All issues / Volume 14 (2020) / Issue 4 (April)
This is an editorial article. It has no abstract.
In the present work, a new strategy for preparing authentic two-way shape memory polymer was proposed by using a conventional crosslinked polyurethane (PU) containing crystalline poly(ε-caprolactone) (PCL) as the proof-ofconcept material. Lauroyl peroxide (LPO) was added as a chemical crosslinker for inducing secondary crosslinking during the programming. Having been stretched and heat treated without additional ingredients and chemicals, the trained PU showed the desired two-way shape memory effect. The crosslinking network created by LPO was successfully converted into internal stress supplier, which represents the core progress of this research. As the temperature changed, the reversible melting/re-crystallization of the crystalline phases elaborately cooperated with the compressed crosslinking network, leading to the implementation of two-way shape memory effect. Through the optimization of the LPO quality, an average reversible strain of up to ~21% in the direction of stretching was measured. In principle, all semi-crystalline polymers can be imparted with two-way shape memory effect following the above-proposed method. Given the great convenience of material selection, preparation, programming and application, the current research may have opened a new way for the production and usage of the smart materials in practice.
In recent years, researchers across the globe have switched from synthetic fibers to natural fibers for the fabrication of composites. The desirable properties of natural fibers which attracted researchers, as well as academicians, are its low density, easy availability, environmentally friendly nature, biodegradability, and high specific strength. Hence in the last decade, there is tremendous progress in the development of natural fiber-reinforced composites for various industrial applications. The current review focused on the recent progress in natural fiber-reinforced polymer composite. The natural fibers discussed in this review are derived from leaves, namely pineapple, sisal, and abaca. The extraction and processing of these fibers are briefly outlined. The properties and application of natural fiber-reinforced composites are also addressed in this review. One of the drawbacks of natural fiber is its poor compatibility with the polymer matrix. The different treatment methods to improve the fiber-matrix interaction are also summarized in the present review.
In this article, we report a detailed study on the kinetics of the reaction of 1,3-xylylene diisocyanate (1,3-XDI) with alcohols of varying alkyl chain lengths and with low molecular weight polyols such as monomethoxylated polyethylene glycol (mPEG) and polytetrahydrofuran (PTHF) in toluene, in the absence of a catalyst. It was found that the pseudo-firstorder rate coefficient increased with the increasing alkyl chain length (k1,app was found to be 0.0166 and 0.0341 min–1 for 1-propanol and 1-hexanol, respectively) and the reactivities of the 1,3-xylyene diisocyanate towards the alcohols and polyols are between those of the aromatic and alkyl isocyanates. The activation energies were also determined and were found to vary from 25.6 to 38.6 kJ/mol (from 1-propanol to 1-hexanol). According to further kinetic studies, the rate coefficients k1,app and k2,app for the reaction of 1,3-XDI with PTHF were determined to be (1.13±0.01)・10–2 and (1.28±0.01)・10–2 min–1, respectively. It was also found that the rate constants did not depend significantly on the length of the polymer chain. However, significantly lower rate coefficients were obtained for the 1,3-XDI-mPEG reaction (k1,app = (2.64±0.03)・10–3 min–1 and k2,app = (2.45±0.03)・10–3 min–1). Besides, the dependence of the pseudo-first-order rate coefficient on the concentration of the alcohols was also studied, and based on these findings a reaction mechanism is proposed.
With the help of three-dimensional (3D) printing technique, complicated and sophisticated structured shape memory polymers (SMPs) devices could be obtained, which have drawn tremendous attention in recent years. However, there is technical limitation for 3D photo-polymerization printing technique to prepare multi-responsive SMP devices. A major problem for this is that functional fillers added to the photo-curing resins disturb or even inhibit their photo polymerization process. Herein, we demonstrated that the fused deposition modeling (FDM) technology seems more promising for fabrication of multi-responsive SMP devices. In this research, SMP devices are printed using a new material, polycyclooctene (PCO), whose application in FDM printing is never reported in the previous work. A universal and facile method, gammaray irradiation, is used to realize the crosslinking of printed structures. The printing quality and thermo-responsive speed of SMPs can be improved by incorporation of highly thermal conductive fillers (hexagonal boron nitride, BN) to PCO; in addition, after the addition of multiwalled carbon nanotubes (MWCNTs), the 3D SMP devices obtained multi-responsive ability owing to the enhanced electrical conduction and light absorption of the composite materials.
Biodegradable poly(carbonate anhydrides) based on poly(trimethylene carbonate) were synthesized by ring-opening polymerization. The hydroxyl-terminated oligomers were acid-functionalized with succinic anhydride and subsequently methacrylated to obtain photo-crosslinkable three-armed macromers with anhydride bonds. The degree of acid-functionalization was over 96%, and the degree of methacrylation was over 80%. The macromers were photo-crosslinked into networks at room temperature in a nitrogen atmosphere. In vitro degradation experiments in phosphate-buffered solution (pH 7.4) at 37 °C showed that the networks degraded rapidly with complete mass loss within 72 h. For networks prepared from the higher molecular weight, the presence of an alkenyl chain in the anhydride bond resulted in lower water uptake and consequently in a decrease in the degradation rate. The degradation mechanism of these networks showed characteristics of surface erosion.
In this paper, the thermal mending potential of a cyclic olefin copolymer (COC) in a thermosetting matrix was evaluated for the first time. Epoxy/COC blends were prepared by mechanically mixing a COC powder (particle size lower than 300 μm) at different concentrations (10, 20, 30 and 40 wt%) in an epoxy resin in the uncured (liquid) state. The presence of a homogenous dispersion of COC particles within the epoxy matrix after curing was confirmed by optical microscopy analysis on polished specimens. Thermogravimetric tests evidenced a positive contribution of the thermoplastic COC phase on the thermal degradation resistance of the blends. The fracture toughness of the blends (both under quasi-static and Charpy impact conditions) increased with the addition of COC. Healing tests were performed on broken samples after healing under a compressive stress of 15 MPa at 190 °C for 1 hour. A significant increase of the healing efficiency with the COC content was detected, and healing efficiency values very near to 100% were obtained for a COC content of 40 wt%.
The tensile and thermo-mechanical properties, as well as the fracture mechanical behavior of a high strength epoxy/amine system modified with particulates of a block copolymer (BCP), a core shell rubber (CSR) and a mixture of them, were investigated at 23 °C. The results show that the fracture energy was increased by more than 700% with a filler content of 12 wt% BCP and by more than 600% with a filler content of 12 wt% CSR particles. The content of BCP and CSR particles influences the final morphology and, thus, also the tensile properties, fracture toughness and thermo-mechanical behavior of the modified systems. The toughening mechanisms induced by the BCP and CSR particles were identified as (a) localized plastic shear yielding around the particles and (b) cavitation of the particles followed by plastic void growth of the epoxy polymer. The fracture toughness and fracture energy were co-related to the plastic zone size for all systems modified. These mechanisms were modeled using the Hsieh et al.  approach which also allows calculating the values of GIc of the differently modified polymers. Excellent agreement was found between the predictions and the experimentally measured fracture energies.