The effect of crystallinity of HDPE precursor film on the properties of the resultant radiation-grafted anion-exchange membranes

Abstract

An advantage of using pre-irradiation grafting for the synthesis of anion exchange membranes (AEMs) is it allows the use of pre-made commercial substrates as the precursor film. To consider scaling-up radiation grafted AEM (RG-AEM) production, it is vital to understand how variation in manufacturing of the precursor film impacts the final RG-AEM properties. In this study, it is shown that commercially supplied 10 m length rolls of high density polyethylene (HDPE) films vary in crystallinity both within and between rolls (to a maximum of 17%). Consequently, the degree of grafting (DoG), ion exchange capacity (IEC), and conductivity of the resultant RG-AEMs were impacted. A negative correlation between crystallinity and DoG and conductivity was observed with Pearson R values of −0.5 and −0.6, respectively. A threshold effect was observed around 82% crystallinity: above this crystallinity value, the variations in DoG and conductivity decreased. A similar effect was also observed for IEC at a crystallinity threshold of approximately 81%. These findings suggest that fabrication of consistent property RG-AEMs requires a precursor film crystallinity of greater than 81%. Finally, a positive correlation was observed between crystallinity and water uptake (WU, Pearson R = +0.7), with more crystalline precursor film resulting in increased WU being observed in the resultant RG-AEMs. This counter intuitive correlation shows that alongside variations in bulk degrees of crystallinity values (considered in this paper), investigations will be required to account for crystallite size and distributions that can impact the hydration channel morphology within the RG-AEMs.