Formation Of Stereocomplex Crystals Research Articles

Enhancing the foamability of poly(L-lactide-co-ε-caprolactone) through formation of stereocomplex crystal by introducing of poly(D-lactic acid)

The bio-elastomer poly (L-lactide-co-ε-caprolactone) (PLCL) with linear random molecular chain structure exhibits low viscosity and melt strength, resulting in poor foamability, cell merging, or collapse during the foaming process. Consequently, PLCL foams have large cell sizes and low cell densities, limiting their use as biodegradable elastomeric foams. To enhance the foamability of PLCL, poly (D-lactic acid) (PDLA) was incorporated into the PLCL matrix. Stereocomplex crystals (Sc-crystals) formed between the L-LA segments of PLCL and the molecular segments of PDLA. The effects of varying PDLA content on the basic properties and foaming behavior of the blends were investigated. Results indicate that optimal Sc-crystal formation occurs in the PLCL/PDLA-10 blend. The presence of Sc-crystals enhances melt strength and facilitates cell nucleation, reducing cell size from 351.2 to 11.8 μm and increasing cell density from 1.4 × 104 to 1.7 × 108 cells/cm³. Consequently, the mechanical properties of the foam are significantly improved. The Sc-crystal formation between PLCL and PDLA provides an effective method to enhance the processability of linear chain copolymers containing L-lactide.

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.

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.

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.

Formation of Stereocomplex Crystal and Its Effect on the Morphology and Property of PDLA/PLLA Blends.

Stereocomplex-polylactic acid (SC-PLA) is obtained in poly(d-lactic) acid/poly(l-lactic) acid (PDLA/PLLA) blends under adjusting processing conditions. It is found that the degree of crystallinity of overall SC-PLA is up to 43.7% in PDLA/PLLA blends of 1:1 mass ratio. Formation of stereocomplex (SC) crystals forces molecular chains in the blends to be more closely arranged and further enhances interaction between molecular chains, thus forming a physical cross-linking network in the SC crystals, resulting in the blends having a special microstructure. The mechanism of formation of the SC crystal physical cross-linking network is elucidated by dielectric spectroscopy, and the relationships between homocomplex (HC) crystals, SC crystals, and amorphous regions in the blends are also analyzed. Interestingly, mechanical properties of the blends are significantly improved due to formation of an SC crystal cross-linking network.

Formation of PLA stereocomplex crystals during melt-blending of asymmetric PLLA/PDLA/PMMA blends of varying miscibility

Polylactide (PLA) enantiomers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) were melt-blended with poly(methyl methacrylate) (PMMA) and compression-molded at a temperature between the melting points of PLA homocrystals (Tm,HC) and stereocomplex (SC) crystals (Tm,SC), causing the selective formation of SC crystallites. The degree of crystallinity of the SC crystals (χc,SC) did not change with the PMMA weight fraction but did vary with changes in the weight ratio of PLLA to PDLA. From differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements, the miscibility of PLA and PMMA was confirmed, but the formation of SC crystals during melt-blending induced phase separation into PLA-rich and PMMA-rich phases. The formation of homocrystals was hindered by increases in the weight fraction of PMMA and χc,SC. The thermal and viscoelastic properties of the PLLA/PDLA/PMMA blends were also affected by the PMMA weight fraction and χc,SC. According to the DMA results, the storage modulus of the ternary blends with higher χc,SC values showed a gentler decrease at the glass transition temperature; the ternary blends also exhibited a higher storage modulus than the PLLA/PMMA blends at high temperatures near Tm,HC. PDLA weight fraction dependence of stereocomplex crystallinity χc,SC for PLLA/PDLA/PMMA blends with PDLA weight fractions of 5, 10, and 20 wt%.

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

It is an effective strategy to improve the heat resistance of polylactide (PLA) via forming stereocomplex crystals (SCs) in poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blend. In this work, pentearythritol (PENTA), which is an environment-friendly polyhydroxy compound for food packaging, is selected to facilitate the SC crystallization and suppress the HC crystallization for HMW PLLA/PDLA blend. Impressively, only 3 wt% PENTA is added, the exclusive SCs with high content (Xc,SC = 50%) can be obtained, which exhibits a high promotion efficiency of crystallization of SCs. Interestingly, with PENTA content ascending, the crystallinity of SCs increases drastically first and then in a relatively slow manner. Furthermore, Miscibility with high content of PENTA leads to a relatively looser packing of SC lamella in the blend confirmed by scanning electron microscope (SEM) due to intercalation of PENTA between crystal lamella. The mechanism of how PENTA facilitates the formation of SCs at molecular level is also proposed in this work.

Relationship between the crystallization behavior of poly(ethylene glycol) and stereocomplex crystallization of poly(L‐lactic acid)/poly(D‐lactic acid)

AbstractPoly(l‐lactic acid) (PLLA) is a good biomedical polymer material with wide applications. The addition of poly(ethylene glycol) (PEG) as a plasticizer and the formation of stereocomplex crystals (SCs) have been proved to be effective methods for improving the crystallization of PLLA, which will promote its heat resistance. In this work, the crystallization behavior of PEG and PLLA/poly(d‐lactic acid) (PDLA) in PLLA/PDLA/PEG and PEG‐b‐PLLA/PEG‐b‐PDLA blends has been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. Both SCs and homocrystals (HCs) were observed in blends with asymmetric mass ratio of PLLA/PDLA, while exclusively SCs were observed in blends with approximately equal mass ratio of PLLA/PDLA. The crystallization of PEG was only observed for the symmetric blends of PLLA39k/PDLA35k/PEG2k, PLLA39k/PDLA35k/PEG5k, PLLA69k/PDLA96k/PEG5k and PEG‐b‐PLLA31k/PEG‐b‐PDLA27k, where the mass ratio of PLLA/PDLA was approximately 1/1. The results demonstrated that the formation of exclusively SCs would facilitate the crystallization of PEG, while the existence of both HCs and SCs could restrict the crystallization of PEG. The crystallization of PEG is related to the crystallinity of PLLA and PDLA, which will be promoted by the formation of SCs. © 2017 Society of Chemical Industry

Effect of the melt‐mixing condition on the physical property of poly(l‐lactic acid)/poly(d‐lactic acid) blends

ABSTRACTThe effect of the mixing condition in a mill‐type mixer on the thermal property and the crystal formation of the poly(l‐lactide)/poly(d‐lactide) blends is investigated. The blends melt‐mixed at 200 and 210 °C under application of a high shear flow tend to show a single melting peak of the stereocomplex crystal (SC) in the differential scanning calorimetry first and second heating processes without indicating the trace of the melting of homo‐chiral crystal. The mixing at an elevated temperature causes a serious thermal degradation. Further kneading of the blends at an elevated temperature higher than Tm of SC causes the transesterification between the same enatiomeric chains forming block copolymers of l‐ and d‐chains. This block copolymer acts as a nucleating agent of SC and the compatibilizing agent between poly(l‐lactide) and poly(d‐lactide) and promotes the formation of SC. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45489.

Inducing Stereocomplex Crystals by Template Effect of Residual Stereocomplex Crystals during Thermal Annealing of Injection-Molded Polylactide

In the present work, the injection-molded poly(l-lactide)/poly(d-lactide) parts were thermally treated at nearly below the melting point of stereocomplex crystals (SCs) (∼210 °C) to prepare polylactides (PLAs) with good heat resistance, and it is found that exclusive formation of SC is effectively achieved. It is worth mentioning that injection-molded PLAs used in thermal treatment were first prepared by employing a high mold temperature (120 °C) to ensure proper crystallinity and SC content, which makes sure that no warpage of PLA was noticed during thermal treatment. Consequently, superbly heat resistant injection-molded parts with Vicat softening temperature of ∼200 °C were successfully achieved. Meanwhile, the template effect of residual SCs, as evidenced by in situ X-ray diffraction results, was proposed to explain the high efficiency of thermal treatment. Specifically, the residual SC in PLA melts with perfect surface and well-defined lattice parameters can act as template to guide formation of new SCs.

Thermostable and Impermeable “Nano-Barrier Walls” Constructed by Poly(lactic acid) Stereocomplex Crystal Decorated Graphene Oxide Nanosheets

In contrast to the relatively clear understanding of epitaxial crystallization induced by one-dimensional nanofillers, the underlying interfacial interactions between polymer crystals and two-dimensional graphene oxide (GO) nanosheets are something of a mystery. Here, the GO-assisted formation of poly(lactic acid) (PLA) stereocomplex crystals (SCs) is disclosed from the quantitative structural analysis to the direct morphological observations at multiscale and the interaction mechanism at the molecular level. It is unexpected to observe that the edges of GO featuring rich grooves and ultralow thickness were ready to induce a layer of ordered lamellae, in clear contrast to the random growth of lamellae on the basal planes. The origin of GO-induced crystallization was appraised from the interaction point of view as indicated by the evident red-shift of a set of functional groups in the Fourier transform infrared spectroscopy spectra. More importantly, the GO nanosheets, albeit presented at an extremely low content (0.05 wt %), decorated by the preferred formation of SCs enabled the simultaneous enhancement of gas barrier properties and resistance to heat distortion. Specifically, the unique combination of greatly improved heat deformation temperature (HDT) and low oxygen permeability coefficient (PO2) for the composite crystallized at 165 °C was demonstrated (146.5 °C and 0.95 × 10–15 cm3 cm cm–2 s–1 Pa–1), outperforming pure PLA with an increment of 75% and a decrease of 77% in HDT and PO2, respectively. The proposed methodology affords elucidation of well-tailored thermal and barrier properties, which may motivate further extension of this rational design to other material combinations.

Solution crystallization analysis of poly(lactic acid) by Scalls: A facile approach for thermal analysis of polymers in solution

Non-isothermal solution crystallization and dissolution behaviour of both enantiomers (D and L) of poly(lactic acid) (PLA) and their blends were studied by the unique Solution Crystallization Analysis by Laser Light Scattering (Scalls) method. For the first time, we have investigated the crystallization of this biopolymer in solution, as well as the subsequent dissolution or “solution melting”. It was found that addition of the D-enantiomer (PDLA) to the L-enantiomer (PLLA) in solution resulted in the formation of stereocomplex crystals (SC), and the nucleation-effect of the crystals was intensified with increase in PDLA content, leading to an earlier onset of crystallization and increased crystallization peak area. Differential Scanning Calorimetry (DSC) analysis confirmed the formation of SC during solution crystallization. Large re-crystallization events were seen for the pristine polymers, indicative of their low crystallization rates. Overall, results obtained by Scalls provided promising information regarding PLA crystallization kinetics, which significantly influences practical applications of this biopolymer.

Precursors in stereo-complex crystals of poly(L-lactic acid)/poly(D-lactic acid) blends under shear flow

To improve the mechanical and the thermal performance of poly(lactic acid) materials, this work focuses on the formation of stereo-complex crystals by blending poly(L-lactic acid) (PLLA) with poly(D-lactic acid) (PDLA). The resulting structure was analyzed using time-resolvedin situX-ray scattering, optical microscopy, differential scanning calorimetry and viscoelastic measurements. The objective of this study is to investigate the effect of shear flow imposed prior to crystallization on higher-order structure formation and acceleration of stereo-complex crystal growth of PLLA and PDLA blends using a wide spatial scale analysis and viscoelastic measurements. Density fluctuations of 100 nm scale were observed prior to nucleation byin situsimultaneous wide- and small-angle X-ray scattering measurements. These density fluctuations grew with time and the intensity increased with increasing shear rate. Furthermore, the results revealed that the PLLA and PDLA chains were only partially interpenetrated; consequently, stereo-complex crystals could grow only in the mixed PLLA/PDLA phase. The correlation length of density fluctuation prior to nucleation was strongly dependent on the mixed phases.

Temperature dependence of crystalline transition of highly-oriented poly(l-lactide)/poly(d-lactide) blend: In-situ synchrotron X-ray scattering study

Formation of stereocomplex crystals (sc-crystals) of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blend has been recognized as a unique opportunity to dramatically improve the heat-resistant of poly(lactic acid) (PLA). In this study, we investigated the dynamic formation and transition of sc-crystals in PLLA/PDLA drawn film with a combination of in-situ synchrotron wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The correlation between sc-crystals content and the competing formation of α crystals (stable phase of PLA) in homopolymers during continuous heating and cooling processes was also studied. It was found that at room temperature, the original PLLA/PDLA drawn film consisted of only α crystals, however, with temperature increasing, two populations of sc-crystals were formed at different temperatures from the oriented amorphous region and the molten α crystals in the highly-oriented sample, respectively. Furthermore, new types of sc-crystals and α crystals with the orientation perpendicular to the original sc-crystals were formed during subsequent cooling process. On the basis of the X-ray scattering and DSC data, a schematic model for crystallization and oriented variation concerning sc-crystals and α crystals was proposed.