Surface Modification Of Carbon Fiber Based On Dopamine Self-polymerization

Carbon fibers are widely used in composite materials due to their excellent mechanical properties,and carbon fiber reinforced resin matrix composites are widely used in various fields,among which epoxy resin matrix is the most widely applied,and polyimide resin has received significant attention in the aerospace industry by virtue of its excellent heat resistance.However,because of the high inertness of carbon fiber surfaces after high temperature carbonization（graphitization）treatment during the production process,the interfacial bonding ability between the resin matrix and the carbon fiber is affected to some extent by the,thus limiting the application scenario of the composites.Dopamine（DA）,with its excellent properties,can adhere to the surface of most organic and inorganic materials to achieve the effect of surface activation,and the diversity of functional groups in the chemical structure which forms an active platform can undergo secondary modification.The branched polyethyleneimine（PEI）structure is rich in primary amine functional groups and presents high reactivity,which is a commonly used modifier.While the-NH₂tubular functional groups in the structure can not only participate in the curing reaction of epoxy resin,but also have good reactivity with polyimide resin matrix.Therefore,this paper intends to use DA modification and PEI secondary functionalization based on DA modification platforms to activate the carbon fiber surfaces,analyze the influence of modification conditions on the structural properties of modified carbon fibers,investigate the modification mechanism,and clarify the optimal modification conditions for DA and PEI.The specific research results obtained are as follows:1.Four types of carbon fibers with different surface physical and chemical structures were modified using DA under the same conditions.The changes in the surface chemical structure,physical morphology and interfacial modification effect of carbon fiber before and after modification were analyzed.It was found that the surface physical structure of carbon fibers had no significant effect on the modification effect,while the type and content of surface functional groups were the dominant factors affecting the modification effect.DA or polydopamine（PDA）mainly underwent a Schiff base reaction with the-C=O,-COO-functional groups on the surface of carbon fibers through their-NH₂in the molecules,and the formation of stable chemical bonds on the surface of carbon fibers had a chemical grafting modification effect.2.Changing the mass ratio of dopamine/carbon fiber,adding sodium periodate,adjusting the reaction temperature and modification time all had effects on DA-modified carbon fiber.The carbon fiber was modified with DA alone at 25 ^oC for 24 h.It was found that the optimal modification ratio of DA to carbon fiber was 1:5,and its total surface energy reached 46.92 m N.m^-1after modification,which was 93.64%higher than that of the unmodified carbon fiber.The bond strength with epoxy resin was effectively improved,and the interfacial shear strength（IFSS）reached 63.72 MPa,an increase of43.03%.Sodium periodate oxidant was introduced into the DA solution to promote the self-polymerization rate of DA,and also produce oxidation effect on the-OH functional groups on the carbon fiber surfaces to generate-C=O and-COO-functional groups,which increased the binding sites of DA on the carbon fiber surfaces and improved the modification effect.By regulating and controlling the concentration,modification time and modification temperature of sodium periodate,it was clear that the most practical conditions for DA modified carbon fibers in the presence of sodium periodate were:sodium periodate concentration of 2 g/L,modification time of 15 min and modification temperature of 70 ^oC;The IFSS of the obtained carbon fibers with epoxy resin E51 was increased to 65.32 MPa,which was 47.35%higher than that of the unmodified treated carbon fibers.Due to too low concentration of sodium periodate or short modification time,the grafting efficiency of DA was low.Too high concentration of sodium periodate or too long modification time would cause excessive oxidation of DA（PDA）grafted on the surface of carbon fibers,resulting in the conversion of polar functional groups to non-polar functional groups and reducing its IFSS with the epoxy resin matrix.3.In order to improve the IFSS of carbon fiber and polyimide resin,PEI was used to modify the surface of carbon fibers,and it was found that the modification effect was not obvious because of the low reactivity of PEI itself with the surface of carbon fiber.However,the introduction of DA into PEI solution can improve the modification effect.By exploring the modification effects at different times under the conditions of m _PEI:m _DAof 6:1 and m _PEI:m _DAof 1:1 to analyze the respective mechanisms of action,it was found that different mass ratios of PEI to DA in solution resulted in different polymerization processes,leading to different modification effects.Compared with unmodified carbon fibers and pure DA modified carbon fibers,the IFSS of modified carbon fibers with epoxy resin or polyimide resin obtained under both mass ratio conditions were further improved,and the IFSS of both modified carbon fibers and polyimide resins had a phenomenon of first increasing and then decreasing with time.When m _PEI:m _DAwas 6:1 and the modification time is 60 min,the IFSS of the samples with polyimide resin reached the maximum value;When m _PEI:m _DAwas 1:1,and the IFSS of the samples with polyimide resin reached the maximum value at a modification time of 15 min.Under both conditions,the reason why the IFSS of modified carbon fibers and polyimide resins decreased over time was that the functionalized functional groups in the PEI structure grafted on the carbon fiber surface were masked by DA（PDA）molecules at a later stage.Using DA and PEI to modify carbon fibers in two steps,compared with the one-step modification of carbon fiber by PEI/DA,PEI can be grafted on the outermost layer of carbon fibers,and making surface structural characteristics easier to control,making it possible to fully utilize the specific functionalization role of PEI,so that the surface structural characteristics can be more easily controlled.The maximum IFSS of carbon fiber and polyimide resin obtained by the two-step modification reached 73.94 MPa,which was31.61%higher than that of unmodified carbon fibers,23.09%higher than that of pure DA modified carbon fibers（60.07 MPa）,and 10.87%higher than that of one-step method（66.69 MPa）.Compared with the PEI/DA one-step modification DA-PEI,the structural properties of PEI was easier to exploit by two-step modification.