Prototype of Low Thermal Expansion Materials: Fabrication of Mesoporous Silica/Polymer Composites with Densely Filled Polymer inside Mesopore Space

A prototype of novel low thermal expansion materials using mesoporous silica particles is demonstrated. Mesoporous silica/polymer composites with densely filled polymer inside the mesopore space are fabricated by mechanically mixing both organically modified mesoporous silica and epoxy polymer. The mesopores are easily penetrated by polymers as a result of the capillary force during the mechanical composite processing. Furthermore, we propose a new model of polymer mobility restriction using mesoporous silica with a large pore space. The robust inorganic frameworks covering the polymer effectively restrict the polymer mobility against thermal energy. As a result, the degree of total thermal expansion of the composites is drastically decreased. From the mass‐normalized thermal mechanical analysis (TMA) charts of various composites with different amounts of mesoporous silica particles, it is observed that the coefficient of thermal expansion (CTE) values gradually increase with an increase of the polymer amount outside the mesopores. It is proven that the CTE values in the range over the glass‐transition temperatures (T_g) are perfectly proportional to the outside polymer amounts. Importantly, the Y‐intercept of the relation equation obtained by a least‐square method is the CTE value and is almost zero. This means that thermal expansion does not occur if no polymers are outside the mesopores. Through such a quantative discussion, we clarify that only the outside polymer affects the thermal expansion of the composites, that is, the embedded polymers inside the mesopores do not expand at all during the thermal treatment.     

Unusual reinforcement of silicone rubber compounds containing mesoporous silica particles as inorganic fillers

We fabricate mesoporous silica/silicone composites in a simple way and systematically examine their thermal stability, swelling characteristic, mechanical strength, and transparency. Simple calculations show that more than 90 vol% of mesopores are filled with silicone rubbers. Compared to non-porous silica/silicone composites, mesoporous silica/silicone composites showed a lower coefficient of linear thermal expansion (CTE). In addition, dramatic improvements of the tensile strength and Young's modulus are obtained with mesoporous silica/silicone composites. Furthermore, mesoporous silica/silicone composites show higher transparency than non-porous silica/silicone composites.     

Thermal and mechanical properties of particulate fillers filled epoxy composites for electronic packaging application

Epoxy composites containing particulate fillers‐fused silica, glass powder, and mineral silica were investigated to be used as substrate materials in electronic packaging application. The content of fillers were varied between 0 and 40 vol%. The effects of the fillers on the thermal properties—thermal stability, thermal expansion and dynamic mechanical properties of the epoxy composites were studied, and it was found that fused silica, glass powder, and mineral silica increase the thermal stability and dynamic thermal mechanical properties and reduce the coefficient of thermal expansion (CTE). The lowest CTE value was observed at a fused silica content of 40 vol% for the epoxy composites, which was traced to the effect of its nature of low intrinsic CTE value of the fillers. The mechanical properties of the epoxy composites were determined in both flexural and single‐edge notch (SEN‐T) fracture toughness properties. Highest flexural strength, stiffness, and toughness values were observed at fillers content of 40 vol% for all the filled epoxy composites. Scanning electron microscopy (SEM) micrograph showed poor filler–matrix interaction in glass powder filled epoxy composites at 40 vol%. Copyright © 2007 John Wiley & Sons, Ltd.     

Thermally stable polymer composites with improved transparency by using colloidal mesoporous silica nanoparticles as inorganic fillers

The colloidal mesoporous silica nanoparticles with small particle sizes (namely, CMS) are used as inorganic fillers of polymers (i.e. epoxy and silicone). From simple calculation, almost all polymers are estimated to be confined in the mesopores. To clarify the superiority of CMS over nonporous silica particles and mesoporous silica particles with much larger size (TMPS-4) as inorganic fillers, a systematic study on mechanical strength and transparency of polymer-silica nanocomposites was conducted. Compared with nonporous silica particles, similar to TMPS-4, CMS shows a greater effect on lowering the CTE. In addition, obtained polymer-CMS nanocomposites show improved transparency than polymer-TMPS-4 nanocomposites.     

Effect of the presence of silica nanoparticles in the coefficient of thermal expansion of LDPE

The effect of the presence of alumina microparticles and silica nanoparticles on the coefficient of thermal expansion (CTE) of films of low density polyethylene (LDPE) based composites was investigated. A new method based on the use of an atomic force microscope (AFM) is proposed for measuring nano-thermal expansion of films to finally obtain the CTE in polymer based materials. Nanocomposites based on silica nanoparticles and LDPE were prepared by mixing those constituents by high energy ball milling (HEBM). Pure alumina microparticles come from the milling tools used to mix the components of the composites. When silica nanoparticles are used as nanofiller of LDPE the effectiveness on reducing the CTE (about a 40% of CTE reduction) is higher than that obtained when high amount of alumina microparticles are present in the LDPE. Only when high amount of silica nanoparticles and low amount of alumina microparticles are present, the reduction of CTE expected from the Levin model is in accordance with the experimental results. This effect was associated to the high surface to volume ratio of nanoparticles considering uniform dispersions of them within the polymer. The region of polymer between particles must be so thin (few nanometers) that constraint effects must play an important role on reducing the chain mobility and therefore the thermal expansion.     

The influence of mesoporous silica modified with phosphorus and nitrogen‐containing hyperbranched molecules on thermal stability,combustion behavior,and toxic volatiles of epoxy resin

A hybrid material (HPPA‐SH‐mSiO₂) containing multiple flame‐retardant elements was synthesized and characterized using NMR, FTIR, and XPS techniques. This hybrid was synthesized by the “thiol–ene” click reaction of thiol‐functionalized mesoporous silica (SH‐mSiO₂) with ene‐terminated hyperbranched polyphosphate acrylate (HPPA).When 2 wt% HPPA‐SH‐mSiO₂ hybrid was added to an epoxy matrix, thermogravimetric analysis (TGA) showed that the incorporation of HPPA‐SH‐mSiO₂ increased the thermal stability of epoxy resin composites. Moreover, the combustion behavior of epoxy composites was investigated using a cone calorimeter, and the results show that the PHRR and THR of EP/HPPA‐SH‐mSiO₂ composites clearly decreased by 28.7% and 16%, respectively. Volatile toxic compounds such as aromatic compounds, CO, carbonyl compounds, and hydrocarbons were identified using TGA‐infrared spectroscopy coupling technique. The effect of HPPA‐SH‐mSiO₂ hybrids on the removal of toxic volatiles was also investigated. Functionalized mesoporous silica and polymer composites have potential applications.     

Anomalous reinforcing effects in cellulose gel-based polymeric nanocomposites

Cellulose-synthetic polymer nanocomposite films were prepared by immersion of cellulose gel in polymer solutions followed by dry casting. The cellulose hydrogel was prepared from aqueous alkali-urea solution. As the synthetic polymer, polystyrene (PS) and poly(methyl methacrylate) (PMMA) were used. The polymer content could be changed between 10 and 80% by changing polymer concentration of immersing solution. While the mechanical properties of the cellulose-PMMA composite films showed a nearly linear dependence on PMMA content, those of cellulose-PS composites showed an anomalous behavior; both tensile strength and Young’s modulus showed prominent maxima at 15–30 wt% PS contents. This anomaly may have resulted from the specific interaction between the aromatic ring of PS and the hydrophobic plane of the glucopyranoside. Both PMMA and PS composite films showed significant improvements in dimensional thermal stability; up to 25 wt% synthetic polymer content, the coefficient of thermal expansion (CTE) was as low as ca. 30 ppm/K, about 1/3 of the pure polymers. This indicates that the regenerated cellulose network is effective in suppressing thermal expansion of the synthetic polymers.     

Fabrication of Microporous Metal–Organic Frameworks in Uninterrupted Mesoporous Tunnels: Hierarchical Structure for Efficient Trypsin Immobilization and Stabilization

Hierarchically porous metal–organic frameworks (HP-MOFs) are promising in various applications. Most reported HP-MOFs are prepared based on the generation of mesopores in microporous frameworks, and the formed mesopores are connected by microporous channels, limiting the accessibility of mesopores for bulky molecules. A hierarchical structure is formed by constructing microporous MOFs in uninterrupted mesoporous tunnels. Using the confined space in as-prepared mesoporous silica, highly dispersed metal precursors for MOFs are coated on the internal surface of mesoporous tunnels. Ligand vapor-induced crystallization is employed to enable quantitative formation of MOFs in situ, in which sublimated ligands diffuse into mesoporous tunnels and react with metal precursors. The obtained hierarchically porous composites exhibit record-high adsorption capacity for the bulky molecule trypsin. The thermal and storage stability of trypsin is improved upon immobilization on the composites.     

Rice Husk‐derived Hierarchical Micro/Mesoporous Carbon–Silica Nanocomposite as Superior Filler for Green Electronic Packaging Material

In this study, a carbon‐controllable hierarchical micro/mesoporous carbon–silica material derived from agricultural waste rice husk was easily synthesized and utilized as filler in an epoxy matrix for electronic packaging applications. Scanning electron microscopy, thermogravimetric analysis, and N₂ adsorption/desorption isotherms were used to characterize the morphology, thermal stability, carbon content, and porous structural properties, respectively, of the as‐obtained carbon–silica material, namely rice husk char (RHC ). As a filler material, the uniformly dispersed RHC filler in the epoxy/RHC composite was easily prepared through hydrogen bonding of the silanol group of silica with the epoxy matrix. For electronic packaging applications, the thermal conductivity and thermomechanical properties (storage modulus and coefficient of thermal expansion) of the epoxy/RHC composites improved with increasing carbon content. Moreover, loading of the 40% RHC filler substantially enhanced the storage modulus of the epoxy/RHC composite (5735 MPa ) compared to the epoxy with 40% commercial silica filler (3681 MPa ). Considerable commercial potential is expected for the carbon–silica composite because of the simple synthesis process and outstanding performance of the prepared packaging material.     

Fabrication of Microporous Metal–Organic Frameworks in Uninterrupted Mesoporous Tunnels: Hierarchical Structure for Efficient Trypsin Immobilization and Stabilization

Hierarchically porous metal–organic frameworks (HP‐MOFs) are promising in various applications. Most reported HP‐MOFs are prepared based on the generation of mesopores in microporous frameworks, and the formed mesopores are connected by microporous channels, limiting the accessibility of mesopores for bulky molecules. A hierarchical structure is formed by constructing microporous MOFs in uninterrupted mesoporous tunnels. Using the confined space in as‐prepared mesoporous silica, highly dispersed metal precursors for MOFs are coated on the internal surface of mesoporous tunnels. Ligand vapor‐induced crystallization is employed to enable quantitative formation of MOFs in situ, in which sublimated ligands diffuse into mesoporous tunnels and react with metal precursors. The obtained hierarchically porous composites exhibit record‐high adsorption capacity for the bulky molecule trypsin. The thermal and storage stability of trypsin is improved upon immobilization on the composites.     

AFM substantiation of the fracture behavior and mechanical properties of sol–gel derived silica packed epoxy networks

Silica packed epoxy networks are prepared in two steps via in situ, solvent free sol–gel processing of tetraethoxysilane in liquid epoxy monomer and curing the mixture with a flexible diamine afterwards. The influence of filler content and processing conditions on the mechanical properties and the fracture behavior is studied by means of the static mechanical analysis and AFM characterization of the pristine and the fractured polymer surfaces, and a mechanism to enhance polymer strength and toughness is proposed. The in–situ evolution and packing of silica nanostructures into epoxy networks influences the overall morphology and performance of polymers under high stress. It is found that smaller silica domains distributed at the molecular level cause efficient crack distribution by absorbing energy and thus improve the strength and toughness of silica packed epoxy polymers.     

Structural and compositional effects on thermal expansion behavior in polyimide substrates of varying thicknesses

Polyimide films with thicknesses ranging from 6 μm to 80 μm were prepared with a solvent casting method to explore film thickness effects on the in-plane thermal expansion coefficient (CTE). In the case of polyimide films composed of bulky and flexible molecular units, CTE is consistent regardless of film thickness. In contrast, films with rigid and planar molecular structure show CTE increase according to the increase of film thickness up to 40–50 μm, which then plateau for thicker films. It is apparent that the film thickness dependent thermal expansion originates from complex effects of molecular orientation, charge transfer complex formation, and crystal formation as a function of film thicknesses, through characterization on UV–Vis absorption, crystalline structure, glass transition behavior, and optical retardation. These results provide insight into the design of polymer structures for flexible display substrates that require appropriate CTE values.     

Fabrication of hollow multifunctional spheres containing MCM-41 nanoparticles and magnetite nanoparticles using layer-by-layer method

Macroscopic mesoporous silica spheres have been fabricated by alternatively depositing preformed MCM-41 nanoparticles and polyelectrolytes onto polystyrene lattices. High surface area hollow mesoporous spheres were obtained by removal of the core by solvent or calcination. Further, the versatility of the layer-by-layer (LBL) method was extended to fabricate magnetite-mesoporous silica composites by depositing magnetite and MCM-41 nanoparticles onto polystyrene beads. Such high surface area composites are important since the mesopores can be used for encapsulation of varied materials like enzymes and drugs while the presence of magnetite ensures application in biocatalysis and separation under magnetic field.     

Preparation, microstructure, and properties of novel low-κ brominated epoxy/mesoporous silica composites

Brominated epoxy resin (BER) composites containing various amounts of SBA-15 and SBA-16 types mesoporous silicas were prepared through the thermal curing with 3-methyl-tetrahydrophthalic anhydride, and their morphologies, dielectric constants (κ), thermal properties and mechanical properties were studied. The investigation suggested that the dielectric constant could be reduced from 4.09 of the pure thermosetting BER to 3.74 and 3.7 by incorporating 3 wt.% SBA-15 and SBA-16, respectively. The reduction of the dielectric constant is attributed to the incorporation of the air voids (κ = 1) stored within the mesoporous silica materials, the air volume existing in the gaps on the interfaces between the mesoporous silica and the BER matrix, and the free volume created by introducing large-sized domains. The BER/mesoporous silica composites also present stable dielectric constants across the wide frequency range. An improvement of thermal stability of the BER is achieved by incorporation of the mesoporous silica materials. The enhanced interfacial interaction between the surface-modified mesoporous silica and the BER matrix has also led to an improvement of the toughness.     

Controlled post‐synthesis grafting of thermoresponsive poly(N‐isopropylacrylamide) on mesoporous silica nanoparticles

Ordered mesoporous silica nanoparticles with pore diameter of 5 nm were synthesized by modification of the sol‐gel synthesis method. Post‐synthesis two‐step grafting of thermoresponsive poly(N‐isopropylacrylamide) inside the mesopores of the nanoparticles was carried out by distillation–precipitation polymerization of the methacryloxy‐functionalized mesoporous nanoparticles with N‐isopropylacrylamide monomer. A precise control on the quantity of the grafted polymer was achieved by changing the ratio of monomer to methacryloxy‐functionalized nanoparticles. The polymer‐grafted hybrid nanoparticles obtained were fully characterized by infrared spectroscopy, X‐ray diffraction, dynamic light scattering, transmission electron microscopy, thermal, and gas‐volumetric analyses, which clearly showed presence and thermoresponsive behavior of the polymer inside the mesopores with the preservation of the characteristic mesoporous structure of the nanoparticles. Copyright © 2015 John Wiley & Sons, Ltd.     

Adsorption and desorption behaviors of DNA with magnetic mesoporous silica nanoparticles

The interaction between DNA and mesopores is one of the basic concerns when mesoporous silica nanoparticle (MSN) is used as a DNA carrier. In this work, we have synthesized a type of mesoporous silica nanoparticle that has a Fe(3)O(4) inner core and mesoporous silica shell. This magnetic mesoporous silica nanoparticle (denoted as M-MSN) offers us a convenient platform to manipulate the DNA adsorption and desorption processes as it can be easily separated from solution by applying a magnetic field. The DNA adsorption behavior is studied as a function of time in chaotropic salt solution. The maximum amount of adsorbed DNA is determined as high as 121.6 mg/g. We have also developed a method to separate the DNA adsorbed onto the external surface and into the mesopores by simply changing temperature windows. The desorption results suggest that, within the whole adsorbed DNA molecules, about 89.5% has been taken up by M-MSN mesopores. Through the dynamic light scattering experiment, we have found that the hydrodynamic size for M-MSN with DNA in its mesopores is higher than the naked M-MSN. Finally, the preliminary result of the adsorption mechanism study suggests that the DNA adsorption into mesopores may generate more intermolecular hydrogen bonds than those formed on the external surface.     

Controllable and repeatable synthesis of thermally stable anatase nanocrystal-silica composites with highly ordered hexagonal mesostructures

In this article, we report a controllable and reproducible approach to prepare highly ordered 2-D hexagonal mesoporous crystalline TiO2-SiO2 nanocomposites with variable Ti/Si ratios (0 to infinity). XRD, TEM, and N2 sorption techniques have been used to systematically investigate the pore wall structure, and thermal stability functioned with the synthetic conditions. The resultant materials are ultra highly stable (over 900 degrees C), have large uniform pore diameters (approximately 6.8 nm), and have high Brunauer-Emmett-Teller specific surface areas (approximately 290 m2/g). These mesostructured TiO2-SiO2 composites were obtained using titanium isopropoxide (TIPO) and tetraethyl orthosilicate (TEOS) as precursors and triblock copolymer P123 as a template based on the solvent evaporation-induced co-self-assembly process under a large amount of HCl. Our strategy was the synchronous assembly of titanate and silicate oligomers with triblock copolymer P123 by finely tuning the relative humidity of the surrounding atmosphere and evaporation temperature according to the Ti/Si ratio. We added a large amount of acidity to lower condensation and polymerization rates of TIPO and accelerate the rates for TEOS molecules. TEM and XRD measurements clearly show that the titania is made of highly crystalline anatase nanoparticles, which are uniformly embedded in the pore walls to form the "bricked-mortar" frameworks. The amorphous silica acts as a glue linking the TiO2 nanocrystals and improves the thermal stability. As the silica contents increase, the thermal stability of the resulting mesoporous TiO2-SiO2 nanocomposites increases and the size of anatase nanocrystals decreases. Our results show that the unique composite frameworks make the mesostructures overwhelmingly stable; even with high Ti/Si ratios (> or =80/20) the stability of the composites is higher than 900 degrees C. The mesoporous TiO2-SiO2 nanocomposites exhibit excellent photocatalytic activities (which are higher than that for commercial catalyst P25) for the degradation of rhodamine B in aqueous suspension. The excellent photocatalytic activities are ascribed to the bifunctional effect of highly crystallized anatase nanoparticles and high porosity.     

高性能可回收环氧树脂及其复合材料的制备与性能研究

采用戊二酸酐为固化剂,乙酰丙酮锌为催化剂制备了一种综合性能优异的高性能可回收环氧树脂.系统研究了固化剂及催化剂含量对树脂结构、热学及动态性能的影响,实现了树脂组成的优化设计.基于酯交换反应的热可逆性,制备的vitrimer树脂通过物理热压方法可实现良好回收,力学强度保持率可达80%.采用RTM工艺制备的碳纤维织物增强vitrimer树脂复合材料表现出与传统热固性树脂基复合材料相当的力学性能,并且通过醇类溶剂热降解树脂的方法,可实现复合材料中碳纤维的高效无损回收,回收率近100%.     

Three-phase cyanate ester composites with fumed silica and negative-CTE reinforcements

Three-phase cyanate ester adhesives have been developed using a bisphenol E cyanate ester resin, fumed silica, and negative-CTE (coefficient of thermal expansion) reinforcements: short carbon fiber or zirconium tungstate (ZrW₂O ₈ ). Fumed silica was used to impart thixotropic behavior on the resin and decrease settling in the adhesives. The cured composites were evaluated using various thermal analysis techniques for their thermal-mechanical properties. Composites with short carbon fiber showed enhanced modulus and decreased thermal expansion (70% reduction for 20 vol%) and showed little phase separation. While settling of the dense ceramic particles could not be completely eliminated for the zirconium tungstate composites through rheological modification of the adhesive with added fumed silica, a reduction in CTE of 84% was achieved in the composite (58 vol%) compared to the neat resin. In addition, the effect of thermal history on the cure and temperature induced ZrW₂O₈ phase transitions, and their corresponding influence on thermal strains vs. temperature, are examined by thermomechanical analysis.     

Molecular Composites Comprising Rodlike Polyamides and Vinyl Polymers

Abstract

At present we have strong evidence that several members of a series of wholly-aromatic, para-linked, rodlike polyamides, polyesters, and polyesteramides form molecular composites with certain flexible-chain, thermoplastic polymers over a wide range of compositions. This paper reports on the initial results of an investigation of intermolecular interactions using spectroscopy and various scattering techniques as well as characterization of some of the mechanical and optical properties of these materials. The composites are made by two techniques: 1) photo-polymerization of a homogeneous solution of a rodlike polymer in a monomer containing a photoinitiation; 2) solvent evaporation from homogeneous solutions of very limited combinations of solvent, rodlike polymers and flexible polymers. While both of these techniques produce optically clear, nonscattering films of various thicknesses over the entire compositional range, e.g., 1–99 wt% of rodlike polymer, the latter is generally more convenient and has been used extensively in this study. Optical and electron microscopy, wide angle light scattering, and spectroscopic and thermal analysis support the view that these polymer combinations are truly molecularly dispersed.