| Flexible polymer nanocomposites exhibit excellent conductivity and sensing performance under large deformation,and have extremely significant and widespread applications in new wearable electronic devices,electronic skins/dexterous hands/intelligent prostheses,humanmachine interfaces,human health monitoring,etc.Different from traditional composite materials,the effect of interfacial interaction between nanofillers and polymers are greatly enhanced due to the increase of the surface area-to-volume ratio of the fillers at the nanoscale.Especially,the interfacial confinement effect on the nearby polymers is significant,generating an interphase region where the material properties can be dramatically different from the polymer matrix.These distinctive material properties will cross the length scale to exert significant impacts on the overall performances of nanocomposites through the percolation process.The confinement effect and mechanical behavior of the "nanofiller-polymer"interface thus have attracted considerable attention from domestical and international researchers.Although it is widely accepted that the interfacial confinement effect originates from interfacial interactions,there still exist some major issues,such as the coupling mechanism between interfacial interaction and physical changes of local polymer,the mechanistical pathway to generat the coupling and the key parameter that can be used to characterize the coupling effects.A comprehensive and in-depth exploration of these issues has important guiding significance for formulating the mechanical theory of nanocomposites,developing novel nanocomposites and studying their mechanical behavior and performance.Accordingly,an effort has been made to address the above issues in the following five aspects by combining the molecular dynamics(MD)simulations with theoretical mechanics models.Firstly,the coupling relationship between the interfacial interaction and physical properties of local polymer has been revealed.In this process,the cohesive model was employed to define the mechanical parameters characterizing interfacial interaction,e.g.,cohesive force and energy.Specifically,the mechanical responses of polyethylene to interfacial interaction has been explored based on the continuum mechanics model and the physical origin of interfacial confinement effect has been uncovered by MD simulations.It is found that the interfacial interaction between nanoparticle and polymer results in a peak compressive stress of the order of polymer yield stresses in the matrix.The concentrated compressive stress then leads to a peak density and the upshift of the glass transition temperature in the interphase.The increase of local density subsequently triggers a redistribution of the matrix density.A sinusoidal fluctuation with attenuation amplitudes is eventually achieved for the density profile.These results demonstrate that the interfacial confinement effect originates from the mechanical response of the polymer to the "nanofiller-polymer" interfacial interaction.Herein,the mechanical parameters quantifying the coupling effect of interfacial forces and polymer physics have also been identified.The hydrostatic stress defined as the average of the three primary stresses using the theory of continuum mechanics is found to serve as a key mechanical parameter to measure the confinement effect of interfacial interaction on nearby polymers.It thus plays an important role in studying the nanoscale-interfacial confinement effect.Hydrostatic stress is found to change the occupied volume of the polymer,restrict the mobility of the polymer chain,and eventually achieve a strong coupling effect with the material properties of the interphase,e.g.,glassy transitional temperature.Subsequently,the study has explored the impact of the shape and size of nanofillers and environmental temperature on the interfacial confinement effect and coupling process.The density and atomic stress distribution of the interphase are firstly characterized by MD simulations.Herein the interfacial interaction obtained by the cohesive model is combined with the stress state of interphase given by the continuum mechanics model.This enables one to analyse the influence of various factors on the coupling relationship of "interfacial interaction-interphase properties".The results show that the decrease of the surface curvature of nanofillers due to the changes in shape or size would increase the compressive hydrostatic stress in the interphase.Under such as stress,the free volume of the interphase decreases and the mobility of polymer chain reduces,which in turn results in a significant increase in the peak density and local glass transition temperature.In the meantime,the elevated temperature can trigger a redistribution of the interfacial interaction between nanofillers and polymer,reducing the concentration of the hydrostatic stress in the interphase and thus weakening the nanoscale confinement effect of the interface.The impact of these factors on the interphase will ultimately change the overall physical properties of the nanocomposites.Next,the study is focused on the mechanical response and physical changes of the interphase under the coupling effect of interfacial interaction and external loads.The radial mass density distribution of the interphase and interfacial force distribution under uniaxial load are calculated by MD simulations and their correlation is analyzed.The results show that the ellipsoidal evolution of the interface under uniaxial strain results in continuous changes in the distribution of interfacial interaction and overall weakens the interfacial confinement effect.Uniaxial tensile strain therefore can reduce the peak density of interphase and reshape the geometric distribution of density in radial and circumferential directions.These changes imply that the physical properties of the interphase,such as elastic modulus,glass transition temperature and electron tunneling energy barrier,exhibit a significant dependence on external strain.After that,the mechanical behavior of the nanoparticle-polymer interface under uniaxial load is studied.The shape changes of the interface are characterized by MD simulations,and the cohesive model is used to define the key parameters characterizing the interfacial interaction and mechanical behavior,i.e.,the cohesive stress σosi that can characterize the interaction between nanoparticle and polymer,and the equivalent elastic modulus Yint that can describe the interfacial adhesion states.It is found that the interface transfers the external load from the polymer to the nanoparticle via the cohesive stress σosi.With rising strain,three states of stability,metastability and instability characterized by positive,zero and negative Yint,respectively,are achieved for the nanoparticle-polymer interface.At the metastable state,interfacial debonding begins on both sides of the ellipsoidal interface and then extends towards the center with increasing strain at the unstable state. |
