Shape memory polymers heating

Thermal techniques, in nondestructive evaluation, 17 420-421. See also Heat entries Heating entries Thermal-transfer printing, 9 242, 338 Thermal transfer processes, 19 320 Thermal transition, in shape-memory polymers, 22 357-358, 359t, 360, 361-362... 

Other materials have been found that have shape memories. Shape-memory polymers are plastics that have properties similar to SMAs. Pliable materials have recently been found that can be stretched up to twice their normal length, yet regain their shape when heated. Having a smart plastic material opens up applications of a softer, more flexible nature, such as clothing. 

Researchers who study polymers are looking for novel ways to incorporate shape memory into these materials. Shape-memory polymers that respond to heat have been discovered, and in 2005, Andreas Eend-lein at the Institute of Polymer Research in Teltow, Germany, Massachusetts Institute of Technology researcher Robert Langer, and their colleagues found a shape-memory polymer that is responsive to light. 

Medical applications of biodegradable shape memory polymers include their use for removing blood clots formed during strokes. Preshaped foams can be used to fill cranial aneurisms. Loosely tied sutures made from fibers that have been stretched at 50 °C will tighten when heated just above room temperature. 

Shape memory polymers are defined by their ability to store and recover strains when subjected to a particular thermo-mechanical cycle. Shape-memory polymers can recover their original shape by being heated above their transition temperature, which are defined by different phases in the materials. In particular, the shape-... 

To date, heat-triggered shape memory polymers have had the greatest share of research and application adaptation. However, trigger mechanisms could also be chemo-responsive, e.g., water, ethanol, and pH change photo-responsive, e.g., UV or IR, including radio waves and/or mechano-responsive, e.g., stretching, impact, etc. 

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. 

Besides the intrinsic conductive polymers, some deformable polymers, such as shape-memory polymers, are usually activated by heating. After incorporating with conductive fillers, such as carbon nanomaterials, they can be simulated by the electricity through Joule heating (Liu et al., 2009 Hu and Chen, 2010 Koerner et al., 2004). This kind of electro thermally active polymer composites can produce expansion/contraction and bending behaviors upon with the electricity. Moreover, these actuators can work durably... 

Presently, the study of shape memory polyurethane (SMPU) has been widely conducted. For common elastic fibers, the elasticity should be defined as the instant recoverability of the length on release of the deforming stress. The recovery in shape memory fibers is the ability of deformed fibers to recover under external stimulus such as heat or chemicals. In this case, the external stimulus is a must. Figure 3.1 shows the net-points and switches which response to the stimulus (Hu et al., 2012). The elongation of shape memory polymer fiber are a little less than spandex, but mechanical... 

Shape memory polymers make up another class of injectable biomaterials for vascular applications, yet are relatively new in the field of endovascular embolization. Shape memory polymers are chemically structured so that they are able to reversibly take on a different physical shape in response to some stimuli (Small et al, 2007). Usually these different shapes include a compact form and an expanded form of the polymer. In the case of endovascular embolization, the expanded polymer can be pre-formed to fit specific contours of an individual aneurysm (Ortega et al, 2007). Upon interacting with some type of stimuli, such as heat or cold, the material is compacted into a shape that can be delivered through a microcatheter. The process of using shape memory polymers to embolize an aneurysm is shown in Fig. 7.5, along with samples of expanded SMPs (Ortega et al, 2007). 

Three-dimensional (3-D) plot of the shape memory cycle for (a) a shape memory polymer (SMP) and (b) vulcanized natural rubber. The star indicates the start of the experiment (initial sample dimensions, temperature, and load). Both the SMP and the rubber were deformed under constant loading rate at constant temperature. The deformation step was then followed by a cooling step under constant load. At low temperature, the load was removed and shape fixing was observed for the SMP, but an instant recovery was seen for natural rubber. Shape recovery of the primary equilibrium shape was obtained by heating the SMP. (Adapted from Liu, C., Qin, H., and Mather, P. T. 2007. Review of progress in shape-memory polymers, journal of Materials Chemistry 17 1543-1558. Copyright Royal Society of Chemistry. Reproduced with permission.)... 

Buckley, P. R., McKinley, G. H., Wilson, T. S., Small, W., Benett, W. ]., Bearinger, J. P., McElfresh, M. W., and Maitland, D. J. 2006. Inductively heated shape memory polymer for the magnetic actuation of medical devices. IEEE Transactions Biomedical Engineering 53 2075-2083. 

Under specific stimulus, shape memory materials could move from a temporary shape to their original shape. The stimulus could be light, pH, or electric or magnetic field, but the most common shmulus is heat. In this case, a shape memory polymer (SMP) possesses a switch transihon temperature. When the SMP is subject to deformation, its cross-linking structure could store internal stress if it is cooled below this switch temperature. When the polymer is heated above this temperature, it returns to its original shape. Shape memory polymer blends could be achieved using irradiahon. 

Stimuli-responsive materials have sparked enormous interest in recent years due to their potential applications in micro-machines, soft robots, biomedical systems, etc. [1-6]. A variety of intelligent polymeric materials such as shape memory polymers [7, 8], polymer gels [9, 10], conducting polymers [11, 12], and dielectric elastomers [13,14] have been developed for these applications. Compared to other stimulus-driven methods including pressure [15], heat [16, 17], electric field... 

When the shape memory polymer is heated above the glass transition temperature of the hard segment, the material can be shaped. This original shape can be memorized by cooling the shape memory polymer below the glass transition temperature of the hard segment. 

An ideal shape-memory polymer for a given application has a small set of parameters that determine the usefulness, or otherwise, for a given application (1) The fractional recoverable strain the maximum fraction of the imposed strain that can be recovered when triggered. (2) The temperature of maximum recovery rate Tmax for a given rate of heating a measure of the temperature required to trigger... 

Fig. 2 Literature and patent analysis for the time period 1907-2008 according to pubUcadon date for the scientific publications and filing date for patents/patent applications. In all cases plural and singular forms of search terms were considered, (a) Result of literature search for shape-memory polymer in CAPlus database performed with Scifinder on March 12th, 2009 white - publications in English, gray - publications in Chinese or Japanese, black - other languages (b) result of two literature searches shape-memory gel, heat shrinkable polymer (all languages) performed in CAPlus database with Scifinder on March 12th, 2009 black - shape-memory gel, white - heat shrinkable polymer (c) result of patent search (issued patents and patent applications) for shape-memory polymer performed in DEPATIS database on Feb 27th, 2009 white - shape-memory polymer (d) result of patent search (issued patents and patent applications) for shape-memory gel, heat shrinkable article performed in DEPATIS database performed on Feb 27th, 2009 black - shape-memory gel, white - heat shrinkable article ...

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