Ht. Dong et al., Molecular dynamics simulations and structural comparisons of amorphous poly(ethylene oxide) and poly(ethylenimine) models, POLYMER, 42(18), 2001, pp. 7809-7817
Poly(ethylene oxide) (or PEG, (-CH2CH2O-)(n)) and poly(ethylenimine) (PEI,
(-CH2CH2NH-)(n)) have been suggested as host polymers for solid polymer ele
ctrolytes in high energy-density batteries. Four repeat-unit models for amo
rphous PEO and PEI (CH3X(CH2CH2X)(4)CH3, X = O in PEG-4 and NH in PEI-4) we
re studied by molecular dynamics (MD) simulations at 300 K. Analysis of ave
rage chain dimensions indicates that PEI-4 adopts a more compact structure
than PEO-4. The characteristic ratios of 4.9 +/- 0.1 for PEG-4 and 3.1 +/-
0.1 for PEI-4 are consistent with experiment and with theoretical predictio
ns. Dihedral angles along the C-X-C-C atom sequence favor the trans (T) con
formation 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 PEG-4 and 66 +/- 2% in PEI-4.
The TTT conformation becomes much less populous in PEI-4 (1.6 +/- 0.4%) tha
n in PEG-4 (20 +/- 2%) while the TGG conformation has a larger population i
n PEI-4 (24 +/- 2%) than in PEG-4 (6 +/- 2%). Radial distribution function
analysis reveals that intra-chain H-bonds exist between two adjacent NH gro
ups 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-ch
ain 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% PEG-4 and 8.2% PEI-4 chains ar
e found to be in the helical form and an insignificantly amount of PEI-4 is
found in double-stranded helices in amorphous PEI-4. (C) 2001 Published by
Elsevier Science Ltd.