Investigation Of Mechanical-electro-thermal Behavior Of Polymer Composites Based On Quasi-molecular Chain Model

Functional polymers and their composites can change shape or have functional response under external stimulations(force field,temperature field,electromagnetic field,chemical field,etc.,).Polymer based composites have great application potential in aerospace,intelligent bionics,machinery,new energy and other fields because of their excellent properties,such as large deformation,light weight,fast reaction and high energy density compared with traditional(e.g.,metals and ceramics).Because of the variety of polymer based composites,complexity of their microstructure and the inevitable multi-field coupling in practical application process,how to improve their multi-field coupling performances through structural optimization are the most important problems that should be solved.In this paper,taking functional polymers and their composites as main research object,the multi-field coupling theory is expanded by combining quasi-molecular chain model and viscoelastic theory.The concept of quasi-molecular has been proposed based on that size of molecular chain structure has extended from microscopic size to mesoscopic size,and the idea that physical properties,combination modes and structure forms of molecular are affected by its polymer material,geometry morphology and internal fillings;the physical properties of a single chain in quasi-molecular chain structure are redefined by analyzing the macroscopic physical phenomena of a single material,such as stress-strain relationship and temperature effect.On this basis,physical response and related optimization of quasi-molecular chain structures under physical fields are further analyzed,e.g.,force field,electric field and temperature field.The main works of this paper are follows:Firstly,a viscoelastic quasi-molecular chain model with quasi-truss structure was proposed by combining Kelvin-Voigt model,and mechanical properties of the polymer were studied.According to the deformation mechanism of the polymer specimen in actual uniaxial tensile test,the actual deformation of the polymer was equivalent to the ideal uniform deformation and the non-uniform deformation by taking into account the effect of fixed boundary and free boundary.Furthermore,the difference between real stress and tested stress of specimens with different sizes under different strain conditions was analyzed,and the effects of geometric size,strain level and pre-stress on the real stress was discussed qualitatively and quantitatively.Finally,numerical simulations of single step relaxation,loading-unloading,tensile-creep-relaxation and multi-step relaxation test of samples were conducted,the results show that the simulation results of the model were more reasonable than the predicted results of Yeoh model and Lochmatter model.Secondly,the viscoelastic quasi-molecular chain model based on distribution probability function was proposed by analyzing microstructure of polymer and integrating the quasi-molecular chain structure.The model can characterize the mechanical properties of dielectric elastomer(DE)effectively,including large deformation.The electromechanical coupling of DE can be realized by applying a voltage on the surface of DE which can be regarded as pressure effect,i.e.,Maxwell stress.Then,the effects of pre-stretch,voltage and geometry size on the electromechanical coupling of DE actuators were studied.For a DE actuator with complex geometrical morphologies or complex loading processes,the surface charge redistribution was discussed,and the Maxwell stress was recalculated based on the surface charge density.Simulation results show that this method was also suitable for shear-tensile tests.Thirdly,the effects of surface geometry,internal filler and local deformation on the distribution of chain structure of polymer composites were analyzed according to the viscoelastic molecular chain model quantitatively.Also,the applied field function can be corrected by local probability density function.Then,the electromechanical coupling model of polymer matrix composites was improved based on statistical mechanics and probability density function of molecular chain.Tensile tests and electro-mechanical coupling tests of typical dielectric thin film VHB4910 were simulated.Furthermore,the mechanical and electrical properties of CaCu3Ti4O12(CCTO)reinforced polydimethylsiloxane(PDMS)composite were studied.In addition,the shape memory process of shape memory polymer(SMP)and its composites was studied based on a viscoelastic quasi-molecular chain model including temperature effect,and shape memory effect(SME)was optimized.Structure of quasi-molecular chain model was composed of thermal and mechanical part:the properties of thermal part and mechanical part were defined by analogy with the macroscopic thermal-mechanical response of SMP.Then,the material parameters were determined by calculating the instantaneous elastic modulus of the molecular chain structure at different temperatures,and the following variables were considered in the simulations,such as chain length,chain number and distribution function.The thermal-mechanical coupling process of typical SMP polyurethane composites was simulated and compared with the experimental data to verify the applicability of the proposed model.In addition,SME of shape memory polymer composites was optimized by changing the geometry size,crosslinking density and internal filler of the model.The results show that increasing the crosslinking density of quasi-molecular chain can enhance the SME of polymer composites.Finally,a three-phase quasi-molecular chain model consisting of main chain,auxiliary chain and filler was established to investigate the hydrophobicity and permeability of polymer composite membranes.In this model,the local distribution probability density of the molecular chain was recalculated considering the interaction between internal fillers and chains,e.g.,repulsive and attractive.In this model,a global functionwas introduced to characterize the interaction between water and chain of polymer,which can be uniquely determined by the molecular chain structure and physical properties.Based on this model,the physical properties of silicon dioxide(SiO2)reinforced polyvinylidene fluoride(PVDF)composite membranes were investigated,e.g.,mechanical properties,hydrophobicity and permeability.Finally,performance optimization of the composite membranes was discussed by changing the position of filler and the effect of filler on the molecular chain.It is hoped that the model will provide an effective theoretical tool for the development of polymer composites.