Hierarchical Structure Regulation Of Carbon Fiber Surface And Its Effect On Composite Interface

Polyacrylonitrile(PAN)-based carbon fiber reinforced resin composites have been widely used in aerospace,industrial production,civil life and other fields because of their excellent properties,such as high specific strength,high specific modulus,corrosion resistance and so on.The mechanical properties of carbon fiber composite are closely related to the interfacial bonding between carbon fiber and resin which is influenced by the surface structure of carbon fiber,so it is necessary to study the surface physical and chemical structure of carbon fiber.Starting from the intrinsic surface structure of carbon fiber,this paper focused on the regulation of hierarchical structure of carbon fiber surface in order to achieve interfacial modification of composite.The quantitative regulation mechanism of the surface physical structure of carbon fiber was explored.The correlation between surface physical structure,surface chemical structure and electrochemical grafting of carbon fiber was also established.Then the electrochemical grafting effect and surface chemical structure of carbon fiber were regulated by adjusting electrode potential of carbon fiber.All the interfacial properties of above carbon fibers under the step-by-step regulation of the hierarchical surface structure were characterized.The main results are as follows:A quantitative characterization method of surface physical structure of carbon fiber was established based on image processing technology.Edge differential detection method was used to automatically identify the boundary of carbon fiber section.By finding the smallest.circumscribed convex polygon of fiber section and the minimum circumscribed rectangle of grooves,the groove width,depth,contour length and area for 20 monofilaments were calculated.The average groove size and groove size distribution of carbon fibers can be used to comprehensively compare the difference of surface physical structures for different carbon fibers,and the distribution of groove size accords with the lognormal distribution.The overall roughness of fiber which is calculated from all the grooves on the carbon fiber surface and the groove shape index rectangularity are defined.It indicates that both groove shape and groove size contribute greatly to the surface roughness of carbon fiber.The quantitative regulation of the surface physical structure of carbon fiber was realized,and the quantitative relationship between the surface physical structure of carbon fiber and the interfacial properties of composite was established.The groove structure of polyacrylonitrile-based carbon fiber is inherited from the surface groove structure of polyacrylonitrile precursor fiber.Results show that the surface roughness of carbon fiber decreased and then increased when the coagulation bath temperature increased from 20℃ to 55℃.It was related to the combined effect of rupture of precursor surface during extrusion swell and phase separation in the coagulation bath.When the concentration of ammonium ion in the coagulation bath was within the range of 0～1×10-4.7 mol/L,the double diffusion process in the coagulation bath was affected by improving the hydrophilicity of the spinning solution,and the carbon fiber with a smaller groove size was prepared by adding ammonia in the coagulation bath.The increase of the groove size on the carbon fiber surface,especially the groove depth,can effectively improve the physical meshing between the carbon fiber and the resin.The interfacial shear strength of carbon fiber composites increased from 53.1 MPa to 60.4 MPa.The surface chemical structure of carbon fiber was further modified by electrochemical grafting based on the regulation of surface physical structure.The effect of surface physical structure on the electrochemical grafting and the effect of electrochemical grafting on the composite interface under the influence of different surface physical structure were analyzed.It was found that the type and content of defective carbon structure,the ratio of oxygen to carbon and the content of C-O on the surface of carbon fiber were increased after the electrochemical grafting of poly(glycidyl methacrylate)(PGMA).The extent of grafting on the rough carbon fiber surface was higher because of the rupture mechanism during grooves formation and the current tip effect.The increase of surface roughness of carbon fiber led to the increase of physical meshing between carbon fiber and resin.In addition,the wettability and chemical combination between rough carbon fiber and resin was improved more greatly because of the higher degree of electrochemical grafting.Thus,the interfacial properties of composite were significantly improved after electrochemical grafting onto the carbon fiber with higher roughness and the interfacial shear strength increased to 70.2 MPa.The electrode potential was reduced based on the galvanic effect,and the electrochemical grafting effect and surface chemical structure of carbon fibers were greatly improved.Carbon fiber and metals with different electrode potential were used to form combined electrodes in order to regulate the electrode potential of carbon fiber.Results showed that galvanic coupling formed in the combined anode when the potential difference between carbon fiber and metal was higher than 0.05 V.The electrode potential of carbon fiber was reduced due to cathodic polarization.The electrode potential of carbon fiber after galvanic coupling was proportional to the self-corrosion potential of metals,because there was the stronger galvanic effect and more significant cathodic polarization of carbon fiber between carbon fiber and metal with lower self-corrosion potential.By applying the electrode potential regulation of carbon fiber in the electrochemical grafting of PGMA,the surface oxygen content and epoxy group content of grafted carbon fiber increased as the electrode potential of carbon fibers dropped to-0.72 V.The grafting amount and grafting rate of PGMA on the carbon fiber surface was significantly increased and the interfacial shear strength of carbon fiber composite was remarkably promoted from 47.6 MPa to 81.4 MPa,increased by 71%.