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Published February 20, 2007 | public
Journal Article

Self-Assembly and Stress Relaxation in Acrylic Triblock Copolymer Gels

Abstract

The structure and relaxation behavior of thermoreversible gels made with poly(methyl methacrylate)−poly(n-butyl acrylate)−poly(methyl methacrylate) [PMMA−PnBA−PMMA] triblock copolymers in 2-ethylhexanol, a midblock selective solvent, were studied by small-angle X-ray scattering (SAXS) and rheology. Effects of endblock length, endblock fraction, and gel concentration on the gel properties were investigated. A dramatic decrease in SAXS intensity was observed over a 20 °C interval where the gel transitions smoothly from elastic to viscous behavior. SAXS patterns were fit with a Percus−Yevick disordered hard-sphere model from which aggregation number and average domain spacing were calculated. Aggregation number increases with increasing gel concentration and endblock length. Increasing the endblock length from 9K to 25K increases the relaxation time of a gel with a polymer volume fraction of 0.15 by a factor of 10^6. For a given triblock endblock fraction and molecular weight, the micelle aggregation number is strongly correlated to the gel relaxation time. Arrhenius behavior with an effective activation energy of ~550 kJ/mol was observed for all triblocks and concentrations. This very high effective energy barrier describes gels relaxation behavior over a 40 °C temperature range, where the relaxation times vary by a factor of 10^(10).

Additional Information

© 2007 American Chemical Society. Received August 29, 2006; Revised Manuscript Received December 7, 2006; Published on Web 01/24/2007. We are grateful to Kuraray Co. of Japan for providing the triblocks used in these studies and to Steven Weigand for all his help collecting the SAXS data. This material is based upon work supported under a National Science Foundation Graduate Research Fellowship and by the Northwestern University Materials Research Center, through the NSF MRSEC program DMR-0520513. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) Synchrotron Research Center located at Sector 5 of the Advanced Photon Source. DND-CAT is supported by the E.I. DuPont de Nemours & Co., The Dow Chemical Company, the U.S. National Science Foundation through Grant DMR-9304725, and the State of Illinois through the Department of Commerce and the Board of Higher Education Grant IBHE HECA NWU 96. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract W-31-10.

Additional details

Created:
August 19, 2023
Modified:
October 26, 2023