Structural basis for unique hierarchical cylindrites induced by ultrahigh shear gradient in single natural fiber reinforced poly(lactic acid) green composites.

A local shear flow field was feasibly generated by pulling the ramie fiber in single fiber reinforced poly(lactic acid) (PLA) composites. This was featured by an ultrahigh shear gradient with a maximum shear rate up to 1500 s(-1), a level comparable to that frequently occurring during the practical polymer processing. To distinguish shear-induced self-nucleation and ramie fiber-induced heterogeneous nucleation, the shear history was classified by pulling the fiber for 5 s (pulled sample) and pulling out the fiber during 10 s (pulled-out sample), while the static fiber-induced crystallization was carried out as the counterpart. As a result of the ultrahigh shear gradient, the combination of primary shear-induced nucleation in the central region and secondary nucleation in the outer layer assembled the unique hierarchical superstructures. By comparing the architectural configurations of interphases formed in the static, pulled, and pulled-out samples, it was shown that the hierarchical cylindrites underwent the process of self-nucleation driven by the applied shear flow, very different from the formation of fiber-induced transcrystallinity (TC) triggered by the heterogeneous nucleating sites at the static fiber surface. The twisting of transcrystallized lamellae may take place due to the spatial hindrance induced by the incredibly dense nuclei under the intense shearing flow, as observed in the synchrotron X-ray diffraction patterns. The influence of chain characteristics on the crystalline morphology was further explored by adding a small amount of poly(ethylene glycol) (PEG) to enhance the molecular mobility of PLA. It was of interest to find that the existence of PEG not only facilitated the growth rates of TC and cylindrites but also improved the preferential orientation of PLA chains and thus expanded the ordered regions. We unearthed lamellar units that were composed of rich fibrillar extended chain crystals (diameter of 50-80 nm). These results are of importance to shed light on tailoring crystalline morphology for natural fibers reinforced green composite materials. Of immense practical significance, too, is the crystalline evolution that has been tracked in the simple model penetrated with an ultrahigh shear gradient, which researchers have so far been unable to replicate during the practical melt processing, such as extrusion and injection molding.

Shear flow and carbon nanotubes synergistically induced nonisothermal crystallization of poly(lactic acid) and its application in injection molding.

The effect of shear flow and carbon nanotubes (CNTs), separately and together, on nonisothermal crystallization of poly(lactic acid) (PLA) at a relatively large cooling rate was investigated by time-resolved synchrotron wide-angle X-ray diffraction (WAXD) and polarized optical microscope (POM). Unlike flexible-chain polymers such as polyethylene, and so on, whose crystallization kinetics are significantly accelerated by shear flow, neat PLA only exhibits an increase in onset crystallization temperature after experiencing a shear rate of 30 s(-1), whereas both the nucleation density and ultimate crystallinity are not changed too much because PLA chains are intrinsically semirigid and have relatively short length. The breaking down of shear-induced nuclei into point-like precursors (or random coil) probably becomes increasingly active after shear stops. Very interestingly, a marked synergistic effect of shear flow and CNTs exists in enhancing crystallization of PLA, leading to a remarkable increase of nucleation density in PLA/CNT nanocomposite. This synergistic effect is ascribed to extra nuclei, which are formed by the anchoring effect of CNTs' surfaces on the shear-induced nuclei and suppressing effect of CNTs on the relaxation of the shear-induced nuclei. Further, this interesting finding was deliberately applied to injection molding, aiming to improve the crystallinity of PLA products. As expected, a remarkable high crystallinity in the injection-molded PLA part has been achieved successfully by the combination of shear flow and CNTs, which offers a new method to fabricate PLA products with high crystallinity for specific applications.

IRAP: An efficient retrotransposon-based electrophoretic technique for studying genetic variability among geographical isolates of Schistosoma japonicum.

In the present study, a inter-retrotransposon-amplified polymorphism (IRAP) technique, based on retrotransposons, was used to examine genetic variability among Schistosoma japonicum isolates from different provinces in mainland China. Of the 15 primers screened, 5 produced highly reproducible IRAP patterns. Using these primers, 54 discernible DNA fragments were generated with 40 (74.07%) being polymorphic, indicating considerable genetic variation among the examined S. japonicum isolates. The primer LTR-11 was found to be able to differentiate male and female parasites, producing one constant specific band for female S. japonicum isolates. The percentages of polymorphic bands (PPB) among all parasites, among isolates from mountainous provinces and among those from the lake/marshland areas were 74.07, 48.15, and 66.67%, respectively. UPGMA analysis revealed that the IRAP profiles could group S. japonicum isolates in mainland China into two clades (mountainous and lake/marshland types), and samples from the same geographical origins clustered together. These results demonstrated that the IRAP technique is suitable for studying genetic diversity and population structures, and also provides an effective technique for studying sex differentiation of S. japonicum.

From Nanofibrillar to Nanolaminar Poly(butylene succinate): Paving the Way to Robust Barrier and Mechanical Properties for Full-Biodegradable Poly(lactic acid) Films.

The traditional approach toward barrier property enhancement of poly(lactic acid) (PLA) is the incorporation of sheet-like fillers such as nanoclay and graphene, unfortunately leading to the sacrificed biocompatibility and degradability. Here we unveil the first application of a confined flaking technique to establish the degradable nanolaminar poly(butylene succinate) (PBS) in PLA films based on PLA/PBS in situ nanofibrillar composites. The combination of high pressure (10 MPa) and appropriate temperature (160 °C) during the flaking process desirably enabled sufficient deformation of PBS nanofibrils and retention of ordered PLA channels. Particularly, interlinked and individual nanosheets were created in composite films containing 10 and 20 wt % PBS, respectively, both of which presented desirable alignment and large width/thickness ratio (nanoscale thickness with a width of 428±13.1 and 76.9±8.2 µm, respectively). With the creation of compact polymer "nano-barrier walls", a dramatic decrease of 86% and 67% in the oxygen permeability coefficient was observed for the film incorporated with well-organized 20 wt % PBS nanosheets compared to pure PLA and pure PBS (1.4 and 0.6×10(-14) cm3·cm·cm(-2)·s(-1)·Pa(-1)), respectively. Unexpectedly, prominent increases of 21% and 28% were achieved in the tensile strength and modulus of composite films loaded 20 wt % PBS nanosheets compared to pure PLA films, although PBS intrinsically presents poor strength and stiffness. The unusual combination of barrier and mechanical performances established in the fully degradable system represent specific properties required in packaging beverages, food and medicine.