Effect of constrained amorphous region on properties of acid-base polyelectrolyte membranes based on sulphonated poly(ether ether ketone) and a nonconjugated diamine

The phenomenon of polymer chains being restrained from participating in relaxation mechanisms has been discussed for a number of situations in the field of polymer science. In the present study we invoke the same as a mechanistic origin of property improvement in an acid-base polymeric membrane for fuel cell application. Sulfonated polyether ether ketone (sPEEK) and a non conjugated diamine (3, 3′ dichloro 4,4′ diaminodiphenylmethane) were used as the acid and base components, respectively to prepare the membranes by a facile solvent casting process. The ionic cross-link induced constrained amorphous region (C) in the acid-base membranes is quantified as a function of basic component loading from the relative depression in the dissipation factor, tan δ using dynamic mechanical analysis (DMA); found to be a staggering 62% at 25% basic component loading. The significantly higher C values are attributed to strong acid-base interactions as established from morphological investigations by field emission scanning electron microscopy (FESEM), FTIR studies and IEC measurements. Consequent to the existence of constrained amorphous region of reduced mobility, significant improvement in thermomechanical properties, (~5 fold increase in storage modulus, 2 fold increase in tensile strength and ~100 °C increase in glass transition temperature compared to pristine sPEEK) thermal, dimensional as well as oxidative stability, of the acid-base membranes are observed compared to the pristine SPEEK membrane. We envisage that the hitherto unexplored acid-base membranes derived from an acidic polymer and a small molecule non-conjugated diamine can also be suitable candidates for fuel cell applications. In addition these fundamental studies are expected to provide insights into mechanistic origins of the property improvements in acid-base membranes.