Depth Profile of Optically Recorded Patterns in Light-Sensitive Liquid Crystal Elastomers

We investigated nonlinear absorption and photobleaching processes in a liquid crystal elastomer (LCE) doped with light-sensitive azobenzene moiety. A conventional one-dimensional holographic grating was recorded in the material with the use of two crossed UV laser beams and the angular dependence of the diffraction efficiency in the vicinity of the Bragg peak was analyzed. These measurements gave information on the depth to which trans to cis isomerisation had progressed into the sample as a function of the UV irradiation time. Using a numerical model that takes into account the propagation of writing beams and rate equations for the local concentration of the absorbing trans conformer, we computed the expected spatial distribution of the trans and cis conformers and the shape of the corresponding Bragg diffraction peak for different irradiation doses. Due to residual absorption of the cis conformers the depth of the recording progresses logarithmically with time and is limited by the thermal relaxation from the cis to trans conformation.Comment: 19 pages (incl. figs), 6 figure

Peculiar behaviour of optical polarization gratings in light-sensitive liquid crystalline elastomers

The angular dependence of the diffraction efficiency of volumetype holographic gratings recorded in a single-domain light-sensitive liquid crystalline elastomer was investigated. Usually this dependence is expected to be very similar for intensity gratings and for polarization gratings. However, our measurements resolved a profound difference between the two types of the gratings: a typical Bragg peak of the diffraction efficiency is observed only for intensity gratings, while polarization gratings exhibit a profound dip at the Bragg angle. The appearance of this dip is explained by strongly anisotropic optical absorption of the actinic light during the recording process

Spontaneous Liquid Crystal and Ferromagnetic Ordering of Colloidal Magnetic Nanoplates

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magneto-mechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in absence of a field. Here we report a fluid suspension of magnetic nanoplates which spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth's magnetic field

Splay nematic phase

Different liquid crystalline phases with long-range orientational but not positional order, so-called nematic phases, are scarce. It rarely occurs that a new nematic phase is discovered and such event is inevitably accompanied by a great interest. Here, we describe a transition from uniaxial to novel nematic phase characterized by a periodic splay modulation of the director. In this new nematic phase, defect structures not present in the uniaxial nematic are observed, which indicates that the new phase has lower symmetry than the ordinary nematic phase. The phase transition is weakly first order with a significant pretransitional behavior, which manifests as strong splay fluctuations. When approaching the phase transition, the splay nematic constant is unusually low and goes towards zero. Analogously to the transition from the uniaxial nematic to the twist-bend nematic phase, this transition is driven by instability towards splay orientational deformation, resulting in a periodically splayed structure. And, similarly, a Landau-de Gennes type of phenomenological theory can be used to describe the phase transition. The modulated splay phase is biaxial and antiferroelectric

Ferroelectric nematic liquid crystalline phases

Recent experimental realization of ferroelectric nematic liquid crystalline phases stimulated material development and numerous experimental studies of these new phases, guided by their fundamental and applicative interest. In this Perspective, we give an overview of this emerging field by linking history and theoretical predictions to a general outlook of the development and properties of the materials exhibiting ferroelectric nematic phases. We will highlight the most relevant observations up-to-date, e.g., giant dielectric permittivity values, polarization values an order of magnitude larger than in classical ferroelectric liquid crystals, and NLO coefficients comparable to several ferroelectric solid materials. Key observations of anchoring and electro-optic behavior will also be examined. The collected contributions lead to a final discussion on open challenges in materials development, theoretical description, experimental explorations, and possible applications of the ferroelectric phases.Comment: 28 pages, 17 Figure

Učinki plazemske obdelave na sorpcijo vode v viskoznih vlaknih

We investigated water sorption in viscose nonwoven fibres manufactured by Tosama d.d. with the surface density of 175 g/m2. A comparison between untreated fibres and by oxygen plasma treated fibres was made using optical polarization microscopy. Plasma treatment was done for 10 minutes at pressure of 75 Pa at current of 250 mA at the power of 500 W. Swelling was characterized by measurements of fibre diameter. Modifications of intensity of the polarized light transmitted through the fibre were measured as a function of time of exposure to water. Characteristic swelling and intensity modification times were resolved for untreated and oxygen plasma treated fibres. The swelling time of oxygen plasma in comparison to untreated plasma is reduced by the factor of 0.54 and intensity change time by the factor of 0.4. From the characteristic swelling and intensity change times it was concluded that oxygen plasma treatment of viscose increases the speed of water sorption.Proučevali smo sorpcijo vode pri vlaknih v viskozni kopreni, ki jo proizvaja podjetje Tosama d.d., s površinsko gostoto 175 g/m2. Primerjali smo neobdelana in s kisikovo plazmo obdelana vlakna z uporabo optično-polarizacijske mikroskopije. Plazemska obdelava je trajala 10 minut pri tlaku 75 Pa, toku 250 mA in pri moči 500 W. Nabrekanje viskoznih vlaken smo opazovali preko sprememb premera vlakna. Z optično polarizacijsko mikroskopijo smo merili spremembe intenzitete polarizirane svetlobe, ki potuje skozi vlakno med močenjem. Iz meritev na neobdelanih in s plazmo obdelanih vlaknih smo ocenili karakteristične čase nabrekanj vlaken in spremembe intenzitete polarizirane svetlobe, ki potuje skozi vlakna. Pri s plazmo obdelanih vlaknih se časi nabrekanja zmanjšajo za faktor 0.54 in čas spremembe intenzitete za 0.4. Iz meritev smo zaključili, da obdelava s kisikovo plazmo pohitri proces sorpcije vode

Magnetic polydomain liquid crystal elastomers – synthesis and characterisation

Although liquid crystal elastomers (LCE) are a fascinating class of materials with interesting thermomechanical properties of their own, the aim is to enhance their performance or add new features, e.g. response to external stimuli. The generally weak response of organic materials can be significantly intensified by mixing nano-sized magnetic particles into the host polymer matrix. An alternative approach is chemically coupling the nanoparticles to the elastomer. We achieved this by bonding functionalised magnetic nanoplatelets to the backbone of a main-chain LCE and obtained polydomain magnetic liquid crystal elastomers. We measured the magnetisation curves in samples, which were exposed to either small or large magnetic fields – their response being a consequence of partial particle reorientation or magnetic moment flipping. In contrast to the samples, which were exposed to small magnetic field and in which the remanent magnetisation can be reset to zero by heating the sample, the samples with flipped magnetisation within the platelets cannot be reversed into the original state. Coupling of magnetic and mechanical properties shows a slight magneto-elastic response at elevated temperatures and a significant inverse magneto-elastic effect: the magnetisation caused by mechanical stretching is almost equal to the magnetisation caused by an external magnetic field