拉曼光谱分析炭纤维表面的微观结构

任桂知, 陈淙洁, 邓李慧, 全海宇,2, 吕永根,3, 吴琪琳,3

(1.纤维材料改性国家重点实验室,上海 201620；2.Texas Tech University,Department of Chemistry and Biochemistry,Lubbock,Texas79409,USA；3.东华大学材料学院,上海 201620)

拉曼光谱分析炭纤维表面的微观结构

任桂知1, 陈淙洁1, 邓李慧1, 全海宇1,2, 吕永根1,3, 吴琪琳1,3

(1.纤维材料改性国家重点实验室,上海 201620；2.Texas Tech University,Department of Chemistry and Biochemistry,Lubbock,Texas79409,USA；3.东华大学材料学院,上海 201620)

采用拉曼光谱技术研究了PAN基炭纤维表面微观结构的异质性。借助于自制的旋装装置,实现了单根炭纤维纤维的旋转,利用拉曼面扫描技术获得了纤维整个外表面的拉曼光谱。通过分峰数据处理,得到II/IG、IA/IG、IDi/IG与ID/IG的分布,发现这些结构参数具有较大的波动性,说明炭纤维表面微观结构是不均匀的。进一步也计算出纤维表面的晶粒尺寸La在0.7-2.9 nm间变化,结构缺陷有沿着纤维轴向取向的趋势。通过拉曼旋转扫描,揭示出了炭纤维表面的复杂微观结构。

拉曼光谱；炭纤维；表面微观结构

1 Introduction

The properties of polymer fibers are determined by their structure,especially surface microstructure. For example,the tensile strengths of carbon fibers (CFs)are limited by a spectrum of defects which are distributed randomly along the fiber axial.The probability of encountering a severe flaw becomes greater as the test length of the filament increases［1］.Defects and heterogeneity on the surface of fibers are incontrovertible and they are the most important factors affecting fiber properties［2-4］.Slender granule-shaped domain on the longitudinal surface of PAN-based CFs was revealed by scanning tunneling microscope,and the smaller or slender is the domain,the higher tensile strength is the fibers［5］.Internal and surface flaws have been identified as source of failure of PAN-based CFs by scanning electron microscopy of their fracture surfaces after tension failure［1］. Besides, surface characteristics of fibers play a key role on fiber-matrix interfacial adhesion and have an important impact on mechanical and ablative properties of fiber-reinforcedcomposites.Montes-Morán et al.［6］have demonstrated that there are good correlation between fibre-matrix interfacial shear strength and the degree of surface order.Therefore,it is of great significance to give a deep insight into surface microstructure heterogeneity of fibers.

Raman spectroscopy,sensitive to the sp2and sp3geometries of carbon,has been used to characterize the microstructure of carbon materials［7-10］.For CFs, ID/IG,the intensity ratio of two major Raman bands (D and G bands),is proposed as one of the most important parameters to evaluate the microstructure heterogeneity［11］.It has also been demonstrated that the intensity of the D band is lower for the skin than the core of CFs［12］.Kobayashi［13,14］have recently used synchrotron micro-beam X-ray scattering and micro-Raman spectral measurements to characterize stress distribution at the various parts of PAN-based CFs caused by structural heterogeneity.Although the Raman spectroscopy has been employed widely to correlate the microstructure to mechanical properties of CFs,previous studies mainly focus on the spectral signals of individual points.To our knowledge,publications rarely report the microstructure heterogeneity on the whole surface of monofilament.The fact that the surfaces of fibers are cylindrical rather than flat, which has hindered a further study.

We made a rotating device and used it for characterization of CF monofilament.Fiber monofilaments can be rotated for 360°with the device and thus Raman spectra can be obtained on the whole cylindrical surface of a monofilament.This makes characterization of the microstructure heterogeneity of a fiber more representative than focus on individual points.

2 Experimental

2.1 Material and equipments

As-received PAN-based CFs(7 μm in diameter, Toray Co.Ltd,Japan)were used in this work.

The morphology of the CFs was evaluated using a scanning electron microscope(SEM,JSM-5600 LV,JEOL,Japan).

Raman scattering measurements were conducted with a confocal Raman system(Renishaw InVia Reflex) under ambient condition.The system was equipped with a Leica microscope,a two-dimensional charge couple device camera and an automated stage with a minimum step size of 0.1 μm.High-resolution gratings(1 800 lines mm-1)were used with additional band pass-filter optics,and the laser excitation wavelength was 532 nm(argon ion).All measurements were made with a backscattering mode using a 50×microscope objective with a NA value of 0.75. The illuminated area was less than 1 μm2.A laser power of 5%maximum intensity was employed to get the best signal and to minimize any heating effects. Each point was collected with a step size of 1 μm,an exposure time of 10 s and a repetition of 10.The spectrum range collected was from 1 000 to 1 900 cm-1,within the first-order Raman spectrum of graphite-based materials［15,16］.

The atomic force microscope(AFM)used is a NanoScopeⅣ made in Veeco Company,it works with an elasticity coefficient around 48 N/m and a resonant frequency about 330 kHz.

2.2 Rotation methods

A coordinate paper frame(Fig.1(c))was used to fix the fiber monofilament straightly by adhering two ends of the fiber to the paper cut empty in the center,which was then attached to the clapping fixture of the rotating device as shown in Fig.1b.The device rotating is mainly comprised of a crank,a driving gear,a transmission shaft and a driven gear, which was placed under the objective lens for measurement during rotating(Fig.1(a)).

Fig.1 (a)The rotating device under the objective lens；(b)The diagram of the self-made rotating device；(c)Paper frame used to fix and straighten fiber monofilament.

The detailed testing procedure is described in Fig.2.We chose a line parallel to the fiber axis as the first line(denoted as Line1),then 30 points were recorded from P(1,1)to P(1,30)with a step size of1 μm.When the scanning of Line1 was finished,the laser went back to the beginning position and the crank was rotated by an angle of about 18°,followed by the detection of Line 2.The same procedure was repeated until the monofilament was rotated for 360°. Assuming that the cylindrical surface of fiber monofilament can be unfolded,all the scanned points can be displayed in a 30 μm×20 μm microregion as shown in Fig.3.In this way,whole cylindrical surface of a single fiber could be determined point by point from the Raman spectra.

Fig.2 The scanning procedure on the cylindrical surface of a fiber.

Fig.3 The scanned points on the unfolded surface of a single fiber.

3 Results and discussion

3.1 The heterogeneity of graphitization degree on whole surface of a carbon fiber

Each Raman spectrum exhibits a hump-like shape with peaks at～1 360 cm-1and ～1 580 cm-1as shown in Fig.4.The spectrum was deconvoluted with four peaks located at～1 200 cm-1(D′′band),～1 360 cm-1(D band),～1 500 cm-1(A band)and～1 580 cm-1(G band)to the peak intensities［9,17］.

According to previous researches on carbonbased materials,D band is usually attributed to an A1gmode and/or to the breakdown of translational and local lattice symmetries,while G band is widely considered as the intrinsic band of graphitic structure［18-20］.“A”band is correlated to sp3-like structures originated from amorphous carbon or some kind of organic functional groups［21］.D′band is tentatively related to be a signature of sp3hybridization formed by carbon atom and a small quantity of heteroatom［9］.

With the help of this self-made rotation device, point-to-point variationsofmicrostructure on the whole cylindrical surface were derived from these spectroscopic data for the first time.

Fig.4 Raman spectra for point P(1,1).

The point-to-point variations of ID/IGwere plotted against the position of scanned points as shown in Fig. 5(a).One can recognize that ID/IGdoes not change obviously in area B,but does significantly in both area A and C.This reveals the existence of microstructure heterogeneity on the whole surface of CFs.It is also worth noting that disorder structure in area A and C orientates along the axial direction.This can be partly proved by SEM observation,which demonstrates that the defects on surface area A，also are aligned along fiber length(shown in Fig.6).Although the scanned areas of SEM and Raman spectroscopy are not exactly the same,these consistent results reveal somewhat defect distribution feature.Likewise,ID/IG,ID′/IG,IA/ IG,and IDi/IGfollow the similar trend as can be displayed in Fig.5.Again,it is obvious that the graphitization degrees in area A,B and C are quite different, indicating the surface heterogeneity on CFs.

3.2 Crystallite parameters

Along fiber axis,some strips made of graphite clusters and grains are observed by AFM,as shown in Fig.7.The graphite cluster in-plane correlation length can be calculated from the ID/IGratio using the relationship developed by Tuinstra Keonig［15］:

Based on the above results,the distribution of Laon the whole cylindrical surface of a CF can be achieved as shown in Fig.8.The Lavalues of most scanned position in area B is around 2.2 nm,which agrees well with the results of Lespade et al.［22］.However,the Lavalues in both area A and C are relatively lower,suggesting that a highly complicated fine structure is formed on CFs,which consists of the crystalline zone with various sizes and amorphous region.

Fig.5 The distributions of(a)ID/IG,(b)ID′/IG,(c)IA/IGand(d)IDi/IGon the whole surface of PAN-based CF.

Fig.6 SEM image of PAN-based CF surface.

Fig.7 AFM image of PAN-based CF.

Fig.8 The distribution of Lavalues on cylindrical surface of PAN-based CF.

One possible reason of all these structural heterogeneity is that the main structure of CFs is a turbostratic graphitic structure rather than a perfect graphite structure.At the same time,there are inherent flaws in the CF precursor and new defects introduced in subsequent pre-oxidation and carbonization［11,23］.

4 Conclusions

The microstructure heterogeneity of PAN-based CF monofilament has been characterized by using Raman spectroscopy with the aid of rotating device tocover the whole cylindrical surface.The Raman intensity ratios relative to the G band were investigated, showing that the IA/IGchanges more significantly than the other ratios and that the quantity of sp3structure fluctuates significantly along the axial direction of CF monofilament.Moreover,the heterogeneous microstructure tends to orient along the fiber axial direction.The distribution map of Lasuggests that CFs have a highly complicated fine structure.

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Microstructural heterogeneity on the cylindrical surface of carbon fibers analyzed by Raman spectroscopy

REN Gui-zhi1, CHEN Cong-jie1, DENG Li-hui1, QUAN Hai-yu1,2, LU Yong-gen1,3, WU Qi-lin1,3
(1.State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,Shanghai201620,China; 2.Texas Tech University,Department of Chemistry and Biochemistry,Lubbock,Texas79409,USA; 3.College of Materials Science and Engineering,Donghua University,Shanghai201620,China)

A polyacrylonitrile-based carbon fiber monofilament was characterized by a confocal micro Raman spectrometer with the aid of a stage that allowed the axial rotation of the fiber so that the whole surface area could be examined.Results indicate that disorder is localized and aligned along the axial direction of the fiber.Lavalues in defective regions are relatively lower than in others.The changes in the amount of amorphous carbon in different regions are significant.

Raman spectroscopy；Carbon fiber；Surface microstructure

WU Qi-lin.E-mail:wql@dhu.edu.cn

TQ342.+74

A

国家自然科学基金(60975059);同济大学先进土木工程材料教育部重点实验室(201301);上海市教育委员会科研创新重点项目(14ZZ069).

WU Qi-lin.E-mail:wql@dhu.edu.cn

1007-8827(2015)05-0476-05

Foundation item:National Natural Science Foundation of China(60975059)；Key Laboratory of Advanced Civil Engineering Materials,Tongji University(201301)；Research and Innovation Project of Shanghai Municipal Education Commission (14ZZ069).

10.1016/S1872-5805(15)60202-5

Received date:2015-03-08 Revised date:2015-10-08

English edition available online ScienceDirect(http://www.sciencedirect.com/science/journal/18725805).