Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

Effect of stereo-complexation on crystallization behavior and barrier properties of poly-lactide

The unique stere-complex crystal formed by poly(ʟ-lactide)/poly(ᴅ-lactide) (PLLA/PDLA) has a significant impact on properties of poly-lactide materials and is considered an effective means to improve the barrier properties of poly-lactide (PLA). In this work, poly-lactide films with different aggregate structures were prepared and the relationship of aggregate structure and barrier properties were explored. The results show that the crystal structure including crystallinity and crystal forms can be controlled by adjusting the isothermal crystallization time and crystallization temperature during the molding process. PLLA/PDLA composite films contain both homochiral crystallites and stereo-complex crystallites, and there is a synergistic crystallization effect between the two of them, which provides the composite films with high crystallinity and excellent barrier properties. Compared to the PLLA with homochiral crystallites, the PLLA/PDLA composite film with only stereo-complex crystallites exhibits higher barrier properties. The linear correlation between the crystallinity and the barrier properties is weak due to the changes in crystallization behavior and then the structure of poly-lactide caused by stereo-complexation. The linear correlation between the crystallinity and the barrier properties of the blend film is strong in the low crystallinity but weak at high crystallinity. Compared to homochiral crystallites, stereo-complex crystallites exhibits lower crystallinity dependence. It has been proven that different crystal forms have different design ideas for preparing high-barrier films, but the stereo-complexation resulting from the intermolecular forces between PLLA and PDLA having complementary chemical structure, is an effective method for enhancing the barrier performances of poly-lactide sustainably.

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.

Enhanced stereocomplex crystalline polylactic acids in melt processed enantiomeric bicomponent fiber configurations

The formation of stereocomplex crystalline domains in the bicomponent fiber melt spinning of enantiomeric polylactic acids (PLAs) is systematically explored and enhanced. Here we report a polycrystalline morphology where distinctly different crystalline regions are formed and aligned along the longitudinal direction of the fiber. This approach employs side-by-side and sheath-core bicomponent melt spinning configurations where the two components are the enantiomeric pairs of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). We demonstrate the formation of the PLA stereocomplexes at the junction interphase through the melt spinning process which subsequently crystallize into a round fibers with stereocomplex and homogeneous crystal lamella morphologies. The fiber morphologies and crystallinities of the melt processed fiber are substantially different from the solution based bicomponent spinning system reported in the prior literature. Furthermore, the different molecular weight in the PLLA/PDLA pairing are found to be crucial to the structural development and properties of the PLA polycrystalline materials. The solid-state annealing does not change the crystal distribution of the crystalline domains and stereocomplex crystalline state, it just enhances the homo-crystallinity in the peripheral of the bicomponent fibers.

Continuous Spectrum of Morphologies and Phase Behavior across the Contact Zone from Poly(l-lactide) to Poly(d-lactide): Stereocomplex, Homocrystal, and Between

The enantiomeric ratio is a key factor affecting the crystallization behavior and morphology of poly-l-lactide/poly-d-lactide (PLLA/PDLA) blends. Despite a number of studies on crystallization of nonequimolar PLLA/PDLA blends, a full picture of the effect of the L/D ratio is still lacking. Here, we put the two enantiomers in contact and allow interdiffusion above the melting point of the stereocomplex crystal (SC) to prepare samples with a continuously changing L/D ratio from enantiopure PLLA (ratio 0/100) to enantiopure PDLA (100/0). Using polarized optical microscopy, atomic force microscopy, and microbeam X-ray diffraction, the continuous spectrum of morphologies and phase behaviors across the contact zone is investigated. The blend morphology shows clear evidence of "poisoning by purity" of SC crystallization at all blend compositions. The low birefringence of the 50/50 SC is found to be due to the meandering of broken edge-on lamellae. Its further decrease to near zero as L/D deviates further away from 50/50 is explained by transition from radial edge-on lamellae to fully random meandering lamellae, then to mixed flat-on lamellae, and finally to submicron-sized axialites. In comparison with the smooth and straight homocrystal (HC) lamellae of pure enantiomers, the lamellae in the blends often have serrated edges caused by pinning by rejected excess enantiomer acting as an impurity during lamellar growth. A feature of the binary phase diagram is pure enantiomers acting as an impurity to the SC and counter-enantiomer acting as an impurity to homocrystallization of the enantiomers. Crystallization was found to be most suppressed at 99% enantiomeric purity, where the amount of the counter-enantiomer is insufficient for creation of SC nuclei and HC growth is inhibited by the small amount of the enantio-impurity. These and other intriguing results are less likely to be noticed without the continuous composition gradient of the contact sample.

Stereocomplexation of enantiomeric star-shaped poly(lactide)s with a chromophore core

Herein, we aim to investigate the influence of the cooling rate from the melt on stereocomplex formation of equimolar blends of enantiomeric star-shaped poly(lactide)s with a dipyridamole core.

Poly(lactic acid) stereocomplexes: A decade of progress

Upon blending enantiomeric poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] and poly(d-lactide) (PDLA) [i.e., poly(d-lactic acid) (PDLA)] or synthesis of stereo block poly(lactide) [i.e., poly(lactic acid) (PLA)], a stereocomplex (SC) is formed. PLA SC has a higher melting temperature (or heat resistance), mechanical performance, and hydrolysis-resistance compared to those of neat PLLA and PDLA. Because of such effects, PLA SC has been extensively studied in terms of biomedical and pharmaceutical applications as well as commodity, industrial, and environmental applications. Stereocomplexation stabilizes and strengthens PLA-based hydrogel or nanoparticles for biomedical applications. Stereocomplexation increases the barrier property of PLA-based materials and thereby prolongs drug release from PLA based materials. In addition, PLA SC is attracting significant attention because it can act as a nucleating agent for the widely used biobased polymer PLLA and thereby the heat resistance of PLLA-based materials can be enhanced. Interestingly, a wide variety of SCs other than PLA SC are found to have been formed in the enantiomeric substituted PLA blends and stereo block substituted PLA polymers. In the present review article, a decade of progress in investigation of PLA SCs is summarized.

Effect of Stereochemistry on Directed Self-Assembly of Poly(styrene-b-lactide) Films on Chemical Patterns

We demonstrated here for the first time that the stereochemistry of polylactide (PLA) blocks affected the assembly behaviors of PS-b-PLA on chemical patterns. Two PS-b-PLA block copolymers, where the PLA block is either racemic (PDLLA) or left-handed (PLLA), were synthesized and directed to assemble on chemical patterns with a wide range of L_s/L₀. PS-b-PDLLA was stretched up to 70% on chemical patterns, while PS-b-PLLA was only stretched by 20%. The assembly behavior of PS-b-PDLLA was different from AB diblock copolymer, but similar to that of ABA triblock copolymer. The high stretchability might be attributed to the formation of stereocomplexes in PDLLA blocks. Compared to ABA triblock copolymers, stereocomplexed diblock copolymers have much faster assembly kinetics. This observation provides a new concept to achieve large process windows by the introduction of specific interactions, for example, H-bonding, supramolecular interaction, and sterecomplexation, between polymer chains.

Powder metallurgy inspired low-temperature fabrication of high-performance stereocomplexed polylactide products with good optical transparency

Stereocomplexation between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) provides an avenue to greatly enhance performance of eco-friendly polylactide (PLA). Unfortunately, although the manufacturing of semicrystalline polymers generally involves melt processing, it is still hugely challenging to create high-performance stereocomplexed polylactide (sc-PLA) products from melt-processed high-molecular-weight PLLA/PDLA blends due to the weak crystallization memory effect of stereocomplex (sc) crystallites after complete melting as well as the substantial degradation of PLA chains at elevated melt-processing temperatures of ca. 240-260 °C. Inspired by the concept of powder metallurgy, here we report a new facile route to address these obstacles by sintering of sc-PLA powder at temperatures as low as 180-210 °C, which is distinctly different from traditional sintering of polymer powders performed at temperatures far exceeding their melting temperatures. The enantiomeric PLA chain segments from adjacent powder particles can interdiffuse across particle interfaces and co-crystallize into new sc crystallites capable of tightly welding the interfaces during the low-temperature sintering process, and thus highly transparent sc-PLA products with outstanding heat resistance, mechanical strength, and hydrolytic stability have been successfully fabricated for the first time.

WITHDRAWN: PLA Stereocomplexes: A Decade of Progress

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Thermally stable polylactides by stereocomplex formation and conjugation of both terminals with bio-based cinnamic acid derivatives

A series of polylactide stereocomplexes with conjugation of both terminals showed high thermal stability and photoreactivity, and are useful as high performance polymers and photoreactive polymers based on natural products.

Thermal and Mechanical Properties of Biodegradable Star-Shaped/Linear Polylactide Stereocomplexes

The 6-arm star-shaped poly(L-lactide) (6PLL)/linear poly(D-lactide) (1PDL) stereocomplex films were prepared by the solvent casting method. The influences of the 6PLL/1PDL blend ratios (75/25, 50/50, and 25/75 w/w) and 1PDL molecular weights (15,000, 30,000, and 60,000 g/mol) on the thermal and mechanical properties of the stereocomplex films were investigated. The 6PLL and 1PDLs had a single melting temperature (Tm) of homocrystallites at 174°C and 167°C, respectively. The 6PLL/1PDL blends had twoTms: a lowerTmof homocrystallites (160–173°C) and a higherTmof stereocomplex crystallites (219–228°C). The stereocomplex crystallinity and mechanical properties of the 6PLL/1PDL blend films were higher than those of 6PLL and 1PDL and were the highest in the 50/50 (w/w) blend ratio. However, the stereocomplex crystallinity of the 50/50 (w/w) 6PLL/1PDL blend films decreased and the mechanical properties increased as the molecular weight of 1PDL increased.

Efficient stereocomplex crystallization in enantiomeric blends of high molecular weight polylactides

Stereocomplex crystallization at high temperatures in PLLA/PDLA blends can be improved selecting conditions that reduce the loss of optical purity arising from transesterification reactions.