Characteristic dynamic rheological responses of nematic poly(p-phenylene terephthalamide) and cholesteric hydroxypropyl cellulose phases

The liquid crystalline (LC) behavior of two lyotropic systems, poly(p-phenylene terephthalamide) (PPTA) solutions in sulfuric acid and hydroxypropyl cellulose (HPC) solutions in water, was compared on a dynamic rheological basis. The critical concentration (C*) curves showed much sharper transitions in PPTA solutions than in HPC solutions. The C* and saturation point (B-point) were observed at 7 and 10.5 wt% for nematic PPTA solutions and at 40 and 53 wt% for cholesteric HPC solutions. Both lyotropic systems gave low yield stress (τy) below 1 Pa at the B-point. With further increases in concentration, the τy values of nematic PPTA and cholesteric HPC solutions were 461.48 and 6.03, respectively. In the loss tangent (tanδ) curves versus frequency, the tanδ of PPTA was monotonically decreased, but that of HPC was increased in the early stage of frequency, and this trend was more noticeable at higher concentrations. Two systems showed similar behaviors in molecular stress relaxation but different orientation relaxation time behaviors. With increased strain, the orientation relaxation time (λe) of the PPTA solutions was slightly increased, but that of the HPC solutions was decreased, and their λe values converged at 100% strain.