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Understanding the shape memory behavior of thermoplastic polyurethane elastomers with coarse-grained molecular dynamics simulations Md Salah Uddin1 and Jaehyung Ju2 1 Department of Mechanical and Energy Engineering, University of North Texas Denton, TX 76203, USA 2

University of Michigan - Shanghai Jiao Tong University Joint Institute Shanghai, 200240, China ABSTRACT We perform molecular dynamics (MD) simulations to understand thermally triggered shape memory behavior of a thermoplastic polyurethane (TPU) elastomer with an enhanced coarse-grained (CG) model. Hard and soft phases of shape memory polymers (SMPs) are known as fixed and reversible phase, respectively. Fixity depends on the content of hard segments due to their restricted mobility. On the contrary, recovery depends on the dynamic motion of the soft segments as well the degree of cross-linking, which is also affected by the quantity of hard segment. Several CG models of the TPU are constructed varying the weight percentage of soft segments to observe their effects on shape recovery and fixity. All of the models are equilibrated at 300K (above glass transition, Tg: 200-250 K) and deformed under uniaxial loading with NPT (isothermal-isobaric) ensembles. The deformed state is cooled to 100K (below Tg) and further equilibrated to estimate the shape fixity. Shape recovery is predicted by heating and equilibrating the structures back to 300K. By the end of this study, we may answer how much the shape fixities and recoveries are changed for varying concentration of hard segments from thermomechanical cycles with CGMD simulations. INTRODUCTION The shape memory effect (SME) of polymers is exhibited with entropy dominated shape transformation triggered by external stimuli such as heat, light, electric field, magnetic field, and solvent exposure associated with a fixed phase (physical or chemical crosslinking) and a reversible phase [1-8]. Temporary deformation applied to the polymers above their glass/melting transition temperatures can be held by cooling the polymers down below the transition temperature and recovered again by heating the polymers up above the transition by suppressing and triggering the molecular mobility, respectively [1-2]. The application of SMPs has become important in several sectors such as packaging, assembling devices, textiles (wrinkle free fabrics), membranes, medical devices and implants, self-deploying sun-sails or antennas, morphing structures, hinges, and booms for aerospace vehicles [3-4]. Along with different types of polymeric materials, block copolymers with segmented molecular structures such as shape memory polyurethanes (SMPUs) consisting of a polyether or polyester soft phase, a diisocyanate based hard phase, and an interphase have become very popular in these applications [9]. The rigidly fixed phase plays the role of physical cross-linking and the soft reversible phase acts for the switching or triggering the shape memory effect [10-11]. Along with the interplay between the hard and soft segments, the shape memory effect