The role of annealing in enhancing crystallinity, mechanical properties, piezoelectricity, and air filtration performance of polylactic acid nanofibers

Abstract

Annealing is a widely adopted post-treatment technique for enhancing the properties of nanofibers without altering their chemical structure. This study investigates the effect of annealing on the surface morphology, crystallinity, mechanical properties, and piezoelectric performance of polylactic acid (PLA) nanofibers produced via the electro-blow spinning (EBS) process. Two distinct PLA grades, 4043D PLA (NatureWorks) and L130 PLA (TotalCorbion) were annealed at temperatures ranging from 80 °C to 140 °C for 24 h. The annealed PLA nanofibers were then thoroughly characterised to identify the most suitable annealing temperature for improving their performance without causing structural defects. Characterisation results revealed a noticeable increase in crystallinity from 25 % to 39 % for 4043D PLA and from 45 % to 59 % for L130 PLA when annealed at 100 °C. At the same annealing temperature, piezoelectric output voltage showed an increase from 20 to 38 mV and from 30 to 52 mV for 4043D PLA and L130 PLA, respectively. The annealed PLA nanofibers were tested for air filtration to correlate the changes in nanofibers properties upon annealing with their filtration performance as air filters. Filtration experiments were performed according to the European standard for respiratory protective air filters (EN149) while Corona discharge was applied to impart long-lasting electrostatic charge onto nanofibers. The results demonstrated that L130 PLA nanofibers achieved a high filtration efficiency of around 99 %, while pressure drop was remarkably reduced from 850 to 405 Pa after annealing. The 4043D PLA nanofibers showed a reduction in filtration efficiency to 93.9 % at higher annealing conditions which was subsequently improved with Corona discharge treatment. The enhanced surface charge induced by Corona discharge enabled sustained electrostatic attraction which further improved the filtration efficiency without affecting the pressure drop. This study introduces annealing as a non-destructive and effective modification method for optimising the performance of biodegradable PLA-based air filters.