NATIONAL LABS: RESEARCH HIGHLIGHTS 2014 CURRENT SCIENCE, VOL. 108, NO. 6, 25 MARCH 2015 1072 *For correspondence. (e-mail: navinchand15@yahoo.co.in) CSIR–Advanced Materials and Processes Research Institute, Bhopal Navin Chand* and S. A. R. Hashmi CSIR–Advanced Materials and Processes Research Institute, Habiganj Naka, Bhopal 462 026, India Shape memory polymers (SMP) are being investigated for improving their recovery stresses to broaden the area of their application in engineering applications. The conventional shape memory creation procedure of SMP was modified, that shows significant improve- ments in recovery stresses. The conventional deforma- tion steps were replaced by progressive stretch-relax– stretch scheme of deformation. Keywords: Carbon nanotubes, polyurethane, recovery stress, recovery strain, shape memory polymers. A modified procedure of creation of shape memory effects in polymers and their composites Shape memory polymers (SMPs) have numerous advan- tages like high recoverable strain, low cost, easy form- ability and response to a wide range of stimuli, including heat, moisture, solvent or change in pH value, light, stress, etc. SMPs possess exceptional shape memory strain, but their low mechanical strength, particularly low recovery stress often results in limited applications. To overcome these difficulties various functional fillers, including carbon nanotubes (CNTs) have been added to the SMP matrix and are being studied 1–4 . Interestingly, shape memory effect (SME) is not an in- trinsic performance of a polymer, but it results from the combination of molecular architecture, morphology and shape memory creation procedure (SMCP) of a poly- mer 5,6 . A modification in SMCP was introduced by re- searchers at CSIR-AMPRI (Advanced Materials and Processes Research Institute), Bhopal to improve energy stored during deformation at high temperature. The con- ventional deformation step which is required to fix a tem- porary shape of the SMP was replaced by progressive stretch–relax–stretch (PSRS) scheme of deformation, to obtain higher values of recovery-stress in the composites. The conventional SMCP approach involves (i) heating the sample to a temperature T H which is higher than the glass transition temperature, T g , (ii) deforming/stretching to a certain level, (iii) bringing down the temperature be- low T g without relaxing the deformation strain and (iv) removal of imposed strain (clamps, etc.) and allowing the sample to relax and attain a fixed length l f or a temporary shape. The sample is now ready to evaluate SMEs as shape memory has been created in the specimen. As men- tioned in step (ii), the sample was stretched continuously to a maximum length l s at temperature T H in conventional SMCP, whereas under the modified SMCP the deforma- tion step (ii) was replaced by the PSRS scheme of defor- mation. Accordingly, the maximum strain in step (ii) was attained in several steps with intermittent relaxation. In other words a small strain is followed by relaxation and stretching, and further followed by a second relaxation and stretching. The process continues until the maximum designed strain is achieved. Steps (iii) and (iv) remain the same as in conventional method for creating shape mem- ory in the polymeric materials. The steps involved in modified SMCP, PSRS scheme and SME determination are shown in Figure 1 by three- dimensional representative curves obtained experimen- tally for SMTPU. The processes involved in each step are: (a) stretching at 70C, (b) cooling to room tempera- ture by maintaining the deformed strain, (c) relaxing and fixing the temporary shape at room temperature, (d) heat- ing to 70C by maintaining the fixed strain and (e) con- strained recovery at 70C by decreasing the strain until the sample recovers completely. Suffixes 1–3 in the figure represent the first, second and third progressive thermomechanical cycle. Experimentally, the multi-walled CNT (MWCNT) re- inforced shape memory polyurethane (SMTPU) films were cast to observe this phenomenon for composites having different loadings of MWCNT. Figure 2 shows the stretching stresses as well as the recovery stresses for 0, 1, 2, 3 and 5 phr MWCNT reinforced SMTPU. Stretch- ing curves show the stresses developed during stretching of samples at 70C. The samples were brought to room temperature at 25C under the same clamped conditions and thereafter were relaxed to attain a temporary de- formed length that was slightly less than that of the stretched length. The samples were reclamped without applying any stress and the temperature of bath was brought to 70C. Under this condition recovery stresses developed in the test specimen, which were determined using the test set-up developed for the purpose in CSIR- AMPRI 7 . The slope of the curves increased with the increase in MWCNT content (Figure 2), indicating an increased