Absence of Ionomer Peaks in X-ray Scattering Does Not Imply Absence of Nanophase Separation in Ionomers

Abstract

A peak at low wavevectors in the X-ray scattering of hydrated, ionic polymers is typically indicative of nanophase separation between the hydrophobic and hydrophilic domains. Here we use atomistic molecular dynamics simulations to show that low intensity or absent ionomer peaks in the X-ray scattering from hydrocarbon-based ion exchange membranes do not necessarily indicate a lack of nanophase separation. Instead, there is often a lack of contrast between the hydrophobic and hydrophilic domains in these systems, due to the similar X-ray atomic form factors of carbon and oxygen. We show that when the backbone hydrogen atoms in three simulated ionic polymers scatter like fluorine instead of like hydrogen, large ionomer peaks are observed. Similarly, when we assume the water in these membranes is fully deuterated, calculations of the expected small angle neutron scattering also show clear ionomer peaks. The correlation lengths calculated from neutron scattering agree with those from the water partial X-ray structure factors and follow expected trends with increasing water content.