Impact of degree of sulfonation on microstructure, thermal, thermomechanical and physicochemical properties of sulfonated poly ether ether ketone

Effect of degree of sulfonation on dynamic mechanical properties, dehydration rate, crystallinity, thermal stability and other fuel cell properties (e.g. proton conductivity, ion exchange capacity, water uptake capacity, etc.) of SPEEK membrane are studied and correlated with microstructure. With increasing degree of sulfonation (DS), alpha-relaxation peak shifts from 135 to 166 degrees C, while beta-relaxation changes from 76 to 37 degrees C. This peak attribution indicates a plasticizing effect of -SO3H group within the crystalline region. Thermal stability of SPEEK decreases while peak degradation temperature for desulfonation and glass transition temperature increase from 320-340 and 191-210 degrees C, respectively with increasing DS from similar to 72 to similar to 128%. Likewise, small angle X-ray scattering peak at scattering vector of 0.2 (A) over circle (-1) shifts to 0.28 (A) over circle (-1) with an associated increase in intensity due to the evolution of microstructure. Increase in number of scattering centers with progressive sulfonation leads to ionic aggregation, which is the main responsible factor for such evolution. Ionic aggregation is further confirmed by the occurrence of two closely spaced melting peaks in the range of 322-328 and 361-365 degrees C in differential scanning calorimetry analysis. Dehydration rate gradually increases with increasing DS. Ion exchange capacity, proton conductivity, water uptake and dehydration rate increase from similar to 0.49 to similar to 2.43 mmol g(-1), similar to 1.07 x 10(-4) to similar to 2.9 x 10(-2) Scm(-1), similar to 20.36 to similar to 135.3% and similar to 0.296 to similar to 0.51 sec(-1), respectively with increase in DS from similar to 72 to similar to 128%. Microstructure become more homogeneous (less phase separated) at DS of similar to 128% resulting in continuous ionic channels for proton conduction. (C) 2016 Elsevier Ltd. All rights reserved.