Shape-memory cycle

Shape-memory cycle of a thermo-responsive SMP. The typical shape-memory cycle of a thermo-responsive SMP consists of the following steps 1) start with a SMP in its original shape (permanent shape) 2) heat the SMP above its thermal transition temperature (Ttrans) and deform the SMP by applying an external force, cool well below 3 rans and remove the constraint to obtain the temporary shape with energy stored and 3) heat the pre-deformed SMP above Ttrans at which point the SMP releases the stored energy and recovers the permanent shape (shape recovery). Reprinted and adapted by permission from Cambridge University Press." ... 

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.)... 

Fig. 19 Mechanical-viscoelastic model of Lin and Chen (1999) with two Maxwell models to describe SME in segmented PUs. (a) General model, (b) Change of the model in the shape-memory cycle, (c) Shape-memory behavior for two PU samples. Solid lines indicate the recoverable ration curves of the model. Taken from ref. [36], Copyright 1999. Reprinted with permission of John WUey Sons, Inc.

The DMTA was mainly used to test the shape memory cycle and to quantitatively compare each sample by calculating a percent fixation, percent recovery and fill factor. As the concentration of ZnOl increased, the percent recovery decreased, and the percent fixation showed no particular trend. The fill factor is a number between 0-1 that can be used to compare the shape memory performance, based on how well the material fixes and how fast it recovers when heated above Tc.. 1 is perfect shape memory, where the shape completely recovers at Tc. The fill factor is calculated from the area below the curve of a graph of temperature versus length. Fig. 4. The fill factor is the ratio of the area under the actual recovery curve divided by the ideal response represented by the rectangular box shown in Fig. 4. As the concentration of ZnOl increased for ionomer/FAS SMPs, the fill factor decreased. 

Small Angle X-Ray Scattering was preformed on all samples, throughout the shape memory cycle. With the samples, there was no orientation in the pre-stretched and the recovered samples however, there is definite orientation on the fixed sample, indicated by the darker regions seen in Fig. 6. Most samples were consistent with this however a few of them did not recover completely... 

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