Structure and Morphology of Poly(lactic acid) Stereocomplex Nanofiber Shish Kebabs

Significantly High Melting Temperature of Homopolymer Crystals Obtained in a Poly(l-Lactic Acid)/Poly(d-Lactic Acid) (50/50) Blend

The isothermal crystallization of a poly(l-lactic acid) (PLLA)/poly(d-lactic acid) (PDLA) (50/50) blend, neat PLLA, and neat PDLA, was studied at different crystallization temperatures (110 °C, 150 °C, 170 and 180 °C) for different durations (1-300 min) by means of differential scanning calorimetry (DSC), polarized optical microscope (POM) observations, and time-resolved wide-angle X-ray diffraction (WAXD). The effects of both the isothermal crystallization temperature and the duration of the isothermal crystallization were investigated for the blend specimens fully crystallized at these crystallization temperatures. The formation of homopolymer crystallites (HC) was confirmed at the isothermal crystallization temperature of 170 °C, which was previously considered too high for its formation, after 70 min had elapsed from the temperature stabilization. Moreover, the melting temperature of the formed HC was found to be significantly high (Tm = 187.5 °C) compared to the one obtained during the nonisothermal DSC measurement of the same specimen of the PLLA/PDLA (50/50) blend, as well as the neat PLLA and PDLA specimens. To the best of our knowledge, this extremely high Tm (=187.5 °C) for HC has never been reported before.

Selectively multilayered distribution of stereocomplex crystallite and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) ribbons to achieve highly ductile and strong poly(l-lactide) composites

Blending poly(l-lactide) (PLLA) with elastic polymers is an efficient way to obtain highly ductile materials (> 300 %), but it is accompanied by a significant reduction in strength. In this work, a special alternating multilayered composites with alternating stereocomplex crystallite (SC) (PLLA/poly(d-lactide) (PDLA) layer) and highly oriented Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) ribbons (PLLA/PHBV layer) is in situ constructed during laminated structuring process. Experimental results show that in situ formed PHBV ribbons are limitedly distributed in the thickness direction and align parallel to the layer interfaces. More interestingly, not only highly oriented shish crystals but also sparse lamellae of PLLA, which are arrested by SC, shish crystals, and PHBV ribbons, are in situ formed. Compared with sea-island structured composites prepared by traditional compression molding, the alternating multilayered composites show an increase in elongation at break from 8.7 % to 345.1 % and an increase in yield strength from 61.4 MPa to 73.2 MPa. During the tensile testing, the PLLA/PHBV layers firstly form micro-fibrils and micro-voids, driving the molecular chains of the PLLA/PDLA layer to respond in time to external forces through stress transfer of rich continuous layer interfaces. Since shear yielding and plastic deformation can easily penetrate the entire matrix, the alternating multilayered composites go a brittle-ductile transformation and the ductility is improved significantly. The increased strength of the alternating multilayered material is ascribed to the stiff shish crystals and SC. This work provides important guidance for the durable application of strong and ductile PLLA-based materials.

Electrically conductive crystalline polylactide nonwovens obtained by electrospinning and modification with multiwall carbon nanotubes

Polylactide nonwovens were electrospun from solutions and then crystallized, one in the α-form, and another, S-PLA made of poly(l-lactide) and poly(d-lactide) 1:1 blend, in scPLA crystals with high melting temperature, close to 220 °C. To make the nonwovens electrically conductive, they were coated with multiwall carbon nanotubes (MWCNT) by padding with an aqueous dispersion of MWCNT or dip-coating in this dispersion. The electrical conductivity evidenced the formation of the electrically conductive MWCNT network on the fiber surfaces. Depending on the coating method, the surface resistivity (R_s) of S-PLA nonwoven of 1.0 kΩ/sq. and 0.09 kΩ/sq. was reached. To study the effect of surface roughness, before the modification the nonwovens were etched with sodium hydroxide, which additionally made them hydrophilic. The effect of etching depended on the coating method and led to an increase or decrease of R_s, in the case of padding or dip-coating, respectively. All MWCNT-modified nonwovens, unetched and etched, were hydrophobic with water contact angles of 138-144°. Scanning electron microscopy corroborated the presence of MWCNT on the fiber surfaces. The impedance spectroscopy confirmed the dominant role of the network of MWCNT direct contacts on the electrical properties of MWCNT-modified nonwovens in a broad frequency range.

New opportunities for sustainable bioplastic development: Tailorable polymorphic and three-phase crystallization of stereocomplex polylactide by layered double hydroxide

Stereocomplexation between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) is a promising sustainable approach and gaining momentum to overcome the shortcomings of polylactide (PLA) for its use as a replacement for fossil-based plastics. Filler addition in tailoring the crystallization of stereocomplex PLA (SC-PLA) attracts extensive attention; however, research has primarily focused on the heterogeneous nucleation effect of filler. The impact of filler on the chain behavior of SC-PLA during crystallization has not been exclusively discussed, and the rigid amorphous fraction (RAF) development remains unknown. In this study, the crystallization of PLLA/PDLA blends was modified by low loading of layered double hydroxide (LDH) (≤ 1 wt%) with the proposed local effect of such filler, and additional RAF development was incurred. In the early stage of crystallization, LDH facilitates the pairing of PLLA and PDLA and arrests the ordered SC pairs during the dynamic balance between the separation and pairing of racemic segments. This explains the severely suppressed homochiral (HC) crystallization, promoted SC crystallization, and additional RAF formation driven by the nucleation-induced chain ordering. This work, for the first time, highlights the role of LDH in creating SC-PLA with tailorable polymorphism and RAF, where the mechanism can be extended to other filler-type nucleator systems.

Governing effects of melt viscosity on fire performances of polylactide and its fire-retardant systems

Extreme flammability of polylactide (PLA) has restricted its real-world applications. Traditional research only focuses on developing new effective fire retardants for PLA without considering the effect of melt viscosity on its fire performances. To fill the knowledge gap, a series of PLA matrices of varied melt flow index (MFI) with and without fire retardants are chosen to examine how melt viscosity affects its fire performances. Our results show that the MFI has a governing impact on fire performances of pure PLA and its fire-retardant systems if the samples are placed vertically during fire testing. PLA with higher MFI values achieves higher limiting oxygen index (LOI) values, and a lower loading level of fire retardants is required for PLA to pass a UL-94 V-0 rating. This work unveils the correlation between melt viscosity and their fire performance and offers a practical guidance for creating flame retardant PLA to extend its applications.

Lamellar crystal-dominated surfaces of polymer films achieved via melt stretching-induced free surface crystallization

Lamellar crystal-dominated (LCD) surfaces hold great superiority and broad prospects in polymer surface engineering. The key to this is avoiding the formation of an amorphous phase in the interlamellar region. Here we give a first report of achieving LCD surfaces of polyethylene films via melt stretching-induced free surface crystallization. We demonstrate that the resultant surface is constructed directly by orientated and edge-on lamellae within a surface depth of tens to hundreds of nanometers, while the normally existing amorphous phase is avoided. The crystallization-driven formation of the LCD surface has been ascribed to the heterogeneous chain dynamics of a melt free surface, that is, high chain mobility, low viscosity and loose chain entanglement, which facilitates the complete chain disentanglement during crystallization. In addition, we confirm that the surface morphology is controllable with respect to lamellar orientation, spacing and depth by changing the melt stretching strain or quenching the deformed melt. Meanwhile, owing to a possible kinetics competition between crystallization and chain disentanglement, the structural spacing of surface lamellae holds a positive correlation with the lamellar depth. Since free surface effects are immanent in polymer materials, the currently proposed melt processing strategy is demonstrated to be transferable to other semicrystalline polymers.

Core-Shell Nanofibers with a Shish-Kebab Structure Simulating Collagen Fibrils for Bone Tissue Engineering

The repair of bone defects is one of the great challenges facing modern orthopedics clinics. Bone tissue engineering scaffold with a nanofibrous structure similar to the original microstructure of a bone is beneficial for bone tissue regeneration. Here, a core-shell nanofibrous membrane (MS), MS containing glucosamine (MS-GLU), MS with a shish-kebab (SK) structure (SKMS), and MS-GLU with a SK structure (SKMS-GLU) were prepared by micro-sol electrospinning technology and a self-induced crystallization method. The diameter of MS nanofibers was 50-900 nm. Contact angle experiments showed that the hydrophilicity of SKMS was moderate, and its contact angle was as low as 72°. SK and GLU have a synergistic effect on cell growth. GLU in the core of MS was demonstrated to obviously promote MC3T3-E1 cell proliferation. At the same time, the SK structure grown on MS-GLU nanofibers mimicked natural collagen fibers, which facilitated MC3T3-E1 cell adhesion and differentiation. This study showed that a biomimetic SKMS-GLU nanofibrous membrane was a promising tissue engineering scaffold for bone defect repair.

Stereocomplexed electrospun nanofibers containing poly (lactic acid) modified quaternized chitosan for wound healing

Skin damage, especially the extensive full-thickness wound, is seriously affecting people's daily life and health. Meanwhile, wound healing is always challenged by bacterial infection. In this study, for the purpose of developing a disinfectant wound dressing, we designed a novel multi-functional nanofiber mats via electrospinning combining chitosan derivations and stereocomplex crystallite (SC). The SC membrane of poly (lactic acid)/chitosan derivatives were prepared via warming at 80 °C for 1 h. The thermal and mechanical properties of the heated mats were strengthened owing to the formation of SC, which restricted the lactide chains mobility. In vivo wound healing test revealed that the SC mats have better wound repair ability than the control group with a wound healing rate of 100 % within 15 days. In a word, the biomass-based mats with enhanced thermal and mechanical properties, antibacterial effect and antioxidant activity, providing a potential multi-functional platform for designing of disinfectant wound dressings.

Construction of stereocomplex granular dams in luminescent biopolymer systems

Lessons from natural and artificial granular dams inspired the design of dam structures of stereocomplex granules in multicolor emissive biopolymer systems.