Effects Of Moisture Ingress On The Bond Between Carbon Fiber And Epoxy Resin Investigated With Molecular Dynamics Simulation

Considering the long service life of FRP in extensive applications under extreme environment,it is inevitably to have a high requirment for its duribility.The water effect on the properties of the interface for FRP material is vital and difficult.Interfaces between CF and resin matrix are intrinsically inhomogeneous and anisotropic.Interface bonds include interdiffusion of elements,molecular entanglement,mechanical interlocking,electrostatic atraction,chemical reaction,etc.It is hard to achieve the mechanisms of the interaction between the interface through tranditional experimental methods and theories.In this paper,molecular dynamics methods are used to study the interface adhesion of CF/epoxy interface under dry and water ingress environment.The main results and conclusions are as follows:First,the atomic model of the carbon fiber / epoxy interface at the nanometer scale was established through Molecular Dynamics method.The configures of the epoxy resin under equilibrated state on the surface of the carbon fiber w ere calculated and in the environment of drying and water ingress environment respectively.As shown,with the intermolecular forces,water molecules are adsorbed onto the surface of the carbon fiber,immersed into the gap between the epoxy resin and the carbon fiber epoxy resin molecules.Thus leading to part of Epoxy resin chains detached from the carbon fiber surface.Water effectively deteriorate the epoxy resin and carbon fiber interface.Second,A quantitative analysis of the surface free energy variation of the carbon fiber/epoxy resin interface with and without water ingress w as performed using the enhanced sampling method metadynamics.Epoxy molecules were pulled away from the surface of the carbon fiber.The potential mean force of the CF/epoxy interface in the process were calculated by Metadynamics simulation.Compared to the dry environment,a reduction by 58% of the surface free energy was found with saturated water uptake.This results was comparable with that by the microbond test,in which carbon fiber/ epoxy samples were respectively treated with various immersion in water.Third,to study the mechanisms of moisture uptake-induced carbon fiber/ epoxy interface adhesive degradation,the effects of Van der Waals force and Coulomb forces between water and interface molecules were studied.The simulation results show that the van der Waals force between the water molecule and the epoxy resin molecules will slow down the movement of the molecular segments of the epoxy resin and prevent the debonding of the epoxy resin from the surface of the carbon fiber.Under the action of Coulomb force,hydrogen bonds are formed between water molecules and epoxy resin molecules.Epoxy molecules are promoted to escape from the surface of carbon fiber.During the process,water molecules immerse into the gap between epoxy resin and carbon fiber surfac.Intermolecular force b etween water and epoxy further expand the gap between the epoxy resin and carbon fiber molecules,reducing the interface adhesion.The simulated results are in good agreement with the water degradation results through micro-debonding experiment between carbon fiber and epoxy resin.As revealed,the Coulomb forces between water and epoxy molecules play a critical role in the interfacial debonding process.