Electro-and Solution-active Shape Memory Polymer Blends And Their Thermodynamic Constitutive Equation

Shape memory polymers (SMPs) offer a number of potential technical advantages that compared with its counterparts, namely shape memory alloys and shape memory ceramics, including high recoverable strain (more than 100%), low density, ease of processing and the ability to tailor the recovery temperature, programmable and controllable recovery behavior, and most important, low cost. These amazing advantages enable such materials to have a high innovation potential in application. The shape memory effect in SMPs can be utilized in many fields, from aerospace engineering, automobile, communications to biomedicine and many important fields. Although SMPs have found a few applications, they have not fully reached their technological potential. Largely due to that the actuation of shape recovery in thermal-responsive SMPs is normally driven by external heat. Another important issue is that there is a few of researches aimed at more complex shape memory behaviors resulting from lack of theoretical development and a short history of SMPs being as a novel smart materials. The mechanism behind these features can not be explored and discovered. As a result, the development and application of SMPs is seriously limited.On this motivation, the first aim of this project is to demonstrate the electro- activated shape memory effect of SMPs nanocomposites by blending hybrid filler and carbon nanopaper, respectively. In subsequence, solution-driven SMPs have been discovered and achieved on mixing with solvent. Finally, a thermodynamic constitutive equation is deduced and constructed to theoretically depict the shape memory behaviors of SMP composites or blends.The present work firstly studies the electro-activated shape memory behaviors of SMP nanocomposites by blending hybrid filler or carbon nanopaper in the third chapter of the dissertation. The morphology, glass transition temperature, thermomechanical and electrical properties of the SMP nanocomposites have been studied by the scanning electron microscope (SEM), optic microscope (OP), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), static mechanical test frame with a series of digital controllers, four-point probe measurement method and Van der Pawn method, respectively. The shape recovery of SMP nanocomposite driven by electrically resistive Joule heating is therefore demonstrated and recorded. And an infrared video camera is used to monitor the temperature distribution and shape recovery simultaneously. Based on these experimental results, I analyze and discuss the correlations among the component type, fraction, electrical properties and shape memory behavior of SMP composite.In chapter 4, the plasticizing effect, theory of polymer solution, rubber elastic theory and relaxation theory are employed to systemically prove that the thermal-responsive SMP can have solution-driven shape memory effect. For the styrene-based SMP, the demonstration of chemo-responsive shape memory behavior has been conduct on mixing with N,N-dimethylformamide (DMF) solvent through chemically conjugated interaction. Alternatively, it also can be carried out on mixing with toluene solvent through physical swelling effect. After being immersed into solvent, the thermomechanical behaviors and change in chemical structure of SMP are performed on thermal gravimetric analysis (TGA), DSC, DMTA and Fourier transform infrared spectroscopy (FTIR). In summary, the mechanism behind these new features is the imbibed solvent molecules have a plasticizing effect on the polymer network in the form of diffusion. The plasticizing effect makes the interactive forces among tangled polymer molecules depressed, resulting in the flexility and motion capability of polymer molecules improved. The solvent molecules then have a chemical or physical interaction with the polymer molecules. All these interactions between solvent molecules and polymer molecules will inductively make the transition temperature of polymer reduced. When the switching temperature of SMP arrives at the room temperature or lower, the shape memory effect of SMP is therefore induced, resulting from the stored strain energy is released in polymer molecules. As a result, the SMP will relax to its original shape from the temporary (or deformed) shape.For the thermo-responsive SMP, the shape memory effect obeys with the relaxation theory and its Eyring equation. It indicates that the shape memory effect is only determined by two parameters, the temperature and internal cohesive energy (namely chemical potential). The present chapter 5 aims at the shape memory effect of SMP induced by inductively lowering the chemical potential instead of temperature heating. Based on the solution theory of polymer and thermodynamic of polymer solution, there will be changes in entropy and free-energy functions, as well as other related thermal parameters, when a polymer is on mixing with a solvent or solid. In combination of thermodynamic equation of polymer solution and Eyring equation, we can qualitative separate the effect of volume fraction of solvent molecule (or polymer molecule), molar volume ratio of polymer molecule to solvent molecule and Flory-Huggins solubility parameter on the chemical potential of SMP. Finally, the free-energy equations of polymer solution are employed to construct the polymer's chemical potential-stretch and blend's stress- strain constitutive equations, which can be used to theoretically describe the behaviors of SMP blends in respect to chemo-mechanical couplings.