Simple method for preparation of porous polyimide film with an ordered surface based on in situ self-assembly of polyamic acid and silica microspheres

In this Article, we addressed a facile method for the fabrication of porous polyimide film with an ordered surface based on the solvent-evaporation-assisted in situ self-assembly of polyamic acid (PAA, precursor of polyimide) and silica microspheres during vacuum-drying of PAA/silica colloid solution. Hydroxyl groups on the surface of silica microspheres have strong hydrogen-bonding with PAA chains, which improve the dispersion of silica microspheres in PAA/DMF solution and further help the self-assembly of PAA/silica colloid solution via solvent evaporation. The approach is simple, neither the preparation of special template nor complex preparation process and precise control over condition is necessary. Furthermore, the method could be employed for mass production of ordered porous polyimide films, and by changing the content and size of silica microspheres, the pore size and porous structure of the porous polyimide films could be tunable. The wettability behavior of the as-prepared porous polyimide films is also studied; the ordered surface topography of the porous polyimide films could change the wettability from hydrophilicity to hydrophobicity.     

Mechanically stable antireflection and antifogging coatings fabricated by the layer-by-layer deposition process and postcalcination

Complexes of poly(diallyldimethylammonium chloride) (PDDA) and sodium silicate (PDDA-silicate) are alternately deposited with poly(acrylic acid) (PAA) to fabricate PAA/PDDA-silicate multilayer films. The removal of the organic components in the PAA/PDDA-silicate mulilayer films through calcination produces highly porous silica coatings with excellent mechanical stability and good adhesion to substrates. Quartz substrates covered with such porous silica coatings exhibit both antireflection and antifogging properties because of the reduced refractive index and superhydrophilicity of the resultant films. A maximum transmittance of 99.86% in the visible spectral range is achieved for the calcinated PAA/PDDA-silicate films deposited on quartz substrates. The wavelengths of maximum transmittance could be well tailored by simply changing the deposition cycles of multilayer films. The usage of PDDA-silicate complexes allows for the introduction of high porosity to the resultant silica coatings, which favors the fabrication of antireflection and antifogging coatings with enhanced performance. Meanwhile, PDDA-silicate complexes enable rapid fabrication of thick porous silica coatings after calcination because of the large dimensions of the complexes in solution. The easy availability of the materials and simplicity of this method for film fabrication might make the mechanically stable multifunctional antireflection and antifogging coatings potentially useful in a variety of applications.     

Nanotexture optimization by oxygen plasma of mesoporous silica thin film for enrichment of low molecular weight peptides captured from human serum

This study investigated the optimization of mesoporous silica thin films by nanotexturing using oxygen plasma versus thermal oxidation. Calcination in oxygen plasma provides superior control over pore formation with regard to the pore surface and higher fidelity to the structure of the polymer template. The resulting porous film offers an ideal substrate for the selective partitioning of peptides from complex mixtures. The improved chemico-physical characteristics of porous thin films (pore size distribution, nanostructure, surface properties and pore connectivity) were systematically characterized with XRD, Ellipsometry, FTIR, TEM and N(2) adsorption/desorption. The enrichment of low molecular weight proteins captured from human serum on mesoporous silica thin films fabricated by both methodologies were investigated by comparison of their MALDI-TOF MS profiles. This novel on-chip fractionation technology offers advantages in recovering the low molecular weight peptides from human serum, which has been recognized as an informative resource for early diagnosis of cancer and other diseases.     

Preparation and dielectric properties of polyimide/silica nanocomposite films prepared from sol–gel and blending process

Using poly(amic acid) (PAA) as a precursor followed by thermal imidization, the polyimide/silica nanocomposite films were prepared via an improved sol–gel process and a blending process, respectively. FT‐IR, TEM and TGA measurements were used to characterize the structure and properties of the obtained films. The results confirmed that the introduction of silica did not yield negative effects on the conversion of the PAA precursor to the polyimide. With the increase of silica content, the aggregation of silica appeared in the polyimide matrix, and the thermal stability decreased slightly for both kinds of films. The dielectric constant (ε) of both films increased slowly with the increase of the silica concentration. The dielectric constant of the obtained polyimide/silica nanocomposite films displayed good stability within a wide range of temperatures or frequency. Based on modeling relation between ε and silica content, the difference in dielectric properties for two kinds of nanocomposites are discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

In situ monitoring of atomic layer deposition in nanoporous thin films using ellipsometric porosimetry

Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO(2) in a mesoporous silica film.     

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.     

Control of prolonged drug release and compression properties of ibuprofen microspheres with acrylic polymer, eudragit RS, by changing their intraparticle porosity [corrected]

Prolonged-release spherical micro-matrices of ibuprofen with Eudragit RS were prepared using a novel emulsion-solvent diffusion method. Those particles were termed "microspheres" due to their characteristic sponge-like texture and unique dissolution and compression properties unlike conventional microcapsules or microspheres. The internal porosity of microspheres could be easily controlled by changing the concentration of the drug and the polymer in the emulsion droplet (ethanol). With lower concentration of ibuprofen in the ethanol, the resultant microspheres had a higher porosity, about 50%. The drug release rate from the microspheres was interpreted by the Higuchi model of spherical matrices, which depended only on their internal porosity of the microspheres when size distribution and drug content were the same. The tortuosities in the microspheres were found to be almost constant (3-4) irrespective of porosity, suggesting the same internal texture. Microsphere compressibility was much improved over the physical mixture of the drug and polymer owing to the plastic deformation of their sponge-like structure. The more porous microspheres produced stronger tablets [corrected].     

Facile synthesis of monodisperse microspheres and gigantic hollow shells of mesoporous silica in mixed water-ethanol solvents

Mesoporous silica materials with a variety of morphologies, such as monodisperse microspheres, gigantic hollow structures comprising a thin shell with a hole, and gigantic hollow structures consisting of an outer thin shell and an inner layer composed of many small spheres, have been readily synthesized in mixed water-ethanol solvents at room temperature using cetyltrimethylammonium bromide (CTAB) as the template. The obtained mesoporous silica generally shows a disordered mesostructure with typical average pore sizes ranging from 3.1 to 3.8 nm. The effects of the water-to-ethanol volume ratio (r), the volume content of tetraethyl orthosilicate TEOS (x), and the CTAB concentration in the solution on the final morphology of the mesoporous silica products have been investigated. The growth process of gigantic hollow shells of mesoporous silica through templating emulsion droplets of TEOS in mixed water-ethanol solution has been monitored directly with optical microscopy. Generally, the morphology of mesoporous silica can be regulated from microspheres through gigantic hollow structures composed of small spheres to gigantic hollow structures with a thin shell by increasing the water-to-ethanol volume ratio, increasing the TEOS volume content, or decreasing the CTAB concentration. A plausible mechanism for the morphological regulation of mesoporous silica by adjusting various experimental parameters has been put forward by considering the existing state of the unhydrolyzed and partially hydrolyzed TEOS in the synthesis system.     

聚合物模板微球表面电性对中空二氧化硅微球形貌的影响

分别以过硫酸钾和偶氮二异丁基脒盐酸盐为引发剂,以聚乙烯吡咯烷酮(PVP)为分散剂,在水中引发苯乙烯聚合制备了2种表面分别带负电性和正电性基团的聚苯乙烯(PS)模板微球.在氨水催化下,利用正硅酸乙酯的水解缩合,形成PS/SiO_2复合微球,去除模板后得到中空SiO_2微球,并对其进行FTIR、电子显微镜、TGA以及氮气吸附等分析表征.结果表明,PS模板微球表面的电性决定了OH-的分布,从而导致PS模板微球表面SiO_2壳层不同的形成机制.当以表面带负电的PS微球为模板时,可得到树莓状的中空SiO_2微球;而以表面带正电的PS微球为模板时,得到是表面光滑的,具有介孔结构的中空SiO_2微球.     

Assembly of multilayered films using well-defined, end-labeled poly(acrylic acid): influence of molecular weight on exponential growth in a synthetic weak polyelectrolyte system

We report on the influence of polyanion molecular weight on the growth and structure of multilayered thin films fabricated from poly(allylamine) (PAH) and well-defined, end-labeled poly(acrylic acid) (PAA) synthesized by atom transfer radical polymerization. We observed striking differences in the growth of PAH/PAA films fabricated using well-defined PAA compared to films fabricated using higher molecular weight, commercially available PAA. Past studies demonstrate that the thicknesses of PAH/PAA films increase as linear functions of the number of PAH and PAA layers deposited over a broad range of pH (e.g., from pH 2.5 to 4.5). We observed the thicknesses of films fabricated using solutions of PAH and PAA adjusted to pH 7.5 and 3.5, respectively, to increase in a nonlinear manner. Films fabricated using well-defined, low molecular weight samples of PAA under these conditions increased in thickness exponentially. Experiments using samples of PAA having substantially non-overlapping molecular weight distributions demonstrated a clear relationship between the molecular weight of PAA and rates of film growth. We also used confocal microscopy, in combination with fluorescently end-labeled samples of PAA, to characterize the location of PAA in these assemblies. The results of these experiments, when combined, support the general conclusion that PAA is able to penetrate or diffuse into these films over large distances during assembly. The mechanism of growth for these films thus appears similar to that recently reported for the exponential growth of films fabricated using a variety of biologically relevant polyelectrolytes. The use of living/controlled methods of polymerization to synthesize well-defined samples of PAA facilitates an interpretation of these differences in film behavior as arising largely from differences in polymer molecular weight and polydispersity. This work provides insight into the assembly and structure of a well-studied weak polyelectrolyte film system and illustrates the potential of living/controlled methods of polymerization to contribute to the characterization and understanding of the physical properties of these ionically cross-linked materials.     

Biomimetic deposition of silica templated by a cationic polyamine-containing microgel

We report using poly(acrylamide-co-2-(dimethylamino)ethyl methacrylate, methyl chloride quaternized) cationic microgels as a porous colloidal template for biomimetic in situ silica mineralization, allowing the well-controlled synthesis of submicrometer-sized hybrid microgel--silica particles and porous silica particles by subsequent calcination. The microgels were prepared by inverse emulsion polymerization in the presence of a bisacrylamide cross-linker. Silica deposition was achieved by simply stirring an aqueous mixture of the microgel particles and tetramethyl orthosilicate (TMOS) at 20 degrees C for 30 min. No experimental evidence was found for nontemplated silica, which indicated that silica deposition occurred exclusively within the cationic microgel template particles. The resulting microgel-silica hybrid particles were characterized by electron microscopy, dynamic light scattering, FT-IR spectroscopy, 1H NMR and solid-state 29Si magic angle spinning NMR spectroscopy, thermogravimetry, aqueous electrophoresis, and surface area measurements. Aqueous electrophoresis studies confirmed that the hybrid microgel-silica particles had positive zeta potentials over a wide pH range and isoelectric points could be tuned by varying the synthesis conditions. This suggests that these particles could form complexes with DNA for improved gene delivery. The porosity of the calcined silica particles could be controlled by varying the amount of TMOS, suggesting potential encapsulation/controlled release applications.     

pH-Induced antireflection coatings derived from hydrogen-bonding-directed multilayer films

Hydrogen-bonding-directed layer-by-layer assembled films, based on polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles and poly(4-vinylpyridine) (P4VP), were successfully fabricated in methanol. Varying the PAA content in the PS-b-PAA micelles afforded control over the film growth properties, especially the multilayer film thickness. Interestingly, antireflection films with refractive indices that could be tuned between 1.58 and 1.28 were obtained by treatment with an aqueous HCl solution (pH 2.27), and the transmittance obtained was as high as 98.4%. In acid solution, the pyridine group was protonated, destroying the hydrogen bonding between P4VP and PAA. A concomitant pH-induced polymer reorganization in the multilayers resulted in a porous honeycomb-like texture on the substrate.     

Low dielectric constant nanocomposite thin films based on silica nanoparticle and organic thermosets

Low dielectric constant (low-k) nanocomposite thin films have been prepared by spin coating and thermal cure of solution mixtures of one of two organic low-k thermoset prepolymers and a silica nanoparticle with an average diameter of about 8 nm. The electrical, the mechanical, and the thermomechanical properties of these low-k nanocomposite thin films have been characterized with 4-point probe electrical measurements, nanoindentation measurements with an atomic force microscope, and specular X-ray reflectivity. Addition of the silica nanoparticle to the low-k organic thermosets enhances both the modulus and the hardness and reduces the coefficient of thermal expansion of the resultant nanocomposite thin films. The enhancements in the modulus of the nanocomposite thin films are less than those predicted by the Halpin-Tsai equations, presumably due to the relatively poor interfacial adhesion and/or the aggregation of the hydrophilic silica nanoparticles in the hydrophobic organic thermoset matrices. The addition of the silica nanoparticle to the low-k organic thermoset matrices increases the relative dielectric constant of the resultant nanocomposite thin films. The relative dielectric constant of the nanocomposite thin films has been found to agree fairly well with an additive formula based on the Debye equation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1482–1493, 2007     

Synthesis of porous carbon balls from spherical colloidal crystal templates

Spherical inverse opal (IO) porous carbon was produced utilizing silica colloidal crystal spheres as templates. The spherical colloidal crystals were obtained through the self-assembly of monodisperse particles inside an emulsion droplet with confined geometry. The templates were inverted using a carbon precursor, phenol-formaldehyde (PF) resol. We demonstrated a two-step synthesis involving the subsequent infiltration of the PF resol precursor into the spherical colloidal crystal template and a one-step synthesis using a silica colloidal solution containing dissolved PF resol. In the former case, the sizes of the IO carbon balls were controlled by the size of the colloidal crystal templates, and diameters of a few micrometers up to 50 μm were obtained. The average diameter of the macropores created by the silica particles was 230 nm. Moreover, meso-/macroporous IO carbon balls were created using block-copolymer templates in the PF resol. In the one-step synthesis, the concentration of PF resol in the colloidal solution controlled the diameter of the IO carbon balls. IO balls smaller than 3 μm were obtained from the direct addition of 5% PF resol. The one-step synthesis produced rather irregular porous structures reflecting the less ordered crystallization processes inside the spherical colloidal crystals. Nitrogen adsorption and cyclic voltammetry measurements were conducted to measure the specific area and electroactive surface area of the IO carbon balls. The specific area of the mesopores-incorporated IO carbon balls was 1.3 times higher than that of bare IO carbon balls. Accordingly, the meso-/macroporous porous carbon balls exhibited higher electrocatalytic properties than the macroporous carbon balls.     

Synthesis of SiO2@SiO2 core-shell microspheres using urea-formaldehyde polymers as the templates for fast separation of small solutes and proteins

The monodisperse superficially porous core-shell silica microspheres (CSSMs) with controllable shell thickness and pore size were synthesized by an improved polymerization-induced colloid aggregation (PICA) approach for fast separation of small solutes and proteins.     

Fluorescence staining and confocal laser scanning microscopy study of hydrogen-bonded poly(vinylpyrrolidone)/poly(acrylic acid) film

Poly(vinylpyrrolidone) (PVPON) and poly(acrylic acid) (PAA) were layer-by-layer (LBL) assembled to prepare the thin films based on hydrogen-bonding complexation. The hydrogen-bonded PVPON/PAA films were incubated in acidic, neutral and basic vapors separately. To study the morphologies after incubation, the films were stained by pH-sensitive fluorescent dyes using chemical and physical ways, and investigated with confocal laser scanning microscope (CLSM). The chemical way (labeling) was covalently linking fluoresceinamine (FAM) to some monomer units of PAA while the physical way was adsorbing rhodamine B (RB) molecules from dilute solution. Atomic force microscope (AFM) was combined with CLSM to find that after incubation in neutral or basic vapor the hydrogen-bonded PVPON/PAA films form porous structure and the pores are through the whole film.     

Microstructure and properties of polyamideimide/silica hybrids compatibilized with 3-aminopropyltriethoxysilane

In this work, we prepared and characterized polyamideimide (PAI)/silica hybrids compatibilized with 3-aminopropyltriethoxysilane (APTES). PAI/silica nanohybrid thin films were prepared using an in situ sol-gel process, followed by thermal imidization. We have investigated the microstructures and properties of the PAI/silica hybrids using FT-IR spectroscopy, X-ray diffraction, small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). We also measured their tensile properties, thermal properties, refractive indices, and dielectric constants. In general, the properties of the PAI/silica hybrids were optimized when the silica content was 6 wt.%.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Adsorption porosimetry application in the study of a porous structure of thin layers

The method of adsorption porosimetry (AP) has been long known in physical chemistry of porous sorbents, but its practical application was limited to studies of massive samples of such materials as silica gels, zeolites, etc. We have developed a technique to study the porous structure of dielectric thin films (d ～ 100 nm). It is stated that it provides us with quite novel information which permits the prediction of properties of dielectric layers and makes it possible on this basis to solve optimization problems of technological processes. The use of ellipsometry has further extended the potential of this method, and currently it is a powerful nondestructive technique to measure porosity and the size distribution of pores in thin layers for the purposes of modern electronics.     

Non‐surfactant template‐directed synthesis of SiO2‐polyimide hybrid self‐standing films with ordered mesoporous structure

The silica‐PI hybrid self‐standing films with ordered mesoporous structure have been prepared by using dibenzoyl‐L ‐tartaric acid (L ‐DBTA) as non‐surfactant template under mild sol–gel route. Polyimide matrix was obtained from polyamic acid (PAA) via thermal imidization process and the template was removed in this process. The PI‐based hybrid film with 20 wt% SiO₂ obtained from DBTA presented the ordered mesoporous channels with average pore size of about 2.0 nm and BET surface area of 1167 m²/g. FTIR and SEM studies indicated that the hydrogen bond interaction between the carboxylic groups of DBTA and benzamide bonds of PAA made the PAA possibly participate in the assembly process of the aggregates of the non‐surfactant template molecules. The mechanical, thermal and some physical properties of these hybrid films materials were also characterized. Copyright © 2010 John Wiley & Sons, Ltd.