Relationship Between Materials Properties and Shape Memory Behavior in Epoxy-Amine Polymers
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Relationship Between Materials Properties and Shape Memory Behavior in Epoxy-Amine Polymers Ingrid A. Rousseau* and Tao Xie General Motors Corp., Technical R&D Center, Materials and Processes Lab., 30500 Mound Road, M.C.: 480-106-710, Warren MI 48090-9055, U.S.A. * Corresponding author: [email protected] ABSTRACT Although epoxy-based polymers remain infrequently used as shape memory polymers (SMP’s), they are a promising base material for highly demanding applications due to their intrinsic physical properties and ease of processing. A series of epoxy SMP’s was synthesized with varying mechanical properties and with glass transition temperatures ranging from 31 to 93 o C, tunable via the variations of the molecular structures. The influence of chemical structures and physical properties of these epoxy SMP’s on their shape memory (SM) behavior is examined in detail along with the impact of the shape memory cycling conditions. While the results show that lower crosslink densities and/or higher molecular flexibility/mobility leads decreased SM performance, at low crosslink density the effect of molecular flexibility/mobility becomes dominant in influencing the SM response. INTRODUCTION Shape memory polymers (SMP’s) have attracted much interest over the past decade as alternative solutions to other smart materials. These polymers can be controlled to exist in either a permanent or temporary shape by the combination of an imposed deformation and a triggering stimulus. While SMP’s low cost, easy processing, and low density, amongst other qualities, made them materials of choice for the development of new technologies [1-6], their lower mechanical and thermal properties, and their reduced durability compared to their metal alloys analogues (i.e., shape memory alloys), require essential improvements [7-11]. Because glassy thermoset SMP’s can be expected to perform better than other SMP classes [11], and because the development of epoxy-SMP’s has remained infrequent, we chose to study the SM behavior of epoxy thermosets and the impact of their molecular structures (crosslink density, crosslinker functionality, and chain flexibility) on their shape memory effect. Their shape memory performance was quantified under varying experimental conditions (i.e., deformation load, recovery heating rate, number of SM cycles, holding times in the deformed or temporary shape above and below their transformation temperature). EXPERIMENT: SHAPE MEMORY CYCLE CHARACTERIZATION The shape memory (SM) cycles were measured using the Q800 DMA. The values for the setting temperature (Ts), the deformation temperature (Td), the transformation temperature (Ttrans), and the storage modulus at Ts (E’s) and at Td (E’d) were determined from the thermomechanical data [12]. The definition and quantification of the SM performance of a material were discussed in greater details in previous reports [11, 12]. The response temperature (Tr), the shape fixity (Rf), the shape recovery (Rrmin and Rrεu), the recovery speed (Vr) and recovery time (tr)
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