Study on Phase Transformation Behavior of Strain-induced PLLA Mesophase by Polarized Infrared Spectroscopy

The structural transformation of mesophase to crystalline phase of strain-induced poly(L-lactic acid) has been investigated by differential scanning calorimetry(DSC) and in situ temperature dependent polarized Fourier transform infrared(FTIR) spectroscopy. It is found that, as the drawing temperature increases, melting of strain-induced mesophase in the heating process can remarkably interfere the crystallization behavior subsequently. Coupling with in situ polarized FTIR, from 60 °C to 76 °C, the mesophase melts partially rather than completely melting, and changes immediately to three-dimensional ordered structure. Of particular note, through monitoring the subtle spectral change in the critical phase transformation temperature from 60 °C to 64 °C, it is clearly demonstrated that relaxation of oriented amorphous chains initially takes place prior to the melting of mesophase.

High Performance of PVA Nanocomposite Reinforced by Janus-like Asymmetrically Oxidized Graphene:Synergetic Effect of H-bonding Interaction and Interfacial Crystallization

Macromolecule nanocrystal network and strong interfacial interaction are always beneficial to enhance the mechanical property of polymer-based nanocomposites.Poly(vinyl alcohol)(PVA),a typical biocompatible semicrystalline polymer,is an ideal candidate for preparing high performance polymer-based nanocomposites.However,the rich hydrogen bonds between PVA matrix and graphene oxide(GO)can disrupt the formation of PVA nanocrystal network.Thus,it remains a great challenge to achieve both strong and tough PVA-GO nanocomposites.Herein,by introducing a novel Janus-like amphiphilic graphene oxide(JGO),both hydrogen bonding and interfacial crystallization have been constructed between JGO sheets and PVA matrix.Benefiting from amphiphilic interfacial interaction and the enhanced crystal network,both PVA-JGO dried films and their swollen hydrogel films show superior mechanical properties than those of traditional PVA-GO nanocomposites.PVA-JGO dried films exhibit a 264%improvement of toughness at a JGO loading of 1 wt%.Meanwhile,the corresponding PVA-JGO swollen hydrogel films display simultaneous improvement of nearly 8 times increase of tensile strength and 20 times increase of toughness compared to traditional PVA-GO nanocomposite.This work indicates multiple interfacial interactions and macromolecule crystal networks can be concurrent in PVA nanocomposites by innovative modification of nanofillers,providing a new strategy to construct PVA nanocomposites with high strength and high toughness.The integration of outstanding mechanical and swelling resistance properties on PVA-JGO nanocomposite films render their promising applications,such as packaging and toughening hydrogel materials.

Cellulose Nanocrystals-mediated Phase Morphology of PLLA/TPU Blends for 3D Printing

Incorporation of nanoparticles into polymer blend to obtain finely dispersed morphology has been considered as an effective strategy to prepare nanocomposites. Owing to the renewable and degradable characters, cellulose nanocrystals(CNCs) have been proposed to tailor the phase morphology of poly(L-lactic acid)(PLLA) blend for producing high-performance fused deposition modeling(FDM) consumables. However,the main challenge associated with the ternary systems is the dispersion of the highly hydrophilic CNCs in non-polar PLLA blend by industrial melt blending without involving solution. Herein, with poly(vinyl acetate)(PVAc) modified CNCs powder(a mixture of PVAc grafted from CNCs and PVAc homopolymer latex), the selective dispersion of CNCs in PLLA has been achieved by simple melt processing of PLLA/TPU(polyether polyurethane)/CNCs blend. This results in the ultra-fine TPU droplets at nanoscale in PLLA and improves the melt processibility of composites in FDM due to the decreased viscosity ratio of the dispersed/matrix and the enhanced melt elasticity of PLLA. Combined with the intensive shear and continuous stretch effect during FDM, aligned TPU nanofibers(TNFs) were in situ formed along the elongational flow direction during deposition, which in turn contributed to the improvement of PLLA/TPU/CNCs with 5 wt% filler loading in tensile ductility by 418%, inter-layer adhesion strength and notched impact toughness by 261% and 210%, respectively, as well as achieved good dimensional accuracy and very fine surface quality.