Bernard Nysten

Regular arrays of highly ordered ferroelectric polymer nanostructures for non-volatile low-voltage memories

Ferroelectric nanostructures are attracting tremendous interest because they offer a promising route to novel integrated electronic devices such as non-volatile memories and probe-based mass data storage. Here, we demonstrate that high-density arrays of nanostructures of a ferroelectric polymer can be easily fabricated by a simple nano-embossing protocol, with integration densities larger than 33 Gbits inch(-2). The orientation of the polarization axis, about which the dipole moment rotates, is simultaneously aligned in plane over the whole patterned region. Internal structural defects are significantly eliminated in the nanostructures. The improved crystal orientation and quality enable well-defined uniform switching behaviour from cell to cell. Each nanocell shows a narrow and almost ideal square-shaped hysteresis curve, with low energy losses and a coercive field of approximately 10 MV m(-1), well below previously reported bulk values. These results pave the way to the fabrication of soft plastic memories compatible with all-organic electronics and low-power information technology.

Chitosan nanoparticles for siRNA delivery: Optimizing formulation to increase stability and efficiency

This study aims at developing chitosan-based nanoparticles suitable for an intravenous administration of small interfering RNA (siRNA) able to achieve (i) high gene silencing without cytotoxicity and (ii) stability in biological media including blood. Therefore, the influence of chitosan/tripolyphosphate ratio, chitosan physicochemical properties, PEGylation of chitosan as well as the addition of an endosomal disrupting agent and a negatively charged polymer was assessed. The gene silencing activity and cytotoxicity were evaluated on B16 melanoma cells expressing luciferase. We monitored the integrity and the size behavior of siRNA nanoparticles in human plasma using fluorescence fluctuation spectroscopy and single particle tracking respectively. The presence of PEGylated chitosan and poly(ethylene imine) was essential for high levels of gene silencing in vitro. Chitosan nanoparticles immediately released siRNA in plasma while the inclusion of hyaluronic acid and high amount of poly(ethylene glycol) in the formulation improved the stability of the particles. The developed formulations of PEGylated chitosan-based nanoparticles that achieve high gene silencing in vitro, low cytotoxicity and high stability in plasma could be promising for intravenous delivery of siRNA.

Measurement of elastic modulus of nanotubes by resonant contact atomic force microscopy

A resonant contact atomic force microscopy technique is used to quantitatively measure the elastic modulus of polymer nanotubes. An oscillating electric field is applied between the sample holder and the microscope head to excite the oscillation of the cantilever in contact with nanotubes. The nanotubes are suspended over the pores of a membrane. The measured resonance frequency of this system, a cantilever with the tip in contact with a nanotube, is shifted to higher values with respect to the resonance frequency of the free cantilever. It is experimentally demonstrated that the system can simply be modeled by a cantilever with the tip in contact with two springs. The measurement of the frequency shift thus enables the direct determination of the spring stiffness, i.e., the nanotube stiffness. The method also enables the determination of the boundary conditions of the nanotube on the membrane. The tensile elastic modulus is then simply determined using the classical theory of beam deflection. The obtained results fairly agree to previously measured values using nanoscopic three points bending tests. It is demonstrated that resonant contact atomic force microscopy allows us to quantitatively measure the mechanical properties of nanomaterials

Surface and Bulk Collapse Transitions of Thermoresponsive Polymer Brushes

We elucidate the sequence of events occurring during the collapse transition of thermoresponsive copolymer brushes based on poly(di(ethyleneglycol) methyl ether methacrylate) chains (PMEO2MA) grown by atom-transfer radical polymerization (ATRP). The collapse of the bulk of the brush is followed by quartz crystal microbalance measurements with dissipation monitoring (QCM-D), and the collapse of its outer surface is assessed by measuring equilibrium water contact angles in the captive bubble configuration. The bulk of the brush collapses over a broad temperature interval (approximately 25 degrees C), and the end of this process is signaled by a sharp first-order transition of the surface of the brush. These observations support theoretical predictions regarding the occurrence of a vertical phase separation during collapse, with surface properties of thermoresponsive brushes exhibiting a sharp variation at a temperature of T(br)(surf). In contrast, the bulk properties of the brush vary smoothly, with a bulk transition T(br)(bulk) occurring on average approximately 8 degrees C below T(br)(surf) and approximately 5 degrees C below the lower critical solution temperature (LCST) of free chains in solution. These observations should also be valid for planar brushes of other neutral, water-soluble thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM). We also propose a way to analyze more quantitatively the temperature dependence of the QCM-D response of thermoresponsive brushes and deliver a simple thermodynamic interpretation of equilibrium contact angles, which can be of use for other complex temperature-responsive solvophilic systems.

Urea potentiometric enzymatic biosensor based on charged biopolymers and electrodeposited polyaniline

A potentiometric biosensor based on urease was developed for the quantitative determination of urea concentration in aqueous solutions for biomedical applications. The urease was either physisorbed onto an electrodeposited polyaniline film (PANI), or immobilized on a layer-by-layer film (LbL) assembled over the PANI film, that was obtained by the alternate deposition of charged polysaccharides (carboxymethylpullulan (CMP) and chitosan (CHI)). In the latter case, the urease (Urs) enzyme was either physically adsorbed or covalently grafted to the LbL film using carbodiimide coupling reaction. Potentiometric responses of the enzymatic biosensors were measured as a function of the urea concentration in aqueous solutions (from 10(-6) to 10(-1) mol L(-1) urea). Very high sensitivity and short response time were observed for the present biosensor. Moreover, a stability study showed a higher stability over time for the potentiometric response of the sensor with the enzyme-grafted LbL film, testifying for the protective nature of the polysaccharide coating and the interest of covalent grafting.

The temperature variation of the thermal conductivity of benzene-derived carbon fibers

The temperature variation of the in-plane thermal conductivity of benzene-derived carbon fibers (BDF) measured from 5 to 300K is reported and discussed. Very high thermal conductivity values-comparable to that of the best HOPG heat treated at the same temperature-are found. The data confirms the high structural order previously reported for BDF.

Characterization of the molecular structure of two highly isotactic polypropylenes

Two polypropylenes, PP1 and PP2, produced with different heterogeneous Ziegler-Natta catalytic systems were studied in this work. Preliminary characterization of the non-fractionated materials showed that a low difference in their average tacticity (PP2 > PP1) leads to an important modification of their rigidity properties. In order to establish correlation between the molecular structure parameters and the rigidity properties of these polymers, fractionation of the materials according to crystallizability was performed by means of temperature rising elution fractionation (TREF). Analysis of the fractions of both PP1 and PP2 was carried out by means of C-13 NMR, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results first showed that TREF does not strictly fractionate PP according to tacticity, but according to the longest crystallizable sequence in a chain. C-13 NMR, SEC and DSC analysis of the fractions demonstrated that the inter-chain tacticity distributions of the polypropylenes is affected by the change of the polymerization conditions, which, in turn, modifies the rigidity properties of the materials. Some results also seem to indicate that the intrachain tacticity distributions are different for the two PP. An AFM study of the elastic modulus was carried out for the first time on the TREE fractions. It showed that the rigidity or the fractions strongly increases as the TREF elution temperature increases in accordance with a concomitant increase of isotacticity and the crytallinity of the fractions. PP2 TREE fractions were, moreover, found to exhibit a higher elastic modulus than PP1 TREF fractions at all elution temperatures. This study allowed us to further identify the TREF fractions that were responsible for differences in rigidity. To summarize, it is shown how the experimentally observed increase of the average rigidity of one of these two polypropylenes can be rationalized via information collected from a TREF fractionation

Nano-structured polymer blends: phase structure, crystallisation behaviour and semi-crystalline morphology of phase separated binary blends of poly(ethylene oxide) and poly(ether sulphone)

The crystallisation behaviour and morphology of binary phase separated crystalline/amorphous blends of poly(ethylene oxide) (PEO) and poly(ether sulphone) (PES) are investigated. The crystallisation behaviour of the PEO/PES blends changes strongly with demixing time and temperature. A method is proposed to determine the composition and amount of PEO-rich phase in the partially demixed blend systems from the dynamic crystallisation behaviour. The results are in agreement with the structure development as predicted for spinodal decomposition that proves the validity of this method. The phase and semi-crystalline morphologies are visualised by scanning electron microscopy and atomic force microscopy. The semi-crystalline morphology is also studied by real-time small angle and wide angle X-ray scattering. It is shown that, depending on the blend composition, a spherulitic or isolated lamellar crystalline morphology is formed in the demixed PEO/PES blends.

Collagen adsorption on poly(methyl methacrylate): net-like structure formation upon drying

Collagen adsorbed on poly(methyl methacrylate) (PMMA) was investigated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and water contact angle measurement, after either fast drying or slow drying. Quantification by :XPS of the amount of adsorbed collagen is shown to be influenced by the drying procedure; This is explained by AFM images, which reveal the formation of a net-like structure at slow drying rates. Comparison between the surface organization, as observed by AFM, surface models provided by XPS and contact angle measurements, indicates that chemically heterogeneous surfaces are produced at slow drying rates, PMMA being exposed at the outermost surface in the holes left by the collagen net

Elastic modulus of halloysite nanotubes

This work reports measurements of the elastic modulus of halloysite nanotubes. Nanoscale three-point bending tests were performed on individual nanotubes using an atomic force microscope. Nanotubes exhibit elastic behaviour at small deformations. The stiffness of the tubes, and hence their elastic modulus, was deduced from force curve measurements using an appropriate mechanical model. The boundary conditions were also identified by recording the stiffness profile of a tube along its suspended length. An average elastic modulus of 140 GPa is obtained for a set of tubes with outer diameters ranging between 50 and 160 nm. Moreover, the elastic modulus increases with decreasing outer diameter, with a steep jump below 50 nm. The size dependence of the elastic modulus may be attributed to: (i) surface tension effects for thinner tubes and (ii) a non-negligible contribution of shear deformations to the total deflection for larger tubes.