Thermomechanical Cyclic Properties by Different Shape Memory Processing in Polyurethane Shape Memory Polymer

Shape Memory Polymers (SMP) are a kind of materials that are capable of change its shape when an external stimulus is applied. The polymer can be processed into their permanent shape by conventional techniques and can be deformed in a temporary shape in a step called programming. The programming includes heating up the material to a temperature , deforming and cooling down to a temperature  in order to fix the temporary shape. This shape is retained until the stimulus is applied allowing the SMP to return to its originally shape, this is called Shape Memory Effect (SME).

SME is strongly influenced by different tests conditions as the programming temperature (T_H) and the maximum stress (s_max) applied. Varying these test conditions it’s possible to alter some thermomechanical parameters such as the maximum deformation or shape recovery ratio (SRR). SME also varies significantly with thermomechanical cycling (TC). The SRR decreases in the first cycles while irrecoverable deformation increases. Both parameters show an asymptotic behaviour and a stabilisation in the last cycles. For these reasons, it’s important to study how TC affects the overall macroscopic behaviour of SMP.

The main objective of this study is to determine the influence of the programming and recovery temperature and the s_max in the thermomechanical parameters under TC. Other parameters such as the slope of the stretching step are also studied in order to analyse its influence in the overall mechanical behaviour. The analysis focuses on finding the TC conditions that stabilize the material in fewest cycles and obtaining the best mechanical properties over the highest number of TC.

To accomplish this, successive TC were performed in an amorphous shape memory polyurethane Tecoflex^® (TFX EG-72D). The TC is performed stretching at different T_H and at different s_max and evaluating the test parameters obtained for each cycle.