Crystalline Strucutral Control And Preparation Of High-Melting-Point Materials For Stere-Ocomplexed Poly (Lactic Acid)

Stereocomplexed poly(lactic acid)(SC-PLA)has a melting point of 230?,which is about 50?higher than that of conventional homocrystalline poly(lactic acid)(PLA,160?170?).Therefore,stereocomplex(SC)crystallization can significantly enhance the thermal resistance of PLA materials.Also,SC-PLA has high mechanical properties,good hydrolysis resistance and solvent resistance than the homocrystalline PLA.The chain structure,blend ratio or block length ratio of PLLA and poly(D-lactic acid)(PDLA),crystallization conditions,and addition of additives are main factors that affect the SC crystallization of PLLA/PDLA blends or stereoblock copolymers.The SC crystallization between enantiomeric PLLA and PDLA can be well controlled by changing such factors,enabling the preparation of SC-PLA materials with better physical performances.First,the crystallization kinetics,polymorphic crystalline structure,crystalline structural reorganization,and spherulite morphology of high-molecular-weight(HMW)PLLA/PDLA blends with different PLLA/PDLA ratios were systematically investigated.With increasing the PDLA content from 0 to 50 wt%,the glass transition temperature(Tg),melting temperature of SCs(Tm,SC)and homocrystallites(HCs,Tm,HC)change little;while the crystallinity of SCs(Xc,SC)and the relative fraction of SCs(fSC)increase.When the PLLA/PDLA ratio is closing to 50/50,SCs can be preferentially formed in the PLLA/PDLA blends.In-situ wide-angle X-ray diffraction(WAXD)results suggest that,when the PLLA/PDLA blend ratio approaches to 50/50 or the crystallization temperature is low,the HC-to-SC melt-recrystallization proceeds more easily in the heating process.The spherulite morphology becomes irregular with the decrease of PDLA content,because of the phase transition in crystallization process and the dilution effect of excess PLLA on SC crystallization.In order to enhance the SC crystallization ability,poly(L-lactic acid)-b-poly(D-lactic acid)(PLLA-b-PDLA)diblock copolymers with different molecular weight were prepared.The effects of molecular weight and chain topology structure on the SC crystallization kinetics,crystalline structure,thermal and mechanical properties were investigated.The HMW linear,3-and 6-arm star-shaped PLLA-b-PDLAs were synthesized via the two-step ring-opening polymerization(ROP)of L-or D-lactide with 1-dodechanol,glycerol,and D-sorbitol as initiators,respectively.The synthesized linear,3-and 6-arm star-shaped PLLA-b-PDLAs exhibit faster crystallization rate than the HMW PLLA/PDLA equivalent blend and they crystallize exclusively in SCs under all the conditions investigated.This could be caused by the stronger interaction between enantiomeric PLLA and PDLA blocks caused by the covalent bonding.The chain topological structure influences the crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly.The crystallization temperature(Tc),melting temperature(Tm),degree of crystallinity(Xc),spherulite growth rate(G),crystallite size,long period(LP),and crystalline layer thickness of PLLA-b-PDLA decrease with increasing the branching number,because of the retarding effect of chain branching on crystallization.All the linear,3-and 6-arm star-shaped PLLA-b-PDLAs exhibit higher storage modulus at high temperature(200?)than the homocrystalline PLLA,which is caused by the formation of SCs with high melting point.Third,the well-defined PLLA-b-PDLA copolymers with different molecular weight(Mn=10?110 kDa)and PLLA/PDLA block length ratios(51/59?22/81)were synthesized via click-chemistry.The effects of molecular weight and PLLA/PDLA block length ratio on the crystallization kinetics,polymorphic crystalline structure,crystalline structural reorganization and thermal mechanic properties of PLLA-b-PDLAs were explored.It was found that all PLLA-b-PDLA copolymers exhibit fast crystallization and exclusively form SCs during the cooling process.Under the isothermal melt crystallization,SCs are easily formed in the PLLA-b-PDLA copolymers with similar PLLA and PDLA lengths at all the investigated Tcs;while the PLLA-b-PDLA copolymer with a PLLA/PDLA block length ratio of 22/81 crystallizes in both HCs and SCs at Tc<160?.The Xc,SC of PLLA-b-PDLA copolymers with similar PLLA and PDLA block lengths first increases and then decreases with increasing the molecular weight;the highest Xc,SC of 53.3%is found for the PLLA-b-PDLA with a moderate molecular weight(Mn=63 kDa).The LP increases with increasing the molecular weight,Tc,and PLLA/PDLA block length ratio(from 22/81 to 51/59).Because of the formation of high-melting-point SCs,all the HMW PLLA-b-PDLAs exhibit higher storage moduli at high temperature(200?)than the homocrystalline PLLA.Fourth,the effects of flexible poly(ethylene glycol)(PEG)midblock on the crystallization kinetic,polymorphic crystalline structure,and thermal mechanical properties of HMW PLLA/PDLA equivalent blends were investigated.The HMW poly(L-lactide)-poly(ethylene glycol)-poly(L-lactide)(PLLA-PEG-PLLA)and poly(D-lactide)-poly(ethylene glycol)-poly(D-lactide)(PDLA-PEG-PDLA)triblock copolymers(Mn>60 kDa)bearing different PEG midblock lengths,PLLA and PDLA end block lengths were prepared.The crystallization rate and the crystallinity,content of SCs enhance as the length of PEG midblock increases or the lengths of PLLA,PDLA end blocks decease,because the flexible PEG midblock improves the diffusion ability of PLLA and PDLA chains and decreases the diffusion pathway between PLLA and PDLA in SC crystallization.The synergistic effects of SC crystallization and plasticizing effect of flexible PEG midblocks offer the PLLA-PEG-PLLA/PDLA-PEG-PDLA blends higher tensile strength,larger elongation-at-break,and better thermal resistance than the corresponding PLLA-PEG-PL-LA copolymers.Finally,the heterogeneous nucleation method was used to promote the SC crystallization of HMW PLLA/PDLA blends.A novel sc-type nucleator,i.e.,zinc phenylphosphonate(PPZn),was found;it can promote the SC crystallization and suppress the HC crystallization of HMW PLLA/PDLA blend.The effects of PPZn on the nonisothermal and isothermal crystallization kinetics,crystalline structure,polymorphic structural transition,spherulitic morphology,and thermal mechanical properties of PLLA/PDLA blends were studied.PPZn not only accelerates the crystallization rate but also selectively promotes the formation of SCs for the HMW PLLA/PDLA blends in nonisothermal and isothermal crystallizations.After the addition of 1 wt%PPZn,the crystallization half-time(t1/2)of PLLA/PDLA blends decreases from 15.2 to 0.28 min at Tc=140?;the nucleation density increases and the spherulite size decreases.Due to the enhanced SC crystallization,the storage modulus of HMW PLLA/PDLA blend at high temperatures(170?200?)increases significantly after the addition of PPZn.