Condensed Structurl Effects Of Stereocomplexation And Amphiphilic Copolymer Chains Structural Design For Polylactide

Poly（lactide）（PLA）,one of the most important biodegradable polyesters,has great potential in many applications fields,such as green plastics,biomedicine,and so on.However,homocrystalline PLA suffers from“improvable”limitations which have hindered the commercialization of PLA in industries.It is well known that copolymers and blends are two common but effective approaches to improve the properties of polymers matrix.We synthetized abundant of PLA chains with different molecular weight and different compositions to improve the performance of PLA through the PLLA/PDLA blends;meanwhile,the crystallization,the phase-transform,and the morphology structural evolution of blends and copolymers were investigated.Firstly,we quantitatively prepared the symmetric PLLA/PDLA blends by taking advantage of convenient homocrystalline PLA to investigate the crystallization temperature（T_c）effect during the nonisothermal melt crystallization and the isothermal cold crystallization（120-200°C）processes.We observed that crystallinity and relative of stereocomplexes（SCs）increase with T_cand the SCs are exclusively formed at T_c>180°C in medium/high-molecular weight（M/H-MW）PLLA/PDLA racemic blends.Secondly,we further explored the asymmetric PLLA/PDLA blends with various MW and blend ratios.For the MW of the enantiomeric chains are over 6 k Da but less than 41 k Da,most of the chain segment keep the non-entanglement conformation to form more ordered SC-form with less entanglement or the amorphous region than for the MW of the enantiomeric chains over41 k Da.On the other hand,when MWs of one of the enantiomers is more than 41 k Da and less than 210 k Da,the objective conditions are provided to the formation of multiple stereocomplexation.When MWs of the enantiomeric chain are over 41 k Da and less than 210k Da,due to the low chain diffusion ability and restricted intermolecular crystal nucleation/growth,SCs coexist with homocrystals（HCs）.Afterwards,poly（ethylene glycol）（PEG）and glucose as the branched point were introduced to increase the complexity of PLA chain segment to investigate the crystallization behaviors and morphologies of PLLA through the branched PEG-b-PLLA copolymers with various architectures and different compositions.When the weight fraction of PLLA is low,the PEG segment as the diluting agent increases the flexibility and chain diffusion ability of PEG-b-PLLA-g-glucose copolymers and meanwhile,which also improves the formation ofα-form to the extensive melting process of PLLA matrix during second subsequent heating.Moreover,the initial decomposition temperature(T_5%)of PEG-b-PLLA-g-glucose copolymers was significantly reduced,due to the enhanced transesterification of the PLLA segment at both ends under the low melting PEG to promote the thermal degradation of copolymers.When the PLLA content reaches a certain level,due to PEG segment as a plasticizer to enhance the interaction（H-bonds）between PLLA chains,T_5%is slightly improved during the heating process.In addition,with increase of the branched PEG-b-PLLA grafted segments,the morphological structure and surface morphology of PEG-b-PLLA-g-glucose copolymers were change from the spherical micelle and irregular surfaces to tightly fused circular lamellas and corrugated dense surfaces,respectively.Finally,the branched PDLA/MPEG-b-PLLA-g-glucose blends with different architectures and different MPEG-b-PLLA grafted copolymer contents were prepared based on the above experimental results.The isothermal crystallization behavior and corresponding morphology evolution of PDLA blends with PLLA_6.7k and MPEG-b-PLLA_6.7k-g-glucose with different architectures（1A-5A）and different MPEG-b-PLLA_6.7k-grafted contents were investigated.The introduction of small amounts of the branched MPEG-b-PLLA_6.7k-g-glucose into PDLA blends to the formation of SCs during quenching from 240°C.Meanwhile,the formation of SCs in between the chain branched structure of MPEG-b-PLLA_6.7k-g-glucose and PDLA chains acting as the role of physical crosslinking points slow down the motion of PDLA chains,but the SCs could act as a heterogeneous nucleating agent for the late formation of HCs in the blend system,accelerating the crystallization kinetics of HCs through enhancing the nucleation density.It is worth to be noted that the blends of PDLA/5A-MPEG-b-PLLA_6.7k-g-Glu own the more intensive SCs in the small range due to the more branched architecture in the small range.Due to the complexity of the branched chains of MPEG-b-PLLA_6.7k-g-glucose,demonstrating the different nucleation and growth behaviors of the SC and the HC during the isothermal crystallization.For PDLA/MPEG-b-PLLA_6.7k-g-glucose blends,the mobility of SCs in the blend system with the branched MPEG-b-PLLA_6.7k-g-glucose and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a T_c of 130°C.