Syunichi Hayashi

Shape recovery and irrecoverable strain control in polyurethane shape-memory polymer

Abstract In shape-memory polymers, large strain can be fixed at a low temperature and thereafter recovered at a high temperature. If the shape-memory polymer is held at a high temperature for a long time, the irrecoverable strain can attain a new intermediate shape between the shape under the maximum stress and the primary shape. Irrecoverable strain control can be applied to the fabrication of a shape-memory polymer element with a complex shape in a simple method. In the present study, the influence of the strain-holding conditions on the shape recovery and the irrecoverable strain control in polyurethane shape-memory polymer is investigated by tension test of a film and three-point bending test of a sheet. The higher the shape-holding temperature and the longer the shape-holding time, the higher the irrecoverable strain rate. The equation that expresses the characteristics of the irrecoverable strain control is formulated.

Influence of strain-holding conditions on shape recovery and secondary-shape forming in polyurethane-shape memory polymer

It was found in previous work on the thermomechanical properties of the polyurethane-shape memory polymer foam that the shape fixity and shape recovery become imperfect and that secondary-shape forming appears, depending on the strain-holding conditions. The main factors of the strain-holding conditions which affect the secondary-shape forming are the holding temperature, holding time and holding strain. In the present study, the influence of the strain-holding conditions on the shape recovery and secondary-shape forming was investigated for the polyurethane-shape memory polymer film. It was found that the secondary-shape forming appears markedly if the holding temperature is higher than the glass transition temperature and does not appear if the holding temperature is lower than the glass transition temperature. The rate of secondary-shape forming increases with an increase in the holding time if the holding temperature is higher than the glass transition temperature. The irrecoverable strain increases in proportion to the holding strain.

Bending Actuation Characteristics of Shape Memory Composite with SMA and SMP

A shape memory composite (SMC) belt with a TiNi-shape memory alloy (SMA) wire fiber and a polyurethane-shape memory polymer (SMP) sheet matrix is fabricated. The bending actuation characteristics of the belt are investigated by the thermomechanical tests. The results obtained are summarized as follows. (1) Residual deflection close to the maximum deflection is obtained by cooling under constant maximum deflection. The residual deflection disappears by heating under no load. Both the rate of shape fixity and the rate of shape recovery are close to 100%. (2) Recovery force appears by heating under constant residual deflection. The recovery force is 93–94% of the maximum force. The development of high functionality of SMC elements is expected by various combinations of SMAs and SMPs.

Shape fixity and shape recovery of polyurethane shape-memory polymer foams

Abstract The thermomechanical properties of polyurethane shape memory polymer (SMP) foams were investigated experimentally. The results obtained can be summarized as follows. (1) By cooling the foam after compressive deformation at high temperature, stress decreases and the deformed shape is fixed. Stress decreases markedly in the region of temperature below the glass transition temperature Ts during the cooling process. (2) By heating the shape-fixed foam under no load, the original shape is recovered. Strain is recovered markedly at the temperature region in the vicinity of Tg. (3) The ratio of shape fixity is 100 per cent and that of shape recovery 98 per cent. Neither ratio depends on the number of cycles. (4) Recovery stress increases by heating under constraint of the fixed shape. Recovery stress is about 80 per cent of the applied maximum stress. Relaxed stress at high temperature is not recovered. (5) The shape deformed at high temperature is maintained for six months under no load at Tg’60 K without depending on maximum strain, and the original shape is recovered by heating thereafter. (6) If the deformed shape is kept at high temperature, the original shape is not recovered. The factors influencing the shape irrecovery are the holding conditions of strain, temperature, and time.

Hydrogen-Induced Blistering of SiC: The Role of Post-Implant Multi-Step Annealing Sequences

Hydrogen-exfoliation has become a viable approach to transfer SiC thin layers onto different substrate materials. However, little attention has been paid to the exfoliation-inducing annealing conditions. To investigate the annealing conditions, 4H SiC wafers were implanted with either 2.5×1016 H2 + cm-2 or 5.0×1016 cm-2at 37 KeV. Post-implant, multi-step annealing sequences were examined in order to promote more efficient blistering, and it was found that a low temperature initial annealing step (T ≈ 500°C) can decrease the annealing time necessary in the high temperature regime; this was attributed to a nucleation of hydrogen induced platelet defects during the low temperature annealing regime and efficient splitting during a higher temperature (900 °C) anneal. This process is similar to what is observed for InP and Si exfoliation, except that the annealing processes occur at higher temperature.

Influence of Strain-Holding Conditions on Shape Recovery and Secondary-Shape Forming in Polyurethane-Shape Memory Polymer

By applying strain at high temperature to polyurethane-shape memory polymer film, followed by holding the strain under various thermomechanical conditions, the behavior of strain recovery by heating after these processes was investigated. In the experiments, the influence of the strain-holding conditions on shape recovery and secondary-shape forming was discussed. The results obtained are summarized as follows. (1) If strain is held at high temperature for short time, irrecoverable strain starts to appear at the holding time th =0.5 h and strain is not recovered at all at th =8 h in the care of holding temperature Th = Tg + 20 K. In the case of Th = Tg + 10 K, irrecoverable strain appears for short holding time if initial strain is large and the rate of secondary-shape forming S is 42% at th =8 h. (2) If initial strain of 50% is held at low temperature for long time, strain becomes not to be recovered and S is 93% at th=12 h in the care of Th = Tg + 10 K. In the care of Th = Tg, the increasing rate of S increases if th becomes longer than 40 h. If Th is lower than Tg - 10 K for th=72 h, strain is recovered perfectly by heating and secondary-shape forming does not appear.

Effects of Crystallinity on Hydrogen Exfoliation of GaN Layers

Exfoliation by hydrogen implantation of GaN grown on sapphire was successfully accomplished for the purpose of wafer bonding and layer transfer. Hydrogen ions were implanted at 60 keV, with low doses of 2.5 and 5.0x10 H2 /cm. High resolution x-ray diffraction and crosssectional transmission electron microscopy were used to investigate splitting kinetics. High temperature exfoliation (>400 °C) was determined to depend on the diffusion of hydrogen. Low temperature exfoliation (<400 °C) was found to be limited by the crystallinity of the GaN. Extremely defective GaN layers were unable to be exfoliated in this lower temperature regime which was attributed to hydrogen trapping from a high density of dislocations.