Covalent phosphorylation of poly(vinyl alcohol) eliminates phosphoric acid leaching without reducing proton donor...

Phosphoric acid-doped poly(vinyl alcohol) membranes suffer from poor acid retention under operational conditions. This phenomenon occurs because phosphoric acid is bound to these materials via hydrogen bonding alone, with an estimated 66% loss due to leaching by aqueous washing. A potential solution involves covalent phosphorylation, i.e., the formation of a chemical ester bond at the hydroxyl moieties of the PVA polymer. However, there was uncertainty regarding whether the inductive effect introduced by this modification would inhibit the acidic nature of P-OH moieties, thus affecting the proton conductivity. The present study provides insight into this issue via computer simulation. The covalent P-O-C linkage (bond distance = 1.612 Å; Mayer bond order = 1.015) demands +251.9 kJ/mol for cleavage, which represents 3.3 times the energy (75.9 kJ/mol) required for the desorption of hydrogen-bonded H3PO4 molecules. Single-point calculations at def2-TZVP confirm and strengthen this ratio to 4.6× . Contrary to expectation, however, the esterification reaction of one of the hydroxyl moieties did not adversely affect the remaining P-OH moiety: the O-H bond order shifted only by 0.007 units, and the proton transfer barrier rose marginally (+0.4 kJ/mol). The 65-atom crosslinking membrane cluster comprising two covalently phosphorylated polyvinyl alcohol (PVA) units, a citric acid crosslink, free H₃PO₄, and explicit water molecules demonstrates that the covalent phosphate engages dynamically in the hydrogen bonding network. This has been verified using a 500 ps molecular dynamics simulation at 353 K, in which the mean square displacement of the phosphorus atom achieves only 0.83 Å2 after 500 ps; this value is two orders of magnitude smaller than the diffusion coefficient. Further evidence for structural immobilization is the invariant peak in the P-O-C radial distribution function at 4.25 Å.

All density functional theory calculations were performed using ORCA 6.1.1 with the B3LYP functional, Grimme's D3BJ dispersion correction, and the 6-31G* basis set; the RIJCOSX approximation was applied to monomer-scale systems. Eight calculations were carried out on gas-phase cluster models ranging from an isopropanol-H₃PO₄ monomer pair to a 65-atom crosslinked membrane fragment comprising two covalently phosphorylated PVA units, citric acid crosslinks, free H₃PO₄, and explicit water molecules. Desorption and proton-transfer potential energy surfaces were obtained as relaxed scans; Mayer bond orders and CHELPG electrostatic charges were computed at all optimised geometries. A 500 ps classical molecular dynamics simulation was performed using LAMMPS (29 Sep 2021 release) with the OPLS-AA force field and the SPC/E water model in the NVT ensemble at 353.15 K (Nosé-Hoover thermostat, 100 fs damping, 1.0 fs timestep) on a periodic cell of 547 atoms; long-range electrostatics were treated by PPPM with a 7.0 Å real-space cutoff.