Properties of polyimide hybrids with mixed metal oxide

Polyimide/silica–titania (PI/SiO₂–TiO₂) hybrid films were prepared via an in situ sol–gel process. The PI precursor, poly(amic acid) (PAA), which contains 2,2'‐bis[4‐(4‐aminophenoxy)‐phenyl]propane (p‐BAPPP), 3,3',4,4'‐ benzophenetetracarboxylic anhydride (BTDA) and 3‐aminopropyltrimethoxysilane (APrTMOS), was first synthesized; this was followed by the addition of phenyltrimethoxysilane (PTMS) and/or tetraethyl orthotitanate (Ti(OEt)₄) to fabricate PI/SiO₂–TiO₂ films. The relative content of SiO₂ to TiO₂ has remarkable effects on the crosslink structure and resultant properties of the hybrids. XPS results confirm that the amount of Si on the surface of the hybrids is higher than that in the bulk. The distribution of Ti in the hybrid films is contrary to the above trend because of the formation of three‐dimensional Si? O? Si, Si? O? Ti, and Ti? O? Ti networks. The SiO₂ content of the hybrids containing only silica significantly affects their refractive index, contact angle, and dielectric constant. The films with added PTMS show higher contact angles than pure PI because nonpolar segments, ? C₂H₆ or benzene groups, tend to distribute on the surface. Upon the addition of (Ti(OEt)₄), some hydrophilic segments on the surface of the hybrids are induced because of the formation of a crosslinked structure. The denser crosslinked molecular structure, and consequently lower CTE and higher T_g are obtained from hybrids containing more TiO₂. By comparing the above properties and flexibility, the best composition of metal oxides (SiO₂/TiO₂) in hybrids is 20/80. That is, an optimum ratio of metal oxides in PI hybrids induces superior properties for advanced practical applications. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

One‐step synthesis of graphene nanoplatelets/SiO2 hybrid materials with excellent toughening performance

Graphene nanoplatelets (GNPs)/SiO₂ hybrid materials had been prepared successfully via chemical grafting in one step. Herein, SiO₂ particles and GNPs were connected by poly acryloyl chloride (PACl). The results from Fourier transform infrared spectroscopy showed that functionalized GNPs and SiO₂ particles had been successfully bridged with chemical bonds like O CO and Si O C. And the nanostructure of hybrid materials was characterized by scanning electron microscopy and transmission electron microscopy. All the images indicated that SiO₂ particles were grafted on the surface of GNPs successfully. Moreover, the result of Raman spectroscopy showed that carbon atoms of GNPs became much more disorder, due to destroying the carbon domains during the process of chemical drafting. Meaningfully, the results from tensile tests indicated that Graphene/SiO₂ hybrid materials had better toughening effect on epoxy composites than graphene oxide and SiO₂ particles. POLYM. COMPOS., 36:907–912, 2015. © 2014 Society of Plastics Engineers     

Synthesis and characterization of nanocomposite of polyimide–silica hybrid from nonaqueous sol–gel process

Polyimide–silica (PI–SiO₂) hybrids with a nanostructure was obtained using the nonaqueous sol–gel process by polycondensation of phenyltriethoxysilane in a polyamic acid solution. Self‐catalyzed hydrolysis of phenyl‐substituted akoxysilane and modification on the polyimide structure are applied and result in highly compatible PI–SiO₂ hybrids. Transparent PI–SiO₂ with a high silica content of about 45% was thus obtained. The prepared PI–SiO₂ films were characterized by infrared spectrometry, ²⁹Si‐NMR, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. These characterizations showed the silica influence on the properties of the hybrid. The thermal expansion coefficient of the PI–SiO₂ and the temperature correlation were also established for probing the potential for application. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1609–1618, 2000     

Mechanical Properties and Flame Retardancy of Rigid Polyurethane Foams Containing SiO2 Nanospheres/Graphene Oxide Hybrid and Dimethyl Methylphosphonate

Halogen-free flame-retardant rigid polyurethane foams were prepared using the combination of SiO₂ nanospheres/graphene oxide hybrid and a phosphorus-containing flame retardant, dimethyl methylphosphonate. The flame retardancy, mechanical, and thermal properties of flame-retardant rigid polyurethane foams containing dimethyl methylphosphonate and SiO₂ nanospheres/graphene oxide were investigated. The results demonstrated that the combination of dimethyl methylphosphonate and SiO₂ nanospheres/graphene oxide enhanced flame retardant and mechanical properties of rigid polyurethane foam greatly compared with pure rigid polyurethane foam and dimethyl methylphosphonate-modified foam. Morphological study indicated that the partial substitution of dimethyl methylphosphonate with SiO₂ nanospheres/graphene oxide led to smaller cell sizes and more uniform cell sizes of dimethyl methylphosphonate-modified rigid polyurethane foams.     

Dielectric and conduction properties of polyimide/silica nano‐hybrid films

Dielectric and conduction properties of polyimide/silica nano‐hybrid films were investigated with the silica content and the testing frequency, using a small electrode system. The hybrid films were prepared through sol‐gel process and thermal imidization, by using pyromellitic dianhydride and 4,4′‐oxydianiline as polyimide precursors, and tetraethoxysilane and methyltriethoxysilane as silica precursors. The dielectric coefficient of PI/SiO₂ films was monotonically increased with increasing silica content, and decreased with increasing testing frequency. The dielectric loss of PI/SiO₂ films had no obvious changes with increasing silica content, but monotonically increased with increasing testing frequency. These can be contributed to the different quantity and migration chunnels of current carriers, which were mainly influenced by a few of complicated factors. There were remarkable differences between conduction property of PI/TEOS‐SiO₂ films and PI/MTEOS‐SiO₂ films because of the different size and dispersion status of silica particles in the polyimide matrix. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Atomic oxygen erosion resistance of titania–polyimide hybrid films derived from titanium tetrabutoxide and polyamic acid

A series of polyimide/titania (PI/TiO₂) hybrid films have been successfully synthesized based on titanium tetrabutoxide (Ti(OEt)₄), 3,3′,4,4′‐bezonphenone tetracarboxylic dianhydride (BTDA), 4,4′‐oxydianiline (ODA), and 1,3‐bis(aminopropyl) tetramethyldisiloxane (APrTMOS) by a sol–gel process. The atomic oxygen (AO) exposure tests were carried out using a ground‐based AO effects simulation facility. The chemical structure of PI/TiO₂ films was characterized by Fourier transform‐infrared (FT‐IR) spectroscope before and after AO exposure. The glass transition temperature (T_g) and mechanical properties were examined by dynamic mechanical analysis (DMA) and universal mechanical testing machine, respectively. The tensile strength and elongation of the hybrid film decreased with the increase of TiO₂ content, whereas the T_g increased with the increase of TiO₂ content. The effects of TiO₂ content on the morphology and structure evolvement of PI/TiO₂ hybrid films were also investigated using field emission scanning electron microscopy (FE‐SEM) and X‐ray photoelectron spectroscope (XPS), respectively. The results indicated that a TiO₂‐rich layer resulting from the Ti(OEt)₄ formed on the PI film after AO exposure, which decreased the mass loss rate and obviously improved the AO resistance of PI films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Tough hybrid hydrogels based on simultaneous dual in situ sol–gel technique and radical polymerization

A tough hybrid hydrogel has been developed by dual in situ sol–gel reaction of γ-ethacryloxypropyltrimethoxysilane (MPTMS) and tetrabutyl titanate, as well as in situ radical polymerization of acrylamide (AM) and MPTMS. In this hydrogel, covalently bonded SiO₂ and TiO₂ nanoparticles were used as inorganic filler and multifunctional crosslinker. Nano-TiO₂ was bonded onto the surface of SiO₂ by forming Ti O Si bonds and SiO₂ bonded with polymer chains by the formation of C O Si bonds, which were confirmed by Fourier transform infrared and X-ray photoelectron spectroscopy technology. Transmission electron microscopy images revealed that SiO₂ and TiO₂ tended to construct a distinct rod-like structure in poly(AM) matrix. This specific microstructure enhanced the mechanical properties of hydrogel. The compressive stress of the gel reached up to 9.49 MPa, and the compressive fracture energy was as high as 5307.73 J m⁻². This strategy provided a probable method for the preparation of tough soft materials with potential applications in chemical machinery and actuators. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47742.     

A new method of preparing superabsorbent PVF porous foam through the simultaneous acidification of water glass solution–aspect of environmental protection

To reduce the wastewater pollution problem, silica particles that have resulted from simultaneous sulfuric acidification of water glass solution serve as the pore‐forming agent for preparing superabsorbent PVF/SiO₂ foam in this study. This is a departure from the traditional porous PVF/starch foam's manufacture method. The pore structure of PVF/SiO₂ foam is very different from that of PVF/starch foam. The effect of the concentration of these pore‐forming agents on the pore structure, mechanical modulus, and water adsorption capacity of PVF/starch and PVF/SiO₂ foams are investigated in this study. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39894.     

Organic–inorganic composite materials from acrylonitrile–butadiene–styrene copolymers (ABS) and silica through an in situ sol‐gel process

Nonbonded and chemically bonded organic–inorganic composite materials, ABS/SiO₂ and ABS Si(OCH₃)₃/SiO₂, were prepared by the sol‐gel processing of tetraethoxysilane (TEOS) in the presence of ABS and trimethoxysilyl functionalized ABS, ABS Si(OCH₃)₃, under the catalization of NH₄F. The ABS Si(OCH₃)₃ was obtained by oxidizing the cyano group in ABS with hydrogen peroxide, then subsequently underwent ring‐opening reaction with 3‐glycidoxypropyltrimethoxysilane (GPTS). The ABS Si(OCH₃)₃/TEOS sol‐gel liquid solution system, in which the ABS chains formed the covalent bonds with silica network and helped fix the polymer chains in the silica network, had a shorter gelation time than that of the ABS/TEOS system, which linked ABS chains to the silica network only by hydrogen bonding the cyano groups in ABS to the silanol groups. The morphology and properties of composite were characterized by scanning electron microscopy (SEM), differential scanning calorimeter (DSC), tensile tests, and thermogravimetry. It was found that the composite prepared from ABS Si(OCH₃)₃ had higher tensile strength, glass transition point (T_g), thermal stability, and more homogeneous morphology because of the existence of the covalent bond between ABS chains and silica network that increased the compatibility between the organic and inorganic phases. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 275–283, 2000     

Study on the mechanical properties of hybrid reinforced rigid polyurethane composite foam

The mechanical properties of hybrid reinforced rigid polyurethane (PU) foams were investigated with the reinforcing agent SiO₂ and fibers. The effect of content of SiO₂ and fibers and the effect of length of fibers on the properties of the PU composite foam were emphatically analyzed. The experiment results show that the tensile strength of the PU composite foam is optimal when the content of SiO₂ and glass fiber is 20 and 7.8%, respectively. Furthermore, the reinforcing effect of glass fiber, Nylon‐66 fiber, and PAN‐matrix carbon fiber were compared and the results show that the tensile strength of the PU composite foam reinforced with 3–5% carbon fiber is optimal. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1493–1500, 2004     

In situ sol–gel route to novel sulfonated polyimide?SiO2 hybrid proton‐exchange membranes for direct methanol fuel cells

Polyimides (PIs) as high‐performance organic matrices are used in the preparation of PI composites because of their excellent mechanical, thermal and dielectric properties. The sol–gel method is a promising technique for preparing these PI composites due to the mild reaction conditions and the process being controllable. Although sulfonated polyimide (SPI) proton‐exchange membranes have attracted much attention recently, studies on preparing SPI‐based hybrid proton‐exchange membranes for fuel cells have been rare. A series of SPI? SiO₂ hybrid proton‐exchange membranes were prepared from amino‐terminated SPI pre‐polymers, 3‐glycidoxypropyltrimethoxysilane (KH‐560) and tetraethylorthosilicate through a co‐hydrolysis and condensation process using an in situ sol–gel method. The reactive silane KH‐560 was used to react with amino‐terminated SPI to form silane‐capped SPI in order to improve the compatibility between the polymer matrix and the inorganic SiO₂ phase. The microstructure and mechanical, thermal and proton conduction properties were studied in detail. The hybrid membranes were highly uniform without phase separation up to 30 wt% SiO₂. The storage modulus and tensile strength of the hybrid membranes increased with increasing SiO₂ content. The introduction of SiO₂ improved the methanol resistance while retaining good proton conductivity. The hybrid membrane with 30 wt% SiO₂ exhibited a proton conductivity of 10.57 mS cm^?1 at 80 °C and methanol permeability of 2.3 × 10^?6 cm² s^?1 possibly because the crosslinking structure and SiO₂ phases formed in the hybrids could retain water and were helpful to proton transport. Copyright © 2010 Society of Chemical Industry     

A novel approach to prepare PBT nanocomposites with elastomer‐modified SiO2 particles

Poly(butylenes terephthalate) (PBT)/SiO₂ nanocomposites with uniform dispersion, strong interfacial adhesion, and improved mechanical properties have been prepared by a novel approach. Ethylene‐methyl acrylate‐glycidyl methacrylate (E‐MA‐GMA) elastomer chains were first chemically grafted onto the surface of SiO₂ nanoparticles. Fourier transform infrared spectra result shows that elastomer‐modified SiO₂ nanoparticles exhibit absorption at 2963–2862 cm⁻¹ of the stretching modes of C H, which suggests the reaction between the hydroxyl groups of SiO₂ surface and epoxy groups of E‐MA‐GMA. And the binding energy of Si2p and O1s of the elastomer‐modified SiO₂ shifts to lower binding energy, which further confirms the formation of Si O C bonds. This surface treatment allows SiO₂ nanoparticles homogeneously dispersing in PBT matrix. The morphology with loose aggregates contains networked SiO₂ particles with an interparticle distance ranging from 0 to 30 nm. As a result, the storage modulus and the tensile properties of PBT/E‐MA‐GMA‐SiO₂ nanocomposites are higher than those of pure PBT and PBT with untreated SiO₂. The incorporation of E‐MA‐GMA‐modified SiO₂ particles increases the tensile strength and modulus to 58.4MPa and 2661MPa respectively, which is 8% and 16% higher than those of pure PBT. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Fabrication and Dielectric,Mechanical, and Thermal Properties of Low-Density Polyethylene (LDPE) Composites Containing Surface-Passivated Silicon (Si/SiO2 Core/Shell Nanoparticles)

Composites of low-density polyethylene containing between 1 and 5?wt% of Si/SiO₂ core/shell nanoparticles were prepared by ball milling method. The thermal, mechanical, and dielectric properties of composites were investigated in terms of composition, frequency, and temperature. The results showed that the dielectric permittivity increased smoothly with a rise of Si/SiO₂ particle. The dielectric permittivity and loss decreases and increases with temperature, respectively. The resistance of composites to erosion due to partial discharge was significantly improved by adding nanoparticles. The results have demonstrated that ball milling was an effective method for producing relatively homogeneous nanocomposite up to 4?wt% Si/SiO₂.     

Structure and dielectric properties of polyimide/silica nanocomposite nanofoam prepared by solid‐state foaming

In this article, polyimide (PI)/silica nanocomposite nanofoams were prepared by solid‐state foaming using supercritical CO₂ as foaming agent. To control the cell size and morphology of the PI/silica foam, the silica nanoparticles as nucleating agent were in situ formation from TEOS via sol‐gel process, which make the silica nanoparticles homogeneously dispersed in PI matrix. The resulting PI/silica nanocomposite nanofoams were characterized by scanning electron microscopy (SEM), the image analysis system attached to the SEM and dielectric properties measurements. In PI/silica nanocomposite nanofoams, one type of novel morphology was shown that each cell contained one silica nanoparticle and many smaller holes about 20–50 nm uniformly located in the cell wall. This special structure could visually prove that the nucleation sites during foaming were formed on the surface of nucleating agents. Compared with those of neat PI foam, the cell size of PI/silica nanocomposite nanofoams was smaller and its distribution was narrower. The dielectric constant of PI/silica nanocomposite nanofoams was decreased because of the incorporation of the air voids into the PI/silica nanofoams. While the porosity of PI/silica nanocomposite nanofoam film was 0.45, the dielectric constant of the film (at 1 MHz) was reduced from 3.8 to about 2.6. Furthermore, the dielectric constant of PI/silica nanofoam films remained stable across the frequency range of 1×10²～1×10⁷ H_Z. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42355.     

Tribological properties of the polyacrylate/PTFE coating modified by POSS in the space environment

The irradiation conditions in the low earth orbit (LEO) severely inhibit the development of polymeric materials for solid lubrication coatings used on the external surfaces of spacecraft. To solve the problem, octavinyl polyhedral oligomeric silsesquioxanes (OvPOSS) were covalently grafted onto poly(methyl/butyl methacrylate) composites (PMB). The results showed that the appropriate incorporation of OvPOSS (10 wt %) significantly reduced the friction coefficient and improved the wear resistance of the OvPOSS/PMB composite coatings. Furthermore, the impact of OvPOSS on the tribological properties of PMB/polytetrafluoroethylene (PTFE) lubrication coatings in the space environment was investigated. In particular, the degradations, mass losses, surface morphologies, and chemical compositions of POSS/PMB/PTEF composite coatings were characterized under ultraviolet (UV), electric irradiation (EI), and atomic oxygen (AO). The results indicated that OvPOSS provides numerous Si O Si bonds in the polymer matrix that improve the resistance to UV and EI. Besides, a passivating SiO₂ layer was formed to prevent further erosion and degradation of the underlying PMB and PTFE components during AO irradiation. Particularly, the wear resistance of OvPOSS/PMB/PTFE coatings under AO irradiation increased significantly compared with the pristine PMB/PTFE coating. Overall, our results indicate that POSS-containing composites are a good prospective material for space application in the LEO. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48730.     

Proton exchange composite membranes comprising SiO2, sulfonated SiO2, and metal–organic frameworks loaded in SPEEK polymer for fuel cell applications

Proton exchange membrane fuel cell (PEMFC) is a promising technology that offers a clean and efficient renewable energy source. The hybrid SiO₂, sulfonated SiO₂ (S SiO₂), and metal–organic framework-5 (MOF-5) incorporated sulfonated poly (ether ether ketone) (SPEEK) ternary composite membranes are fabricated using dry phase inversion technique for PEMFC. The membrane performance is evaluated in terms of water uptake, ion exchange capacity, methanol permeability, and proton conductivity (PC) measurements. The morphological study of fabricated membranes was carried out using scanning electron microscopy and atomic force microscopy analysis. The mechanical stability is strengthened up to 30–40%, and the PC gets enhanced with the incorporation of MOF-5, achieving simultaneous improvement in proton conduction and membrane stability. The PC of the ternary SPEEK/S SiO₂/MOF-5 membrane is 3.69 × 10⁻³ S cm⁻¹, 32% more than the neat membrane. A significant increase in selectivity of 23% is observed by incorporating S SiO₂ and MOF-5 fillers when compared with the neat membrane. The synergistic effect of MOF-5 and S SiO₂ in the ternary membrane has significantly improved water retention and proton conductivity. The functional  SO₃H groups of SiO₂ and MOF-5 bonded via acid–base electrostatic interactions with the SPEEK; enhances proton conduction accompanied by suppressing the methanol penetration through membranes.     

Preparation and characterization of POSS‐SiO2/cyanate ester composites with high performance

In this article, a hybrid filler based on polyhedral oligomeric silsesquioxane and silica, coded as POSS‐SiO₂, has been successfully synthesized. The structure of POSS‐SiO₂ was studied by Fourier‐transform infrared spectra, X‐ray diffraction, and scanning electron microscopy. Then the POSS‐SiO₂ was compounded with dicyclopentadiene bisphenol dicyanate ester (DCPDCE) resin to prepare composites. The effects of POSS‐SiO₂ on the curing reaction, mechanical, thermal, dielectric and tribological properties of DCPDCE resin were investigated systematically. Results of differential scanning calorimetry show that the addition of POSS‐SiO₂ can facilitate the curing reaction of DCPDCE and decrease the curing temperature of DCPDCE. Compared with pure DCPDCE resin, the impact and flexural strengths of the composites materials are improved markedly with up to 72 and 52% increasing magnitude, respectively. Meanwhile, the POSS‐SiO₂/DCPDCE systems exhibit lower dielectric constant and loss than pure DCPDCE resin over the testing frequency from 10 to 60 MHz. In addition, the thermal stability and tribological properties of POSS‐SiO₂/DCPDCE composites are also superior to that of pure DCPDCE resin. POLYM. COMPOS., 36:1840–1848, 2015. © 2014 Society of Plastics Engineers     

Preparation and characterization of silica/polyimide nanocomposite films based on water‐soluble poly(amic acid) ammonium salt

In this article, a series of new silica/polyimide (SiO₂/PI) nanocomposite films with high dielectric constant (>4.0), low dielectric loss (<0.0325), high breakdown strength (288.8 kV mm⁻¹), and high volume resistivity (2.498 × 10¹⁴ Ω m) were prepared by the hydrolysis of tetraethyl orthosilicate in water‐soluble poly(amic acid) ammonium salt (PAAS). The chemical structure of nanocomposite films compared with the traditional pure PI was confirmed by Fourier transform infrared spectroscopy and X‐ray diffraction patterns. The results indicated that both the PAAS and the polyamide acid (PAA) material were effectively converted into the corresponding PI material through the thermal imidization and the amorphous SiO₂ was embedded in the nanocomposite films without structural changes. Thermal stability of the nanocomposite films was increased though mechanical property was generally decreased with increasing the mass fraction of SiO₂. All the nanocomposite films exhibited an almost single‐step thermal decomposition behavior and the average decomposition temperature was about 615°C. It was concluded that the effective dispersion of SiO₂ particles in PI matrix vigorously improved the comprehensive performance of the SiO₂/PI nanocomposite films and expanded their applications in the electronic and environment‐friendly industries. POLYM. COMPOS., 38:774–781, 2017. © 2015 Society of Plastics Engineers     

Molecular dynamics simulation study on the structure and properties of polyimide/silica hybrid materials

A type of polyimide/silica (PI/SiO₂) copolymer model was established through the dehydration of tetraethyl orthosilicate molecules (TEOS) and bonding to a silane coupling agent. The content of SiO₂ was controlled by adjusting the number of molecules which bound to the TEOS. Finally, the silica was formed into a hybrid model (hybrid PI/SiO₂) with a small molecule embedded in the PI. The model was optimized by geometric and molecular dynamics and the changes in the model structure, Young's modulus, shear modulus, and glass-transition temperature (T _g) were analyzed. The results showed that the density and cohesive energy density of the composites could be improved by doping SiO₂ in PI. Young's modulus and shear modulus of PI/SiO₂ hybrid materials were higher than undoped PI. The tensile strength reached 568.15 MPa when the doping content was 9%. Therefore, the structure design and content control of SiO₂ was an effective way to improve the performance of a PI/SiO₂ composite. The variation of T _g and tensile strength of PI/SiO₂ hybrid composites is consistent with that of PI/SiO₂ composite synthesized in real experiment, which will be a convenient method for new material design and performance prediction. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47335.     

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