Substantially Enhanced Stereocomplex Crystallization of Poly(L-lactide)/Poly(D-lactide) Blends by the Formation of Multi-Arm Stereo-Block Copolymers

Abstract

Stereocomplex-type polylactide (SC-PLA) created by alternate packing of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains in a crystalline state has emerged as a growingly popular engineering bioplastic that possesses excellent hydrolytic stability and thermomechanical properties. However, it is extremely difficult to acquire high-performance SC-PLA products via melt-processing of high-molecular-weight PLLA/PDLA blends because both SC crystallites and homocrystallites (HCs) are competitively formed in the melt-crystallization. Herein, a facile yet powerful way was employed to boost SC formation by introducing trace amounts of some epoxy-functionalized small-molecule modifiers into the enantiomeric blends during reactive melt-blending. The results show that the SC formation is considerably enhanced with the in situ generation of multi-arm stereo-block PLA copolymers, based on the reaction between epoxy groups of the modifiers and hydroxyl end groups of PLAs. More impressively, it is intriguing to find that the introduction of only 0.5 wt% modifiers can induce exclusive SC formation in the blends upon isothermal and non-isothermal melt-crystallizations. The outstanding SC crystallizability might be attributed to the suppressing effect of such unique copolymers on the separation of the alternately arranged PLLA/PDLA chain segments in molten state as a compatibilizer. Furthermore, the generation of these copolymers does not result in a significant increase in melt viscosity of the blends. These findings suggest new opportunities for the high-throughput processing of SC-PLA materials into useful products.

Significantly High Melting Temperature of Homopolymer Crystals Obtained in a Poly(l-Lactic Acid)/Poly(d-Lactic Acid) (50/50) Blend

“…Furthermore, the longer durations resulted in remarkably high T m of 186.2 °C (180 min) and 187.5 °C (300 min), which are more than 10 °C higher than the T m of HC obtained for the same specimen during the nonisothermal DSC measurement (Figure ) with the same heating rate for the DSC measurement. This T m is well beyond the common range of HC melting point, namely, 170 °C–180 °C, as reported in the previous studies of PLLA/PDLA 50/50 blends. – In their review, Saeidlou et al stated that the melting temperature of PLA increases with molecular weight for low M n but reaches an asymptotical value at M n higher than 100 kg/mol. This is in reference to Jamshidi et al, who reported a T m of 184 °C for PLLA at infinitely high molecular weight.…”

“…Therefore, promoting the stereocomplexation of PLA is one of the best approaches to ameliorate its poor properties, such as hydrolysis resistance, thermal stability and heat distortion temperature. – However, the SC formation is usually accompanied by the HC formation . Many studies attempted to form SC exclusively through the addition of nucleation agents or plasticizers, , the formation of multiarm stereoblock copolymers, and by processing with the aid of ionic liquids …”

“… 6 – 8 However, the SC formation is usually accompanied by the HC formation. 9 Many studies attempted to form SC exclusively through the addition of nucleation agents 10 or plasticizers, 11 , 12 the formation of multiarm stereoblock copolymers, 13 and by processing with the aid of ionic liquids. 14 …”

Effect of multi‐epoxy compatibilizers with branched structures on enhancing mechanical and compatibility of PLA/starch composite

“…This might be because the star-shaped branched structure had a stronger crosslinking ability due to its stronger ability to link molecular chains with the increase in the number of arms. 16 Therefore, it could be concluded that a better improvement in compatibility occurs with the compatibilizer of the star-shaped branched structure.…”

“…Diao X et al used epoxy‐functionalized small molecule modifiers with different arm numbers to promote SC crystallization of PLLA/PDLA. It is found that with the increase of the arm number of epoxy‐functionalized small molecule modifiers, its entanglement ability is improved, which makes the crystallization ability of PLLA/PDLA blends increase 16 . However, there are few reports about the effect of using compatibilizers with different branched structures on the properties of PLA‐based blends.…”