Mechanical Properties of Poly(N-isopropyl-acrylamide-co- itaconic acid) Hydrogels JOSE ´ R. QUINTANA, NORA E. VALDERRUTEN, ISSA KATIME Grupo de Nuevos Materiales, Departamento de Quı ´mica Fı ´sica, Facultad de Ciencias, Campus de Leioa, Universidad del Paı ´s Vasco, Apartado 644, 48080 Bilbao, Spain Received 16 July 2001; accepted 27 November 2001 Published online 2 July 2002 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/app.10852 ABSTRACT: Hydrogels of N-isopropylacrylamide and itaconic acid were synthesized with different monomer ratios and with two crosslinking agent concentrations. The different xerogels were immersed in water and the swelling process was conducted up to equilibrium conditions at two temperatures (22 and 37°C). These temperatures are lower and higher than the transition temperature shown by PNIPA hydrogels. The mechanical properties of the different solvated hydrogels were examined by oscillatory shear measurements at 22 and 37°C. The copolymer volume fraction and the elastic storage modulus of the hydrogels decreased as the itaconic acid percentage in the copolymer increased. This behavior was attributed to the higher hydrophilic character of the itaconic acid comonomer. Effective crosslinking density, molar mass between crosslinks, and the polymer–solvent interaction parameter were determined from the experimental values of the elastic storage moduli and the copolymer volume fractions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2540 –2545, 2002 Key words: hydrogels; copolymerization; crosslinking; drug delivery systems; swell- ing INTRODUCTION Polymers and copolymers that exhibit the ability to absorb a considerable quantity of water with- out actual dissolution are termed hydrogels. Such materials are usually crosslinked and therefore can be considered as water-swollen polymer net- works. Hydrogels have become of major interest because polymers are increasingly used in medi- cal applications. Hydrogels are being used nowa- days for membranes, catheters, contact lenses, and drug-delivery systems. 1–4 The equilibrium water content of the gel is a basic property, given that several useful applica- tions require a suitable water content. Hydrogels with a higher water content are generally more advantageous because they show a higher perme- ability and biocompatibility. However, on increas- ing the water content the mechanical strength of the hydrogel decreases. To obtain a gel with a high water content and an acceptable mechanical strength, a composite molecular structure is nec- essary. This structure will consist of a hydrophilic component that absorbs large amounts of water and a hydrophobic component that improves the mechanical strength. 5 In recent years different types of hydrogels sensitive to temperature or pH have been synthe- Correspondence to: I. Katime (qfpkaami@lg.ehu.es). Contract grant sponsor: Ministerio de Ciencia y Tecnolo- gı ´a; contract grant number: MAT2000-0768-C02-02. Contract grant sponsor: Vicerrectorado de Investigacio ´ n de la Universidad del Paı ´s Vasco. Contract grant sponsor: Departamento de Educacio ´n, Uni- versidades e Investigacio ´n del Gobierno Vasco. Contract grant sponsor: Programa de Ciencia y Tecnologı ´a para el Desarrollo (CYTED). Contract grant sponsor: COLCIENCIAS. Journal of Applied Polymer Science, Vol. 85, 2540 –2545 (2002) © 2002 Wiley Periodicals, Inc. 2540