Abstract
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. 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; crystalline forms and thermal properties were assessed using X-ray diffractometry, Raman spectrometry, differential scanning calorimetry (DSC) as well as modulated DSC. 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 localized thermomechanical analysis and static tensile test, respectively.
Highlights
A wide variety of value-added biopolymer systems, that have been gaining ground in several industry segments, have to be adjusted to certain processing and application requirements
Microfibrous poly(lactic acid) (PLA) nonwoven mats were successfully manufactured by high-speed electrospinning, reaching a remarkable productivity of 40 g h−1
Conventional differential scanning calorimetry (DSC) showed that solvent-induced crystallization progresses 2–3 times more rapidly; besides, the large surface-area-to-volume ratio further increases the efficiency of the methods
Summary
A wide variety of value-added biopolymer systems, that have been gaining ground in several industry segments, have to be adjusted to certain processing and application requirements. Annealing (recrystallization) is one of the frequently utilized modifications that usually takes place during or after manufacturing By this method, the crystallinity of polymer material can be enhanced through the increased macromolecular mobility above Tg. In particular, the design of poly(lactic acid) (PLA) products requires special attention to its crystalline phase mainly due to the inherently. PLA blends with a higher amount of l-lactide and a lower d-lactide content are used [8] The properties of these semi-crystalline PLA grades can be tailored by varying the ratio of the two isomers, adjusting the molecular regularity of the PLA chains. By this means, a grade with lower d-lactide content has a higher maximum crystalline fraction
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