Research On Mechanical Behavior Of Shape Memory Polymers In Multi-transition

Shape memory polymer is a kind of widely used smart materials.Due to its advantages of light weight,large deformation,good biocompatibility and low processing cost,it has been preliminarily applied in aerospace deployable structure,biomedical devices,etc.With the development of 3D printing technology,the simple 3D printing samples are no longer satisfied with needs,but hoped to be comparable to or even beyond traditional processing methods in producing components.In recent years,3D printing technology has shown unique advantages in functionalization and intellectualization,among which4 D printing is one of the most potential technologies.4D printing technology is based on 3D printing of smart materials,which can realize 3D printing structure to change shape or other properties under specific stimulation.As a relatively developed smart material,shape memory polymer has more advanced progress in 4D printing.Due to the thermosetting shape memory polymers cannot be directly printed by fused deposition method,shape memory polymers with variable original shape have attracted more and more attention,including thermoplastic shape memory polymers and a class of thermadapt shape memory polymers with dynamic covalent networks.4D printing uses the residual stress stored in the printing and post-processing stage to achieve reshaping,which requires the quantitative description of the mechanical behavior of shape memory polymer.The traditional research on the mechanical behavior of shape memory polymers mainly focuses on the amorphous thermosetting shape memory polymers.Compared with thermosetting shape memory polymers,there are several transition behaviors,such as glass transition,crystallization transition and viscous flow transition.The deformation history of shape memory polymer at different temperatures will have a long-term impact on the mechanical behavior of materials,so it is necessary to establish a comprehensive constitutive model with physical significance.In order to solve the problem of mechanical analysis in 4D printing process,the constitutive models of amorphous thermoplastic shape memory polymer and crystalline thermadapt shape memory polymer were developed.Firstly,the basic framework of the constitutive model of shape memory polymer is established by using the concept of multi branch model and intermediate configuration.The constitutive models of Maxwell model,standard linear solid model and generalized Maxwell model under small deformation and finite deformation are derived by using Clausius-Duhem inequality,and the calculation equations of elastic constitutive relation are given.Secondly,a thermodynamical frame finite deformation constitutive model of amorphous thermoplastic shape memory polymer is developed.The model adopts four element rheological model,including two Maxwell branches of network flow and chain flow.Arrhenius equation and WLF equation are used to describe the glass transition and viscous flow transition,respectively.The isotropic eight chain model established by Arruda Boyce is improved,and the concept of effective unit cell is proposed to predict the elastic anisotropy evolution behavior of shape memory polymers.Finally,a thermodynamical framework finite deformation constitutive model of crystalline thermadapt shape memory polymer is developed.Based on the model in Chapter3,the rule of phase transition is proposed for inelastic deformation of glassy phase,which includes shape memory effect and stacking of crystal segment.The model can well predict the thermal remodeling process with elastic evolution,two-way shape memory effect and other thermodynamic test results.