Controlling the Physical Properties of Random Network Based Shape Memory Polymer Foams
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Controlling the physical properties of random network based shape memory polymer foams Pooja Singhal1, Thomas S. Wilson2 and Duncan J. Maitland1, 2 1 Texas A&M University, Biomedical Engineering Department. 337 Zachry Engineering Center, MS 3120, College Station Texas- 77843. 2 7000 East Avenue, Lawrence Livermore National Laboratory, Livermore, CA- 94551. ABSTRACT Shape memory polymers (SMPs) are increasingly being considered for use in minimally invasive medical devices. For safe deployment of implanted devices it is important to be able to precisely control the actuation temperature of the device. In this study we report the effect of varying monomer composition on the glass transitions/actuation temperatures (Tg) of novel low density shape memory foams. The foams were based on hexamethylenediisocyanate (HDI), triethanolamine (TEA) and tetrakis (2-hydroxyl propyl) ethylenediamine (HPED), and were produced via a combination of chemical and physical blowing process. The process for postfoaming cleaning was also varied. Foams were characterized by DSC, DMA, and for shape memory. No clear trends were observed for foam samples without cleaning, and this was attributed to process chemicals acting as plasticizers. In foams cleaned via washing and/or sonication, the Tg was observed to decrease for compositions that were higher in the TEA content. Also, no change in shape memory behavior was observed for varying compositions. This work demonstrates the ability to tailor actuation transition temperature while maintaining shape memory behavior for low density foams suitable for aneurysm treatment. INTRODUCTION Shape memory polymers are synthetic polymeric materials that can be deformed to a stable secondary shape from their original primary shape. They retain the secondary shape until an appropriate stimulus is provided. Their response to a variety of stimuli, i.e. light, heat, electricity etc. has been reported 1-3. Amongst these, thermally responsive SMPs are the most widely studied polymers. Thermal shape-memory behavior has been observed in a number of polymer systems comprised of covalently crosslinked amorphous or semi-crystalline networks as well as physically crosslinked glassy or semi-crystalline block copolymers 4. Considering a potential deployment in human body, several safer methods of heating the SMPs have been investigated as possible triggering mechanisms for these materials, for e.g. electric, electromagnetic and radio frequency 5-7 . But still, thermal damage to tissue is a concern in medical procedures, and it is best to minimize it. Hence, owing to their promising future in biomedical applications 8, controlling the actuation temperature to be close to body temperature is an important requirement for safe deployment of biomedical devices based on these SMPs. Here we report the effect of changing monomer functionality on the shape memory behavior of highly crosslinked amorphous polymers with a very regular network structure 9. These materials
are synthesized from hexamethylenediisocyanate
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