M. T. Cidade

Shear induced textures of thermotropic acetoxypropylcellulose

Acetoxypropylcellulose is a thermotropic cellulose derivative which is cholesteric from below room temperature up to 170 °C. Its textures during and after shear flow were studied by optical microscopy and small-angle light scattering (SALS) light scattering. The main features are similar to those occurring in hydroxypropylcellulose solutions, i.e., a polydomain texture evolving into a striated texture, as seen by optical microscopy. Elliptical or else four lobes with a streak are seen in the SALS patterns. Defects appearing as short dark lines not aligned with the flow direction are shown to be responsible for the existence of the four lobes of the SALS patterns. In addition these defects seem to be located where the band texture develops during relaxation.

Cross-linked hydroxypropylcellulose films: mechanical behaviour and electro-optical properties of PDLC type cells

We study the effect of the amount of cross-linking agent upon the mechanical and electro-optical behaviour of several cells prepared from hydroxypropylcellulose (HPC) cross-linked with 1,4 diisocianatobutane (BDI) (0.0%, 3.1%, 7.6%, 12.3% w/w). The tensile properties and the sol/gel fractions were obtained as a function of the amount of the cross-linking agent used to prepare the solid films. The Youngs modulus appears to be constant, over the range of concentrations studied. The electro-optical cells prepared with the nematic liquid crystal (E7) were analysed by light transmission. Several trends were observed, e.g. the cross-linking agent, at the percentages used, has a strong influence on the cells contrast but not on the cells maximum transmission or turn on voltage, while the film thickness shows preponderantly its influence on the cells maximum transmission and turn on voltage. The mechanical properties of the HPC films are important for a recent application that is now being developed [Liq. Cryst., in press], where a flexible substrate is used. The results obtained point out ways for the realisation of an optimum electro-optical cell.

The use of rheology in the study of the composition effects on the fresh behaviour of hydraulic lime grouts for injection of masonry walls

The injection of grouts inside multi-leaf stone masonries is a technique widely used for structural consolidation. To ensure an adequate flow of the grout inside the masonry, it is crucial to assure good fresh grout properties, such as good rheological behaviour. The scope of this paper is to provide preliminary indications and valuable data about the effects of specific hydraulic lime grout composition on their rheological behaviour with the purpose of a successful injection process. Through the use of rotational rheometer together with the Taguchi method, it was possible to study the influence of water/binder ratio, the type and dosage of superplasticizer and the partial replacement of hydraulic lime by silica fume, upon the grout rheological properties. The study leads to the conclusion that polycarboxylate-based superplasticizers present better performance from a rheological point of view than the naphthalene-based superplasticizer and that the water/binder ratio and superplasticizer dosage are the most determinant factors in the fresh grout rheological behaviour. On the other hand, silica fume dosage turned out to be the factor with the least contribution to improve the grout rheological behaviour compared to the other two factors reported in this study. The results summarised in this paper are part of a larger study and precede the analysis of the performance of those grouts when injected into different porous media that simulate old masonries.

Electro-rheology study of a series of liquid crystal cyanobiphenyls: experimental and theoretical treatment

In this work we study the electro-rheological behaviour of a series of four liquid crystal (LC) cyanobiphenyls with a number of carbon atoms in the alkyl group, ranging from five to eight (5CB–8CB). We present the flow curves for different temperatures and under the influence of an external electric field, ranging from 0 to 3 kV mm−1, and the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory of Leslie–Ericksen for low molecular weight nematic LCs. In our analysis, the director alignment angle is only a function of the ratio between the shear rate and the square of the electric field – boundary conditions are neglected. By fitting the theoretical model to the experimental data, we are able to determine some viscosity coefficients and the dielectric anisotropy as a function of temperature. To interpret the behaviour of the flow curves near the nematic–isotropic transitions, we apply the continuum theory of Olmsted–Goldbart, which extends the theory of Leslie–Ericksen to the case where the degree of alignment of the LC molecules can also vary.

Electrorheological properties of polyaniline-vanadium oxide nanostructures suspended in silicone oil

In the present work, organic/inorganic hybrid nanostructures comprised of polyaniline and vanadium oxide were synthesized via a simple hydrothermal technique. The polyaniline/vanadium oxide hybrid morphology was tailored from rods to spheres by controlling the relative concentration of the reactants. The synthesized composites were characterized by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM) for structural and morphological analyses. Electrorheological (ER) properties of the as-prepared nanocomposites suspended in silicone oil were investigated by a rotational viscometer under both steady and dynamic shear. The ER activity of the composite material suspensions showed higher ER effects for the product with the rod-like structures than for the product with the sphere-like structures. The typical ER behaviour showed by the polyaniline-vanadium oxide nanocomposites demonstrated their potential application as an ER smart material.

Polyurea dendrimer for efficient cytosolic siRNA delivery

The design of small interfering RNA (siRNA) delivery materials showing efficacy in vivo is at the forefront of nanotherapeutics research. Polyurea (PURE-type) dendrimers are ‘smart’ biocompatible 3D polymers that unveil a dynamic and elegant back-folding mechanism involving hydrogen bonding between primary amines at the surface and tertiary amines and ureas at the core. Similarly, to a biological proton pump, they are able to automatically and reversibly transform their conformation in response to pH stimulus. Here, we show that PURE-G4 is a useful gene silencing platform showing no cellular toxicity. As a proof of concept we investigated the PURE-G4-siRNA dendriplex, which was shown to be an attractive platform with high transfection efficacy. The simplicity associated with the complexation of siRNA with polyurea dendrimers makes them a powerful tool for efficient cytosolic siRNA delivery.

Synthesis and characterization of novel liquidcrystalline polyurethanes

Abstract Novel polyurethanes have been synthesized by the condensation of two rigid diols, biphenyl-4,4′-diol (BPH(4,4′)D) and 4-hydroxyphenyl-4-hydroxybenzoate (4HPH4′HB), with the flexible hexamethylene diisocyanate (HDI). Their phase diagrams were established by means of differential scanning calorimetry, nuclear magnetic resonance and polarizing optical microscopy. Poly(4HPH4′HB/HDI) was found to display a nematic phase between 140 and 199°C; poly (BPH(4,4′)D/HDI) also exhibits a mesomorphic phase, but so far the nature of this phase has not been established.

An electro-rheological study of the nematic liquid crystal 4-n-heptyl-4'-cyanobiphenyl

An experimental and theoretical study of the electro-rheological effects observed in the nematic phase of 4-n-heptyl-4′-cyanobiphenyl has been conducted. This liquid crystal appears to be a model system, in which the observed rheological behaviour can be interpreted by the Leslie–Ericksen continuum theory for low molecular weight liquid crystals. Flow curves are illustrated at different temperatures and under the influence of an external electric field ranging from 0 to 3 kV mm−1, applied perpendicular to the direction of flow. Also presented is the apparent viscosity as a function of temperature, over similar values of electric field, obtained at different shear rates. A master flow curve has been constructed for each temperature by dividing the shear rate by the square of the electric field and multiplying by the square of a reference value of electric field. In a log–log plot, two Newtonian plateaux are found to appear at low and high shear rates, connected by a shear-thinning region. We have applied the Leslie–Ericksen continuum theory, in which the director alignment angle is a function of the electric field and the flow field boundary conditions are neglected, to determine viscoelastic parameters and the dielectric anisotropy.

Flexible cellulose derivative PDLC type cells

In this work we perform a study of 250 ≈ μm thick flexible electro-optical PDLC type cells made from a biocompatible cellulose derivative film and several conductive substrates. The deposition of an ITO layer by reactive thermal evaporation on a polymeric substrate was referred to in the literature very recently and this type of coated substrate was used in the present work. In order to consider the influence of the substrates on the electro-optical behaviour of the cells, five cells were made using different substrates (three flexible polymers and two glass for comparison). Three of the substrates were coated under the same conditions, and the other two were commercially available substrates.

Electro-Optical Properties of Cellulose Based PDLC Type Cells: Dependence on the Type of Diisocyanate Cross-Linking Agent Used

Abstract We have studied the effect of different diisocyanate cross-linking agents on the electro-optical properties of cellulose derivative based PDLC type cells and on the dielectric properties of the films used on their preparation. Each optical cell was assembled with a cross-linked film produced with a different type of diisocyanate and with the nematic liquid crystal (E7). One was assembled with a non cross-linked film. The six different cells were then analysed by light transmission and the results were correlated with the dielectric properties of the cellulosic films used in each one of them. It was found that the cell with the higher contrast is the one prepared with the film cross-linked with 1,4-diisocyanatobutane (BDI) [6]. Based on the results obtained, we propose a mechanism to describe the electro-optical behaviour of these systems.