Supermolecular Structure, Free Volume, and Glass Transition of Needleless Electrospun Polymer Nanofibers
T.M. Kotresh, R. Ramani, Nirban Jana, Sathiadasan Minu, R. Indu Shekar, Renjith Ramachandran
Abstract
Polymer nanofibers are known to possess highly aligned polymer chains, and the existence of open spaces/free volumes within them is something difficult to envisage. Herein, we have carried out a thorough free volume investigation in three different needleless electrospun polymer nanofibers, viz., polyamide-6 (PA-6), polyacrylonitrile (PAN), and polystyrene (PS), using the positronium annihilation lifetime spectroscopy technique. The lifetime results were supported by Doppler broadening of annihilation radiation measurements. For comparison, studies were also carried out on solution-cast films of these polymers. The free volumes in polar polymer nanofibers were found to be localized mainly at the chain ends, contributing to supernumerary free volumes, and the free volume contents in them were surprisingly higher than their bulk films. The free-volume-based structural studies were supplemented by X-ray diffraction, and their thermal stability was found by thermogravimetric analysis. The glass-transition temperature (Tg) values of nanofibers in the case of polar polymers (PA-6 and PAN) are higher than their corresponding solution-cast films as measured by dynamic mechanical thermal analysis (DMTA) measurements. The present study reveals that the free volume and Tg of polar polymer nanofibers bear a correlation, which is distinct from the generally known chain end–free volume theory, thus posing the need for further research on the glass-transition process in them. Furthermore, a drastic reduction in Tg of PA-6 nanofibers by ca. 80 °C from its granular form due to the influence of an acidic solvent has been found for the first time using a low-temperature DMTA study. In-depth understanding of these aspects improves the fundamental knowledge of the internal structure of nanofibers at the molecular level and serves as a key for achieving the long-term goal of controlling their properties for specific applications like dye filtration and energy storage.