Zhi-xing Zhang

Thermal and electroactive shape memory behaviors of poly(L-lactide)/thermoplastic polyurethane blend induced by carbon nanotubes

Poly(L-lactide) (PLLA) based shape memory materials (SMP) recently attracted much attention due to their great potential application in biomedical materials. In this work, we introduced carbon nanotubes (CNTs) into a blend of PLLA/thermoplastic polyurethane (TPU) through a simple melt compounding processing to develop a new kind of PLLA SMP. The effects of CNTs on morphologies of blend composites, the selective location of CNTs, the dynamic mechanical properties of samples and the microstructure of CNTs in the blend composites were characterized using a scanning electron microscope (SEM), a transmission electron microscope (TEM) and rheological measurements, respectively. The results demonstrated that CNTs selectively located in the TPU component and induced apparent change of the blend composite morphology. High content of CNTs also formed the percolated network structure in the material, which resulted in the dramatic decrease of electrical resistivity. The shape memory behaviors of the samples were comparatively investigated in two different conditions, i.e. thermally activated and electrically activated conditions. It was demonstrated that CNTs prevents the thermally activated shape recovery process of the blend composites, especially when CNTs formed the percolated network structure. However, with the aid of electrical actuation, largely accelerated shape recovery process and enhanced degree of shape recovery were achieved for the blend composites containing 5 wt% CNTs, especially at relatively low recovery temperatures. The main mechanism was attributed to the largely enhanced electrical conductivity, which provides more Joule heating in the sample. This work provides an alternative way to develop PLLA-based shape memory materials through a simple melt compounding processing.

Largely improved fracture toughness of an immiscible poly(L-lactide)/ethylene-co-vinyl acetate blend achieved by adding carbon nanotubes

In this work, different contents of CNTs were introduced into an immiscible poly(L-lactide)/ethylene-co-vinyl acetate (PLLA/EVA) blend that exhibited a sea-island structure to further demonstrate the toughening probability of CNTs on the immiscible polymer blends. The fracture toughness was evaluated through impact measurements. The impact-fractured surface morphologies as well as the morphological changes of the blend induced by adding CNTs were characterized using scanning electron microscope. Furthermore, rheological properties and glass transition behaviors of samples were comparatively investigated to better understand the toughening mechanisms. The results demonstrated that although the presence of CNTs resulted in the formation of EVA particles with irregular shape and simultaneously increased diameters and matrix ligament thickness, which were usually thought to be unfavorable for the improvement of fracture toughness according to the Wus toughening mechanism, the blend composites still exhibited largely enhanced fracture toughness compared with the binary blend, and the impact strength increased gradually with increasing content of CNTs. The plastic deformation ability of PLLA matrix during the fracture process was greatly enhanced, especially when CNTs formed the physical network structure. Further results demonstrated that the glass transition behavior of EVA particles was greatly influenced by CNTs. Then, the toughening mechanism was proposed on the basis of the morphological changes of EVA particles, the formation of CNT physical network structure and the glass transition behavior of EVA. The other mechanical properties were also measured and analyzed. This work further demonstrated the toughening effect of CNTs on the immiscible polymer blends, and the methodology can be widely adopted in industry application.

Tunable shape memory behaviors of poly(ethylene vinyl acetate) achieved by adding poly(L-lactide)

In this work, different contents of poly(L-lactide) (PLLA) (20–50 wt%) were introduced into poly(ethylene vinyl acetate) (EVA) to prepare the samples with a tunable shape memory behavior. Morphological characterization demonstrated that with increasing PLLA content from 20 to 50 wt%, the blend morphology changed from sea-island structure to cocontinuous structure. In all the samples, PLLA was amorphous and it did not affect the crystallization of polyethylene part in the EVA component. The presence of PLLA greatly enhanced the storage modulus of samples, especially at relatively low temperatures. The shape memory behaviors of samples were systematically investigated and the results demonstrated that the EVA/PLLA blends exhibited a tunable shape memory effect. On one hand, PLLA accelerated the shape fixation and enhanced the fixity ratio of samples. On the other hand, PLLA reduced the dependence of shape fixity of samples on fixity temperatures. Specifically, for the first time, a critical recovery temperature was observed for the immiscible shape memory polymer blends. In this work, the critical recovery temperature was about 53 °C. At recovery temperature below the critical value, the blends exhibited smaller recovery ratios compared with the pure EVA, however, at recovery temperature above 53 °C, the blends exhibited higher recovery ratios.

Electrically/infrared actuated shape memory composites based on a bio-based polyester blend and graphene nanoplatelets and their excellent self-driven ability

Multi-stimuli-responsive shape memory polymers (SMPs) have a wide range of potential applications in many fields. In this work, novel SMPs with electrically actuated and infrared-actuated shape memory effects (SMEs) have been developed through the incorporation of graphene nanoplatelets (GNPs) into the poly(caprolactone) (PCL)/polyurethane (PU) blend through a mechanical compounding process. The dispersion of GNPs, the morphology of the blend composites and the crystalline structures of the PCL component were systematically investigated. The results demonstrated that although GNPs did not influence the miscible state of the blend, at a relatively high load they formed a percolated network structure, which apparently influenced the crystalline structure of the PCL component. The electrical resistivity measurements showed that the blend composites had excellent electrical conductivity properties, and the percolation threshold was about 1.62 wt%, which endowed the samples with excellent electrically actuated SMEs. By detection of the variation of the sample surface temperature under the infrared (IR) illumination conditions, the excellent photothermal effect of the blend composites was demonstrated. A largely accelerated shape recovery process under the IR illumination conditions was achieved for the blend composite samples. Specifically, by constructing the oriented structure in the sample and then introducing IR illumination, the sample could accomplish the complex deformation, exhibiting the excellent self-driven ability. The dual-responsive SMEs of the PCL/PU/GNP blend composites endow the material with wide potential applications in many fields, such as smart switches, robot hands and biomedical devices.