Shape Memory Behavior of Ultra-High Molecular Weight Polyethylene
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Shape Memory Behavior of Ultra-High Molecular Weight Polyethylene Sergey Kaloshkin1, Aleksey Maksimkin1, Maria Kaloshkina1, Mihail Zadorozhnyy1 and Margarita Churyukanova1 1 National University of Science and Technology "MISIS", Leninsky prosp., 4, Moscow, 119049, Russia ABSTRACT Shape memory effect in pure ultra-high molecular weight polyethylene (UHMWPE) has been studied using dynamic mechanical analysis. Temperature dependencies of properties that define functional characteristics of shape memory polymers (SMP) such as recovery stress, recovery strain and activation temperature of transition were determined for UHMWPE. The recovery stress in UHMWPE deformed by 200% achieved rather high values, up to 7 MPa. INTRODUCTION Shape memory polymers, SMP, are smart materials that can be programmed to change shape when exposed to an external stimulus. SMP can take two or more [1] shapes: one permanent and one or more temporary. Temporary shape is created by deformation of a pre-fabricated sample. Temporary shape can be maintained for a long period of time until the sample is exposed to an external stimulus that the particular material was designed for: heat, electricity, light, magnetism, and even moisture [2]. After activation of shape recovery processes the polymer goes back to its permanent shape. The deformation-recovery cycle can be repeated multiple times. SMP are used in heat-shrinkable tubes, packaging, smart fabrics, deployable aerospace structures, electrical industries, biomedical devices [3], etc. Cross-linked polyethylene is a widely used shape-memory material. The material can be deformed up to 300% of the permanent shape dimensions above the melting temperature and should then be fixed firmly during cooling to “memorize” the temporary shape [4]. Upon reheating polyethylene returns to its permanent shape, developing recovery stress of up to 3 MPa [3]. Ultra-high molecular weight polyethylene, UHMWPE, stands out in this family of materials due to its high molecular mass, excellent mechanical and tribological properties. It has been widely used as biocompatible antifriction material in joint replacements [5]. Shape memory effect of UHMWPE has only been mentioned a few times in literature over the several past decades. The authors of studies [6,7] used the shape-memory property of UHMWPE to remove the effect of plastic deformation of the polymer surface after tribological tests to see the actual wearing of the material. Study [8] looked at memory effects related to restoring equilibrium viscoelastic properties in polyethylene and UHMWPE. Literature does not have data on activation temperature of transition, recovery stress and recovery strain for UHMWPE that could allow comparing it to other SMP. The current work is aimed at studying the main properties of UHMWPE characterizing its shape-memory behavior.
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EXPERIMENTAL DETAILS Shape memory effect was studied using Ticona GUR 4120 UHMWPE powder with an average molecular weight of 5·106 g/mol. UHMWPE powder was thermo pressed into samples with dimensions of 78
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