Soft Glass Rheology in Liquid Crystalline Gels Formed by a Monodisperse Dipeptide

Abstract

Thermal and extensive rheological characterization of a nematic liquid crystal gelated with a novel monodisperse dipeptide, also a liquid crystal, has been carried out. For certain concentrations, the calorimetric scans display a two-peak profile across the chiral nematic-isotropic (N*-I) transition, a feature reminiscent of the random-dilution to random-field crossover observed in liquid crystal gels formed with aerosil particles. All samples show shear thinning behavior without a Newtonian plateau region at lower shear rates. Small deformation oscillatory data at lower frequencies exhibit a frequency dependence of the storage (G') and loss (G'') moduli that can be described by a weak power-law, characteristic of soft glassy rheological systems. At higher frequencies, while lower concentration composites have a strong frequency dependence with a trend for possible crossover from viscoelastic solid to viscoelastic liquid behavior, the higher-concentration gels show frequency-independent rheograms of entirely elastic nature G' > G''. The plateau modulus of G' is described by a power-law with an exponent again common to soft materials, such as foams, slurries, etc. Other features which are a hallmark of such materials observed in the present study are: (i) above a critical strain, a strain softening of the moduli with a peak in the loss modulus, (ii) power-law variation of the storage modulus in the nonlinear viscoelastic regime, and (iii) absence of Cox-Merz superposition for the complex viscosity. An attractive feature of these gels is the fast recovery upon removal of large strain and qualitatively different temporal behavior of the recovery between the low and high concentration composites, with the latter indicating the presence of two characteristic time scales.

Phase Behavior and Rheological Analysis of Reverse Liquid Crystals and W/I2 and W/H2 Gel Emulsions Using an Amphiphilic Block Copolymer

“…This is in agreement with some experimental results, 41 which found this behavior of G'' in the low frequency range in liquid crystalline gel systems. In this case, G' was predicted to be constant.…”

Liquid-crystal mediated nanoparticle interactions and gel formation

“…The resulting materials have a wide variety of possible applications from displays to liquid-crystalline semiconductors 7 to sensors. Systems coupling physical gels with liquid crystalline systems have been prepared using physical and chemical gelators 7,8 at the molecular level, with a gel structure determined by the relative temperatures of gelation, T gel and the isotropic-nematic (IN) phase transition T IN . Systems which gel above the IN transition exhibit a random network structure, while systems gelling below this transition experience a) Electronic mail: depablo@uchicago.edu phase separation into filaments aligned with the nematic director.…”

Effect of Atomic‐Scale Differences on the Self‐Assembly of Thiophene‐based Polycatenars in Liquid Crystalline and Organogel States

“…At low g values, both the samples show strain-independentbehavior for G' and G'',w ith G' > G''.This solid-like behavior is typical of ag el structure. [19] The variation in G' and G'' with angular frequency (w)o btained by keeping the strain amplitude constant in the LVRi ss hown in Figure22b.B oth G' and G'' show almostn ov ariation with w,w itha ne lastic response (G' > G'')o ver the entiref requency range studied, which confirms the gel nature of the samples. G' shows am onotonic decrease for both samples, whereas G'' for 2c passes through am aximum before decreasing.…”

“…The latter feature is associated with soft glassy rheological( SGR) behavior [18] universally seen in materials such as foams,s lurries, and pastes. [19] In the steady-statem easurements the shear viscosity (h)w as determined as the samples were subjectedt oi ncreasing shear rate (ġ). [19] The variation in G' and G'' with angular frequency (w)o btained by keeping the strain amplitude constant in the LVRi ss hown in Figure22b.B oth G' and G'' show almostn ov ariation with w,w itha ne lastic response (G' > G'')o ver the entiref requency range studied, which confirms the gel nature of the samples.…”

“…The magnitude of G', ad irect measure of the mechanical stiffness, was found to be lower and the LVRl onger for 1c.T hese features indicate that although 2c is as tiff gel (higher G'), it is less robust than 1c.In both gels, G' crosseso ver G'' at higher g values (above g c ), which signals ad eformation-driven transition from av iscoelastic solid to av iscoelastic liquid. [19] The variation in G' and G'' with angular frequency (w)o btained by keeping the strain amplitude constant in the LVRi ss hown in Figure22b.B oth G' and G'' show almostn ov ariation with w,w itha ne lastic response (G' > G'')o ver the entiref requency range studied, which confirms the gel nature of the samples. [17] Af urther gel collapse and recovery test was done by performing step-strain measurements (Figure 23).…”