Controlling Crystallinity in Graft Ionomers, and Its Effect on Morphology, Water Sorption, and Proton Conductivity of Graft Ionomer Membranes

• Published in: Chemistry of materials 2013-05, Vol.25 (9), p.1935-1946
• Main Authors: Yang, Ami C. C, Narimani, Rasoul, Zhang, Zhaobin, Frisken, Barbara J, Holdcroft, Steven
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm4005932

To gain insight into the role of crystallinity and morphology on proton transport through solid polymer electrolytes, we synthesized graft copolymers, poly(vinylidene difluoride-co-chlorotrifluoroethylene)-g-polystyrene [P(VDF-co-CTFE)-g-PS], consisting of a hydrophobic, fluorous backbone and styrenic graft chain of varied length (DPstyrene = 39, 62, and 79), by graft atom transfer radical polymerization (ATRP). The polystyrene graft chains were subsequently sulfonated to different degrees to provide three series of polymers with controlled ion exchange capacity (IEC). The crystallinity and morphology of solution-cast membranes were examined by XRD and TEM, respectively. The grafting of the parent side chain is found to hinder crystallization of the fluorous backbone and the impact of the degree of sulfonation of the side chain on the crystallinity of the polymer is dependent on the graft length: No impact is found for medium and long graft lengths, but for short graft length copolymers (PS39), the degree of crystallinity in the sulfonated membranes is twice that of the unsulfonated membrane. A phase-separated morphology consisting of 2–5 (±1) nm ion-rich domains is observed for all of the graft copolymers. These graft copolymers allow access to very high IEC membranes (>3 mmol/g), which are insoluble in water. The shorter graft length series, P(VDF-co-CTFE)-g-SPS39, swells less in the intermediate IEC range (