Effect of functionalized graphene oxide with a hyperbranched cyclotriphosphazene polymer on mechanical and thermal properties of cyanate ester composites

Abstract

In this paper, graphene oxide (GO) was functionalized with a hyperbranched cyclotriphosphazene polymer, which was synthesized by the repeated reactions of hexachlorotriphosphazene with hexamethylenediamine. Subsequently, the resultant functionalized GO was incorporated into dicyclopentadiene bisphenol dicyanate ester (DCPDCE) to prepare composites. Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron spectroscopy were employed to examine the surface functionalization of GO. The effects of functionalized GO on the curing reactivity, mechanical, dielectric, thermal and water resistent properties of DCPDCE resin were investigated systematically. Results show that the addition of modified GO can facilitate the curing reaction of DCPDCE. Meanwhile, the appropriate content of modified GO can enhance the mechanical properties including impact and flexural strengths of DCPDCE resin. When modified GO content is 0.6 wt%, the corresponding composite exhibits slightly higher dielectric constant but lower dielectric loss than pure DCPDCE resin over the testing frequency from 10 to 60 MHz. In addition, the thermal stability and moisture resistance of modified GO/DCPDCE nanocomposties are also superior to that of pure DCPDCE resin.

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1. Introduction

Cyanate ester resin is one of the most important thermosetting resins. The cured cyanate ester resins exhibit good mechanical properties, superior dielectric properties, excellent moisture resistance and low volume shrinkage, etc.^1,2 These unique properties of cyanate ester resins make them excellent candidates for many cutting-edge applications such as high-frequency numeric printed circuit board, radomes, and the matrix resin in structural composites for aircraft and in high temperature encapsulation.^3–5 It is well known that cyanate ester monomers can be polymerized in the presence of heat and catalysts to form three-dimensional networks of oxygen-linked triazine ring through the cyclotrimerization reaction of three cyanate ester groups. However, the cured cyanate ester resin consists of stiff network of triazine groups with highly cross-linking density, resulting in its brittleness that restricts its further prosperity into the advanced industrial applications. Therefore, it has been a hot topic during the last decade to modify and toughen cyanate ester resins. In recent years, many researches have focused on improving their performance by introducing different types of fillers, such as nanoclays,^6,7 carbon nanotubes,^8,9 carbon fibers^10,11 and graphene nanosheets.^12,13

Graphene oxide (GO), sharing the same framework as graphene, bears abundant oxygen-containing groups (e.g. hydroxyl, epoxide, and carbonyl groups) on its basal planes and edges.¹⁴ These oxygen functional groups on the GO surface provide versatile sites for chemical functionalization, imparting improved compatibility of GO in polymeric matrixes. GO has attracted increasing interest as a filler for polymer nanocomposites due to its good dispersive capacity, excellent mechanical properties, unique thermal properties and low manufacturing costs, etc.^15,16 Polymer–GO nanocomposites have been intensively studied in the recent years with the aim to obtain high-performance materials.^17–20 For example, Wang et al.²¹ prepared GO/polybenzimidazole composites by a solvent-exchange method, which resulted in a increase of 17% and 33% in the Young's modulus and tensile strength of the composite with the addition of 0.3 wt% GO. Lin et al.¹³ prepared phenyl isocyanate functionalized GO to reinforce bisphenol A dicyanate ester resin, and found that the addition of functionalized GO was beneficial to improve the mechanical, thermal and tribological properties of the composites. Yang et al.²² investigated the effects of GO on the properties of polystyrene, and concluded that the incorporation of GO could increase the glass transition temperature, storage modulus and thermal stability of the polystyrene/GO composites. These interesting reports show that GO can be used as an effective filler for the reinforcement of cyanate ester resins. However, the actual performances of GO–filled polymer composites are lower than the anticipated value estimated from the ultrahigh surface area and superior mechanical properties of GO,^23,24 which is due to insufficient polymer–GO interactions. Therefore, better dispersion of GO in the polymeric hosts constitute the main challenge to use it in nanocomposites, the control of the interfacial interaction being crucial.^25–27

Cyclotriphosphazene derivatives are typical classes of organic–inorganic compounds with a planar non-delocalized cyclic ring consisting of alternating N and P atoms.²⁸ Because of the versatility of cyclotriphosphazene chemistry, the cyclotriphosphazene ring of high stability allows a wide range of functional groups to be attached onto cyclotriphosphazene.²⁹ Hexachlorotriphosphazene with six remarkably reactive P–Cl groups is a most important class of cyclotriphosphazene that can be easily reacted with a large variety of different nucleophiles to give rise to several different classes of cyclo-and polyphosphazenes.³⁰ Surface functionalization of conventional fillers using hexachlorotriphosphazene as coupling agent has been carried out to modify the polymeric resins to improve the interfacial properties of composites, and thus opening for these materials very valuable applicative domains, due to the high reactivity of P–Cl with the free hydroxyl groups or amine groups on the material surface.^31–33 Hyperbranched polymers are special kinds of polymers with highly branched structure. Comparing with their linear analogues, they possess considerably lower viscosity, better solubility, and extremely high density of functional groups, so they can be utilized advantageously as surface modifiers.³⁴ Anchoring the hyperbranched polymers on the silica nanoparticles,³⁵ carbon nanotubes^36,37 and graphene nanosheets^38,39 has been studied for improving solubility of the inorganic materials, and the obtained hybrid nanoparticles exhibited some novel properties. Therefore, in order to develop GO-based cyanate ester nanocomposites with high performance, hyperbranched cyclotriphosphazene polymer functionalized GO may be a useful method for achieving homogeneous dispersion of GO sheets in matrix.

To our best knowledge, reports on the modification of cyanate esters are mostly focused on bisphenol A dicyanate esters, novolac cyanate esters and bisphenol E cyanate esters. However, comparing to them, dicyclopendiene bisphenol dicyanate esters, bearing the hydrophobic cycloaliphatic backbone, possess better dielectric properties, lower moisture absorption, superior thermal cycling tolerance, and better retention of mechanical properties at high temperature.^40,41 Unfortunately, few works have been reported on the studies of DCPDCE. Herein, we propose a facile method to modify dicyclopentadiene bisphenol dicyanate ester by introducing GO functionalized by a hyperbranched cyclotriphosphazene polymer which contains active –NH₂ groups, expecting dispersal of GO in dicyclopentadiene bisphenol dicyanate ester resins. The effects of functionalized GO on the curing reaction, mechanical, dielectric, thermal and water resistent properties of dicyclopentadiene bisphenol dicyanate ester resins were investigated to develop high performance materials.

2. Experimental

2.1. Reagents and materials

The GO nanosheets were produced from natural graphite flakes by the modified Hummer's method.⁴² Dicyclopentadiene bisphenol dicyanate ester (DCPDCE) was purchased from Jiangdu Wuqiao Resin Plant (Jiangsu, China), the structure of DCPDCE was shown in Fig. 1. Hexachlorotriphosphazene was purchased from aladdin industrial corporation (Shanghai, China). γ-Aminopropyltriethoxysilane (KH-550) was purchased from Jingzhou Jianghan Fine Chemical Co. Ltd (Hubei, China). Ethanol was purchased from Tianjin Tianli Chemical Reagents Co., Ltd. Diethyl ether was purchased form Tianjin Fuyu Fine Chemical Co. Ltd (Hebei, China). Triethylamine and hexamethylenediamine were purchased from Guangdong Guanghua Sci-Tech Co., Ltd (Guangdong, China). Other reagents were all commercial products with analysis grades.

Fig. 1 The structure of DCPDCE.

2.2. Preparation of KH-550 grafted GO (NH₂–GO)

GO (0.25 g), distilled water (5 ml) and ethanol (60.0 ml) were added into a glass breaker, followed by ultrasonication with a power of 300 W for 30 min. Then the mixture was transferred to a 250 ml four-mouth flask holding a nitrogen inlet, mechanical stirrer, reflux-condenser, constant-pressure funnel and thermometer. At 25 °C, a mixture of KH-550 (0.25 g) and ethanol (20.0 ml) was slowly added into the flask through the constant-pressure funnel. The reaction mass was heated to 78 °C and maintained at that temperature for 6 h. The synthetic route was shown in Fig. 2. After the reaction, the mixture was filtrated and washed with acetone to remove the unreacted KH-550. Finally, the resulting product was collected and dried in a vacuum oven at 50 °C for 12 h. The resulting product was abbreviated NH₂–GO.

Fig. 2 The synthesis route of KH-550 grafted GO.

2.3. Preparation of hyperbranched cyclotriphosphazene polymer grafted GO (HBPGO)

Grafting of hyperbranched cyclotriphosphazene polymer from the GO surfaces was achieved by repeating two reactions: (1) the reaction of cyclotriphosphazene with GO–NH₂ and (2) the reaction of terminal chlorotriphosphazene groups with hexamethylenediamine.

The first step of the reaction was carried out as follows: in a 250 three-mouth round-bottom flask holding a mechanical stirrer, reflux-condenser, and thermometer, GO–NH₂ (1.0 g), hexachlorotriphosphazene (0.15 g), diethyl ether (60.0 ml) and triethylamine (5 ml) were added. Then the reaction mass was heated to 25 °C at which the reaction continued for 6 h. After the reaction, the mixture was filtered and washed with copious amounts of ethanol.

The second reaction step was carried out as follows: In a 250 three-mouth round-bottom flask holding a mechanical stirrer, reflux-condenser, and thermometer, GO obtained from the first step, hexamethylenediamine (0.27 g), diethyl ether (60.0 ml) and triethylamine (5 ml) were added, and the mixture was stirred at 60 °C for 5 h. After the reaction, the mixture was filtered and washed with copious amounts of ethanol. Both the first and second reaction steps were repeated three times to grow the cyclotriphosphazene polymer from the GO–NH₂ surfaces. The resulting product was abbreviated HBPGO. The synthetic route of HBPGO was shown in Fig. 3.

Fig. 3 The synthesis route of HBPGO.

2.4. Preparation of HBPGO/DCPDCE nanocomposites

The DCPDCE was heated to 100 °C in a glass beaker and kept at this constant temperature until melting. The appropriate amount of HBPGO was then carefully mixed with the melted DCPDCE using a mechanical high shear dispersion process. The mixture, consisting of prepolymer and HBPGO, was heated to 140 °C in an oil bath and kept at this temperature for 15–20 min with stirring. Then the mixture was put into a preheated mold with release agent followed by degassing at 140 °C for 1 h in a vacuum oven. After that, the mixture was cured and post-cured via the procedures of 160 °C/1 h + 180 °C/1 h + 200 °C/2 h + 220 °C/2 h and 240 °C/2 h, respectively. Finally, the mold was cooled to room temperature and demolded to get the samples of HBPGO/DCPDCE systems.

The samples of pure DCPDCE resin were prepared in the same manner as above. All samples were dried at 120 °C under vacuum for 6 h and kept in a dry environment prior to testing.

2.5. Characterization

Fourier Transform Infrared (FTIR) spectrum was recorded between 400 and 4000 cm⁻¹ with a resolution of 2 cm⁻¹ on a Nicolet FTIR 5700 spectrometer (USA). The samples were mixed with potassium bromide (KBr) powder, to form the homogeneous mixtures using a grinder, and then the mixtures were compression moulded at 10 bar pressure to make a thin disc for the test.

X-ray photo-electron spectroscopy (XPS, Thermal Scientific K-Alpha XPS spectrometer) was used to investigate the surface elemental composition of GO, NH₂–GO and HBPGO. The analysis was performed under 1027 Torr vacuum with an AlKa X-ray source using a power of 200 W.

X-ray diraction (XRD) investigation was carried out using a X′ Pert Pro MPD diffractometer with Cu Kα radiation (λ = 0.154178 nm). The tube voltage was 36 kV, and the current was 20 mA. Scans were taken over the 2θ range of 5 to 85° with the scanning rate of 0.02° s⁻¹.

Transmission electron microscopic (TEM) images were obtained by a HITACHIS-600 (Japan) transmission electron microscope operating at 200 kV.

Gel time was measured on a temperature-controlled hot plate by the standard knife method, the time required for the resin to stop legging and becomes elastic is recorded as the gel time.

Impact strength was determined according to GB/T 2571-1995. Samples were cut into strips of (80 ± 0.02) × (10 ± 0.02) × (4 ± 0.02) mm³ by a cutting machine. The impact strength tests were performed using a Charpy impact machine tester (XCJ-L, China). Five samples were tested for each composition, and the results are presented as an average for tested samples.

Flexural strength was measured according to GB/T2570-1995. Samples were cut into strips of (80 ± 0.02) × (15 ± 0.02) × (4 ± 0.02) mm³. The flexural tests were performed using an electronic universal testing machine (RIGER-20, China) at a crosshead speed of 2 mm min⁻¹. Five samples were tested for each composition, and the results are presented as an average for tested samples.

Scanning electron micrographs (SEM) were performed on a HITACHIS-570 instrument. For SEM samples preparation, the fracture surface of the specimens was sputtered with a thin layer (about 10 nm) of gold by vapor deposition on a stainless steel stub using a vacuum sputter coater.

Thermal gravimetrical analysis (TGA) tests were performed by using Perkin Elmer TGA-7 (USA) at a heating rate of 10 °C min⁻¹ in a nitrogen atmosphere from 25 to 800 °C.

The dielectric constant and loss factor were measured by a high frequency QBG-3 Gauger and a S914 dielectric loss test set (China) at the frequency range from 10 MHz to 60 MHz. The sample dimension was (25 ± 0.02) × (25 ± 0.02) × (3 ± 0.02) mm³. For each condition, five samples were tested the data was averaged.

The water absorption of a sample was determined by swelling the sample in distilled water for 48 h at 100 °C. The sample dimension was (10 ± 0.02) × (10 ± 0.02) × (3 ± 0.02) mm³. The percentage of water absorbed by the specimen is calculated using equation. Percentage of water absorption = (w₂ − w₁)/w₁. Where w₁ is the initial weight of the sample and w₂ is the weight of the sample after immersion in water for 48 h at 100 °C.

3. Results and discussions

3.1. Characteristics of HBPGO

In order to investigate the KH550 modification and hyperbranched cyclotriphosphazene polymer functionalization of GO in detail, XPS is employed. Fig. 4 provides the survey spectra of GO, NH₂–GO, and HBPGO. In the XPS spectrum of GO, two obvious peaks are observed at 287.0 eV and 534.0 eV, corresponding to C 1s and O 1s, respectively. For the sample of NH₂–GO, new slight reflections are found at 104 eV, 160 eV and 405 eV besides the signals of C ls and O ls, corresponding to Si 2p, Si 2s and N 1s, respectively. The Si element and N element derive from the grafted KH-550, confirming the success of the modification. XPS survey of HBPGO shows significant amount of N 1s comparing to that of NH₂–GO, which is attributed to the grafted hexamethylenediamine. Moreover, new peaks of P 2 s and P 2p appear in the XPS spectrum of HBPGO, which is originated from grafted hexachlorotriphosphazene. These results verified the success of hyperbranched cyclotriphosphazene polymer functionalization of GO sheets by chemical reaction.

Fig. 4 Board scan XPS spectra of GO (a), NH₂-GO (b) and HBPGO (c).

Further evidence on the successful grafting of the hyperbranched cyclotriphosphazene polymer onto the GO can be demonstrated by FTIR spectra, as shown in Fig. 5. Fig. 5a shows the FTIR spectrum of the GO. Intense absorption peak at 3421 cm⁻¹ represents hydroxyl (–OH) groups. Peaks at 1626 cm⁻¹, 1055 cm⁻¹ and 1737.79 cm⁻¹ are generated by C=C, C–O and C=O vibrations, respectively. Peaks at 864 cm⁻¹ and 1250 cm⁻¹ are generated by bending and stretching vibrations of epoxy groups. Therefore, it can be concluded that the oxygen-containing functional groups of GO mainly are hydroxyl, epoxy and carboxyl groups. In the case of KH550 grafted GO, as seen in Fig. 5b, the peaks at around 3434 cm⁻¹, 1735 cm⁻¹ and 1632 cm⁻¹, which are assigned to –OH, C=O and C=C, respectively, continue to be observed. New peaks at 2923 cm⁻¹ and 2860 cm⁻¹ are generated by the stretching vibrations of methyl groups and methylene groups, respectively, which are derived from the alkyl chains of the silane moieties. Moreover, new peak at 1130 cm⁻¹ corresponding to Si–O–C appears, which suggests that the hydrolyzation reactions between alkoxy groups in KH-550 and hydroxy groups in GO took place. And an additional new peak at 1055 cm⁻¹ corresponding to Si–O–Si vibration can be observed, which is caused by the hydrolyzation reactions among alkoxy groups in KH-550. Low degree self-polymerization of KH-550 is inevitable because the reactivity of –Si(OH)₃ in KH-550 is very high. These changes indicate that GO has been effectively functionalized by KH-550. After the grafting of the hyperbranched cyclotriphosphazene polymer on the surface of NH₂–GO, a obvious change can be observed in the FTIR spectrum as shown in Fig. 5c, a new peak is found at 1200 cm⁻¹, which is ascribed to P=N–P vibration. These results also provide an evidence of the successful covalent grafting of the hyperbranched cyclotriphosphazene polymer onto the GO through the chemical reaction.

Fig. 5 FTIR spectra of GO (a), NH₂-GO (b) and HBPGO (c).

The GO, NH₂–GO and HBPGO structures were further investigated by X-ray diffraction (XRD), and spectra are shown in Fig. 6. The XRD spectrum of GO exhibits a sharp diffraction peak at 2θ = 11.36°, corresponding to a d-spacing of 0.78 nm. Whereas the strong diffraction peak of the NH₂–GO shifts to a smaller 2θ = 10.04° (Fig. 6b), corresponding to a d-spacing of 0.88 nm. This interlayer spacing is larger than that of GO, which implies that KH-550 successfully inserted in GO via hydrolytic condensation. It is interesting to note that the sharp diffraction peak disappears in the XRD profile of HBPGO. In contrast, HBPGO shows a weak, broad peak at 2θ = 9.80°, suggesting that the hyperbranched cyclotriphosphazene polymer grafted on the surface of GO disordered the stacking structure of GO sheets. The high molecular weight of the hyperbranched polymer exfoliates the GO sheets and restrains their reaggregation.

Fig. 6 XRD spectra of GO (a), NH₂–GO (b) and HBPGO (c).

The nanosheets morphology of GO and HBPGO were investigated by TEM imaging as displayed in Fig. 7. TEM analysis (Fig. 7a) shows that the GO nanosheets are very thin and have some wrinkles and folded regions, which is consistent with the morphology typically reported in the literature.^15,19,39,44 In contrast, HBPGO (Fig. 7b) exhibits a different morphology. Compared with GO, the nanosheets of HBPGO become less transparent, and some black regions on the surface of HBPGO can be observed. The black regions of HBPGO can be attributed to the hyperbranched polymer layer attached onto the GO surface from both sides.

Fig. 7 TEM images of GO (a) and HBPGO (b).

3.2. Curing behavior of HBPGO/DCPDCE system

Gel time is generally used to evaluate the curing behavior of a resin, a shorter gel time indicates a bigger curing activity. Fig. 8 depicts the gel time of DCPDCE resin and HBPGO/DCPDCE systems at different temperatures, it can be observed that the addition of HBPGO can effectively decrease the gel time of DCPDCE, indicting that the addition of HBPGO can catalyze the gelation of DCPDCE. This phenomenon is mainly contributed to the intensive promotion of the curing reaction of DCPDCE by –OH groups in the molecule of HBPGO,¹² and the reaction between active –NH₂ at the end of molecular chains of HBPGO with –OCN in DCPDCE.⁴³

Fig. 8 Dependence of gel time on temperature for DCPDCE and HBPGO/DCPDCE systems.

3.3. Mechanical properties of HBPGO/DCPDCE system

The impact and flexural strengths of HBPGO/DCPDCE systems are shown in Fig. 9 and 10, respectively. Compared to the pure DCPDCE, the HBPGO/DCPDCE systems show increased impact and flexural strengths. The impact and flexural strength are increase by 42% and 36% for the system with 0.6 wt% HBPGO. The increases in mechanical properties can be understood from the following reasons.

Fig. 9 Relationship of impact strength on HBPGO content for HBPGO/DCPDCE systems.

Fig. 10 Relationship of flexural strength on HBPGO content for HBPGO/DCPDCE systems.

Firstly, the chain of hyperbranched cyclotriphosphazene polymer grafted on the surface of GO nanosheets can bring stronger mutual exclusion and steric hindrance effect, thus the restacking of GO nanosheets is restrained and the agglomeration tendency of GO in matrix can be controlled effectively, which results in the efficient load transfer from the DCPDCE matrix to the GO sheets. The increased dispersibility of GO sheets in DCPDCE matrix is the most important factor for determining the mechanical properties of the composites. Secondly, the reaction between –NH₂ and –OCN is very easy to happen upon mild heating. In our experiment, we prepared HBPGO/DCPDCE composites employing the method of melt casting and then curing. This process will make sure that the extent of reactions between –NH₂ and –OCN in DCPDCE is nearly 100%, thus leading to improved interfacial bonding strength between GO and matrix. For the HBPGO/DCPDCE system, the probability of forming a strong organic–inorganic combination will be enhanced. Moreover, –NH₂ groups in HBPGO can react with –OCN groups to form isoureas that contain the flexible chains, improving the impact strength.⁷ Therefore, the mechanical properties of HBPGO/DCPDCE system are increased as the contents of HBPGO from 0.0 wt% to 0.6 wt%. However, when the fillers content is high enough (>0.6 wt%), the mechanical properties of the composites decrease with the increasing concentration of fillers. This occurs because further loading causes the GO sheets to stack together, reducing the improvement return in mechanical properties.⁴⁴

In order to further confirm the effect of HBPGO on the toughness of DCPDCE resin, SEM images of the fracture surfaces of samples after impact tests are taken and shown in Fig. 11, it can be observed that pure DCPDCE resin has a smooth and river-like fracture surface (Fig. 11a), exhibiting a typical brittle feature. While with the addition of HBPGO into DCPDCE resin, the fracture surfaces become rougher and are accompanied with more ductile sunken areas, which is consistent with the improved impact strength of the nanocomposites. In the case of 0.2 wt% HBPGO/DCPDCE system, as shown in Fig. 11b, the surface becomes coarse and some ductile sunken regions can be observed, which can absorb the energy of fracture and hinder the crack propagation. For the 0.6 wt% HBPGO/DCPDCE system, as shown in Fig. 11c, the fracture surface of the composite is much rougher than those of pure DCPDCE and 0.2 wt% HBPGO/DCPDCE system, and there exist large amount of ductile sunken areas, exhibiting a typical rough feature. It is also noted that there exists no obvious aggregates on the fracture surfaces of 0.2 and 0.6 wt% HBPGO/DCPDCE systems, which suggests that the appropriate amount of HBPGO has well compatibility with DCPDCE matrix. However, when the concentration of HBPGO is 0.8 wt%, as shown in Fig. 11d, large clusters in the matrix appear due to the aggregation of HBPGO at high concentration, which will lead to more rapid crack initiation and impact failure. Therefore, the impact strength of the nanocomposite with high HBPGO content is decreased. The features of the fracture surfaces of HBPGO/DCPDCE systems accord well with the mechanical properties.

Fig. 11 SEM images of the fracture surfaces of the HBPGO/DCPDCE systems (a) 0.0 wt%, (b) 0.2 wt%, (c) 0.6 wt%, (d) 0.8 wt%.

3.4. Dielectric properties of HBPGO/DCPDCE system

Fig. 12 shows overlay plots of dependence of dielectric constant on frequency for pure DCPDCE resin and modified systems. It can be seen that the dielectric constants of composites slightly enlarge as the loading of HBPGO increases, and has a remarkable jump when the loading of HBPGO is 0.8 wt%. Meanwhile, the dielectric constants of HBPGO/DCPDCE systems remain stable over the testing frequency band from 10 to 60 MHz. When the content of HBPGO is low (≤0.6 wt%), the slight increase in dielectric constant may be explained by the following reasons. Firstly, HBPGO contain polar –NH₂ and –OH groups, higher content of HBPGO can result in higher dielectric constant of DCPDCE. Secondly, additional product from the reaction between –NH₂ and –OCN increases the polarity, it also declines the degree of polar symmetry, leading to larger dielectric constant.⁴⁵ Thirdly, the increase of the content of HBPGO increases the proportion of the interface between HBPGO and DCPDCE matrix, which leads to enhance the dipole of the interface which in turn leads to a slight increase of dielectric constant values. When the content of HBPGO is higher (>0.6 wt%), much more aggregated HBPGO exists in the composites, and a bad adhesion brings the much stronger interfacial polarization, resulting in much higher dielectric constant. That's why the 0.8 wt% HBPGO/DCPDCE system has much higher dielectric constant than other systems.

Fig. 12 Dielectric constants of DCPDCE resin and HBPGO/DCPDCE systems versus frequency.

In the practical applications of high performance polymer composites, low dielectric loss is highly desirable and has been actively pursued.^46–49 Fig. 13 displays the effect of HBPGO on the dielectric loss of HBPGO/DCPDCE systems. It can be seen that all the composites exhibit lower dielectric loss that pure DCPDCE resin except 0.8 wt% HBPGO/DCPDCE system. It is well known that dielectric properties of a hybrid depend on the orientation and relaxation of dipoles in the applied electric field and the accumulation of space charges. The process of dipole polarization is accompanied by the movement of polymer-chain segments, whereas the accumulation of space charges is closely related to the interfacial adhesion between fillers and the matrix. As discussed earlier, strong interfaces between DCPDCE resin and HBPGO are achieved through the chemical reactions between –NH₂ groups in HBPGO molecules and –OCN groups in DCPDCE resin. Therefore, the mobility of chain segments will be restricted, thus reducing the relaxation loss of conductive dipole segment. As a result, the dielectric loss factors of HBPGO/DCPDCE system are decreased as the contents of HBPGO from 0.0 wt% to 0.6 wt%. However, when the content of HBPGO is high enough to form aggregates, the interfacial adhesion between HBPGO and DCPDCE matrix will become poor, leading to the increase of the dielectric loss of composite. So the 0.8 wt% HBPGO/DCPDCE system has much bigger dielectric loss than other systems.

Fig. 13 Dielectric loss factors of DCPDCE resin and HBPGO/DCPDCE systems versus frequency.

3.5. Thermal properties of HBPGO/DCPDCE system

Fig. 14 displays TGA and corresponding differential thermogravimetric (DTG) thermograms for pure DCPDCE and 0.6 wt% HBPGO/DCPDCE system. Compared to that of pure DCPDCE, the TGA curve of the HBPGO/DCPDCE system shifts toward higher temperature and the onset temperature of thermal degradation for the HBPGO/DCPDCE system is increased from 384 °C for pure DCPDCE to 406 °C. The peak decomposition temperature of the DTG curve represents the temperature at which the maximum weight loss rate is reached. The peak decomposition temperature of the HBPGO/DCPDCE system appears at about 450 °C and is increased by about 20 °C compared to that of pure DCPDCE. These results indicates that the addition of HBPGO improves the thermal stability of DCPDCE resin. On one hand, the homogeneously distributed HBPGO sheets in the matrix can efficiently avoid heat concentration upon external thermal exposure. On the other hand, the good interfacial interactions between HBPGO and DCPDCE resin can also provide contribution to the increase in thermal stability. Specifically, a strong interfacial adhesion is beneficial in restricting the segmental motions and thus increasing the energy consumption of polymer chains degradation.

Fig. 14 Overlay TGA and DTG curves of DCPDCE and 0.6 wt% HBPGO/DCPDCE system.

3.6. Water absorption of HBPGO/DCPDCE system

One of the advantages of DCPDCE resin is its low water absorption, and the saturated water absorption of DCPDCE resin is only 1.4 wt%,⁴⁰ which is much lower than those of bismaleimide resins (4–5 wt%)⁵⁰ and tetrafunctional epoxy resins (5–6 wt%).⁵¹ Outstanding moisture resistance is a very important property of a material, especially those requiring stably high performance,^52–54 because in general absorbed water will decline almost all properties of the original material including thermal, mechanical and dielectric properties, etc. Therefore, less water absorption is one important target of developing new DCPDCE composites with high performance. Fig. 15 gives the water absorption values of HBPGO/DCPDCE systems and that of pure DCPDCE resin for comparison. It can be seen that the water absorption decreases from 0.57 to 0.53 wt% with the small addition (0.2 wt%) of HBPGO into DCPDCE resin, and which continually decreases with the continuous increase of HBPGO content in HBPGO/DCPDCE systems. In the case of 0.6 wt% HBPGO/DCPDCE system, its water absorption is only 0.38 wt%, which is much lower than those of pure DCPDCE resin and reported DCPDCE composites. For example, Jia et al.⁵⁵ prepared a modified DCPDCE resin by carboxyl terminated butadiene-acrylonitrile (CTBN) rubber, and found that the modified DCPDCE resin has increased toughness and storage modulus. However, the addition of CTBN tends to deteriorate the water-resistance property. The water absorption of CTBN/DCPDCE composite exceeds 0.75 wt%. The improvement of water-resistant property of HBPGO/DCPDCE systems may result from two facts. Firstly, GO with distinctive two-dimensional and layered structure possesses very high barrier properties,⁵⁶ which is beneficial to improve the water-resistant property of DCPDCE matrix. Secondly, a large number of functional groups on the HBPGO surface can enhance inorganic–organic phase compatibilization at the interface, which can effectively prevent water from entering the network of nanocomposties.

Fig. 15 Water absorption of DCPDCE resin and HBPGO/DCPDCE systems.

4. Conclusions

A kind of high-performance polymer composite has been fabricated using DCPDCE resin as matrix and GO as filler employing the method of melting mixing, in which the GO was modified with a hyperbranched cyclotriphosphazene polymer before use in order to improve its dispersability and compatibility with the DCPDCE matrix. The incorporation of HBPGO can catalyze the reaction of DCPDCE resin. When HBPGO content is 0.6 wt%, HBPGO/DCPDCE system shows the maximum impact strength and flexural strength, which are 42% and 36% higher than that of pure DCPDCE resin, respectively. And the proper addition of HBPGO can not significantly sacrifice the low dielectric constant of pure DCPDCE resin. Moreover, the HBPGO/DCPDCE systems exhibit better thermal stability and moisture resistance than pure DCPDCE resin. Functionalized GO with hyperbranched cyclotriphosphazene polymer presented herein will provide an effective strategy to improve the mechanical properties and thermal stability in nonpolar polymers.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06411a

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