Study of Odd−Even Effect of Flexible Spacer Length on the Chain Dynamics of Main-Chain Thermotropic Liquid-Crystalline Polymers Using High-Resolution Solid-State ¹³C Nuclear Magnetic Resonance Spectroscopy

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 13 C nuclear magnetic resonance (NMR) spectroscopy. The temperature dependences of 13 C NMR spectra and spin-lattice relaxation time (T1) were measured for PSHQn over the temperature range of 293-520 K. Line broadening of R-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 10 4 -10 6 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 1 H line width in two-dimensional wideline separation (WISE) NMR spectra. It was found that the 13 C 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 10 8 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.

Poly(epichlorohydrin) modified with 3,4,5‐tris(dodecyloxy)benzoate: The structure and dynamics of the aliphatic side chains in the columnar mesophase

“…The values were considerably higher for the longer components of C 3 and C 10 , and this reflects the restricted mobility of these atoms in the LC phase. In this case, the activation energies are indeed comparable to the values found in the literature for the motion of the α‐spacer carbon in the nematic phase 39, 41…”

Direct Observation of Odd−Even Effect in Dilute Polymeric Solutions: A Time-Resolved Fluorescence Anisotropy Study

“…At this temperature (ca. 270 K) rotations seem to cease and the κ 2 ) (cosθ da -3 cosθ d cosθ a ) 2 (3) aliphatic chains freeze in a nearly all staggered conformation. This conformation, however, leads to entirely different mutual orientation of the fluorophore dipoles in the odd and even polymers, as shown in Figure 2b.…”

“…Actually most known series of compounds having alkyl chains with variable numbers of methylene groups [-CH 2 ) n -] display an odd-even effect in their phase transition temperatures, 1 optical characteristics, etc. [2][3][4][5] For these solidstate affected characteristics, many experimental techniques have been employed, all pointing out to the different adopted shapes of alkyl chains in their all-staggered conformations. 6 Especially in those cases in which mesogenic units are separating subsequent aliphatic spacers it has been long assumed that the final packing ability of the mesogenic parts in one or the other conformation will eventually control the final bulk properties of the material.…”

“…The so-called “odd−even” effect of the number of methylene groups in aliphatic chains is a well-known and extensively demonstrated phenomenon. − In the overwhelming majority of these cases this effect has been observed in the solid state and has been mainly attributed to packing differences in the crystal structure between chains having an odd or even number of methylene groups. Actually most known series of compounds having alkyl chains with variable numbers of methylene groups [−CH 2 ) n −] display an odd−even effect in their phase transition temperatures, optical characteristics, etc. − For these solid-state affected characteristics, many experimental techniques have been employed, all pointing out to the different adopted shapes of alkyl chains in their all-staggered conformations .…”

Effects of Carbon Atom Parity and Alkyl Side Chain Length on the Crystallization and Morphology of Biscarbamates, A Set of Model Compounds for Polyurethanes

“…This effect has been observed in the solid state, solution phases, and solid/liquid interfaces and has been mainly attributed to packing differences in the crystal structures between chains having odd or even numbers of carbon atoms. Although liquid crystals, as a class of molecules, are known to exhibit such an odd–even effect, others, e.g., polyurethanes, n -alkanes, carboxylic acids, alcohols, bifunctional hydroxycarboxylic acids, etc., also show the odd–even effect as a function of carbon atom parity. In the overwhelming majority of the literature, this effect is discussed with respect to the carbon atom parity in the spacer groups .…”