Hao Xiu

Tailoring Impact Toughness of Poly(l-lactide)/Poly(ε-caprolactone) (PLLA/PCL) Blends by Controlling Crystallization of PLLA Matrix

Melt blending poly(L-lactide) (PLLA) with various biodegradable polymers has been thought to be the most economic and effective route to toughen PLLA without compromising its biodegradability. Unfortunately, only very limited improvement in notched impact toughness can be achieved, although most of these blends show significant enhancement in tensile toughness. In this work, biodegradable poly(ε-caprolactone) (PCL) was used as an impact modifier to toughen PLLA and a nucleating agent was utilized to tailor the crystallization of PLLA matrix. Depending on the nucleating agent concentrations in the matrix and mold temperatures in injection molding, PLLA/PCL blends with a wide range of matrix crystallinity (10-50%) were prepared by practical injection molding. The results show that there is a linear relationship between PLLA matrix crystallinity and impact toughness. With the increase in PLLA crystalline content, toughening becomes much easier to achieve. PLLA crystals are believed to provide a path for the propagation of shear yielding needed for effective impact energy absorption, and then, excellent toughening effect can be obtained when these crystals percolate through the whole matrix. This investigation provides not only a new route to prepare sustainable PLLA products with good impact toughness but also a fresh insight into the importance of matrix crystallization in the toughening of semicrystalline polymers with a flexible polymer.

Significantly Improving Oxygen Barrier Properties of Polylactide via Constructing Parallel-Aligned Shish-Kebab-Like Crystals with Well-Interlocked Boundaries

Recently, some attempts have been made to enhance the gas barrier properties of semicrystalline polymers by precisely controlling the arrangement of their impermeable crystalline lamellae. However, it is still a great challenge to achieve regular arrangement of the lamellae along the direction perpendicular to the gas diffusion path, especially using conventional polymer processing technologies. This work presents a novel and simple strategy to dramatically improve oxygen barrier performance of biobased and biodegradable polylactide (PLA) through constructing parallel-aligned shish-kebab-like crystals with well-interlocked boundaries with the aid of a highly active nucleating agent. The nucleating agent was introduced into PLA by melting compounding and the sheet-like specimens were fabricated by compression molding. We demonstrate that the fibrillar nucleating agent dispersed in PLA melt can serve as shish to induce the change of crystallization habit of PLA from isotopic spherulitic crystals to unique shish-kebab-like crystals and the shear flow in the compression molding can induce the highly ordered alignment of the nucleating agent fibrils as well as the subsequent shish-kebab-like crystals along the direction parallel to the sheet surface. More importantly, the growing lamellae are found to interpenetrate and tightly interlock with each other at the boundary regions of the shish-kebab-like crystals in the later stage of the crystallization, forming a densely packed nanobrick wall structure to prevent gas molecules from permeating through the crystals and thus imparting the PLA sheets with unprecedentedly low oxygen permeability. This work provides not only a successful example of preparing semicrystalline polymer with super gas barrier properties by tailoring crystal superstructure but also a promising route to rapidly fabricate high-performance food packaging materials via industrially meaningful melt processing.

Towards high-performance poly(L-lactide)/elastomer blends with tunable interfacial adhesion and matrix crystallization via constructing stereocomplex crystallites at the interface

In this work, we report a facile strategy to prepare super-tough and heat-resistant poly(L-lactide) (PLLA) blends by constructing stereocomplex (sc) crystallites with dual interfacial adhesion enhancer/matrix crystallization accelerator functionality at the interface of the blends of PLLA/ethylene copolymer. To exploit the dual functionality, poly(D-lactide) grafted ethylene–acrylic ester copolymer (EMA-g-PDLA) capable of collaborating with the PLLA matrix to form the sc crystallites was first prepared via melt coupling reaction between end groups (carboxyl and hydroxyl) of PDLA and excess epoxy group of EMA–glycidyl methacrylate copolymer (EMA–GMA). During subsequent melt-blending of PLLA with the prepared EMA-g-PDLA, sc crystallites are formed at the interface. The results show that, compared with PLLA/EMA–GMA and PLLA/EMA-g-PLLA blends, injection molded PLLA/EMA-g-PDLA blends have much higher impact toughness and heat resistance because the interface-localized sc crystallites can induce substantial enhancement in both interfacial adhesion and matrix crystallinity. More interestingly, by modulating the amount of sc crystallites at the interface of the blends, optimum impact toughness can be achieved due to the optimization of interfacial strength and matrix crystallinity. This work provides a new concept for the fabrication of high-performance PLLA blends by tailoring matrix and interface properties with the aid of sc crystallites.

Formation of new electric double percolation via carbon black induced co-continuous like morphology

A new electric double percolation was realized via carbon black self-networking induced co-continuous like morphology (composed of disconnected PU clusters and bands) in polylactide/poly(ether)urethane (PLA/PU) blends. As a result, a simultaneous improvement in electrical conductivity and impact toughness of the blends without compromising strength and modulus has been achieved.

Largely reinforced polyurethane via simultaneous incorporation of poly(lactic acid) and multiwalled carbon nanotubes

In this study, we simultaneously introduced both poly(lactic acid) (PLA) and multiwalled carbon nanotubes (CNTs) into the polyurethane (PU) matrix via melt blending, to achieve balanced mechanical properties and good conductivity. Different contents of PLA (0–30 wt%) and CNTs (0–3.0 wt%) were used in this work. A significant improvement in tensile strength at 300% strain and Youngs modulus were observed, which could not be obtained by incorporating either PLA or CNTs with PU separately. Particularly, the ternary composites containing a large amount of PLA (30 wt%), 70PU/30PLA/CNTs composites, exhibit superior mechanical properties compared to other composites with less PLA content but the same amounts of CNTs. Moreover, the ternary composites showed better electrical conductivity compared with the binary counterpart. SEM observations demonstrated that PLA particles and CNTs are separately dispersed in the PU matrix. It was found that PLA particles remain spherical and their size increases with increasing PLA content up to 30 wt%, while a network structure of CNTs is formed with increasing its content, which was also confirmed via dynamic rheological analysis. Interestingly, some CNTs were seen to be located in the interfaces between PLA particles and the PU matrix for 70PU/30PLA/CNTs composites, namely, some CNTs exhibit a nano-bridge effect between PU and PLA. We hypothesized that the nano-bridge structure of CNTs in the composites could mainly contribute to the observed enhancement of mechanical properties and electrical conductivity.