Thermal and mechanical properties of fluoroaramid-silica nanocomposites

Fluoroaramids have been used as an attractive matrix polymer for composites due to their excellent mechanical and surface properties. Properties of these polymers can be improved further by dispersing silica in these matrices at a nano-scale via the sol–gel process. The role of interfacial interaction on the thermal and mechanical properties in such hybrids has been investigated in the present work. Two types of hybrids have been prepared; one using the aramid matrix with pendant alkoxy groups on the chain and other without. Silica network was developed by addition of tetraethoxysilane and its subsequent hydrolysis and condensation in the polymer matrix. Well dispersed inorganic domains of nanometer scale were obtained in case of matrix with pendant alkoxy groups on the chain, which showed larger increase in the α- and β-relaxation temperatures, storage modulus and thermal stability as compared to the matrix without alkoxy groups. The role of interfacial interaction, and its effect on properties on the fluoroaramid-silica hybrid composites has been discussed.     

Synthesis and Properties of Hyperbranched Polyimides Combined with Silica

Summary: The novel hyperbranched polyimide - silica hybrid materials containing theoretically 16 wt% of an inorganic phase were prepared via a sol-gel process. An amine terminated polyimide precursor (hyperbranched polyamic acid) was prepared from commercially available monomers 4,4′,4″-triaminotriphenylmethane and 4,4′-oxydiphthalic anhydride in molar ratio 1:1. Tetramethoxysilane and/or 3-glycidoxypropyltrimethoxysilane (also used as a coupling agent) were used as silica precursors. During thermal exposition the polyimide precursor was transformed to hyperbranched polyimide and hydrolyzed alkoxy groups reacted mutually to form silica. The final products were self-standing films, whose structure was characterized by using IR and ¹³C and ²⁹Si solid state NMR spectroscopy. The influence of the amount of silica and/or coupling agent on their structure and thermal properties was described.     

Preparation and Properties of Aramid-Silica Hybrids with Inter-Phase Bonding Through (3-glycidoxypropyl)-trimethoxysilane

Nano-composites from aramid-silica system have been prepared via sol-gel process. Poly (phenyleneterephthalamide) copolymer chains were prepared by reacting a mixture of p- and m-phenylenediamines with terephthaloyl chloride in dimethylacetamide used as solvent. The sol-gel process in the polymer matrix was carried out through hydrolysis and condensation of a mixture of tetraethoxysilane and (3-glycidoxypropyl) trimethoxysilane. The latter was used to develop linkage, on one hand with silica network structure using alkoxy groups and on the other hand with aramid chains at its secondary amine groups through glycidal groups of silane. Mutual interaction between the two disparate phases aramid and silica network was thus created. Thin films of the composites containing different proportions of silica ranging from 5.0 to 25.0-wt% were cast by the solvent elution technique. The α-relaxation temperature associated with the glass transition was measured by the dynamic mechanical thermal analysis. The results showed an increase in the glass transition temperature from 328°C for the pure aramid to 352°C for the hybrid materials containing 25-wt% silica, an indicative of the increased interfacial interaction between the two phases. Films having relatively low silica content were flexible and transparent, but those with high silica content were opaque and brittle. These films were tested for their tensile strength, modulus and toughness. The mechanical strength of the composites as compared to the pure aramid increased initially but with further addition of silica the strength decreased. The initial increase can be explained due to increased interfacial interaction between the two phases, however agglomeration of silica particles was responsible for decreasing strength at higher silica contents.     

Poly (ether amide) and silica nanocomposites derived from sol–gel process

The sol–gel derived chemically combined organic–inorganic nanocomposites were synthesized from poly(etheramide) and tetraethoxysilane. Reaction of a mixture of 4-aminophenyl ether and 1,3-phenyldiamine with terephthaloyl chloride (TPC) in dimethylacetamide (DMAc) produced the amide chains. These chains were modified with carbonyl chloride end groups using a slight excess of diacid chloride and were then reacted with aminophenyl trimethoxysilane (APTMOS), where the amine group reacted with carbonyl chloride end groups. Hydrolysis/condensation of tetraethoxysilane (TEOS) and alkoxy groups present in APTMOS developed bonding between the polyamide chains and inorganic silica network generated in situ. By changing the relative proportions of the polymer solution and the amount of TEOS, the composition of hybrid films was varied. Thin hybrid films with various concentrations of silica network obtained after evaporation of the solvent were subjected to mechanical, dynamic mechanical thermal and morphological measurements. The results indicate a gradual increase in the modulus (3.84 GPa) and tensile strength (121 MPa) up to 15-wt.% silica relative to the pure polyamide. The elongation at break point and toughness gradually decrease with addition of silica content. These hybrids were found to be thermally stable up to a temperature of 500 °C. The weight retained above 800 °C was roughly proportional to amount of silica in the matrix. The glass transition temperature and the storage moduli increased with increasing silica concentration. The maximum increase in the T _g value (358 °C) was observed with 15-wt.% silica. Scanning electron micrographs indicated the uniform distribution of silica in the composites with an average particle size ranging from 9 to 47 nm.     

Electrochemistry of Prussian Blue in silica sol-gel electrolytes doped with polyamidoamine dendrimers

The inclusion of a generation-4 polyamidoamine (G4-PAMAM) dendrimer in a silica sol-gel yielded a solid electrolyte that was used to encapsulate Prussian Blue (PB), iron(III) hexacyanoferrate(II), and cobalt hexacyanoferrate. The PB was synthesized in the doped silica by sequential immersion of a monolith in 0.1 M K₄Fe(CN)₆, water, and 0.1 M FeCl₃. Inclusion of G4-PAMAM resulted in a nanoporous anion-exchange material with a capacity of 10.1 mmol g^–1, which is about four times greater than the capacity of silica alone. Relative to its G0 counterpart, the G4-PAMAM doped silica increased the rate of formation of PB by a factor of ca. 20. The solid state voltammetry of PB in the doped silica had the usual features for this compound. At 0.1 V vs. a Ag quasi-reference electrode, a reversible reduction was seen; the relationship between current and scan rate was that for a surface-confined redox couple. The quasi-reversible oxidation of PB was observed at 0.85 V. Inclusion of G4-PAMAM increased the lifetime of silica as a solid electrolyte from a few days to at least three months. Raman microprobe mapping analysis demonstrated that PB was homogeneously distributed across the entire width (ca. 1 mm) of the G4-doped monolith with 20-h immersions. Electronic Publication     

Interlayer esterification of layered silicic acid-alcohol nanostructured materials derived from alkoxytrichlorosilane

Layered silicic acid-organic nanohybrid materials consisting of long-chain alkoxy groups attached to thin silica layers have been prepared via esterification of a layered silicic acid-alcohol nanostructured material derived from hexadecoxytrichlorosilane (C(16)H(33)OSiCl(3)). The esterification reaction was performed by heating the layered composite. The detailed characterization of the product heated at 80 degrees C revealed that the interlayer alcohol molecules partly ( approximately 50%) reacted with the interlayer surface silanol groups to form alkoxy groups. Unreacted alcohol molecules were removed by tetrahydrofuran (THF) treatment to form a novel alkoxylated layered silica material. This product retains its structure up to 120 degrees C and has a higher stability in organic solvents if compared with the layered silicic acid-alcohol nanocomposite before esterification, whose structure collapsed over 100 degrees C. Furthermore, various alcohols can be adsorbed into the esterified nanohybrid with the expansion of the interlayer spacing.     

Microwave-assisted organic functionalization of silica surfaces: Effect of selectively heating silylating agents

Microwave (MW)-assisted (2.45 GHz) organic functionalization of silica surfaces was investigated using (3-chloropropyl)dimethylsilanes with alkoxy, allyl, or aryl leaving groups in heptane or toluene at 80 °C. ²⁹Si and ¹³C CP/MAS spectroscopy confirmed the 3-chloropropyldimethylsilyl moiety was covalently grafted onto silica for all the samples. The effect of MW irradiation on the loading amount strongly depended on the leaving group as well as the solvent; using methoxysilane and p-anisylsilane in heptane caused a distinct acceleration. The correlation with the dielectric loss factors of the silylating agents suggested that the MW acceleration effect resulted from selectively heating the strongly MW-absorbing silylating agent. For the grafting reaction in toluene, the MW effect was not observed possibly because toluene masked the selective heating effect of the silylating agent.     

Application of solid-state NMR (13C and 29Si CP/MAS NMR) spectroscopy to the characterization of alkenyltrialkoxysilane and trialkoxysilyl-terminated polyisoprene grafting onto silica microparticles

The solid-state Nuclear Magnetic Resonance (NMR) was used to characterize surfaces of silica gels chemically modified by alkenyltrialkoxysilanes and trialkoxysilyl terminated 1,4-polyisoprenes. The formation of covalent bonds created between alkoxy functional groups from alkenyltrialkoxysilane or trialkoxysilyl-terminated 1,4-polyisoprene and silanol groups on silica was clearly demonstrated by means of ¹³C and ²⁹Si CP/MAS NMR spectroscopy. Quantitative data, including calculation of the grafting yields in relation with the initial silanol concentrations, were also obtained by using solid-state ²⁹Si-NMR leading to a final well-defined characterization of the silica surfaces. A relatively good agreement was noticed between the grafting yields calculated from ²⁹Si-NMR spectra and those determined from other analytical techniques such as Wijs titration or elementary analysis. The reactivity of the various silica silanols towards each coupling agent was clearly characterized and estimated, as were the proportions of the various grafted structures formed at the silica surface. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36 : 437–453, 1998     

Preparation of Silica/Poly(tert‐butylmethacrylate) Core/Shell Nanocomposite Latex Particles

Nanocomposite latex particles, with a silica nanoparticle as core and crosslinked poly(tert‐butylmethacrylate) as shell, were prepared in this work. Silica nanoparticles were first synthesized by a sol‐gel process, and then modified by 3‐(trimethoxysilyl)propyl methacrylate (MPS) to graft C?C groups on their surfaces. The MPS‐modified silica nanoparticles were characterized by elemental analysis, FTIR, and ²⁹Si NMR and ¹³C‐NMR spectroscopy; the results showed that the C?C groups were successfully grafted on the surface of the silica nanoparticles and the grafted substance was mostly the oligomer formed by the hydrolysis and condensation reaction of MPS. Silica/poly(tert‐butylmethacrylate) core/shell nanocomposite latex particles were prepared via seed emulsion polymerization using the MPS‐modified silica nanoparticle as seed, tert‐butylmethacrylate as monomer and ethyleneglycol dimethacrylate as crosslinker. Their core/shell nanocomposite structure and chemical composition were characterized by means of TEM and FTIR, respectively, and the results indicated that silica/poly(tert‐butylmethacrylate) core/shell nanocomposite latex particles were obtained.     

On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes: selective esterification of simple 2-hydroxycarboxylic acids

The interactions of a range of monocarboxylic acids with tetramethoxysilane Si(OMe)(4) (TMOS), in methanol (MeOH), have been investigated by using (1)H, (13)C and (29)Si solution-phase NMR spectroscopy and electrospray mass spectrometry (ESMS). Si(OMe)(4) acts as a catalyst/reagent in the selective methylation of 2-hydroxycarboxylic acids (2HOAs) in MeOH at room temperature: glycolic acid, lactic acid and 2-hydroxybutyric acid are esterified more than a hundred times faster in MeOH and Si(OMe)(4) than in MeOH alone. No acceleration of methylation is observed for carboxylic acids lacking the 2-hydroxy group. Methylation of the 2HOAs is associated with the condensation of individual siloxane units to form oligomers. A mechanism is proposed in which 2HOAs attach to silicon via the alkoxy group, then subsequently via the carboxyl group in an intramolecular rearrangement to form an unstable and reactive cyclic intermediate. This intermediate may lead to accelerated methylation of the carboxylic acid via nucleophilic attack of MeOH at the carbonyl group, while a separate reaction pathway leads to condensation of silanols and/or alkoxysilanes leading to oligosiloxanes. The mechanism has implications for the use of 2HOAs as templates in sol-gel silica preparation.     

Laterally attached liquid-crystalline polymers as stationary phases. Effect of temperature in RP HPLC and comparison with a commercial C18 stationary phase

Summary Specific side-on-fixed liquid-crystalline polymers (SOLCP) have been synthesized for use in silica-modified stationary phases in high-performance liquid chromatography (HPLC). The mesogenic side group of the SOLCP is composed of three benzoate-type phenyl rings with terminal alkoxy chains and is laterally linked to a polysiloxane backbone via an alkyl ester spacer arm. The dependence of the logarithm of the retention factor on the reciprocal temperature showed that the liquid-crystalline anisotropic order was conserved in the small pores (200 ? diameter) of the silica gel. The first-order nematic-isotropic transition is lost and probably becomes second-order. Adsorption enthalpies for the liquid-crystalline stationary phases have been measurement for three polycyclic aromatic hydrocarbon isomers (ortho-terphenyl, triphenylene, and chrysene) and compared with those for a commercial C₁₈ phase. The adsorption enthalpies never exceeded 30 kJ mol⁻¹, i.e. ten times the thermal agitation energy,RT. They were always less on the SOLCP stationary phase than on the C₁₈ column, emphasizing the more rigid structure of the liquid crystalline phase and its mechanism based upon adsorption. Better separation of steroids, pesticides and amino acids were obtained with the LCP-coated silica than the commercial bonded C₁₈ column. Four small peptides were successfully separated by using pure water as mobile phase.     

Dependence of surface properties of silylated silica on the length of silane arms

Amorphous precipitated Zeosil 1165 MP silica was silylated with low grafting degrees of organosilicons bearing different alkoxy and hydrocarbon tails, like monomethoxy(dimethyl)octadecylsilane (DMODMS), monomethoxytrimethylsilane (TMMS), trimethoxymercaptopropylsilane (MPTS), and 3-octanoylthio-1-propyltriethoxysilane (NXT^?). Thermogravimetry and Elemental Analysis were used to determine the degree of silane grafting and the final number of free silanol OH groups/nm² on the modified Zeosil surface. Free energy, enthalpy and entropy of adsorption of hydrocarbon probes were determined by Inverse Gas Chromatography at infinite dilution and dispersive component, $\gamma_{s}^{d}$ , and specific interaction parameter, I ^sp , of the surface tension of the silica surface were calculated. Silylation changes the hydrophilic character of Zeosil silica to the hydrophobic one, on increasing the grafting degree and, mainly, the length of hydrocarbon tail of the silane molecule (DMODMS and NXT^?). The long hydrocarbon tails practically shield the silica particle surface and the adsorbed probes preferentially interact with them. In the case of TMMS-Zeosil the adsorbed probes practically interact with the silica surface, with loss of entropy well above that of the bare silica, while being equal the values of the enthalpy of adsorption. All the other modified silicas show loss of entropy lower than that of bare silica. Steric hindrance, played by the presence of methyl groups of TMMS, is suggested to reduce the freedom of translational and rotational movements of the adsorbed probe.     

The Effect of Nanoscaled Metal Oxide Sols on the Structure and Properties of Glycidoxypropyltrimethoxysilane Derived Sols and Gels

Glycidoxypropyltrimethoxysilane (GPTS) is frequently used as precursor for the preparation of sol-gel derived nanoscaled hybrid polymers. The influence of nanoscaled metal oxide sols of silica, boehmite, zirconia and ceria on reactions of GPTS in ethanolic hydrolysates and in corresponding gels (epoxide ring-opening, condensation degree) was examined by liquid- and solid-state ¹³C and ²⁹Si NMR with regard to a better correlation between structure and material properties. Generally, a higher condensation degree of RSi(O_0.5)₃ units of GPTS is found after addition of metal oxide sols compared to GPTS without additives. The metal oxide sols (10 mole% series) cause an epoxide ring-opening up to 20% in GPTS hydrolysates after 24 h. A nearly complete ring opening was found in the boehmite and silica containing hybrid gels whereas gels containing ceria and other types of silica only show a low degree of ring-opening. The results show an accelerated ring-opening with increasing content of AlO/OH-species in silica sols. ¹³C NMR studies reveal that the epoxide ring-opening does not completely lead to polyether structures but to considerable amounts (up to 40%) of ethylether groups which can influence the material properties (hardness).     

NIR studies of the surface modification in silica fillers

Methods of estimating the degree of condensation of the surface silanol groups of silica due to its modification by silane coupling agents are reported.Also, a procedure for estimating the surface silanol groups for the pre- and post-modified silicas for the NIR 7326 cm^–1 band is given.Using electron microscope studies and heats of immersion of silica surfaces, the silane effect on agglomeration of silica particles and, thus, on the physicochemical properties of its surface has been demonstrated.     

Modification of aminopropyl silica gel with some chelating agents and their effect on its stability against hydrolysis

The stability of aminopropyl-silica gel (AP-SG) against hydrolysis was investigated after modification by 2,3-butanedione monoxime (BDMO), 8-hydroxyquinoline-2-carboxaldehyde (HQC) or isatin (Is) by Schiff’s base condensation reaction. The hybrid modified silica gel (HMSG) compounds; BDMO-SG, HQC-SG and Is-SG were characterized by IR, thermogravimetric and elemental analyses, magic angle spinning-¹³C-nuclear magnetic resonance, pH-metric titration and inductively coupled plasma–optical emission spectrometry-monitored silica hydrolysis. The stability of the HMSG’s was found to be dependant on the type of the functionalization group, which may protect the silica surface via; (1) the delocalization of the free lone pair of electrons on nitrogen of the propylamine group in the aromatic rings, which reduces local basicity experienced on the silica surface, (2) the hydrophobic nature of the organic substrates, which hinders the attack of hydroxide ions and water molecules, and (3) the bulky organic substrate, which hinders nucleophilic attack on silicon. HQC-SG is recommended as an optimal modification to meet the requirements of stability, capacity and separation efficiency of Hg(II) at pH 4.1.     

A novel approach to produce monodisperse hollow pure silica spheres

In this work, we report an efficient method to produce pure hollow silica spheres (HSS) using phenyltrimethoxysilane (PTMS) compound. The production of HSS was carried out via hydrolysis of PTMS in the aqueous media and followed by a condensation reaction to form silica spheres with phenyl groups. The product was then calcined to remove phenyl groups and obtain pure silica spheres with >95% fine structure. The chemical nature of pure silica was confirmed by Fourier transforms infrared spectroscopy. The calcined HSS were stable beyond the temperature of 900 °C as confirmed by thermal gravimetric analysis (TGA). The calcined spheres preserved their spherical appearance and hollow core as shown by SEM and TEM micrographs. Interestingly, the average size of the spheres was reduced significantly after calcination from 760 to 510 nm, confirming further the removal of phenyl groups. The calcined HSS offered much higher surface area (A_s) when analysed by BET; A_s for calcined product was ～406 and mere ～4.8 m²/g for uncalcined HSS. Finally, drug release study of cisplatin/HSS showed over 45% of steady cumulative release for 72 h. The prepared HSS can be dispersed in water opening the possibility of many novel bio/non-bio applications.     

Solid polymer electrolytes I,preparation, characterization,and ionic conductivity of gelled polymer electrolytes based on novel crosslinked siloxane/poly(ethylene glycol) polymers

A series of crosslinked siloxane/poly(ethylene glycol) (Si–PEG) copolymers were synthesized from the reactive methoxy‐functional silicone resin (Si resin) and PEGs with different molecular weights via two kinds of crosslinking reactions during an in situ curing stage. One of the crosslinking reactions is the self‐condensation between two methoxy groups in the Si resin, and another one is an alkoxy‐exchange reaction between the methoxy group in the Si resin and the OH group in PEG. The synthesized crosslinked copolymers were characterized by Fourier transform infrared spectroscopy, DSC, and ¹³C NMR. The crosslinked copolymers were stable in a moisture‐free environment, but the Si? O? C linkages were hydrolyzed in humid conditions. The gel‐like solid polymer electrolytes (SPEs) were prepared by impregnating these crosslinked Si–PEG copolymers in a propylene carbonate (LiClO₄/PC) solution. The highest conductivity reached 2.4 × 10^?4 S cm^?1 at 25 °C and increased to 8.7 × 10^?4 S cm^?1 at 85 °C. The conductivities of these gel‐type SPEs were affected by the content of LiClO₄/PC, the molecular weights of PEGs, and the weight fraction of the Si resin. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2051–2059, 2004     

Synthesis and properties of poly(ether ether ketone)-block-sulfonated polybutadiene copolymers for PEM applications

A novel series of sulfonated block copolymers were successfully synthesized by the condensation of modified poly(ether ether ketone) (PEEK) and polybutadiene (PB), followed by the selective post-sulfonation of PB blocks using acetyl sulfate as the sulfonating reagent. The sulfonic acid groups were only attached onto PB segments due to the high reactivity of double bonds to sulfonating reagent. The degree of sulfonation was controlled by changing the feed ratio of sulfonating reagent to block copolymer. PEEK-b-sPB could be easily cast into flexible and transparent membranes. The obtained membranes exhibited good thermal stability and satisfied mechanical properties. Tensile test showed the incorporation of sulfonate groups into PB blocks resulted in an increase in tensile strength and a decrease in elongation at break. TEM images revealed the existence of ionic spherical domains with the average sizes of 50-100 nm. Some of these small domains further aggregated to form large hydrophilic regions. The proton conductivity values were measured in the range of 10⁻² S/cm in water and increased with increasing IEC and temperature.     

Studies of the characteristic and reaction mechanism of silica xerogel by sol–gel method catalyzed by alkylbiguanide as a strong organic base

Treatment of tetraethyl orthosilicate with 1,2-diisopropyl-4,4,5,5-tetra-methyl biguanide (A) as a highly strong base immediately gave silica gel by means of hydrolysis and condensation reaction at room temperature. The resulting wet gel was transparent and showed high density after dryness. From the results of gas adsorption and BET analysis, silica gel obtained by the treatment of strong base A had larger specific surface area and pore volume than silica gel that was prepared by a regular or less strong base such as tetramethylammonium hydroxide (TMAH). FTIR analysis revealed that the peak strength of Si-OH bond at 960?cm^?1 of silica gel prepared by highly strong base A was smaller than that of TMAH. To understand the mechanism behind such difference, a mixture of diphenylsilandiol and dimethoxydiphenylsilane were reacted with highly strong base A, and the resulting products comprised linear-chain siloxane oligomer and octaphenylcyclotetrasiloxane. Our results indicate that silanol generated by hydrolysis of TEOS is activated by A and the activated silanol undergoes subsequent direct reaction with unhydrolyzed alkoxy silane to give condensation products in ethanol. Such a direct polycondensation between silanol and alkoxy silane brought by highly strong base A led to three-dimensional crosslinking having a higher bulk density of silica gel.

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通过RAFT聚合制备SiO2/接枝共聚物纳米杂化粒子

以二氧化硅(SiO2)纳米粒子表面键接的二硫代苯甲酸酯作为可逆加成-断裂-链转移(RAFT)聚合反应的链转移剂, 在室温下引发苯乙烯和马来酸酐进行表面RAFT交替共聚反应, 制得了SiO2/苯乙烯-alt-马来酸酐杂化材料. 通过聚氧化乙烯(PEO)的羟基与马来酸酐的酯化反应, 将PEO接枝到SiO2纳米粒子的表面, 增加了硅粒子的生物相容性. 用FTIR, TGA和TEM对杂化材料的结构、组成和形貌进行了表征.