186 Research Article Received: 16 March 2010 Revised: 18 April 2010 Accepted: 10 May 2010 Published online in Wiley Online Library: 21 September 2010 (wileyonlinelibrary.com) DOI 10.1002/pi.2925 Comparative investigations of radiation-grafted proton-exchange membranes prepared using single-step and conventional two-step radiation-induced grafting methods Mohamed Mahmoud Nasef, a,b* Hamdani Saidi a,b and Khairulzaman Mohd Dahlan c Abstract Proton-exchange membranes containing poly(styrene sulfonic acid) grafts hosted in poly(vinylidene fluoride) (PVDF) films were prepared using two radiation-induced grafting methods: a single-step grafting method (SSGM) involving grafting of sodium styrene sulfonate onto electron beam (EB)-irradiated PVDF films and a conventional two-step grafting method (CTSGM) in which styrene monomer is grafted onto EB-irradiated PVDF films and subsequently sulfonated. Differential scanning calorimetry, universal mechanical testing and scanning transmission electron microscopy were used to evaluate the thermal, mechanical and structural changes developed in the membranes during the preparation procedures. Physicochemical properties such as water uptake, hydration number and ionic conductivity were studied as functions of ion-exchange capacity and the results obtained were correlated with the structural changes accompanying each preparation method. Membranes obtained using the SSGM were found to have superior properties compared to their counterparts prepared using the CTSGM suggesting the former method is more effective than the latter for imparting desired functionality and stability properties to the membranes. c  2010 Society of Chemical Industry Keywords: single-step radiation-induced grafting method; conventional two-step grafting method; poly(vinylidene fluoride)-graft- poly(styrene sulfonic acid); physicochemical properties INTRODUCTION Polymer electrolytes are a class of materials that are receiving increasing attention due to their wide-ranging applications in various electrochemical devices and chemical processes. Proton- exchange membranes which serve as separators and proton conductors in proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are currently the most widely researched solid polymer electrolytes. Many studies using various approaches have been conducted to identify new proton- conducting polymer materials possessing high ionic conductivity coupled with chemical stability and cost effectiveness. 1–9 This was prompted by the inherently high cost of commercially available perfluorosulfonic acid membranes (e.g. Nafion and Aciplex) arising from the fluorine chemistry used in their production. Radiation-grafted proton-exchange membranes containing sulfonic acid have been recently investigated as interesting alternative cost-effective proton-conducting materials after their potential to replace commercial membranes in PEMFCs and DMFCs had been explored. 10–13 The advantages of these membranes are derived from the simplicity of their preparation method, the ability to tailor their properties by controlling the reaction parameters, the absence of membrane shaping problems as preparation starts with a polymer film already in a membrane form and their low cost. 14 Radiation-grafted proton-exchange membranes are commonly prepared by grafting of styrene, styrene/crosslinker mixtures or substituted styrene monomers onto fluorinated polymer films, followed by a sulfonation reaction to confer on the polystyrene-grafted films pendant ionic sites. 10–14 Sulfonation is commonly performed using a strong sulfonating agent such as chlorosulfonic acid diluted with chemical-resisting solvents (e.g. 1,2-dichloromethane) under controlled conditions. However, achieving a degree of sulfonation of 100%, i.e. every benzene ∗ Correspondence to: Mohamed Mahmoud Nasef, Institute of Hydrogen Econ- omy, Universiti Teknologi Malaysia, 54000 Kuala Lumpur, Malaysia. E-mail: mahmoudeithar@fkkksa.utm.my a Institute of Hydrogen Economy, Universiti Teknologi Malaysia, 54000 Kuala Lumpur, Malaysia b Faculty of Chemical and Natural Resources Engineering, 81310 UTM Skudai, Johor, Malaysia c Radiation Processing Technology Division, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor, Malaysia Polym Int 2011; 60: 186–193 www.soci.org c  2010 Society of Chemical Industry