Mechanism Of Optimizing The Elongation And Crystallization Of PLLA By P4HB And COF

Poly(lactic acid)(PLLA)is a biodegradable polymer that has been widely used in many fields.However,its inherent drawbacks of brittleness and poor crystallization performance have limited its further development.Therefore,improving the mechanical and crystallization properties of PLLA has important theoretical and practical significance.In this study,another biodegradable polymer,poly(4-hydroxybutyrate)(P4HB),was used as a toughening agent to prepare PLLA/P4 HB blends through melt blending.It was found that P4 HB could significantly improve the tensile properties of PLLA,and the blend with a weight ratio of 7:3 of PLLA to P4 HB exhibited a fracture elongation of up to 249.0%.However,the results of dynamic mechanical analysis indicated poor compatibility between PLLA and P4 HB.Furthermore,reactive toughening agent,glycidyl methacrylate-styrene-methyl methacrylate reduced glycerol ester(MSG),and non-reactive toughening agent,polyoxymethylene(POM),were introduced into the PLLA/P4HB=7/3 blend.The results showed that the epoxy groups in the MSG molecular chains can undergo ring-opening reaction with the hydroxyl or carboxyl end groups of PLLA and P4 HB molecular chains to in-situ form graft copolymers of PLLA and P4 HB as toughening agents,which distribute at the interface between the PLLA and P4 HB phases and enhance the interfacial interaction between them.When the MSG content was 2 wt%,the blend exhibited a yield strength of 30.0 MPa,a tensile strength of 34.3 MPa,and an elongation at break of 328.5%.In contrast,the C-H groups in the POM molecular chain can form hydrogen bonds with the carbonyl groups in the PLLA and P4 HB molecular chains,distributing at the phase interface of PLLA and P4 HB to improve the interfacial adhesion between the two phases.Scanning electron microscopy(SEM)observation showed that the phase interface of the PLLA/P4 HB blend became more blurred after adding POM,indicating a good toughening effect.When the POM content was 3 wt%,the yield strength was 35.4 MPa,the fracture strength was 37.3 MPa,and the fracture elongation was 303.0%.Moreover,both toughening agents could inhibit the physical aging problem of the decrease in fracture elongation of the PLLA/P4 HB blend with time,improving the tensile stability of the blend.The three-dimensional covalent organic framework material(COF-300)was introduced into PLLA through a solution blending method,and it was found that COF-300 can induce PLLA crystallization.During isothermal crystallization at125 °C,COF-300 did not affect the crystal form of PLLA,and α-crystals were formed in both the pure PLLA and PLLA/COF-300 composite.When the isothermal crystallization temperature was 85 °C,pure PLLA mainly formed δ-crystals,while the addition of COF-300 induced PLLA to mainly form α-crystals,indicating that the introduction of COF-300 can induce the transformation of PLLA from δ-crystals to α-crystals.