Shaping helical electrospun filaments: a review.

Nature abounds with helical filaments designed for specific tasks. For instance, some plants use tendrils to coil and attach to the surroundings, while Spiroplasma, a helical bacterium, moves by inverting the helical handedness along the filament axis. Therefore, developing methods to shape filaments on demand to exhibit a diversity of physical properties and shapes could be of interest to many fields, such as the textile industry, biomedicine or nanotechnology. Electrospinning is a simple and versatile technique that allows the production of micro and nanofibres with many different helical shapes. In this work, we review the different electrospinning procedures that can be used to obtain helical shapes similar to those found in natural materials. These techniques also demonstrate that the creation of helical shapes at the micro/nanoscale is not limited by the chirality of the building blocks at the molecular level, a finding which opens new horizons on filament shaping.

Hygroscopic study of hydroxypropylcellulose : Structure and strain-induced birefringence of capillary bridges.

The hygroscopic method developed previously for studies of lyotropic liquid crystals is used for the first time in experiments with millimetric capillary bridges made of a hydroxypropylcellulose/water mixture. Composition of such very small samples is controlled via humidity of the surrounding air. By a slow and well-controlled drying of initially isotropic samples, the isotropic/anisotropic phase transition is crossed and polydomain pseudo-isotropic capillary bridges are prepared. Kept in an atmosphere of constant humidity, these bridges are stretched and the strain-induced birefringence [Formula: see text] n is measured as a function of the draw ratio [Formula: see text] . The variation of [Formula: see text] n with [Formula: see text] is interpreted in terms of an affine uniaxial deformation of the initial pseudo-isotropic texture.

¹H-²H cross-relaxation study in a partially deuterated nematic liquid crystal.

A detailed study of the cross-relaxation effects between the ¹H and ²H spins systems is presented in the nematic phase of a 5-cyanobiphenyl (5CB) liquid crystal, partially deuterated at α position (5CB-αd2). The proton spin-lattice relaxation time was measured at a frequency range from 5 kHz to 100 MHz at a temperature 5 K below the nematic-isotropic phase transition. In the low frequency domain, the spin-lattice relaxation rate (T1⁻¹) dispersion clearly differs from that of the fully protonated 5CB homologue. At two distinct frequencies, T1⁻¹ presents two distinct local maxima and for low frequencies T1⁻¹ presents a stronger frequency dependence when compared with what is observed for 5CB. The T1⁻¹ dispersion obtained for 5CB-αd2 for frequencies above 60 kHz was interpreted in terms of the relaxation mechanisms usually accepted to interpret the spin-lattice relaxation in nematic phases in general and 5CB in particular. For lower frequencies it was necessary to consider cross-relaxation contributions between the proton and deuterium reservoirs. A detailed model interpretation of the deuterium quadrupolar dips with respect to the proton-spin relaxation is presented. The analysis of the quadrupolar relaxation independently confirms that the order director fluctuations is the dominant mechanism of proton relaxation in the low frequency domain.

Longitudinal versus polar wrinkling of core-shell fibers with anisotropic size mismatches.

We consider a fiber made of a soft elastic material, encased in a stiff elastic shell (core-shell geometry). If the core and shell dimensions are mismatched, e.g., because the core shrinks while the shell does not, but the two remain attached, then an elastic instability is triggered whereby wrinkles may appear on the shell. The wrinkle orientation may be longitudinal (along the fiber axis), polar (along the fiber perimeter), or a mixture of both, depending on the fiber's geometrical and material parameters. Here we investigate under what conditions longitudinal or polar wrinkling will occur.

Micro- and nanofibers and liquid crystals for light-scattering shutters: simulation of electro-optical properties.

This work demonstrates the feasibility of using polymeric micro- and nanofiber-composed films and liquid crystals as electrically switchable scattering light shutters. We present a concept of electro-optic device based on an innovative combination of two mature technologies: optics of nematic liquid crystals and electrospinning of nanofibers. These devices have electric and optical characteristics far superior to other comparable methods. The simulation presented shows results that are highly consistent with those of experiments and that explain the working mechanism of the devices.

Nanocrystalline cellulose applied simultaneously as the gate dielectric and the substrate in flexible field effect transistors.

Cotton-based nanocrystalline cellulose (NCC), also known as nanopaper, one of the major sources of renewable materials, is a promising substrate and component for producing low cost fully recyclable flexible paper electronic devices and systems due to its properties (lightweight, stiffness, non-toxicity, transparency, low thermal expansion, gas impermeability and improved mechanical properties).Here, we have demonstrated for the first time a thin transparent nanopaper-based field effect transistor (FET) where NCC is simultaneously used as the substrate and as the gate dielectric layer in an 'interstrate' structure, since the device is built on both sides of the NCC films; while the active channel layer is based on oxide amorphous semiconductors, the gate electrode is based on a transparent conductive oxide.Such hybrid FETs present excellent operating characteristics such as high channel saturation mobility (>7 cm(2) V (-1) s(-1)), drain-source current on/off modulation ratio higher than 10(5), enhancement n-type operation and subthreshold gate voltage swing of 2.11 V/decade. The NCC film FET characteristics have been measured in air ambient conditions and present good stability, after two weeks of being processed, without any type of encapsulation or passivation layer. The results obtained are comparable to ones produced for conventional cellulose paper, marking this out as a promising approach for attaining high-performance disposable electronics such as paper displays, smart labels, smart packaging, RFID (radio-frequency identification) and point-of-care systems for self-analysis in bioscience applications, among others.

Improved mechanical stability of acetoxypropyl cellulose upon blending with ultranarrow PbS nanowires in Langmuir monolayer matrix.

Cellulose and cellulose derivatives have long been used as membrane fabrication. Langmuir monolayer behavior, which naturally mimics membranes, of acetoxypropyl cellulose (APC) and lead sulfide (PbS) nanowire mixtures at different volume ratios is reported. Surface pressure (π)-area (A) isotherms of APC and PbS nanowires mixtures at different volume ratios show a gradual decrease in the monolayer area with increasing volume fraction of PbS nanowires. Change of surface potential with monolayer area at different volume ratios also reveals a gradual increase in the surface potential indicating incorporation of PbS nanowires within APC matrix. The compressibility and elastic constants measurements reveal an enhancement of the elasticity upon incorporation of PbS nanowires up to certain volume fractions. An enhancement in stability of the blend is observed upon PbS nanowire incorporation to the APC matrix. Rheological measurements also support the robustness of the mixture of APC and PbS nanowires in 3D bulk phase. Such robust ultrathin films of cellulose based-nanowire blend obtained by means of the Langmuir technique may lead to novel routes for designing cellulosic-based thin films and membranes.

Luminescent elastomeric Janus particles.

We report on a low-cost and low-tech method for the preparation of luminescent micro- and millimeter elastomeric particles with asymmetric morphology. The method of fabrication consists in UV-irradiating soft urethane/urea fluorescent spheres, which are then extracted in toluene and dried. Wrinkles appear on the irradiated portions of the particles surfaces, and the spatial periodicity can be controlled with variation in UV irradiation time and the amount of the luminescent compound. The spheres are thus composed of an urethane/urea network in which the tris(8-hydroxyquinolinato)aluminum (Alq3) fluorescent compound was incorporated. The asymmetric morphology and the optical properties of the resultant particles have been confirmed by scanning electron microscopy, atomic force microscopy, optical microscopy, and UV-Vis spectrophotometry. The system shows negligible leaching, and the encapsulation of the Alq3 without recourse to covalent bonding to the polymeric matrix has the advantage of allowing the tuning of the spheres morphology and fluorescence.

Fast field-cycling NMR relaxometry study of chiral and nonchiral nematic liquid crystals.

We present a proton NMR relaxometry study of the molecular dynamics in three liquid crystalline systems: 4'-n-pentyl-4-cyanobiphenyl (5CB), (S)-4'-(3-methylpentyl)-4-cyanobiphenyl (5CB*), and a 12% weight mixture of 5CB* in 5CB. The proton spin-lattice relaxation time (T1) was measured as a function of temperature and Larmor frequency in the isotropic, nematic, chiral nematic (N*), and smectic A phases of these liquid crystalline systems. A unified relaxation model was used to analyze the molecular dynamics, considering local molecular rotations/reorientations, translational self-diffusion, and collective motions as the relaxation mechanisms that contribute most effectively to the T1(-1) relaxation. Additionally, in the chiral nematic phase a fourth relaxation mechanism associated with the rotations induced by the translational diffusion along the helical axis (RMTD) was included in the model. All experimental results were consistently analyzed taking into account the physical parameters known for 5CB. The global analysis of the experimental results shows that the RMTDs are associated with the pitch value measured for the N* phases and that its contribution to the T1(-1) dispersion is observed at low frequencies. The T1(-1) dispersion in the smectic A phase of 5CB* is strongly dominated by the layer undulations relaxation mechanism over a broad frequency range from the low kilohertz regime to tens of megahertz. It was the first time such behavior was observed in a low molecular weight liquid crystalline system.

Cellulose-based liquid crystalline photoresponsive films with tunable surface wettability.

We report on a new type of liquid crystalline cellulosic films with light controllable reversible wettability. The films are prepared from a thermotropic cellulose derivative functionalized with azo-containing groups. These groups exhibit dynamic changes in interfacial properties in response to UV irradiation. The UV irradiation induces trans-to-cis isomerization in the azobenzene moiety, which causes a conformational change in the upper molecular layers of the thin films. These changes originate a hydrophobic to comparatively hydrophilic transformation of the surface. The reversible wettability of the surface results from the cis/trans photo and thermal isomerization. The UV-vis absorption spectra, as well as contact angle measurements with UV irradiation, clearly support the understanding of the phenomenon. This type of surface design enables the amplification of molecular level conformational transitions to macroscopic changes in interface properties using the means of isomerism. This opens new opportunities in surface engineering using eco-friendly cellulose manipulation.

Free-standing urethane/urea elastomer films undoped and doped with ferro-nano-particles.

We report on an experimental study of the structures presented by urethane/urea elastomeric films without and with ferromagnetic nanoparticles incorporated. The study is made by using the X-ray diffraction, nuclear magnetic resonance (NMR), optical, atomic and magnetic force (MFM) microscopy techniques, and mechanical assays. The structure of the elastomeric matrix is characterized by a distance of 0.46 nm between neighboring molecular segments, almost independent on the stretching applied. The shear casting performed in order to obtain the elastomeric films tends to orient the molecules parallel to the flow direction thus introducing anisotropy in the molecular network which is reflected on the values obtained for the orientational order parameter and its increase for the stretched films. In the case of nanoparticles-doped samples, the structure remains nearly unchanged although the local order parameter is clearly larger for the undoped films. NMR experiments evidence modifications in the molecular network local ordering. Micrometer size clusters were observed by MFM for even small concentration of magnetic particles.

Study of micro and nano surface structures from UV irradiated urethane/urea elastomers.

In this work we address new results obtained with a thin free standing flexible film (approximately 120 microm) of a urethane/urea copolymer related to the formation of micro and nano size structures [M.H. Godinho, A.C. Trindade, J.L. Figueirinhas, L.V. Melo, P. Brogueira, Synthetic Metals, 147(1-3), 209 (2004); M.H. Godinho, A.C. Trindade, J.L. Figueirinhas, L.V. Melo, P. Brogueira, Molecular Crystals and Liquid Crystals (2005)]. The copolymer was synthesized from a polypropylene oxide-based prepolymer with three isocyanate terminal groups (PU) and polybutadienediol (PBDO) with PBDO content of 40% wt. After casting and curing the film was cut into different samples and each exposed to UV radiation for different periods of time; 23, 25, 26, 31 and 49 h (lambda=254 nm) and later extracted with toluene and dried. The dried films were then studied by polarising optical microscopy (POM), small angle light scattering (SALS) and the surfaces exposed to UV radiation analyzed by means of atomic force microscopy (AFM). Before extraction with toluene a nanometer-flat surface, characterized by a mean roughness value Ra=0.59 nm, was obtained. Depending on exposure time to UV radiation and after extraction with toluene a corrugated surface, with features mum-sized in all axes, resulting in an increase of the overall mean roughness value to Ra=50.7 nm, starts to develop after 25 h of exposure time. This work gives evidence of the non-monotonous time behavior of the wrinkled surface growth that develops under the action of ultraviolet radiation. As the exposure time increases the free-standing films directly exposed surfaces show a decreasing density of the structures observed and an increasing characteristic peak-to-valley height. The peak-to-valley height measured for samples exposed for 23, 25, 26, 31 and 49 h, respectively 193, 383, 381, 1550 and 2039 nm and the corresponding mean roughness values are Ra=50.7 nm, 105.4, 116.8, 438.3 and 515.4 nm, respectively. Between 26 and 31 h exposure time a leap in both values, peak-to-valley and Ra, was observed. The sudden increase in these values is correlated to fabrication of wrinkles by uniaxially stretching PU/PBDO elastomer films.

Staphylococcus epidermidis adhesion on modified urea/urethane elastomers.

Block urea/urethane co-polymer films present elastomeric properties with the possible tuning of their surface properties within a wide range and are therefore considered relevant surfaces for possible medical applications. In particular, thin free standing films of urea/urethane elastomers with two soft segments, polypropylene oxide and more hydrophobic polybutadiene, develop multistable states with surface topography features with remarkable regularity. Moreover, complex surface structures may be obtained by UV radiation treatment followed by suitable mechanical action and also by extraction of the elastomer with a suitable solvent. In the present work, different modified elastomer samples were assayed for Staphylococcus epidermidis adhesion during 2 h and the extent of bacterial adhesion was evaluated by automatic cell enumeration. Bacterial adhesion assays demonstrate that the typical trend relating the increase in the number of adhered bacteria with the increase of the surface roughness does not hold for all materials. Results may be interpreted taking into account both the surface topography and the different types of micro-phase segregation of hydrophobic and hydrophilic parts of the elastomer.

Tuneable micro- and nano-periodic structures in a free-standing flexible urethane/urea elastomer film.

We have studied the control and manipulation of tuneable equilibrium structures in a free-standing urethane/urea elastomer film by means of atomic force microscopy, small-angle light scattering and polarising optical microscopy. The urethane/urea elastomer was prepared by reacting a poly(propyleneoxide)-based triisocyanate-terminated prepolymer (PU) with poly(butadienediol) (PBDO), with a weight ratio of 60% PU/40% PBDO. An elastomer film was shear-cast onto a glass plate and allowed to cure, first in an oven, then in air. Latent micro- and nano-periodic patterns are induced by ultra-violet (UV) irradiation of the film and can be "developed" by applying a plane uniaxial stress or by immersing the elastomer in an appropriate solvent and then drying it. For this elastomer we describe six pattern states, how they are related and how they can be manipulated. The morphological features of the UV-exposed film surface can be tuned, reproducibly and reversibly, by switching the direction of the applied mechanical field. Elastomers extracted in toluene exhibit different surface patterns depending upon the state in which they were developed. Stress-strain data collected for the films before and after UV irradiation reveal anisotropy induced by the shear-casting conditions and enhanced by the mechanical field. We have interpreted our results by assuming the film to consist of a thin, stiff surface layer ("skin") lying atop a thicker, softer substrate ("bulk"). The skin's higher stiffness is hypothesised to be due to the more extensive cross-linking of chains located near the surface by the UV radiation. Patterns would thus arise as a competition between the effects of bending the skin and stretching/compressing the bulk, as in the work of Cerda and Mahadevan (Phys. Rev. Lett. 90, 074302 (2003)). We present some preliminary results of a simulation of this model using the Finite Element package ABAQUS.