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In this work, the silane coupling agents with 4‐carboxyphenyl (PhCOOH), amino, phenyl, 4‐aminophenyl and 4‐phenylmethylketone were employed to modify the interfaces of the polyimide/SiO2 (PIS) nanocomposites, and the effects of the interfacial structures on the dielectric and the mechanical properties for the PI‐based composites were predicted by molecular dynamics simulation. The results showed that the interface structure with PhCOOH simultaneously elevated the static dielectric permittivity (εs) and Young modulus of the PI‐based composites. Due to enhanced intrinsic polarization of the SiO2 particles and the interface polarization in the composites, the εs of PIS‐PhCOOH composites (6.66) was 28% higher than that of PIS (5.21). Young modulus of PIS‐PhCOOH (5.09 GPa) was higher than that of PIS (3.25 GPa), which was attributed to the increase of the number of hydrogen bonds and non‐bond interaction. In addition, the length of PhCOOH was found to be larger than the thickness of excluded volume region, showing that part of PhCOOH could be embedded into the gaps among PI chains and the interfacial load transfer efficiency was reinforced. These results provided an understanding of the atomic‐scale interfacial structures, which was beneficial for the design and preparation of the nanocomposites with superior dielectric and mechanical performances. [ABSTRACT FROM AUTHOR]

A simple and low‐cost approach was proposed to fabricate poly(vinylidene fluoride) (PVDF) and polyimide‐poly(amidic acid) (PI‐PAA) all‐organic hybrid films with excellent dielectric performance by solution casting and thermal imidization process. The results showed that the introduction of PVDF and PAA molecular chains into PI matrix significantly enhanced the dielectric permittivity and energy density of PI‐based composites. The permittivity and energy density of 15 wt% PI‐PAA/PVDF composites were 4.83 at 103 Hz and 5.55 J cm−3, which were 52% and 107% higher than that of PI (3.18 at 103 Hz, 2.68 J cm−3). In addition, the dielectric loss of 15 wt% PI‐PAA/PVDF composites was still maintained at a low level (0.009 at 103 Hz). The molecular dynamics results showed that the polarization strength of composites mainly originated from the intrinsic polarization strength. The intrinsic polarization strength of PI‐PAA/PVDF in the composites was 1010 times higher than interfacial polarization strength, and interfacial polarization was negligible. There was no significant difference between the self‐diffusion coefficients of 15 wt% PI‐PAA/PVDF and PI‐PAA, indicating that the relaxation polarization of polymer molecular chains was suppressed. This work provides an effective way for the preparation of dielectric for all organic high voltage direct current system. [ABSTRACT FROM AUTHOR]

In this work, the tensile strength of polyimide/silica composites with the covalently bonded interface (bonded PI/SiO2) is investigated by molecular dynamic simulation. It is found that the nanofiller with smaller size can bring out a larger number of hydrogen bonds and interfacial non‐bond energy in the composites, resulting in higher tensile strength. As the immobilization of the PI chains in the vicinity of SiO2, the covalently bonded interface is found to offer a greater reinforcing effect than the unbonded interface that is confirmed by the self‐diffusion coefficient. The tensile strength of 9 wt.% bonded PI/SiO2 composites is 11.34% higher than that of the unbounded composites. The tensile strength of PI/SiO2 composites is enhanced with the increase of SiO2 concentration up to critical mass percent (Xc), beyond which it will be decreased. To quantitatively predict Xc of PI/SiO2 composites, an empirical equation based on the non‐bond energy of the composites is proposed. The empirical equation showed that the Xc of PI/SiO2 composites ranged from 8.03 to 10.36 wt.%, which is consistent with experimental values. These results provided the understanding of size‐dependent covalently bonded interface structure, which would be beneficial to the design of nanocomposites with excellent mechanical performances. [ABSTRACT FROM AUTHOR]

Using coupling agent isocyanatopropyltriethoxysilane (ICTOS) to modified nano-TiO2, the polyimide (PI) with different titanium dioxide (TiO2) contents (0, 1, 2, 3, 4, and 5 wt %) doped nano-composites were prepared by sol–gel method (PI/TiO2 ICTOS composites). The effect of ICTOS modification on polarization and time-to-breakdown properties of composites were investigated by thermally stimulated depolarization current (TSDC) method, dielectric, and Corona aging measurements. The TSDC spectra show that ICTOS modification enhanced α-peak intensity and make β-peak disappear in composites. Relevant trap parameters were calculated by an approximate model, and the results indicate that introduction of ICTOS is effective for the charge carrier traps, activation energy distribution in composites. Corona aging measurement show corona resistance was also sufficient improved in PI/TiO2 ICTOS composites. The changes of activation energy and intensity of traps in composites may be responsible for the corona resistance. [ABSTRACT FROM AUTHOR]

ABSTRACT Hybrid polyimide (PI)/titanium dioxide (TiO2) films were prepared by in situ polymerization and sol-gel and in-sol methods (where in-sol method indicates that in situ polymerization and the sol-gel method were used in the same samples). The mechanical and electrical properties were found to be sensitive to the processing methods and the dispersion of nano titanium dioxide (nano-TiO2) in the PI matrix. For the PI/TiO2 films prepared by the in situ polymerization method, their tensile strength increased with increasing TiO2- in situ ('TiO2- in situ' is 'the TiO2 nano-particles prepared by in situ polymerization method') concentration. However, the optimal corona lifetime of the PI/TiO2 films was 15 min at 20 kHz and 2 kV because of poor dispersion. For the PI/TiO2 films prepared by the sol-gel method, the corona lifetime reached 113 min because of superior dispersion and a tensile strength of about 19.63 MPa. A balance of mechanical and electrical performances was achieved with the in-sol method. The corona-resistant life of the PI/TiO2 films was 43 min, which was about six times longer than that of the neat PI. Their tensile strength was 83.5 MPa; these films showed no decrease in this value compared with the pure PI films. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44666. [ABSTRACT FROM AUTHOR]

The hybrid films of well dispersed SiO nanoparticle in polyimide (PI/SiO) were prepared through the sol-gel processing. The pure PI was synthesized from 4, 4′,-oxydianaline (ODA) and pyromellitic dianhydride (PMDA). The residual amino groups which were controlled by the reactant stoichiometric ratio reacted with the coupling agent isocyanatopropyltriethoxysilane (ICTOS) to provide an organic-inorganic bonding. A series amount of tetraethoxysilane (TEOS) was then added, thus producing chemically bonded composite films. The films with different silica contents (0 wt %, 5 wt %, 8 wt %, 10 wt %, 13 wt %, and 15 wt %) were characterized by a variety of techniques including FT-IR, SEM, TGA, and tensile analysis. The results indicated that the chemical interaction between the two phases resulted in the formation of well-dispersed nanoparticles (20-30 nm) in the PI matrix, and superior thermal stability of hybrid films. The tensile measurement showed that the mechanical properties were markedly improved with incorporating various amount of silica, and the maximum of tensile strength was 105.4 MPa with10 wt % of silica concentration. [ABSTRACT FROM AUTHOR]

This work reports the advanced potassium sodium niobate (K0.5Na0.5NbO3, KNN)/polyimide (KNN/PI) functional hybrid films with 5 vol%, 10 vol%, 20 vol%, 30 vol%, and 40 vol% KNN loaded. The structure, morphology, and dielectric properties of hybrid films have been measured. The results show that the KNN fillers are well dispersed in the PI matrix. An internal boundary layer capacitance structure is formed in the composite films. The dielectric permittivity of the film is as high as 22 when the concentration of KNN filler reaches 40 vol% at 102 Hz, which is five times larger than that of a pure PI matrix. Meanwhile, the loss tangent is lower than 0.05. In addition, the conductivity is lower than 10−8 (S m−1) at 102 Hz, which indicates the excellent insulation and is important for the practical use of this kind of KNN/PI hybrid films. It is believed that the KNN/PI hybrid films could be applied in future electronic fields. POLYM. COMPOS., 35:969-974, 2014. © 2013 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

ABSTRACT Polyimide (PI) films filled with K0.5Na0.5NbO3 (KNN50) particles at different weight 0, 5, 10, and 15 wt % had been prepared by in-situ dispersion polymerization process. The thermally stimulated current (TSDC) method was used to investigate the charge carrier trap levels and their distribution of the composite films. The TSDC spectra show that pure film has [ABSTRACT FROM AUTHOR]

The effect of covalent bonds and the SiO2 concentration on the interface structure and the performance of polyimide/SiO2 (PI/SiO2) composites is investigated by molecular dynamic simulation. To enhance the interface interaction, the silica coupling agent is treated as a bridge to connect PI chains with SiO2 particles. The SiO2 content is controlled by adjusting the number of PI chains. The results show that the bonded interface structure not only increases the interfacial non‐bond energy and the number of the hydrogen bonds, but also strengthens the tensile strength, Young's modulus, shear modulus, and the glass transition temperature of the composites. As the SiO2 concentration increases, the non‐bond interaction energy between each PI chain and SiO2 particle increases and the non‐bond interaction energy in the bonded composites are higher than that of the unbonded. The number of hydrogen bonds in the bonded composites is sensitive to the SiO2 concentration, but there is no obvious relationship in the unbonded composites. Young's modulus and shear modulus of PI/SiO2 composites are higher than that of the unbonded at the same doping concentration. Therefore, establishing the bonded interface structure is an effective way to improve the interface stability and the properties of composite materials. [ABSTRACT FROM AUTHOR]

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 (Tg) 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 Tg 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. Periodic unit cell of hybrid PI/SiO2 model. [ABSTRACT FROM AUTHOR]