Precursors in stereo-complex crystals of poly(L-lactic acid)/poly(D-lactic acid) blends under shear flow

Abstract

To improve the mechanical and the thermal performance of poly(lactic acid) materials, this work focuses on the formation of stereo-complex crystals by blending poly(l-lactic acid) (PLLA) with poly(d-lactic acid) (PDLA). The resulting structure was analyzed using time-resolved in situ X-ray scattering, optical microscopy, differential scanning calorimetry and viscoelastic measurements. The objective of this study is to investigate the effect of shear flow imposed prior to crystallization on higher-order structure formation and acceleration of stereo-complex crystal growth of PLLA and PDLA blends using a wide spatial scale analysis and viscoelastic measurements. Density fluctuations of 100 nm scale were observed prior to nucleation by in situ simultaneous wideand small-angle X-ray scattering measurements. These density fluctuations grew with time and the intensity increased with increasing shear rate. Furthermore, the results revealed that the PLLA and PDLA chains were only partially interpenetrated; consequently, stereo-complex crystals could grow only in the mixed PLLA/PDLA phase. The correlation length of density fluctuation prior to nucleation was strongly dependent on the mixed phases.research papers

Nanostructures of Stereocomplex Polylactide in Poly(l-lactide) Doped with Poly(d-lactide)

“…While remarkable observations have been reported on the quiescent stereocomplexation of PLA, increasing attention was drawn to the favoring effect of flow existing in realistic polymer processing . Not only the amount but also the T m of sc ‐PLA was significantly increased in such cases, which was attributed to the flow enhanced mixing and disentanglement of molecular chains . Nevertheless, details are still lacking on the structural formation and evolution of sc ‐PLA under flow, to the best of our knowledge.…”

Enhancement of stereocomplex formation in poly(l-lactide)/poly(d-lactide) mixture by shear

“…As such, it is anticipated that higher shear rate is particularly effective in promoting the crystallization of SC. Another possible reason for the enhancement of SC formation might be the phase mixing enlarged by shear as hypothesized by Matsuba et al [35].…”

“…Such anisotropy became more evident with increasing shear rate. Matsuba et al [35] studied the isothermal crystallization of PLLA/ PDLA equimolar blends at 200 C under shear rate/strain of 25 s À1 / 50,000% and 75 s À1 /75,000%. At~60s after the application of shear, only weak orientation was observed by SAXS and became gradually weaker, which was considered to be caused by the growth of isotropic SC.…”

“…PLLA with a small amount of SC was more sensitive to flow-induced crystallization [36]. Recently, Matsuba et al [35] studied shear-induced crystallization of equimolar PLLA/PDLA at 200 C, which was above the melting temperature (T m ) of a crystal. Enhanced SC nucleation density and shorter induction time were observed upon applying a shear field.…”

Dynamic Monte Carlo simulations of competition in crystallization of mixed polymers grafted on a substrate

“…Thus, nowadays, many researchers focus on the formation mechanism of the stereocomplexation between enantiomeric PLAs, and different methods to improve the content of SCs have been proposed. For instance, the formation of SCs can be promoted by applying a shearing field, adding specific nucleating agents, or one‐dimensional fillers, such as carbon nanotubes, synthesizing polymers with new topology, and changing crystallization temperatures …”