Proton-conducting polymer electrolytes based on phosphoric acid

The synthesis, thermal, mechanical and conduction properties of blends of a cationic polyelectrolyte, poly(diallyldimethylammonium-dihydrogenphosphate ), 'PAMA+ H2PO4-, and phosphoric acid are reported. Blends of 'PAMA-H(2)PO( 4)(-)xH(3)PO(4)' with 0.5 less than or equal to x less than or equal to 2.0 can be cast into amorphous films, which are stable up to 150 degrees C. DS C results show that the softening temperatures of the blends decrease from 126 degrees C for x = 0.5 to -23 degrees C for x = 2.0. The dc conductivity increases with x and reaches 10(-4) S/cm at ambient temperature and 10(-2) S/cm at 100 degrees C for PAMA(+)H(2)PO(4)(-)2H(3)PO(4). The H-1- and P-31 -self-diffusion coefficients of PAMA(+)H(2)PO(4)(-)xH(3)PO(4) for x = 1,2 w ere determined by PFG-NMR. D-(H) is always at least one order of magnitude larger than D-(P), which means that inter-phosphate H+ transfer plays an im portant role in the samples. D-(H) coincides quite well with the conductivi ty diffusion coefficient D-sigma which is obtained from the conductivity da ta via the Nernst-Einstein relationship, assuming that the charge carrier c oncentration is equal to the repeat unit concentration. Since with the same assumption the charge carrier concentration is only (3x + 2)(-1) times the concentration of all the phosphate protons, the result D-sigma/D-(H) appro ximate to 1 is rather surprising and indicative of cooperative proton trans fer which does not contribute to the conductivity. D-(H)/D-(P) in the blend s is much larger than D-(H)/D-(P) in pure phosphoric acid. This means that the phosphate moieties are considerably more immobilized in the blends as c ompared to H3PO4. This immobilization effect is more pronounced in blends w ith low phosphoric acid content and decreases with increasing temperature. (C) 1999 Elsevier Science B.V. All rights reserved.