Abstract
The influence of macromolecular architecture on shear-induced crystallization of poly(L-lactide) (PLLA) was studied. To this aim, three star PLLAs, 6-arm with Mw of 120 and 245 kg/mol, 4-arm with Mw of 123 kg/mol, and three linear PLLAs with Mw of 121, 240 and 339 kg/mol, were synthesized and examined. The PLLAs were sheared at 170 and 150 °C, at 5/s, 10/s and 20/s for 20 s, 10 s and 5 s, respectively, and then cooled at 10 or 30 °C/min. Shear-induced crystallization during cooling was followed by a light depolarization method, whereas the crystallized specimens were examined by DSC, 2D-WAXS, 2D-SAXS and SEM. The effect of shear depended on the shearing conditions, cooling rate and polymer molar mass but it was also affected by the macromolecular architecture. The shear-induced crystallization of linear PLLA with Mw of 240 kg/mol was more intense than that of the 6-arm polymer with similar Mw, most possibly due to its higher Mz. However, the influence of shear on the crystallization of the star polymers with Mw close to 120 kg/mol was stronger than on that of their linear analog. This was reflected in higher crystallization temperature, as well as crystallinity achieved during cooling.
Highlights
In recent decades, bio-based polymers derived from annually renewable resources have drawn increasing attention [1], as biomass is the only source of available renewable carbon
The crystallization was followed by a light depolarization technique, whereas the crystallized specimens were ex-situ examined with scanning electron microscopy (SEM), differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (2D-SAXS and 2DWAXS)
The scattering patterns were recorded with Pilatus 100K solid-state detector from Dectris (Baden, Switzerland). To reveal their internal structure, selected sheared PLLA specimens were cut across their thickness parallel to the shearing direction, and the exposed surfaces were analyzed with scanning electron microscopy (SEM) using Jeol JSM-5500 LV (Tokyo, Japan)
Summary
Bio-based polymers derived from annually renewable resources have drawn increasing attention [1], as biomass is the only source of available renewable carbon. An increase of Tg , cold crystallization peak temperature, Tcc , accompanied by a decrease of melting peak temperature, Tm , and crystal growth rate of 3-arm PLLAs, with Mn in the range of 13 to 63 kg/mol, were observed, as compared with those of linear PLLAs [21]. It was found [23] that for 1-, 2-, 4- and 6-arm PLLAs, Tm , Tcc , and crystallinity, χc , decreased with increasing number of arms at a fixed Mn. Recently, Bojda et al [24] synthesized three star PLLAs, two 6-arm with weight average molar masses, Mw , of 120 and 245 kg/mol and one 4-arm with Mw of 123 kg/mol, and compared their crystallization with that of linear ones with Mw of 121, 291 and 339 kg/mol. The crystallization was followed by a light depolarization technique, whereas the crystallized specimens were ex-situ examined with scanning electron microscopy (SEM), differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (2D-SAXS and 2DWAXS)
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