Gelatine/silicate interactions: from nanoparticles to composite gels

The possibility to design new composites associating biopolymers with mineral phases relies on the understanding and control of their mutual interactions. In this work, aqueous solutions of gelatine and sodium silicate were mixed at pH 5, 37 degrees C and left to stand at 20 degrees C for 1 day. At low gelatine and high silicate contents, precipitates were obtained, containing a fixed silicon/polymer molar ratio. Scanning electron microscopy (SEM) reveals that they are formed of large aggregates of platelets, constituted of closely-packed nanoparticles. For high gelatine contents, composite gels were formed consisting of silica particles dispersed in the biopolymer matrix. Swelling studies indicate that the addition of silica decreases the stability of the gels by inducing gelatine depletion in solution. Similar experiments conducted at pH 7 show that at this pH, silicates are more effective at precipitating gelatine. A model is proposed for the formation of the composites, based on the electrostatic interactions arising between silicates and polymer chains. These results are discussed in the context of hybrid biomaterials design and biosilicification processes.     

Silica-surfactant-polyelectrolyte film formation: evolution in the subphase

We have previously reported that robust mesostructured films will grow at the surface of alkaline solutions containing cetyltrimethylammonium bromide (CTAB), polyethylenimine (PEI), and silica precursors. Here we have used time-resolved small-angle X-ray scattering to investigate the structural evolution of the micellar solution from which the films form, at several different CTAB-PEI concentrations. Simple models have been employed to quantify the size and shape of the micelles in the solution. There are no mesostructured particles occurring in the CTAB-PEI solution prior to silica addition; however, after the addition of silicate species the hydrolysis and condensation of these species causes the formation of mesophase particles in a very short time, much faster than ordering observed in the film at the interface. The mesophase within the CTAB-PEI-silica particles finally rearranges into a 2D hexagonal ordered structure. With the aid of the previous neutron reflectivity data on films formed at the air/water interface from similar solutions, a formation mechanism for CTAB-PEI-silica films at the air/water interface has been developed. We suggest that although the route of mesostructure evolution of the film is the same as that of the particles in the solution, the liquid crystalline phase at the interface is not directly formed by the particles that developed below the interface.     

Polymer nanocomposites with “ultralow” refractive index

Nanocomposites of gelatine and gold were prepared by reduction of AuCl₄H in a gelatine solution, followed by a spin coating process and a brief washing. The thickness of the films thus obtained was typically 200–400 nm. The surface of the nanocomposites was investigated by scanning electron microscopy (SEM) and surface profilometry. The size of the gold particles is in the range of ca. 2–10 nm, as elucidated by transmission electron microscopy (TEM). Refractive indices were measured by ellipsometry at wavelengths of 632.8 and 1300 nm. The refractive index depends linearly on the volume fraction of gold, at least at 1300 nm. The lowest refractive indices measured are 1.008±0.060 at 1300 nm and 0.963±0.062 at 632.8 nm (limits are 95% confidence interval). These values are, to our best knowledge, the lowest reported for a polymer or polymer composite.     

Growth and spectroscopic characterization of CdSe nanoparticles synthesized from CdCl2 and Na2SeSO3 in aqueous gelatine solutions

CdSe nanoparticles were synthesized in comparatively mild conditions from Na₂SeSO₃ and CdCl₂ in aqueous gelatine solutions. Kinetics of the formation and growth of CdSe nanocrystals as well as the effect of various parameters of reacting mixture on the size of CdSe nanocrystals are investigated. Optical properties of thin gelatine films, containing CdSe nanoparticles of different size, are characterized using absorption and Raman spectroscopy.     

Preparation and structure of chitosan-silica organic-inorganic hybrid films

Organic-inorganic hybrid films are prepared via the hydrolysis and polycondensation of tetraethoxysilane in chitosan solutions. It is shown that the composition of reagents, the time of synthesis, the crosslinking of chitosan macromolecules via amino groups by glutaric aldehyde, and the conditions of drying affect structurization of the resulting films. With the use of optical polarization microscopy and atomic force microscopy, it is found that the films contain silica particles with a wide size distribution. The crosslinking of the chitosan matrix by glutaric aldehyde results in the predominant formation of inorganic nanoparticles in the films. A good sorption capacity of the films for copper ions in solutions is found. It increases by a factor of 1.7 as the content of the silica in the films increases to 30 wt %.     

Size control of nanostructured silica using chitosan template and fractal geometry: effect of chitosan/silica ratio and aging temperature

The uses of low cost, renewable, environmentally friendly chitosan biopolymer as the structural template to control the size of silica particles in the range of nanometer scales are attractive for their practical industrial applications. In this paper, the nanostructured silica was synthesized using sodium silicate as the silica source and chitosan as the template under mild conditions. Effects of chitosan/silica ratio and aging temperature on the formation and the control of nanostructured silica was investigated by using thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), N₂-sorption measurement, and transmission electron microscopy (TEM). It was found that the silica products were composed of the aggregates of primary silica nanoparticles and nanostructured silica units. At low aging temperature, the size of nanostructured silica was decreased when increasing the chitosan/silica ratio from 0.1 to 0.4. In contrast, the reverse trend was observed at the chitosan/silica ratio of higher than 0.4. The increase of aging temperature led to the formation of larger primary silica nanoparticles and nanostructured silica, and also promoted the formation of silica/chitosan composites. The fractal dimension calculated using modified FHH method found the linear correlation at two different regimes which might reflect the aggregates of silica products at different length scales.     

聚酰亚胺/二氧化硅/银杂化薄膜的制备及其结构与性能研究

采用溶胶-凝胶和原位自金属化相结合方法制备聚酰亚胺(PI)/二氧化硅(SiO2)/银(Ag)三元复合薄膜,通过红外(FTIR)、透射电镜(TEM)、动态粘弹谱仪(DMTA)等系统地考察了热处理过程中杂化薄膜结构形态变化以及SiO2含量对金属银向基体表面迁移情况和杂化薄膜各种性能的影响.实验结果表明,在热处理过程中可以同时完成聚酰胺酸的亚胺化、SiO2粒子的形成及银的还原,并且可以通过改变热处理温度和时间或改变SiO2含量来控制银粒子向聚合物基体表面的迁移.     

Synthesis of vinyl polymer-silica colloidal nanocomposites prepared using commercial alcoholic silica sols

The surfactant-free synthesis of vinyl polymer-silica nanocomposite particles has been achieved in aqueous alcoholic media at ambient temperature in the absence of auxiliary comonomers. Styrene, methyl methacrylate, methyl acrylate, n-butyl acrylate, and 2-hydroxypropyl methacrylate were homopolymerized in turn in the presence of three commercially available ultrafine alcoholic silica sols. Stable colloidal dispersions with reasonably narrow size distributions were obtained, with silica contents of up to 58% by mass indicated by thermogravimetric analysis. Particle size distributions were assessed using both dynamic light scattering and disk centrifuge photosedimentometry. The former technique indicated that the particle size increased for the first 1-2 h at 25 degrees C and thereafter remained constant. Particle morphologies were studied using electron microscopy. Most of the colloidal nanocomposites comprised approximately spherical particles with relatively narrow size distributions, but in some cases more polydisperse or nonspherical particles were obtained. Selected acrylate-based nanocomposites were examined in terms of their film formation behavior. Scanning electron microscopy studies indicated relatively smooth films were obtained on drying at 20 degrees C, with complete loss of the original particle morphology. The optical clarity of solution-cast 10 microm nanocomposite films was assessed using visible absorption spectrophotometry, with 93-98% transmission being obtained from 400 to 800 nm; the effect of long-term immersion of such films in aqueous solutions was also examined. X-ray photoelectron spectroscopy studies indicated that the surface compositions of these nanocomposite particles are invariably silica-rich, which is consistent with their long-term colloidal stability and also with aqueous electrophoresis measurements. FT-IR studies suggested that in the case of the poly(methyl methacrylate)-silica nanocomposite particles, the carbonyl ester groups in the polymer are hydrogen-bonded to the surface silanol groups. According to differential scanning calorimetry studies, the glass transition temperatures of several poly(methyl methacrylate)-silica and polystyrene-silica nanocomposites can be either higher or lower than those of the corresponding homopolymers, depending on the nature of the silica sol.     

Control of the morphology of nanostructured particles prepared by the spray drying of a nanoparticle sol

The control of the morphology of nanostructured particles prepared by the spray drying of nanoparticle sol was investigated experimentally and the results are qualitatively explained based on available theory. A theoretical analysis indicates that the structural stability of the droplet and the hydrodynamic effects during the drying process play important roles in controlling the morphology of the resulting particles. The size of the sol in the droplet, droplet size, viscosity of droplet, drying temperature, gas flow rate, and addition of surfactant are all crucial parameters that affect the morphology of particles. Experimentally, nanostructured silica particles were prepared from a nanosize silica sol under various preparation conditions. Doughnut-shaped particles can be produced when the droplet size is large, in conjunction with high temperature, high gas flow rate and in the presence of an added surfactant. Appropriate choice of the spray drying method permits control of the particle size and shape, ranging from spheres to ellipsoids as well as doughnut-shaped particles by varying the preparation conditions. The results open a new route to controlling the formation of a wide variety of nanostructured particles.     

Preparation of thick silica films in the presence of poly(acrylic acid) by using electrophoretic sol-gel deposition

Thick silica films were fabricated by electrophoretic sol-gel deposition of silica particles on a stainless steel sheet. Using sols prepared by the sol-gel method with poly(acrylic acid) (PAA) films of ca. 25 m in thickness were prepared with no cracks. The films were shown to be agglomerates of monodispersed silica particles with PAA. The size of the silica particles decreased with an increase in the added amount of PAA. The deposited weight was considerably larger for the films with PAA than that of the films without PAA.     

Formation and structure of polyelectrolyte and nanoparticle multilayers: effect of particle characteristics

The formation and structure of multilayer films containing a cationic polyelectrolyte and anionic silica nanoparticles were studied by means of ellipsometry and atomic force microscopy. Three types of silica particles of different sizes were examined. The density and thickness of the films as well as the adsorption kinetics appear to be strongly dependent on the choice of particle; smaller particles favor the formation of smooth and dense films with a higher content of the inorganic component.     

液化膜对胶体悬浮液粘度的影响

Viscosity is one of the most important properties of colloids in mixing, transportation, stabilization, energy consumption, and so on. According to Einstein‘s viscosity equation, the viscosity of a colloidal dispersion increases with the increase of particle concentration. And the equation can be applicable to all micro-particle dispersions, because the effect of solvation films coated on particles can be neglectable in that case. But with the decrease of particle size to nano-scale, the formation of solvation films on nano-particles can greatly affect the viscosity of a dispersion, and Einstein‘s equation may not be applicable to this case. In this work, one kind of micro-size silica particle and two kinds of nano-size silica particles were used to investigate the effect of solvation films on dispersion viscosity, dispersed in water and ethyl alcohol solvents, respectively. The results of theoretical calculation and experimental investigation show that the increase of viscosity is contributed from solvation films by more than 95 percent for nano-particle dispersions, while less than 10 percent for micro-particle dispersions.     

Rheological behavior of thermoreversible kappa-carrageenan/nanosilica gels

The rheological behavior of silica/kappa-carrageenan nanocomposites has been investigated as a function of silica particle size and load. The addition of silica nanoparticles was observed to invariably impair the gelation process, as viewed by the reduction of gel strength and decrease of gelation and melting temperatures. This weakening effect is seen, for the lowest particle size, to become slightly more marked as silica concentration (or load) is increased and at the lowest load as particle size is increased. These results suggest that, under these conditions, the particles act as physical barriers to polysaccharide chain aggregation and, hence, gelation. However, for larger particle sizes and higher loads, gel strength does not weaken with size or concentration but, rather, becomes relatively stronger for intermediate particles sizes, or remains unchanged for the largest particles, as a function of load. This indicates that larger particles in higher number do not seem to increasingly disrupt the gel, as expected, but rather promote the formation of stable gel network of intermediate strength. The possibility of this being caused by the larger negative surface charge found for the larger particles is discussed. This may impede further approximation of neighboring particles thus leaving enough inter-particle space for gel formation, taking advantage of a high local polysaccharide concentration due to the higher total space occupied by large particles at higher loads.     

Silica Films Containing Ordered Pores Prepared by Dip Coating of Silica Nanoparticles and Polystyrene Beads Colloidal Mixture

Silica films containing three dimensionally (3D) ordered pores were prepared by a simple dip coating method. A colloidal sol containing silica particles in the nanometer size range and a polystyrene latex (PSL) colloidal sol containing particles of tens of nanometers to one micrometer in size were used as precursors. The pore periodicity, which in turn produces the dielectric periodicity, can be easily altered by changing the size of the PSL beads. Films having a high surface smoothness were obtained by using small silica particles, large PSL particles, and a low withdrawal speed in the dip-coating. When the films were irradiated with a white light source, the reflective spectrum was changed by varying the incident angle, indicating its possible use as a monochromator. The change in the reflective spectrum was explained using effective medium approximation combined with a simple Bragg reflection equation.     

PVC-silica hybrids: effect of sol–gel conditions on the morphology of silica particles and thermal mechanical properties of the hybrids

Hybrid materials from poly(vinyl chloride) (PVC) and silica have been prepared using different conditions by the sol–gel technique. In situ generation of silica network in the PVC matrix was carried out by hydrolysis/condensation of tetraethoxysilane (TEOS) in the matrix. Morphology of the silica particles produced in hybrid films was studied by scattering electron microscopy. The shape of silica particle produced in the matrix was modified by carrying out the sol–gel process under steam on the hybrid films using TEOS. The films were subjected to strain conditions during this process, which produced lamellar shaped particles in the matrix. It was possible to produce platelet type of structure with different aspect ratio by changing the composition and the stress conditions on the films during the steaming process. Addition of a very small amount of γ-glycidoxypropyltrimethoxysilane as compatibilizer drastically reduced the silica particles size in the matrix to nano-level. Thermal–mechanical properties of some of these hybrids were studied and related to the composition, structure and inter-phase interaction between the silica and the matrix.     

Regulation of cellular responses to macroporous inorganic films prepared by the inverse-opal method

Regenerative medicine for repairing damaged body tissues has recently become critically important. Cell culture scaffolds are required for the control of cell attachment, proliferation, and differentiation in in vitro cell cultures. A new strategy to control cell adhesion, morphology, and proliferation was developed by culturing mouse osteoblast-like MC3T3-E1 cells on novel cell culture scaffolds fabricated using ordered nanometer-sized pores (100, 300, 500, and 1000 nm). Results of this study indicate that after 72 h of incubation, the number of cells cultured on a silica film with a pore size of 1000 nm was similar to or slightly lower than that cultured on a non-porous control silica film. Films with 100-500 nm pore sizes, however, resulted in the cell growth inhibition. Morphology of the cultured cells revealed increased elongation and the formation of actin stress fibers was virtually absent on macroporous silica films with 100-500 nm pore size. Vinculin molecules expressed in cells cultured on the non-porous silica films showed many clear focal adhesions, whereas focal contacts were insufficiently formed in cells cultured on macroporous films. The influence of hydroxyapatite (HAp) and alumina scaffolds on the behavior of MC3T3-E1 cells was also evaluated. The proliferation rate of MC3T3-E1 cells cultured on HAp films with 1000 nm pore size was increased to approximately 20% above than that obtained of cells cultured on non-porous HAp films. These results demonstrate that the pore size and constituents of films play a role in controlling the morphology and proliferation rate of MC3T3-E1 cells.     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. II. Self-organization of cationic polymer particles on polymer films and wettability control with particle monolayers

Self-organization of cationic polymer particles through hydrophobic interaction on polymer films in aqueous system and characteristic properties of the resulting particle monolayers were investigated. Cationic polymer particles bearing quaternary ammonium groups on their surfaces effectively self-organized on polymer films. With an increase of the particle surface charge density, the surface coverage and average aggregate size (N _a) decreased. The surface coverage control was accomplished by tuning the ionic strength of the media. The wettability of polymer films for water was imparted by the formation of particle monolayers on them. Annealing of the particle monolayers resulted in the increase of the adhesive strength, while the wettability for water was lost. Further improvements of both wettability and adhesive strength of particle monolayers were achieved by the immobilization of silica colloids on the particle monolayers. This method would be effective for the hydrophilization of polymer films.     

Effect of anionic surfactants on synthesis and self-assembly of silica colloidal nanoparticles

Effects of the anionic surfactants, sodium dodecyl sulfate and sodium oleate, on the formation and properties of silica colloidal nanoparticles were investigated. At a concentration of approximately 1 x 10(-3) M, adsorption of anionic surfactants increased particle size, monodispersity, and negative surface charge density of synthesized silica particles. As uniformity of particle size and particle-particle interactions increase, colloidal photonic crystals readily self-assemble without extensive washing of the synthesized silica nanoparticles. The photonic crystals diffract light in the visible region according to Bragg's law. The assembled colloidal particle arrays exhibit a face-centered cubic structure in dried thin films. This study offers a new approach for producing ordered colloidal silica thin films.     

Ionic transport across tailored nanoporous anodic alumina membranes

Monodispersed silica particles with bimodal size distribution were successfully prepared through adding an ethanol (EtOH) solution containing tetraethylorthosilicate (TEOS) dropwise into an ammonia EtOH solution at a constant low rate. The effects of the reaction parameters such as ammonia/ethanol ratio, feeding rate of TEOS solution, reaction temperature, and time on the size and size distribution of the as-obtained particles were investigated. Based on these phenomena, a modified LaMer model of nucleation and growth mechanism was proposed to reasonably explain the formation of the as-obtained silica particles with bimodal size distribution. The as-prepared monodispersed silica particles with bimodal size distribution can be directly fabricated into binary colloidal crystals with small particles surrounding large particles by evaporation-induced cooperative self-assembly. This suggests that the method reported here provides a straightforward and effective route to the in situ fabrication of novel binary colloidal crystals and their replicated patterns in one reaction system.     

The use of monodispersed colloids in the polishing of copper and tantalum

The properties of abrasive particles play a significant role in chemical mechanical polishing (CMP) of metal and dielectric films in semiconductor device manufacturing. This study investigates the effects of the particle size, shape, and hardness of abrasives on the polishing of copper and tantalum films in the presence of different slurry chemistries. Well-defined dispersions of uniform particles, including spherical silica of varying diameters, hematite of different shapes, and hematite cores coated with silica of different thicknesses, were prepared and used to polish blanket films of Cu and Ta. It was shown that the total surface area of the solids in the slurry controlled the rate of material removal by pure silica for both Cu and Ta, while the surface quality of the polished films was better when higher silica content was used. The shape or the aspect ratio of hematite particles had a minor effect on the removal rate. In contrast, when hematite particles coated with silica were employed in the polishing of Cu and Ta, the polish rate decreased with increasing thickness of the shell.