Systematic studies on structure–properties relationship of main chain discotic liquid crystalline polyethers: effects of the spacer lengths and substitution positions

Abstract

The elucidation of structure–properties relationship influenced by spacer lengths and substitution positions is of crucial importance for the application of discotic liquid crystalline polymers (DLCPs), but few systematic investigations have been reported. Herein, an exhaustive study was carried out in order to gain a deeper insight into the structure–properties relationship of main chain DLC polyethers P_a,b-n; these were regiospecifically synthesized via polycondensation by changing spacer length n from 8 to 12 and varying substitution positions a,b- indicated 2,3-, 2,6-, 2,7-, and 3,6-substitution positions. To our surprise, when the spacer length was twice that of the side chain, the polymers P_a,b-10 displayed the highest order in each series. This was attributed to the perturbations from steric hindrance and space filling effects being minimized. It was disclosed that angular rotation of a triphenylene unit prevailed in the main chain DLCPs and increasing of the rotation freedom in the angular range was desirable for achieving higher order; this was directly confirmed by investigating the charge carrier mobilities of P_2,3-10, P_2,6-10, P_2,7-10 and P_3,6-10. Results showed that P_3,6-10 displayed the highest order in variant substitution positions. More inspiring, the charge carrier mobility of P_3,6-10 could reach the value 1.9 × 10⁻³ cm² V⁻¹ s⁻¹, which was comparable to that of the corresponding monomer hexapentyloxytriphenylene (HAT5). This could provide a practical way for conductive polymers design by incorporating a liquid crystal unit into the main chain to establish charge transportation properties via self-assembling into ordered structures.