Weifu Dong

Crystallization modification of poly(lactide) by using nucleating agents and stereocomplexation

Abstract Poly(lactide), PLA, as one of the most promising biopolymers, has been receiving increasing attention in recent years because of its excellent performances in renewability, mechanical properties, biocompatibility and biodegradability. However, its application is limited by its brittleness and low heat distortion temperatures (HDT). The low HDT mainly results from a low crystallization rate and lack of crystallinity after fast processing, e.g. injection molding. Consequently, considerable attention was paid, in recent years, to achieve fast(er) crystallization of PLA. In here, we briefly review the research progress in the crystallization modification of PLA notably by means of adding nucleating agents and stereocomplexation.

Rheological and structural characterization of HA/PVA-SbQ composites film-forming solutions and resulting films as affected by UV irradiation time.

Hyaluronan (HA)/poly (vinyl alcohol) bearing styrylpyridinium groups (PVA-SbQ) composites film-forming solutions were prepared by a negatively charged HA and an oppositely charged PVA-SbQ. The rheological properties and structural characterization of HA/PVA-SbQ composites in aqueous solution were investigated. Zeta potential measurements and TEM were utilized to explore the formation of HA/PVA-SbQ complex micelles in aqueous solution. UV spectra and DLS experiments confirmed that the micelles are photo-crosslinkable. HA/PVA-SbQ composites films were prepared by a casting method. The microstructure and properties of the film were analyzed by SEM, optical transmittance, DSC, XRD and tensile testing. The crosslinked HA/PVA-SbQ composites films exhibited higher UV light shielding and visible light transparency and better mechanical and water vapor barrier properties as well as thermal stability than the uncrosslinked HA/PVA-SbQ composites films, indicating the formation of three-dimensional network structure. This work provided a good way for increasing the mechanical, thermal, water vapor barrier, and optical properties of HA materials for the packaging material.

A Novel UV-Shielding and Transparent Polymer Film: When Bioinspired Dopamine–Melanin Hollow Nanoparticles Join Polymers

Ultraviolet (UV) light is known to be harmful to human health and cause organic materials to undergo photodegradation. In this Research Article, bioinspired dopamine-melanin solid nanoparticles (Dpa-s NPs) and hollow nanoparticles (Dpa-h NPs) as UV-absorbers were introduced to enhance the UV-shielding performance of polymer. First, Dpa-s NPs were synthesized through autoxidation of dopamine in alkaline aqueous solution. Dpa-h NPs were prepared by the spontaneous oxidative polymerization of dopamine solution onto polystyrene (PS) nanospheres template, followed by removal of the template. Poly(vinyl alcohol) (PVA)/Dpa nanocomposite films were subsequently fabricated by a simple casting solvent. UV irradiation protocols were set up, allowing selective study of the extra-shielding effects of Dpa-s versus Dpa-h NPs. In contrast to PVA/Dpa-s films, PVA/Dpa-h films exhibit stronger UV-shielding capabilities and can almost block the complete UV region (200-400 nm). The excellent UV-shielding performance of the PVA/Dpa-h films mainly arises from multiple absorption because of the hollow structure and large specific area of Dpa-h NPs. Moreover, the wall thickness of Dpa-h NPs can be simply controlled from 28 to 8 nm, depending on the ratio between PS and dopamine. The resulting films with Dpa-h NPs (wall thickness = ∼8 nm) maintained relatively high transparency to visible light because of the thinner wall thickness. The results indicate that the prepared Dpa-h NPs can be used as a novel UV absorber for next-generation transparent UV-shielding materials.

All cellulose composites based on cellulose diacetate and nanofibrillated cellulose prepared by alkali treatment

Cellulose diacetate (CDA) composite films were synthesized by incorporation of nanofibrillated celluloses (NFCs), using a casting/evaporation technique. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) characterization indicated the presence of interaction and certain compatibility between CDA and NFCs. The CDA and NFCs were converted to cellulose II by an alkaline treatment which resulted in the formation of all cellulose composite (ACC) films, as indicated by the FTIR and XRD characterization. Significant changes were observed in their structural, thermal, mechanical, and optical transparency properties. The SEM images revealed the excellent compatibility among the constituents of the ACC films. The tensile strength, Youngs modulus, and strain at break of the ACC films with 15wt% NFCs content were increased to 102.3MPa, 5.8GPa, 14.7% respectively. This work provides a promising pathway for manufacturing high performance ACC materials.

Preparation and properties of thermoplastic poly(caprolactone) composites containing high amount of esterified starch without plasticizer

Based on stearyl chloride and native starch, esterified starch were prepared and the chemical structure was characterized by (1)H NMR and FTIR. It was found that stearyl chloride was an efficient agent to fabricate esterified starch with high degree of substitution (DS). During the melt blending of esterified starch (80 wt%) and poly(caprolactone) (PCL, 20 wt%), it was shown the torque of PCL/esterified starch was much lower than that of PCL/native starch without any plasticizer, and further decreased with increasing DS. Compared with PCL/native starch, the tensile properties of PCL/esterified starch composites were significantly enhanced. The tensile strength and elongation at break were increased from 2.7 MPa to 56% for PCL/native starch composites to 9.1 MPa and 626% for PCL/esterified starch ones with DS of 1.50, respectively. SEM observation revealed the esterified starch particles in matrix became smaller and more uniform. In addition, the water resistance and hydrophobic character of PCL/esterified starch composites were improved. PCL composites containing 80 wt% esterified starch with favorable mechanical properties would have great potential applications in broad areas.

Interfacial modification on polyhydroxyalkanoates/starch blend by grafting in-situ

The interfacial adhesion between polyhydroxyalkanoates (PHAs) and native starch is poor. To improve the interfacial adhesion, PHAs were in-situ grafted onto starch using dicumyl peroxide (DCP) as a free radical initiator. The grafting reaction was carefully characterized and confirmed by gel analysis and Fourier transform infrared spectroscopy (FT-IR). The gel yield of the PHAs/starch/DCP blend increased with the DCP concentration up to 2wt%. Meanwhile, obvious plastic deformation (stretched fibrils) was observed at the interface in the PHAs/starch/DCP blend in comparison with complete interfacial debonding in the PHAs/starch physical blend. The improved interfacial adhesion after grafting was further confirmed by a reduction in adhesion factor (Af) obtained from dynamic mechanical analysis (DMA). The mechanical strength and the crystallization rate of the PHAs were deteriorated after incorporation of starch, and were backed up by the interfacial improvement. A linear relationship between the mechanical properties and the gel yield was discovered. In addition, the PHAs/starch/DCP blend exhibited higher decomposition active energy (Ea) and thus better thermal stability in comparison with the PHAs and the PHAs/starch physical blend. Therefore, this study provides a simple route to utilize low-cost starch as a component in biopolymer blend.

Enhanced crystallization kinetics of poly(lactide) with oxalamide compounds as nucleators: effect of spacer length between the oxalamide moieties

Poly(lactide), PLA, suffers from low crystallization rate. To speed up the crystallization rate, oxalamide compounds with the formula of C6H5NHCOCONH(CH2)nNHCOCONHC6H5 (n = 2, 4, 6, 8, 12) are synthesized as nucleating agents (NA(n)). Their thermal properties and nucleation efficiency can be tailored by tuning the aliphatic spacer length, i.e., –(CH2)n–. The melting temperatures and crystallization temperatures of the NA(n) decrease monotonically with increasing n value while the thermal decomposition temperature remains constant. The aliphatic spacer length (n) of NA(n) influences obviously the crystallization kinetics of PLA, resulting in nucleation efficiency in the sequence of NA(2) > NA(4) > NA(12) ≈ NA(8) > NA(6). Polarized optical microscopy results show that NA(n) could self-assemble into needle-like superstructures which subsequently promote the crystallization of PLA macromolecules. The differences in superstructural geometry and the nucleation activity of NA(n) are responsible for the variation in the nucleation efficiency.

Cellulose-g-poly(d-lactide) nanohybrids induced significant low melt viscosity and fast crystallization of fully bio-based nanocomposites

Comb-like nanocrystal cellulose graft poly (d-lactide) (PDLA), i.e., NCC-g-PDLA nanohybrids were synthesized and compounded with poly (l-lactide) (PLLA) and poly (hydroxyalkanoate)s (PHA) to make fully biobased nanocomposites. Surprisingly, the complex viscosity of the PLLA/PHA melts was reduced by more than one order of magnitudes, viz. from 4000 to 100Pas by incorporation of 2-4wt% of the NCC-g-PDLA nanohybrids. Meanwhile, the crystallization of the PLLA component was accelerated by the NCC-g-PDLA nanohybrids due to the strong interaction between PDLA and PLLA macromolecules. The significant reduction in melt viscosity associating with unique core-shell-like microstructures due to the synergetic effect of the NCC-g-PDLA nanohybrids and the PHA would facilitate the preparation of complex-shaped biomass articles and fibers under low(er) pressure and temperatures, which is beyond pure academic interest.

High-performance poly(lactide) composites by construction of network-like shish-kebab crystals

Poly(lactide), PLA, suffers from brittleness and poor heat-resistance. Herein, a novel strategy to in situ fabricate high-performance PLA composites is reported for the first time, i.e., by solid-state uniaxial stretching in the presence of N1,N1′-(ethane-1,2-diyl)bis(N2-phenyloxalamide) (OXA) as needle-like nucleation templates. As a consequence, a tensile strength of >120 MPa, an elongation at break of >25% and a heat-resistant temperature of >150 °C of the PLA composites are simultaneously achieved. The superior performances are ascribed to unique interlocked network-like shish-kebab crystal structures that formed upon solid-state hot-stretching in the presence of OXA. This research may create a new type of sustainable engineered plastic.

Long-chain branching hydrogel with ultrahigh tensibility and high strength by grafting via photo-induced polymerization

A novel long-chain branching (LCB) hydrogel, polyacrylamide grafted from poly(2-acrylamido-2-methylpropanesulfonic acid) via photo-induced polymerization, has been developed with excellent mechanical properties such as ultrahigh tensibility and high strength. Investigations of tensile and compressive properties indicate that the Youngs modulus, ultimate strength and strain, toughness, compressive modulus and stress are significantly improved by introduction of LCB. Scanning electron microscopy (SEM) reveals that the amount of embedded micro-network structures in LCB gels is increased with an increase in the length of LCB. The compressive loading–unloading behaviour shows that the area of the hysteresis loop (the dissipated energy) for the first compressive cycle increases with the introduction of LCB and is higher than that for subsequent cycles. The energy dissipation mechanism is demonstrated for a better understanding of LCB gels. Furthermore, rheological measurements are also studied. Dynamic shear measurements show that both the storage modulus and the complex viscosity increase in the presence of LCB.