Molecular dynamics simulations and structural comparisons of amorphous poly(ethylene oxide) and poly(ethylenimine) models

Poly(ethylene oxide) (or PEO, (CH2CH2O)n) and poly(ethylenimine) (PEI, (CH2CH2NH)n) have been suggested as host polymers for solid polymer electrolytes in high energy-density batteries. Four repeat-unit models for amorphous PEO and PEI (CH3X(CH2CH2X)4CH3, X=O in PEO-4 and NH in PEI-4) were studied by molecular dynamics (MD) simulations at 300 K. Analysis of average chain dimensions indicates that PEI-4 adopts a more compact structure than PEO-4. The characteristic ratios of 4.9±0.1 for PEO-4 and 3.1±0.1 for PEI-4 are consistent with experiment and with theoretical predictions. Dihedral angles along the CXCC atom sequence favor the trans (T) conformation while the XCCX sequence favors the gauche (G) conformation for both models. The TGT conformation along the CXCCXC sequence is found to have the largest population, 59±3% in PEO-4 and 66±2% in PEI-4. The TTT conformation becomes much less populous in PEI-4 (1.6±0.4%) than in PEO-4 (20±2%) while the TGG conformation has a larger population in PEI-4 (24±2%) than in PEO-4 (6±2%). Radial distribution function analysis reveals that intra-chain H-bonds exist between two adjacent NH groups of PEI-4 chains. All intra-chain H-bonds are found to be nonlinear and longer than typical H-bonds. These results are consistent with an ab initio study of a methyl capped monomer of PEI, dimethylethylenediamine. Inter-chain H-bonds are also found in the amorphous state of PEI-4 and appear to be more like typical linear H-bonds. Only 5.6% PEO-4 and 8.2% PEI-4 chains are found to be in the helical form and an insignificantly amount of PEI-4 is found in double-stranded helices in amorphous PEI-4.