Carbon nanotubes (CNTs), one-dimensional carbon allotrope used as filler, attracted many researchers owing to its remarkable structure and excellent properties [1
], which were considered as an ideal reinforcement for the fabrication of CNT/polymer composites. However, CNTs are easily aggregated and difficult to dispersed in the polymer matrix owing to their strong intermolecular van der Waals interactions and high aspect ratio. Moreover, the weak interfacial interaction between CNTs and the polymer matrix effectively limits the load transfer from the matrix to the CNTs. So, uniform dispersion of CNTs in the polymer matrix and strong interfacial interaction between them are the main challenges for the CNT/polymer composite, which can be used in the technological sectors and branches of industry depending on the CNTs unique properties [5
Currently, chemical modification on the surface of CNTs could enhance the dispersion and interfacial interaction between the CNTs and polymer matrix. Gulotty et al. [6
] reported that the carbon nanotubes with carboxylic functionalization improved their dispersion properties in the polymer nanocomposites, and the coupling interaction between them. Szatkowski et al. [7
] reported that the functionalized carbon nanotubes with hydroxyl groups (–OH) were homogeneously dispersed in the polyurethane (PU) matrix and the strong interaction was existed between the PU structure and –OH groups on the functionalized CNTs. However, the covalent functionalization is a double-edged sword because it can reduce aggregation and facilitate CNT interfacing with other materials, but it can also have detrimental effects on CNT properties if not controlled [8
]. Furthermore, in situ polymerization is considered as the appropriate method for producing the CNT/polymer composites with excellent properties by means of ultrasonication [10
]. Saeed et al. [12
] reported that the functionalized multi-walled carbon nanotubes with aromatic amine (COC6
) groups into nylon-6 matrix have homogeneous dispersion by in situ polymerization and enhanced the specific strength and modulus compared to the neat nylon-6 nanofibers. Zhang et al. [13
] obtained the uniform dispersion of functionalized CNTs containing hydroxyl groups in the polyacrylonitrile (PAN) matrix and strong interfacial interaction between them through aqueous deposition polymerization. Ultrasonication as an auxiliary method is used to improve the dispersion of CNTs in the polymer matrix. However, the long ultrasonication process and the high power would shorten the length of CNTs [14
]. Therefore, the ultrasonic parameters play an important role in utilizing the excellent properties of CNTs in composite materials.
PAN is the main precursor for manufacturing carbon fiber with excellent mechanical properties, and belongs to a semi-crystalline polymer which could change the crystal structure during processing of the PAN precursor fiber [16
]. After stabilization and carbonization, PAN precursor fiber finally becomes the carbon fiber with outstanding tensile strength, which is widely used in sports, aerospace, automobile, industry and so on. The addition of CNTs into PAN polymer could change the crystal structure [17
], and the functionalized groups on the surface of CNTs also influence the interfacial interaction between CNTs and PAN polymer [19
]. Newcomb et al. [20
] obtained the PAN/CNT composite fibers by gel spun and regarded that the addition of CNTs into PAN matrix increased the crystallinity, crystal size and mechanical properties compared to the PAN fibers. He groups [21
] reported that PAN/functionalized CNTs with carboxyl acid groups (–COOH) prepared by plasticized spinning exhibited excellent tensile strength and modulus compared to the precursor fibers, and the CNTs–COOH initiated the crystallization and improved the crystal size. Quan et al. [22
] prepared the CNT/PAN nascent composite fiber by wet-spinning and regarded that the addition of CNT into PAN nascent fibers increased the degree of crystallinity, crystal size and crystal orientation compared to the control PAN nascent fibers, and the composite fibers had higher tensile strength and tensile modulus than the PAN fibers.
However, few studies are reported on rheological properties of functionalized CNT/PAN concentrated solutions by in situ polymerization. Rheological properties of the solutions can reflect the internal structure and processability, which can provide information about the network structure, dispersion state of the CNTs, and the interfacial interaction between CNTs and polymers [23
]. At low frequency, the rheological behaviors reflect the relaxation and motion of polymer chains. At high frequency, the rheological behaviors are corresponding to the movement of polymer chains within small time. So, rheological behaviors of various CNT/polymer composite have been widely studied [27
]. Kim et al. [32
] reported that the addition of amino-CNTs into poly(methyl methacrylate) (PMMA) composite increased the viscosity, storage modulus and loss modulus compared with the control PMMA polymer, which were ascribed to the strong interaction between amino-CNTs and PMMA matrix.
In this paper, we studied the effect of amino-functionalized multi-wall carbon nanotubes (amino-CNTs) on the rheological behavior of PAN/dimethyl sulfoxide (DMSO) concentrated solutions by in situ polymerization and the crystal structure of PAN precursor fibers by wet-spinning method. The effects of amino-CNTs on the rheological behaviors were examined by the parallel plate rheometer. The dispersion of amino-CNT into PAN concentrated solutions and morphology of composite fibers were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. The interfacial interaction between amino-CNTs and PAN polymer in composite fibers was characterized by the Raman spectroscopy and Fourier transform infrared (FTIR) spectrum. The degree of crystallization, crystal size and crystal region orientation of amino-CNT/PAN composite fibers were measured by the different X-ray diffraction (XRD), respectively. The cyclization reaction was tested in the differential scanning calorimetry (DSC) in nitrogen atmosphere. The mechanical properties were measured by monofilament tensile testing machine.
In this paper, the effects of amino-CNTs on the rheological behavior of PAN concentrated solutions prepared by in situ polymerization were studied. The addition of amino-CNTs into the PAN concentrated solutions with 22% concentration by mass have the higher shear viscosity and show earlier shear thinning behavior compared to the control PAN concentrated solutions, indicating that the network structures were formed between the amino-CNTs and the PAN macromolecular chains and the amino-CNTs could be homogeneously dispersed in the PAN concentrated solutions. The amino-CNT/PAN composite concentrated solutions with higher storage modulus at low frequency suggested that there existed network structures and the uniform dispersion of amino-CNTs in the composite concentrated solutions, which were also confirmed by the amino-CNT/PAN composite concentrated solutions having lower tangent loss angle at the same conditions. The composite fibers containing 1% amino-CNT by mass have 46.5% degree of crystallization, 77.7 Å crystal size and 0.914 orientation of crystal region, which is lower than the control PAN precursor fibers with 50.5% degree of crystallization, 83.4 Å crystal size and 0.943 orientation of crystal region. These results coming from the XRD analysis suggested that the addition of amino-CNTs hindered the orientation and crystallization of the PAN chains during the processing of stretching. The results from FTIR and Raman spectroscopy indicated that the amino-CNTs had interfacial interaction with the PAN macromolecular chains, and affected the orientation of nitrile groups, which made that the ratio value of A∥/A⊥ for the control PAN precursor fibers increased from 1.61 to 2.30 for the amino-CNT/PAN composite fibers. The SEM images for the amino-CNT/PAN composite fibers prepared by wet-spinning method exhibit the diameter of about 10.5 μm, which is larger than the control PAN precursor fibers with diameter of about 8.0 μm. This is ascribed to the composite solutions having higher viscosity than the PAN concentrated solutions. The initial temperature of cyclization reaction for composite fibers is 258.3 °C, which is significantly lower than that of the control PAN precursor fibers with 266.5 °C. The addition of amino-CNTs into the PAN precursor fibers also decreases the maximum temperature from 279.4 °C to 276.4 °C. The cyclization of composite fibers have larger exothermic temperature range and lower evolved heat than the PAN precursor fibers. Furthermore, the tensile strength and tensile modulus of the amino-CNT/PAN composite fibers are 0.83 GPa and 28.37 GPa, respectively, which is higher than the control PAN precursor fibers with the tensile strength of 0.74 GPa and the tensile modulus of 16.03 GPa.