Transient shear response of liquid crystal‐forming hydroxypropyl cellulose solution in dimethylacetamide. I. Stress growth and relaxation behavior

Abstract

SYNOPSISThe stress growth and relaxation behavior of liquid crystalline hydroxypropyl cellulose solutions in dimethylacetamide was characterized by using exponential functions. The parameters evaluated for the stress growth and relaxation processes were compared and the shear history effect on the parameters was determined. The exponential functions proposed were valid for our system. The isotropic solutions had one retardation time and one relaxation time, whereas the liquid crystalline solutions had plural retardation and relaxation times. The concentration dependence of the parameters for the stress growth process was similar to that for the stress relaxation process and to that for the steady-state shear viscosity. The stress growth and relaxation behavior for the liquid crystalline solutions was originated from the change (deformation or decrease) in liquid crystalline domains. The deformation of liquid crystalline domains with shear seemed to be slower than the recovering of the domains to original shape. The stress growth process was a progressive event, whereas the relaxation process was a sudden event. Stress relaxation behavior for the liquid crystalline solutions was sensitive to the shear history. 0 1994

Rheological behavior and microstructure of 2,2′‐dioxy‐ 1,1′‐binaphthylphosphazene R/S random copolymer

“…4) confirming the capacity of ( R/S PBNP) to form lyotropic liquid‐crystalline systems. Figure 4 reveals a grainy texture resembling literature results obtained at rest (not sheared samples) with lyotropic solutions of cellulose derivatives,6, 36, 45, 53 polyisocyanates,4 and poly(benzyl glutamate) 50, 54. The rheo‐optical study of aqueous solutions of hydroxypropylcellulose lead Guido et al45 to associate the grainy texture with the aforementioned Region I observed at low shear rates or oscillation frequencies.…”

“…Currently, we have not an explanation for the narrowness of our biphasic region. In any case, low ratios between respective minimum and maximum viscosity concentrations have been reported in the literature for solutions of poly(50% butyl + 50% p ‐anisole‐3‐propyl isocyanate),4 trimethylsilyl cellulose,7 ethyl cellulose,9 and hydroxypropyl cellulose 36…”

Structural Color Out of the Blue: A Quantitative Framework for the Self-Assembly Kinetics of Cholesteric Cellulosic Mesophases

“…To regain the original texture, order must be restored on both length scales. Building on previous studies, we calculated two constants, τ 1 and τ 2 , by fitting each stress–time curve to two exponential equations, where one applies in the early time regime of the test and the other in the later stage, to decouple the recovery behaviors of these different length scales. ,,,,, In fitting this data, we took τ 1 and τ 2 to be essentially independent of each other in EC–AA mesophases, an assumption which is consistent with the existing literature describing two separable time scales of phase restructuring. ,,, For all cellulose chain lengths, τ 1 decreased when the shear strain or strain rate is raised (Figure d,e). , However, the relationship between M.W. and τ 1 is nonmonotonic.…”

“…Our values for τ 1 were lower than those reported for HPC-dimethylacetamide or EC-m-cresol mesophases, which hover around 10 s, but we attributed this difference to the sensitivity of this time constant to the M.W., shear strain, and shear rate of the experiment. 25,86 Faster relaxation times have been measured in other polymeric LC phases, such as poly(γ-benzyl-L-glutamate)-m-cresol which undergoes its initial recovery in 0.1−0.25 s, so the τ 1 values calculated for the EC−AA system are on a reasonable order of magnitude. 39 Visually, the slope of the shear stress vs time curves on long time scales was relatively independent of the M.W., which was registered in the values of τ 2 and suggested that either the second relaxation occurs on a time scale longer than that of these experiments or it reflects that the process of domain recovery is independent of EC chain length (Figure S15).…”