A series of polymer electrolyte gels was made by cross-linking linear poly(ethylenimine) hydrochloride (LPEI-HCI, DP ca. 2000) malonaldehyde (generated in situ) in aqueous solutions containing varying concentrations of phosphoric acid (H3PO4) in the range 0.35 <= x <= 4.16 mol/polymer repeat unit. After dehydration at 150 degrees C, four-probe a.c. conductivity measurements were carried out in the 60 degrees C range under reduced pressure and with no humidification. Conductivity at 110 degrees C ranged from 2 x 10(-7) S/cm for cross-linked PEI-HCI x=0.35 H3PO4 to 8 x 10(-3) S/cm for x=4.16 H3PO4. The apparent energy of activation for conduction decreased with increasing concentration and was lowest (11 kJ/mol) for the sample with x=4.16 H3PO4. The highest conductivity observed was 1.1 x 10(-2) for the x = H3PO4 sample at 150 degrees C. Infrared spectroscopy of the LPEI-HCl shows a distinct, broad band between 2200 and 2500 cm(-1) due to NH2+, which decreases increasing cross-linking but reappears with increasing amounts of phosphoric acid. A second, very broad feature has an extensive, high wing that contains a relatively smaller amount of N-H stretching contributions from covalently bonded hydrogen. Bands in the P-O region (1200-900 cm(-1)) and the O-P-O bending region (700-400 cm(-1)) do not indicate the presence of discrete PO43- units, even at H3PO4 concentrations. Although the spectra are consistent with such species as phosphate dimers, trimers, or even polyphosphates, it is possible at this time to rule out a distribution of protonated PO4 units, i.e. HuPO(4)((n-3)). Only a few bands of the cross-linked PEI host frequency shifts and intensity changes with addition of the H3PO4 and subsequent heating. These bands occur in the range from 700 to 900 cm and are comprised of a complex mixture of C-N stretching, CH2 rocking, and C-C stretching motions. This suggests that the oxygen atoms of phosphate entities interact with the protonated NH groups oil the PEI backbone via a hydrogen bonding interaction. (c) 2005 Elsevier B.V. All rights reserved.