An efficient, food contact accelerator for stereocomplexation of high-molecular-weight poly( -lactide)/ poly( -lactide) blend under nonisothermal crystallization

Bio-based poly(lactic acid) foams with enhanced mechanical and heat-resistant properties obtained by facilitating stereocomplex crystallization with addition of D-sorbitol

The preparation of bio-based poly(lactic acid) (PLA) foams with high mechanical properties and heat resistance is of great significance for environmental protection and green sustainable development. In this paper, D-sorbitol (DS) containing six hydroxyl groups was introduced into poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blends for first time to promote the formation of stereocomplex (SC) crystals, which could improve the foaming behavior and enhance mechanical properties and heat resistance of PLA foams. The results showed that DS could improve the formation efficiency and crystallinity of SC crystals by enhancing the hydrogen bonding between the enantiomeric molecular chains. Furthermore, the compression modulus and interactions Vicat softening temperature of the PLLA/PDLA/DS blend foam increased about 854% and 16% compared to the pure PLLA foam, respectively. Besides, when the annealing process was introduced, the compression and heat resistance of the PLA foams increased further. This study provided a feasible strategy for the preparation of bio-based and biodegradable PLA foams with outstanding compressive and heat resistance properties.

Experimental evidence for immiscibility of enantiomeric polymers: Phase separation of high-molecular-weight poly(ʟ-lactide)/poly(ᴅ-lactide) blends and its impact on hindering stereocomplex crystallization

Although polymers tend not to mix, it remains challenging to characterize the immiscibility of enantiomeric poly(ʟ-lactide) (PLLA) and poly(ᴅ-lactide) (PDLA), particularly with equivalent and high molecular weight (high MW), which frustratingly disfavors the exclusive stereocomplexation. By introducing a random copolymer (PLC) of ʟ-lactide and caprolactone to form binary blends with PLLA and PDLA, the phase behavior of high-MW PLLA/PDLA blends was investigated mainly by using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). DSC results showed that PLLA/PLC blends exhibited a single glass transition temperature (Tg), which depended on the blending ratio and precisely corresponded with the theoretical values calculated from the Fox equation. In comparison, PDLA/PLC blends showed composition-dependent heat-capacity increment at two unchanged Tg values of pure PLC and PDLA. AFM observation revealed that PLC is completely miscible with PLLA at high MW but is immiscible with PDLA, logically suggesting immiscibility of high-MW PLLA and PDLA. Moreover, AFM results demonstrated that high-MW PLLA/PDLA blends exhibited spherical droplets in asymmetric blends and bicontinuous interpenetrating worm-like patterns in symmetric counterparts, showing distinct and well-defined interfaces, confirming the microphase separation. Additionally, different MWs fundamentally led to significant differences in miscibility, which consequently affected the crystallization behaviors of PLLA/PDLA blends. This work provides evidence for (im)miscibility and its crucial impact on the crystallization of PLLA/PDLA blends and has important implications for understanding the stereocomplexation of polymers.

Remarkably enhanced stereocomplex crystallization of high-molar-mass enantiomeric polylactide blends by adding double-grafted copolymers

Stereocomplex (SC) crystallization can prominently improve the physico-chemical properties of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blends, yielding a novel polylactide (PLA) material. However, the predominant formation of SC crystals in the melt-processing of high-molar-mass (high-MW, >100 kg/mol) enantiomeric PLA blends remains a huge challenge due to the competition between SC crystallization and homocrystallization. Herein, double-grafted copolymer having both PLLA and PDLA side chain has been designed and synthesized as an efficient crystallization promoter for the harvest of SC crystals in the high-MW PLLA/PDLA blends. The results show that, with the addition of such a copolymer, the blends can preferentially crystallize into SC crystals in both isothermal and non-isothermal conditions. Promisingly, the SC crystals can be exclusively formed by adding only small amounts (e.g., 0.5 wt%) of the copolymer, without the formation of any homocrystals. This interesting observation can be interpreted by the crucial role of the unique copolymer in suppressing the phase separation of the opposite PLA enantiomers upon melting as an efficient compatibilizer and then encouraging the generation of alternatingly arranged PLLA/PDLA chain clusters favored for SC nucleation and crystal growth. These findings provide new inspiration for the development of high-performance PLA with desirable SC crystallizability.

Effect of chitin nanocrystals on stereocomplexation of poly( -lactide)/poly( -lactide) blends

Using polysaccharide nanocrystals such as chitin nanocrystals (ChNCs) as nanofiller for biodegradable aliphatic polymers is an attractive way of developing all-degradable nanocomposites. Crystallization study is vital for well regulating final performance of these type polymeric nanocomposites. In this work, ChNCs were incorporated with the poly(l-lactide)/poly(d-lactide) blends and as-obtained nanocomposites were used as target samples for the study. The results showed that ChNCs acted as nucleating agent, promoting the formation of stereocomplex (SC) crystallites and accelerating overall crystallization kinetics as a result. Therefore, the nanocomposites possessed higher SC crystallization temperatures and lower apparent activation energy as compared to the blend. However, the formation of homocrystallites (HC) was dominated by nucleation effect of SC crystallites and accordingly, the fraction of SC crystallites reduced more or less in the presence of ChNCs, despite the nanocomposites possessed higher rate of HC crystallization. This study also provided valuable information on accessing more applications of ChNCs to be used as SC nucleator for polylactide.

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

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.

Promoted formation of stereocomplex in enantiomeric poly(lactic acid)s induced by cellulose nanofibers

Stereocomplex (SC) crystallization between enantiomeric poly(_L-lactic acid) (PLLA) and poly(_D-lactic acid) (PDLA) is believed to yield poly(lactic acid) (PLA) with superior physiochemical properties. However, homocrystallization (HC) crystallites are inevitably generated in the PLLA/PDLA blends. Herein, we report a simple approach to fabricate PLLA/PDLA racemic blends with high contents of SC crystallites by introducing cellulose nanofibers (CNFs). The isothermal crystallization results revealed that the half-crystallization time of the PLLA/PDLA blend was significantly decreased by adding CNFs. Additionally, with the incorporation of 3 wt% modified CNFs, the PLLA/PDLA blend was overwhelmingly crystallized into SC crystallites with no HC crystallite formation. Based on Fourier transform infrared spectroscopy findings, it was speculated that the preferred SC crystallization of PLLA/PDLA/CNF was caused by enhanced interchain molecular interactions between CNFs and PLA. This work presents a feasible and efficient method to fabricate PLA with exclusively SC crystallites, which possesses great potential for producing high-performance PLA materials.

Effect of Storage Conditions on the Thermal Stability and Crystallization Behaviors of Poly(L-Lactide)/Poly(D-Lactide)

Polylactide (PLA) is a biodegradable thermoplastic aliphatic polyester. The thermal stability and crystallization behavior of PLA are extremely sensitive to storage, processing, and usage conditions. This work systematically studied the thermal stability and crystallization behavior of poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and a PLLA/PDLA (LD) blend, which were stored under two sets of laboratory storage conditions: (1) stored in a vacuum-free desiccator and (2) stored in vacuum-sealed bags. Both were stored at room temperature for 3 years. Gel permeation chromatography results revealed that the PLLA, PDLA, and LD samples hydrolyzed slowly when stored in vacuum-sealed bags and degraded significantly when stored in a vacuum-free desiccator; this process significantly reduced the thermal stability of the samples stored in the vacuum-free desiccator. Owing to hydrolysis, the levorotation and dextrorotation (L- and D-) molecular chains were shortened; consequently, more nuclei were formed, and this caused the melting points of the PLLA, PDLA, and LD samples to decrease and the melting enthalpy of the crystals in these samples to increase. Wide-angle X-ray diffraction analysis revealed that when the L- and D- molecular chains were packed side by side to form stereocomplex crystals and the randomly arranged L- and D- molecular chains were easy hydrolyzed and degraded, this interfered with the formation of homocrystals in LD. When PLLA, PDLA, and LD samples are stored in a vacuum-free desiccator, they will be significantly hydrolyzed, resulting in the formation of only stereocomplex crystals, and no homocrystals are observed.