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College Park, Maryland      June 6 - 10 , 2004

M2-B6 (12:00 PM): USANS Investigation of Poly (N-isopropylacrylamide) Gels Prepared from Synchrotron-Radiation-Induced Polymerization in a Retrograde-Precipitation Environment

V. R. Tirumala (Advanced Photon Source, Argonne National Laboratory, Argonne, IL), L. Guo (Intense Pulsed Neutron Source, Argonne National Lab), G. T. Caneba (Department of Chemical Engineering, Michigan Technological University, Houghton MI 49931), D. C. Mancini (Advanced Photon Source, Argonne National Laboratory, Argonne, IL), P. Thiyagarajan (Intense Pulsed Neutron Source, Argonne National Lab), J. G. Barker (National Institute of Standards and Technology)

Poly (N-isopropylacrylamide) belongs to the class of thermoreversible hydrogels that can respond to a change in ambient temperature with a change in their physical characteristics. Due to their reversible response and hydrophilic behavior, hydrogels based on poly (N-isopropylacrylamide) are widely researched for applications ranging from controlled drug delivery to microfluidic valves and actuators. The stimuli-response behavior of these gels, which depends on their synthesis route, is therefore, of utmost importance. Conventional synthesis of hydrogels results in a nanoporous morphology that decreases the permeability of water and, therefore, their response rates. Gel response can thus be improved by introducing macropores into the polymer network. The morphology of the gel networks can be characterized by neutron scattering in the presence of a deuterated solvent.

We have recently prepared poly (N-isopropylacrylamide) hydrogels by a novel synchrotron-radiation-induced polymerization. The monomers were solution polymerized by exposing to x-rays from 2-BM-B bending magnet, at the Advanced Photon Source. The time of exposure affects the crosslinking density, since monomer crosslinking and polymer chain scission occur simultaneously during irradiation. The gels were also prepared above their lower critical solution temperature in water, which results in a microporous morphology that is evident only when the gels are swollen. Such unique morphology of these gels should enable them to be ultrafast in their stimuli-response. The inherent pore structure of the gels was characterized below and above their phase transition temperature by ultra-small-angle neutron scattering. The correlation length of the gel network was observed to be power-law dependent on the absorbed dose. The average pore size of the gels was also found to reversibly change by ~ 200 , depending on the monomer concentration and the absorbed dose.

Acknowledgments

Work at the Advanced Photon Source was supported by the Department of Energy, Office of Science, BES, under contract No. W-31-109-ENG-38. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

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