Synthesis and Characterization of Poly(vinylidene fluoride)-g-sulfonated Polystyrene Graft Copolymers for Proton Exchange Membrane

Abstract

A series of poly(vinylidene fluoride)-g-sulfonated polystyrene (PVDF-g-SPS) graft copolymers were systematically synthesized and examined with the focus of understanding how the polymer microstructure (backbone molecular weight, graft density, graft length, sulfonic acid concentration, ion exchange capacity, etc.) affects their morphology, water uptake, and proton conductivity under various environmental conditions (temperature and relative humidity). The synthesis of these relatively well-defined graft structures involves three reaction steps, including the preparation of PVDF copolymers containing a few mol% of chlorotrifluoroethylene (CTFE) units, the ATRP graft-from reaction to incorporate several polystyrene side chains, and the subsequent sulfonation reaction on the PS side chains. The PVDF-g-SPS graft copolymer with a combination of a high PVDF backbone molecular weight (M n >300 000 g/mol), very low SPS graft density (0.3 mol%), and high graft length (SPS content >30 mol%) presents an interesting material for the proton exchange membrane (PEM). This graft copolymer self-assembles into a microphase-separated morphology with randomly oriented long-range lamella/cylinder ionic channels (with small widths) that are imbedded in the hydrophobic semicrystalline PVDF matrix. This graft copolymer morphology offers a high ion exchange capacity (IEC = 2.75 mmol/g) and resistance to excessive water swelling, which yields notably higher proton conductivity than Nafion under 30−120 °C and high humidity conditions.

Nanostructure, Morphology, and Properties of Fluorous Copolymers Bearing Ionic Grafts

“…From Arrhenius plots (not shown), the activation energy for proton transport are found to be 27.5, 17.3, and 18.3 kJ/ mol for the graft membranes possessing IECs of 1.72, 1.93, and 2.24 mmol/g, respectively. These values are comparable with that found for Nafion 117, 14.1 kJ/mol (reported values are 7.8, 90 9.6, 88 and 13.5 91 kJ/mol), and with the reported values of 12.9 kJ/mol for P(VDF)-g-SPS by Zhang et al 72 The activation energies for proton conduction through P(VDF-co-CTFE)-g-SPS membranes show decreasing values (from 27.5 to 18.3 kJ/mol) as IEC increases. This observation agrees with the experimental data for P(VDF-co-HFP)-b-SPS diblock copolymer membranes where the activation energy is reported to fall from 25.7 to 17.1 kJ/mol as IEC increases from 0.72 to 1.18 mmol/g.…”

Ion Transport by Nanochannels in Ion-Containing Aromatic Copolymers

“…108 108,110 Moreover, sulfonic acids are considered as strong acids in PEMs (the pK a of aryl sulfonic acid has been estimated to be on the order of −1), 110 while quaternary ammoniums are comparatively weak bases with a pK b on the order of 4. Therefore, dissociation of the quaternary ammonium ions presents a considerable theoretical barrier for efficient hydroxide conduction in an AEM.…”

Compositional effect on the properties of sulfonated and nonsulfonated polymer blend membranes for direct methanol fuel cell

“…Moreover, it is possible to predict sorption properties such as water, methanol uptake by measuring IEC which corresponds to degree of sulfonation which determines sorption properties. 22 Thus, in this paper IEC was compared with water uptake and methanol uptake ( Figure 10).…”