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 asymmetr...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.展开更多
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 ...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.展开更多
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 cryst...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.展开更多
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 ...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.展开更多
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 ...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.展开更多
基金supported by the National Natural Science Foundation of China(No.21404050)the Research Foundation of Jiangsu University(No.14JDG059)+2 种基金Hao also thanks the supports from Postdoctoral Science Foundation of China(No.2019M651478)Natural Science Foundation of Jiangsu Province(No.BK20190866)Natural Science Foundation of the Higher Education Institutions of Jiangsu Provinee(No.18KJB150009).
文摘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.
基金financially supported by the National Natural Science Foundation of China(No.21604016)National Undergraduate Innovation Training Program(No.201610657004)
文摘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.
基金financially supported by the National Natural Science Foundation of China(No.21873083).
文摘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.
基金financially supported by the National Natural Science Foundation of China(Nos.52173020 and 21404050)the simulation work is also supported by the Open Research Fund of CNMGE Platform&NSCC-TJ(No.CNMGE202101013)Hao also thanks the supports from the Natural Science Foundation of Jiangsu Province(No.BK20190866)。
文摘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.
基金supported by the National Natural Science Foundation of China(Nos.21134002 and 21104003)National Basic Research Program of China(973 Program:2009CB825300)
文摘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.
