Multiblock Copolymers Of PEG And Biodegradable Flexible Polyesters As Modifiers Of PLLA And Compatibilizers Of PLLA Blends

Biodegradable material polylactide?PLA?is produced from plant starch,and has good biological compatibility,excellent mechanical strength,transparency,and processability.However,PLA also has inherent drawbacks including poor ductility and toughness and slow crystallization which limit its application in a wider field.To improve ductility and toughness of PLA without sacrificing environmental friendly performance,plasticizers or biodegradable flexible polyesters are frequently employed to melt blend with PLA.Among various plasticizers,polyethylene glycol?PEG?is in common use.But PLA/PEG blends often suffer from poor tensile strength and modulus and poor performance stability resulted from migration of low molecular weight PEG and slow crystallization and phase separation of high molecular weight PEG PLA toughened by impact modifiers often suffer from poor compatibility.In order to prepare plasticized and/or toughened PLA with more balanced properties,a multiblock copolymer composed of PLA-miscible PEG segments and PLA-immiscible biodegradable flexible polyester segments were designed to be plasticizing and toughening modifiers of PLA.In these multiblock copolymers,the PEG segments are expected to act as plasticizers of PLA to improve the ductility of PLA,and act as a compatibilizer of the polyester segments as well so as to improve the toughness of PLA effectively.Besides,the multiblock copolymers are expected to be compatibilizers of PLA/PBAT?poly?butylene adipate-co-terephthalate??blends in this work as well.PLLA?poly L-lactide?4032D purchased from NatureWorks was selected as the unmodified material in this work.To toughen and plasticize PLLA,a completely amorphous polyester poly?1,2-propyleneco-adipate??PPA?was used as the biodegradable flexible polyester segments first.E_6kP30,E_6kP50 and E5kP80,in which the PPA content were of 30 wt%,50 wt%,and 80 wt%respectively,were synthesized via chain extension/coupling with PEG6000(E_6k)using hexamethylene diisocyanate?HDI?as a chain extender.E_6kP multiblock copolymers were melt blended with PLLA.The DSC results show the crystallization of PEG6000 can be effectively inhibited by chain extension.According to the mechanical property results,plasticization-dominant PLLA materials?elongation-250%?were obtained when the amount of PEG segments was high in the blends and showed better mechanical performance stability compared with the PEG plasticized PLLA materials.Toughening-dominant materials?impact strength 33 kJ/m²?were obtained when the blends contained more PPA segments.Plasticized-toughened PLLA materials were obtained?250%,14 kJ/m²?for the blends containing 30 wt%E_6kP30(L70E_6kP3030),and it exhibited excellent comprehensive mechanical properties.The scanning electron microscopy?SEM?results show that,the PPA segments which acted as a toughening modifier aggravated in the blends and formed dispersed phase particles and the PEG segment displayed a compatibilizing effect.When the dosage of E_6kP is 30 wt%,elongated PPA particles/cavities can be found in the PLLA continuous phase,and the corresponding materials show much higher impact strength.As expected,E_6kP shows plasticizing and toughening effect on PLLA.Secondly,in order to prepare modified PLLA materials with better performance,PBAT was used as the biodegradable flexible polyester segments as PBAT is a crystalline aliphatic and aromatic copolyester.HO-terminated low molecular weight PBAT were chain extended with PEG6000(E_6k)first and the resulting multiblock copolymers E_6kBs were melt blended with PLLA.According to the mechanical properties results,it can be found that the ductility and impact toughness of PLLA is significantly enhanced when using 30 wt%E_6kB.PLLA/E_6kB blends can maintain high strength of 24-31 MPa and high modulus of 1.2-2.0 GPa as well.L70E_6kB5030 shows the elongation at break>200%and maintains the impact strength of 17 kJ/m².L70E_6k8030 exhibits superior impact strength of 43 kJ/m² and retains elongation of 150%and it shows better mechanical property stability.In comparison,PLLA/E_6kB blends exhibit better comprehensive mechanical properties than PLLA/E_6kP.Thirdly,PLLA and PBAT?2003F Biocosafe,supplied by Xin Fu Corp.?blends were modified using EB multiblock copolymers as compatibilizers.Among various EB copolymers with different chain structure and compositions,E_0.6kB₈₀ was found to be the one gave the best modification results.For the 80PLLA/20PBAT blends,the material strength and rigidity decline gradually with the amount of E_0.6kB₈₀ increases from 1phr to 10phr while the elongation remains unchanged?-230%?and the impact strength shows an improvement.For the 20PLLA/80PBAT blends,the modulus reduced gradually with the increase of E_0.6kB₈₀ content,and elongation at break increased and strength at break shows the highest value of 21 MPa at a dosage of 5phr.The SEM observation indicates that E_0.6kB₈₀ is able to locate on the interface of PLLA and PBAT,thus resulting in an improvement of interface adhesion and compatibility of PLLA and PBAT.