Effect of chemical crosslinking on the free-strain recovery characteristics of amorphous shape-memory polymers
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Effect of chemical crosslinking on the free-strain recovery characteristics of amorphous shape-memory polymers Alicia M. Ortega1, Christopher M. Yakacki2,3, Sean A. Dixon2,3, Alan R. Greenberg1, and Ken Gall2,3,4 1 Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, U.S.A. 2 MedShape Solutions Inc., Atlanta, GA 30318, U.S.A. 3 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A. 4 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A. ABSTRACT The goal of this study is to investigate the fundamental relationship between the extent of crosslinking and shape-memory behavior of amorphous, (meth)acrylate-based polymer networks. The polymer networks were produced by copolymerization of tert-butyl acrylate (tBA) and poly(ethylene glycol) dimethacrylates of differing molecular weights (PEGDMA). Polymer compositions were tailored via the amount (weight percent (wt%)) and molecular weight of the PEGDMA crosslinking agents added to produce four materials with varying levels of crosslinking (0, 2, 10, and 40 wt% crosslinking agent corresponding to 0, 0.6, 3.2, and 16.6 mole%) and nearly equal glass transition temperatures (Tg). The effect of crosslinking on deformation limits and free-strain recovery is evaluated. Near complete strain recovery was demonstrated by all materials; however, absolute recovery strain decreased with increasing crosslinking due to a corresponding decrease in strain-to-failure. The results provide insights regarding the link between polymer structure, deformation limits, and strain-recovery capabilities of this class of shape-memory polymers. An improved understanding of this relationship is pivotal for optimizing system response for a wide range of shape-memory applications. INTRODUCTION Shape-memory materials are defined by their ability to recover to an original, permanent shape from a temporary, stored shape with the application of an external stimulus such as an increase in temperature. Shape-memory polymers have been proposed for use in a number of applications such as microfluidic devices [1], stents [2-3], blood clot removal devices [4-5], and orthopedic fixation devices [6], all of which take advantage of the actuation capabilities of these functional materials. Amorphous, chemically crosslinked networks are one class of polymeric materials that have been shown to evidence shape-memory behavior [8]. Listed advantages of this class of shape-memory polymers include good shape recovery, tunable recovery work capacity (by varying crosslinking degree), and no chain slippage [8]. Polymer systems in which the physical, thermomechanical, and shape-memory properties of the polymers could be easily and systematically modified (tuned) have been suggested for use in shape-memory polymer design since they would allow for the control of properties to meet the different requirements associated with a wide range of potential applications [7-8]. Polymer systems based on t
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