This Review discusses stimuli-responsive liquid crystalline polymer networks and elastomers as materials with programmable mechanics for use in functional devices. Liquid crystals are the basis of a pervasive technology of the modern era. Yet, as the display market becomes commoditized, researchers in industry, government and academia are increasingly examining liquid crystalline materials in a variety of polymeric forms and discovering their fascinating and useful properties. In this Review, we detail the historical development of liquid crystalline polymeric materials, with emphasis on the thermally and photogenerated macroscale mechanical responses — such as bending, twisting and buckling — and on local-feature development (primarily related to topographical control). Within this framework, we elucidate the benefits of liquid crystallinity and contrast them with other stimuli-induced mechanical responses reported for other materials. We end with an outlook of existing challenges and near-term application opportunities.

Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers

A review of stimuli-responsive shape memory polymer composites

An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.

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Bent-core liquid crystals: polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems

Electrically conductive polymeric materials have recently attracted considerable interest from academic and industrial researchers to explore their potential in biomedical applications such as in biosensors, drug delivery systems, biomedical implants and tissue engineering. Conventional conductive homopolymers such as polypyrrole and PEDOT show promising conductivity for these applications, however their mechanical properties, biocompatibility and processability are often poor. This has led to more recent attention being directed towards conductive polymeric composites comprised of biostable/biocompatible polymers with dispersed conductive fillers such as graphene, carbon nanotubes and metallic nanoparticles. The major objective of this paper is to provide an up to date review of the recent investigations conducted in the development of conductive polymer composites focussing on the methods of their preparation, underlying concepts of their conductivity and the ways to tailor their properties. Furthermore, recent progress made in conventional conducting polymers and their composites/blends for biomedical applications is also discussed.

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Electrically conductive polymers and composites for biomedical applications

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Ionic Liquid Crystals: Versatile Materials.

This work reports a new approach to extract the maximum chemical information from the absorption spectrum of extra virgin olive oils (EVOOs) in the 390–720 nm spectral range, where “oil pigments” dominate the light absorption. Four most important pigments, i.e., two carotenoids (lutein and β-carotene) and two chlorophylls (pheophytin-a and pheophytin-b), are chosen as reference oil pigments, being present in all the reported analytical data regarding pigments of EVOOs. The method allows the quantification of the concentration values of these four pigments directly from the deconvolution of the measured absorption spectrum of EVOOs. Advantages and limits of the method and the reliability of the pigment family quantification are discussed. The main point of this work is the description of a fast and simple method to extract of such information in less than a minute, through the mathematical analysis of the UV–vis spectrum of untreated samples of oil.

Extraction of pigment information from near-UV vis absorption spectra of extra virgin olive oils.

NMR in chiral and achiral smectic phases: Structure, orientational order and dynamics

The need for mechanical manipulation during the curing of conventional liquid crystal elastomers diminishes their applicability in the field of shape-programmable soft materials and future applications in additive manufacturing. Here we report on polymer-dispersed liquid crystal elastomers, novel composite materials that eliminate this difficulty. Their thermal shape memory anisotropy is imprinted by curing in external magnetic field, providing for conventional moulding of macroscopically sized soft, thermomechanically active elastic objects of general shapes. The binary soft-soft composition of isotropic elastomer matrix, filled with freeze-fracture-fabricated, oriented liquid crystal elastomer microparticles as colloidal inclusions, allows for fine-tuning of thermal morphing behaviour. This is accomplished by adjusting the concentration, spatial distribution and orientation of microparticles or using blends of microparticles with different thermomechanical characteristics. We demonstrate that any Gaussian thermomechanical deformation mode (bend, cup, saddle, left and right twist) of a planar sample, as well as beat-like actuation, is attainable with bilayer microparticle configurations. Liquid crystal elastomers are promising for building actuators due to excellent thermomechanical response, but it is challenging to prepare them at macroscale with arbitrary shape. Here, the authors overcome this restriction by doping microparticles to the polymer matrix without employing mechanical stressing.

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Polymer-dispersed liquid crystal elastomers.

In this work the orientation of banana-shaped molecules in the magnetic field is investigated. A new and original hypothesis of the peculiar aggregation of banana-shaped molecules in the nematic phase in the presence of a magnetic field is here reported. Deuterium NMR measurements on two selectively deuterium labelled mesogens and preliminary calculations of the magnetic susceptibility anisotropy of the aromatic core of these bent molecules are reported and discussed to support our hypothesis.

How do banana-shaped molecules get oriented (if they do) in the magnetic field?

Within the apicultural products, the honey bee-pollen is growing in commercial interest due to its high nutritional properties. For the first time, bee-pollen samples from Tuscany (Italy) were studied to evaluate botanical origin, phytochemical composition and antioxidant activity. The investigated pollen loads were composed of three botanical families: Castanea, Rubus and Cistus. The highest levels of proteins and lipids were detected in Rubus pollen. Castanea pollen contained greater polyphenols, flavonoids and anthocyanins content, while the highest flavonols level was detected in Cistus pollen. These results were also confirmed by front-face fluorescence spectroscopy, used here, for the first time, as a fast tool to characterize bee-pollens.

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Valentina Domenici

Papers

Extraction of pigment information from near-UV vis absorption spectra of extra virgin olive oils.

NMR in chiral and achiral smectic phases: Structure, orientational order and dynamics

Polymer-dispersed liquid crystal elastomers.

How do banana-shaped molecules get oriented (if they do) in the magnetic field?

Phytochemical composition and antioxidant activity of Tuscan bee pollen of different botanic origins