Unveiling chain–chain interactions in CO2-based crystalline stereocomplexed polycarbonates by solid-state NMR spectroscopy and DFT calculations

CO2-based stereocomplexed polycarbonates derived from the intermolecularly interlocked interaction between the enantiopure polymers with the opposite configuration exhibit high crystallinity, excellent thermal and mechanical stabilities. Deep insights into the mechanism of stereocomplexation are of particular importance to the design and manufacture of new promising and sustainable polycarbonates with enhanced physicochemical properties. Our solid-state NMR experiments linking with DFT computations clearly reveal the specific chain-chain interactions in a typical stereocomplexed poly（4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0] octane carbonate）（PCXC）.13C CP/MAS NMR,1H DUMBO MAS NMR and 13C/1H relaxation-time measurements indicate that the formation of stereocomplex reduces the local mobilities of carbonyl, methine and methylene groups in each chain of PCXC significantly. Through a combination of two-dimensional 1H-13C HETCOR NMR and DFT calculation analysis, the cis-/trans-conformations and packing models of PCXC chains in the amorphous, enantionpure isotactic and stereocomplexed polycarbonates are identified. The splitting of 13C and 1H NMR chemical shifts of methine groups in the backbone carbon region demonstrates the ordered interlock interactions between the R-and S-chain in the stereocomplexed PCXC.

Stereocomplex Formation of Atactic Poly(methyl mcthacrylate)

Stereocomplexes formed in atactic poly(methyl methacrylatc) (α-PMMA) films cast form different solvents were studied by means of Fourier transform infrared spectroscopy (FTIR). and differential scanning calorimetry (DSC). The growth of stereocomplex was a function of annealing temperature and annealing time. respectively.

Toughening of Poly (L-lactide) with Branched Multiblock Poly (ε-caprolactone)/poly (D-lactide) Copolymers

A total biodegradable elastomer,branched multiblock poly( ε-caprolactone)/poly( D-lactide)( BMCD) was prepared using 3-isocyanatopropyltriethoxysilane( IPTS) as a coupling agent.To improve the toughness of poly( L-lactide)( PLLA),PLLA/BMCD blends were prepared via a simple solvent evaporation method at various BMCD loadings. Tensile test showed that the elongation at break of PLLA blends increased to 50. 97% and104. 55% at the loadings of 5% and 7%( mass fraction) BMCD respectively, with no sacrifice of their biodegradability. This approach allowed for simultaneous control of mechanical and biodegradable properties of PLLA with a few additives in actual production. Furthermore, UV-VIS test showed that the light transmittance of the films at the loadings of 5%( mass fraction)BMCD was almost the same as pure PLLA at 400 nm.

Synthesis and Characterization of Poly（Lactide）s Using Novel Solid Cloisite Modified Tin Catalyst

A novel cloisite modified solid catalyst was prepared in a single step from commercially available starting materials for the first time. The ring opening polymerization of L-lactide and D-lactide using this cloisite modified solid catalyst resulted in homopolymers of 75,000 and PDI = 1.6 and the maximum molecular weight （Mw） i.e. 180,000 with PDI = 1.9 were obtained. The catalytic activity ofcloisite modified solid catalyst was compared with the conventional stannous octoate catalyst and found superior to stannous octoate in all respect such as conversion, molecular weight and molecular weight distribution etc.. Moreover, the maximum molecular weight i.e. 180,000 was obtained at 220 ℃, whereas, transesterification reaction predominate in presence of stannous octoate The linear structure was confirmed by quantitative ^13C NMR Spectroscopy. Blend films were obtained by casting mixed solutions of poly （D-lactide） and poly （L-lactide） at various compositions, and stereocomplex was formed at 50/50 composition with molecular weight of 75,000.

Impact of Annealing on the Melt Recrystallization of a-PDLA/α-PLLA Double-layered Films

Poly(lactic acid)(PLA) as a bio-based polymer with biodegradability and biocompatibility has attracted much attention. To manipulate its properties for different applications, regulation of crystal structure and crystalline morphology becomes an attractive research topic. In this work, the structure evolution of layered samples containing an amorphous poly(D-lactide)(PDLA) layer and a crystalline poly(L-lactide)(PLLA)layer with highly oriented edge-on α lamellar crystals after annealing at 150 ℃ or/and after melt-recrystallization has been studied by AFM, FTIR,and TEM combined with electron diffraction. The results demonstrate that melt recrystallization of the as-prepared sample leads to the formation of abundant randomly oriented PLA stereo-complex(PLA SC) crystals. Annealing at 150 ℃ results in the formation of a small amount of oriented PLA SC crystals at the interface. These PLA SC crystals show great impact on the recrystallization behavior of sample after melting at 190 ℃ and then crystallizing at 90 ℃. First, they impede the mutual diffusion of the overlying PDLA and underlying PLLA, and thus reduce their stereocomplexation ability as manifested by the decreased amount of PLA SC crystals. Second, they act as substrate to initiate the epitaxial crystallization of the overlying PDLA and underlying PLLA, which ensures the production of a highly oriented structure of PDLA and PLLA after melt recrystallization again.

High Performance Polylactide Toughened by Supertough Polyester Thermoplastic Elastomers:Properties and Mechanism

It is a challenge to develop a biodegradable toughener to toughen polylactic acid(PLA)with both high strength and high toughness,since toughness and strength are mutually exclusive.Here,a series of supertough polyester thermoplastic elastomers(TPEs),poly(L/D-lactide)-b-poly(ε-caprolactone-co-δ-valerolactone)-b-poly(L/D-lactide)s(PLLA-PCVL-PLLA,L-TPEs or PDLA-PCVL-PDLA,D-TPEs),were prepared and blended with a PLLA matrix to toughen PLLA.The mechanical properties of PLLA could be regulated in a wide range by changing blending ratios and TPE structures.For PLLA blends toughened by L-TPEs,the highest elongation at break is up to 425%with the tensile strength of 33.1 MPa and the toughness of 104 MJ/m3.By the stereocomplex crystallization of PLA(sc-PLA),the tensile strength of the PLLA/D-TPE blends further increased to 41.8 MPa with a similar elongation at break(418%)and the toughness up to 128 MJ/m3.The detailed characterizations revealed a toughening mechanism:(I)the added soft segments increased the ductility of the PLLA matrix,(II)the PLLA segments of L-TPEs increased the compatibility between TPEs and PLLA matrix,and(III)the formation of sc-PLA between the PDLA segments in D-TPE and PLLA provided higher tensile strength by enhancing the strength of the crystal skeleton.The toughened PLA using TPEs can maintain original non-toxic and degradable properties,and be applied potentially in surgical sutures,and 3D-printed scaffolds.

A Facile Strategy to Enhance the Formation of Stereocomplex Crystallites in Poly(L-lactic acid)/Poly(D-lactic acid) Blend with High Molecular Weights

Blending of poly(levorotatory-lactic acid) (PLLA) and poly(dextrorotatory-lactic acid) (PDLA) produces the stereocomplex crystallites (PLA SC), which present higher melting temperature and mechanical properties than that of neat PLLA or PDLA. However, in the PLLA/PDLA blends with higher molecular weights, the phase separation occurs and the SC exhibits weak memory after melting, which lead to a small amount of SC together with a large amount of homochiral crystallites (HC) develop during crystallization from the melt. In this study, a small content of graphite oxide was blended with PLLA and PDLA to form ternary blends, and it was exciting to find that the formation of SC was enhanced gradually with the content of graphite oxide. The SC exclusively developed when 2 wt% graphite oxide was incorporated into the PLLA/PDLA, and the crystallinity with ∼50% was received even during fast cooling from the melt (−50 ℃/min). The acceleration formation of SC was speculated due to the interaction between PLA molecular chains and the hydroxyl groups on the surface of graphite oxide and the obstruction of proliferation of graphite oxide.

Significantly Improved Stereocomplexation Ability in Cyclic Block Copolymers

Stereocomplex crystallization in cyclic polymer blend and cyclic block copolymers was investigated by means of dynamic Monte Carlo simulations.Five polymer systems(linear polymer blend,linear diblock copolymer,cyclic polymer blend,cyclic diblock copolymer and tetrablock copolymer)were established.It was interestingly found that the cyclic polymer blend exhibited the weakest stereocomplexation ability,while the two cyclic block copolymers showed stronger stereocomplexation ability than the linear diblock copolymer.This abnormal improved stereocomplexation ability of the cyclic block copolymers can be attributed to the synergy between the ring chain topology and the block copolymer structure.Compared with the linear block copolymers,the ring chain topology confined segmental motions of cyclic polymer chains to smaller regions,and then the segments belonging to the different blocks in the cyclic block copolymers have more chance to contact with each other.In this way,the cyclic block copolymers had better miscibility between segments belonging to different types of blocks,leading to the stronger stereocomplexation ability.

Sustainable Blends of Poly(propylene carbonate)and Stereocomplex Polylactide with Enhanced Rheological Properties and Heat Resistance

Sustainable blends of poly(propylene carbonate)(PPC)and stereocomplex polylactide(sc-PLA)were prepared by melt blending equimolar poly(L-lactic acid)(PLLA)and poly(D-lactide acid)(PDLA)with PPC to form sc-PLA crystals in situ in the melt blending process.Differential seanning calorimetry analysis revealed that only sc-PLA,no homo-crystallization of PLLA or PDLA,formed in the PPC matrix as the sc-PLA con tent was more than 10 wt%.Very in triguingly,scan ning electronic microscopy observati on showed that sc-PLA was evenly dispersed in the PPC phase as spherical particles and the sizes of sc-PLA particles did not obviously increase with in creasing sc-PLA con tent.As a con seque nee,the rheological properties of PPC were greatly improved by incorporation of sc-PLA.When the sc-PLA con tent was 20 wt%,a percolati on n etwork structure was formed,and the blends showed solid-like behavior.The sc-PLA particles could reinforce the PPC matrix,especially at a temperature above the glass transition temperature of PPC.Moreover,the Vicat softening temperature of PPC/sc-PLA blends could be increased compared with that of neat PPC.

Poly(β-cyclodextrin)-mediated Polylactide-cholesterol Stereocomplex Micelles for Controlled Drug Delivery

A series of host-guest interaction-adjusted polylactide stereocomplex micelles was prepared via the self-assembly of 4-armed poly（ethylene glycol）-block-poly（L-lactide/D-lactide）-cholesterol （4-armed PEG-b-PLLA/PDLA-CHOL） and poly（β-cyclodextrin） （PCD） with the molar ratios of CHOL/β-CD at 1：0.5, 1 ：l, and 1：2 in an aqueous environment. The hydrodynamic diameters of the micelles ranged from 84.1 nm to 107 nm depending on the molar ratio of CHOL/β-CD. It was shown that the micelle with the largest proportion of PCD possessed excellent abilities in drug release, cell internalization as well as proliferation inhibitory effect toward human A549 lung cancer cells. The results demonstrated that the stereocomplex and host-guest interactions-mediated PLA micelles exhibited great potential in sustained drug delivery.

Stereocompfexed Poiy(iactide) Composites toward Engineering Plastics with Superior Toughness, Heat Resistance and Anti-hydrolysis

Polyl(lactide),PLA,suffers from bitleness and low heat deflection temperature(HDT),which limits its application as an engineering plastic.In this work,poly(L-lactide)/poly(D-lactide)/ethylene-vinyl acetate glycidyl methacrylate random copolymer(PLLA/PDL A/EVM-GMA=1/1/x)composites were prepared by melt blending,and the in situ formed EVM-g PLA copolymers improved the compatibility between PLA and EVM-GMA.Subsequently,the blends were subjected to a two-step annealing process during compression molding,i.e.first annealing at 120℃ to rapidly form a certain amount of stereocomplex(sc)crystallites as nucleation sites,and then annealing at 200℃ to guide the formation of new sccrystallites.Both differential scanning calorimetry(DSC)and wide angle X-ray dffrction(WAXD)measurements confirmed the formation of highly stereocomplexed PLA products.Mechanical results showed that the PLLA/PDLA blend with 20 wt%of EVM-GMA had a notched impact strength up to 65 kJ/m2 and an elongation at break of 48%,while maintaining a tensile strength of 40 MPa.Meanwhile,dynamic mechanical analysis(DMA)and heat deflection tests showed that the PLA composite had an HDT up to 142℃ which is 90℃ higher than that of normal PLA products.Scanning electron microscopy(SEM)confirmed the fine dispersion of EVM-GMA particles,which facilitated to understand the toughening mechanism.Furthermore,the highly stereocomplexed PLA composites simultaneously exhibited excellent chemical and hydrolysis resistance.Therefore,these fascinating properties may extend the application range of sc PLA material as an engineering bioplastic.

The Stereocomplex Formation and Phase Separation of PLLA/PDLA Blends with Different Optical Purities and Molecular Weights

In this study, the poly（L-lactide）/poly（D-lactide） （PLLA/PDLA） blends with different optical purities of PLLA and various molecular weights of PDLA are prepared by solution mixing, and the stereocomplex formation and phase separation behaviors of these blends are investigated. Results reveal that optical purity and molecular weight do not vary the crystal structure of PLA stereocomplex （sc） and homochiral crystallites （hc）. As the optical purity increasing in the blends, the melting temperature of sc （Tsc） and the content of sc （AHsc） increased, while the melting temperature of hc （Thin） hardly changes, although the content ofhc （AHhm） decreased gradually. The Tsc and AHsc are also enhanced as the molecular weight of PDLA reduces, and the AHhm reduces rapidly even though the Thin does not vary apparently. With lower optical purities of PLLA and higher molecular weights of PDLA, three types of crystals form in the blends, i.e., PLA sc, PLLA hc and PDLA hc. As molecular weight decreases and optical purity enhances, the crystal phase decreases to two （sc and PDLA hc）, and one （sc） finally. This investigation indicates that the phase separation behavior between PLLA and PDLA in the PLLA/PDLA blends not only depends on molecular weights, but also relies on the optical purities of polymers.

Enhanced Crystallization Rate of Poly(L-lactide)/Hydroxyapatite-graftpoly(D-lactide) Composite with Different Processing Temperatures

Hydroxyapatite-graft-poly(D-lactide)(HA-g-PDLA)was synthesized by ring-opening polymerization with HA as initiator and stannous octanoate(Sn(Oct)2)as catalyst.Thermogravimetric analysis(TGA)and Fourier transform infrared spectroscopy(FUR)results indicate that PDLA chains are successfully grafted onto HA particles by covalent bond.Under two different processing temperatures(190 and 230℃),the effect of the grafted PDLA chains on the crystallization behavior of poly(L-lactide)/HA-g-PDLA(PLLA/HA-g-PDLA)composite was investigated in the current study,comparing to neat PLLA and its four composites(PLLA/HA,PLLA/HA-g-PLLA,PLLA/PDLA,and PLLA/HA/PDLA).The crystallization rate of PLLA/HA-g-PDLA composite is highly enhanced comparing to PLLA,PLLA/HA and PLLA/HA-g-PLLA composites in which there are no stereocomplex(SC)crystallites.In addition,when the processing temperature rises from 190℃ to 230℃,the acceleration of PLLA crystallization in PLLA/HA-g-PDLA composite is not influenced so much as other composites containing SC crystallites,such as PLLA/HA/PDLA or PLLA/PDLA.The differential scanning calorimetry(DSC)results demonstrate that even without SC crystallites,the crystallization of PLLA can still be accelerated a lot in this composite.This may be related to the interaction between the grafted PDLA chains and the amorphous PLLA chains in PLLA/HA-g-PDLA composite.The isothermal crystallization kinetics studies indicate that the nature of nucleation and crystal growth of PLLA/HAg-PDLA composite are more likely 3D crystalline growth with heterogeneous nucleation mode,which are different from PLLA or other composites.This investigation could shed new light on the application of PLLA/HA composites.

Stereocomplexation and Mechanical Properties of Polylactide-bpoly(propylene glycol)-b-polylactide Blend Films: Effects of Polylactide Block Length and Blend Ratio

In the present work, poly（propylene glycol） （PPG） was block copolymerized to form polylactide-poly（propylene glycol）-polylactide （PL-PPG-PL） triblock copolymers for preparing flexible stereocomplex PL （scPL） blend films. The scPL blend films were prepared by solution blending of poly（L-lactide）-PPG-poly（L-lactide） （PLL-PPG-PLL） and poly（D-lactide）- PPG-poly（D-lactide） （PDL-PPG-PDL） triblock copolymers before film casting. The influences of PL end-block lengths （2 ×10^4 and 4×10^4 g/tool） and blend ratios （75/25, 50/50 and 25/75 W/W） on the stereocomplexation and mechanical properties of the blend films were evaluated. From DSC and WAXD results, the 50/50 blend films had complete stereocomplexation. Phase separation between the scPL and PPG phases was not observed from their SEM images. The tensile stress and elongation at break increased with the sterecomplex crystallinities and PL end-block lengths. The PPG middle-blocks enhanced elongation at break of the scPL films. The results showed that the PL-PPG-PL triblock structures did not affect stereocomplexation of the PLL/PDL block blending. In conclusion, the phase compatibility and flexibility of the scPL films were improved by PPG block copolymerization.

STEREOREGULAR POLY(CYCLOHEXENE CARBONATE)S:UNIQUE CRYSTALLIZATION BEHAVIOR

An example of crystalline CO2-based polymer from the asymmetric alternating copolymerization of CO2 and cyclohexene oxide is reported. Isotacticity of poly（cyclohexene carbonate） （PCHC） has the critical influence on the crystallinity, and only copolymers with a isotacticity of more than 90% are crystallizable. The stereoregular PCHC is a typical semi-crystalline thermoplastic, and possesses a high melting point （Tin） of 215-230℃ and a decomposition temperature of ca. 310℃. The spherulitic morphology of （R）-PCHC grows in a clockwise spiral from a center, and that of （S）-PCHC is a counterclockwise spiral, while the stereocomplex of （S）-PCHC/（R）-PCHC （1/1 mass ratio） presents lath-like dendritic crystal. The novel crystalline CO2-based polycarbonate represents a rare example of optically active polymers with unique crystallization behavior. Our findings reflect the critical influence of stereoregularity on the crystallization for this kind of polymeric materials, and may lead to developments of thermal-resistance CO2 copolymers for application in engineering thermoplastics.

Stereocomplex Crystallization in Asymmetric Diblock Copolymers Studied by Dynamic Monte Carlo Simulations

Stereocomplex crystallization in asymmetric diblock copolymers was studied using dynamic Monte Carlo simulations,and the key factor dominating the formation of stereocomplex crystallites(SCs)was uncovered.The asymmetric diblock copolymers with higher degree of asymmetry exhibit larger difference between volume fractions of beads of different blocks,and local miscibility between different kinds of beads is lower,leading to lower SC content.To minimize the interference from volume fraction of beads,the SC formation in blends of asymmetric diblock copolymers was also studied.For the cases where the volume fractions of beads of different blocks are the same,similar local miscibility between beads of different blocks and similar SC content was observed.These findings indicate that the volume fraction of beads of different blocks is a key factor controlling the SC formation in the asymmetric diblock copolymers.The SC content can be regulated by adjusting the difference between the contents of beads of different blocks in asymmetric diblock copolymers.

Mixing of Racemic Poly(L-lactide)/Poly(D-lactide) Blend with Miscible Poly(D,L-lactide):Toward All Stereocomplex-type Polylactide with Strikingly Enhanced SC Crystallizability

Stereocomplex-type polylactide(SC-PLA)consisting of alternatively arranged poly(L-lactide)(PLLA)and poly(D-lactide)(PDLA)chains has gained a good reputation as a sustainable engineering plastic with outstanding heat resistance and durability,however its practical applications have been considerably hindered by the weak SC crystallizability.Current methods used to enhance the SC crystallizability are generally achieved at the expense of the precious bio-renewability and/or bio-degradability of PLAs.Herein,we demonstrate a feasible method to address these challenges by incorporating small amounts of poly(D,L-lactide)(PDLLA)into linear high-molecular-weight PLLA/PDLA blends.The results show that the incorporation of the atactic PDLLA leads to a significant enhancement in the SC crystallizability because its good miscibility with the isotactic PLAs makes it possible to greatly improve the chain mixing between PLLA and PDLA as an effective compatibilizer.Meanwhile,the melt stability(i.e.,the stability of PLLA/PDLA chain assemblies upon melting)could also be improved substantially.Very intriguingly,SC crystallites are predominantly formed with increasing content and molecular weight of PDLLA.More notably,exclusive SC crystallization can be obtained in the racemic blends with 20 wt%PDLLA having weight-average molecular weight of above 1×10^(5)g/mol,where the chain mixing level and intermolecular interactions between the PLA enantiomers could be strikingly enhanced.Overall,our work could not only open a promising horizon for the development of all SC-PLA-based engineering plastic with exceptional SC crystallizability but also give a fundamental insight into the crucial role of PDLLA in improving the SC crystallizability of PLLA/PDLA blends.

Influence of Interfacial Enantiomeric Grafting on Melt Rheology and Crystallization of Polylactide/Cellulose Nanocrystals Composites

Incorporating the surface-grafted cellulose nanocrystals(CNCs)with enantiomeric polylactide(PLLA or PDLA)is an effective and sustainable way to modify PLLA,but their difference in promoting matrix crystallization is still unrevealed.In this study,the CNCs with identical content and length of PLLA and PDLA(CNC-g-L and CNC-g-D)were prepared and blended with PLLA.The rheological properties of PLLA/CNC-gD are greatly improved,indicating that the stereocomplexation can significantly improve the interfacial strength as compared with the conventional van der Waals force in PLLA/CNC-g-L.Surprisingly,the matrix crystallizes at a higher rate in PLLA/CNC-g-L than PLLA/CNC-g-D.PLLA/CNC-g-L15 reaches its half crystallinity in 8.26 min while a longer period of 13.41 min is required for PLLA/CNC-g-D15.POM observation reveals that the superior crystallization behavior in PLLA/CNC-g-L is originated from its higher nucleation efficiency and faster growth rate.The formation of low content of sc-PLA at the interface can restrict the diffusion of PLLA but contribute less to generate crystalline nuclei,which synergistically leads to the retarded crystallization kinetics in PLLA/CNC-g-D.Revealing the mechanism of different interfacial enantiomeric grafting on the melt rheology and crystallization of PLLA is of great significance for the development of high-performance polylactide materials.

Can Classic Avrami Theory Describe the Isothermal Crystallization Kinetics for Stereocomplex Poly(lactic acid)?

Classic Avrami model and its modifications have found diverse applications in describing the thermal and phase behaviors of inorganic metals and organic polymers. The direct introduction of classic Avrami equation to offer quantitative analyses of crystallization kinetic parameters for enantiomeric poly（lactic acid） （PLA） blends may, however, lead to contradictory conclusions. As revealed by this study, during the characterization of isothermal melt and cold crystallization for stereocomplex PLA containing equal-weight poly（L-lactic acid） and poly（D-lactic acid）, the kinetic parameters yielded by Avrami equation are not in line with the classic crystallization hypotheses or the direct morphological observations. The underlying mechanisms, to some extent, lie in the generation of stereocomplex crystals （SCs） during the cooling/heating which affects the subsequent crystallization dynamics. The huge gap between the melting enthalpies of 100% crystalline SCs （142 J/g） and homo-crystals （HCs, 93 J/g） is most likely responsible for the confusing kinetic parameters acquired from the deduction of Avrami equation, which is based on the integration of enthalpies as a function of crystallization time. This prompts for great care that the classic Avrami equation is not applicable to accurately describe the crystallization kinetics of stereocomplex PLA, given the generation of SCs prior to crystallization and the coexistence of HCs and SCs during crystallization.

Fractionated Crystallization Kinetics and Polymorphic Homocrystalline Structure of Poly(_(L)-lactic acid)/Poly(_(D)-lactic acid)Blends:Effect of Blend Ratio

Stereocomplex(SC)crystallization has been an effective way to improve the physical performances of stereoregular polymers.However,the competition between homo and SC crystallizations can lead to more complicated crystallization kinetics and polymorphic crystalline structure in stereocomplexable polymers,which influences the physical properties of obtained materials.Herein,we select the medium-molecular-weight(MMW)poly(L-lactic acid)/poly(D-lactic acid)(PLLA/PDLA)asymmetric blends with different PDLA fractions(f_(D)=0.01-0.5)as the model system and investigate the effects of f_(D) and crystallization temperature(T_(c))on the crystallization kinetics and polymorphic crystalline structure.We observe the fractionated(i.e.,multistep)crystallization kinetics and the formation of peculiar β-form homocrystals(HCs)in the asymmetric blends under quiescent conditions,which are strongly influenced by both f_(D) and T_(c).Precisely,crystallization of β-form HCs is favorable in the MMW PLLA/PDLA blends with high f_(D)(≥0.2)at a low T_(c)(80-100℃).It is proposed that the formation of metastable β-form HCs is attributed to the conformational matching between β-form HCs and SCs,and the stronger constrain effects of precedingly-formed SCs in the early stage of crystallization.Such effects can also cause the multistep crystallization kinetics of MMW PLLA/PDLA asymmetric blends in the heating process.