Self-Assembly of Amphiphilic Biotransesterified ß-Cyclodextrins: Supramolecular Structure of Nanoparticles and Surface Properties.

A series of ß-cyclodextrin (ßCD) amphiphilic derivatives with varying degrees of substitution were prepared by acylating ßCDs on their secondary face using thermolysin to catalyze the transesterification. After dissolution in acetone, the ßCD-Cn derivatives (n = 8, 10, 12, 14) were nanoprecipitated in water, where they self-organized into structured particles that were characterized using cryo-transmission electron microscopy (cryo-TEM) images and small-angle X-ray scattering (SAXS) data. Two types of morphologies and ultrastructures were observed depending on the total degree of substitution (TDS) of the parent derivative. The molecules with TDS < 5 formed nanospheres with a multilamellar organization, whereas those with TDS > 5 self-assembled into barrel-like (n = 8, 10, 12) or more tortuous (n = 14) particles with a columnar inverse hexagonal structure. In particular, faceted ßCD-C14 particles (TDS = 7) appeared to be composed of several domains with different orientations that were separated by sharp interfaces. Ultrastructural models were proposed on the basis of cryo-TEM images and the analysis of the contrast distribution in different projections of the lattice. Complementary compression isotherm experiments carried out at the air-water interface also suggested that differences in the molecular conformation of the series of derivatives existed depending on whether TDS was lower or higher than 5.

Dynamic Structuration of Physical Chitosan Hydrogels.

We studied the microstructure of physical chitosan hydrogels formed by the neutralization of chitosan aqueous solutions highlighting the structural gradients within thick gels (up to a thickness of 16 mm). We explored a high polymer concentrations range (Cp ≥ 1.0% w/w) with different molar masses of chitosan and different concentrations of the coagulation agent. The effect of these processing parameters on the morphology was evaluated mainly through small-angle light scattering (SALS) measurements and confocal laser scanning microscopy (CLSM) observations. As a result, we reported that the microstructure is continuously evolving from the surface to the bulk, with mainly two structural transitions zones separating three types of hydrogels. The first zone (zone I) is located close to the surface of the hydrogel and constitutes a hard (entangled) layer formed under fast neutralization conditions. It is followed by a second zone (zone II) with a larger thickness (∼3-4 mm), where in some cases large pores or capillaries (diameter ∼10 µm) oriented parallel to the direction of the gel front are present. Deeper in the hydrogel (zone III), a finer oriented microstructure, with characteristic sizes lower than 2-3 µm, gradually replace the capillary morphology. However, this last bulk morphology cannot be regarded as structurally uniform because the size of small micrometer-range-oriented pores continuously increases as the distance to the surface of the hydrogel increases. These results could be rationalized through the effect of coagulation kinetics impacting the morphology obtained during neutralization.

Tuning the Hydrophilic/Hydrophobic Balance to Control the Structure of Chitosan Films and Their Protein Release Behavior.

The control over the crystallinity of chitosan and chitosan/ovalbumin films can be achieved via an appropriate balance of the hydrophilic/hydrophobic interactions during the film formation process, which then controls the release kinetics of ovalbumin. Chitosan films were prepared by solvent casting. The presence of the anhydrous allomorph can be viewed as a probe of the hydrophobic conditions at the neutralization step. The semicrystalline structure, the swelling behavior of the films, the protein/chitosan interactions, and the release behavior of the films were impacted by the DA and the film processing parameters. At low DAs, the chitosan films neutralized in the solid state corresponded to the most hydrophobic environment, inducing the crystallization of the anhydrous allomorph with and without protein. The most hydrophilic conditions, leading to the hydrated allomorph, corresponded to non-neutralized films for the highest DAs. For the non-neutralized chitosan acetate (amorphous) films, the swelling increased when the DA decreased, whereas for the neutralized chitosan films, the swelling decreased. The in vitro release of ovalbumin (model protein) from chitosan films was controlled by their swelling behavior. For fast swelling films (DA = 45%), a burst effect was observed. On the contrary, a lag time was evidenced for DA = 2.5% with a limited release of the protein. Furthermore, by blending chitosans (DA = 2.5% and 45%), the release behavior was improved by reducing the burst effect and the lag time. The secondary structure of ovalbumin was partially maintained in the solid state, and the ovalbumin was released under its native form.

Effect of enzyme concentration of the morphology and properties of enzymatically triggered peptide hydrogels.

We have recently shown that thermolysine, a protease enzyme obtained from Bacillus thermoproteolyticus rokko , can be used to trigger the gelation of FEFK (F, phenylalanine; E, glutamic acid; K, lysine) tetrapeptides through reverse hydrolysis and formation of longer peptide sequences, mainly octapeptides, that self-assemble readily. In this article we investigate the effect of enzyme concentration on the morphology and properties of enzymatically triggered peptide hydrogels using HPLC, FTIR, real-time SAXS, TEM, and shear rheology. We have shown that the enzyme concentration, Cenz, does not affect the final composition of the samples. Instead, this is dictated by the initial tetrapeptide concentration, C0, suggesting the existence of a chemical equilibrium. We went on to show that Cenz does not affect the self-assembly of these peptides at a molecular level either nor the structure of the fibrillar network formed at the nanometer scale. Interestingly, the mechanical properties were found to be affected by Cenz, where the shear moduli of the hydrogels were found to increase with increasing Cenz. These results suggest that morphological differences between the hydrogels at the microscale are at the origin of their difference in mechanical properties. In this paper, we propose a morphological model in which denser network regions are found around the enzymes, resulting in the creation of heterogeneous networks. These were confirmed by TEM measurements. The existence of these denser network regions will result in the reinforcement of the hydrogels, thus, explaining the high shear moduli obtained increasing Cenz.

Reorientation of cellulose nanowhiskers in agarose hydrogels under tensile loading.

Agarose hydrogels filled with cellulose nanowhiskers were strained in uniaxial stretching under different humidity conditions. The orientation of the cellulose whiskers was examined before and after testing with an X-ray laboratory source and monitored in situ during loading by synchrotron X-ray diffraction. The aim of this approach was to determine the process parameters for reorienting the cellulose nanowhiskers toward a preferential direction. Results show that a controlled drying of the hydrogel is essential to establish interactions between the matrix and the cellulose nanowhiskers which allow for a stress transfer during stretching and thereby promote their alignment. Rewetting of the sample after reorientation of the cellulose nanowhiskers circumvents a critical increase of stress. This improves the extensibility of the hydrogel and is accompanied by a further moderate alignment of the cellulose nanowhiskers. Following this protocol, cellulose nanowhiskers with an initial random distribution can be reoriented toward a preferential direction, creating anisotropic nanocomposites.

Influence of monovalent ions on density fluctuations in hydrothermal aqueous solutions by small angle X-ray scattering.

Synchrotron small angle X-ray scattering measurements on water and alkaline bromine aqueous solutions (XBr, with X = Li, Rb, or Cs) were carried out from ambient to supercritical conditions. The temperature was increased from 300 to 750 K along several isobars between 24 and 35 MPa. The correlation length and the structure factor were extracted from the data following the Ornstein-Zernike formalism. We obtained experimental evidence of the shift of the critical point and isochore and their dependence on the ions concentration (0.33 mol/kg and 1.0 mol/kg). We also observed that the size of the density fluctuations and the structure factor increase with the presence of the ions and that this effect is positively correlated with the atomic number of the cation. These behaviors were compared with ZnBr(2) and NaCl systems from the literature.

Nano-organization of amylose-b-polystyrene block copolymer films doped with bipyridine.

This paper discusses the self-assembly of rod-coil amylose-b-polystyrene (Mal-b-PS) block copolymer thick and thin films. The nano-organization falls in an interdomain spacing d of about 10 nm, much smaller than flexible-flexible petrol block copolymer systems. Additionally, hydrogen-bonding interactions between carbohydrate rods (amylose) and 4',4-bipyridine (bipy) molecules induces phase transitions. Indeed, adding bipy in maltooctadecaose-block-polystyrene (Mal18-b-PS) copolymers results, at room temperature, in the formation of a lamellar phase having Mal18 bipy-rich nanodomains instead of hexagonal close-packed (HCP) of cylinders made of Mal18, whereas a coexistence of Mal7bipy-rich cylindrical and spherical nanodomains are formed from maltoheptaose-b-polystyrene (Mal7-b-PS) copolymers instead of a poorly organized array of Mal7-based cylinders. On heating, the Mal7bipy-b-PS system shows more rich phase behavior as compared to the Mal7-b-PS one due to weakening of hydrogen bonding with temperature. Such a system is of great interest in developing active layers in light-emitting diodes (LEDs) or in photovoltaic cells to realize devices with an optimal structure, that is, having large interface area and domain size with similar exciton diffusion length (10 nm).

Aqueous suspensions of natural swelling clay minerals. 1. Structure and electrostatic interactions.

In this article, we present a general overview of the organization of colloidal charged clay particles in aqueous suspension by studying different natural samples with different structural charges and charge locations. Small-angle X-ray scattering experiments (SAXS) are first used to derive swelling laws that demonstrate the almost perfect exfoliation of clay sheets in suspension. Using a simple approach based on geometrical constraints, we show that these swelling laws can be fully modeled on the basis of morphological parameters only. The validity of this approach was further extended to other clay data from the literature, in particular, synthetic Laponite. For all of the investigated samples, experimental osmotic pressures can be properly described by a Poisson-Boltzmann approach for ionic strength up to 10(-3) M, which reveals that these systems are dominated by repulsive electrostatic interactions. However, a detailed analysis of the Poisson-Boltzmann treatment shows differences in the repulsive potential strength that are not directly linked to the structural charge of the minerals but rather to the charge location in the structure for tetrahedrally charged clays (beidellite and nontronites) undergoing stronger electrostatic repulsions than octahedrally charged samples (montmorillonites, laponite). Only minerals subjected to the strongest electrostatic repulsions present a true isotropic to nematic phase transition in their phase diagrams. The influence of ionic repulsions on the local order of clay platelets was then analyzed through a detailed investigation of the structure factors of the various clay samples. It appears that stronger electrostatic repulsions improve the liquidlike positional local order.

Micron range morphology of physical chitosan hydrogels.

Physical chitosan hydrogels are potential biomaterials for several biomedical applications, such as wound healing, tissue repair, and drug delivery. Controlling the microstructural organization of chitosan gels is one of the keys for monitoring the physical, mechanical, and biological properties. As a result, the main objective of the present work was to explore the microstructural organization of chitosan hydrogels in relation with the processing conditions of gelation. For this purpose, different gelation routes were studied, that is, chitosan solution neutralization of an aqueous or hydroalcoholic solution and neutralization of an alco-gel. Overall, the resulting morphology after processing was determined by the medium viscosity during neutralization and the nature and concentration of the base. The effect of these processing parameters on the morphology was evaluated mainly through small angle light scattering (SALS) measurements including in situ measurements during chitosan neutralization. As a result, we reported different bulk microstructures consisting in 200-400 nm aggregates (primary particles) agglomerated into micrometer range clusters or arranged into more organized structures, that is, forming microchannels (4-6 µm). We thus established a qualitative and quantitative relation between supramolecular morphology and gelation conditions of chitosan hydrogels.

Continuum of structural organization from chitosan solutions to derived physical forms.

The structural organization of chitosan, a cationic polyelectrolyte, in aqueous solutions of high ionic strength, is investigated by quasi-elastic light scattering and wet scanning transmission electron microscopy. The formation of submicrometric chain aggregates driven by hydrophobic interactions is evidenced. These heterogeneities are at the core of the multiscale morphology of physical hydrogels processed from this polysaccharide. Therefore, a close structural relationship exists between the initial solution and the final hydrogel.

B-->A Allomorphic transition in native starch and amylose spherocrystals monitored by in situ synchrotron X-ray diffraction.

The B-->A phase transition in native starch granules and spherocrystals prepared from DP 20-40 synthetic amylose chains was investigated in situ at intermediate moisture content (20-30%) by wide-angle synchrotron X-ray scattering, using a temperature-controlled pressure cell. The transition in native starch was monitored at hydrostatic pressures of 1.6-11.0 MPa and occurred in a temperature range of 90-110 degrees C. The transition temperature increased with increasing amylose content and the transition was incomplete in amylose-rich starch. The B-->A transition in highly crystalline amylose spherocrystals was monitored at pressures between 2.0 and 28.5 MPa. The transition temperature was higher than in native starch, ranging from 125 to 135 degrees C. At 2.0 MPa, after conversion, the hydrated spherocrystals melted at 185 degrees C. Surprisingly, at the same pressure, in excess water, the spherocrystals did not solubilize but converted to allomorph A at 100 degrees C and melted at 160 degrees C. For all samples, the transition occurred in a matter of minutes and a higher pressure decreased the transition temperature. For the first time, thermal expansion coefficients were estimated for A- and B-amylose at intermediate moisture. A strong thermal anisotropy was observed for A-amylose, the expansion being higher along the b-axis than along the a-axis of the monoclinic unit cell. This anisotropy was attributed to the fact that, in the b-direction, amylose double helices lie at the same height along the chain axis while, in the a-direction, they are more closely packed in a zigzag fashion.

The shape and size distribution of crystalline nanoparticles prepared by acid hydrolysis of native cellulose.

The shape and size distribution of crystalline nanoparticles resulting from the sulfuric acid hydrolysis of cellulose from cotton, Avicel, and tunicate were investigated using transmission electron microscopy (TEM) and atomic force microscopy (AFM) as well as small- and wide-angle X-ray scattering (SAXS and WAXS). Images of negatively stained and cryo-TEM specimens showed that the majority of cellulose particles were flat objects constituted by elementary crystallites whose lateral adhesion was resistant against hydrolysis and sonication treatments. Moreover, tunicin whiskers were described as twisted ribbons with an estimated pitch of 2.4-3.2 microm. Length and width distributions of all samples were generally well described by log-normal functions, with the exception of tunicin, which had less lateral aggregation. AFM observation confirmed that the thickness of the nanocrystals was almost constant for a given origin and corresponded to the crystallite size measured from peak broadening in WAXS spectra. Experimental SAXS profiles were numerically simulated, combining the dimensions and size distribution functions determined by the various techniques.

Morphology and mechanical properties of chitosan fibers obtained by gel-spinning: influence of the dry-jet-stretching step and ageing.

The morphology and mechanical properties of chitosan fibers obtained by gel-spinning are reported. The objectives were both to understand how the microstructure of the fibers could be modified and how the mechanical properties were improved by means of a dry-jet-stretching step. A highly deacetylated chitosan (degree of acetylation=2.7%) from squid-pens, with a high weight-average molecular weight (M(w)=540,000g/mol) was dissolved in an aqueous acetic acid solution, spun using gaseous ammonia as the coagulant, and then directly dried under hot air. A "post-drying" of 1week was necessary to stabilize the fibers in ambient atmosphere. A dry-jet-stretching ratio applied during the monofilament coagulation (maximal value of 4.3) allowed us to increase the density and favor the orientation of chains along the fiber axis. This allowed us to improve the mechanical properties of the fibers (Young's modulus of 82g/denier and tenacity of 2g/denier). The ageing in ambient atmosphere played an important role in the crystalline microstructure in relation to: the kinetics of ammonium acetate hydrolysis, the formation of a weak fraction of the anhydrous allomorph of chitosan, and an increase of the crystallinity index, whereas the Young's modulus was increased and the tenacity was slightly lowered. In addition, gel-jet-stretched or dry-jet-stretched fibers could be stored at least 3months in ambient atmosphere without any significant degradation.

Hydrothermal Transformation of Wood Cellulose Crystals into Pseudo-Orthorhombic Structure by Cocrystallization.

The ultrastructural transformation of wood cellulose crystals by hydrothermal treatment was followed by synchrotron and standard X-ray scattering experiments. When treated at 200 °C for 2 h in the presence of an excess of water, a significant sharpening of the equatorial reflections of crystalline cellulose was observed, and the average crystallite size, estimated from the X-ray line broadening, was twice as large as that of untreated wood cellulose. During the treatment, the cellulose structure was converted from the native monoclinic form of cellulose I into a pseudo-orthorhombic system, coined as cellulose I', a transformation occurring only with an excess of water, above 180 °C and after more than half an hour. In situ experiments indicated that the increase of crystallite size was likely due to cocrystallization of individual crystallites rather than to the crystallization of the amorphous domains of cellulose.

Varying the counter ion changes the kinetics, but not the final structure of colloidal gels.

We show that, while the gelation of colloidal silica proceeds much faster in the presence of added KCl than NaCl, the final gels are very similar in structure and properties. We have studied the gelation process by visual inspection and by small angle X-ray scattering for a range of salt and silica particle concentrations. The characteristic times of the early aggregation process and the formation of a stress-bearing structure with both salts are shown to collapse onto master curves with single multiplicative constants, linked to the stability ratio of the colloidal suspensions. The influence of the salt type and concentration is confirmed to be mainly kinetic, as the static structure factors and viscoelastic moduli of the gels are shown to be equivalent at normalized times. While there is strong variation in the kinetics, the structure and properties of the gel at long-times are shown to be mainly controlled by the concentration of particles, and hardly influenced by the type or the concentration of salt. This suggests that the differences between gels generated by different salts are only transient in time.

Fine microstructure of processed chitosan nanofibril networks preserving directional packing and high molecular weight.

Crystalline chitosan nanofibril networks were prepared, preserving the native structural packing and the polymer high molecular weight. The fine microstructure of the nanomaterial, obtained by mild hydrolysis of chitosan (CHI), was characterized by using synchrotron small- and wide-angle X-ray scattering (SAXS and WAXS), transmission electron microscopy (TEM) and electron diffraction. Hydrolysis of chitosan yielded a network of crystalline nanofibrils, containing both allomorphs of chitosan: hydrated and anhydrous. The comparison of WAXS data in transmission and reflection mode revealed the preferential orientation of the CHI crystals when subjected to mechanical compression constrains. The results are in agreement with the existence of a network nanostructure containing fiber-like crystals with the principal axis parallel to the polymer chain axis. The evolution of the CHI allomorphic composition with temperature was studied to further elucidate the mechanism of structural transitions occurring during CHI nanofibril network processing.

Scattering from dilute ferrofluid suspensions in soft polymer gels.

Small angle neutron and x-ray scattering methods are used to investigate the structure of dilute suspensions of two different ferrofluid systems dispersed in soft polyacrylamide hydrogels. It is found that the particles in the fluid are fractal aggregates composed of smaller particles of radius ca. 5 nm. The fractal dimension is strongly dependent on sample, taking the value 1.7 in the first sample and 2.9 in the second sample. In the presence of a magnetic field the aggregates orient, but are restricted in both their translational and rotational freedom. The effect of the gel elasticity is treated as a hindrance to the orientation process.

Thermoreversible gelation in aqueous binary solvents of chemically modified agarose.

The thermoreversible gelation of chemically modified agarose has been studied in aqueous binary solvents (dimethyl sulfoxide and a series of formamide) by differential calorimetry, mechanical testing, and small-angle neutron scattering. The temperature-composition phase diagrams have been established. It is concluded that gelation is promoted by the formation of ternary complexes modified agarose/water/cosolvent, wherein the cosolvent mediates the interaction between chains through the formation of electrostatic interactions.

Hybrid nanocomposites with tunable alignment of the magnetic nanorod filler.

For many important applications, the performance of polymer-anisotropic particle nanocomposite materials strongly depends on the orientation of the nanoparticles. Using the very peculiar magnetic properties of goethite (α-FeOOH) nanorods, we produced goethite-poly(hydroxyethyl methacrylate) nanocomposites in which the alignment direction and the level of orientation of the nanorods could easily be tuned by simply adjusting the intensity of a magnetic field applied during polymerization. Because the particle volume fraction was kept low (1-5.5 vol %), we used the orientational order induced by the field in the isotropic phase rather than the spontaneous orientational order of the nematic phase. At the strongest field values (up to 1.5 T), the particles exhibit almost perfect antinematic alignment, as measured by optical birefringence and small-angle X-ray scattering. The results of these two techniques are in remarkably good agreement, validating the use of birefringence measurements for quantifying the degree of orientational order. We also demonstrate that the ordering induced by the field in the isotropic suspension is preserved in the final material after field removal. This work illustrates the interest, for such problems, of considering the field-induced alignment of anisotropic nanoparticles in the isotropic phase, an approach that is effective at low filler content, that avoids the need of controlling the nematic texture, and that allows tuning of the orientation level of the particles at will simply by adjusting the field intensity.

Spinning of hydroalcoholic chitosan solutions.

We investigated the spinning of hydroalcoholic chitosan solutions. The dope composition was optimized in order to obtain a continuous alcogel fiber by water evaporation on heating the extruded hydroalcoholic solution. This alcogel fiber was then neutralized in aqueous alkali baths and washed in water to eliminate the residual alcohol and salts before final drying. Depending on the alcohol content in the filament at the neutralization step, on specific alcohol-chitosan interactions and on the nature and concentration of the coagulation base, the process yielded semicrystalline chitosan fibers with different proportions of anhydrous and hydrated allomorphs. Contrarily to the classical annealing method, the formation of mainly anhydrous crystals was obtained without significant molecular weight decrease by neutralizing the polymer in hydrophobic conditions. The control of allomorph content was shown to be related to the hydrophobicity of the solvent (alcohol fraction) at the neutralization step.