Abstract
The odd−even effect of flexible spacer length on the molecular dynamics of poly[(phenylsulfonyl)-p-phenylenealkylenebis(4-oxybenzoate)]s (PSHQn) having varying lengths (n) of methylene groups of flexible spacers and bulky pendent side groups was investigated using high-resolution solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The temperature dependences of 13C NMR spectra and spin−lattice relaxation time (T1) were measured for PSHQn over the temperature range of 293−520 K. Line broadening of α-CH2 and rigid core carbons, due to the interference between proton decoupling and local polymer chain motion about the preferred direction of alignment accompanying the fluctuation of the rigid core, was observed in the nematic phase. The local polymer chain motion in the nematic phase was found to vary over a frequency range of 104−106 Hz. The activation energy (77−104 kJ/mol) of this motion was found to increase with decreasing number of methylene groups and exhibits odd−even fluctuations. The motions of the local polymer chains in the nematic phase were also observed via the 1H line width in two-dimensional wideline separation (WISE) NMR spectra. It was found that the 13C NMR T1 of flexible spacer carbon was dominated by the fast trans−gauche exchange. The frequency of the trans−gauche exchange was found to occur above 108 Hz in the nematic mesophase, since T1 increased with increasing temperature. The activation energy (6.5−9.8 kJ/mol) of the trans−gauche exchange estimated from T1 was found to increase with decreasing number of methylene groups and exhibits odd−even fluctuations. The odd−even fluctuations of the activation energy of local polymer chain motion and trans−gauche exchange are indicative of the differences in molecular packing between PSHQn having odd-numbered n and PSHQn having even-numbered n.
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