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Register a new userPredicted field-induced hexatic structure in an ionomer membrane
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Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion$^{tiny textregistered}$-like ionomer by the imposition of a strong electric field. We observe the formation of novel structures aligned along the direction of the applied field. The polar head groups of the ionomer side chains aggregate into clusters, which then form rod-like formations which assemble into a hexatic array aligned with the direction of the field. Occasionally these lines of sulfonates and protons form a helical structure. Upon removal of the electric field, the hexatic array of rod-like structures persists, and has a lower calculated free energy than the original isotropic morphology.
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This simulation study investigates the dependence of the structure of dry Nafion$^{tinytextregistered}$-like ionomers on the electrostatic interactions between the components of the molecules. In order to speed equilibration, a procedure was adopted which involved detaching the side chains from the backbone and cutting the backbone into segments, and then reassembling the macromolecule by means of a strong imposed attractive force between the cut ends of the backbone, and between the non-ionic ends of the side chains and the midpoints of the backbone segments. Parameters varied in this study include the dielectric constant, the free volume, side-chain length, and strength of head-group interactions. A series of coarse-grained mesoscale simulations shows the morphlogy to depend sensitively on the ratio of the strength of the dipole-dipole interactions between the side-chain acidic end groups to the strength of the other electrostatic components of the Hamiltonian. Examples of the two differing morphologies proposed by Gierke and by Gebel emerge from our simulations.
We present detailed systematic studies of structural transformations in thin liquid crystal films with the smectic-C to hexatic phase transition. For the first time all possible structures reported in the literature are observed for one material (5 O.6) at the variation of temperature and thickness. In unusual modulated structures the equilibrium period of stripes is twice with respect to the domain size. We interpret these patterns in the frame work of phenomenological Landau type theory, as equilibrium phenomena produced by a natural geometric frustration in a system having spontaneous splay distortion.
When an electric field is applied to an electrolyte-saturated polymer gel embedded with charged colloidal particles, the force that must be exerted by the hydrogel on each particle reflects a delicate balance of electrical, hydrodynamic and elastic stresses. This paper examines the displacement of a single charged spherical inclusion embedded in an uncharged hydrogel. We present numerically exact solutions of coupled electrokinetic transport and elastic-deformation equations, where the gel is treated as an incompressible, elastic Brinkman medium. This model problem demonstrates how the displacement depends on the particle size and charge, the electrolyte ionic strength, and Youngs modulus of the polymer skeleton. The numerics are verified, in part, with an analytical (boundary-layer) theory valid when the Debye length is much smaller than the particle radius. Further, we identify a close connection between the displacement when a colloid is immobilized in a gel and its velocity when dispersed in a Newtonian electrolyte. Finally, we describe an experiment where nanometer-scale displacements might be accurately measured using back-focal-plane interferometry. The purpose of such an experiment is to probe physicochemical and rheological characteristics of hydrogel composites, possibly during gelation.
The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH) and SWCNT functionalized with Fe$_3$O$_4$ nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was $phi_1$ = 10$^{-4}$ and $phi_2$ = 10$^{-3}$. Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Freederiksz transition threshold) has been observed, which is not present in the undoped nematic.
Elastic crystalline membranes exhibit a buckling transition from sphere to polyhedron. However, their morphologies are restricted to convex polyhedra and are difficult to externally control. Here, we study morphological changes of closed crystalline membrane of super-paramagnetic particles. The competition of magnetic dipole-dipole interactions with the elasticity of this magnetoelastic membrane leads to concave morphologies. Interestingly, as the magnetic field strength increases, the symmetry of the buckled membrane decreases from 5-fold to 3-fold, to 2-fold and, finally, to 1-fold rotational symmetry. This gives the ability to switch the membrane morphology between convex and concave shapes with specific symmetry and provides promising applications for membrane shape control in the design of actuatable micro-containers for targeted delivery systems.
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