Stereocomplex Crystallization Research Articles

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.

Effect of electrospun stereocomplex PLA fibers and modified cellulose nanocrystals on crystallization of poly(L‐lactic acid)

AbstractPoly(L‐lactic acid) (PLLA) is a biodegradable polymer that has attracted increasing attention due to its fantastic properties. Nevertheless, the disadvantages of PLLA, such as low crystallization rate and brittleness, limit its wide commercial applications. In the present work, electrospun stereocomplex PLA fibers (Sc‐fibers) and PLLA grafted cellulose nanocrystals (g‐CNCs) are prepared as nano‐fillers and then blended with PLLA matrix to prepare different nanocomposites. Both of nano‐fillers would accelerate the crystallization rate of PLLA, and as expected, g‐CNCs have much stronger nucleation ability towards PLLA than Sc‐fibers. Nevertheless, the crystallization of PLLA will decrease to some extent in the presence of both nano‐fillers of g‐CNCs and Sc‐fibers due to the preferential formation of stereocomplex crystals on the surfaces of CNCs. Moreover, the presence of nano‐fillers would enhance the mechanical properties of PLLA, such as the elongation at break can increase by four times after incorporating both g‐CNCs and Sc‐fibers. This work provides a new consideration for designing different nano‐fillers, especially for polymer matrices with low crystallization ability and brittleness.

Competition effect of solid-state stretching induced orientation and phase separation on stereocomplex crystallization of PLLA/PDLA during annealing

Benefiting from its superior thermal and mechanical properties, the stereocomplex crystal (SC) of PLLA/PDLA is catching increasing interest. In this work, a systematic investigation was performed to reveal the effect of solid-state stretching on the formation of SC during annealing. Results show that as the stretching temperature was close to the glass transition temperature (Ts = 60 °C), the polymer chains orientation was increased gradually with draw ratio (λ). As a result, the crystallization induction time (t0) was shortened, the growth rate (Gr) of SC was enlarged, and the crystallinity of SC (XSC) was elevated as a result of orientation-induced crystallization. Whereas, the saturation of t0, Gr, and XSC appeared quickly when λ reached 2.7, although the orientation of polymer chains continued to increase. The reason is that in addition to orientation, strong phase separation between PLLA and PDLA was also triggered therein. The phase separation prohibited greatly the crystallization of SC due to a long time would be needed for PLLA or PDLA chains to diffuse to a distance close enough to activate the hydrogen-bonding interaction, which is the prerequisite of SC crystallization. When the stretching temperature was elevated to 75 °C, the phase separation became much weaker. So XSC was higher compared to that of the blend stretched at 60 °C to the same λ. Moreover, in the annealing temperature from 180 °C–190 °C, t0 was much smaller for the blend stretched at 75 °C since the crystallization of SC in this temperature range is a diffusion-controlled process.

Effects of PDLA molecular weight on the crystallization behaviors and rheological properties of asymmetric PDLA/PLLA blends

The molecular weight of poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) affects the performance of their blends by controlling the formation and characteristics of stereocomplex (SC) crystals. In this work, the crystallization behaviors and rheological properties of asymmetric PDLA/PLLA (5/95, w/w) blends were carefully investigated considering the molecular weight of PDLA (Mn(PDLA) = 1.5 × 103–18.7 × 103 g/mol) when the molecular weight of PLLA was selected as 5.0 × 104 g/mol. The results revealed that PDLA holding too low or too high Mn(PDLA) was unfavorable to the formation of SC crystallites owing to their weak interaction or difficult diffusion in the PLLA matrix, and the optimal Mn(PDLA) was ca.5.4 × 103 g/mol. Given this optimal Mn(PDLA), the composite viscosity and tensile viscosity of the obtained PDLA/PLLA blend were increased by 2–3 orders of magnitude and 10 times compared with neat PLLA, respectively, due to the formation of SC crystals with a high content (ΔHm-SC of 26.0 J/g). Besides, this PDLA/PLLA blend showed a fast crystallization rate and strong solid-like viscoelastic behavior. This work revealed the regular correspondence between Mn(PDLA) and the performance of asymmetric PDLA/PLLA blends.

Critical Size and Formation Mechanism of Secondary Nuclei in Melt-Crystallized Polylactide Stereocomplex Crystals

Revealing the molecular mechanism and determining the length scale of the rate-limiting ordering process have been a challenge in the field of polymer crystallization. Previously, we proposed a theory to determine the size of critical nuclei in single-component crystals from the variation of nucleation kinetics with different dilution ratios. Here, we further extend this theory to polylactide (PLA) stereocomplex crystals (SCs) consisting of poly(l-lactide) and poly(d-lactide). By diluting only l-lactide units or both l-lactide and d-lactide units via random copolymerization, the number of l-lactide units and that of two types of units in a critical secondary nucleus were obtained. The results show that a critical secondary nucleus of an SC bulk crystal consists of 22–27 l-lactide units and the same number of d-lactide units when crystallized from melt at temperatures ranging from 140 to 170 °C. These repeating units form two poly(l-lactide) stems and two poly(d-lactide) stems. The four stems in a secondary nucleus of SC are chosen from approximately three to four chains. According to the number of chains, at most one-third of PLA chains has once adjacent re-entry folding in the secondary nuclei of SC crystallized from melt. These results provide rich information on the ordering process during the melt crystallization of SC crystals and are beneficial for understanding the crystallization mechanism.

Poisoning by Purity: What Stops Stereocomplex Crystallization in Polylactide Racemate?

Formation of stereocomplex crystals (SC) is an effective way to improve the heat resistance and mechanical performance of poly(lactic acid) products. However, at all but the slowest cooling rates, SC crystallization of a high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemate stops at a high temperature or does not even start, leaving the remaining melt to crystallize into homochiral crystals (HC) or an SC-HC mixture on continuous cooling. To understand this intriguing phenomenon, we revisit the SC crystallization of both high- and low-molecular-weight PLLA/PDLA racemates. Based on differential scanning calorimetry (DSC), supplemented by optical microscopy and X-ray scattering, we concluded that what stops the growth of SC is the accumulation of the nearly pure enantiomer, either PDLA or PLLA, that is rejected from the SC ahead of its growth front. The excess enantiomer is a result of random compositional fluctuation present in the melt even if the average composition is 1:1. The situation is more favorable if the initial polymer is not fully molten or is brought up to just above the melting point where SC seeds remain, as proven by DSC and X-ray scattering. Moreover, we find that not only is SC growth poisoned by the locally pure enantiomer but also that at lower temperatures, the HC growth can be poisoned by the blend. This explains why SC growth, arrested at high temperatures, can resume at lower temperatures, along with the growth of HC. Furthermore, while some previous works attributed the incomplete SC crystallization to a problem of primary nucleation, we find that adding a specific SC-promoting nucleating agent does not help alleviate the problem of cessation of SC crystallization. This reinforces the conclusion that the main problem is in growth rather than in nucleation.

Toughened transparent poly(L-lactic acid)/poly(D-lactide)-b-poly(butadiene)-b-poly(D-lactide) blended film with balanced strength

Tough and transparent poly(L-lactic acid)/poly(D-lactide)-b-poly(butadiene)-b-poly(D-lactide) (PLLA/PDLA-PB-PDLA) film with balanced strength was obtained. The PDLA-PB-PDLA triblock copolymer was firstly synthesized by ring-opening polymerization (ROP) of D-lactide using bihydroyl-terminated polybutadiene as macroinitiator, and then the obtained triblock copolymer was mixed with PLLA in solution. The blended films kept in good transparence even when the loading of PDLA-PB-PDLA reached to 15 wt%. Meanwhile, the elongation at break of the PLLA/PDLA-PB-PDLA film increased by ∼10 times without a significant reduction in tensile strength as compared with that of pure PLLA. The mechanical property was ascribed to the synergetic effect of the flexible PB chains and the stereocomplex crystals formed between PLLA matrix and PDLA chains. “Salami-like” dispersed-phase with nanometer scales was observed, which would contribute to the transparency of the blended film. The tensile fractured morphologies indicated that shear yield of the matrix and cavitation might be the major toughening mechanism of PLLA/PDLA-PB-PDLA film.

Stereocomplex crystallization in cyclic polymer blends studied using dynamic Monte Carlo simulations

Molecular simulation was used to explore the mechanism of the stereocomplex crystallization in cyclic polymer blends.

Local segmental miscibility dominating stereocomplex crystallization in polymer blends

The influence of segmental miscibility on the formation of stereocomplex crystals, while excluding other relevant factors, was studied for the first time.

Cooling rate guiding contradictory effect of thermal treated temperature on the crystallization of PLA racemic blends upon cooling

In this work, cooling rate guiding contradictory effect of thermal treated temperature on the crystallization of PLA racemic blends upon cooling was reported. In addition, it was found that, the higher cooling rate is more favorable for the formation of stereocomplex crystals (SCs). The separation between the enantiomeric chains in the racemic melt was proposed as the reason for the contradictory effect of the thermal treated temperature on the crystallization enthalpy of PLA racemic blends upon cooling. At last, by using Flash DSC, the crystallization kinetics of PLA racemic blends after thermal treated at different temperatures were compared, and the results verified the influence of the separation between the enantiomeric chains on the contradictory effect of the thermal treated temperature.

Role of the Branched PEG-b-PLLA Block Chain in Stereocomplex Crystallization and Crystallization Kinetics for PDLA/MPEG-b-PLLA-g-glucose Blends with Different Architectures.

The isothermal crystallization behavior and corresponding morphology evolution of poly(d-lactic acid) (PDLA) blends with PLLA6.7k or MPEG-b-PLLA6.7k-g-glucose with different architectures and different PLLA-grafted copolymer contents were investigated. The formation of stereocomplexes (SCs) in between the chain branched structure of MPEG-b-PLLA6.7k-g-glucose and PDLA chains acting as the physical crosslinking points slows down the motion of PDLA chains, but the SCs could act as a heterogeneous nucleating agent for the late formation of homocrystals (HCs) in the blend system, accelerating the crystallization kinetics of HCs through enhancing the nucleation density. For PDLA/MPEG-b-PLLA6.7k-g-glucose blends, the mobility of SCs in the blend system and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a Tc of 130 °C. The evolution of the crystal growth and structure during the isothermal crystallization process by rheometry has revealed the interesting role of the branched chains of MPEG-b-PLLA6.7k-g-glucose in the mechanism of the crystallization in PDLA blends.

Decomposition Behavior of Stereocomplex PLA Melt-Blown Fine Fiber Mats in Water and in Compost

This study introduces systematic and comparative investigations of various PLA fine fiber mats prepared by melt blowing. A series of PLLA and PDLA melt-blown fibers from various L and D enantiomers blends were produced. Their morphological, mechanical, and thermal properties were studied, and their decomposition in water and compost was investigated. It was found that the 1:1 ratio blend with stereocomplex crystals had an 80% lower average fiber diameter, 60% higher specific strength and better thermal stability than the PLLA and PDLA fiber mats. In the case of composting, the crystalline peak melting temperature, crystallinity, and thermogravimetric decomposition temperatures marginally decreased after 14 days. The high surface of the fine fiber mats played a crucial role in fast decomposition, as they entirely disintegrated in less than only 40 days. In the case of water, the homocrystalline domains were more susceptible to hydrolysis than the stereocomplex ones. All the PLA fiber mats underwent decomposition and extensive disintegration for 70 days in water. Hydrolysis reduced the amorphous and crystalline fraction of the fibers via surface and bulk erosion, while the decomposition of stereocomplex-crystalline-rich domains mainly exhibited surface erosion. Findings revealed that high porosity and the high surface area of PLA melt-blown fine fiber mats undergo fast decomposition in compost and in water.Graphical