Enantiomeric Polymers Research Articles

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.

Photoiniferter RAFT Polymerization of Optically Active Hydrophilic Vitamin B5 Analogous Methacrylamide

Chiroptical polymers are interesting due to diverse applications in the field of chromatography, biomedicines, and catalysis. This study describes the synthesis of new types of optically active polymers of vitamin B5 analogous methacrylamide (B5AMA) using a photoiniferter polymerization approach at ambient temperatures. Enantiomers of B5AMA monomer ((R) and (S)) were prepared by the ring opening method and were characterized for their optical activity. The monomers were then polymerized by a photoiniferter polymerization approach using two different thiocarbonylthio (TCT) compounds as iniferters. Both TCTs showed controlled polymerization of B5AMA enantiomers and yielded polymers of opposite optical rotation. The enantiomeric polymers of B5AMA exhibited a complementary mirror-image Cotton effect to each other in the circular dichroism (CD) spectra, and 13C NMR analysis revealed the formation of syndiotactic polymer structures. To the best of our knowledge, this is the first study that depicts the successful synthesis of optically active macromolecules by a photoiniferter polymerization approach.

Stereocomplex crystallization of chiral hydroxyalkanoic acid-based biodegradable alternating copolyesters with two types of chiral centers and opposite monomer configurational combinations of L-D and D-L (S-R and R–S)

Stereocomplex (SC) crystallization of chiral copolyesters was normally reported for those having the two types of chiral centers and identical monomer configurational combinations (MCCs) of L-L and D-D (or S–S and R-R). Herein, we report for the first time the SC crystallization of chiral hydroxyalkanoic acid-based biodegradable alternating copolyesters with the two types of chiral centers and the opposite MCCs of L-D and D-L (or S-R and R–S). Poly(l-lactic acid-alt-d-2-hydroxybutanoic acid) [P(LLA-alt-D-2HB)] with MCC of L-D and poly(d-lactic acid-alt-l-2-hydroxybutanoic acid) [P(DLA-alt-L-2HB)] with MCC of D-L were synthesized and their SC crystallization was confirmed by wide-angle X-ray diffractometry. The crystallizability was not enhanced by SC crystallization in enantiomeric polymer blending compared to homocrystallization in unblended specimens. The melting temperature was not elevated by SC crystallization as in the normal case of SC crystallization reported previously for chiral homopolyesters and copolyesters. However, the fact that only SC crystallization occurred without homo-crystallization in the enantiomeric blend indicates that interaction between the enantiomeric polymer chains were stronger than that between the identical polymer chains. It is expected that the newly proposed novel combinations of chiral hydroxyalkanoic acid-based alternating copolyesters with the two types of chiral centers and the opposite MCCs of L-D and D-L (S-R and R–S) can facilitate the preparation of biodegradable materials having a variety of physical properties and biodegradability.

Stereocomplex formation and hierarchical structural changes during heating of supramolecular gels obtained by polylactide racemic blends

Solvents that are capable of forming crystalline complexes with poly(ʟ-lactide) (PLLA) have been known to form thermoreversible gels. However, the gelation behavior of linear PLLA/poly(ᴅ-lactide) (PDLA) blends has not been studied. Herein, the formation of supramolecular gels using PLLA/PDLA blends in N,N-dimethylformamide (DMF) is reported. The structural changes of these gels were investigated in multiple length scales during heating. In blend gels, both homopolymers crystallize into the ε form. On heating, DMF molecules that are present in the crystal lattice of the ε form are expelled out by keeping the lamellar structure intact and the structure transformed to the α form. At 45 °C, stereocomplex (SC) crystals are also formed due to the activated thermal motion of enantiomeric chains in the presence of solvent. The α form became unstable at ∼75 °C and the enantiomeric polymer chains reorganized into the SC. In the case of low molecular weight blend gels, high chain mobility and chain diffusion ability resulted in the more stable SC. A structural model is proposed to explain the phase transition behavior of blend gels. This study not only enhances the fundamental understanding of SC formation but also provides opportunities to exploit the molecular recognition in helical polymers.

Synthesis, stereocomplex crystallization, homo-crystallization, and thermal properties and degradation of enantiomeric aromatic poly(lactic acid)s, poly(mandelic acid)s

Enantiomeric aromatic poly(lactic acid)s, i.e., poly(l-mandelic acid) (PLMA) and poly(d-mandelic acid) (PDMA), with different number-average molecular weight (Mn) values were synthesized by polycondensation and their stereocomplex (SC) crystallization and homo-crystallization by solvent-evaporation, as well as thermal properties and degradation were investigated in detail. The SC crystallization was confirmed by wide-angle X-ray diffraction and differential scanning calorimetry. However, Fourier-transfer infrared spectroscopy could not identify SC crystallization, excluding the lowered peak width of the carbonyl group. Predominant SC crystallization was observed for high molecular weight PLMA/PDMA (Mn = 1.7 × 104 and 1.5 × 104 g mol–1, respectively) blends and low molecular weight PLMA/PDMA (Mn = 5.8 × 103 and 7.2 × 103 g mol–1) blends, except for low molecular weight PLMA/PDMA blend with a PLMA fraction of 75%, where both SC crystallization and homo-crystallization occurred. This can be explained by the lower crystallizability of homo-crystallites at high Mn values compared to that of SC crystallites. The glass transition temperatures of poly(mandelic acid)s (87 °C − 112 °C) were higher than those previously reported for poly(lactic acid)s (PLAs) (approximately 60 °C), poly(2-hydroxybutanoic acid)s [P(2HB)s] (24 °C − 44 °C), and poly(phenyllactic acid) (32 °C − 47 °C). In marked contrast with the results previously reported for enantiomeric PLAs and P(2HB)s, melting temperatures of SC crystallites of PMAs (105 °C – 127 °C) were lower than those of homo-crystallites (162 °C − 180 °C). The thermal degradation temperatures at 10% weight loss for unblended PLMA, PDMA, and their (50/50) blend (290 °C − 313 °C) were much higher than those reported for unblended enantiomeric PLAs, and their (50/50) blend (218 °C − 243 °C) and similar to those reported for enantiomeric P(2HB)s, and their (50/50) blend (300 °C − 330 °C). The thermal degradation profiles and temperatures of unblended PLMA, PDMA, and their blend were similar with each other, whereas the activation energy for thermal degradation (ΔEtd) of PLMA/PDMA blend (136.7− 163.0 kJ mol–1) was between those of unblended PLMA (117.7 − 154.4 kJ mol–1) and unblended PDMA (196.3 − 226.6 kJ mol–1). These results contrast with those previously reported for enantiomeric PLAs and P(2HB)s, wherein ΔEtd values were increased by enantiomeric polymer blending. Furthermore, the crystallizability of aromatic poly(mandelic acid) and poly(phenyllactic acid) was compared.

Stereocomplex and individual crystallization behavior of symmetric or enantiomeric substituted Poly(lactic acid)s random copolymers with high crystallizabilities

Stereocomplex (SC) crystallization is reported for the first time for the enantiomeric blends of random copolymers of l-configured poly(l-2-hydroxybutanoic acid-co-l-2-hydroxy-3-methylbutanoic acid) [P(L-2HB-co-L-2H3MB)] (49/51) and d-configured poly(d-2-hydroxybutanoic acid-co-d-2-hydroxy-3-methylbutanoic acid) [P(D-2HB-co-D-2H3MB)] (47/53) with high individual crystallizabilities. Unblended l-configured P(L-2HB-co-L-2H3MB) and d-configured P(D-2HB-co-D-2H3MB) formed individual crystallites through purification, solvent evaporation, and melt crystallization. Three types of individual crystallites (provisionally named as α-, β-, and γ-forms) were observed in the unblended copolymers depending on the crystallization methods. SC crystallites were formed in the blends through solvent evaporation and melt crystallization method. All types of monomer units cocrystallized in the individual and SC crystallites. The melting and equilibrium-melting temperatures of SC crystallites in the blends were 192.5–203.7 °C and 205.2 °C, respectively, which were significantly higher than the melting temperatures of individual crystallites in the unblended copolymers (61.6–133.1 °C). The maximum radial growth rate of SC crystallites (13.5 μm min−1) was 10 and 27 times higher than those of individual crystallites of the unblended copolymers (1.4 and 0.5 μm min−1), indicating that SC crystallization increased the crystallization rate. SC crystallization proceeded via regimes II and III. The copolymer crystallizability, monomer cocrystallizability, and crystallization rate of P(L-2HB-co-L-2H3MB) (49/51) and P(D-2HB-co-D-2H3MB) (47/53) are compared with those reported for the series of symmetric or enantiomeric polymer blends of poly(l-lactic acid-co-l-2-hydroxybutanoic acid) (56/44) and poly(d-lactic acid-co-d-2-hydroxybutanoic acid) (52/48) with lowindividualcrystallizabilities and poly(l-lactic acid-co-l-2-hydroxy-3-methylbutanoic acid) (47/53) and poly(d-lactic acid-co-d-2-hydroxy-3-methylbutanoic acid) (47/53) with noindividualcrystallizability.

Small amounts of poly(d-lactic acid) on the properties of poly(l-lactic acid)/microcrystalline cellulose/ poly(d-lactic acid) blends

Stereocomplexation is utilized to improve and control the mechanical and thermal properties of poly(lactic acid) (PLA) where enantiomeric polymers poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) form stereocomplexation. This research reports the effects of minor amounts of PDLA on the properties of PLLA/microcryslalline cellulose (MCC)/PDLA blends. Blends of PLLA/MCC/PDLA are prepared with constant percentage of MCC and different percentage of PLLA and PDLA by solution casting methods and are characterized by FTIR for the confirmation of blends compatibility, WAXS and SEM for the analysis of morphology, TGA and DTA for thermal properties measurement. Shifting of peaks of FTIR profiles observe in blends with and without PDLA due to hydrogen bonding which helps to have compatibility of blends component. XRD analysis show three new peaks after adding PDLA in PLLA/MCC blends confirm the formation of stereocomplex (SC) crystalline structure. TGA and DTG results indicate that PDLA can improve the heat resistivity of the PLLA/MCC blends by increasing its degradation temperature. DTA peaks profile also ensure the development of thermal properties. Image of SEM show the improvement of surface morphology.

Phosphoglycerol-type wall and lipoteichoic acids are enantiomeric polymers differentiated by the stereospecific glycerophosphodiesterase GlpQ

The cell envelope of Gram-positive bacteria generally comprises two types of polyanionic polymers linked to either peptidoglycan (wall teichoic acids; WTA) or to membrane glycolipids (lipoteichoic acids; LTA). In some bacteria, including Bacillus subtilis strain 168, both WTA and LTA are glycerolphosphate polymers yet are synthesized through different pathways and have distinct but incompletely understood morphogenetic functions during cell elongation and division. We show here that the exolytic sn-glycerol-3-phosphodiesterase GlpQ can discriminate between B. subtilis WTA and LTA. GlpQ completely degraded unsubstituted WTA, which lacks substituents at the glycerol residues, by sequentially removing glycerolphosphates from the free end of the polymer up to the peptidoglycan linker. In contrast, GlpQ could not degrade unsubstituted LTA unless it was partially precleaved, allowing access of GlpQ to the other end of the polymer, which, in the intact molecule, is protected by a connection to the lipid anchor. Differences in stereochemistry between WTA and LTA have been suggested previously on the basis of differences in their biosynthetic precursors and chemical degradation products. The differential cleavage of WTA and LTA by GlpQ reported here represents the first direct evidence that they are enantiomeric polymers: WTA is made of sn-glycerol-3-phosphate, and LTA is made of sn-glycerol-1-phosphate. Their distinct stereochemistries reflect the dissimilar physiological and immunogenic properties of WTA and LTA. It also enables differential degradation of the two polymers within the same envelope compartment in vivo, particularly under phosphate-limiting conditions, when B. subtilis specifically degrades WTA and replaces it with phosphate-free teichuronic acids.

Thermal properties and degradation of enantiomeric copolyesteramides poly(lactic acid-co-alanine)s

Thermal properties and degradation of enantiomeric random copolyesteramides poly(l-lactic acid-co-l-alanine) [P(LLA-LAL)] and poly(d-lactic acid-co-d-alanine) [P(DLA-DAL)] copolymers with wide alanine unit content ranges from 0 to 21 and 22 mol% were investigated by differential scanning calorimetry and themogravimetry. The total enthalpy of cold crystallization enthalpy and melting enthalpy as an indicator of crystallinity decreased with a decrease in lactic acid unit content or an increase in alanine unit content. Equilibrium melting temperature values of α-form homo-crystallites for the unblended samples decreased with an increase in alanine unit content (157.3–179.6 °C) and those of δ-form homo-crystallites for the unblended L100 and D100 (190.7 and 185.1 °C) were much higher than the reported values (both 172.8 °C, Polymer 2017, 9, 625). Thermal stability was enhanced by enantiomeric polymer blending, although thermal degradation was accelerated by incorporated alanine units. The activation energy for thermal degradation (ΔEtd) of the blend samples decreased with an increase in alanine unit content, whereas that of the unblended samples showed complicated dependence on alanine unit content and became maximum at alanine unit contents of 21 and 22 mol%. The blend samples had higher ΔEtd values comparted to those of the unblended samples, except for the alanine unit contents of 21 and 22 mol%. The reason for the high ΔEtd values of the unblended samples compared with that of the blend sample at the alanine unit contents of 21 and 22 mol% is discussed.

Comprehensive Understanding of Polyester Stereocomplexation.

We report a comprehensive understanding of the stereoselective interaction between two opposite enantiomeric polyesters prepared from the regioselective copolymerization of chiral terminal epoxides and cyclic anhydrides. For many of the resultant polyesters, the interactions between polymer chains of opposite chirality are stronger than those of polymer chains with the same chirality, resulting in the formation of a stereocomplex with an enhanced melting point (Tm) and crystallinity. The backbone, tacticity, steric hindrance of the pendant group, and molecular weight of the polyesters have significant effects on stereocomplex formation. Bulky substituent groups favor stereocomplexation, resulting in a greater rise in Tm in comparison to the component enantiomeric polymers. The stereocomplex assembly of discrete (R)- and (S)-poly(phenyl glycidyl ether-alt-phthalic anhydride)s oligomers revealed that the minimum degree of polymerization required for stereocomplex formation is five. Raman spectroscopy and solid-state NMR studies indicate that stereocomplex formation significantly restricts the local mobilities of C═O and C-H groups along the backbone of chains. The reduced mobility results in the enhanced spin-lattice relaxation time and both 1H and 13C downfield shifts due to the strong intermolecular interactions between R- and S-chains.

Catalyst‐Sidearm‐Induced Stereoselectivity Switching in Polymerization of a Racemic Lactone for Stereocomplexed Crystalline Polymer with a Circular Life Cycle

AbstractConstruction of robust, stereocomplexed (sc) crystalline material, based on a recently discovered infinitely recyclable polymer system, requires blending of enantiomeric polymer chains produced from respective enantiopure, fused six‐five bicyclic lactones. Herein, the stereoselective polymerization of the racemic monomer by yttrium catalysts bearing tetradentate ligands is reported, where the tethered donor sidearm switches the heteroselectivity of the catalyst to isoselectivity when it is changed from the β‐OMe to β‐NMe2 sidearm. The latter catalyst produces an isotactic stereoblock polymer (Pm up to 0.95) that forms the crystalline sc‐material with a Tm of up to 171 °C. This sc‐material can be fully depolymerized back to rac‐monomer in a quantitative yield and purity, thus establishing its circular life cycle.

Catalyst-Sidearm-Induced Stereoselectivity Switching in Polymerization of a Racemic Lactone for Stereocomplexed Crystalline Polymer with a Circular Life Cycle.

Construction of robust, stereocomplexed (sc) crystalline material, based on a recently discovered infinitely recyclable polymer system, requires blending of enantiomeric polymer chains produced from respective enantiopure, fused six-five bicyclic lactones. Herein, the stereoselective polymerization of the racemic monomer by yttrium catalysts bearing tetradentate ligands is reported, where the tethered donor sidearm switches the heteroselectivity of the catalyst to isoselectivity when it is changed from the β-OMe to β-NMe2 sidearm. The latter catalyst produces an isotactic stereoblock polymer (Pm up to 0.95) that forms the crystalline sc-material with a Tm of up to 171 °C. This sc-material can be fully depolymerized back to rac-monomer in a quantitative yield and purity, thus establishing its circular life cycle.

Polylactides Bearing Vanillin at Chain End Provided Dual Dynamic Interactions: Stereocomplex Formation and Nanostructure Control

The chemical modification of the polymer chain end group of polylactide is important for the development of nanostructural materials. Herein, the first example of dual dynamic functionality in polylactides bearing vanillin at the chain end is first demonstrated. The first function, stereocomplex formation of both enantiomeric polymers, results in an enhancement in thermal resistance, while the second creates reversibility through amine–imine transformation. This transformation self‐assembles into interesting morphologies, including raindrop‐like nanoparticles observed on imine formation, and spider‐web‐like fibrous ones observed on aldehyde formation from hydrophobic–hydrophilic stimuli by acid. These properties indicate that the present polymer shows dual dynamic building block moiety properties, composed of the natural products. image

Stereocomplex- and homo-crystallization of blends from 2-armed poly(l-lactide) and poly(d-lactide) with identical and opposite chain directional architectures and of 2-armed stereo diblock poly(lactide)

Ethylene glycol (EG) and succinic anhydride (SA)-based 2-armed PLLA and PDLA correspondingly with tail-to-tail (T–T) and head-to-head (H–H) architectures, together with SA-based linear 2-armed stereo diblock copolymer, were synthesized and the SC crystallization and homo-crystallization behavior of blends of 2-armed PLLA and PDLA with various combinations of molecular architectures, together with that of neat polymers, were investigated. The crystallizability was higher for SC crystalline samples than for homo-crystalline samples during solvent evaporation and slow cooling from the melt. The hydrogen bonds of hydroxyl terminals and/or low molecular weight of EG-based polymer rather than specific chain directional architecture or its combination disturbed and delayed the crystallization, lowering the crystallization temperature (Tc), crystalline], the reciprocal of experimental crystallization half time [1/tc(1/2) (exp)], spherulite growth rate (G), enthalpy of crystallization (ΔHc) for 1st cooling, and total enthalpy of cold crystallization and melting [ΔH(tot)] for 2nd heating of both SC crystalline and homo-crystalline samples, the melting temperature (Tm) for 1st and 2nd heating, nucleus density and the degree of lamella orientation of spherulites of SC crystalline samples. The opposite chain directional architectures of polymers in blends should have increased ΔH(tot) for 1st heating, ΔHc for 1st cooling, ΔH(tot) for 2nd heating, and G of SC crystalline samples. The effects of hydrogen bonds of hydroxyl terminal groups, Mn (or Mn per one arm or block), and chain directional combination were determined by the crystallization temperature range and the presence or absence of a solvent. The Tm values of SC crystalline samples were determined by Mn (or Mn per one arm or block), whereas those of homo-crystalline samples did not show clear dependence on Mn (or Mn per one arm or block) values per one arm, hydroxyl terminal groups (hydrogen bonding), or chain direction. The intermolecular SC crystallization in the enantiomeric polymer blends was favored compared with intramolecular SC crystallization in the stereo diblock copolymer.

Crystalline hetero-stereocomplexed polycarbonates produced from amorphous opposite enantiomers having different chemical structures.

Stereocomplexation is the stereoselective interaction between two opposite enantiomeric polymers through an interlocked orderly assembly. Most studies focus on the stereocomplex formation from the crystalline opposite enantiomers having the identical structure; nevertheless, rare examples were reported regarding the crystalline stereocomplexes from enantiomeric polymers having different chemical structures. Herein we show a strategy for polymer orderly assembly through the formation of crystalline hetero-stereocomplexed polymeric materials by the cocrystallization of amorphous isotactic polycarbonates with different chemical structures and opposite configurations. The behaviors in the crystalline state are significantly different from that of the component enantiomeric polymers or their homo-stereocomplexes. This study is expected to open up a new way to prepare various semicrystalline materials having a wide variety of physical properties and degradability.

Crystalline Hetero‐Stereocomplexed Polycarbonates Produced from Amorphous Opposite Enantiomers Having Different Chemical Structures

AbstractStereocomplexation is the stereoselective interaction between two opposite enantiomeric polymers through an interlocked orderly assembly. Most studies focus on the stereocomplex formation from the crystalline opposite enantiomers having the identical structure; nevertheless, rare examples were reported regarding the crystalline stereocomplexes from enantiomeric polymers having different chemical structures. Herein we show a strategy for polymer orderly assembly through the formation of crystalline hetero‐stereocomplexed polymeric materials by the cocrystallization of amorphous isotactic polycarbonates with different chemical structures and opposite configurations. The behaviors in the crystalline state are significantly different from that of the component enantiomeric polymers or their homo‐stereocomplexes. This study is expected to open up a new way to prepare various semicrystalline materials having a wide variety of physical properties and degradability.

Nanospheres, Nanotubes, Toroids, and Gels with Controlled Macroscopic Chirality

AbstractThe interaction of a highly dynamic poly(aryl acetylene) (poly‐1) with Li+, Na+, and Ag+ leads to macroscopically chiral supramolecular nanospheres, nanotubes, toroids, and gels. With Ag+, nanospheres with M helicity and tunable sizes are generated, which complement those obtained from the same polymer with divalent cations. With Li+ or Na+, poly‐1 yields chiral nanotubes, gels, or toroids with encapsulating properties and M helicity. Right‐handed supramolecular structures can be obtained by using the enantiomeric polymer. The interaction of poly‐1 with Na+ produces nanostructures whose helicity is highly dependent on the solvation state of the cation. Therefore, structures with either of the two helicities can be prepared from the same polymer by manipulation of the cosolvent. Such chiral nanotubes, toroids, and gels have previously not been obtained from helical polymer–metal complexes. Chiral nanospheres made of poly(aryl acetylene) that were previously assembled with metal(II) species can now be obtained with metal(I) species.

Nanospheres, nanotubes, toroids, and gels with controlled macroscopic chirality.

The interaction of a highly dynamic poly(aryl acetylene) (poly-1) with Li(+), Na(+), and Ag(+) leads to macroscopically chiral supramolecular nanospheres, nanotubes, toroids, and gels. With Ag(+), nanospheres with M helicity and tunable sizes are generated, which complement those obtained from the same polymer with divalent cations. With Li(+) or Na(+), poly-1 yields chiral nanotubes, gels, or toroids with encapsulating properties and M helicity. Right-handed supramolecular structures can be obtained by using the enantiomeric polymer. The interaction of poly-1 with Na(+) produces nanostructures whose helicity is highly dependent on the solvation state of the cation. Therefore, structures with either of the two helicities can be prepared from the same polymer by manipulation of the cosolvent. Such chiral nanotubes, toroids, and gels have previously not been obtained from helical polymer-metal complexes. Chiral nanospheres made of poly(aryl acetylene) that were previously assembled with metal(II) species can now be obtained with metal(I) species.

Synergistic effect of stereocomplex crystals and shear flow on the crystallization rate of poly(l-lactic acid): A rheological study

Stereocomplex (SC) is a very interesting crystal modification formed between enantiomeric polymers, such as poly(L-lactic acid) (PLLA) and poly(D-lactic acid). Here, by a rheological approach, we found that SC and shear flow can give rise to a notable synergistic effect on the crystallization rate of PLLA. Moreover, the PLLA containing stereocomplex was found to be more sensitive to shear as the onset crystallization time and half crystallization time are greatly shortened even at a low shear rate, compared with neat PLLA. The synergistic effect on the crystallization rate of PLLA is likely caused by the crosslinking effect of SC crystallites on the relaxation behavior of PLLA and the filler effect of SC crystallites, which, in nature, can enhance the flow-induced crystallization. The results suggest that SC crystallites can be an attractive nucleation agent to promote the crystallization rate of PLLA especially in practical processing and the appearance of the solid crystallites in the melt can speed up the following flow induced crystallization process.

Stereocomplexation of Poly(l-lactide) and Random Copolymer Poly(d-lactide-co-ε-caprolactone) To Enhance Melt Stability

■ INTRODUCTION Stereocomplex polylactide (s-PLA) is a polylactide based macromolecule which has been known as one of the choices to enhance the properties of polylactides which is formed from an enantiomeric polymer blend of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). The s-PLA has melting temperature (Tm) approximately 50 °C higher than the Tm of either PLLA or PDLA. There are so many reports about s-PLA macromolecules development using linear PLLA and PDLA homopolymers. Moreover, there are also s-PLA macromolecules developments using block copolymer, starshaped, and cyclic based polylactide. The melt processing is regular material processing in industry. Unfortunately, s-PLA from linear high-molecularweight polylactide has weakness due to limitation of memory to re-form s-PLA after melted (melt stability). Biela et al. reported the star-shaped s-PLA with has 13 arms or more able to completely melt and perfectly reversible due to hardlock-type interactions. The melt stable star-shaped s-PLA requires starshaped PDLA and PLLA with 13 arms or more. Otherwise, commercial polylactide is mostly in the linear form. Consequently, the star-shaped s-PLA concept is not suitable to apply for commercial polylactide. For these reasons, it is necessary to find a new concept for making melt stable s-PLA which can be applied to the commercial polylactide. Therefore, we studied about the stereocomplexation of PLLA and random copolymer of poly(D-lactide-co-e-caprolactone) (PDLCL) with small content of e-caprolactone (CL). The addition of small amount of caprolactone in the PDLCL is addressed as soft fragment to accelerate the chain movement of PDLA fragments to interact with PLLA chain after melted and “re-assemble” sPLA. In this report, we demonstrated for the first time that small caprolactone content in the PDLCL can enhanced the melt stability of s-PLA.