Relaxation Processes in sheared films of ethyl-cyanoethyl cellulose cholesteric liquid crystalline solutions

Abstract

The relaxation processes in sheared films of ethyl-cyanoethyl cellulose [(E-CE)C]/acrylic acid (AA) cholesteric liquid crystalline (LC) solutions were studied by polarizing optical microscopy (POM) and UV-Vis spectrophotometry. Under shearing normal to the helix axis and above the critical shear rate, the planar texture arrangement of the (E-CE)C/AA cholesteric LC solution was destroyed and transformed to the nematic phase. Observed by POM, the banded texture formed quickly following the cessation of the shear, but it was unstable and disappeared after several minutes. The reflection spectrum of the sheared (E-CE)C/AA cholesteric LC solution film was recorded as a function of relaxation time. It was found that the selective reflection property was lost under the shear, but the shape of the reflection spectrum recovered quickly with cessation of the shear, and the reflection peak in the spectrum became sharper with time, returning to the original form before shearing. A proposed model of the structural transformation during the relaxation was confirmed by additional optical measurement and transmission electron microscopy.

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

“…As the mesophase equilibrated, the peak red-shifted into the visible range and its intensity grew and narrowed, signaling the formation and realignment of cholesteric LC domains. 17,31 Consistent with our static spectra, the reflection peak was broader and less intense for high-M.W. samples due to a wider angular distribution of smaller domains with respect to the optical axis.…”

“…Among the time-dependent processes that influence the morphology and color of cholesteric cellulose mesophases, stress relaxation and solvent evaporation are the most well-studied. Upon the application of shear stress, the initial texture of cholesteric polymer phases is disturbed or lost. − Shear stresses can decrease the cholesteric domain size, create new defects between domains, or incite nematic flow alignment. ,,− Additionally, shear flow can induce a cholesteric–nematic transition referred to as helix uncoiling or helix untwisting. − Multiple time constants characterize the recovery of the polydomain cholesteric structure, which is a reflection of the various ordering length scales in the mesophases and the mechanisms by which the corresponding characteristic structures, domains and helices, may be distorted. ,− Generally, at short time scales, relaxation is governed by rearrangements of individual macromolecules and helices, while longer relaxation times indicate domain recovery. ,,, The time scale of the first relaxation event is inversely related to the shear rate, but the effects of polymer chain mobility on either recovery process remains unclear . Past efforts have focused on coupling rheology with spectroscopy methods to assess helix retwisting and cholesteric domain alignment upon the cessation of shear; however, the results of these experiments remain largely qualitative and removed from the measurable physical and chemical mesophase properties that should influence the kinetics of these processes. ,− Spatiotemporal transformations of structural color and chiral nematic phase arrangement are also observed upon solvent evaporation, with the sample thickness and solvent diffusivity impacting the elapsed time that compromises the morphology and optical behavior. ,− Nonetheless, these studies still fail to study macromolecular motion in the mesophase and its control over the optical properties.…”

“…To demonstrate the universality of our phenomenological framework, we then performed the same experiments on aqueous HPC mesophases and explained why these LCs exhibit structural colors more quickly than the EC–AA materials. Both EC and AA are inexpensive and commercially available precursors, and previous studies have reported on the equilibrium cholesteric LC phase behavior of these mesophases for a given EC molecular weight, allowing us to intelligently build our kinetic insights into foundational knowledge of the static properties. − , Moreover, AA can be polymerized to trap cholesteric domains in the solid state, allowing us to expand the scope of characterization techniques we could employ. ,,− Similarly, the equilibrium optical properties of HPC–water mesophases are well-researched as this cellulosic polymer is affordable, abundant, and nontoxic. ,,,,,,,,,− …”

Construction of CdS quantum dots via a regioselective dendritic functionalized cellulose template

“…2 Regiocontrol of cellulose substitution leads to the design of advanced materials and nano-scale architectures in interdisciplinary research at the interface of organic and supramolecular chemistry. Utilitarian applications of this chemistry include liquid crystalline polymers, 3,4 host-guest assemblies, 5,6 sensor matrices, 7,8 and bioactive materials. [9][10][11] Semiconductor nanocrystals and quantum dots (QDs) have also attracted great interest from the biological and medical communities.…”

Coordinate chloride and water-assisted assembly of novel 3D-cageworks of cadmium(II) complex with 4-[N,N-bis(2-cyanoethyl)]aminopyridine

“…Cyanoethyl derivatives are versatile materials which have widely been used in specific organic synthesis [1][2][3][4], liquid crystal preparation [5], and functional devices creation [6]. However, only a few of examples for their successful uses in supramolecular structure construction have been reported [7][8][9][10].…”