A facile approach for grafting bi-functional groups terminated branched polyphosphazene on carbon fibers

Abstract

We explored a facile method for grafting bi-functional groups terminated branched polyphosphazene on carbon fibers via direct epoxy amination with an aqueous ammonia solution. The branched polyphosphazene with abundant ß-hydroxyl groups and amino groups significantly changes the chemical composition of carbon fibers surface. A significant improvement in interfacial shear strength was obtained from 43.6 MPa for virgin carbon fibers (C.F.) composites to 89.6 MPa for branched polyphosphazene grafted C.F. composites.

Headnote

Received 22 January 2020; accepted in revised form 22 March 2020

Abstract. We explored a facile method for grafting bi-functional groups terminated branched polyphosphazene on carbon fibers via direct epoxy amination with an aqueous ammonia solution. The branched polyphosphazene with abundant ß-hydroxyl groups and amino groups significantly changes the chemical composition of carbon fibers surface. A significant improvement in interfacial shear strength was obtained from 43.6 MPa for virgin carbon fibers (C.F.) composites to 89.6 MPa for branched polyphosphazene grafted C.F. composites.

Keywords: polymer composites, molecular engineering, carbon fibers, interfaces, polyphosphazene

1.Introduction

Carbon fibers (C.F.) reinforced composites can achieve excellent mechanical properties with high specific strength and specific stiffness; hence they have been widely used as structural components [1]. However, the performance of C.F. reinforced composites is, to a large extent, governed by the structure and character of the fiber-matrix interface. Therefore, various strategies such as oxidation, coating, - irradiation [2, 3] introducing carbon nanotubes or graphene oxide [4-7], chemical grafting [8], have been proposed for improving the interfacial properties of composites [9]. In our previous studies [1013], an amine-capped cross-linked polyphosphazenes was grafted or coated on carbon fibers through in situ polycondensation between hexachlorocyclotriphosphazene (HCCP) and 4,4'-oxydiphenylamine (ODA) under mild conditions. However, compared with the oxidized C.F./epoxy resin composites, the highest degree of IFSS improvement for the aminecapped cross-linked polyphosphazenes functionalized carbon fiber was only 43.0% [11]. Additionally, it was found that the cross-linked polyphosphazene layer was brittle owing to the high cross density. This may be the reason for limiting improvement of the interfacial properties. Most recently, it is noteworthy that the chemical grafting of hyperbranched polymers [14-17] or polyamidoamine (PAMAM) dendrimers [18-20] on C.F. has drawn comprehensive attention from researchers. These types of polymers can provide a lot of active polar groups to improve the physical interfacial adhesion or even form strong chemical bonding between fiber and matrix. However, all these chemical modified polymers above contain only mono-functional groups, e.g. amino groups or hydroxyl groups. There are few reports focusing on the modification of carbon fibers with bifunctional groups at the same polymers that can provide more reactive sites to react with epoxy resin. It is expected to gain a better improvement of interfacial properties for the bi-functional groups polymer functionalized carbon fibers.

In the current study, a novel and straightforward approach has been proposed to chemically graft bi-functional groups terminated branched polyphosphazene on carbon fibers via direct epoxy amination method [21] using a six-armed epoxy monomer and aqueous ammonia solution (NH3-H2O). Interestingly, as shown in Figure 1, the grafted branched polyphosphazene has an abundance of hydroxyl groups and amino groups. These two types of active groups simultaneously distributed at the terminus of branched polyphosphazene, which can provide more reactive sites to react with epoxy resin and is expected to increase the chemical bonding density at the interface.

2.Experimental

2.1.Materials

Commercially available T700S carbon-fibers (denoted as C.F., 12 K, 1.80 g/cm3), purchased from Japan Toray Co., Tokyo, Japan. A six-armed epoxy monomer hexa[(4-(2,3-epoxypropyl)-2-methoxy) phenoxy] cyclotriphosphazene (named as EHEP) was synthesized from eugenol and hexachlorocyclotriphosphazene according to our previous work [22]. The chemical structure of EHEP was shown in Figure 1. Ethanediamine (EDA) and thionyl chloride (SOCl2) were purchased from Innochem Co., Beijing, China. The E51 (EEW=200 g/eq) epoxy resin was obtained from Jinhong Co., Zhejiang, China. The hardener 4,4-diaminodiphenyl methane (DDM, AHEW = 50 g/eq) was obtained from Aladdin Co., Shanghai, China. The chemical structures of E51 and DDM are shown in Figure 4a. Aqueous ammonia solution (25% NH3 basis), acetone, dichloromethane, dioxane and other reagents were obtained from Tianjin Damao Co., Tianjin, China, and used as received.

2.2.Bi-functional groups terminated branched polyphosphazene grafting of carbon fibers

The typical procedure of preparation for the bi-functional groups terminated branched polyphosphazene grafting of carbon fibers (denoted as CF-HEA2) were as follow: (1) The commercial C.F. were refluxed in acetone for 48 h to remove the sizing, and then oxidized in concentrated HNO3 at 100 °C for 6 h to introduce oxygen-containing functional groups onto fiber surfaces (denoted as CF-O). EDA grafted C.F. (named as CF-EA) was prepared according to the report in ref [23]. (2) The CF-EA (1.0 g) was immersed in the EHEP solution (0.04 mmol/ml in dioxane), followed by a reflux treatment at 70 °C for 12 h. EHEP grafted CF-EA (denoted as CF-HDE1) was obtained after washing with dioxane and drying in oven at 100°C. (3) To prepare the bi-functional groups terminated cyclotriphosphazene grafted carbon fibers (named as CF-HEA1), epoxy amination of CF-HDE1 was carried out according to the ref [21]. CF-HDE1 (1.0 g) was immersed in 40 ml of dioxane, and NH3-H2O (25% solution, 10 ml) was then added. The reaction was refluxed at 100 °C for 5 h. Subsequently, the CF-HEA1 was taken out and rinsed with deionized water to remove residual NH3-H2O, and then dried at 100 °C. After repeating the reaction of step (2) and step (3), the branched bifunctional groups terminated polyphosphazene grafted carbon fibers was obtained, denoted as CF-HEA2.

2.3.Characterization

The surface topography of carbon fibers was detected by Scanning electron microscope (SEM), which was performed on a Hitachi S-3400 SEM system (Hitachi Co., Japan). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrometer (FTIR) were used to analyze the chemical composition on the surface of C.F. XPS data was recorded on an Escalab 250Xi XPS system (Thermo Fisher, UK). FTIR was recorded on a Nicolet IS-50 FTIR spectrometer system (Thermo Fisher Scientific, USA).

Interfacial shear strength (IFSS) was tested on an interfacial micro-bond evaluation instrument (Model HM 410, Tohei Sanyon Corporation of Japan). The specimens for the microbond test were prepared by the following procedure: A CF single filament was fixed horizontally onto the metal frame using glue. Some epoxy resin droplets were placed against a single filament and cured (Figure 4a). The formulation of the cured epoxy resin was prepared in a 1.0:1.0 molar ratio of the epoxy group to the active amine hydrogen. The specimens were cured at 90 °C for 2 h and 110 °C for 2 h, and then cooled down to room temperature naturally.

3.Results and discussion

The SEM verified the surface topography's changes for the carbon fibers after grafting treatment, and the results are displayed in Figure 2. The oxidized C.F. (CF-O) with a smooth surface was observed. For the CF-EA, there was almost no obvious change on the surface, which was due to the short chain of ethanediamine. After grafting EHEP and amination of epoxy groups with NH3-H2O, it is clear that only a sixarmed compound containing cyclotriphosphazene ring was introduced onto fiber surface (Figure 1). Hence, the surface topography of CF-HEA1 did not change significantly owing to the lack of obvious chemical structure change. However, compared with CF-HEA1, some small particles distributed on the branched polyphosphazene grafted carbon fiber (CFHEA2) surface could be seen as shown in Figure 2d, the size of particles was 80-250 nm, which was determined by SEM photographs. This result suggested that the surface roughness of the carbon fiber increased with repeated grafting of EHEP and amination reactions, which was beneficial to enhancing the mechanical interlocking between C.F. and matrix.

The surface chemical composition of CF-O and modified C.F.s were characterized by XPS, as shown in Figure 3, the detailed results are listed in Table 1. As can be seen in Figure 3a, two characteristic peaks assigned to C1s (~284.8 eV) and O1s (~532.1 eV), and a small amount of N1s (~399.5 eV) were observed in CF-O. In the case of the CF-HEA2, a new signal of P2p peak at 133.5 eV was found beside the signals of C1s, N1s and O1s, and the atom percentage of N1s in CF-HEA2 dramatically increased from 2.7 to 5.1% (Table 1). These results suggested that the carbon fibers were successfully grafted with polyphosphazene, which contained phosphorus and nitrogen. In addition, CF-HEA2 has higher N/C ratio (6.6) and O/C ratio (21.8) than CF-O (N/C = 3.3, O/C = 20.0), suggesting that a large number of polar groups could be incorporated on carbon fibers, which might improve the fiber surface wettability. To further verify the types of functional groups on carbon fibers, we used high-resolution XPS to analyze the surface composition and chemical environment of CF-O and CF-HEA2. Figure 3b shows the N1s XPS spectrum of CF-O, a component peak at 399.2 eV was assigned to C-N=C that was derived from the carbonization procedure of polyacrylonitrile (PAN) based carbon fibers [24]. For the CFHEA2, there was deconvoluted three peaks at 397.2, 399.1 and 401.3 eV in the N1s XPS spectrum (Figure 3 c) [25-27], which were attributed to the phosphazenic nitrogen(-P=N-) of the cyclotriphosphazene, new generated terminal primary amine (-NH2)/secondary amine (-NH-) and hydrogen bonded amines [26-27], respectively, indicating a lot of amino groups were introduced on carbon fibers. Additionally, compared with CF-O (Figure 3d), O1s XPS spectrum of CF-HEA2 revealed new component peaks at 531.6, 532.5 and 533.5 eV [28, 29], which were assigned to (Ph)-O-CH3, secondary alcohol (CH-OH) and (Ph)-O-P, respectively. This observation suggested that a large abundant of ßhydroxyl groups were also incorporated on fiber surface.

Figure 3f shows the FTIR spectrum of CF-HEA2 compared with that of CF-O. There was a typical peak at ~3436 cm-1, which was attributed to O-H stretching vibration due to carboxyl groups and hydroxyl groups on CF-O surface. After grafting branched polyphosphazene on C.F., several new peaks appeared in the FTIR spectrum of CF-HEA2. The characteristic peak at 3431 cm-1 (1), 3171 cm-1 (2) shoulder peak, 1627 cm-1 (3), 1404 cm-1 (4), 1120 cm-1 (5), were assigned to O-H stretching vibration of a secondary alcohol (CHO-H), to N-H stretching vibration of primary amine (CH2-NH2) and secondary amine (CH-NH-CH), to the C=C stretching vibration of the benzene ring, to the O-H formation vibration of a secondary alcohol (CHO-H), to the -P=N- asymmetric stretching vibration of cyclotriphosphazene ring. These features clearly verified that the surface of CF-HEA2 contained both hydroxyl groups and amino groups, corresponding to the XPS results. Hence, based on the XPS and FTIR results, it is confirmed that a large number of hydroxyl groups and amino groups were successfully introduced on carbon fibers through grafting branched polyphosphazene via epoxy amination method and repeating reactions as displayed in Figure 1, which is expected to improve the surface wettability and provide abundant active sites to form strong chemical bonding at the interface of composites.

As displayed in Figure 4, the IFSS results of CF-O/ EP, CF-EA/EP, CF-HEA1/EP and CF-HEA2/EP are 43.6, 65.3 ,76.8 and 89.6 MPa, respectively. It is found that bi-functional groups grafted carbon fibers exhibited higher interfacial shear strength than that of CF-O and CF-EA. CF-HEA2/EP composites yield an IFSS of 89.6 MPa, which has an increase of about 105% in comparison with CF-O/EP composites (43.6 MPa) and about 37% in comparison with CF-EA/EP composites (65.3 MPa). It is noteworthy that the IFSS of CF-HEA2/EP is higher than that of CF-HEA1/EP as a result of more bi-functional groups generated on fiber surface through repeating reaction. Additionally, the degree of interfacial shear strength improvement for the bi-functional groups terminated polyphosphazene grafted carbon fiber composites (CF-HEA2/EP) is also better than those of other polyphosphazenes modified carbon fibers composites reported in our previous studies [11-13]. Furthermore, the advantages of this method for mterfacial improvement can be further verified in contrast with the other parameter appeared in the literature [14, 17, 20]. For example, Shi et al. [14] reported a 54.6% enhancement of IFSS for the hydroxyl-terminated hyperbranched polymer grafted on carbon fiber in comparison with the untreated carbon fibers.

To help to understand the interface behavior and improving mechanism of CF-HEA2/EP composites, we examined the surface topography of single carbonfiber after debonding from the epoxy matrix, as shown in Figure 4c and 4d. It can be clearly seen that there was almost no epoxy resin remained on the debonded CF-O surface, indicating that the matrix completely detached from the fiber surface due to the weak adhesion [12]. By contrast, a substantial amount of the epoxy resin still adhered to the CFHEA2 surface (Figure 4c). The development of this fracture microstructure should be related to the interaction between carbon fiber and matrix. XPS and FTIR results clearly confirmed that a large amount of hydroxyl groups and amino groups distributed on carbon fibers, which can react with epoxy in matrix resin. Moreover, Mora et al. [21] found that the ß-hydroxyl groups of amine afforded good reactivity properties and accelerated the rate of amine curing. These indicated that strong chemical bonding would be formed between fiber and matrix. In addition, the particles on the carbon fiber surface might also enhance the mechanical interlocking on the interface to some extent [4, 5]. Thereby, the strong chemical linking, as well as mechanical interlocking between CF-HEA2 and epoxy, mainly contribute to the obvious improvement of the interfacial strength.

4.Conclusions

In this study, a novel bi-fucntional groups terminated polyphosphazene grafted on carbon fiber was fabricated via the epoxy amination method. A large number of hydroxylamine could facilitate to form strong chemical bonding, the particles of grafted polyphosphazene could also enhance mechanical interlocking between carbon fibers and epoxy resin. The IFSS of CF-HEA2/EP composites could obtain about 105% improvement compared with that of CF-O/EP composites. This processing of HEA2 grafted carbon fibers through epoxy amination method would provide a novel and effective approach to functionalize the carbon fibers and improve the interfacial performance of the composites.

Acknowledgements

This study were financially supported by the Guangzhou Science and Technology Plan Project (No.20180410326), Guangdong Province Research and Development Plan of Key Areas (No.2019B010929001) and the Guangzhou Emerging Industry Development Fund Project of the Guangzhou Development and Reform Commission.

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Footnote

'Corresponding author, e-mail: [email protected]

References

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