Optical and Mechanical Rheometry of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers with Varying Pendant Groups

Abstract

A specially designed flow cell was used to investigate birefringence in transient shear flow, ¢n + (A o ,t), of three semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs), poly((x)-p- phenylene 1,10-decamethylenebis(4-oxybenzoate)) (PxHQ10) with varying pendant side groups (x ) oxyethyleneethoxy, tert-butyl, or phenylsulfonyl). Also, shear rheometry was used to investigate the growth of shear stress U+(A o ,t) and first normal stress difference N1 +(A o ,t) during transient shear flow of the same TLCPs. Upon startup of shear flow, ¢n + (A o ,t) initially increases rapidly, follows two minor overshoots, and then levels off to attain a steady-state value ¢n in all three TLCPs. The steady-state value of ¢n for PSHQ10 (PxHQ10 with x ) phenylsulfonyl pendant side group) was found to be greater than that for PEHQ10 (PxHQ10 with x ) ethoxy pendant side group) and PTHQ10 (PxHQ10 with x ) tert-butyl pendant side group). This behavior is strongly correlated to the large van der Waals volume of phenylsulfonyl pendant side groups in PSHQ10 compared to that of ethoxy pendant side groups in PEHQ10 and tert- butyl pendant side groups in PTHQ10. It is also observed that N1 + (A o ,t) exhibits a very large overshoot followed by an oscillatory decay to attain a steady-state value N1, while U + (A o ,t) shows a very large overshoot followed by a rapid monotonic decrease to attain a steady-state value, U. The peak values of N1 + (A o ,t) and U + (A o ,t) for PSHQ10 were found to be 3-4 times greater than those for both PEHQ10 and PTHQ10, which we again attribute to the bulky phenylsulfonyl pendant side groups in PSHQ10. Only positive values of N1 + (A o ,t) and N1 were observed over the entire range of shear rates investigated. Variations of the ratio N1 +(A o ,t)/U+(A o ,t) with shear strain (A o t) were found to be very similar to variations of ¢n+(A o ,t) with A o t for all three TLCPs investigated, except for that the ratio N1 + (A o ,t)/U + (A o ,t) reached a steady state sooner than ¢n+(A o ,t). In steady-state shear flow, it is observed that the ratio N1/U overlaps ¢n over the entire range of temperatures investigated; specifically, both the ratio N1/U and ¢n remain constant at temperatures far below the clearing temperature (TNI), decrease gradually as the temperature approaches TNI, and drop precipitously to zero value at and near TNI.