Enhancing the fraction of grafted polystyrene on silica hybrid nanoparticles

Polymer-grafted inorganic nanoparticles are being developed for a diverse array of applications, ranging from drug delivery to multifunctional composites. In many instances, performance of these core-shell hybrids is limited by relatively broad distributions of size and composition, as well as the presence of impurities, such as unattached polymer chains. Herein, further synthetic improvements, and associated characterization techniques, to enhance the fraction of the grafted polystyrene shell on silica hybrid nanoparticles are discussed. We found that during surface-initiated atom transfer radical polymerization (SI-ATRP) from the silica nanoparticles, thermal self-initiation of styrene produces unattached polymer chains. Size exclusion chromatography afforded a facile approach to quantify the mass of the unattached polymer, and provide a substantial refinement to estimates of chain graft density beyond traditionally-used approaches, such as thermogravimetry. This fraction of unattached polymer is still present even after post-polymerization work-up via precipitation and re-dissolution. Removal necessitates additional procedures, such as high speed centrifugation. Selection of a lower polymerization temperature, in concert with a more reactive Cu complex, significantly reduces the amount of unattached polystyrene impurity. The improved polymerization conditions and post-polymerization purification provide more refined polystyrene-grafted silica nanoparticles to clarify structure-property relationships of these core-shell hybrids.     

Corrosion protection evaluation of silica/epoxy hybrid nanocomposite coatings to AA2024

Silica-based organic–inorganic hybrid nanocomposite films have been developed by sol–gel method for corrosion protection of AA2024 alloy. The sol–gel films have been synthesized from 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraethylorthosilicate (TEOS) precursors. Interlinked organic–inorganic networks can be formed because of the presence of both epoxy and silicon alkoxide functionalities in the precursor molecules. In order to investigate the effective factors on the properties of organically modified silicates films (Ormosils), different coatings with different organic and hydrolysis water content were developed. The films were prepared by dip-coating technique. The chemical composition and the structure of the hybrid sol–gel films were studied by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM), respectively. The corrosion protection properties of the films were studied by potentiodynamic scanning (PDS) and salt spray tests. The results indicate that the hybrid films provided exceptional barrier and corrosion protection in comparison with untreated aluminium alloy substrate.     

Thermal and electrical behavior of polyimide/silica hybrid thin films

Silica‐containing polyimide films were prepared by sol‐gel technique using a poly(amic acid) and tetraethoxysilane. The poly(amic acid) was synthesized by solution polycondensation reaction of 4,4′‐oxydiphthalic anhydride with 2,6‐bis(3‐aminophenoxy)benzene and an aminosilane coupling agent, 3‐aminopropyltriethoxysilane. The properties of these films, such as water vapors sorption capacity, dynamic contact angles and contact angle hysteresis, thermal, and electrical behavior have been evaluated with respect to their structure. The polymer films exhibited good thermal stability having the initial decomposition temperature above 450°C, glass transition temperature in the range of 223?228°C, and low‐dielectric constant in the range of 2.64?3.16. Two subglass transitions, γ and β, were evidenced by dynamic mechanical analysis and dielectric spectroscopy. The surface morphology and the roughness were investigated by atomic force microscopy and scanning electron microscopy. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Encapsulated-polyoxometalates in surfactant/silica gel hybrid films: Electrochemical behavior and main characteristics

Immobilization of various polyoxometalates (POMs) of different sizes and charges in surfactant/silica gel hybrid films is investigated. A change in the electrochemical behavior is noticed between dissolved and immobilized POMs. This change is attributed to the microenvironment effect that POM clusters experience inside the films. The presence of surfactant Triton X-405 in the matrix is essential for these observations. Results from SEM showed rough and porous morphology for the films prepared with surfactant, while smooth and fractured film structure is observed in absence of surfactant. For POMs such as [P₂W₁₈O₆₂]⁶⁻ and [SiW₁₂O₄₀]⁴⁻, the first two one-electron redox waves merged into one two-electron redox wave. However, for other POMs such as [PW₁₂O₄₀]³⁻, [H₄AsW₁₈O₆₂]⁷⁻, [P₂W₁₅Mo₂V^IVO₆₂]⁷⁻, α₁-[P₂W₁₇O₆₁Fe^III]⁷⁻, α₂-[P₂W₁₇VO₆₂]⁸⁻ and [P₈W₄₈O₁₈₄]⁴⁰⁻, the microenvironment effect results only in a shift of the potentials towards the negative values. The magnitude of the shift in potential is shown to decrease with increasing the POM's charge. Results from Particle size analysis showed that Triton X-405 influences the POM's particle size and with increasing the POM's charge, the POM's particle size in presence of Triton X-405 decreases. The latter is attributed to formation of micelles POM-Triton X-405, which consequently create the microenvironment effect for the immobilized POMs. The main characteristics of the surfactant/silica gel immobilized POM films are explored. The influence of the scan rate demonstrates that the redox behavior of the surfactant/silica gel immobilized composites is fast and involves surface-confined electron transfer processes. The study of these films in different proton concentrations showed that they are stable at low pH, and to obtain a better electron transfer, they have to be stored in the reference medium of 0.5 M H₂SO₄.     

Corrosion inhibition of aluminum by organic coatings formulated with calcium exchange silica pigment

One of the first commercial ion-exchange anticorrosive pigments to be developed was Shieldex^® (Si/Ca). Its proposed corrosion protection mechanism, based on the retention of aggressive cations and the subsequent release of calcium cations, has created certain controversy. A number of studies have focused on the anticorrosive behavior of this pigment on carbon steel and galvanized steel to replace chromates (Cr⁶⁺) as inhibitor pigment, but none has considered its performance on aluminum or aluminum alloys. In this research, alkyd coatings have been formulated with Si/Ca pigment at different concentrations and applied on aluminum 1050 (Al 99.5%) specimens. These specimens have then been subjected to accelerated tests (condensing humidity, salt spray, and Kesternich) and natural weathering in atmospheres of different aggressivity. Corrosion performance has been also evaluated in the laboratory by electrochemical impedance spectroscopy. The study has also considered an organic coating with zinc chromate anticorrosive pigment for comparative purposes. The results obtained with organic coatings formulated with Si/Ca pigments confirm that they provide corrosion protection of the underlying aluminum substrate, even improving the behavior of the reference zinc chromate in some environmental conditions.     

Extraordinary terahertz absorption bands observed in micro/nanostructured Au/polystyrene sphere arrays

Terahertz (THz) time-domain spectroscopy is carried out for micro/nanostructured periodic Au/dielectric sphere arrays on Si substrate. We find that the metal-insulator transition can be achieved in THz bandwidth via varying sample parameters such as the thickness of the Au shell and the diameter of the Au/dielectric sphere. The Au/polystyrene sphere arrays do not show metallic THz response when the Au shell thickness is larger than 10 nm and the sphere diameter is smaller than 500 nm. This effect is in sharp contrast to the observations in flat Au films on Si substrate. Interestingly, the Au/polystyrene sphere arrays with a 5-nm-thick Au shell show extraordinary THz absorption bands or metallic optical conductance when the diameter of the sphere is larger than 200 nm. This effect is related to the quantum confinement effect in which the electrons in the structure are trapped in the sphere potential well of the gold shell.     

Evaluation of interfacial layer properties in the polystyrene/silica nanocomposite

Processing conditions and final mechanical properties of polymer nanocomposites are affected by their interfacial layers behavior. However, it is impossible to determine directly the properties of these layers by dynamic rheometry tests. In this work, the interfacial layers properties are evaluated for polystyrene containing silica nanoparticles by the concept of glass‐transition temperature shift. The samples were prepared via solution‐mixing method and dynamic rheometry was used to determine the viscoelastic behavior of filled polymers in the melt state. This initial step showed that addition of silica particles increased the glass‐transition temperature. By preference, decrease in the filler particle size lead to a drastic increase in the glass‐transition temperature and interfacial layer volume fraction due to relatively high surface area of the small filler particles. Then, in the next step, the viscoelastic properties of interfacial layer have been evaluated on the basis of the properties of neat polystyrene using temperature‐frequency superposition law. For this purpose, the shift factor was calculated from the glass‐transition temperature of the sample with maximum filler content. Finally, the effect of immobilized interfacial layer on the viscoelastic properties of the polymer nanocomposite samples has been estimated. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

In situ sol‐gel process of polystyrene/silica hybrid materials: Effect of silane‐coupling agents

The polystyrene–silica hybrid materials have been successfully prepared from styrene and tetraethoxysilane in the presence of silane‐coupling agents by an in situ sol‐gel process. Triethoxysilyl group can be incorporated into polystyrene as side chains by the free‐radical copolymerization of polystyrene with silane‐coupling agents, and simultaneously polystyrene–silica hybrid materials with covalent bonds between two phases were formed via the sol‐gel reaction. The 3‐(trimethoxysilyl)‐propyl‐methacrylate (MPS) systems were found to be more homogeneous than the corresponding allytrimethoxysilane hybrid system of equal molar content. In the MPS‐introduced system, the thermal properties of the materials were greatly affected by the presence of MPS. FTIR results indicate successful formation of the silica networks and covalent bonding formation of coupling agents with styrene. The homogeneity of polystyrene–silica systems was examined by scanning electron microscope and atomic force microscope. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2074–2083, 2002     

聚苯乙烯/二氧化硅杂化材料的原位制备

采用活性负离子聚合法和末端官能化改性技术合成三乙氧基封端聚苯乙烯,然后用溶胶-凝胶法合成聚苯乙烯(PS)/SiO2杂化材料,用红外光谱、凝胶渗透色谱、差示扫描量热法、热重分析、扫描电子显微镜对PS/SiO2杂化材料的结构和性能进行分析。结果表明:PS/SiO2杂化材料中的PS和SiO2间有化学键相连;PS/SiO2杂化材料中的w(SiO2)为25%时,SiO2是粒径约为180 nm,且粒径分布均匀、有完善球形结构的颗粒,其玻璃化转变温度比纯PS高1.3℃,热稳定性较纯PS高。     

Biological processing of nanostructured silica in diatoms

The most outstanding example of biological processing of silicon occurs in the unicellular algae known as diatoms. The diatom cell wall contains nanostructured silica with features exceeding current manufacturing capabilities, reproduced with exactness in vast numbers. Such structures must result from specific molecular interactions between cellular components and silicon, and larger scale movements of the intracellular compartment where silica polymerization occurs, the silica deposition vesicle (SDV). New insights into diatom silicification have arisen from recent characterization of the molecular components involved. We have isolated genes encoding silicic acid transporters (SITs) responsible for transport across the cell’s lipid bilayer membrane. The SITs are the only proteins shown to specifically interact with silicon, and a major goal is to identify amino acids responsible for silicic acid recognition and binding. Long-chain polyamines, both free and peptide-attached, apparently induce silica polymerization in diatoms. These compounds have not been shown to have direct control over the formation of larger-scale structure, but observations suggest some involvement. Movements and molding of the SDV during silicification, driven by the cytoskeleton, are major determinants of silica macrostructure. We have applied and are developing techniques to elucidate the molecular mechanisms underlying diatom silicification. This investigation could inspire biomimetic approaches, or lead to the specific manipulation of silicified diatom structures for direct application in nanostructured materials syntheses. These materials are not limited to being silica-based; recent work using shape-preserving displacement reactions has converted diatom silica into an inorganic metal oxide, while maintaining the detailed silica morphology.     

紫外光固化二氧化硅/丙烯酸酯亲水杂化薄膜

紫外光照射下制备了用于改善玻璃表面亲水性的二氧化硅/丙烯酸酯透明杂化薄膜,其水接触角小于5°,具有优异的亲水性。讨论了反应时间、反应温度、丙烯酸羟丙酯用量和正丙醇用量与薄膜亲水性的关系。通过SEM对薄膜表面形貌进行了研究,发现薄膜具有多孔结构,SiO₂溶胶粒子均匀分布在膜层中。研究表明,以硅溶胶（ml）与丙烯酸羟丙酯（mol）配比为50∶0.15在40℃时反应1 h制备杂化溶胶,且涂膜液用20%（质量）正丙醇稀释时所制备的杂化薄膜亲水性最好。     

Nanoindentation on hybrid organic/inorganic silica aerogels

The hybrid organic/inorganic silica aerogels experiment a drastic mechanical change into rubber behaviour in relation with the pure inorganic silica aerogel as a brittle material. Aerogels were prepared by sol–gel process and drying by venting off the supercritical ethanol, no degradation of the organic polymer was detected. TEOS (tetraethoxysiloxane) and PDMS (polydimethylsiloxane) were used as inorganic and organic precursors, respectively. Depth sensing nanoindentator was used to study the mechanical properties, which is extremely sensitive to small loads (1 mN) and penetration depths (10 nm). The TEOS inorganic clusters and the polymer crosslinking degree influence the microstructure of the hybrid aerogels. Surface indentations maps reveal the different heterogeneities such as the tough silica matrix, the softness of the elastic polymer chains and the plastic microcracks in pores. The values obtained are compatible with the macroscopic ones resulting from uniaxial compression. Creep tests confirm that the compliance parameter increases with the polymer content and results can be theoretically modeled by the Burger model.     

Preparation and properties of cellulose/silica hybrid composites

Cellulose/silica hybrid (CSH) composites were prepared from bleached Eucalyptus globulus kraft pulp or primary treatment sludge cellulose fibers by a sol–gel method, using solely tetraethoxysilane or with the addition of trimethoxyoctylsilane as the silica precursors and heteropoly acid (H₃PW₁₂O₄₀) as the catalyst. Silica aging was accomplished during the hot pressing of preformed CSHs plates, and the properties of the resulting materials have been evaluated. The incorporation of a silica network in cellulosic materials increased their dimensional stability significantly during soaking in water, hydrophobicity, thermal stability, and bending strength. The thermal conductivity of these hybrid materials is comparable to commercially available insulation foams. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Preparation and properties of polyimide/silica hybrid nanocomposites

In this study, a commercially available nano‐sized silica (SiO₂) was surface‐modified via esterification with oleic acid (OA), a relatively inexpensive and hydrophobic modifier. The surface‐modified silica (SiO₂‐OA) nanoparticles were used to disperse in the poly(amic acid) solutions of a commercial polyimide (PI), used for two‐layer film, and thermally imidized to form a series of PI/silica nanocomposites. The effects of the addition of SiO₂‐OA nanoparticles on the properties of the as‐prepared PI/silica nanocomposites were studied. The results indicated that the as‐prepared PI/silica nanocomposites exhibited improvements in the dynamic mechanical property, thermal stability, water resistance, and thermal expansion. POLYM. COMPOS. 28:575–581, 2007. © 2007 Society of Plastics Engineers     

Synthesis and characterization of polyimide/silica hybrid nanocomposites

Polyimide/silica (PI/SiO₂) hybrid nanocomposites were prepared by the sol‐gel process directly from a soluble polyimide. This soluble PI was synthesized from a diamine with a pendant phenyl hydroxyl group, 4,4′‐diamino‐4″‐hydroxy triphenyl methane (DHTM) and a dianhydride, pyromellitic dianhydride (PMDA), followed by cyclodehydration. Three ways of preparing PI/SiO₂ hybrid nanocomposites were investigated in this study. Two of them used the coupling agent 3‐glycidyloxy propyl trimethoxysilane (GPTMOS) to enhance the compatibility between PI and silica. The coupling agent can react with the PI to form covalent bonds. The structures of the modified hybrid nanocomposites were identified with a FTIR, whereas the size of the silica in polyimides was characterized with a scanning electron microscope. The size of silica particles in the modified system was <100 nm. The covalently bonded PI/SiO₂ hybrid nanocomposites prepared by the novel third approach exhibited good transparency when the silica content was <15%. Moreover, their thermal and mechanical properties exhibited a significant improvement. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 382–393, 2004     

Synthesis of polystyrene/silica gel polymer hybrids by in-situ polymerization method

Summary. Homogeneous polystyrene and silica gel polymer hybrids were prepared by in-situ radical polymerization method. Styrene monomer was introduced into a sol-gel reaction mixture of tetramethoxysilane (TMOS) and the polymerization was initiated by Azobisisobutyronitrile (AIBN), while sol-gel reaction of TMOS proceeded to form a silica gel. The homogeneity of the hybrids was found to be dependent on the amount of the acid catalyst. The homogeneity was confirmed quantitatively by measuring porosity of charred hybrids with nitrogen porosimetry method. It was found that polystyrene was dispersed at a nano-meter level in the silica gel matrix. Received: 8 July 1997/Revised: 29 July 1997/Accepted: 1 August 1997     

铝在高压直流输电系统中高电压高温下的腐蚀行为研究

高压直流输电换流阀的内冷水系统中,铝散热器在高压高温下会腐蚀进而引起均压电极发生氢氧化铝结垢,导致生产异常。此状态下的铝的腐蚀行为少有报道。本文研究了与散热器相同材质的铝电极在高电压、高温下于蒸馏水和弱碱性溶液中的腐蚀行为及其电化学反应规律。理清了铝的电化学腐蚀行为,建立铝在中性溶液中失去电子的反应的模型。建立了铝电极的腐蚀反应的等效电路,计算了各历程的阻抗,发现腐蚀的控制步骤为OH^-穿过沉淀层。研究表明,在3.35mmol/L氨水溶液中具有最低的腐蚀趋势,其腐蚀电位为-0.731 V,腐蚀电流密度为7.667×10^-7 A/cm²,塔菲尔斜率为5.328。此时电化学反应的阻抗最大,其腐蚀速度最低。铝电极表面会覆盖氢氧化铝等腐蚀产物,其限制离子的扩散,减缓腐蚀趋势。由此建议在保证低电导率的前提下调整内冷水的pH,来改善换流阀内冷水系统的防腐能力。     

铝/铅-碳化钨-氧化铈复合电极的耐腐蚀性能

以1060铝为基体,在由Pb(CH3COO)2 220 g/L,HBF4170 g/L,H3BO314g/L,明胶2g/L,十六烷基三甲基溴化铵0.5～1.0 g/L组成的基础镀液中,电沉积制得Pb-WC-CeO2复合镀层.通过测定其作阳极电解锌时的塔菲尔曲线,研究了WC和CeO2颗粒的质量浓度、电流密度、温度及搅拌速...     

Stress‐relaxation behavior of natural rubber/polystyrene and natural rubber/polystyrene/natural rubber‐graft‐polystyrene blends

We studied the stress‐relaxation behavior of natural rubber (NR)/polystyrene (PS) blends in tension. The effects of strain level, composition, compatibilizer loading, and aging on the stress‐relaxation behavior were investigated in detail. The dispersed/matrix phase morphology always showed a two‐stage mechanism. On the other hand, the cocontinuos morphology showed a single‐stage mechanism. The addition of a compatibilizer (NR‐g‐PS) into 50/50 blends changed the blend morphology to a matrix/dispersed phase structure. As a result, a two‐step relaxation mechanism was found in the compatibilized blends. A three‐stage mechanism was observed at very high loadings of the compatibilizer (above the critical micelle concentration), where the compatibilizer formed micelles in the continuous phase. The aged samples showed a two‐stage relaxation mechanism. The rate of relaxation increased with strain levels. The aging produced interesting effects on the relaxation pattern. The rate of relaxation increased with temperature due to the degradation of the samples. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Silica/polystyrene and silica/polystyrene‐b‐polymethacryloxypropyltrimethoxysilane hybrid nanoparticles via surface‐initiated ATRP and comparison of their wettabilities

Both silica/polystyrene (SiO₂/PS) and silica/polystyrene‐b‐polymethacryloxypropyltrimethoxysilane (SiO₂/PS‐b‐PMPTS) hybrid nanoparticles were synthesized via surface‐initiated atom transfer radical polymerization (SI‐ATRP) from SiO₂ nanoparticles. The growths of all polymers via ATRP from the SiO₂ surfaces were well controlled as demonstrated by the macromolecular characteristics of the grafted chains. Their wettabilities were measured and compared by water contact angle (WCA) and surface roughness. The results show that the nanoparticles possess hydrophobic surface properties. The static WCA of SiO₂/PS‐b‐PMPTS hybrid nanoparticles is smaller than that of SiO₂/PS hybrid nanoparticles, meanwhile, the surface roughness of SiO₂/PS‐b‐PMPTS hybrid nanoparticles is yet slightly rougher than that of SiO₂/PS hybrid nanoparticles, which shows that the combination and competition of surface chemistry and roughness of a solid material can finally determine its wettability. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers