Chemical modification of copolyimides with bulky pendent groups: Effect of modification on solubility and thermal stability E.M. Maya * , A.E. Lozano, J. de Abajo, J.G. de la Campa Dpto. de Quı ´mica Macromolecular, Instituto de Ciencia y Tecnologı ´a de Polı ´meros, Consejo Superior de Investigaciones Cientı ´ficas, c/Juan de a Cierva 3, Madrid 28006, Spain Received 1 December 2006; received in revised form 18 December 2006; accepted 15 January 2007 Available online 15 August 2007 Abstract Two novel copolyimides bearing bulky pendent groups have been prepared by chemical modification of a copolyimide precursor containing carboxylic acid groups. The incorporation of the bulky groups was achieved by esterification of the copolyimide containing carboxylic acid groups with 4-tert-butylbenzyl alcohol. By controlling the carboxylic acid/alcohol ratio, two different degrees of modification were obtained. The actual composition of the modified polymer was estimated by 1 H NMR. They showed better solubility than the copolyimide precursor, where 100% modification yielded the copolyimide with the highest solubility properties. Thermal analyses indicated that the incorporation of bulky pendent groups has a moderate effect on decreasing the thermal degradation temperature and the glass transition temperature. Ó 2007 Published by Elsevier Ltd. Keywords: Polyimides; Bulky pendent groups; Esterification; Solubility; Thermal properties 1. Introduction Aromatic polyimides have great potential as high-perfor- mance polymers because of their good thermal and chemical stabilities, excellent mechanical properties, and high glass transition temperatures [1]. However, it is also well known that an important drawback of polyimides is their poor solubil- ity, which makes their processing to membranes difficult. For this reason, a goal in the chemistry of polyimides is to improve their solubility in organic solvents, which means their process- ability, while maintaining their inherent properties. Several modifications of the structure of polyimides, such as introduc- tion of hinge linkages in the main chain [2e4] or the incorpo- ration of asymmetric monomers [5,6], have been extensively adopted to address this problem. Another interesting approach is the introduction of pendent groups in the polymer that can disrupt chain packing efficiency and increase the solubility. The general way to introduce bulky pendent groups into polyimides is by using monomers that bear those groups through a polycondensation reaction. Thus, bulky groups such as phenyl [7], tert-butyl [8] or cardo [9] have been suc- cessfully introduced into polyimides through monomers con- taining those functionalities. However, some monomers are difficult to synthesize or purify, which results in polyimides with low molecular weights and, as a consequence with bad film-forming ability. A very convenient approach to overcome these problems consists of the reaction of a bulky group-con- taining reagent with a high molecular weight pre-formed pol- yimide. Recently, we have reported a set of polyimides containing halogen atoms in the repeat unit that could be re- placed through Pd-catalyzed coupling reactions to introduce side bulky groups [10]. Nevertheless, the halogenated and modified polyimides showed only moderate molecular weights, thus giving films with poor mechanical properties. In this work, a random copolyimide containing free carbox- ylic acid groups, PI-0 (Scheme 1) has been synthesized and used as precursor to introduce bulky pendent groups. Polyimides and copolyimides containing free carboxylic groups have previously been used in many instances to obtain crosslinked polymers for * Corresponding author. Tel.: þ34 91 562 2900x350; fax: þ34 91 564 4853. E-mail address: evamaya@ictp.csic.es (E.M. Maya). 0141-3910/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2007.01.042 Available online at www.sciencedirect.com Polymer Degradation and Stability 92 (2007) 2294e2299 www.elsevier.com/locate/polydegstab