Gan-Ji Zhong

Formation of Shish-Kebabs in Injection-Molded Poly(l-lactic acid) by Application of an Intense Flow Field

Unlike polyolefins (e.g., isotactic polypropylene), it is still a great challenge to form rich shish-kebabs in biodegradable poly(L-lactic acid) (PLLA) because of its short chain length and semirigid chain backbone. In the present work, a modified injection molding technology, named oscillation shear injection molding, was applied to provide an intense shear flow on PLLA melt in mold cavity, in order to promote shear-induced crystallization of PLLA. Additionally, a small amount of poly(ethylene glycol) (PEG) with flexible chains was introduced for improving the crystallization kinetics. Numerous shish-kebabs of PLLA were achieved in injection-molded PLLA for the first time. High-resolution scanning electronic microscopy and small-angle X-ray scattering showed a structure feature of shish-kebabs with a diameter of around 0.7 μm and a long period of ~20 nm. The wide-angle X-ray diffraction results showed that shish-kebabs had more ordered crystalline structure of α-form. A significant improvement of the mechanical properties was obtained; the tensile strength and modulus increased to 73.7 and 1888 MPa from the initial values of 64.9 and 1684 MPa, respectively, meanwhile the ductility is not deteriorated. Interestingly, when shish-kebabs form in the PLLA/PEG system, a bamboo-like bionic structure comprising a hard skin layer and a soft core develops in injection-molded specimen. This unique structure leads to a great balance of mechanical properties, including substantial increments of 26, 20, and 112% in the tensile strength, modulus, and impact toughness, compared to the control sample. Further exploration will give a rich fundamental understanding in the shear-induced crystallization and morphology manipulation of PLLA, aiming to achieve superior PLLA products.

Cellulose composite aerogel for highly efficient electromagnetic interference shielding

An ultra-light and highly conductive cellulose composite aerogel was fabricated by a simple, efficient and environmentally benign strategy. The scaffold structure was well designed from nanofibrillar networks to nanosheet networks by controlling the concentration of cellulose in the sodium hydroxide/urea solution. The obtained conductive aerogel was first reported as an electromagnetic interference shielding material; it exhibits an electromagnetic interference (EMI) shielding effectiveness of ∼20.8 dB and a corresponding specific EMI shielding effectiveness as high as ∼219 dB cm3 g−1 with microwave absorption as the dominant EMI shielding mechanism in the microwave frequency range of 8.2–12.4 GHz at a density of as low as 0.095 g cm−3. This result demonstrates that this type of green conductive aerogel has the potential to be used as lightweight shielding material against electromagnetic radiation, especially for aircraft and spacecraft applications.

Graphene Oxide Nanosheet Induced Intrachain Conformational Ordering in a Semicrystalline Polymer

The physical origin of graphene oxide nanosheet (GONS)-driven polymer crystallization was studied from the perspective of intrachain conformational ordering. Time-resolved Fourier-transform infrared spectroscopy indicated that both conformational ordering and crystallization of isotactic polypropylene (iPP) were obviously accelerated by the presence of GONSs, indicating their efficient nucleation activity for iPP crystallization. Furthermore, the ordering of long helical segments occurred prior to the crystallization of iPP, as revealed by two-dimensional correlation infrared analysis. Compared to pure bulk system, the presence of GONSs was in favor of the formation of long ordering segments, especially at the early stage, accompanied by considerable enhancement of the crystallization kinetics. GONS-driven iPP crystallization was suggested to be attributed to this GONS-induced intrachain conformational ordering.

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.

Toward Stronger Transcrystalline Layers in Poly(l-lactic acid)/Natural Fiber Biocomposites with the Aid of an Accelerator of Chain Mobility

Formation of transcrystalline layer probably enhances the interfacial adhesion of poly(L-lactic acid) (PLLA)/natural fiber biocomposites as confirmed by this work. We found that a crystallization accelerator, poly(ethylene glycol) (PEG), improved chain mobility of PLLA and thus enhanced the growth kinetics of ramie fiber-induced transcrystallinity (TC). The direct observation of polarized optical microscopy during isothermal crystallization revealed that large-sized TC with rapid growth was produced after adding PEG. It could be exemplified by the case at 125 °C that the growth rate of TC developed in PLLA10 (containing 10 wt % PEG) achieved 6.1 μm/min, which was nearly triple that of pure PLLA (2.1 μm/min). And interestingly enough, spherulitic nucleation proceeding was largely restricted because it was difficult to fulfill the critical size for stable nuclei due to the increased chain mobility. Meanwhile, combining the effective nucleation activity of ramie fibers and acceleration virtue of PEG offered the chance to form prevailing TC texture, instead of rich spherulites dominated in pure PLLA. The local structure (including lamellar structure and molecular orientation) of transcrystalline layers was further determined, which indicated that TC presented α crystal form and random lamellar packing derived from the moderate nucleating ability. To our surprise, the single fiber reinforced composite samples containing prevailing TC textures achieved remarkably higher strength compared to that of pure PLLA samples with poorly developed transcrystalline layers, as demonstrated by the single-fiber pull-out test.

Tuning the Superstructure of Ultrahigh-Molecular-Weight Polyethylene/Low-Molecular-Weight Polyethylene Blend for Artificial Joint Application

An easy approach was reported to achieve high mechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE)-based polyethylene (PE) blend for artificial joint application without the sacrifice of the original excellent wear and fatigue behavior of UHMWPE. The PE blend with desirable fluidity was obtained by melt mixing UHMWPE and low molecular weight polyethylene (LMWPE), and then was processed by a modified injection molding technology-oscillatory shear injection molding (OSIM). Morphological observation of the OSIM PE blend showed LMWPE contained well-defined interlocking shish-kebab self-reinforced superstructure. Addition of a small amount of long chain polyethylene (2 wt %) to LMWPE greatly induced formation of rich shish-kebabs. The ultimate tensile strength considerably increased from 27.6 MPa for conventional compression molded UHMWPE up to 78.4 MPa for OSIM PE blend along the flow direction and up to 33.5 MPa in its transverse direction. The impact strength of OSIM PE blend was increased by 46% and 7% for OSIM PE blend in the direction parallel and vertical to the shear flow, respectively. Wear and fatigue resistance were comparable to conventional compression molded UHMWPE. The superb performance of the OSIM PE blend was originated from formation of rich interlocking shish-kebab superstructure while maintaining unique properties of UHMWPE. The present results suggested the OSIM PE blend has high potential for artificial joint application.

Easy alignment and effective nucleation activity of ramie fibers in injection-molded poly(lactic acid) biocomposites.

The poly(lactic acid) (PLA)/ramie fiber biocomposites were fabricated, which exhibited considerable reinforcement effect comparable to the glass fiber at the same loading. The attempts were made to understand the flow-induced morphology of ramie fibers and PLA crystals in the injection-molded PLA/ramie fiber biocomposites, thus revealing its relationship to biocomposite mechanical properties. The polarized optical microscopy (POM) and two-dimensional wide-angle X-ray diffraction (2D-WAXD) were for the first time used to determine the distribution of nature fibers, which interestingly showed the ramie fibers aligned well along the flow direction over the whole thickness of injection-molded parts, instead of skin-core structure. This easy alignment of ramie fibers during the common processing was ascribed to the intrinsically high flexibility of ramie fibers and strong interfacial interaction between PLA chains and cellulose molecules of ramie fibers. Both 2D-WAXD and differential scanning calorimeter (DSC) measurements suggested that the PLA matrix in its ramie biocomposites had rather high orientation degree and crystallinity, which was attributed to effective heterogeneous nucleation induced by ramie fibers and local shearing field in the vicinity of fiber surface. Remarkable improvement of mechanical and thermo-mechanical properties was achieved for PLA/ramie fiber biocomposites, without sacrifice of toughness and ductility. Addition of 30wt% ramie fibers increased the tensile strength and modulus of PLA/ramie fiber biocomposites from 65.6 and 1468 MPa for pure PLA to 91.3 and 2977 MPa, respectively. These superior mechanical properties were ascribed to easy alignment of ramie fibers, high crystallinity of PLA, and favorable interfacial adhesion as revealed by scanning electron microscopy (SEM) observation and theoretical analysis based on dynamic mechanical analysis (DMA) data.

Ultra-low gas permeability and efficient reinforcement of cellulose nanocomposite films by well-aligned graphene oxide nanosheets

Cellulose is often considered to be an ideal candidate for biodegradable packaging films, but its main weakness is its poor gas barrier performance. We used a simple, efficient, low cost, recyclable, non-toxic and environmentally friendly processing solvent (an aqueous solution of NaOH/urea) to fabricate graphene oxide nanosheet (GONS)/regenerated cellulose (RC) nanocomposite films with an ultra-low O2 permeability and high mechanical performance. Transmission electron microscopy and two-dimensional wide-angle X-ray diffraction measurements showed that the GONSs were fully exfoliated, homogeneously dispersed and highly aligned along the surface of the cellulose nanocomposite films. Rheological and Fourier transform infrared spectroscopy measurements demonstrated the existence of strong H-bonding interactions between the GONSs and the cellulose matrix. A significant improvement in the barrier properties of the regenerated cellulose nanocomposite films was achieved. The O2 permeability coefficient was reduced by about 1000 times relative to the pure regenerated cellulose film at a low GONS loading of 1.64 vol%. The tensile strength and Youngs modulus of the regenerated cellulose nanocomposite films were enhanced by about 67 and 68%, respectively, compared with the RC film. The theoretical simulation results of the Cussler and Halpin–Tsai models consistently confirmed that the GONSs tended to align parallel to the film surface; this was probably induced by gravitational forces and further consolidated by hot pressing. The work presented here indicates that this simple and environmentally friendly method is an effective strategy to design highly aligned nanofillers in polymer nanocomposite films. The cellulose nanocomposite films obtained have excellent potential as packaging materials for protecting perishable goods susceptible to O2 degradation.

Suppressing the skin-core structure of injection-molded isotactic polypropylene via combination of an in situ microfibrillar network and an interfacial compatibilizer.

Injection-molded semicrystalline polymer parts generally exhibited a so-called skin-core structure basically as a result of the large gradients of temperature, shear rate, stress, and pressure fields created by the boundary conditions of injection molding. Suppression of the skin-core structure is a long-term practical challenge. In the current work, the skin-core structure of the conventional injection-molded isotactic polypropylene (iPP) was largely relieved by the cooperative effects of an in situ microfibrillar network and interfacial compatibilizer. The in situ poly(ethylene terephthalate) microfibrils of 1-8 μm in diameter and large aspect ratios of above 40 tended to entangle with each other to generate a microfibrillar network in the iPP melt. During injection molding, the iPP molecules experienced confined flow in the microchannels or pores formed by the microfibrillar network, which could redistribute and homogenize the flow field of polymer melt. Addition of the compatibilizer, glycidyl methacrylate-grafted iPP, restrained the molecular orientation but facilitated preservation of oriented molecules due to the chemical bonds at the interface between PET microfibrils and iPP. The cooperative effects of in situ microfibrillar network and interfacial compatibilizer led to almost the same molecular orientation across the whole thickness of the injection-molded parts. Additionally, the content of β crystals in different layers of injection-molded iPP parts depended on the combined effects of the molecular orientation, the amount of oriented crystals, and the crystallization time between 105 and 140 °C. The presence of the interfacial compatibilizer facilitated formation of the β crystals because of preservation of the oriented molecules.

Role of ion-dipole interactions in nucleation of gamma poly(vinylidene fluoride) in the presence of graphene oxide during melt crystallization.

The crystallization behavior and crystalline structure of poly(vinylidene fluororide) (PVDF) in the presence of graphene oxide (GO) platelets were investigated using time-resolved Fourier transformation infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), as well as differential scanning calorimetry (DSC). It is shown that GO platelets induce the formation of γ phase when crystallizing from solution, but only α phase forms from melt crystallization. The crystallization kinetics of α phase is promoted due to heterogeneous nucleation ability of GO, which is probably originated from a weak π-dipole interaction between GO and PVDF. Intriguingly, after introduction of strong ion-dipole interactions between GO and PVDF by addition of an ionic surfactant (cetyltrimethylammonium bromide, CTAB), a significant amount of γ crystals are obtained during isothermal melt crystallization. Time-resolved FTIR results further provide a detailed evolution of the γ phase formation, and there are two distinct stages during the melt crystallization in the PVDF/GO composites in the presence of CTAB, i.e., a simultaneous growth of γ and α phases in the first stage, and a solid α to γ transition in the second stage. These results may provide a facile routine to manipulate the crystalline structure in PVDF/GO composites, and thus to gain desirable properties.