Xiaodong Qi

Water-induced shape memory effect of graphene oxide reinforced polyvinyl alcohol nanocomposites

A novel water-induced shape memory polymer based on polyvinyl alcohol (PVA) was prepared by introducing graphene oxide (GO). Due to the strong hydrogen bonding interaction between PVA and GO, some additional physically cross-linked points could be formed in PVA, which largely improved shape memory properties of PVA. Solvent-induced shape memory behavior was observed by immersing PVA/GO nanocomposites in water. The water-induced shape recovery was due to the decrease of glass transition temperature and storage modulus. This could be explained by the swelling plasticizing effect of water on PVA, as indicated by the obvious expansion in volume of PVA. On the other hand, the weakened hydrogen bonding between PVA and GO was also observed after immersing the PVA/GO nanocomposites in water. Thus both the plasticizing effect and the competitive hydrogen bonding were the two main reasons for the shape recovery of PVA/GO nanocomposites. This study provides a framework for developing new shape memory polymers (SMPs) and for better understanding the shape recovery mechanism in solvent-induced SMPs.

Simultaneous reinforcing and toughening of polyurethane via grafting on the surface of microfibrillated cellulose.

In the present work, a series of thermoplastic polyurethane (TPU)/microfibrillated cellulose (MFC) nanocomposites were successfully synthesized via in situ polymerization. TPU was covalently grafted onto the MFC by particular association with the hard segments, as evidenced by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The adequate dispersion and network structure of MFC in the TPU matrix and the strong interfacial interaction through covalent grafting and hydrogen bonding between MFC and TPU resulted in significantly improved mechanical properties and thermostability of the prepared nanocomposites. The tensile strength and elongation-at-break of the nanocomposite containing only 1 wt % MFC were increased by 4.5-fold and 1.8-fold compared with that of neat TPU, respectively. It was also very interesting to find that the glass transition temperature (Tg) of TPU was decreased significantly with the introduction of MFC, indicating potential for low-temperature resistance applications. Most importantly, compared with TPU nanocomposites reinforced with other nanofillers, the TPU/MFC nanocomposites prepared in this work exhibited excellent transparency and higher reinforcing efficiency.

Microfibrillated cellulose-reinforced bio-based poly(propylene carbonate) with dual shape memory and self-healing properties

A novel, biologically friendly polymer with shape memory and self-healing properties based on poly(propylene carbonate) (PPC)/microfibrillated cellulose (MFC) was prepared. MFC was first modified by a one-step mechanical–chemical approach involving ball milling and esterification reaction. In this way, MFC could be incorporated into PPC at up to 20 wt% with excellent dispersion. The formation of the “MFC network” structure in the PPC matrix was verified by scanning electron microscopy, and the strong interfacial interaction between PPC and MFC was confirmed by X-ray photoelectron spectroscopy. The incorporation of MFC not only significantly enhanced the mechanical strength and thermal stability of the polymer, but also acted as a physical cross-linker that could improve the shape memory property of PPC at specific contents (5–10 wt%). More importantly, due to the shape memory effect and the reinforcement of MFC fibres, the polymer composites also showed enhanced scratch resistance and scratch self-healing behaviour. Our work provides an approach to tune the shape memory behaviours of polymer composites and may contribute to the application of PPC in the field of smart materials.

Preparation of polyvinylidene fluoride/expanded graphite composites with enhanced thermal conductivity via ball milling treatment

In recent decades, great efforts have been devoted to prepare materials with enhanced thermal conductivity due to the growing interest in thermal conductive materials. Herein, we illustrate a facile strategy to improve the thermal conductivity of polyvinylidene fluoride/expanded graphite (PVDF/EG) composites by pre-treatment of EG via ball milling. Before incorporating EG into PVDF via conventional melt processing, EG powders were treated by shear-force-dominated ball milling. In this way, the loose and porous vermicular structure of EG could be effectively destroyed and exfoliated to graphite nanosheets (GNSs). As a result, the PVDF/GNSs composites show improved thermal conductivity owing to their larger specific surface area. With the filler content fixed at 15 wt%, the thermal conductivity of treated PVDF/GNSs composites can reach 1.29 W m−1 K−1, 42.5% higher than that of PVDF/EG (0.90 W m−1 K−1). Moreover, the electromagnetic interference (EMI) shielding property and tensile strength of PVDF/CNSs composites are also remarkably improved. Our work proves to be a simple and easily industrialized method for EG treatment which has great potential for improving the thermal conductivity of polymer composites in lighting devices and electromagnetic shielding applications.

Microfibrillated cellulose reinforced bio-based poly(propylene carbonate) with dual-responsive shape memory properties

A novel biodegradable polymer-based composite with excellent dual-responsive shape memory properties based on poly(propylene carbonate) (PPC)/microfibrillated cellulose (MFC) was prepared. MFC was modified by a one-step mechanical–chemical approach involving ball milling and an esterification reaction to improve its dispersion. The shape memory properties of PPC/MFC-BR composites and PPC/unmodified MFC composites were compared, and the former showed better shape memory properties due to the uniform dispersion of MFC-BR which ensured a higher fraction of the interfacial zone than unmodified MFC. Here, MFC-BR fibers act as multifunctional cross-links, reinforcing fillers, and relaxation retarders. In addition, the composites with a MFC-BR content of 5–10 wt% showed a good shape memory effect upon exposure to water at 30 °C due to the hydrophilicity of MFC-BR. The mechanism was mainly ascribed to water molecules destroying the hydrogen bonding at the polymer–filler interfaces, which reduced the glass transition temperature and increased the flexibility of the polymer chains. Our work provides a composite approach to tune the shape memory behavior of polymer composites and may contribute to the application of PPC in the smart materials field.

Multishape and Temperature Memory Effects by Strong Physical Confinement in Poly(propylene carbonate)/Graphene Oxide Nanocomposites

The importance of filler-matrix interactions is generally recognized for mechanical property enhancement; their direct impact by physical confinement on diverse functional properties has remained poorly explored. We report here our effort in achieving versatile shape memory performances for a biodegradable poly(propylene carbonate) (PPC) matrix containing high contents of graphene oxide (GO). The excellent dispersion in the entire filler range (up to 20 wt %) allows precise morphological tuning, along with physical filler-matrix interactions, contributing overall to a strong nanoconfinement effect that positively affects the thermomechanical properties of nanocomposites. Only one glass-transition temperature (Tg) of PPC is detected when the GO content is below 10 wt %, corresponding to a slightly confined system, whereas two distinct Tgs are observed with a GO content over 10 wt %, corresponding to a highly confined system. As such, a tunable multishape memory effect can be achieved simply by tuning the filler contents. A dual-shape memory effect (DSME) is observed for a slightly confined system, whereas a triple-shape memory effect (TSME) can be achieved by deformation at two distinct Tgs for a highly confined system. More importantly, it is interesting to find that the switch temperature (Tsw) evolves linearly with the programing temperature (Tprog) for both slightly and highly confined systems, with Tsw ≈ Tprog for a highly confined system but Tsw < Tprog for a slightly confined system. Our work suggests a highly flexible approach to take advantage of the strong nanoconfinement effect by tuning the content of GO within a single polymer to access versatile SMEs, such as DSME and TSME, and the temperature memory effect.

A facile way to large-scale production of few-layered graphene via planetary ball mill

In the field of polymer/graphene nanocomposites, massive production and commercial availability of graphene are essential. Exfoliation of graphite to obtain graphene is one of the most promising ways to large-scale production at extremely low cost. In this work we illustrate a facile strategy for mass production of few-layered (≤ 10) graphene (FLG) via the newly explored ball milling. The achieved FLG concentration was determined by UV/Vis spectroscopy. The formation of FLG was proved by measuring the flake thickness by atomic force microscopy (AFM). Further Raman spectral studies indicated that the crystal structure of exfoliated flakes was preserved satisfactorily during this shear-force dominating process. To increase the maximum concentration obtainable, it’s critical to make a good parameter assessment. N-methylpyrrolidone (NMP) was used as a dispersing medium and the effect of milling parameters was systematically and quantitatively investigated, thus providing a criterion to optimize the milling process. We established the optimal values for solvent volume and initial weight of graphite. As for milling time, the production of FLG was enhanced with continuous milling according to the power law, but not linearly with increasing milling time. Moreover, the possible mechanism involved in milling process was also explored. Our work provides a simple method for graphite exfoliation and has great potential for improving thermal and electrical conductivity of polymer composites in the fields of engineering.

The different effect of reduced graphene oxide and graphene oxide on the performance of chitosan by using homogenous fillers

How reduced graphene oxide (rGO) and graphene oxide (GO) affect the performance of chitosan (CS) nanocomposites is discussed in this paper. A special two-step reduction method was used to prepare the CS–rGO nanocomposite films, firstly, the CS–GO nanocomposite films were prepared by the solution casting method, and secondly, the prepared CS–GO films were immersed into a reducing agent aqueous solution in which the reducing agents could diffuse onto the surface of the GO sheets and then reduce them. So this method could avoid the phenomena of aggregation, morphology change and rearrangement of the GO sheets, which would happen if they are directly reduced in the CS–GO solution with or without a surfactant. The results show that the loading of the two kinds of fillers can enhance the tensile strength of the nanocomposites, but the mechanism is different, one reason is due to the strong interfacial interaction between GO and CS, and another one may be due to the high mechanical strength of rGO and the recrystallization of the CS matrix during the reduction process. This work provides a new way to analyze the interfacial interaction between the filler and CS matrix, and also could be used in other polymer systems to find the essential mechanism of how the filler could affect on the mechanical properties of nanocomposites.

Preparation of Polylactide/Poly(ether)urethane Blends with Excellent Electro-actuated Shape Memory via Incorporating Carbon Black and Carbon Nanotubes Hybrids Fillers

In this work, hybrid conductive fillers of carbon black (CB) and carbon nanotubes (CNTs) were introduced into polylactide (PLA)/thermoplastic poly(ether)urethane (TPU) blend (70/30 by weight) to tune the phase morphology and realize rapid electrically actuated shape memory effect (SME). Particularly, the dispersion of conductive fillers, the phase morphology, the electrical conductivities and the shape memory properties of the composites containing CB or CB/CNTs were comparatively investigated. The results suggested that both CB and CNTs were selectively localized in TPU phase, and induced the morphological change from the sea-island structure to the co-continuous structure. The presence of CNTs resulted in a denser CB/CNTs network, which enhanced the continuity of TPU phase. Because the formed continuous TPU phase provided stronger recovery driving force, the PLA/TPU/CB/CNTs composites showed better shape recovery properties compared with the PLA/TPU/CB composites at the same CB content. Moreover, the CB and CNTs exerted a synergistic effect on enhancing the electrical conductivities of the composites. As a result, the prepared composites exhibited excellent electrically actuated SME and the shape recovery speed was also greatly enhanced. This work demonstrated a promising strategy to achieve rapid electrically actuated SME via the addition of hybrid nanoparticles with self-networking ability in binary PLA/TPU blends over a much larger composition range.

Gradient Polydopamine Coating: A Simple and General Strategy toward Multishape Memory Effects

Multi shape memory polymers (multi-SMPs) exhibit many potential applications such as aerospace, soft robotics, and biomedical devices because of their unique abilities. Although many works are done to broaden the preparations of multi-SMPs, the desire to a simple and versatile strategy as well as more complex shapes still exists. Moreover, a light-induced SMP shows more advantages than a thermal-induced one in many practical working circumstances. Herein, inspired by strong adhesion and efficient photothermal conversion of polydopamine (PDA) coating, we report a more simple and facile approach to prepare light-induced multi-SMPs by introducing a gradient PDA coating on a dual-SMP through time-controlled dipping. The photothermal converting properties with varying thicknesses of PDA under the tunable near-infrared light source are investigated. Then, light-induced multishape memory effects based on gradient PDA coatings are illustrated, where three designs of multi-SMPs - rectangle, triangle, and cross are prepared and demonstrated. Also, the evolutions of coating morphology during shape shifting are carefully studied. Finally, we present few complex designs of patterns and shapes as well as a design of potential application for the highly controllable smart devices. This strategy demonstrates a very simple and general strategy to design and prepare the light-induced multi-SMPs with complex shapes based on any thermal-responsive dual-SMPs.