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

Abstract

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 300K. 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 CXCC atom sequence favor the trans (T) conformation while the XCCX sequence favors the gauche (G) conformation for both models. The TGT conformation along the CXCCXC 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.