Studies on Dopamine-Based Carbon Fiber Surface Modification and Performance

Author:Liu Zuo

Supervisor:wang xin ling yang bin


Degree Year:2018





Carbon fiber(CF),as a new generation of high-performance fibers,has been widely used to reinforce resin matrix composites,due to its outstanding properties,such as light weight,high specific strength and modulus,excellent electrical and thermal conductivity.The composite interphase plays a key role in connecting reinforcement with matrix and ensuring the efficient stress transfer and the fibers-matrix interaction at the interface is a decisive factor for developing high-performance composites.Unfortunately,owing to its unique turbostratic graphitic structure and low specific surface area,low surface energy,chemical inertness,pristine CF often exhibits insufficient mechanical interlock,poor compatibility and weak chemical linkage with polymer resin.Thereby,excellent interfacial adhesion could not be expected and the interface becomes the weak point,which directly degrades the ultimate performance of the resulting composites and also restricts the use and performance of CF as structural and functional materials.The interfacial optimization is usually performed by CF surface functionalization to regulate CF surface microstructure and physico-chemical properties.In this paper,we made use of the polymerization and adhesive properties of dopamine(DA),and developed dopamine co-polymerization and co-deposition techniques starting with the versatility,simplicity and reactivity of DA surface chemistry.On one hand,mediated by polydopamine(PDA),organic functional polymer polyethyleneimine(PEI)was introduced onto CF surface,to control the interfacial bonding and interfacial crystallization behaviors;On the other hand,the fiber reinforcement was designed and modified from different scales through the incorporation of carbon nanotube(CNT)nano-component;in addition,conductive polymer polypyrrole(PPy)and CNT were loaded onto the CF substrate to build PPy-CNT-CF composite fiber electrode,and explored its application in flexible supercapacitor fields.The conclusions are as follows:(1)A PEI-PDA hybrid layer with rich amino groups was coated onto CF surface by the in-situ oxidative co-polymerization of PEI and DA via a facile one-step dip-coating method.The effect of mixture composition,PEI molecular weight on the co-polymerization reaction were discussed.The introduction of PEI suppressed the PDA aggregates and accelerated the polymerization process,besides,the lower the molecular weight,the faster the reaction,and a perfect coating was obtained with the PEI/DA ratio of 2:1.The changes on fiber surface morphology,surface chemical conditions and surface wettability before and after amine functionalization were characterized through SEM,AFM,XPS,Raman and surface energy tests.The obtained CF with PEI-PDA coating exhibited larger surface roughness,abundant active groups and bonding sites for interfacial adhesion,higher surface energy and polarity,better wettability between fiber and resin,besides,the higher the PEI molecular weight,the more obvious the improvement.Micro de-bonding method was used to test the interfacial shear strength(IFSS)between single fiber and epoxy resin and the interfacial interaction mechanism was discussed.The functionalized CF was incorporated into two kinds of resin matrix,Polyurethane(PU)and Polypropylene(PP),respectively,and their macro static(tensile,bending,impact)and dynamic mechanical behaviors were systematically investigated.SEM fracture morphology was observed to get more interfacial information and the relationship between the interfacial properties and bulk performance were revealed,and it was indicated that chemical bonding at the interface was the key factor to influence the interfacial strength.The apparent activation energy was calculated to quantitatively characterize the interfacial interaction by Arrhenius equation,and the remarkably improved interfacial properties was attributed to the synergistic effect of chemical bond,hydrogen bond,π-πcoupling and physical entanglement among molecular chains.(2)The crystal growth and melting process of polymer/single fiber system was in situ monitored by Polarized Light Microscope(POM).The amino functionalized CF(PEI-PDA-CF)could induce Polylactic Acid(PLA)molecules to form special interfacial transcrystallinity structure along the fiber axis through strong interfacial interaction in the isothermal crystallization temperature range of 120-135°C.The mechanism of transcrystallinity phenomenon was discussed and the larger amino amount on fiber surface could bring about stronger heterogeneous nucleation capability,higher nucleation density,hence,denser and better crystal structure.In addition,the heterogeneous nucleation capability of PEI-PDA-CF has certain universality,which could also induce other polymer,such as PP,PA6 and PBT to form transcrystallinity structure at the interfaces though physical entanglement or hydrogen interaction.The effect of crystallization temperature on the crystal morphology,crystal thermal stability and crystal growth kinetics were investigated in detail.The nucleation density greatly decreased with the increase of crystallization temperature,while the crystal melting point increased,the crystal radial growth rate reached the maximum at 125°C,which was 2.26μm/min.The DSC non-isothermal and isothermal kinetics methods were used to analyze the crystallization and melting behaviors of composite materials.The results showed that the strong heterogeneous nucleation effect reduced the PLA nucleation activation energy and the crystal growth barrier,thus enhanced the crystallization capacity and significantly accelerated the PLA crystallization process,therefore,PLA could crystallize under faster cooling rate and higher temperature and achieved a larger crystallinity degree.The macroscopic mechanical data of PLA composites displayed that the interfacial crystallization effectively enhanced the interfacial adhesion between the fiber and resin,which significantly promoted the bending and impact performance.(3)The chemical grafting method and dopamine-assisted co-deposition technique were used to construct three kinds of multi-scale structure of CF/CNT reinforcement(CF-g-CNT,CF@CNT,CF@OCNT),with PEI molecules of different molecular weights as the bridge between pristine CF and amino or carboxyl functionalized CNT(CNT@PDA or OCNT).When the PEI molecular weight was 10k,the grafting degree and uniformity of CNT on CF surface was the best.The surface physico-chemical characteristics and monofilament tensile strength of the above reinforcements were systematically compared.CF@CNT has realized the increase of the fiber specific surface area and surface active groups,without damage to the structure and strength of fiber itself,meanwhile,maintained the high aspect ratio of CNT as well as resolved the dispersion problem of nano-reinforement.CF@CNT reinforcement increased the physical and chemical bonding of fiber and resin,improved the interface condition,thus promoted the macro flexural and impact properties,dynamic mechanical properties and thermal stability of PLA composites.Apparent activation energies of different composites were calculated to quantitatively characterize the interfacial interaction and the interfacial enhancement mechanism were illuminated.Finally,a series of PLA composites with different CF@CNT content were prepared,the relationship between rheological,mechanical and thermal properties of composite materials and the filling amount were analyzed.(4)With pyrrole(Py)monomer instead of PEI molecule,Polypyrrole(PPy)and CNT were directly loaded onto CF substrate surface by a facile in situ polymerization co-deposition method to prepare of self-supported flexible composite electrode CF@PPy-PDA-CNT.CF@PPy-PDA and CF@PDA-CNT electrodes with single active material(PPy or CNT)were also prepared as comparisons.The surface morphology,specific surface area and mechanical strength of the different electrodes were systematically compared.The results showed that PPy and CNT formed continuous three-dimensional network structure on the fiber surface by the PDA connection,which endowed the CF@PPy-PDA-CNT electrode high specific surface area,good electrical conductivity and excellent tensile strength.The electrochemical properties of the above fiber electrode was measured under three electrode system with KOH and H3PO4 as electrolytes,respectively.The results showed that the stability and capacitance performance of fiber electrodes were better in H3PO4 electrolyte,and CF@PPy-PDA-CNT electrode exhibited the highest electrochemical activity,compared to single PPy or CNT component loaded electrodes.Solid flexible supercapacitor were prepared with composite fibers as electrodes and their application were explored.The results showed that the supercapacitor based on CF@PPy-PDA-CNT electrode displayed superior electrochemical performance and cycle stability with a specific capacitance of 426.9 uF/cm under a current density of 6.67μA/cm,and the capacitance retention rate was as high as 85%after1000 cycles.The fiber supercapacitor also showed high flexibility and still maintained high electrochemical performance in the different bending states,which was expected to be used in flexible wearable electronic devices.