Structural Basis for Unique Hierarchical Cylindrites Induced by Ultrahigh Shear Gradient in Single Natural Fiber Reinforced Poly(lactic acid) Green Composites

Abstract

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.

Beyond a Model of Polymer Processing-Triggered Shear: Reconciling Shish-Kebab Formation and Control of Chain Degradation in Sheared Poly(l-lactic acid)

“…It appears that compact fibrillar chain bundles orderly assemble into the shish, in close resemblance to the shear-driven assembly of fibrillar extended chain crystals unveiled in the sheared PLLA by gently pulling the fiber which shares the strong shear rate and short duration. 6 Figure 3b,c again confirms the hierarchical structure of cylindrites in the samples sheared at 135 and 140°C, consisting of threadlike shish that elicits the compact alignment of kebabs in both sides. Furthermore, the shish structure is essentially associated with the organization of fibrillar extended chain bundles, unaltered by the crystallization temperature.…”

Poly(lactide)-g-poly(butylene succinate-co-adipate) with High Crystallization Capacity and Migration Resistance

“…1D-WAXD intensity profiles were extracted from the 2D-WAXD. Typical peaks of PLLA α-crystal assigned to lattice planes (010), (200)/(110), (203) and (015) were located at 2θ = 14.8°, 16.7°, 19.0° and 22.3°, respectively [18,38]. In earlier studies, three strong diffraction peaks at 2θ = 19.3°, 21.5°, and 22.4° corresponding to (020), (021), and (110) planes respectively have been assigned for crystalline neat PBSA [39].…”

Correlations between microstructure of α‐row nuclei and polymorphism of shear‐induced iPP/carbon fiber cylindrite

“…To precisely investigate the mechanism of shear‐induced polymorphs of iPP, various devices for imposing shear to the polymer melts have been developed. Among them, a fiber‐pulling technique was a commonly used approach to simulate shear field due to its convenience, manipulability, and validity …”