The demand for new types of actuators continues to grow, and novel approaches have been made possible by the advent of new materials and fabrication strategies. Self-powered actuators have attracted significant attention owing to their ability to be driven by elements in ambient environments. This type of actuator can be used in flexible strain sensors, artificial muscles, soft robotics, and smart breathing textiles. However, petrochemical-based polymers are generally environmentally unfriendly and cause ecological problems. The use of biodegradable polymers is one of the preferred solutions to ecological problems. Polylactic acid is a biodegradable and biocompatible polymer with a high potential. In this study, nanoclay reinforced polylactic acid/thermoplastic polyurethane was used as a precursor. The yarn that was produced was highly twisted. The twisted yarn was then shaped into a coiled structure via mandrel annealing. An apparatus was designed to investigate the thermal actuation behavior of twisted coiled yarn in an isometric state. The blocked force and free stroke were calculated in an isometric state by using linear material equations. The thermal actuation behavior of the twisted coiled yarn was also studied in the isotonic state. This precursor exhibited a considerable two-way shape-memory effect in a twisted coiled structure. It also showed a significant reversible contraction stroke within the ambient temperature range. The theoretical stroke was determined using two different models: the force–stroke equation and spring mechanics. The theoretical results were compared with the experimental results, which revealed acceptable agreement between the theoretical and experimental values.
Shape-memory materials are a promising new class of smart materials with many applications such as strain sensors, artificial muscles, and smart breathing textiles. These materials are subjected to force and extension in situ. Thus, the time-dependent behavior of these materials can play an important role in their long-term performance. The present study was conducted to investigate the time-dependent behavior of a type of shape-memory fabric. Nanoclay-reinforced polylactic acid/thermoplastic polyurethane was used as the precursor. The yarn that was produced was highly twisted. The twisted yarn was then shaped into a coiled structure by mandrel annealing. This yarn was then used to produce knitted fabric. The fabric was examined under both cold (25 °C) and hot (50 °C) conditions. The fabric contracted in hot water in the course direction but did not show a significant contraction in the wale direction. It returned to its original width in cold water. This effect was observed repeatedly over several cycles. This shows that the knitted fabric composed of the precursor twisted-coiled yarn exhibited low-temperature actuation and reversible two-way shape-memory behavior. A value of %16 was calculated for the contraction stroke along the course direction. The stress relaxation behavior of the two-way shape-memory fabric was then studied and analyzed. For this purpose, four different viscoelastic models were considered: the standard linear model (a), Burgers model (b), Jeffrey model (c), and Kelvin-Voigt-Maxwell model (d). We used the curve fitting procedure to find the best fit to the experimental data based on the least-squares method. The results showed that the Kelvin-Voigt-Maxwell model (model d) exhibited a higher and more acceptable regression coefficient (R2) than the other three models. The Jeffrey model showed the lowest regression coefficient (R2), thus confirming that it is not suitable for explaining the relaxation behavior of the fabric. However, a limitation of Model (d) is that it is not in line with the experimental loading stage. To modify the model, we propose the replacement of the dashpot with a dynamic frictional element. The results indicate that the proposed dynamic friction model can eliminate the limitations of the dashpot body during the loading stage.
Shape memory polymers are strong competitors against shape memory alloys in many similar applications due to their inherent properties. Numerous studies have already reported shape memory properties of pure and blended polymer systems. However, further research is required to address property–structure interplays in nano‐reinforced shape memory composites of low programming temperature with specialty applications including wearing smart cloths, advanced medical devices and biomedical shape memory scaffolds. This research aims to evaluate the overall interplay amongst thermal, structural and viscoelastic properties of nanoclay reinforced polylactic acid (PLA)/thermoplastic polyurethane (TPU) shape memory blends whilst the use of a plasticizer is hypothesized to modify the programming temperature of the fabricated parts. To achieve this, the melt mixing method was utilized in fabrication of nanoclay filled PLA/TPU blends of 60/40 ratio as the optimized reference system. Polyethylene glycol (PEG) was used to reduce the stimulation temperature down to around 42°C. The results showed that the optimal thermal, structural and shape memory properties are obtained when 5 wt% of nanoclay and 10 wt% of PEG are incorporated into the reference blend exhibiting the targeted stimulus temperature as supported by the thermal, structural and shape memory response of the parts. The contact angles, surface energy values and transmission electron microscopy analysis further confirmed the improvement in the miscibility of PLA/TPU blends correlated to the overall shape memory response of the fabricated parts.
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