Effects of thermal annealing and solvent-induced crystallization on the structure and properties of poly(lactic acid) mi
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Effects of thermal annealing and solvent‑induced crystallization on the structure and properties of poly(lactic acid) microfibres produced by high‑speed electrospinning Dániel Vadas1 · Zsombor K. Nagy1 · István Csontos1 · György Marosi1,2 · Katalin Bocz1 Received: 22 July 2019 / Accepted: 10 December 2019 © The Author(s) 2020
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. Keywords Poly(lactic acid) · High-speed electrospinning · Polymorphs
Introduction 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
* György Marosi [email protected] 1
Department of Organic Chemistry and Technology, Faculty of Chemical Technology and Biotechnology, Budapest University of Technology and Economics, Műegyetem rkp. 3., Budapest 1111, Hungary
Budapest, Hungary
2
slow crystallization kinetics of the polymer [1–5]. Inter alia, thermal history has a direct influence on the crystallinity of PLA and, consequently, on its properties altogether. Numerous studies showed that by exploiting the advantages of thermal annealing, Young’s modulus and heat distortion temperature (HDT) can be effectively improved; moreover, even increased tensile st
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