The Study On Odd-odd Aliphatic Polyester Block Poly(Lactic Acid)

Aliphatic polyesters are derived from renewable plant resources and have good biocompatibility and biodegradability.Polylactic acid(PLA),as a widely used aliphatic polyester,has extremely high strength and modulus,but has poor toughness and the elongation at break is less than 10%,which limits the processing application of PLA.Studies have shown that the toughness of PLA can be significantly improved by copolymerizing with amorphous polymers.The odd-odd polyester is an aliphatic polyester that having poor crystallinity and a low glass transition temperature(Tg).Therefore,it is concluded that the whole biobased copolymer that copolymerized by PLA and odd-odd polyester not only has good biocompatibility and biodegradability,but can also significantly improves the toughness of PLA.The thesis first synthesized a series of odd-odd aliphatic polyesters and studied the influence of chemical structure on thermodynamic properties.The polyesters structure were confirmed by nuclear magnetic resonance spectroscopy(~1H NMR).Non-isothermal differential scanning calorimetry(DSC)studies have shown that at the crystallization and heating rate of10 ~oC/min,the polyesters containing the side methyl group exhibit an amorphous state,while the linear aliphatic polyesters have the relatively good crystallinity,except for poly(propylene glutarate)(PPG).What's more,with the chain length increases,the crystallinity of the polyesters increases.In addition,the Tg values of the polyesters were analyzed and the results showed that the chain length,spatial effect,symmetry and other factors of the monomer will affect the flexibility of the polymer chain and further affect the values of Tg.In addition,for the polyesters synthesis from propylene glycol and three diacids with different chain length:PPG,poly(propylene pimelate)(PPP)and poly(propylene sebacate)(PPA),the multiple melting behaviors after isothermal crystallization were compared studied and the mechanism was proved.Moreover,the Avrami model was successfully applied to the isothermal crystallization kinetics of PPP and PPA.Finally,the polarized polyester(POM)test of the linear polyesters were carried out to study the trend of the spherulite morphology with chain length increas.The Lauritzen-Hoffman model was successfully applied to the spherulite growth kinetics of PPG,PPP and PPA.Through screening,we used poly(propylene glutarate)(PPG)and poly(pentylene pimelate)(PPeP)as a macroinitiator to initiate the ring-opening polymerization of L-lactide to synthesize two PLLA triblock copolymers.The effects of intermediate segments with different crystallinity on the thermodynamic properties and tensile properties of the copolymers were investigated.The DSC results show that both odd-odd polyesters improve the crystallization ability of PLLA.PPG does not crystallize in the copolymer due to its poor crystallinity,and PPeP competes with PLLA crystal in its triblock copolymer.The POM test was carried out at a temperature of 130°C.The patterns show the spherulitic morphology of the PLLA segment.When the length of the PLLA segment increases,the PLLA-b-PPeP-b-PLLA spherulite pattern appears to be defective,indicating that the crystallization ability of the PLLA segment is decreased,which is consistent with the DSC measurement.In addition,the influence of the middle segment on the thermal stability of the copolymer were studied by thermogravimetric analysis(TGA).The results show that the blocks of the odd-odd polyesters significantly improve the thermal stability of the triblock copolymer,and PPeP's lifting effect is remarkable.Finally,the tensile test of the two groups of triblock copolymers showed that the two odd-odd polyesters with different crystallinity significantly improved the toughness of PLLA,with little difference,and the elongation at break was up to 676%.The purpose of toughening polylactic acid is achieved.