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Photoinduced Stress Relaxation and Actuation in Crosslinked Polymers

Christopher N. Bowman, Timothy F. Scott, and Rocky B. Draughon. Chemical and Biological Engineering, University of Colorado, Campus Box 424, Boulder, CO 80309

The inherent limitations in chemically crosslinked polymers include stresses introduced by post gelation volume changes during polymerization and the difficulties in changing the crosslinked polymer's shape without a corresponding loss of material properties or significant stress development. We recently demonstrated [1] a crosslinked polymer which, upon exposure to light, exhibits dramatic stress and/or strain relaxation without any concomitant change in material properties, achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix which then diffuse via addition-fragmentation chain transfer of mid-chain functional groups. Polymers able to undergo actuation in response to light have numerous applications including valves and switches in microdevices and stents and wound closures in the biomedical field. A practical polymeric system would be stable for extended periods under an array of conditions, possess excellent mechanical properties and move rapidly in response to irradiation at long wavelengths. We have used the controlled relief of stress in rubbery, crosslinked polymers to produce dramatic actuation by relieving introduced stress using irradiation. The sensitivity of this method to light is significantly greater than previously developed photoinduced actuation techniques as the stress relief and actuation processes result from the amplification of one introduced radical producing many chain transfer events. [1] T.F. Scott, A.D. Schneider, W.D. Cook and C.N. Bowman, Science, 308, 1615 (2005).