The Mechanical Behaviors Of Shape Memory Polymers And Their Composites

As a typical smart material,shape memory polymer(SMP)has aroused much attention for its multiple actuated methods(thermal,light,magnetic or electrical field,water and solution),large recovery strain(above 400%),low density(1.04g/cm3)and good manufacture,which give SMP great potential applications in the field of textile,biomedical,aerospace and MEMS.Compared with the general polymers,shape memory polymer will highly develop the intelligent systems and structures.In this thesis,we focused on the mechanical properties of epoxy and styrene based shape memory polymer and their composites,and investigated the influence of loading conditions on the mechanical behaviors of shape memory polymer.A new phase transition model was developed to explain the phase transition behavior between the active and frozen phase.In order to describe the mechanical behaviors of shape memory polymers and their composites during the thermal-mechanical cycle,the three dimension constitutive model was proposed and verified through comparing with the experimental results.First,the dynamic and static mechanical analysis experiments were conducted on the epoxy and styrene based shape memory polymers.From the dynamic mechanical analysis experiments,the phase transition temperature was measured,as well as the relationship between the storage modulus and temperature.And the elongation at break and the mechanical behaviors of epoxy and styrene were investigated through the stretching experiments at different strain rates and temperatures.When the specimen were loaded at a constant stress during the heating process,the effect of the stress on the phase transition temperature was also concluded.In addition,the mechanical behaviors of styrene under cyclic loading experiments were tested for their better practical applications.Second,the shape memory effect,which is the key factor of shape memory polymer,was effectively accounted using the phase transition model as reported in the previous publications.Here,we proposed a normal distribution model to describe the phase transitions during the cooling and heating processes.Compared with the other models built before,the newly built normal distribution model can characterize the volume fraction of frozen phase more exactly and all of the parameters appeared have their own physical meanings.Third,the three dimension constitutive model was developed based on the viscoelastic and phase transition theories.In this model,the strain was composed by four parts:the strain of active phase,the strain of frozen phase,the storage strain and the thermal strain.In this paper,the relationships between the strain and stress at each steps including deformed at a high temperature,strain storage and shape recovery during heating process were discussed in detail.And the storage strain,the key factor of shape memory effect,was given as the function of temperature.The rationality and precision of the new constitutive model was finally verified through the comparison with the experiments.Fourth,the applied stress has an unignorably effect on the phase transition temperature as mentioned previously.Through taken the effect of stress on the phase transition temperature into account,the phase transition model were revised,as well as the three dimension constitutive model.As a rate-dependent material,the mechanical properties of shape memory polymer were influenced significantly by the strain rate.Thus the modified constitutive model in which the strain rate was introduced was developed and compared with the experiments.Finally,the plate and tube based the carbon fiber reinforced shape memory polymer composites(SMPC)were fabricated using the filament winding method.And their three point bending experiments at room and high temperatures were completed,as well as the shape recovery experiments at a high temperature.As indicated in the experiments,the SMPC with a higher volume fraction of carbon fiber will perform better mechanical properties.Most important is that all of the deformed specimen can fully recover to their original shape when exposed to a high temperature environment.Based on the bridging model,the constitutive model was proposed to describe the mechanical behaviors and shape memory effect of shape memory polymer composites.All of these efforts indicate that SMPC with a high carbon fiber volume fraction will show their great application prospects in the future.