Electro-optic effects of colloidal crystals of polymer-modified silica spheres immobilized with gelator

Electro-optic responses of colloidal crystals consist of poly(maleic anhydride-co-styrene)-modified silica spheres (P(MA-ST)/SiO₂) in acetonitrile and the crystals immobilized with a gelator, N-benzyloxycarbonyl-L-isoleucilaminooctadecane (Z-L-Ile-C-18), are studied by reflected-light intensity measurements and time-resolved reflection spectroscopy. Application of an alternating electric field deforms P(MA-ST)/SiO₂ crystal lattices reversibly. The response waveforms from the crystals are dependent on the frequency and strength of the applied electric field; similar dependencies have been qualitatively observed for the colloidal crystals consisting of polystyrene or silica spheres in aqueous media in our previous studies. Both gelated and ungelated P(MA-ST)/SiO₂ crystals change the reflection intensity, however, the amplitude is larger for the latter. The small response for the gelated P(MA-ST)/SiO₂ crystals is attributed to the higher elastic modulus (G). The G value of the gelated P(MA-ST)/SiO₂ crystals in acetonitrile is estimated from the change in the inter-sphere distance to be 8.0 Pa, which is about 2.3 and 2.4 times larger than that for ungelated P(MA-ST)/SiO₂ crystals in acetonitrile and colloidal silica crystals in aqueous media, respectively.     

Kinetics analysis of colloidal crystallization of silica spheres modified with polymers on their surfaces in acetonitrile

 The nucleation and growth rates in the colloidal crystallization of silica spheres (136 nm in diameter) modified with polymers on their surface were measured by time-resolved reflection spectroscopy. The polymers were poly(maleic anhydride-co-styrene) [P(MA-ST)] and poly(methyl methacrylate) (PMMA). The induction period for nucleation decreased sharply when the sphere concentration increased. The crystal growth process consisted of a fast growing step leading to metastable crystals (rate v ₁) and a slow growth rate accompanied by the formation of stable crystals. The crystal size of the P(MA-ST)/SiO₂ particles decreased from 0.4 to 0.06 mm, whereas v ₁ increased from 13 to 37 μm/s, when the particle concentration increased. The slow step was also observed for almost all the samples but was not analyzed since the rate was too small. For PMMA/SiO₂ dispersions, the crystal size (0.17–0.3 mm) and v ₁ (43–166 μm/s) did not show any relation to the particle concentration but showed a linear relationship with the molecular weight of PMMA. These results suggest the important role of the excluded-volume effects of the polymer layers around the silica surface. The contribution of the repulsion due to the electrical double layers is still effective in the colloidal crystallization in acetonitrile. Received: 6 June 2001 Accepted: 20 September 2001     

Opal and inverse opal photonic crystals: Fabrication and characterization

Three-dimensional photonic crystals made of close-packed polymethylmethacrylate (PMMA) spheres or air spheres in silica, titania and ceria matrices have been fabricated and characterized using SEM, XRD, Raman spectroscopy and UV–Vis transmittance measurements. The PMMA colloidal crystals (opals) were grown by self-assembly from aqueous suspensions of monodisperse PMMA spheres with diameters between 280 and 415 nm. SEM confirmed the PMMA spheres crystallized uniformly in a face-centred cubic (fcc) array, and UV–Vis measurements show that the colloidal crystals possess pseudo photonic band gaps in the visible and near-IR regions. Inverse opals were prepared by depositing silica (SiO₂), titania (TiO₂) or ceria (CeO₂) in the voids of the PMMA colloidal crystals using sol-gel procedures, then calcining the resulting structure at 550 °C to remove the polymer template. The resulting macroporous materials showed fcc ordering of air spheres separated by thin frameworks of amorphous silica, nanocrystalline titania or nanocrystalline ceria particles, respectively. Optical measurements confirmed the photonic nature of the inverse opal arrays. UV–Vis data collected for the opals and inverse opals obeyed a modified Bragg’s law expression that considers both diffraction and refraction of light by the photonic crystal architectures. The versatility of the colloidal crystal template approach for the fabrication of macroporous oxide structures is demonstrated.     

Poly(styrene) coating of monodispersed colloidal metal oxides by grafting to hydrophilic macromer adsorbed on the surface

Poly(styrene) (PST) coatings of monodispersed colloidal metal oxide particles by surface grafting to poly(N-vinyl-2-pyrrolidone) (ST–PVP) or quaternized poly(4-vinylpyridine) (ST-PVPy(Me)) macromer, having a vinylphenylene end group, were investigated. Radical polymerization of styrene (ST) in ethanolic silica colloid in the presence of ST-PVP successfully led to the formation of monodispersed PST/PVP copolymer/SiO₂composites. The addition of divinylbenzene (DVB) to the reaction system gave SiO₂ composites coated with crosslinked PST. Graft-polymerization of ST to ST-PVP also took place on TiO₂, CeO₂ and Al(OH)₃ colloidal particles in ethanolic solution. However, ST-PVPy(Me) adsorbed on colloidal silica did not effectively graft PST.     

Colloidal crystallization of thermosensitive gel spheres of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) with low degree of cross-linking

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermosensitive gel spheres, poly(N-isopropylacrylamide) ((200–0.5), 318 and 116 nm in the hydrodynamic diameter at 25°C and 45°C, and 0.5% in the degree of cross-linking) were studied. Giant colloidal single crystals formed at very low gel concentrations. Densities of the gel spheres were 0.030 and 0.61 at 25°C and 45°C, respectively. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.015 wt.% at 25°C as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation and anion exchange resins. These results demonstrate that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Extent of the contribution of the electrical double layers on the crystallization increased sharply when the degree of cross-linking increased, the gel spheres shrank, and/or the density of the gel spheres increased.     

Cationic gel crystals and amorphous solids of lightly cross-linked poly(2-vinylpyridine) spheres in the deionized aqueous suspension

Colloidal crystallization and amorphous solidification of deionized suspensions of the polydispersed cationic gel spheres of lightly cross-linked poly(2-vinylpyridine), CAIBA-P2VP (107～113 nm in diameter, ±19～22 nm in dispersity), have been studied from the reflection spectroscopy, morphology, phase diagram, and elastic property. Crystallization takes place even for the polydispersed cationic gel spheres by the significant contribution of the extended electrical double layers formed around the spheres. Critical concentrations of melting coexisted with ion exchange resins were around 0.02 in volume fraction and high compared with those of other cationic and anionic gel crystals examined hitherto. The densities (ρ) of CAIBA-P2VP in suspension state, i.e., weight percent of the gel spheres divided by the corresponding volume percent, was around 0.3. The ρ values decreased sharply with decreasing size of P2VP gel spheres, which supports the small gel spheres containing much water inside and being softer than the large ones. The closest intersphere distances of the crystals and/or amorphous solids were much longer than the hydrodynamic diameters of the gel spheres especially at low sphere concentrations. Fluctuation parameters (b) evaluated from the rigidities of CAIBA-P2VP (0.15～0.28) were large compared with those of gel crystals of large-sized P2VP-based cationic gel spheres, anionic thermosensitive gel spheres of poly(N-isopropylacrylamide) (0.05～0.09) and further much larger than those of typical colloidal hard spheres (around 0.03). The dispersity in sphere size played an important role for distinguishing crystal and amorphous solid. Importance of the extended electrical double layers around the cationic gel spheres is supported in addition to the excluded volume effect of the sphere themselves on the crystallization and/or solidification.     

Colloidal crystallization of thermo-sensitive gel spheres of poly (<Emphasis Type="Italic">N</Emphasis>-isopropyl acrylamide). Influence of gel size

Influence of the gel size on the morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (pNIPAm), was discussed by adding the data of two gel samples of pNIPAm(400–5) and pNIPAm(600–5) of 412 nm (at 25 °C) and 220 nm (at 45 °C) and of 517 nm (at 20 °C) and 294 nm (at 45 °C), respectively. Colloidal single crystals formed, but not so large compared with the giant crystals of small pNIPAm gels reported previously. The suspensions even with ion-exchange resins were turbid and hard to observe the single crystals clearly with the naked eyes as gel size increased. The critical concentration of melting decreased sharply as the suspensions were deionized with coexistence of the mixtures of cation- and anion-exchange resins. The critical concentration increased as the gel size increased and/or dispersion temperature increased. Density of the gel spheres increased as their size increased. These results demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Contribution of the electrical double layers on the crystallization increased sharply as temperature increased and gel concentration decreased, respectively. The contribution also increased slightly as sphere size increased, when comparison was made at the same gel concentration in wt.%. The present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed between the water phase and gel spheres, though the gel spheres contain a lot of water molecules at the inner sphere region.     

Cationic gel crystals of lightly cross-linked poly(2-vinylpyridine) spheres (170∼180 nm in diameter) in the deionized aqueous suspension

Colloidal crystallization of deionized suspensions of the cationic gel spheres of lightly cross-linked poly(2-vinylpyridine), AIBA-P2VP (170～180 nm in diameter) has been studied from the microscopic observation, morphology, phase diagram, and elastic property. Critical concentrations of melting that coexisted with ion-exchange resins were low compared with those without resins and decreased as the degree of cross-linking decreased. The density of a gel sphere in suspension state (ρ), i.e., weight percent of the gel spheres divided by the corresponding volume percent, was between 0.5 and 0.8, and decreased as the degree of cross-linking of the spheres decreased. The ρ values also decreased with decreasing size of gel spheres, which supports the small P2VP gel spheres being softer than the large ones. The closest intersphere distances of the crystals were much longer than the hydrodynamic diameters of the gel spheres especially at low sphere concentrations. Fluctuation parameters evaluated from the rigidities of the crystals of AIBA-P2VP (0.05～0.09) were similar to those of gel crystals of cationic gel spheres of lightly cross-linked poly(2-vinylpyridine) spheres coated with poly(ethylene glycol), 400 nm in diameter, and thermo-sensitive gel spheres of pNIPAm, poly(N-isopropylacrylamide), but larger than those of typical colloidal hard spheres. The stable crystal phase formed beyond the adsorbed monolayer of cationic gel spheres on the surface of the substrate. These experimental findings support important role of the extended electrical double layers around the cationic gel spheres in addition to the excluded volume effect of the sphere themselves on the crystallization.     

Capabilities of polymer-modified monodisperse colloidal silica particles as biomaterial carrier

 Polymer modification of monodispersed colloidal silica (0.5 μm) with poly(maleic anhydride-co-styrene) (P(MA-ST)) and poly (maleic anhydride-co-methyl methacrylate) (P(MA-MMA)) and application of the composite particles to biomaterial carriers were investigated. The reaction of bovine serum albumin(BSA)-immobilized P(MA-MMA)/SiO₂ with the anti-BSA antibody showed higher sensitivity in immunological agglutination test than BSA–P(MA-ST)/SiO₂, though immobilization efficiency of BSA on P(MA-MMA)/SiO₂ was lower than that on P(MA-ST)/SiO₂. Alkaline phosphatase and glucose oxidase immobilized on the composite particles exhibited extremely low activities, but α-chymotrypsin immobilized on P(MA-MMA)/SiO₂ and its derivative particles showed the relative activity of 12.5% and 16.1% to the native enzyme, respectively. Grafting of a hydrophilic polymer of poly(acrylic acid) to P(MA-ST)/SiO₂ let to an increase of the immobilized α-chymotrypsin activity to give the maximum relative activity of 55.5%. Received: 23 August 1996 Accepted: 16 October 1996     

Thermo-sensitive colloidal crystals of silica spheres in the presence of large spheres with poly (N-isopropyl acrylamide) shells

Thermo-sensitive colloidal crystals are prepared simply by mixing colloidal silica spheres and large thermo-sensitive gel spheres. The thermo-reversible change in the lattice spacing of colloidal crystals of monodisperse silica spheres (CS82, 103 nm in diameter) depends on the size of the admixed temperature-sensitive gel spheres. For spheres with sizes less and greater than that of the silica spheres, the lattice spacing upon temperature increase above the lower critical solution temperature of poly(N-isopropyl acrylamide) decreases (cf. Okubo et al. Langmuir 18:6783, 2002) and increases, respectively. A mechanism, which is able to explain these experimental findings, is proposed. Moreover, crystal growth rates and the rigidities of the thermo-sensitive colloidal crystals are studied.     

Surface modification of fine colloidal silica with copolymer silane-coupling agents composed of maleic anhydride

The reactivity of copolymer silane composed of maleic anhydride in the modification of fine colloidal silica was studied. The reaction of colloidal silica of 10 and 45-nm diameter with trimethoxysilyl-terminated poly(maleic anhydride-co-styrene) [P(MA-ST)] and poly(MA-co-methyl methacrylate) in tetrahydrofuran resulted in effective surface modification without particle aggregation. From the results that the reaction using the polystyrene silane of low molecular weight led to partial aggregation, it was suggested that the steric interaction between relatively rigid copolymer chains having a maleic anhydride moiety adsorbed on the silica prevented the aggregation in the reaction. The ²⁹Si cross-polarization magic-angle-spinning NMR spectra of P(MA-ST)-modified silica showed that the polymer silane was bound to the silica surface by the direct reaction with silica hydroxyl groups and via the polymerization. Received: 27 June 2001 Accepted: 6 September 2001     

Giant colloidal crystals of fluorine-containing polymer spheres in exhaustively deionized aqueous suspensions and in the presence of sodium chloride

 Gigantic colloidal single crystals (2–6 mm) are formed for fluorine-containing polymer spheres (120–210 nm in diameter) in exhaustively deionized aqueous suspensions. The spheres used are poly(tetrafluoroethylene) (PTFEA and PTFEB), copolymer of tetrafluoroethylene and perfluorovinylether (PFA) and copolymer of tetrafluoroethylene and perfluoropropylene (PTP). The phase diagrams of these spheres are obtained in the deionized suspensions and also in the presence of sodium chloride for PFA. The critical sphere concentrations of crystal melting (φ _c) for these spheres are around 0.0006 in volume fraction, which are close to, but slightly larger than, those of monodispersed polystyrene spheres (φ _c ≈ 0.00015) and colloidal silica spheres(φ _c = 0.0002–0.0004) reported previously. The crystals are largest when the sphere concentrations are a bit higher than the φ _c value and their size decreases as the sphere concentration increases. Reflection spectra are taken in sedimentation equilibrium as a function of the height from the bottom of the suspension. The static elastic modulus is estimated to be 10.8 and 28.7 Pa for PTFEA and PTP spheres at the sphere concentrations 0.00325 and 0.00322 in volume fraction, respectively. Received: 27 October 1999 Accepted in revised form: 16 November 1999     

Static and dynamic light-scattering of colloidal gases,liquids and crystals

Static and dynamic light-scattering measurements are made for colloidal-crystals,-liquids and-gases of silica spheres, 103 nm in diameter, in the exhaustively deionized suspension and in the presence of sodium chloride. Sharp peaks in the scattering curve are observed, for the first time, for the colloidal crystals in very diluted aqueous suspension. The product of the effective diffusion coefficient and the scattered light intensity is found constant over the whole range of the scattering angle measured for the colloidal crystals and liquids. Three and two dynamic processes have been extracted separately from time profiles of autocorrelation function of colloidal crystals and liquids, respectively from Marquadt histogram analysis. Decay curves of colloidal gases are characterized by a single translational diffusion coefficient,D ₀.D ₀ of the gases is always lower than the calculation from the Stokes-Einstein equation with the true diameter of spheres, and increases as ionic concentration increases. These experimental results emphasize the important role of the expanded electrical double layers on the diffusive properties in the colloidal crystals, liquids and gases.     

Tunable Fabrication of Two‐Dimensional Arrays of Polymer Nanobowls for Biomimic Growth of Amorphous Calcium Carbonate

Two‐dimensional arrays of polymer nanobowls can be fabricated by an oxygen plasma etching technique. The 2D colloidal crystals made of SiO₂@PMMA particles are fabricated by a convective self‐assembly method. The oxygen plasma treatment is applied to the colloidal crystals to selectively etch the PMMA shells. Because the oxygen plasma etching proceeds in a layer‐by‐layer manner from top to bottom, the top parts of the PMMA shells are etched first, and the silica cores are exposed to the atmosphere, which can be removed with HF, leaving the bowl‐shaped PMMA shells to form 2D arrays of polymer nanobowls. The size and packing density of the nanobowl arrays can be tuned with tightly controlled etching time. The polymer nanobowl arrays can also serve as a template to direct the growth of calcium carbonate within the interstice of the nanobowls.     

Another look at the melting temperature of colloidal crystals in the completely deionized suspension

Melting temperature (T _m) of colloidal crystals of monodispersed polystyrene and silica spheres has been measured for thecompletely deionized suspensions as a function of sphere concentration. More than 3 weeks are needed before achievement of the completely deionized state.T _mincreases substantially as the deionization process of the suspension proceeds. The most reliable values ofT _mobserved for the completely deionized suspensions are successfully analyzed again with the theory of Williams et al. The newT _mvalues are compared also with the theory of Robbins et al., which treats the repulsive Yukawa potential between colloidal spheres.     

Textile with silver silica spheres: its antimicrobial activity against <Emphasis Type="Italic">Escherichia coli and Staphylococcus aureus</Emphasis>

The aim of this study was to investigate antimicrobial activity of textiles doped with silver in different forms. Three types of textiles were prepared and examined: textiles doped with commercially available Ag nanoparticles, textiles doped with commercial colloidal silver and textiles doped with silver silica SiO₂/Ag spheres. The specimens of silica submicron spheres were synthesized by the sol–gel method as a matrix for biological active silver. The results of microbiological tests revealed that among three kinds of Ag doped textiles only these doped with SiO₂/Ag spheres are bacteriostatically active. During the experiments minimal inhibiting bacteria growth concentration of active SiO₂/Ag spheres added to textiles was determined.     

Colloidal crystallization of thermo-sensitive gel spheres of poly (N-isopropyl acrylamide). Influence of degree of cross-linking of the gels

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) having degrees of cross-linking 10 and 2?mol.% (pNIPAm(200?C10) and pNIPAm(200?C2)) were studied. Giant colloidal single crystals formed at very low gel concentrations. Critical concentrations of melting increased as the degree of cross-linking decreased in the range from 10 to 0.5?mol.% and/or suspension temperature increased from 20 to 45?°C. The critical concentration decreased sharply as the suspensions were deionized with coexistence of the mixtures of cation- and anion-exchange resins. Density of a gel sphere (gel concentration in weight percent divided by that in volume percent) increased sharply as the degree of cross-linking and/or temperature increased. These results demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded-volume effect of the gels. Most of the researchers including the authors have believed that the crystallization of the gel spheres takes place by the excluded-volume effect. However, the present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed firmly between the water phase and gel spheres, though the gel spheres contain a lot of water molecules in the sphere region.     

Particle monolayer formation at the air–water interface by silica particle with well‐defined grafted polymer

Particle monolayer formation at the air–water interface by polymer‐grafted colloidal silica was investigated. Methyl methacrylate (MMA) was polymerized from initiative bromide groups at colloidal silica surface by atom transfer radical polymerization. We obtained polymer‐grafted silica particle (SiO₂‐PMMA) with relative narrow polydispersity of PMMA. For the polymer‐grafted particle with high graft density, particle monolayer formation was confirmed by π‐A isotherm measurement and SEM observation. Interparticle distance was controllable by surface pressure. Furthermore, grafted polymer chains were suggested to be fairly extended at the air–water interface. However, for the polymer‐grafted particle with low graft density, monolayer structure on substrate showed aggregation and voids. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2789–2797, 2006     

Convectional, sedimentation, and drying dissipative patterns of colloidal silica (183 nm in diameter) suspensions in a glass dish and a watch glass

Convectional, sedimentation, and drying dissipative structural patterns formed during the course of drying aqueous colloidal crystals of silica spheres (183 nm in diameter) have been studied in a glass dish and a watch glass. Spoke-like convectional patterns were observed in a watch glass. The broad ring sedimentation patterns formed especially in a glass dish within 30–40 min in suspension state by the convectional flow of water and colloidal spheres. The macroscopic broad ring drying patterns formed both in a glass dish and a watch glass. The ratio of the broad ring size in a glass dish against the initial size of suspension, i.e., inner diameter of the glass dish, d _f/d _i, in this work, were compared with previous work of other silica spheres having sizes of 305 and 560 nm and 1.2 μm in diameter. The d _f/d _i values in a glass dish increased as sphere concentration increased, but were rather insensitive to colloidal size. The d _f/d _i values on a watch glass also increased as sphere concentration increased, and further increased as sphere size decreased. Segregation effect by sphere size in a watch glass takes place by the balancing between the upward convectional flow of spheres in the lower layers of the liquid and the downward sedimentation of spheres. Colorful microscopic drying patterns formed both in a glass dish and a watch glass.     

Size effect of silica nanoparticles on thermal decomposition of PMMA

The size effect of silica nanoparticles (SiO₂) on thermal decomposition of poly(methylmethacrylate) (PMMA) was investigated by the controlled rate thermogravimetry. Thermal degradation temperature of PMMA–SiO₂ composites depended on both fraction and size of SiO₂, the thermal degradation temperature of 23 nm (diameter) SiO₂–PMMA (6.1 wt%) was 13.5 °C higher than that of PMMA. The thermal stabilities of 17 nm SiO₂–PMMA (3.2 wt%) and 13 nm SiO₂–PMMA (4.8 wt%) were 21 and 23 °C, respectively, higher than that of PMMA without SiO₂. The degree of degradation improvement was increased linearly with the surface area of SiO₂. The number of surface hydroxyl group in unit volume of SiO₂ particle increased with increasing the specific surface area of SiO₂, and the interaction between hydroxide group of SiO₂ and carbonyl group of PMMA had an important role to improve the thermal stability of PMMA.