Stereocomplex Crystallization Research Articles

SC crystals of porous PLA via thermally–induced phase separation: Effects of process conditions, solvent composition and nucleating agent

The formation of stereo-complex crystals (SC) in porous poly (lactic acid) (PLA) material is intricately linked with improving its comprehensive properties. In this work, the effects of process conditions, solvent composition and additional nucleating agent on SC crystals during the fabrication of porous PLA through thermally-induced phase separation method were studied. Crystallization behavior of SC crystals during the pore-forming process is systematically investigated, revealing that higher quenching temperature and longer quenching period facilitate the formation of SC crystals. Additionally, the addition of deionized water to the solvent effectively enhances SC crystals formation by reducing solution viscosity and prolonging the phase separation time. The highest crystallinity of SC crystals (XSC) can be achieved, reaching 30.6%, when the water content is controlled at 1.6%. Furthermore, the introduction of carbon nanotubes (CNTs) as a nucleating agent exhibits a pronounced effect on the formation of SC crystals. When incorporating a 3 wt% content of CNTs, XSC reaches its peak value at 37.7%. This work is helpful to provide reference for improving mechanical properties and heat resistance of porous PLA, thereby expanding its potential outdoor applications.

Phase transitions in mixtures based on poly(lactic acid) and two polymeric nucleants processed under a fast or slow cooling from the melt

AbstractMixtures based on two different poly(lactic acid)s (PLA) matrices and two distinct nucleating agents, incorporated at several contents, were prepared by extrusion. Furthermore, two thermal treatments were applied during the processing of films from the melt: a quenching (Q) and a slow (S) cooling. Crystalline characteristics of the resultant polymeric systems were mainly triggered by the PLA matrix used and by the cooling rate applied, turning out less important the nature of the nucleant. Thus, stereocomplex crystals were developed almost exclusively in the Q samples while coexistence of homo and stereocomplex crystallites was noticed when slow cooling was imposed. The type of homocrystallites present in the specimens was also dependent on the cooling rate applied during film processing. In the Q systems, crystals were grown on heating through a cold crystallization, as observed by differential scanning calorimetry; while they were developed along the cooling stage in the S materials. Influence of these structural characteristics on mechanical response was also evaluated by microhardness measurements.

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.

Toward exclusive stereocomplex crystallization of high-molecular-weight poly(L-lactic acid)/poly(D-lactic acid) blends with outstanding heat resistance via incorporating selective nucleating agents

In high molecular weight poly(L-lactic acid)/poly(D-lactic acid) (HMW PLLA/PDLA) blends, the construction of exclusive stereocomplex crystals (SC) with high crystallinity and strong melt memory remains a great challenge. In the present study, various norbornene dicarboxylate complexes (TMXNa, Mg, Al, or Ca) were employed as the stereo-selective nucleating agents (NAs), and their effect on the crystallization characteristics, rheological behavior, and heat resistance of PLLA/PDLA blends were thoroughly studied. Strikingly, TMX-Al facilitated the construction of exclusive SC with over 50 % crystallinity and excellent melt memory. The dense SC crystals network structure boosted the heat resistance of L/D-xAl blends with a VST as high as 145 °C. The strengthened intermolecular interaction fostered the generation of pre-ordered structure in the melt and enhanced chain interdiffusion, which contributed to intermolecular nucleation and SC crystallization in L/D-xAl blend. This study opens up a new avenue for melt processing and application development of SC-PLA materials.

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.

Preferential formation of the stereocomplex crystals of poly(L‐lactide) and poly(D‐lactide) blend by epoxidized soybean oil under nonisothermal crystallization

AbstractFormation of stereocomplex crystals (SCs) in enantiomeric poly(L‐lactide) (PLLA) and poly(D‐lactide) (PDLA) blends is an effective way to improve the heat resistance and mechanical performance of polylactide (PLA) products. However, obtaining PLAs with high stereocomplex content is not straightforward as the homocrystal and SCs occur simultaneously and compete with one another. In this work, various amounts of epoxidized soybean oil (SO) or SO at 3%–20% by weight were added to equal amounts of the stereoisomers (PLLA/PDLA; 50/50). From differential scanning calorimetry, it was found that epoxidized SO increased the crystallization temperature, facilitated the SC crystallization, and suppressed the homecrystal formation, whereas SO did not affect the crystallization behavior for the racemic blends. Moreover, epoxidized SO reduced the glass transition temperature, cold crystallization temperature, and melting temperature of the racemic blends. We examined how epoxidized SO affected the crystallization of PLLA/PDLA at cooling rates from 2 to 20°C/min. We discovered that epoxidized SO still encouraged SC crystallization. The activation energy, calculated from Takhor and Kissinger equations, indicated that adding 5 wt% epoxidized SO effectively promoted the crystallization and facilitated the SC formation of the PLLA/PDLA blend.

Physical modifications of cellulose nanocrystals by poly(d-lactic acid) and poly(ethylene glycol) (PDLA-PEG-PDLA) block copolymer to improve the mechanical properties of polylactide

As a green and renewable resource, cellulose nanocrystals (CNC) possess excellent mechanical properties, rendering them a favorable option for modifying polymer materials. However, improving their dispersibility in the matrix and enhancing the interface are the crucial to exploit their advantages. The physically modified CNC (mCNC) are prepared by exploiting hydrogen bonding between CNC and the block copolymer of poly(d-lactic acid) and poly(ethylene glycol) (PDLA-PEG-PDLA) through a solvent exchange method. The incorporation of the copolymer not only enhances the dispersity of CNC, but also facilitates the formation of the stereocomplex crystal (SC) structure in PLA, thereby concurrently enhancing the interface. Consequently, the PLA/mCNC nanocomposites demonstrate a notable increase in crystallization ability and the mechanical properties. In comparison to neat PLA, the yield strength, elongation at break, and tensile toughness experienced significant enhancements of 11.2 %, 265.9 %, and 280.2 %, respectively. This facile method provides a good basis for the large-scale application of cellulose nanocrystals.

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.

Stereocomplex PLLA–PBAT copolymer and its composites with multi-walled carbon nanotubes for electrostatic dissipative application

Abstract Because of its low thermal stability and brittleness, both the drawbacks of poly(l-lactide) (PLLA) were solved by forming stereocomplex (ST) and its copolymer with poly(butylene adipate-co-terephthalate) (PLLA–PBAT). In this study, we synthesized PLLA and PLLA–PBAT copolymer by ring-opening polymerization. Both polymers were blended with poly(d-lactide) to form ST crystals. Multi-walled carbon nanotubes (MWCNTs) were added into the polymer matrix at 5 phr by the solvent casting method. The surface resistance of the composite was ≅106 Ω, which is appropriate for electrostatic dissipative purposes. The copolymer and its ST crystallites were confirmed by the peaks in infrared spectra at 922 and 908 cm−1, respectively. The PLLA–PBAT copolymer had 60% lower tensile strength than PLLA and its stereocomplex, but 10% higher elongation at break. The elongation at break of the PLLA–PBAT copolymer/MWCNT composite decreased by 17% while its thermal stability slightly increased when compared to the unfilled copolymer. The melting temperature for both ST PLLA–PBAT copolymers, with and without MWCNTs, was around 225°C, which is 50°C higher than that of the homocrystals. Moreover, the glass transition temperature and crystallinity of the ST PLLA–PBAT copolymer also increased by adding MWCNTs.

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.

Preferential formation of stereocomplex crystals in poly(L-lactic acid)/poly(D-lactic acid) blends by a fullerene nucleator

Selective formation of stereocomplex (sc) crystallization in enantiomeric poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends is considered as one of the most effective and promising way to improve the mechanical and thermal properties of polylactide (PLA) materials. However, homocrystallization (hc) prevails over sc crystallization in high-molecular-weight (HMW) PLLA/PDLA blends. Herein, we propose a simple and straightforward approach for fabricating sc crystallization and suppress hc crystallization for HMW PLLA/PDLA blends through the addition of C70 as a nucleator. Non-isothermal crystallization and wide-angel X-ray diffraction studies demonstrate that, the incorporation of 1 wt% C70 overwhelmingly leads to the formation of sc crystallites, while preventing the formation of hc crystallites. Isothermal crystallization experiments at 140 °C reveal a significant reduction in the half-crystallization period of the PLLA/PDLA blend upon the addition of C70. Fourier-transformed infrared spectroscopy suggests that, the improved intermolecular interactions between PLLA and PDLA chains, as well as the inhibition of molecular chain diffusion and mobility, contribute to the accelerated formation of sc facilitated by C70. The enhanced sc crystallization results in a 15.5 °C higher thermal stability in the as-prepared PLLA/PDLA blend with 1 wt% C70 compared to the neat counterpart.

Fine-tuning heat resistance and impact toughness of natural rubber-toughened PLA at Various degrees of PLA/PDLA stereocomplexation

This study represents a new investigation into the influence of stereocomplex crystal content within the blend system of natural rubber (NR) and polylactic acid (PLA), aiming to concomitantly enhance both the impact strength and thermal properties of PLA. The NR content was fixed at 15 wt%, while the amount of stereocomplex crystals was increased by adjusting the PLA:PDLA ratio from 100:0 to 50:50 corresponding to the PDLA content ranging from 0 – 42.5 wt%. Regarding the effect of NR alone, the incorporation of well-dispersed NR into PLA resulted in a significant improvement in the impact strength with a slight increase in heat resistance of PLA as NR acted not only as a toughening agent but also as a mild nucleating agent for PLA. The addition of PDLA, especially at higher contents, provided higher % crystallinity and crystallization rate of stereocomplex crystals, consequently enhancing the heat resistance of the blends. At the optimal PDLA content of 30 wt%, the molded specimens could successfully withstand a temperature of 100 °C throughout the 24-hour test with a 3-fold higher impact strength than that of neat PLA.

Influence of Molecular Weight on the Enzymatic Degradation of PLA Isomer Blends by a Langmuir System.

Polylactides (PLAs) and lactide copolymers are biodegradable, compostable, and derived from renewable resources, offering a sustainable alternative to petroleum-based synthetic polymers owing to their advantages of comparable mechanical properties with commodity plastics and biodegradability. Their hydrolytic stability and thermal properties can affect their potential for long-lasting applications. However, stereocomplex crystallization is a robust method between isomer PLAs that allows significant amelioration in copolymer properties, such as thermal stability, mechanical properties, and biocompatibility, through substantial intermolecular interactions amid l-lactyl and d-lactyl sequences, which have been the key approach to initial degradation rate and further PLA applications. It was demonstrated that the essential parameters affecting stereocomplexation are the mixing ratio and the chain length of each unit sequence. This study deals with the molecular weight, one of the specific interactions between isomers of PLAs. A solution polymerization method was applied to control molecular weight and chain architecture. The stereocomplexation was monitored with DSC. It was confirmed that the lower molecular weight polymer showed a higher degradation rate, as a hydrolyzed fragment having a molecular weight below a certain length dissolves into the water. To systematically explore the critical contribution of molecular weights, the Langmuir system was used to observe the stereocomplexation effect and the overall degradation rate.

Preparation of stereocomplex polylactic acid fiber and evolution of crystal structures

Stereocomplex polylactic acid (ScPLA) fibers were successfully prepared by mixing equal mass melts of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) by melt spinning process. The evolution of crystal structures of ScPLA fibers was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results show that the homogeneous crystal (Hc) will be converted to stereocomplex crystal (Sc) as the heat setting temperature (HT) increases. The crystallinity curves of Hc and Sc cross into an “X” and the cross point is near the melting point of the Hc. The temperature range to form ScPLA fiber with fully stereoscomplex structures is 187 ℃ to 197 ℃ and the maximum Sc content is about 60%.

Review on Cell Structure Regulation and Performances Improvement of Porous Poly(Lactic Acid).

Recent advances in the cell structure regulation and performances improvement of porous poly(lactic acid) materials (PPMs) are systematically reviewed in this feature article. First, the typical processing methods, including template method, non-solvent induced phase separation, freeze-drying, and supercritical CO2 foaming, of PPMs are introduced emphatically. Their various cell morphologies by different processing methods are summarized: finger-like, honeycomb-like, fiber-like, through cell, open cell, closed cell, ball-like, and flower-like. Meanwhile, the transformation among different cell morphologies as well as the changes in cell size and cell density, having impact on the performances, is described. Second, the influence of stereo-complex crystals on the cell structure of PPMs is emphatically reviewed. Furthermore, the relationships between cell structure and properties that includes mechanical properties, thermal stability, heat insulation, and hydrophobicity, are elaborated. Eventually, the issues of PPMs worthy of further study are discussed.

Effect of chitin nanocrystals on stereocomplexation of poly(l-lactide)/poly(d-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.

Stereocomplex crystals induced outstanding energy storage performance with PLLA/PDLA blend film

Poly(lactic acid)-based (PLA) films with biodegradability and ease of processing are promising dielectrics for application in electrostatic capacitors. Nevertheless, the relative low energy storage capability of the PLA film greatly hampers the development of its further application. Herein, a biodegradable polymer of stereocomplex crystals of poly(lactic acid) is prepared utilizing a solution blending method of the co-blended poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) in a 1:1 ratio. The stereocomplex crystals could prominently improve insulation and mechanical properties of PLLA/PDLA blend films, thus achieving an outstanding energy density (Ud ∼ 13.7 J/cm3) accompanied with high efficiency (η ∼ 92.8%) at a breakdown electric field of 750 MV/m. Additionally, the PLLA/PDLA blend film also exhibit good high-temperature performance and reliability. This work provides a reasonable tailoring strategy for biodegradable polymer dielectrics and demonstrates a great potential of biodegradable polymers for practical applications in environmentally friendly power electronics and electrical systems.

Elevation of Flow Temperature and Structural Change of Poly(Lactic Acid) Gel Induced by Formation of Stereocomplex Crystals

AbstractPoly(lactic acid) gels formed from solutions of poly(L‐lactic acid) (PLLA) and poly(D‐lactic acid) (PDLA). X‐ray diffraction patters indicate the formation of stereocomplex crystals for not only the gel formed from solution containing equimolar PLLA and PDLA, but also the gel formed from solution containing 9:1 of PLLA and PDLA. The flow temperatures of the gels were determined by viscoelastic measurements, and the morphologies of the dried gels were observed by field emission scanning electron microscopy. The flow temperatures of the gels from solutions containing both PLLA and PDLA were higher than those containing only PLLA. Network structures formed by uniformly distributed thin fibers, were observed for the gels formed from solutions with L:D ratios near the equimolar ratio. The dependency of the flow temperature and morphology on the polymer concentration and L:D ratio of the gels was discussed based on the experimental data.

Quasi block copolymers noncovalent bonded by stereocomplex crystals

Self-assembly of block copolymers (BCPs) yields unparalleled opportunities to integrate exquisite characteristics for preparation of advanced nanotechnology materials. The high-yield manufacturing of BCP is extremely desired, yet rarely exploited. Herein, we displayed that stereocomplex crystals (sc) of poly lactide (PLA) enantiomers, acting as pseudo linkage between adjacent polymer chains, enable formation of quasi-block copolymer (qBCPs) by straightforward melt compounding. Identical content of poly (D-lactide) (PDLA) and poly (L-lactide) (PLLA) were pre-grafted onto the backbones of poly styrene (PS) and poly (methyl methacrylate) (PMMA), respectively. Due to the complementary multiple hydrobonding interactions between PDLA and PLLA grafted moieties, PS-“block”-PMMA connected by cocrystallized sc was induced during the subsequent melt processing of PDLA-g-PS/PLLA-g-PMMA, thus allowing local segregation of PS-“block” and PMMA-“block” with nanometer size. The refined morphology, which was resembled with micro-phase separation of classical BCP, can be facilely tuned by varying the length and ratio of enantiomeric PDLA/PLLA grafts. More interestingly, the PS-“block”-PMMA typed qBCPs demonstrated superiority in compatibilizing immiscible PS/PMMA polymer blends, as compared with the classical compatibilizer (PS-block-PMMA) via reactive blending. Overall, our strategy represents a general method to construct noncovalent linked di-block copolymer via industrially relevant melt processing, which is quite challenging to achieve by existing synthetic technologies.

Construction of fully biodegradable poly(L-lactic acid)/poly(D-lactic acid)-poly(lactide-co-caprolactone) block polymer films: Viscoelasticity, processability and flexibility

Development of biodegradable polymer films is essential for sustainable energy conservation and ecological protection. In this work, to improve the processability and toughness of poly(lactic acid) (PLA) films, poly(lactide-co-caprolactone) (PLCL) segments were introduced into poly(L-lactic acid) (PLLA)/poly(D-lactic acid) (PDLA) chains via chain branching reactions during reactive processing, and fully biodegradable/flexible PLLA/D-PLCL block polymer with long-chain branches and stereocomplex (SC) crystalline structure was prepared. Compared with neat PLLA, PLLA/D-PLCL exhibited much higher complex viscosity/storage modulus, lower tanδ values in terminal region and obvious strain-hardening behavior. Through biaxial drawing, PLLA/D-PLCL films were prepared, which showed improved uniformity and non-preferred orientation. With increasing draw ratio, the total crystallinity (Xc) and Xc for SC crystal both increased. By introduction of PDLA, the two phases of PLLA and PLCL penetrated and entangled with each other, and the phase structure transformed from “sea-island” structure to “co-continuous network” structure, which was beneficial for exerting the toughening effect of flexible PLCL molecules on PLA matrix. The tensile strength and elongation at break of PLLA/D-PLCL films increased from 51.87 MPa and 28.22 % of neat PLLA film to 70.82 MPa and 148.28 %. This work provided a new strategy for developing fully biodegradable polymer films with high performance.