Applications Of Shape Memory Polymers Research Articles

Thermomechanics of shape memory polymers: Uniaxial experiments and constitutive modeling

Shape memory polymers (SMPs) can retain a temporary shape after pre-deformation at an elevated temperature and subsequent cooling to a lower temperature. When reheated, the original shape can be recovered. Relatively little work in the literature has addressed the constitutive modeling of the unique thermomechanical coupling in SMPs. Constitutive models are critical for predicting the deformation and recovery of SMPs under a range of different constraints. In this study, the thermomechanics of shape storage and recovery of an epoxy resin is systematically investigated for small strains (within ±10%) in uniaxial tension and uniaxial compression. After initial pre-deformation at a high temperature, the strain is held constant for shape storage while the stress evolution is monitored. Three cases of heated recovery are selected: unconstrained free strain recovery, stress recovery under full constraint at the pre-deformation strain level (no low temperature unloading), and stress recovery under full constraint at a strain level fixed at a low temperature (low temperature unloading). The free strain recovery results indicate that the polymer can fully recover the original shape when reheated above its glass transition temperature ( T g). Due to the high stiffness in the glassy state ( T < T g), the evolution of the stress under strain constraint is strongly influenced by thermal expansion of the polymer. The relationship between the final recoverable stress and strain is governed by the stress–strain response of the polymer above T g. Based on the experimental results and the molecular mechanism of shape memory, a three-dimensional small-strain internal state variable constitutive model is developed. The model quantifies the storage and release of the entropic deformation during thermomechanical processes. The fraction of the material freezing a temporary entropy state is a function of temperature, which can be determined by fitting the free strain recovery response. A free energy function for the model is formulated and thermodynamic consistency is ensured. The model can predict the stress evolution of the uniaxial experimental results. The model captures differences in the tensile and compressive recovery responses caused by thermal expansion. The model is used to explore strain and stress recovery responses under various flexible external constraints that would be encountered in applications of SMPs.

Shape-Memory Polymers for Microelectromechanical Systems

This paper investigates the use of shape-memory polymer thin films in microelectromechanical systems (MEMS). shape-memory polymers possess the capacity to recover large-strain deformations by the application of heat and are candidates for small-scale transduction. The key advantages of shape-memory polymers are their low material/fabrication cost coupled with their simplicity of integration/operation. In the present study, shape-memory polymers are spin coated onto a standard Si wafer and polymerized by thermal annealing. The thermomechanics of strain storage and recovery in the polymer films are studied using instrumented microindentation. The sharp microindents demonstrate full recovery at all load levels, establishing the feasibility of microscale actuation. The microindentation response of the polymer film is shown to depend on temperature and the cooling cycle during indentation. In turn, the subsequent recovery behavior of an indent depends on the thermal history during indentation. Indents performed at higher temperatures are larger in size, but have smaller stored strain energy compared to indents performed at low temperature. The larger stored strain energy in low temperature indents results in lower shape recovery temperatures. The effects of indentation temperature and load are systematically investigated to provide a framework for the use of shape-memory polymers in microsystems. Application of shape-memory polymers is demonstrated through the development of an active microfluidic reservoir. The reservoir was created by indentation at the end of a microfluidic channel and was activated by local heating. The collapse of the filled reservoir caused the motion of fluid down the microfluidic channel.

Thermomechanics of shape memory polymer nanocomposites

Shape memory polymers (SMPs) have the capacity to recover large strains when pre-deformed at an elevated temperature, cooled to a lower temperature, and reheated. The thermomechanical behavior of SMPs can be tailored by modifying the molecular structure of the polymer, or by using the polymer as a matrix for multiphase composites. Here we study the thermomechanics of a SMP polymer and its composites made by adding nano-scale SiC reinforcements. Our experimental study focuses on the thermomechanical behavior in three-point flexure. The results show that the SMP nanocomposites have a higher elastic modulus and are capable of generating higher recovery forces as compared to the SMP resin. When pre-deformed at a temperature well above the glass transition temperature, T g, the stress–strain response at the pre-deformation temperature governs the relationship between the recovery stress/strain and the corresponding constraining strain/stress. When pre-deformed at a temperature below T g, the recoverable stress/strain is not governed by the stress–strain response at the pre-deformation temperature. Rather, a peak recovery stress, which is less than the pre-deformation stress, appears at a temperature near T g. Ramifications of the results on future research efforts and emerging applications of SMPs and their composites are discussed.

Thermomechanical recovery couplings of shape memory polymers in flexure

Shape memory polymers (SMPs) have the capacity to recover large strains whenpre-deformed at an elevated temperature, cooled to a lower temperature andreheated. The thermomechanical recovery behavior of an SMP is examined inthree-point flexure for various pre-deformation and recovery conditions. Resultsindicate that when pre-deformed well above the glass transition temperature,Tg, the stress–strain response at the pre-deformation temperature governsthe relationship between the recovery stress/strain and the correspondingpre-deformation strain/stress. When pre-deformed at a temperature belowTg, the relationship between recoverable stress and strain level in the SMP is not governed bythe stress–strain response of the material at the pre-deformation temperature. Rather, apeak recovery stress, which is less than the applied pre-deformation stress, appears nearTg. Higher cooling rates during constraint lower the temperature necessary for complete shapefixity, but increase the recoverable stress level. Higher heating rates during recovery raisethe recovery onset temperature and decrease the peak recoverable stress. Ramifications ofthe results on future research efforts and emerging applications of SMPs are discussed.

形状記憶ポリマーの特性と応用

形状記憶ポリマーの特性と応用