers have been proposed as shape-memory polymers (SMPs), and many of them are based on polyurethanes. This is because of the intrinsic versatility of segmented copolyurethane systems. By suitable choice of diisocyanate and macrodiol, a wide variation in properties may be obtained, allowing the possibility of tuning the shape-memory response to suit different applications. Usually they are phase-segregated materials. For example, a dispersed rigid phase (usually based on the diisocyanate) provides physical crosslinks, while the macrodiol provides a soft amorphous phase with low glass transition that provides the trigger temperature for shape recovery [63]. 

Abstract This chapter describes polymers that undergo a temperature-induced phase transition in aqueous solution providing an important basis for smart materials. Different types of temperature-responsive polymers, including shape-memory materials, hquid crystalline materials and responsive polymer solutions are briefly introduced. Subsequently this chapter will focus on thermoresponsive polymer solutions. At first, the basic principles of the upper and lower critical temperature polymer phase transitions will be discussed, followed by an overview and discussion of important aspects of various key types of such temperature-responsive polymers. Finally, selected potential apphcations of thermoresponsive polymer solutions will be described. 

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-... 

Shape-memory polymers (SMP) are stimuli-responsive. They have the capability of recovering their shape npon the application of an external stimulus. This external stimulus can be heat or an electrical voltage if the material shows electrical conductivity. Thermoplastic polyurethanes with SMP behavior were discovered at Mitsnbishi Co. in 1988. However, it was since 1996 that their investigation turned out to be more systematic." In this type of TPU, hard segments act as cross-links and are responsible for the permanent shape. Above the melting temperature of the hard segment domains, the polymer can... 

Goo et al. investigated the actuation durability of a conducting shape memory polyurethane/MWNT (CSMPU) actuator and concluded that the number of cycles at breaking decreased, as the actuation temperature increased (108). The possible reason is that more material degradation of CSMPU can be induced due to rapid and large movement of polymer chains as the actuation temperature increases. For a CSMPU actuator, the authors confirmed that an actuation temperature that is higher than the transition temperature produces a rapid response but low durability. 

Hydrogels are another class of polymers with shape memory properties. They are cross-linked polymers with a hydrophilic portion that has high affinity for water and a hydrophobic part that can be controlled by temperature variations. The cross-linked part is responsible for setting the permanent shape at an elevated temperature, whilst the hydrophobic part assumes a secondary shape at a specific or critical temperature. Heating above these temperatures completes the recovery (Liu et al., 2007). 

We have characterized these polymers with respect to their thermorheological response in tension, in the linear viscoelastic regime. Then their performance in a shape-memory sequence was computed from the theory of temperature-dependent linear viscoelasticity, allowing comparisons to be made between them [63]. 

Fig. 12 Scheme of a three-point flexural test, (a) Scheme of the shape-memory effect (SME) in polymers as defined by four critical temperatures. The value of 7 is a material property. Tiow is always less than Tg, whereas Tiugh may be above or below Tg, depending on the desired recovery response. (b) Schematic of the three-point flexure thermomechanical test setup. Taken from lef. [63], Copyright 2005. Reprinted with permission of John Wiley Sons, Inc. 

Tailored characterization methods for the SME were also developed for biomedical applications, such as for stents. A mechanical key characteristic for vascular stents is to withstand the compressive radial stresses over the lifetime of the application, i.e., maintain desirable thermomechanical characteristics with respect to recovery and deployment [63]. In a study on this topic, SME characterization methods were applied to a shape-memory stent from polymer networks, synthesized via photopolymerization of fert-butyl acrylate and PEG dimethacrylate [72]. The free recovery response of polymer stents at body temperature was studied as a function of Tg, crosslinking density, geometrical perforation, and deformation temperature. 

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