1. 1Egyptian Petroleum Research Institute, Nasr City 11727, Cairo, Egypt; fax +20-2-274-7443
2. 2Laboratory of Advanced Polymers & Optimized Materials (LAPOM), Department of Materials Science and Engineering, University of North Texas, 1150 Union Circle # 305310, Denton, TX 76203-5017, USA, http://www.unt.edu/LAPOM/