Mechanical Reinforcement Of Electrospun Polymer Nanofiber Membranes

Optimization of mechanical properties is required in the applications of electrospunnanofibrous matrices.Firstly, thermal annealing strategy is proposed to improve the mechanical properties ofpolyelectrolyte complex nanofiber membranes. The effects of annealing on the structural andmechanical properties of electrospun chitosan-gelatin (CG) nanofiber membranes wereinvestigated using tensile tests, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction(XRD), and differential scanning calorimetry (DSC). Tensile test results showed that annealingprocessing at90°C produced1.3-fold and1.1-fold increase on Young’s modulus and tensilestrength, respectively. By scanning electron microscopy (SEM) observation, it was found thatthat there was a formation of partial interfiber bonding when annealing temperature was elevatedover the glass transition temperature (Tg) of CG nanofibers. FTIR results showed enhancedmolecular interactions within fibers, suggesting that annealing treatment promoted theconjunction between chitosan and gelatin. In contrast, no detectable changes in crystallinity forCG nanofiber specimens were exhibited on XRD patterns following annealing treatment. Inaddition, thermal annealing induced the improvement in thermal stability, aqueous stability andswelling capacity. Therefore, annealing is proved to be an effective strategy for mechanicalenhancement of polyelectrolyte complex nanofibrous scaffolds. The enhanced stiffness andstrength is mainly attributed to the formation of interfiber bonding and strengthened molecularinteractions between chitosan and gelatin.Secondly, mechanical reinforcement of electrospun nanofiber membranes of water-solublepolymer by the incorporation of commercial nanodiamonds (NDs) was studied. Through annanodiamond/poly(vinyl alcohol)(ND/PVA) model system, it is demonstrated that155%improvement of Young’s modulus,89%increase in tensile strength, and336%elevation inenergy to break are achieved by the addition of only2wt%nanodiamond. Fourier transforminfrared spectroscopy (FTIR) results suggest the existence of molecular interactions betweenNDs and PVA matrix, which contributes to the effective load transfer from the polymer matrix tothe fillers. However, higher level of ND addition (>2wt%) aggravates the agglomeration of nanofillers in PVA matrix and offsets the reinforcing effect, as ND agglomerates may act as flawsin composite nanofibers. Furthermore, NDs have optimizing effect on the morphology ofND/PVA nanofibers through increasing the conductivity of the electrospinning solution.Therefore, NDs nanofillers possess the potential to improve the mechanical performance ofwater-soluble polymer-based nanofiber membranes.Finally, mechanical reinforcement of electrospun nanofiber membranes of Poly(L-lactic acid)by the incorporation of natural halloysite nanotubes (HNTs) was studied. Through an halloysitenanotube/Poly(L-lactic acid)(HNT/PLA) model system, it is demonstrated that100%improvement of Young’s modulus,62%increase in tensile strength, and181%elevation inenergy to break are achieved by the addition of only4wt%halloysite nanotube. Fouriertransform infrared spectroscopy (FTIR) results and X-ray diffraction (XRD) suggest theexistence of molecular interactions between HNT and PLA matrix, which contributes to theeffective load transfer from the polymer matrix to the fillers. However, higher level of HNTaddition (>4wt%) aggravates the agglomeration of nanofillers in PLA matrix and offsets thereinforcing effect. In addition, the incorporation of natural halloysite nanotubes (HNTs) inducedthe improvement in thermal stability. Therefore, HNTs nanofillers possess the potential toimprove the mechanical and thermal performance of PLA nanofiber membranes.