Structural investigation of polystyrene grafted and sulfonated polytetra¯uoroethylene) membranes Mohamed Mahmoud Nasef * Chemical Engineering Program, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Received 31 January 2001; received in revised form 14 June 2001; accepted 21 June 2001 Abstract Structural investigation of polystyrene grafted and sulfonated polytetra¯uoroethylene) PTFE) membranes pre- pared by radiation-induced grafting of styrene onto commercial PTFE ®lms and subsequent sulfonation was carried out by dierential scanning calorimetry and X-ray diraction. The eect of the structural changes taking place in the membranes during the preparation procedure grafting and sulfonation) and the variation of the degree of grafting on melting temperature T m ), glass transition temperature T g ), heat of melting DH m ), and degree of crystallinity was studied. The melting temperature T m ) was found to be independent of the degree of grafting unlike glass transition temperature T g ), which was found to be a function of the degree of grafting. Moreover, the degree of crystallinity of the membranes was found to decrease with the increase in the degree of grafting. The results of this work suggest that grafting takes place in the entire amorphous region without any signi®cant disruption in the crystalline structure of PTFE ®lm and the decrease in the degree of crystallinity is mainly attributed to the dilution eect. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Structural investigation; Polystyrene grafted and sulfonated PTFE membranes; DSC; XRD 1. Introduction Sulfonic acid membranes have received a considera- ble attention over the last decade due to their applica- bility to various electrochemical cells such as water electrolyzers and proton exchange membrane PEM) fuel cells [1±3]. In such cells, membrane plays a vital role as it severs as a separator and electrolyte. Na®onâ and Dowâ commercial membranes have been identi®ed for use in PEM fuel cells where high current density and low resistance are required [4]. However, these membranes are expensive and other membranes are needed to pro- mote the PEM fuel cell systems. Many eorts have been devoted world wide to de- velop very speci®c and cost-eective sulfonic acid mem- branes. The latest development of these membranes with respect to methods of preparation, properties and po- tential use in PEM fuel cells have been recently reviewed by Savadogo [5]. Modi®cation of polymer ®lms by ra- diation-induced grafting of chemical functionality is an advantageous technique to prepare these membranes, particularly, in terms of the ability to control the com- position and properties of the membranes by variation of grafting parameters [6]. For stability reasons, sulfonic acid groups have been identi®ed as functional groups and ¯uorinated ®lms as base polymers in these mem- branes. However, grafting of a sulfonated monomer is very dicult due to the high incompatibility between sulfonic acid groups strongly hydrophilic) and the poly- mer ®lms strongly hydrophobic). Hence, styrene is often grafted onto ¯uorinated polymer ®lms to pro- duce graft copolymers that can be easily sulfonated in a post-grafting reaction using chlorosulfonic acid [7]. European Polymer Journal 38 2002) 87±95 www.elsevier.com/locate/europolj * Tel.: +6-5-3721131; fax: +6-5-3721111. E-mail address: mahmoudeithar@mailcity.com M.M. Nasef). 0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0014-305701)00175-6