Rubber_Elasticity_PNIPPA_gels_Macromolecules_Gundogan_2002.ppt

Rubber_Elasticity_PNIPPA_gels_Macromolecules_Gundogan_2002.ppt

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Rubber Elasticity of Poly( N -isopropylacrylamide) Gels at Various Charge Densities Nermin Gundogan, Demet Melekaslan, and Oguz Okay* Department of Chemistry, Istanbul Technical University, 80626 Maslak, Istanbul, Turkey Received January 29, 2002; Revised Manuscript Received April 21, 2002 ABSTRACT: The mechanical properties and the swelling behavior of a series of N -isopropylacrylamide copolymers with the sodium salt of 2-acrylamido-2-methylpropanesulfonic acid were investigated. The increase of the equilibrium gel volume V eq with the charge density fN was considerably weaker ( V eq fN 0.75 ) than expected assuming Gaussian statistics ( V eq fN 1.5 ). This deviation is interpreted as the non- Gaussian elasticity of the hydrogels swollen to equilibrium. The exponent 0.75 found indicates that the elastic free energy Δ G el scales with the linear swelling ratio R as Δ G el R 4 , thus deviating from Δ G el R 2 derived using Gaussian chain statistics. This expression for the elastic free energy explains the behavior of elastic modulus data of the equilibrium swollen hydrogels. Introduction Poly( N -isopropylacrylamide) (PNIPA) gel is a typical temperature-sensitive gel exhibiting volume phase tran- sition at approximately 34 °C. 1,2 The temperature sensitivity of PNIPA gels has been studied both for its fundamental interest and technological application. 3 - 6 These materials are useful for drug delivery systems, separation operations in biotechnology, processing of agricultural products, sensors, and actuators. The ex- perimental studies of PNIPA gels reported in the past focused essentially on their swelling behavior. To in- terpret the swelling data of PNIPA hydrogels, a number of theories were used. 2,7 - 10 However, fitting the existing theories to the swelling data requires PNIPA network cross-link density as an adjustable parameter and does not allow definitive conclusions regarding the suitability of the theories. Such conclusions are more appropriately obtained from independent elasticity measurements. Although much has been reported on the swelling behavior of PNIPA gels, only a few studies reported their mechanical properties. 11 - 13 Takigawa et al. ob- served that the elastic modulus of PNIPA gels in the collapsed state is much higher than in the swollen state due to the formation of physical cross-links at high PNIPA network concentrations. 14 Muniz and Geuskens investigated the elastic modulus of semiinterpenetrated polymer networks of cross-linked polyacrylamide (PAAm) having linear N -isopropylacrylamide (NIPA) homopoly- mer inside. 15 They observed a decrease in the swelling ratio and an increase in the modulus of the networks with an increasing NIPA concentration due to the less hydrophilic character of NIPA homopolymer.
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This note was uploaded on 07/20/2011 for the course EMA 6165 taught by Professor Brennan during the Spring '08 term at University of Florida.

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Rubber_Elasticity_PNIPPA_gels_Macromolecules_Gundogan_2002.ppt

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