Dick J. Broer

New insights into photoactivated volume generation boost surface morphing in liquid crystal coatings

Photoactivated generation of disorder in a liquid crystal network produces free volume that leads to the controlled formation of dynamic corrugations at its surface. The liquid crystal order amplifies the deformation of copolymerized azobenzene, which takes place on molecular length scales, to a micrometre-sized macroscopic phenomenon based on changes in density. We postulate a new mechanism in which continuous oscillating dynamics of the trans-to-cis isomerization of the azobenzene overrules the net conversion, which is currently considered as the origin. This is supported by a significant local density decrease when both the trans and cis isomers are triggered simultaneously, either by dual-wavelength excitation or by the addition of a fluorescent agent converting part of the light to the cis-actuating wavelengths. This new insight provides a general guideline to boost free volume generation leading not only to larger macroscopic deformations but also to controllable and faster non-equilibrium dynamics.

Nanoporous membranes based on liquid crystalline polymers

In the present review, we discuss the most significant recent developments in the field of nanoporous membranes based on liquid crystalline lyotropic and thermotropic polymer networks.

A scattering electro-optical switch based on dendrimers dispersed in liquid crystals

The dispersion of dendrimers in a liquid crystal (LC) is reported as a new approach to LC displays based on light scattering. It is shown that the performance of a fifth-generation palmitoyl-functionalized poly(propylene imine) dendrimer - with 64 legs, as shown in the Figure - as an electro-optical switch is superior to that of polymer-filled nematics and polymer-dispersed LC systems (see also cover).

Viscoelastic liquid crystal colloids for the continuous processing of twisted nematic electro-optical cells

Liquid crystal colloid materials are described based on the liquid crystal (LC) E7 and submicron sized poly(methyl methacrylate-co-divinylbenzene) particles. Application of a thermal treatment to the composite material produces a finely dispersed network of the internally crosslinked polymeric inclusions in the LC-E7. Dynamic rheological measurements on the LC colloids show that the presence of this network imposes pronounced viscoelastic behavior on the material, which may be exploited in the manufacturing of large-area twisted nematic (TN) electro-optical cells via continuous methods as an alternative to the currently available batchwise routes. The electro-optical characteristics of TN cells based on the composite material are approximately comparable to the electro-optical characteristics of a reference cell filled with pure LC E7, which ensures that the largely increased viscoelasticity of the composite does not lead to a degradation of electro-optical properties.

Sub‐5 nm Patterning by Directed Self‐Assembly of Oligo(Dimethylsiloxane) Liquid Crystal Thin Films

Highly ordered nanopatterns are obtained at sub-5 nm periodicities by the graphoepitaxial directed self-assembly of monodisperse, oligo(dimethylsiloxane) liquid crystals. These hybrid organic/inorganic liquid crystals are of high interest for nanopatterning applications due to the combination of their ultrasmall feature sizes and their ability to be directed into highly ordered domains without additional annealing.

A dielectric study on the relaxation and switching behaviour of liquid crystals confined within a colloidal network

The molecular ordering and dynamics of a liquid crystal (LC E7) in the presence of a three-dimensional network of submicron particles have been studied by dielectric relaxation spectroscopy. The field-dependent orientation of the LC was quantified by the director order parameter and modelled by use of a three-phase model. The influence of the colloidal network on the molecular dynamics was assessed from the dielectric spectra, e.g. from the position of relaxation peaks as well as from the strength of the two principal relaxations (α and γ). The spectra changed noticeably upon application of an increasing d.c. bias. A reduction of the threshold field was observed upon addition of colloidal particles to the LC. This was associated with a switching between two metastable states induced by anchoring on the filler particles. Modelled spectra were found to be in good agreement with the experimental data. The modelling showed that the confined LC phase is composed of two fractions, viz. an ordered and a disordered one with different molecular mobilities. Furthermore, switching experiments were conducted at various temperatures in order to evaluate the impact of the colloidal network on the (temperature-dependent) orientational behaviour of the LC molecules. For the colloid-filled LC higher conductivities were found, which gave rise to longer switch-off times.

Orientational properties and dynamics of nematic liquid crystals mixed with dendrimers for electro-optical switches

Novel two-phase materials based on nematic liquid crystals (LC E7) filled with poly(propylene imine) dendrimers of 2–3u2009nm diameter were investigated for application in light-scattering, electro-optical switches. Polarizing optical microscopy, differential scanning calorimetry and rheological experiments all indicate the presence of a pronounced polydomain structure within these LC mixtures. The molecular ordering and dynamics of the dendrimer-filled nematics were studied in more detail by dielectric relaxation spectroscopy. The director order parameter S d was derived from the strength of the two principal molecular relaxations, α and δ, in the presence and absence of an electric field. In addition, a three-phase model was applied for determining the size of the different LC populations in the mixture. A substantial decrease in the fraction of randomly oriented LC was found upon application of a d.c. field. Over a broad temperature range the LC molecular dynamics were assessed from the variation in the strength of the dielectric relaxations as a function of the d.c. bias. A fast response was found at room temperature. Electro-optical switching experiments showed that the dendrimer-filled LCs can be switched rapidly from a light-scattering, opaque state to a more transparent state.

Water-responsive dual-coloured photonic polymer coatings based on cholesteric liquid crystals

This work describes a straightforward method to prepare patterned photonic coatings which alter their colour when exposed to water. Various kinds of dual-coloured patterns were made, which become visible or fade away when placed in water. These effects are reversible and can be repeated many times.

An efficient illumination system for liquid crystal displays incorporating an anisotropic hologram

An anisotropic hologram is combined with an edge-lit planar waveguide to produce an improved liquid crystal display (LCD) backlight. A holographic-polymer dispersed liquid crystal material is exposed to a slanted one-dimensional interference pattern to produce an anisotropic Bragg transmission grating with strong diffraction for P-polarized light and very low diffraction for S-polarized light. While the hologram is recorded at a UV wavelength (351 nm), light at visible wavelengths propagating from the waveguide edge is redirected toward the normal direction. The emission is collimated, polarized, and unidirectional, effectively integrating several functions that are typically embodied in separate optical films in a conventional LCD.

4D printed actuators with soft-robotic functions

Soft matter elements undergoing programed, reversible shape change can contribute to fundamental advance in areas such as optics, medicine, microfluidics, and robotics. Crosslinked liquid crystalline polymers have demonstrated huge potential to implement soft responsive elements; however, the complexity and size of the actuators are limited by the current dominant thin-film geometry processing toolbox. Using 3D printing, stimuli-responsive liquid crystalline elastomeric structures are created here. The printing process prescribes a reversible shape-morphing behavior, offering a new paradigm for active polymer system preparation. The additive character of this technology also leads to unprecedented geometries, complex functions, and sizes beyond those of typical thin-films. The fundamental concepts and devices presented therefore overcome the current limitations of actuation energy available from thin-films, thereby narrowing the gap between materials and practical applications.