Preparation Of Hyperbranched Polyamide-amine And Its Functional Application In Carbon Fiber/Epoxy Resin Composites

In the context of carbon neutrality and peak carbon emission strategies,hydrogen energy stands out as a clean and pollution-free source of high enthalpy,playing a crucial role in the green and low-carbon transformation of energy systems.The 70 MPa IV type hydrogen storage cylinder is a research hotspot in the field of on-board hydrogen storage due to its high hydrogen storage density.However,the large-scale application of Type IV hydrogen storage cylinders still faces challenges due to the high preparation costs and other factors.The cost of carbon fiber composites accounts for more than 75% of the total cost of cylinders.Using large tow fiber with high cost-effectiveness to further reduce costs and increase efficiency is a feasible solution.However,most resin matrices cannot match with commercially available large tow carbon fibers,and cannot meet the performance requirements of IV-type hydrogen storage cylinders for the toughness,flame retardancy of the resin matrix,and strongtough interfaces under actual working conditions.In this application context,this article focuses on common basic issues such as the toughening mechanism of the resin matrix,flame retardant modification method,and synergistic strengthening mechanism of the interface between large tow fiber and matrix.Based on the Michael addition reaction,three types of hyperbranched polyamide-amine(HPAA)with different branching degrees were designed and synthesized through a one-pot method.The correlation mechanism between the branching degree and hydrogen bond density of HPAA was systematically studied,and the functional application of HPAA in large tow carbon fiber composites was studied based on different functional characteristics of HPAA.The research mainly includes:(1)To address the high crosslinking density and brittleness of the resin matrix,the HPAA was selected as a toughening agent and dispersed into the epoxy resin matrix using an in-situ dispersion method.The effects of different HPAA addition contents on the curing activity,thermal stability,and mechanical properties of the resin matrix were investigated.Due to the presence of a large number of terminal amine groups,the incorporation of HPAA significantly reduced the initial curing temperature and reaction activation energy of the HPAA/EP composites,indicating a pronounced curing promotion effect.However,the soft structure of HPAA led to a reduction in the thermal decomposition temperature and glass transition temperature of HPAA/EP composites.It is worth emphasizing that the impact strength of HPAA/EP composites was improved without sacrificing tensile and flexural properties.The most balanced mechanical properties of the HPAA/EP composites were achieved when the HPAA addition was 6.0 wt.%.Compared to the unmodified EP,the tensile modulus,flexural modulus,bending depth,and impact strength of the HPAA/EP composites were increased by 31.6 %,22.2 %,30.3 %,and24.8 %,respectively.The toughening effect of HPAA could be attributed to the increased free volume and the formation of hydrogen bonding between molecular chains within the crosslink network.When subjected to external forces,the free volume could be compressed,and hydrogen bonds broken before covalent bonds,which allowed for greater absorption of impact energy.(2)To address the challenge of simultaneously improving the mechanical properties and flame retardancy of the resin matrix,a nitrogen/phosphorus functionalized graphene oxide(GO)based nano-flame retardant(PN-GOs)was prepared using HPAA and phytic acid(PA)as flame retardant functional components.The morphology,physicochemical properties,and their effects on the mechanical and flame-retardant properties of the resin matrix were systematically studied.The results indicated that the PN-GOs possessed intrinsic reactive interfaces due to the presence of active functional groups within the HPAA and PA structures,which resulted in excellent dispersion and compatibility with EP.The incorporation of PN-GOs at a loading of 1.0 wt.%led to a 20.4 % increase in flexural strength and a 42.7 % increase in impact strength for the PN-GOs/EP nanocomposites.This enhancement in mechanical properties was attributed to the reactive interface and uniform dispersion,which facilitated robust interfacial interactions with the matrix,thereby improving the efficiency of stress transfer.Additionally,the peak heat release rate(p HRR)and total smoke generation rate(TSP)of PN-GOs/EP nanocomposites were reduced by 60.0 and 43.0 %,respectively.The synergistic flame retardant effects of N and P elements promoted the dilution of the gas phase "fuel" and the formation of the dense char layer.(3)To achieve simultaneous improvement in the interfacial strength and toughness between CF and resin matrix,a spider silk-inspired interphase featuring high hydrogen density was constructed by introducing HPAA and GO onto the CF surface.The surface morphology,physicochemical properties,and interfacial mechanical properties of modified fibers were systematically studied.The results showed that the surface roughness,interfacial adhesion force,surface energy,and wetting behavior of the modified fibers(ECF)were improved.The active functional groups on the surfaces of HPAA and GO promoted the formation of a transition layer interface structure,leading to a significant increase in interfacial thickness.Consequently,the interfacial shear strength and toughness of ECF/EP composites were 94.5 % and 110.0 % higher than those of the unmodified fiber/EP composites,respectively.The simultaneous improvement of interfacial strength and toughness was attributed to the formation of the "nano fishing net" microstructure.The active functional groups of HPAA and GO could participate in the formation of the cross-linked network structure,allowing HPAA and GO to be anchored in the "nano fishing net".The anchored GO could cause crack deflection,passivation,and termination,improving stress transfer efficiency.Meanwhile,the hydrogen bonding within and between HPAA molecules could be disrupted before covalent bonding,allowing for the absorption of more energy and the concurrent improvement in toughness.