Rheological aspect on electrospinning of polyamide 6 solutions

Abstract

Polyamide 6 (PA6) solutions in formic acid (FA) and deionized water cosolvent may behave as polyelectrolyte or neutral solutions depending on the cosolvent composition. In this study, both polyelectrolyte and neutral PA6 solutions were prepared for electrospinning, and their spinnability was correlated with their rheological properties. In addition, the effects of PA6 average molecular weight and carbon nanocapsule (CNC) nanoparticle addition on solution rheology and electrospinnability were investigated. Microstructure and thermal properties of the as-spun fibers were identified by wide-angle X-ray diffraction, polarized Fourier infrared spectroscopy, and differential scanning calorimetry (DSC). Due to the chain expansion, polyelectrolyte solutions with 99vol.% FA solvent possess much lower entanglement concentration (ϕe, ∼1wt.%) than neutral solutions (∼7wt.%) prepared by 90 and 85vol.% FA solvent. Compared with the neutral solution, the polyelectrolyte solution is more advantageous because a lower concentration is sufficient to obtain bead-free PA6 fibers. However, at a concentrated regime of 15wt.% solution, the obtained fibers exhibit a larger diameter due to the higher entanglement density. For the crystalline structure, the content and orientation of α-form crystals are higher in the PA6 fibers obtained from the polyelectrolyte than from the neutral solution. When PA6 with a lower molecular weight is used, a higher concentration is required to develop the entangled chains to produce bead-free fibers. Homogeneous PA6 solutions filled with CNCs exhibit more elastic behavior than unfilled solutions due to the presence of the CNC–CNC network, aside from the entangled network of PA6 chains. Electrospinning of the CNC-filled solutions yields PA6 fibers with CNC aggregates protruding from the fiber surface. The inclusion of CNC in the PA6/FA solution produces fibers possessing enhanced α-form crystals with reduced orientation. In all cases, DSC heating traces of the as-spun fibers identify a high melting temperature (HMT) phase of PA6. The amount of HMT phase decreases, provided that more water or CNCs are added into the PA6/FA solution for electrospinning.