Hybrid membranes based on block co-polymer ionomers and silica gel. Synthesis and characterization C. del Rı ´o a, * , J.R. Jurado b , J.L. Acosta a a Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain b Instituto de Cera ´mica y Vidrio (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain Received 7 September 2004; received in revised form 20 February 2005; accepted 9 March 2005 Available online 25 April 2005 Abstract This work reports on the synthesis (via heterogeneous sulfonation) structural and electrical characterization of hybrid membranes based on block co-polymer ionomers (HSBR and EPDM) and silica gel prepared by sol–gel reaction. The structural characterization consisted of the analysis of their thermal and mechanodynamical transitions by DSC and DMA, respectively. The ion-exchange capacity of each polymer was determined by both titration and elemental analysis (EA). The introduction of sulfonic groups was verified by means of infrared spectroscopy (ATR). The electrical characterization was made using ac impedance at different hydration times, the conductivity being calculated from the corresponding impedance spectra. Finally, methanol crossover through the membranes was carried out, comparing the results obtained with Nafion 117. The results indicate the existence of a complex microstructure formed by different phases corresponding to both ionic and non-ionic blocks of the co-polymer as well as the aggregates or clusters owed to the electrostatic interaction among ion pairs. Conductivity values are similar to Nafion and they improve with hydration time for hybrid membranes due to the absorbent nature of the inorganic filler. Likewise, methanol crossover is lower than in Nafion, probably due to the barrier effect exerted by the non-sulfonated blocks of the co-polymer. q 2005 Elsevier Ltd. All rights reserved. Keywords: Block co-polymer ionomers; Ionic conductivity; Methanol crossover 1. Introduction Polymers with ionic groups attached to their structure, more known as ionomers, are indispensable electrolyte materials for numerous electrochemical applications such as batteries, sensors, low temperature fuel cells and in particular, direct methanol fuel cells (DMFC) that are already outlined as a future alternative energy source [1,2]. For this application, Nafion w (Dupont) is still the most frequently used membrane although it has mainly two disadvantages, high cost and high methanol permeability (methanol crossover) [3,4]. Current research is focused on the development of new polymer membranes to make the DMFC an economical and practical option [5–8]. In this sense, our group has been working on the synthesis and characterization of ionomers based on block co-polymers like an alternative to the commercial mem- brane Nafion [9–12]. Block co-polymers are constituted by two or more polymers blocks, placed throughout the same chain following an ordered sequence. When one of these blocks contains ionic groups attached to its structure, then we are talking about block co-polymer ionomers [13–19]. The interest for this type of materials is due, on the one hand, that non-ionic blocks can act as a barrier for methanol crossover and on the other hand, that the ordered microstructure can exert on ionic transport, definitively on the conductivity of the membrane. Thus, in this work we have carried out the synthesis (via heterogeneous sulfonation), the microstructural and elec- trical characterization and also the methanol diffusion through of two ionomers based on block co-polymers. The two most desirable functional groups for reacting with sulfonating agents are double bonds and aromatic groups, particularly phenyl groups. Poly(butadiene–styrene) block co-polymer possesses a two phase microstructure consisting of polystyrene (PS) domains dispersed in a rubbery Polymer 46 (2005) 3975–3985 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2005.03.033 * Corresponding author. Tel.: C34 91 562 2900; fax: C34 91 564 4853. E-mail address: cdelrio@ictp.csic.es (C. del Rı ´o).