Increasing Silica Content Research Articles

Polyamide-6,6/in situ silica hybrid nanocomposites by sol–gel technique: synthesis, characterization and properties

The organic–inorganic hybrid nanocomposites comprising of poly(iminohexamethyleneiminoadipoyl), better known as Polyamide-6,6 (abbreviated henceforth as PA66), and silica (SiO 2) were synthesized through sol–gel technique at ambient temperature. The inorganic phase was generated in situ by hydrolysis–condensation of tetraethoxysilane (TEOS) in different concentrations, under acid catalysis, in presence of the organic phase, PA66, dissolved in formic acid. Infrared (IR) spectroscopy was used to monitor the microstructural evolution of the silica phase in the PA66 matrix. Wide angle X-ray scattering (WAXS) studies showed that the crystallinity in PA66 phase decreased with increasing silica content. Atomic force microscopy (AFM) of the nanocomposite films revealed the dispersion of SiO 2 particle with dimensions of <100 nm in the form of network as well as linear structure. X-ray silicon mapping further confirmed the homogeneous dispersion of the silica phase in the bulk of the organic phase. The melting peak temperatures slightly decreased compared to neat PA66, while an improvement in thermal stability by about 20 °C was achieved with hybrid nanocomposite films, as indicated by thermogravimetric analysis (TGA). Dynamic mechanical analysis (DMA) exhibited significant improvement in storage modulus ( E′) for the hybrid nanocomposites over the control specimen. An increase in Young's modulus and tensile strength of the hybrid films was also observed with an increase in silica content, indicating significant reinforcement of the matrix in the presence of nanoparticles. Some properties of the in situ prepared PA66-silica nanocomposites were compared with those of conventional composites prepared using precipitated silica as the filler by solution casting from formic acid.

Synthesis and characterization of bismaleimide‐polyetherimide‐silica hybrid by sol‐gel process

AbstractA ternary hybrid of bismaleimide‐polyetherimide‐silica (BMI‐PEI‐SiO2) was synthesized by sol‐gel reaction and characterized by scanning electron microscopy (SEM), energy dispersive X‐ray analysis (EDX), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).The results indicated that silica could be introduced into the blend of thermoplastic‐thermosetting system by sol‐gel reaction and dispersed uniformly as nanoparticles. The phase separation of BMI‐PEI was not affected apparently by the presence of silica. The coupling agent 3‐aminopropyl (triethoxy)silane was chosen to enhance the interaction between the polymer‐matrix and silica. Thermal properties and mechanical properties of the hybrids both were improved with increasing silica content. Copyright © 2005 John Wiley &amp; Sons, Ltd.

The experimental silicification of Aquificales and their role in hot spring sinter formation

ABSTRACTArchean microfossils provide some of the earliest physical evidence for life on Earth, yet there remains a great deal of uncertainty regarding which micro‐organisms were actually preserved. Because of the limited cellular detail remaining, interpretation of those microfossils has been based solely on size and morphology. This has led to significant controversy surrounding the presence or absence of cyanobacteria as early as 3.5 billion years. Accordingly, there has been an experimental bias towards studying their silicification. Here we report the very first findings on thermophilic bacteria–silica interactions, and investigate how Sulfurihydrogenibium azorense, a representative of the Aquificales often found as prominent members of modern hot spring vent communities, interacts with highly siliceous hydrothermal fluids. We show that adsorption of silica is limited to silica polymers and colloids, and that the magnitude of silica adsorption is dependent on its chemolithoautotrophic pathway. Intriguingly, when S. azorense is grown as a H2‐oxidizer, it responds to increasing silica concentrations by producing a protein‐rich biofilm that may afford the cells protection against cell wall silicification. Although the biofilms of Aquificales could potentially contribute to or accelerate siliceous sinter formation under certain growth conditions, the cells themselves show a low preservation potential and are unlikely to have been preserved in the ancient rock record, despite phylogenetic analyses suggesting that they represent one of the most primordial life forms.

Physical and Gas Transport Properties of Novel Hyperbranched Polyimide ? Silica Hybrid Membranes

Physical and gas transport properties of novel hyperbranched polyimide – silica hybrid membranes were investigated. Hyperbranched polyamic acid as a precursor was prepared by polycondensation of a triamine monomer, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and a dianhydride monomer, 4,4’-(hexafluoro-isopropylidene)diphthalic anhydride (6FDA), and subsequently modified the end groups by 3-aminopropyltrimethoxysilane (APTrMOS). The hyperbranched polyimide – silica hybrid membranes were prepared using the polyamic acid, water, and tetramethoxysilane (TMOS) via a sol-gel technique. 5 % weight-loss temperature and glass transition temperature of the hyperbranched polyimide – silica hybrid membranes determined by TG-DTA measurement considerably increased with increasing silica content, indicating effective cross-linking at polymer – silica interface mediated by APTrMOS moiety. CO2, O2, and N2 permeability coefficients of the hybrid membranes increased with increasing silica content. It was pointed out that the increased gas permeabilities are mainly attributed to increase in the gas solubilities. On the contrary, CH4 permeability of the hybrid membranes decreased with increasing silica content because of decrease in the CH4 diffusivity and, as a result, CO2/CH4 selectivity of the hybrid membranes remarkably increased. It was concluded that the 6FDA-TAPOB hyperbranched polyimide – silica hybrid membranes have high thermal stability and excellent gas selectivity, and are expected to apply to a high-performance gas separation membrane.

The dissolution rates of natural glasses as a function of their composition at pH 4 and 10.6, and temperatures from 25 to 74°C

Far-from-equilibrium dissolution rates of a suite of volcanic glasses that range from basaltic to rhyolitic in composition were measured in mixed flow reactors at pH 4 and 10.6, and temperatures from 25 to 74°C. Experiments performed on glasses of similar composition suggest that dissolution rates are more closely proportional to geometric surface areas than their BET surface areas. Measured geometric surface area normalized dissolution rates ( r +,geo) at 25°C were found to vary exponentially with the silica content of the glasses. For pH 4 solutions this relation is given by: (A1) log r + , geo ( mol / m 2 / s ) = − 0.03 ⋅ [ SiO 2 ( wt % ) ] − 7.58 , and at pH 10.6 ± 0.2 this relation is given by: (A2) log r + , geo ( mol / m 2 / s ) = − 0.02 ⋅ [ SiO 2 ( wt % ) ] − 7.02. These equations can be used to estimate lifetimes and metal release fluxes of natural glasses at far-from-equilibrium conditions. The lifetime at pH 4 and 25°C of a 1 mm basaltic glass sphere is calculated to be 500 yr, whereas that of a 1 mm rhyolitic glass sphere is 4500 yr. Estimated nutrient release rates from natural glasses decrease exponentially with increasing silica content.

The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism

The internal architecture of the immense volumes of eruptive products in Continental Flood Basalt Provinces (CFBPs) provides vital clues, through the constraint of a chrono-stratigraphic framework, to the origins of major intraplate melting events. This work presents close examination of the internal facies architecture and structure, duration of volcanism, epeirogenetic uplift associated with CFBPs, and the potential environmental impacts of three intensely studied CFBPs (the Parana-Etendeka, Deccan Traps and North Atlantic Igneous Province). Such a combination of key volcanological, stratigraphic and chronologic observations can reveal how a CFBP is constructed spatially and temporally to provide crucial geological constraints regarding their development. Using this approach, a typical model can be generated, on the basis of the three selected CFBPs, that describes three main phases of flood basalt volcanism. These phases are recognized in Phanerozoic CFBPs globally. At the inception of CFBP volcanism, relatively low-volume transitional-alkaline eruptions are forcibly erupted into exposed cratonic basement lithologies, sediments, and in some cases, water. Distribution of initial volcanism is strongly controlled by the arrangement of pre-existing topography, the presence of water bodies and local sedimentary systems, but is primarily controlled by existing lithospheric and crustal weaknesses and concurrent regional stress patterns. The main phase of volcanism is typically characterised by a culmination of repeated episodes of large volume tholeiitic flows that predominantly generate large tabular flows and flow fields from a number of spatially restricted eruption sites and fissures. These tabular flows build a thick lava flow stratigraphy in a relatively short period of time (c. 1–5 Ma). With the overall duration of flood volcanism lasting 5–10 Ma (the main phase accounting for less than half the overall eruptive time in each specific case). This main phase or ‘acme’ of volcanism accounts for much of the CFBP eruptive volume, indicating that eruption rates are extremely variable over the whole duration of the CFBP. During the waning phase of flood volcanism, the volume of eruptions rapidly decrease and more widely distributed localised centres of eruption begin to develop. These late-stage eruptions are commonly associated with increasing silica content and highly explosive eruptive products. Posteruptive modification is characterised by continued episodes of regional uplift, associated erosion, and often the persistence of a lower-volume mantle melting anomaly in the offshore parts of those CFBPs at volcanic rifted margins.

Rapid synthesis of Mn 0.65Zn 0.35Fe 2O 4/SiO 2 homogeneous nanocomposites by modified sol–gel auto-combustion method

MnZn-ferrite/SiO 2 nanocomposites with different silica content were successfully fabricated by a novel modified sol–gel auto-combustion method using citric acid as a chelating agent and tetraethyl orthosilicate (TEOS) as the source of silica matrix. The auto-combustion nature of the dried gel was studied by X-ray diffraction (XRD), Infrared spectra (IR), thermogravimetry (TG) and differential thermal analysis (DTA). Transmission electron microscope (TEM) observation shows that the MnZn-ferrite particles are homogeneously dispersed in silica matrix after auto-combustion of the dried gels. The magnetic properties vary with the silica content. The transition from the ferromagnetic to paramagnetic state is observed by Mössbauer spectra measurement with the increasing silica content. Vibrating sample magnetometer (VSM) shows that the magnetic properties of Mn 0.65Zn 0.35Fe 2O 4/SiO 2 nanocomposites strongly depend on the silica content.

Characterization and properties of a novel organic–inorganic hybrid based on hyperbranched aliphatic polyester prepared via sol-gel process

A series of organic–inorganic hybrids, as a kind of composite material, were prepared based upon hyperbranched aliphatic polyester (Boltorn™ H20), which reacted with succinic anhydride and glycidoxy-propyltrimethoxysilane as a coupling agent, then condensed with pre-hydrolyzed product of tetraethoxysilane via a sol-gel process. Solid 29 Si nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy were used to characterize the condensation degree and Si-2p core-level valence of the hybrids. Both the results have shown that the condensation degree of the hybrids increased with the addition of tetraethoxysilane. The morphological analysis performed by scanning electronic microscopy has shown that the silica particles dispersed homogeneously in the polymer matrix as nanoparticles with the size of 6–9 nm. The T g of hybrids measured by differential scanning calorimetry increased with the increasing silica content, whereas the thermal stability showed not big different except for more residue formed at high silica content. The refractive indices and abrasion resistance of the composite materials increased with the increasing silica contents, as well as the baking temperature.

Surface‐modified silica nanoparticle–reinforced poly(ethylene 2,6‐naphthalate)

AbstractThe effect of stearic acid modification on the dispersity of silica nanoparticles and the adhesion between the filler and polymer matrix with stearic acid concentration were investigated. The wettability of silica nanoparticles was improved by the addition of stearic acid. The presence of adsorbed stearic acid on the surface of the silica nanoparticles reduced the interaction between silica nanoparticles, and reduced the size of agglomerates with increasing concentration. Silica nanoparticle–reinforced poly(ethylene 2,6‐naphthalate) (PEN) composites were melt‐blended to investigate their mechanical properties and the processability of the composites. The torque and total torque values of the composites decreased with increasing silica nanoparticle content. The tensile moduli of the composites reinforced with unmodified silica nanoparticles increased with increasing silica content, whereas the tensile strength and elongation decreased. However, the stearic acid–modified silica nanoparticle–reinforced PEN composites exhibited increased elongation and decreased tensile moduli with increasing content because stearic acid, which adsorbs onto the surface of the silica nanoparticles in layers thicker than a monolayer, acts as a plasticizer during the melt‐compounding stage. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 812–818, 2004

Preparation of “Green” Composites by the Sol-Gel Process: In Situ Silica Filled Natural Rubber

“Green” composites with different amounts of in situ silica nano-particles were prepared by a sol-gel reaction of tetraethoxysilane (TEOS) in natural rubber (NR). The control of swelling degree of TEOS in NR and concentration of n-butylamine in water was useful to change the amount of generated in situ silica in the uncured NR matrix. In situ silica up to 42 parts per hundred rubber by weight (phr) was successfully filled in the NR matrix. The particle size of in situ silica became larger with the increase of silica content from ca. 10 nm to ca. 40 nm for 10 phr--40 phr loadings in the NR matrix, respectively. Even when the amount of in situ silica content was high, the dispersion of in situ silica particles was more homogeneous than that of commercial silica (VN-3). The reinforcement effect of the in situ silica for NR vulcanizates increased with increasing the in situ silica content.

Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, ΔH̄SiO2, of SiO2 in SiO2–M2O (M = Li, Na, K and Cs) binary and SiO2–CaO–Al2O3ternary melts were directly measured by drop‐solution calorimetry at 1465 K and 1663 K. ΔH̄SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that ΔH̄SiO2 is negative due to acid‐base reactions and charge‐coupled substitutions when the melt is composed of fewer Q4 and more Q3 and Q2 species, but positive due to structural strain when the melt is composed of mostly Q4 species. The ΔH̄SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.

Structure of Nanometric Silica Clusters in Polymeric Composite Materials

Composite films were prepared from nanometric silica suspension and poly(vinyl alcohol) (PVA) solution. Both were mixed at increasing silica contents and left until complete evaporation of the solvent. Small angle neutronscattering (SANS) and transmission electron microscopy (TEM) were used to characterize the local and mean particle distribution of silica particles in the film. The interaction between the polymer and the surface was characterized through an adsorption isotherm. The SANS results were processed within the Percus-Yevick approximation. At a given drying stage, the colloidal system undergoes a microphase separation and submicron size clusters are produced. Both SANS and TEM showed the presence of dense and well-ordered clusters in the dry film. The size of the clusters and the surface-to-surface distance between the silica primary particles within the clusters both depend on the initial ionic strength.

Influence of silica and magnesia onin situspinel formation in alumina–magnesia castable

The present study was carried out on the matrix composition of spinel forming high alumina castable to determine the effects of silica, magnesia particle size and temperature on spinel formation, the alumina content of the spinel formed, and associated physical characteristics at high temperature. It was found that an increase in silica content and use of finer magnesia helps to increase spinel formation. However to increase the alumina content of the spinel, increased temperature and magnesia particle size were more effective than an increased amount of silica. Furthermore the permanent linear change and porosity of the matrix are adversely affected when the amount of silica is increased or when magnesia is added in the form of fines. These findings suggest that medium and fine magnesia particles and an optimum amount of silica in the matrix composition of a spinel forming castable are necessary to obtain a matrix with increased alumina rich spinel content with controlled permanent linear change and porosity when exposed to high temperature during service.

Preparation and thermal properties of silica‐graft acrylonitrile‐butadiene‐styrene nanocomposites

AbstractThe synthesis of silica‐graft acrylonitrile‐butadiene‐styrene (ABS) nanocomposites and their thermal properties are studied. The silica nanopowders are treated with silane to introduce reactive groups onto the silica surfaces. The dispersion of modified silica nanoparticles in tetrahydrofuran (THF) is significantly improved. The silica‐graft ABS composites were prepared by an open ring reaction and radical grafting copolymerization of modified silica, styrene, and maleic anhydride (MAH) in ABS/THF solution. Differential scanning calorimetry (DSC) results show that the glass transition temperature (Tg) of ABS‐graft‐SiO2 shifts obviously to high temperatures with increasing silica content. The TGA results indicate that the thermal stability is also improved with the increase of grafted silica. Polym. Eng. Sci. 44:1077–1082, 2004. © 2004 Society of Plastics Engineers.

Preparation and characterization of PVDF/silica hybrid membranes containing sulfonic acid groups

AbstractOrganic–inorganic hybrid membranes of poly(vinylidene fluoride)‐cohexafluoropropylene (PVDF‐HFP) and silica composites containing sulfonic acid groups were prepared via in situ polymerization of tetraethoxysilane (TEOS) and sulfosuccinic acid (SSA) using the sol‐gel process. The membranes containing more sulfonic acid groups showed a higher vapor sorption and greater swelling behavior. The bound and free water content of the membrane is proportional to the SSA concentration. However, the hybrid membranes without SSA do not have free water. The ion conductivity of the membranes is proportional to the SSA concentration. Silica content in the hybrid membrane without SSA had great effect on their mechanical properties. Tensile modulus and yield stress increased and yield strain and elongation at break decreased with increased silica content. However, in the case of the hybrid membrane containing SSA modulus, yield stress decreased and yield strain and elongation at break increased with increased silica content due to the weak interactions between the hydrophobic polymer chain and the hydrophilic group of SSA. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 209–218, 2004

Silica‐reinforced dynamically vulcanized ethylene–propylene–diene monomer/polypropylene thermoplastic elastomers: Morphology, rheology, and dynamic mechanical properties

AbstractAttempts were made to prepare dynamically crosslinked ethylene–propylene–diene monomer/polypropylene (EPDM/PP, 60/40 w/w) blends loaded with various amounts of silica as a particulate reinforcing agent. The dispersion of silica between the two phases under mixing conditions, and also extent of interaction, as the two main factors that influence the blend morphology were studied by scanning electron microscopy. Increasing the silica concentration led to the formation of large‐size EPDM aggregates shelled by a layer of PP. Dynamic mechanical thermal analysis performed on the dynamically cured silica‐loaded blend samples showed reduction in damping behavior with increasing silica content. Higher rubbery‐like characteristics under tensile load were exhibited by the silica‐filled EPDM/PP‐cured blends. However, increasing the silica level to 50 phr led to the enhancement of interface, evidenced by increases in the tensile modulus and extensibility of the blend compared with those of the unloaded sample. Addition of a silane coupling agent (Si69) into the mix improved the mechanical properties of the blend, attributed to the strengthening of interfacial adhesion between the PP matrix and silica‐filled EPDM phase. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2000–2007, 2004

The Influence of Filler-Filler and Filler-Polymer Interactions on the Physical Properties of Silica-Filled Liquid Polyisoprene

Abstract The dynamic rheological properties of uncured dispersions of silica in liquid polyisoprene are measured and are used to assess filler-filler networking. An increase in the dynamic storage modulus (G′) is observed with decreasing polyisoprene molecular weight and with increasing silica concentration. For sufficiently high silica concentrations, a low-frequency plateau is observed in the frequency-dependence of G′, indicating the presence of a volume-spanning filler-filler network. The height of this plateau scales with silica volume fraction to the power of 5.2, consistent with several models describing the elasticity of polymer materials filled with fractal clusters of colloidal particles. Treatment of the silica filler surface with a silane coupling agent leads to a decrease in the modulus of the resulting dispersion. The properties of the filler-polymer interface were further investigated by bound polymer measurements and by solid-state 1H NMR experiments on the bound polymer fractions. The mobility of the polymer segments immediately adjacent to the filler surface is significantly reduced, as evidenced by faster spin-spin relaxation. The bound polymer per m2 of silica surface increases with increasing silica concentration, primarily due to entrapment of loosely bound polymer chains in the filler-filler network.

Thermochemistry of Glasses in the Y2O3—Al2O3—SiO2 System.

Enthalpies of drop solution in molten 2PbO-B2O3 at 1078 K were measured for glasses along the 2YAlO3-3SiO2 and return ½Y3Al5O12-3SiO2 joins. The onset glass transition temperature increases slightly with increasing silica content and Y/Al. Enthalpies of mixing were calculated on the basis of amorphous end members. Strongly negative heats of mixing support the absence of miscibility gaps except possibly for very high silica content, consistent with experimental phase analyses, which indicate much narrower miscibility gaps compared with the phase diagrams calculated on the basis of previous data and the CALPHAD formalism.

Effects of submicrometer particulate silica addenda on the adhesion of micrometer-size particles to a polyester-composite substrate

The force needed to detach five sets of different size particles, having number-averaged diameters between 3.6 and 8.5 µm, from a composite substrate was measured using an ultracentrifuge. In addition to size variations, the asperity concentration for each size particle was adjusted by varying the silica concentration, adjusted so that the surface area concentration at each level was kept constant for the five sizes of particles. Due to the changing silica concentration and particle size, the charge per particle also varied. It was found that the detachment force appeared to be virtually independent of charge, with any correlation actually appearing slightly negative, if anything. However, the detachment force increased monotonically with increasing particle diameter and decreased monotonically with increasing silica concentration. Moreover, upon normalizing the detachment force to the particle diameter and the silica concentration to the surface area concentration of silica, it was found that the detachment force clustered into groups in which the force needed to separate the particle from the substrate depended only on the silica concentration. These results suggest that van der Waals interaction, rather than electrostatic forces, are the dominant mechanism controlling toner adhesion in this instance.

Thermochemistry of Glasses in the Y2O3‐Al2O3‐SiO2 System

Enthalpies of drop solution in molten 2PbO‐B2O3 at 1078 K were measured for glasses along the 2YAlO3‐3SiO2 and return ½Y3Al5O12‐3SiO2 joins. The onset glass transition temperature increases slightly with increasing silica content and Y/Al. Enthalpies of mixing were calculated on the basis of amorphous end members. Strongly negative heats of mixing support the absence of miscibility gaps except possibly for very high silica content, consistent with experimental phase analyses, which indicate much narrower miscibility gaps compared with the phase diagrams calculated on the basis of previous data and the CALPHAD formalism.