Jian-Bing Zeng

Chitin Whiskers: An Overview

Chitin is the second most abundant semicrystalline polysaccharide. Like cellulose, the amorphous domains of chitin can also be removed under certain conditions such as acidolysis to give rise to crystallites in nanoscale, which are the so-called chitin nanocrystals or chitin whiskers (CHWs). CHW together with other organic nanoparticles such as cellulose whisker (CW) and starch nanocrystal show many advantages over traditional inorganic nanoparticles such as easy availability, nontoxicity, biodegradability, low density, and easy modification. They have been widely used as substitutes for inorganic nanoparticles in reinforcing polymer nanocomposites. The research and development of CHW related areas are much slower than those of CW. However, CHWs are still of strategic importance in the resource scarcity periods because of their abundant availability and special properties. During the past decade, increasing studies have been done on preparation of CHWs and their application in reinforcing polymer nanocomposites. Some other applications such as being used as feedstock to prepare chitosan nanoscaffolds have also been investigated. This Article is to review the recent development on CHW related studies.

Ultralow percolation threshold and enhanced electromagnetic interference shielding in poly(L-lactide)/multi-walled carbon nanotube nanocomposites with electrically conductive segregated networks

Electrically conductive segregated networks were built in poly(L-lactide)/multi-walled carbon nanotube (PLLA/MWCNT) nanocomposites without sacrificing their mechanical properties via simply choosing two different PLLA polymers with different viscosities and crystallinities. First, the MWCNTs were dispersed in PLLA with low viscosity and crystallinity (L-PLLA) to obtain the L-PLANT phase. Second, the PLLA particles with high viscosity and crystallinity (H-PLLA) were well coated with the L-PLANT phase at 140 °C which was below the melting temperature of H-PLLA. Finally, the coated H-PLLA particles were compressed above the melting temperature of H-PLLA to form the PLLA/MWCNT nanocomposites with segregated structures. The morphological observation showed the successful location of MWCNTs in the continuous L-PLLA phase, resulting in an ultralow percolation threshold of 0.019 vol% MWCNTs. The electrical conductivity and the electromagnetic interference (EMI) shielding effectiveness (SE) of the composites with the segregated structure are 25 S m−1 and ∼30 dB, showing three orders and 36% higher than that of the samples with a random distribution of MWCNTs with 0.8 vol% of MWCNT loading, respectively. High-performance electromagnetic interference (EMI) shielding was also observed mainly dependent on the highly efficient absorption shielding, which can be achieved by the densely continuous MWCNT networks and the abundant interfaces induced by the segregated structures. Furthermore, the composites with segregated structures not only showed higher Youngs modulus and tensile strength than the corresponding conventional composites, but also maintained high elongation at break because of the continuous and dense MWCNT networks induced by the segregated structures and the high interfacial interaction between H-PLLA and L-PLLA.

Compatibilization strategies in poly(lactic acid)-based blends

Poly(lactic acid) (PLA) is regarded as one of the most promising bio-based and biodegradable polymers, due to its excellent biodegradability, biocompatibility, renewability, high strength, and easy processibility. However, disadvantages such as brittleness and relatively high cost have restricted its applications significantly. Polymer blending provides an economic and efficient way to modify the properties of PLA. Most shortcomings of PLA are theoretically surmountable by blending with abundant polymers with various properties. But, unfortunately, PLA is thermodynamically immiscible with most existing polymers. High-performance PLA-based blends are usually unanticipated by direct blending. In order to obtain PLA-based blends with excellent overall properties, compatibilization is required during polymer blending. Various strategies have been employed or developed to compatibilize PLA blends with different polymers, as reported in recent studies. This article aims to review the development in compatibilization strategies employed in PLA-based blends.

Structure and Properties of Soy Protein/Poly(butylene succinate) Blends with Improved Compatibility

A novel environmentally friendly thermoplastic soy protein/polyester blend was successfully prepared by blending soy protein isolate (SPI) with poly(butylene succinate) (PBS). To improve the compatibility between SPI and PBS, the polyester was pretreated by introducing different amounts of urethane and isocyanate groups before blending. The blends containing pretreated PBS showed much finer phase structures because of good dispersion of polyester in protein. Consequently, the tensile strength and modulus of blends increased obviously. A lower glass transition temperature of protein in the blends than that of the pure SPI, which was caused by the improvement of the compatibility between two phases, was observed by dynamic mechanical analyzer (DMA). The hydrophobicity, water resistance, and moisture absorption at different humidities of the blends were modified significantly due to the incorporation of PBS.

Fully Biobased and Supertough Polylactide-Based Thermoplastic Vulcanizates Fabricated by Peroxide-Induced Dynamic Vulcanization and Interfacial Compatibilization

A fully biobased and supertough thermoplastic vulcanizate (TPV) consisting of polylactide (PLA) and a biobased vulcanized unsaturated aliphatic polyester elastomer (UPE) was fabricated via peroxide-induced dynamic vulcanization. Interfacial compatibilization between PLA and UPE took place during dynamic vulcanization, which was confirmed by gel measurement and NMR analysis. After vulcanization, the TPV exhibited a quasi cocontinuous morphology with vulcanized UPE compactly dispersed in PLA matrix, which was different from the pristine PLA/UPE blend, exhibiting typically phase-separated morphology with unvulcanized UPE droplets discretely dispersed in matrix. The TPV showed significantly improved tensile and impact toughness with values up to about 99.3 MJ/m(3) and 586.6 J/m, respectively, compared to those of 3.2 MJ/m(3) and 16.8 J/m for neat PLA, respectively. The toughening mechanisms under tensile and impact tests were investigated and deduced as massive shear yielding of the PLA matrix triggered by internal cavitation of VUPE. The fully biobased supertough PLA vulcanizate could serve as a promising alternative to traditional commodity plastics.

Morphological regulation improved electrical conductivity and electromagnetic interference shielding in poly(L-lactide)/poly(ε-caprolactone)/carbon nanotube nanocomposites via constructing stereocomplex crystallites

Morphological control of conductive networks in conductive polymer composites has been demonstrated to efficiently improve their electrical performance. Here, morphological regulation used for the formation of conductive networks occurs in poly(L-lactide)/poly(e-caprolactone) (PLLA/PCL) blends when stereocomplex crystallites (SCs) are formed in the PLLA phase. The SCs formed during the melt-processing increase the viscosity and elasticity of the PLLA phase, which makes the PLLA domains shrink and the PCL phase becomes continuous from the previously dispersed phase. As a result, for PLLA/PCL/multi-walled carbon nanotube (MWCNT) nanocomposites, the MWCNTs prefer to disperse in the PCL phase via morphological regulation. The electrical conductivity and the electromagnetic interference (EMI) shielding effectiveness (SE) of the PLLA/PCL/MWCNT nanocomposites can be abruptly increased and attributed to the simultaneous organization of conductive paths when the continuous PCL phase develops. For example, the electrical conductivity and the EMI SE of the PLLA/PCL/MWCNT nanocomposites increased from 2.1 × 10−12 S m−1 and 5.3–8.6 dB to 0.012 S m−1 and ∼17 dB, respectively, with 0.8 wt% MWCNTs when adding 20 wt% poly(D-lactide) (PDLA) to the PLLA phase. Furthermore, the percolation threshold of the nanocomposites was reduced from 0.13 to 0.017 vol% by adding 20 wt% poly(D-lactide) (PDLA) to the PLLA phase.

Unique crystalline/crystalline polymer blends of poly(ethylene succinate) and poly(p-dioxanone): miscibility and crystallization behaviors.

Miscibility and crystallization behaviors of poly(ethylene succinate)/poly(p-dioxanone) (PES/PPDO) blends were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). PES/PPDO blends are completely miscible as proved by the single grass transition temperature (T(g)) dependence of composition and decreasing crystallization temperature of the blends in comparison with the respective component. POM observation suggests that simultaneous crystallization of PES and PPDO components in the blends took place, spherulites of one component can crystallize inside the spherulites of the other component, and the unique interpenetrated crystalline morphology has been formed for the blends in the full composition range. Isothermal crystallization kinetics of the blends was studied by DSC and the data were analyzed by the Avrami equation. The results suggest that the crystallization mechanisms of the blends were unchanged but the overall crystallization rates were slowed down compared with neat PES and neat PPDO. WAXD results indicate that the crystal structures of PES and PPDO did not change in the blends.

In situ formed crosslinked polyurethane toughened polylactide

Polylactide (PLA), a biobased polymer, has a short elongation at break and low impact strength, which restricted its broad application as a commodity polymer. In this paper, super-tough polylactide/crosslinked polyurethane (PLA/CPU) binary blends with CPU dispersed in the PLA matrix were prepared by reactive blending of PLA with poly(ethylene glycol) (PEG) and polymeric methylene diphenylene diisocyanate (PMDI). The in situ polymerization of PEG and PMDI in the PLA matrix formed CPU, and the interfacial compatibilization between PLA and CPU phases occurred by the reaction of NCO groups with terminal hydroxyl groups of PLA, which was confirmed by Fourier transform infrared spectroscopy. The results of a tensile test and a notched Izod impact test suggest that the elongation at break and impact strength were increased to more than 20 and 30 times those of neat PLA, respectively. The effects of PEG molecular weight (namely soft segment length of CPU) and CPU content on the phase morphology and impact strength of PLA/CPU blends were investigated systematically. The optimum CPU particle size for high impact toughness was identified to be 0.7–1.0 μm when the soft segment length and the content of CPU were in the ranges of 1000–2000 g mol−1 and 20–30 wt%, respectively. The compatibility between the dispersed CPU and PLA matrix was studied by dynamic mechanical analysis through the change in glass transition temperatures of PLA and CPU components. The results suggest that the compatibility increased with increasing soft segment length and content of CPU, which was mainly due to the increased plasticization effect. With improved toughness, the PLA/CPU blends could be used as substitutes for some traditional petroleum-based polymers.

Super-tough poly(L-lactide)/crosslinked polyurethane blends with tunable impact toughness

Super-tough poly(L-lactide)/crosslinked polyurethane (PLLA/CPU) blends with a CPU phase dispersed in the PLLA matrix were prepared by reactive blending of PLLA with poly(ethylene glycol) (PEG), glycerol, and 4,4′-methylenediphenyl diisocyanate (MDI). The gel fraction increased while the swelling ratio decreased with increasing glycerol content. FT-IR analysis suggests that interfacial compatibilization between PLLA and CPU occurred via reaction between the hydroxyl group of PLLA and the isocyanate group of MDI. The elongation at break and notched impact strength of PLLA/CPU blends were increased by up to 38 and 21 times those of neat PLLA. The morphology of PLLA/CPU blends plays an important role in notched impact strength and can be controlled by adjusting the content of glycerol. The size of the dispersed CPU phase increased gradually while the notched impact strength increased first and then decreased with increasing glycerol content. Therefore, the notched impact strength can be easily tailored by the content of glycerol of CPU. The optimum size for high impact strength was found to be ∼0.7 μm, which was obtained for the blends with glycerol content in the range of 5 to 10 wt% on the basis of PEG weight. In addition, the effect of glycerol content on the compatibility and rheological properties of PLLA/CPU blends was also investigated.

From miscible to partially miscible biodegradable double crystalline poly(ethylene succinate)-b-poly(butylene succinate) multiblock copolymers

A series of biodegradable double crystalline poly(ethylene succinate)-b-poly(butylene succinate) (PES-b-PBS) multiblock copolymers with various PES and PBS block lengths were successfully synthesized by chain-extension reaction of dihydroxylated poly(ethylene succinate) (HO-PES-OH) and poly(butylene succinate) (HO-PBS-OH) using 1,6-hexamethylene diisocyanate (HDI) as a chain-extender. The compositions and structures were characterized by proton nuclear magnetic resonance spectroscopy (1H NMR). The miscibility of amorphous phase and crystallization behaviors of the two blocks were investigated by standard differential scanning calorimetry (DSC), temperature modulated DSC (TMDSC), polarized light optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). When the block length of PBS and PES were less than 4710 g mol−1, the amorphous phases of the two blocks were miscible. As the block length increased to more than 5430 g mol−1, the amorphous phases of the two blocks changed to be partially miscible, and the miscibility decreased with further increasing the block lengths. The crystallizability of both PBS and PES blocks increased with an increase in size of their blocks. POM observation showed that the copolymers displayed banded spherulitic morphologies, and the crystallization of PES happened in confined space after crystallization of PBS blocks. WAXD analysis suggested that the crystals of the copolymers were composed of crystals of both PES and PBS blocks.