Shape Memory Actuation by Resistive Heating in Polyurethane Composites of Carbonaceous Conductive Fillers
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1129-V12-05
Shape Memory Actuation by Resistive Heating in Polyurethane Composites of Carbonaceous Conductive Fillers I. Sedat Gunes, Guillermo A. Jimenez, Sadhan C. Jana Department of Polymer Engineering, The University of Akron, Akron, OH 44325-0301, U.S.A. ABSTRACT The dependence of electrical resistivity on specimen temperature and imposed tensile strains was determined for shape memory polyurethane (SMPU) composites of carbon nanofiber (CNF), oxidized carbon nanofiber (ox-CNF), and carbon black (CB). The SMPU composites with crystalline soft segments were synthesized from diphenylmethane diisocyanate, 1,4butanediol, and poly(caprolactone)diol in a low-shear chaotic mixer and in an internal mixer. The materials synthesized in the chaotic mixer showed higher soft segment crystallinity and lower electrical percolation thresholds. The soft segment crystallinity reduced in the presence of CNF and ox-CNF; although the reduction was lower in the case of ox-CNF. The composites of CB showed pronounced positive temperature coefficient (PTC) effects which in turn showed a close relationship with non-linear thermal expansion behavior. The composites of CNF and oxCNF did not exhibit PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CNF and ox-CNF showed weak dependence on strain, while that of composites of CB increased by several orders of magnitude with imposed tensile strain. A corollary of this study was that a high level of crystallinity may cause a PTC effect and prevent any actuation through resistive heating. However, a carefully tailored compound which has reduced crystallinity and which requires minimum amount of filler may prevent PTC phenomenon and could supply necessary electrical conductivity over the operating temperature range, while offering enough soft segment crystallinity and rubberlike properties for excellent shape memory function. INTRODUCTION Shape memory polymers (SMPs) are a class of stimuli responsive materials, which recover the original shapes from the states of large deformation when subjected to an external stimulus, such as heat, electrical voltage, magnetic field, etc. [1 -5]. Shape memory functions can be actuated by resistive heating [6-8], if appropriate electrically conductive fillers are combined with electrically non-conductive matrix polymers. A major challenge in material design and proper functioning of SMP composite actuators based on semi-crystalline SMP is the occurrence of positive temperature coefficient (PTC) of resistivity. An otherwise electrically conductive polymer composite transforms into insulator as the electrical resistance increases with temperature. Another critical aspect of shape memory actuation by resistive heating is the dependence of electrical conductivity on applied strain. Note that SMPs experience strains on the order of several hundred percents both during deformation and shape recovery [9-12]. In view of this, the knowledge of temperature and strain dependence of electrical resistivity is of central importance
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