DU Xiaodong()， JIANG Jinhua()*， CHEN Nanliang()， LIU Yanping()

1Engineering Research Center of Technical Textiles，Ministry of Education，Donghua University，Shanghai201620，China2College of Textiles，Donghua University，Shanghai201620，China

Abstract：The application of polyimide(PI)fibers in the field of composite materials has been limited because of their smooth surface and chemical inertness.In order to overcome these problems，oxygen plasma was used to modify the surface of fibers.The single fiber fragmentation test(SFFT)was used to characterize the interfacial adhesion performance of PI fiber as a simple and accurate analysis method.It was found that the interfacial shear strength between the fiber and resin after oxygen plasma modification was increased by 54%compared to the untreated fiber.Meanwhile，the surface micromorphology，chemical composition，wettability of fibers and the interface morphology at the fiber fracture were analyzed by field emission scanning electron microscope(FESEM)，X-ray photoelectron spectroscopy(XPS)，contact angle measurement and polarizing microscope，respectively.All of these results demonstrated that the single fiber fragmentation test for analyzing the interfacial adhesion of PI fibers was effective.

Key words：single fiber fragmentation test(SFFT)；polyimide(PI)fibers；oxygen plasma treatment；interfacial shear strength

Introduction

High-performance polyimide(PI)[1]fibers are considered as a popular material with considerable development potential[2-3]. However， smooth surface and low surface energy of the PI fibers lead to poor interfacial adhesion between the fibers and matrix， which seriously affects the performance of the composites. Oxygen plasma modification was commonly used to improve the interfacial properties of PI fibers[4]. Therefore， the analysis of interfacial adhesion of modified PI fibers is very necessary. The single fiber fragmentation test(SFFT)[5-6]as a test method for testing interfacial shear strength is simple， accurate and repeatable. But it is often used to analyze the interfacial shear strength of brittle fibers such as carbon fibers[7-8]， glass fibers[9-10]rather than flexile fibers owing to low elongation of resin.

In this work， PI fibers were modified by oxygen plasma， and the SFFT was applied to analyze flexile fibers/matrix adhesion before and after modification. The selected resin was the mixture of polyurethane modified epoxy resin and epoxy resin. It had excellent elongation， which changed with the mixing ratio of epoxy resin. Furthermore， the variations of microstructure， chemical composition on the fiber surface， mode of cracks between fiber and the matrix were investigated by using field emission scanning electron microscopy (FESEM)， X-ray photoelectron spectroscopy (XPS) and polarizing microscope.

1 Experimental

1.1 Materials and oxygen plasma treatment

Suplon?fibers were provided by Jiangsu Aoshen Hi-Tech Materials Co.， Ltd.， China. The diameter of single fibers were 12.82 μm. Diameter was taken at least 20 places to measure by polarizing microscope， and the average was calculated by Nanomeasure software.Polyurethane modified epoxy resin and epoxy resin were purchased from Shanghai Science and Technology Development Co.， Ltd.， China.

The oxygen plasma treatment was conducted in a HD-300 low temperature plasma treatment apparatus (Changzhou Zhongke ChangTsi Plasma Processing Apparatus Technology Co.， Ltd.， China) with oxygen pressure value of 10 Pa and power value of 100 W. The parallel Suplon?fibers were treated for 4 min.

1.2 Mechanical properties of PI fibers

The tensile properties of untreated and oxygen plasma treated Suplon?fibers were measured using XQ-1C high-strength high-modulus fiber tensile tester (Shanghai Xinxian Instrument Co.， Ltd.， China.) with stretching distance of 20 mm， and tensile speed of 10 mm/min. The test of each sample was repeated for 40 times. The strength data of PI fibers were very dispersive. Therefore， the Weibull statistical distribution model after taking logarithm could be obtained as follows.

ln ln(1/(1-F(σ)))=lnL+λlnσ-λlnσ0,

(1)

whereF(σ) refers to the fracture probability cumulative distribution function of a PI fiber when the tensile strength is not higher thanσ， andLis gauge length.σ0andλare the Weibull scale parameter and shape parameter， respectively， and they can be estimated by the least square method[11].

1.3 SFFT

According to the standard JIS K6251， dumbbell type specimens were prepared and then tested in the universal testing machine (INSTRON 5967， Instron Engineering Corporation Co.， Ltd.， America). The fiber was broken continuously until it reached with the increase of external force， because the elongation rate of the resin is far higher than that of Suplon?fiber. Then critical fracture length of fibers was observed by the polarizing microscope (ECLIPESLV 100N POL， Nikon Co.， Ltd.， Japan). According to Kelly-Tyson[5]theory， the formulas for calculating interfacial shear strength are as follows.

(2)

(3)

(4)

1.4 Analysis of surface morphology and chemical composition

The surface morphology of untreated and oxygen plasma modified PI fiber was observed by FESEM (S-4800， Hitachi， Japan)， and accelerating voltage was 5.0 kV. The XPS(ESCALAB 250Xi， Thermo Fisher Scientific Inc.， America) was used to analyze the surface chemical composition with a monochromatic Al Kα X-ray source.

1.5 Analysis of surface wettability

The contact angle measurement(OCA15EC， Dataphysics， Germany) was used to deposit a certain amount of deionized water on the fixed fiber， and the liquid was suspended in droplet shape on the fiber. The contact angle of the fiber droplets was calculated by the contact angle analysis software.

2 Results and Discussion

2.1 Effect of oxygen plasma on the mechanical properties of PI fiber

The tensile strength of PI fiber was analyzed by Weibull theory. Figure 1 shows Weibull distribution of the tensile strengths of the untreated and oxygen plasma modified Suplon?fiber. The shape parameter(λ) characterizes the degree of dispersion of the fiber strength. The larger value ofλmeans that the smaller fibers irregularity， and the better mechanical properties.

Fig.1 Linear fitting of ln ln(1/(1-F(σ))) as a function of ln σ

As shown in Fig.1， the Weibull shape parameter of fibers changed from 13.07 to 9.79 after oxygen plasma treatment. It is considered that the defects of the fibers， such as voids and cracks， are more obvious after plasma etching. Therefore， it leads to larger strength irregularity.

In addition， the stress-strain force curves of PI fibers and other tensile results are shown in Figs.2-3. Tensile strength and initial modulus of the fiber after plasma treatment changed from 0.93 GPa to 0.88 GPa， and 9.26 GPa to 8.46 GPa， respectively. It is easy to see that the average strength of Suplon?fiber decreased slightly. Plasma modification less damages to the strength of PI fibers.

Fig.2 Stress-strain force curves of Suplon?fiber before and after oxygen plasma treatment

Fig.3 Tensile strength and modulus of Suplon?fiber before and after oxygen plasma treatment

2.2 Analysis of interface property between PI fiber and resin by the SFFT

The SFFT was used to analyze the influence of oxygen plasma modification on the interfacial bonding properties of PI fibers. As shown in Fig.4(a)， the morphology of the resin around the breakpoints of fiber was hollow. And there was no trace of strong action between untreated Suplon?fiber and resin. That is to say， the untreated Suplon?fiber had debonded and slipped in the matrix. It indicated that the poor interface compatibility between fiber and resin caused by the smooth and chemical inertness of Suplon?fiber. In addition， the cracks at the oxygen plasma modified Suplon?fiber breakages are shown in Fig.4(b)， which indicated that the interface compatibility between the fiber and the resin became stronger. It can be further proved by the results as shown in Table 1， the critical length of PI fiber decreased from 276.41 μm to 196.02 μm after oxygen plasma modification. And the interfacial shear strength of treated fiber was 54% higher than untreated fiber.

(a) Untreated

(b) Oxygen plasma treatment

Fig.4 Interface morphology between fiber and resin at the fiber fracture

Table1Comparison of interfacial shear strength for untreated and oxygen plasma modified Suplon?fiber

Suplon fibersLC/μmτ/MPaAchieved improvement/%Untreated276.4129.88-Oxygen plasma treatment196.0246.1354

2.3 Surface morphology analysis of PI fiber

The FESEM images showed that the surface morphology of oxygen plasma modified PI fiber changed significantly. It can be seen from Fig.5 that the surface of the untreated fiber was smooth. After plasma modification， Suplon?fiber had even and dense microcracks， and the sizes of the cracks were in micron or nanoscale.

(a) Untreated

(b) Oxygen plasma treatment

2.4 Surface chemical composition analysis of PI fiber

(a) Untreated

(b) Oxygen plasma treatment

Fig.6 C 1s core-level spectra of Suplon?fibers

Table2Concentration of functional groups on the untreated and oxygen plasma modified Suplon?fibers surface

Suplon fibersConcentrations of correlative functional groups/%C—CC—NC—OCOUntreated46.335.87.610.3Oxygen plasma treatment31.528.920.419.2

2.5 Surface wettability analysis of PI fiber

The contact microscope photographs of PI fibers before and after modification are shown in Fig.7. The untreated PI fiber exhibited hydrophobic properties with contact angle larger than 90° and water droplets could only be attached to one side of the fiber. However， the surface contact angle of PI fiber modified by plasma for 4 min was reduced to less than 55°， and the surface showed hydrophilic properties.The improvement of hydrophilicity is also one of the reasons for the increase of interfacial shear strength of modified PI fiber.

(a) Untreated

(b) Oxygen plasma treatment

3 Conclusions

In summary，Suplon?fibers were modified by oxygen plasma to develop the effect of modification for overcoming the defect of poor interfacial adhesion， and the SFFT was used for analyzing the interfacial adhesion of flexile fibers as PI fibers. The results showed that the interfacial shear strength between fiber and resin was increased by 54% compared to the untreated fiber after oxygen plasma modification. Moreover， it demonstrated that the SFFT for analyzing the interfacial adhesion of PI fibers was effective.