Functional Fatigue of Shape-Memory Polymers
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Functional Fatigue of Shape-Memory Polymers Christina Schmidt, Klaus Neuking, Gunther Eggeler Ruhr-University Bochum, Institute for Materials, 44780 Bochum, Germany ABSTRACT The present study represents a first step towards an understanding of what we refer to as the functional fatigue behaviour of shape-memory polymers. These materials have a processing shape B and a programmed shape A [1]. And when the material is exposed to an appropriate stimulus (in our case: heating above a critical temperature), a one way effect is observed: A → B (one way effect: 1WE). The objectives of the present study were to find out whether and how often programming can be repeated, whether repeated programming affects the 1WE and how much irreversible strain the material accumulates. We study the effect in dependence of different stress levels, and consider the effect of recovery temperature and recovery time. As a model material we examine the commercial amorphous shape-memory polymer Tecoflex® and subject it to 50 programming/1WE cycles. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. INTRODUCTION Shape-memory polymers (SMP) are polymers that are able to respond to external stimuli. SMP show a one way effect like shape-memory alloys. They can be deformed from a permanent shape to a temporary shape and recover by means of an external stimulus. For this direct thermal activation is the commonly used [2]. In contrast, alternative activation stimuli like inductive heating [3, 4], irradiation [5], magnetic [6] or electric fields [7] and pH changes [8] are not frequently applied. The shape-memory effect of SMP is based on the physical properties of the constituents of the polymer structure, chemistry and morphology. Thermoplastic SMP are typically (phase segregated) multiblock copolymers which consist of at least two types of segments [9-11]. The component with the higher thermal transition Thard, is referred to as hard segment. This hard segment (an elastomer) acts as physical netpoint and represents the part of the network which provides the mechanical strength of the material at temperatures above the transition temperature Ttrans of the SMP (Ttrans < T > Thard). The soft segment (a thermoplastic), having a lower thermal transition Ttrans, stabilizes the hard segment at low temperatures (T < Ttrans) and looses its strength for T > Ttrans. The transition temperature Ttrans of the SMP can be a glass transition or a melting point. The shape-memory effect occurs by exposing the SMP to a specific switching temperature, Tsw. Compared to Ttrans, the switching temperature Tsw is result of a thermomechanical test [12,13]. In its processed shape B, the chains of the shape memory polymer are randomly oriented. Below Tsw, the copolymer shows only little elasticity (high Young’s modulus). In contrast, above Tsw, the soft segment loo
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