Thermodynamic Constitutive Equation For The Complex Molecular Structure System Of Shape Memory Polymers

Emerging smart materials blured the boundary of traditional functional materials and structural materials,which represents the development direction of the next-generation applied materials.Shape memory polymers(SMPs)are one kind of smart materials,whose shape memory behavior can be triggered under light,temperature,solution/water,enzyme and electromagnetic field.Meanwhile,SMPs possess simple preparation process,low manufacturing cost and good biocompatibility.SMPs have been widely studied by researchers from all over the world and applied for aerospace and biomedical fields such as self-expanding structure,deformable wing,drug release and heart stent.To further expand the potential application for SMPs,the development direction of the next-generation SMPs focuses on improving the versatility and enhancing the driving force.To expand the versatility,researchers have developed multi-SMPs and multi-field SMPs.To enhance the driving force,shape memory copolymer systems,fiber-reinforced shape memory composites and nanoparticle-reinforced shape memory composites have been further developed.These two types of materials are prepared by adding new component elements into the traditional SMPs to form a complex macromolecular structure system achieving the function of engineering requirements.However,due to the short research time,the relative working mechanism is not clear,and the corresponding constitutive models are urgently needed.On this motivation,the dissertation starts from the perspective of glass transition,andthermodynamic constitutive equations are established for SMPs with complex molecular structures.In this way,the working mechanisms are explored,and the behavior of macroscopic shape memory performance undergoing preparation and shape recovery are quantitatively described.Based on the in-depth understanding of the working mechanism,this study guides the application and development of SMPs with complex molecular structures.By introducing the Takayanagi principle into the phase transition theory,we exploredthe working mechanism of dual-SMPS composed of one soft component and one hard component.Effects of connection mode and interactions on the thermo-mechanical behavior and SME of dual-SMPs were further discussed.The Takayanagi principle was then extended to study the multi-SME and the segmental decrement of the storage modulus with the increase of temperature.The obtained cooperative relaxation mechanism of multi-components materials lays the foundation for the dissertation.Based on the obtained cooperative relaxation mechanism,a dynamics model of multi-SMPs was established by further combining it with the rheological model.Effects of heating history and external force field on the shape recovery and preparation process of multi-SMPs were firstly investigated,respectively.Subsequently,a thermodynamic model was established to characterise effects of weight fraction and molar mass of monomer of each component on the glass transition temperature(T_g)of shape memory copolymer.The obtained theoretical results were further substituted into the Weibull equation,and the relationship between shape recovery strength and the structural design was established.And then,the shape memory behavior of copolymers with different structural designs were predicted using the obtained parameters,and the enhancement mechanism of shape memory copolymers was revealed.This proposed model provided a theoretical basis for the design of copolymers with high recovery strength.On the other hand,a coupling thermodynamic model was established to reveal the enhancement mechanism of shape memory nanocomposites,which can regulate the thermodynamic behavior of its components,as well as the entanglement and coupling constraint mechanisms behind it.Based on the Adams-Gibbs entropy theory,effects of the mass fraction and size of nanoparticles on the modulus,relaxation time and T_g of the nanocomposites were established from the view of entropy.Furthermore,the viscoelastic relaxation behavior and the recovery behavior of nanocomposites undergoing preparation process were predicted by this model,which provided a solution for the design of the optimal shape memory nanocomposites.Finally,a cooperative model was established for the multi-stimuli responsive SMPs,and effects of bound solvent on the T_g,relaxation time,storage modulus and stored strain of SMPs were systematically studied based on the proposed model.Based on the discovered thermochemical cooperative mechanism,a multi-SMP system driven by thermochemical field is theoretically designed to provide a theoretical basis for improving the versatility of smart materials.