Aijuan Gu

Flame retardancy materials based on a novel fully end-capped hyperbranched polysiloxane and bismaleimide/diallylbisphenol A resin with simultaneously improved integrated performance

A novel fully end-capped hyperbranched polysiloxane (Am-HPSi) with large branching degree and amine-groups was successfully synthesized by a controlled hydrolysis of phenyltrimethoxysilane and γ-aminopropyl triethoxysilane, and its structure was characterized by nuclear magnetic resonance (1H-NMR and 29Si-NMR) and Fourier transform infrared (FTIR) spectra as well as gel permeation chromatography (GPC). In addition, Am-HPSi was used to develop a new modified bismaleimide resin with simultaneously improved flame retardancy and other typical properties. The incorporation of Am-HPSi to 4,4′-bismaleimidodiphenyl methane/2,2′-diallyl bisphenol A (BDM/DBA) resin not only obviously increases the thermal resistance, moisture resistance, impact strength, and dielectric properties, but also remarkably improves the flame retardancy. Specifically, the average heat release rate and total heat release of modified BDM/DBA resin with 10 wt% Am-HPSi are only 37 % and 23 % of that of neat BDM/DBA resin, respectively. A synergistic flame retarding mechanism is believed to be attributed to these results, which includes improving thermal stability, producing non-combustible gas, acting in the condensed phase, and providing a barrier for heat and mass transfer owing to the introduction of Am-HPSi to BDM/DBA resin. These attractive features of Am-HPSi/BDM/DBA resins suggest that the method proposed herein is a new approach to develop high performance resins for cutting-edge industries.

A novel inorganic–organic hybridized intumescent flame retardant and its super flame retarding cyanate ester resins

A unique hybridized intumescent flame retardant (hIFR), of which expandable graphite (EG) is chemically coated by organic chains containing phosphorous and nitrogen elements, was synthesized and characterized. Based on the successful synthesis, hIFR was used to develop modified cyanate ester (CE) resins with super flame retardancy. With the addition of only 5 wt% hIFR into CE resin, the peak and total heat releases significantly reduce to values that are only 32.3 and 23.1% of that of CE resin, respectively; meanwhile the fire performance index and limited oxygen index increase about 2 and 1.4 times. Besides, both smoke and carbon monoxide releases are remarkably reduced. These attractive data are much better than those of the modified CE resin with 5 wt% EG, clearly demonstrating that hIFR is a super flame retardant. Besides the traditional investigations on the structures of chars and cone calorimeter tests, an intensive study on the thermodegradation kinetics was carried out to reveal the mechanism of the outstanding flame retarding performance of the hIFR/CE resins. Different from EG, the unique structure of hIFR provides multi-effects on improving the flame retardancy, they are taking part in the structural formation of a cured network, increasing the thermal stability during the whole process of degradation, and strengthening the ability to form a thermally stable and condensed barrier for heat and mass transfer. These attractive features of hIFR/CE resins suggest that the method proposed herein is a new approach to prepare very effective flame retardants and corresponding super flame retarding resins.

Novel permittivity gradient carbon nanotubes/cyanate ester composites with high permittivity and extremely low dielectric loss

Novel permittivity gradient (e-G) composites based on surface treated multi-walled carbon nanotubes (eMCNTs) and cyanate ester (CE) resin were successfully developed by the gravity sedimentation method. The composites consisting of original multi-walled carbon nanotubes (MCNTs) and CE resin, coded as MCNT/CE, were also prepared for comparison. Each composite was cut into three slices in the direction of thickness for evaluating the difference of dielectric properties over a wide frequency range from 1 to 109 Hz between the whole composites and their slices. Results show that the surface treatment of MCNTs is necessary to form e-G composites because the good dispersion of nanotubes in the resin matrix and the attractive interfacial adhesion between nanotubes and the matrix are key aspects for guaranteeing the gradient distribution of the concentration of nanotubes in the composites. Note that two interesting phenomena were discovered. First, the whole composites show different conductive and dielectric properties from their slices, specifically, the percolation effect does not appear in either whole eMCNT/CE or MCNT/CE composites, while it can be observed in their slices. In addition, the percolation threshold of the eMCNT/CE slice is about 25% lower than that of the MCNT/CE slice. Second, with regard to the whole composite and its slice with the same content of nanotubes, they have a similar dielectric constant, but the dielectric loss factor of the former is remarkably larger than that of the latter; these differences in properties are attributed to the different space distribution of the concentration of nanotubes between the whole composites and slices. These attractive features of eMCNT/CE composites suggest that the method proposed herein is a new approach to develop high performance e-G composites, especially those with high dielectric constant and extremely low dielectric loss for cutting-edge industries.

Unique hybridized carbon nanotubes and their high performance flame retarding composites with high smoke suppression, good toughness and low curing temperature

High curing temperature, poor flame retardancy and high brittleness are three critical disadvantages of the available heat-resistant resins. To simultaneously overcome these problems, unique phosphorus-containing hybridized multi-walled carbon nanotubes (MWCNTs), coded as PMWCNTs, were synthesized through a ring-opening reaction between epoxidated MWCNTs and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The structure of the PMWCNTs were fully characterized. On this basis, novel flame retarding PMWCNT/cyanate ester (CE) composites with simultaneously improved integrated performance were developed. The influence of the loading of PMWCNTs (from 0.5 to 3.5 wt%) on the integrated performance of the composites was systematically investigated. The results show that the curing temperatures of the PMWCNT/CE composites are 45–71 °C lower than that of CE; the impact strengths of the PMWCNT/CE composites are 1.3–1.6 times that of the CE resin. Meanwhile, the PMWCNT/CE composites have very good flame retardancy and smoke suppression, mainly reflected by the remarkably decreased average heat release rate and average smoke production rate. Note that the PMWCNT/CE composites also have much better integrated performance than the DOPO/CE and MWCNT/CE composites. The origin of these interesting results was intensively studied, which was proved to be attributed to the unique structure of the PMWCNTs and their interaction with the CE resin. The investigation provides a new approach to synthesizing hybridized MWCNTs and related high performance resins.

Multi-functional ladderlike polysiloxane: synthesis, characterization and its high performance flame retarding bismaleimide resins with simultaneously improved thermal resistance, dimensional stability and dielectric properties

The development of high-performance flame-retardant polymers with simultaneously increased integrated performance, especially thermal resistance, dimensional stability and dielectric properties, is a challenge. Progress in this field depends greatly on the development of high-performance flame retardants. In the work reported here, a unique ladder-like multi-functional polysiloxane (PN-PSQ), with a large number of amine groups and a phosphaphenanthrene structure, was synthesized through the controlled hydrolysis of self-made phosphorus-containing triethoxysilane and γ-aminopropyl triethoxysilane. A series of PN-PSQ/bismaleimide (BMI) resins was then prepared and their structure and integrated properties investigated. The results show that a small addition of PN-PSQ effectively gives BMI resins an improved curing process, outstanding flame retardant properties, remarkably improved thermal and dimensional stability as well as a decreased dielectric constant and dielectric loss, completely overcoming the critical disadvantages of currently available flame retardants for thermally resistant polymers. For example, for the PN-PSQ5/BMI resin with 5 wt% PN-PSQ, its limited oxygen index and average heat release rate are about, respectively, 1.6 times and 58% of that of BMI resin alone. Compared with BMI resin alone, the glass transition temperature of the PN-PSQ5/BMI resin is increased by about 10 °C. The coefficient of thermal expansion of the former in a glassy or rubbery state and the dielectric constant and loss at 1 MHz decrease by about 10–20%. These attractive performances are attributed to the special structure of PN-PSQ/BMI resins induced by the unique nature of PN-PSQ. This investigation provides a new approach to synthesizing multi-functional polysiloxane and related high-performance resins.

Heat-resistant polyurethane films with great electrostatic dissipation capacity and very high thermally reversible self-healing efficiency based on multi-furan and liquid multi-maleimide polymers

Single-function, low temperature resistance and low self-healing efficiency are three big bottlenecks that restrict the applications of existing reversible self-healing polyurethane (PU) films in leading industries. Herein, starting with synthesizing new polymer with multi-furan rings (FPU) and liquid hyperbranched polysiloxane terminated by multi-maleimide (HSiNCM) as well as using acidified multi-walled carbon nanotubes (aCNTs) as the conductor, a new kind of thermally reversible self-healing PU film that simultaneously has good processing property, high thermal stability and great electrostatic dissipation capacity, coded as PU-DA-CNTs, was designed and synthesized. The thermally reversible ability of PU-DA-CNTs was based on the Diels–Alder (DA) reaction. With about 1.96–4.76 wt% aCNTs, the resultant PU-DA-CNT films have outstanding thermal stability; their initial decomposition temperatures are 283–298 °C, at least about 20–40 °C higher than those of reversible self-healing PU films in the literature; in addition, the cracks on the surface of the PU-DA-CNT film can be self-healed after the film was maintained at 130 °C for 5 min, and the self-healing efficiency of the 1st cycle is as high as 92.54%, almost the highest value for reversible self-healing materials reported to date. Multi-maleimide groups in PU-DA-CNTs provide higher proceeding possibility of the DA reaction between furan rings and maleimide groups, thus leading to a high self-healing efficiency. On the other hand, PU-DA-CNT films have outstanding electrostatic dissipation capacity; their surface resistance, static decay half-life and conductivity are 3.094 × 108 Ω, 0.07 s and 4.116 × 10−8 S cm−1, respectively. These attractive integrated performances of PU-DA-CNT films are proved to be derived from the special structure and advantages of FPU, HSiNCM and aCNTs.

Building a poly(epoxy propylimidazolium ionic liquid)/graphene hybrid through πcation–π interaction for fabricating high-k polymer composites with low dielectric loss and percolation threshold

Sustainability urgently asks for low dielectric loss and a low percolation threshold (fc) while developing high dielectric constant (Hi-k) conductor/polymer composites. In this work, a novel hybridized graphene (PIL–TrGO) was first reported using a two-step process, including the decoration of an epoxy functionalized ionic liquid (IL) on the surface of thermally reduced graphene oxide (TrGO) through πcation–π interaction, followed by in situ polymerization of the IL. The DC conductivity of the PIL–TrGO hybrid is as good as that of TrGO. Then, different loadings of PIL–TrGO were added into cyanate ester (CE) to prepare a series of composites; the TrGO/CE composites were also prepared for comparison. Different from TrGO, the PIL has a large amount of epoxy groups, which guarantees good dispersion of the hybridized graphene in CE matrix, and thus provides the base for transferring the outstanding electrical properties of graphene to the composites. When the loading of fillers approaches the percolation threshold (fc), the dielectric constant and loss at 100 Hz of PIL–TrGO/CE composites are about 13 and 0.57 times that of TrGO/CE composites, respectively, while the fc of PIL–TrGO/CE composites is still as low as 0.94 wt%. The dielectric mechanism was studied by discussing and simulating impedance spectra, and the results show that PIL–TrGO/CE composites possess more micro-capacitor structures than TrGO/CE composites; moreover, the cation–anion charge layers on TrGO surfaces enhance the Maxwell–Wagner–Sillars polarization between PIL–TrGO hybrid and the CE matrix, and then markedly increase the dielectric constant of composites. PILs coated on graphene surfaces act as electron insulative layers and thus decrease dielectric loss induced by the leakage current between conductive carbon layers.

Boost up dielectric constant and push down dielectric loss of carbon nanotube/cyanate ester composites via gradient and layered structure design

How to develop high-k materials with extremely low dielectric loss based on commercially available conductors and polymers is still a big challenge. Here we present a general method that simultaneously increases the dielectric constant ten times and decreases the dielectric loss by five orders of magnitude. By adjusting the prepolymerization time of multi-walled carbon nanotube (MWCNT) and cyanate ester (CE) blends and using the layer-by-layer casting procedure, precisely controllable dispersion and distribution of MWCNTs in polymers were achieved. Consequently, a three-layer material (PE-[g-MWCNT0.5/CE-75%]2) with an optimized prepolymerization degree, consisting of two MWCNT/CE composite layers and one polyethylene (PE) thin film, exhibits a dielectric constant of 1027 and a dielectric loss of 0.02 at 1 Hz. This is one of the best results reported for polymer composites made up of nano-carbon or ceramics to date. The mechanism behind this was elucidated by analyzing the polarization of induced charges and transport of free charges. The formation of vastly interconnected networks of space charge regions, and the existence of a conductor fault and an insulating layer are the main factors that determine an extraordinarily high dielectric constant and extremely low dielectric loss simultaneously.

Low-cost and facile fabrication of titanium dioxide coated oxidized titanium diboride–epoxy resin composites with high dielectric constant and extremely low dielectric loss

A series of titanium dioxide (TiO2) coated titanium diboride (TiB2) particles, On-TiB2, were facilely prepared using an oxidation process; interestingly, the surface morphology of TiO2 on TiB2 can be controlled from some unconnected TiO2 particles to a full coating of TiO2 by simply adjusting the oxidation time. Based on the synthesis of On-TiB2, On-TiB2–epoxy (EP) resin composites with high dielectric constant and very low dielectric loss are developed. The influences of the structure and content of On-TiB2 on the electric and dielectric properties of On-TiB2–EP composites were intensively studied, and the corresponding study on TiB2–EP composites was also carried out for comparison. Results show that the dielectric properties of On-TiB2–EP composites are greatly dependent on the oxidation time for preparing On-TiB2 and the content of On-TiB2 used. 20 min is proved to be the optimum oxidation time, the structure of the resultant fillers (O20-TiB2) shows that lots of TiO2 particles exist on the surface of TiB2 but do not form a full coating. For the composite with 25 vol% O20-TiB2, the dielectric constant is as high as 407 (at 1 Hz), about 1.8 times of the maximum value that can be reached by the TiB2–EP composites; meanwhile, the dielectric loss of the O20-TiB2–EP composite is as low as 1.5 (at 1 Hz), only about 0.06 times of the corresponding value of the TiB2–EP composite. Two different equivalent circuit models are setup for O20-TiB2–EP and TiB2–EP composites to evaluate the nature behind these attractive results.

A facile method to prepare zirconia electrospun fibers with different morphologies and their novel composites based on cyanate ester resin

Using zirconium propoxide as the precursor, a new method combining the advantages of electrospinning and sol–gel approaches is set up to facilely fabricate zirconia (ZrO2) nanofibers with controllable chemical and morphological structures in batch size on a standard electrospinning equipment. Interestingly, zirconia (ZrO2) nanofibers with different morphologies were prepared, the influence of the preparing parameters on the structure of the ZrO2 nanofibers were investigated. Results show that by adjusting the preparing parameter, the ZrO2 nanofibers can be porous or compact fibers, in addition the crystalline structure, and dimensions of pores can be also changed. Based on the successful preparation of these ZrO2 electrospun fibers, novel composites consisting of ZrO2 electrospun fibers and cyanate ester (CE) resin were developed, which show significantly reduced curing temperature compared to CE owing to the presence of hydroxyl groups on the fibers. The dynamic mechanical and dielectric properties of ZrO2/CE composites are closely related to the morphological structure of ZrO2 fibers because the latter determines the chemical structure and crosslinking density of the matrix as well as the interfacial adhesion of the corresponding composites. These interesting results demonstrate that the method proposed herein provide a new approach to design and prepare ceramic nanofibers and corresponding composites with controlled structure and expected performance for cutting-edge industries.