Liquid crystals are often combined with polymers to influence the liquid crystals' orientation and mechanical properties, but at the expense of reorientation speed or uniformity of alignment. We introduce a new method to create self-assembled nematic liquid-crystal gels using an ABA triblock copolymer with a side-group liquid-crystalline midblock and liquid-crystal-phobic endblocks. In contrast to in situ polymerized networks, these physical gels are homogeneous systems with a solubilized polymer network giving them exceptional optical uniformity and well-defined crosslink density. Furthermore, the unusually high-molecular-weight polymers used allow gels to form at lower concentrations than previously accessible. This enables these gels to be aligned by surface anchoring, shear, or magnetic fields. The high content of small-molecule liquid crystal (>/=95%) allows access to a regime of fast reorientation dynamics.

Rectification of the ionic current flowing through nanotubes embedded in a polymeric membrane is achieved by selective adsorption of polycations to the nanotubes' mouths. A one-dimensional model of ionic flux through a nanotube with charged entrance regions qualitatively describes current-voltage curves before and after polycation exposure; reversal potential measurements confirm that charge reversal takes place upon polycation adsorption. The inherent simply of this electrostatic approach makes it attractive in membrane and nanofluidic applications employing rectification.

Rheological properties of triblock copolymers dissolved in a nematic liquid crystal (LC) solvent demonstrate that their microphase separated structure is heavily influenced by changes in LC order. Nematic gels were created by swelling a well-defined, high molecular weight ABA block copolymer with the small-molecule nematic LC solvent 4-pentyl-49-cyanobiphenyl (5CB). The ''B'' midblock is a side-group liquid crystal polymer (SGLCP) designed to be soluble in 5CB and the ''A'' endblocks are polystyrene, which is LC-phobic and microphase separates to produce a physically cross-linked, thermoreversible, macroscopic polymer network. At sufficiently low polymer concentration a plateau modulus in the nematic phase, characteristic of a gel, abruptly transitions to terminal behavior when the gel is heated into its isotropic phase. In more concentrated gels, endblock aggregates persist into the isotopic phase. Dramatic changes in network structure are observed over small temperature windows (as little as 1 uC) due to tccche rapidly changing LC order near the isotropization point. The discontinuous change in solvent quality produces an abrupt change in viscoelastic properties for three polymers having different pendant mesogenic groups and matched block lengths.

Addition of a small‐molecule liquid crystal (5CB) to a cyanobiphenyl‐based side‐group liquid crystal polymer (SGLCP) stabilizes nematic order, increasing the isotropization temperature (TNI) more than 15 °C. Despite synergistic ordering at high concentration, small amounts of polymer destabilize nematic order. Even though TNI(SGLCP) is 27 °C greater than TNI(5CB), 2H NMR shows that the order parameter of the SGLCP is less than that of 5CB at concentrations for which monodomains were accessible (≤10 wt.‐%). The results imply that nematic order is frustrated in the bulk polymer and addition of small molecule LC relaxes this frustration by allowing greater configurational freedom. Conversely, adding small amounts of polymer to the bulk 5CB introduces frustration, resulting in the strong asymmetry of the phase diagram.magnified image

The discontinuous change in solvent quality of a liquid crystal (LC) solvent, 5CB, at the nematicisotropic phase transition produces abrupt changes in the phase behavior of solutions of coil and LC polymers and in the self-assembly of coil-LC block copolymers. Nematic 5CB is strongly selective for a side-group liquid crystal polymer (SGLCP), and isotropic 5CB is a good solvent for both SGLCP and a random coil (polystyrene, PS). In nematic 5CB, unfavorable LC-PS interactions drive phase separation in SGLCP-PS-LC ternary solutions and drive micellization of PS-SGLCP diblocks. In isotropic 5CB, rich phase behavior occurs in both ternary solutions and block copolymer solutions. Despite the fact that isotropic 5CB is a good solvent for both SGLCP and PS, segregation can occur due to the asymmetric solvent effect (i.e., the preference of the solvent for the SGLCP). In concentrated isotropic solutions, unfavorable SGLCP-PS interactions become dominant.

The SANS and rheology experiments reported in the main text were also performed on a set of coil/SGLCP diblocks having mesogenic side-groups attached "side-on", with the mesogen's long axis parallel to the polymer backbone ( Figure S1). Side-on and end-on polymers were synthesized from the same set of four PS-PB diblocks and one PB homopolymer using methods described in Reference 36. Side-on polymer properties are summarized in Table S2.All experimental methods for side-on and end-on polymers were identical except the percentage of reacted 1,2-butadiene monomers in side-on polymers was calculated from the relative intensities of peaks at δ = 4.9 ppm (characteristic of vinylic hydrogens in 1,2-butadiene) and δ = 3.6 ppm (characteristic of hydrogens in the side-on mesogen's alkyl tails). Also, rheological temperature ramps on side-on solutions were performed at a rate of 0.5 °C/min, instead of the rate of 1 °C/min used for end-on polymers. TEM experiments were not performed using side-on polymers.

Nematic liquid-crystal (LC) elastomers and gels have a rubbery polymer network coupled to the nematic director. While LC elastomers show a single, non-hydrodynamic relaxation mode, dynamic light-scattering studies of self-assembled liquid-crystal gels reveal orientational fluctuations that relax over a broad time scale. At short times, the relaxation dynamics exhibit hydrodynamic behavior. In contrast, the relaxation dynamics at long times are nonhydrodynamic, highly anisotropic, and increase in amplitude at small scattering angles. We argue that the slower dynamics arise from coupling between the director and the physically associated network, which prevents director orientational fluctuations from decaying completely at short times. At long enough times the network restructures, allowing the orientational fluctuations to fully decay. Director dynamics in the self-assembled gels are thus quite distinct from those observed in LC elastomers in two respects: they display soft orientational fluctuations at short times, and they exhibit at least two qualitatively distinct relaxation processes.

Self-assembly of coil-SGLCP-coil block copolymers provides a route to model liquid crystalline (LC) gels in concentrations ranging from bulk polymer to gels with as little as 5 wt % polymer in a small-molecule LC host. Triblock copolymers with LC-phobic endblocks associate in a nematic liquid crystal to create crosslinks. An SGLCP midblock allows for solubilization of the network. These materials have a precisely tailored network structure ideal for comparison to theory 1 of liquid crystalline elastomers. In addition, the gel responds readily to external fields since the concentration of polymer can be relatively low. In this report we discuss rheological measurements that demonstrate gelation at low polymer concentrations. We show that the association of the PS blocks at low concentration is driven by the presence of the SGLCP, rather than incompatibility between PS and 5CB. We then discuss the alignment of the gels via shear, magnetic fields, electric fields, and alignment surfaces. Finally, we present results on a distinctive striped texture observed in aligned gels when subjected to an applied electric field or normal force. The exceptional electrooptic and mechano-optic responsiveness of these gels coupled with thermally reversible gelation suggests they would be ideal candidates for use in large-area printable display technology.