Polymer electrolytes based on cross-linked linear poly(ethylenimine) hydrochloride/phosphoric acid systems

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