Abstract
The impacts of local polymer chain conformations on the methine and carbonyl 13C-NMR chemical shifts for polyethylene acrylic acid p(E-AA) copolymers were predicted using ab initio methods. Using small molecular cluster models, the magnitude and sign of the γ-gauche torsional angle effect, along with the impact of local tetrahedral structure distortions near the carbonyl group, on the 13C-NMR chemical shifts were determined. These 13C-NMR chemical shift variations were compared to the experimental trends observed for precise p(E-AA) copolymers as a function acid group spacing and degree of zinc-neutralization in the corresponding p(E-AA) ionomers. These ab initio calculations address the future ability of 13C-NMR chemical shift variations to provide information about the local chain conformations in p(E-AA) copolymer materials.
Highlights
In polyethylene (PE) the 13C-NMR chemical shift difference between the crystalline and amorphous phases are commonly attributed to the γ-gauche effect arising from differences in the local polymer chain conformation [1,2,3,4]
In our original analysis of these 13C-NMR chemical shift trends in the p(E-AA) copolymers and ionomers, we argued that the copolymers. The methine (CH) variations resulted from a γ-gauche effect, and that Zn-neutralization did not have a large influence on the observed shift
Ab initio calculations were used to explore the polymer chain conformational effects on the methine (CH) and carbonyl (COOH) 13C-NMR chemical shifts in polyethylene acrylic acid copolymers. These calculations demonstrate that the observed chemical shift variations could not be directly correlated to the γ-gauche effect for either the CH or the COOH carbons
Summary
In polyethylene (PE) the 13C-NMR chemical shift difference between the crystalline and amorphous phases are commonly attributed to the γ-gauche effect (trans-gauche effect) arising from differences in the local polymer chain conformation [1,2,3,4]. The broad resonance corresponding to the amorphous trans/gauche phase is shifted approximately −2 ppm with respect to the all trans conformation in the crystalline phase [14] For these same precise p(E-AA) copolymers distinct trends in the 13C-NMR chemical shifts were observed for the methine (CH) carbon as a function of the chain length spacing between the carboxylic acid groups, and as a function of the extent of. 2-ethyl-butanoic acid model cluster employed to initially model the 13C-NMR chemical shifts using ab initio methods was too small to eliminate end-group effects from impacting the calculated shifts of the central CH, and only a very limited number of chain conformations were considered in that analysis [14]. The magnitude and sign of the γ-gauche effect were evaluated for both the CH and CO carbons, and are compared to the 13C-NMR experimental results observed in the p(E-AA) copolymers and Zn-neutralized ionomers [14]
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