Effects of thermal annealing and solvent-induced crystallization on the structure and properties of poly(lactic acid) microfibres produced by high-speed electrospinning

This research concentrates on the marked discrepancies in the crystalline structure of poly(lactic acid) (PLA) nano- and microfibres, achieved by different annealing strategies. PLA nonwoven mats were produced by high-speed electrospinning (HSES). The high-speed production technique allowed the manufacturing of PLA microfibres with diameters of 0.25–8.50 µm with a relatively high yield of 40 g h-1. The crystalline content of the inherently highly amorphous microfibres was increased by two methods, thermal annealing in an oven at 85°C was compared to immersion in absolute ethanol at 40°C. The morphology of the fibres was examined by scanning electron microscopy (SEM), crystalline forms and thermal properties were assessed using X-ray diffractometry (XRD), Raman spectrometry, differential scanning calorimetry (DSC) as well as modulated differential scanning calorimetry (MDSC). As a consequence of 45 min heat treatment, the crystalline fraction increased up to 26%, while solution treatment resulted in 33% crystallinity. It was found that only disordered α’ crystals are formed during the conventional heat treatment, however, the ethanol-induced crystallization favours the formation of the ordered α polymorph. In connection with the different crystalline structures, noticeable changes in the macroscopic properties such as heat resistance and mechanical properties were evinced by localised thermomechanical analysis (LTMA) and static tensile test, respectively