Glass Transition Constitutive Model Of Shape Memory Polymers And Its Applications On The Design Of Composites/structures

The viability of animals and plants strongly relies on their ability to adapt and respond to the environment.In recent years,many artificial materials have been found to have the ability to respond to environmental stimuli.Among these materials,the shape memory material is one of the most famous ones.Shape memory materials can change their shape under a given external stimulus,by which the macroscopic structures can achieve a spontaneous deformation.Recently,shape memory polymers have attracted wide attention from many commercial enterprises and academic institutes because of their simple process,diverse stimuli,low cost and great biocompatibility,and therefore have a huge market of commercial application,such as space deployable structures,stents,and deformable devices.In recent years,it has been reported that the shape memory effect of amorphous polymers is closely related to the relaxtion and the glass transition.When the temperature is much higher than the glass transition temperature(_gT)of polymers,its relaxation time is very short,the material shows elastic behaviors,and the deformation history of polymers would not changes with time.However,by cooling the shape memory polymer from high temperature to low temperature,the polymer experiences the glass transition,whose relaxation time becomes very long.In this circumstance,the polymer shows visco-elasticity,and its deformation history experiences a memory degradation with time.Thus,by using the tuning effect of temperature on the relaxation time,the amorphous polymers can achieve the shape memory effect.Under the thermal excitation,a typical shape memory process contains two steps:programming and recovering.During the shape programming process,the polymer was first heated up above its _gT,and deformed to a temporary shape under external loading.After the programming,the polymer was then cooled down below its _gT,during which the glass transition induced an obvious increment of relaxation time,and therefore this temporary shape was memoried by the polymer.During the shape recovering process,heating reduced the relaxation time,and the shape gradually recovered the original one.The reversible change of relaxation time under the thermal excitation makes the mechanical constitutions of polymers very insteresting,which provides a broad stage for the researchers in the field of solid mechanics.Additionaly,a reliable and concise theoretical model paves the way for the design of shape memory composites and devices.With the purpose of exploring the shape memory mechanism of polymers induced by its glass transition and quantitatively describing the continuous variations of material properties during the glass transition,this thesis built a viscoelastic constitutive model for amorphous shape memory polymers,which considers the influence of stimulus-imposing rates(here is the temperature rate).Using this model,this thesis furthur designed and fabricated a kind of two dimensional composites to tune the in-plane properties,and the smart hinges to achieve controllable shape changes.Additionaly,this thesis explored the enhancement mechanism of nanofiber filled shape memory nanocomposites,based on the flow properties of the viscous matrix.Finally,a three-dimensional viscoelastic-plastic-damage model of semicrystalline polymers under a large deformation was built for the more complex semicrystalline shape memory polymers,which provided a theoretical reference for the shape memory process of semicrystalline polymers under a large deformation.First,given the micro motion of molecular chains,a temperature-rate dependent shape memory constitutive model was established based on the generalized Maxwell model.By introducing the entropic temperature,the hysteresis of relaxation time with temperature change was modeled precisely.Comparison with simulation and experiments validated the developed model.Under variable temperature rates,the model can accurately predict both the viscoelastic evolutions and the shape memory behaviors.Then,based on the developed model,a new type of two-dimensional staggered composites was designed and fabricated by using soft inclusions to fill a hard mesh.The in-plane viscoelasticity and fracture behaviors of composites can be tuned by inclusion geometries,material properties and volume fractions.Compared with the traditional staggered composites with hard inclusions in a soft mesh,this new staggered feature gives the composite a wider tunable range of viscoelasticity,higher stiffness and greater tensile toughness.Based on the development of flexible electronics,smart hinges with shape memory polymers and flexible circuits were designed and fabricated.The recovery process is driven by the thermoelectric couple circuit,and simultaneously the recovery stain is monitored by the sensing circuit.Combined with the feedback control program,the recovery angle is precisely controlled.The enhancement mechanism of nanofiber reinforced shape memory composites was investigated by indentation tests.The O&P criterion for indentation pile-up effects,which describes the appearance of material accumulations besides the indent,is invalid for the nanofiller reinforced composites.The enhancement effect of nanofibers strongly depends on the testing scale.When the testing scale is smaller than the nanofibers'skeleton scale,nanofibers cannot effectively prevent the flow of the viscous matrix.Thus,nanofibers can only retard the long-distance motion of viscous matrix,and induce the pile-up effects besides the indent.In the last chapter,we experimentally studied the three dimentional mechanical properties of the semicrystalline polymer Polyether ether ketone(PEEK),and then built a constitutive model considering the viscous-elastic-plastic-damage behaviors.In the experiments,we used a two-step loading,uniaxial tension and transverse compression.In the uniaxial tension,voids initiated and evolved,and a significant volume expansion was observed.After the uniaxial tension,the stretched samples were cut into cubic samples to perform the transverse compression.During the transverse compression,the transverse moduli and yield stress were observed to decrease with the pre-stretching ratio.Based on the experimental observation,both the volume expansion and a decrease of the transverse moduli and yield stress were attributed to the void growth in our constitutive model.Comparison with simulations and experiments in various loading histories,temperature and strain rate validated the developed model,which can effectively predict the three-dimensional mechanical properties of PEEK.