Small-angle light scattering (SALS) has proven to be an effective tool for probing the complex microstructure present in liquid crystal polymers (LCPs) under shear flow. The flow SALS technique probes structures on the length scale of microns which, in the case of LCPs, corresponds to the defect texture. In this work, a model lyotropic LCP system, poly({gamma} benzylglutamate) (PBG) dissolved in m-cresol, has been investigated under steady state flow over several decades in shear rate. Several different features of the SALS patterns are identified and the influence of shear rate on these different features and relationship to observed microstructures and rheological behavior are discussed. The results of this work must be incorporated into texture-based models of the rheological behavior of LCPs; models which are necessary for the intelligent design and processing of LCP and LCP-based materials.
Particle-laden interfaces play a crucial role in engineering stability of multiphase systems. However, a full understanding of the mechanical properties in shear and compression, especially in relation to the underlying microstructural changes, is as yet lacking. In this study, we investigate the interfacial rheological moduli in heterogeneous networks of aggregated 2D suspensions using different deformation modes and relate these moduli to changes in the microstructure.Interfacial rheological experiments were conducted at different surface coverages and clean kinematic conditions, namely in (i) simple shear flow in a modified double wall-ring geometry and (ii) isotropic compression in a custom-built radial trough, while monitoring the evolution of the microstructure.The compressive moduli increase non-monotonically with decreasing void fraction, reflecting the combined effect of aggregate densification and reduction of void structures, with rotation of rigid clusters playing a significant role in closing voids. However, the shear moduli increase monotonically, which correlates with the increase in fractal dimension of the aggregates making up the backbone network. We also observe that these interfaces act as 2D auxetic materials at intermediate coverages, which is surprising given their amorphous structure. This finding has potential implications for the resilience of particle-coated bubbles or droplets subjected to time-varying compression-expansion deformations.
In multiphase materials, structured fluid–fluid interfaces can provide mechanical resistance against destabilization. Coarsening, coalescence, and significant deformation can be stalled with appropriate interfacial rheology and thus preserve interface integrity. Often, interfacial "strength" is generated by dense, packed surface populations, which are challenging to achieve through gradual, equilibrium-limited adsorption. Recent efforts have focused on developing new methods to produce kinetically trapped interfacial structures that possess desirable viscoelasticity or viscoplasticity, sometimes even with sparse populations. In creating these interfaces, we should recognize that the processing history is deterministic and offers alternative handles to engineer useful rheology. In this Perspective, we consider what can be achieved by designing not only the intrinsic qualities of surface-active species but also the process that brings them to the interface. We contrast different classes of processing history through a somewhat historical lens: after creating an interface ("divide"), what ("conquering") strategies exist for populating it with agents that ensure stabilization? Navigating the delicate interplay among property, structure, and processing history is required to improve material and energy use and to realize unique multiphase materials.
Porous polymeric structures with controlled porosity were prepared using a new approach involving solidification of emulsified polymer solutions via phase inversion (SEPPI). The new method starts from a polymeric emulsion for which the presence of nanosized particles or surfactants is crucial. Subsequent solidification of such emulsion is realized by simple contact with a polymer nonsolvent. The resulting solids exhibit spherical pores for which the emulsion droplets act as template. The preparation method allows easy control over pore morphology by tuning a number of easily accessible parameters, mainly at the level of the emulsion itself. A wide variety of polymers, including biocompatible and biodegradable polymers, can thus be turned into porous materials. Two typical applications in controlled release and solvent resistant nanofiltration are presented, illustrating the real practical utility of the presented method. Compared with the commonly used methods to prepare porous polymers, the presented method has a large potential since it (1) is applicable to a wide range of different polymers, (2) shows simply accessible flexibility in structural properties, such as porosity, pore size, pore interconnectivity, and pore wall functionality, (3) involves no chemical reaction in the polymer hardening process, and (4) allows creation of porous materials with an asymmetric structure.
Experimental observations of sustained oscillations of both shear stress and first normal stress differences are reported in flowing liquid crystalline polymers in a limited range of shear rates. The results can be described by considering the response of a rigid-rod model. Depending on the initial conditions, the frequency spectrum of the stress signal contains either one or two characteristic frequencies. This can be explained by the occurrence of either pure "wagging" or the coexistence of wagging and "log-rolling" behavior of the director.
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