Study On Plasticization,crystallization Behavior And Mechanical Properties Of Poly(L-lactide)

Poly(L-lactide)(PLLA),a renewable,biodegradable and biocompatible polymer with high strength and modulus,can be proposed as an excellent substitute for petroleum-based plastics.However,the intrinsic brittleness and slow crystallization rate of PLLA have restricted its broader applications.In this work,the new plasticizers poly(triethylene glycol adipate)(PTGA1K and PTGA14K with number-average molecular weight of 1000 and 14000 g/mol)were first synthesized by polycondensation reaction and then blended with PLLA through a solution and casting process.The phase behavior,crystallization behavior and mechanical properties of PLLA/PTGA blends were investigated in depth.For the PLLA/PTGA1K system,when the PTGA1K content was not higher than 15 wt%,the single glass transition temperature suggested the miscibility between PLLA and PTGA1K.While two composition dependent glass transition temperatures were found when the PTGA1K content was higher than 15 wt%,indicating the partial miscibility between PLLA and PTGA1K.With the increase of PTGA1K content,the gradually decreased equilibrium melting point of PLLA also confirmed the miscibility between PLLA and PTGA1K.The crystallization behavior results showed that PTGA1K increased the isothermal crystallization rate of PLLA by accelerating its spherulitic growth without changing the crystallization mechanism and crystal structure of PLLA.Compared to neat PLLA,the tensile strength and modulus of the blended samples decreased,but the elongations at break significantly increased due to the plasticizing effect of PTGA1K.As to the PLLA/PTGA14K system,the blended samples showed a single glass transition temperature when the content of PTGA14K was 10 wt%;however,two composition dependent glass transition temperatures were found when the content of PTGA14K increased to 15 wt%and above.PLLA was completely miscible with PTGA14K when the content of PTGA14K was 10 wt%;however,they were partially miscible when the content of PTGA14K was 15 wt%and above.The equilibrium melting point of PLLA gradually decreased with increasing the content of PTGA14K,suggesting the miscibility between PLLA and PTGA14K.Similar to PTGA1K,PTGA14K was an effective plasticizer for PLLA.Without changing the crystallization mechanism and crystal structure of PLLA,PTGA14K increased the isothermal crystallization rate of PLLA by accelerating the spherulitic growth of PLLA and improved the mechanical properties by enhancing the elongation at break of PLLA.Besides,PTGA14K showed the better effect than PTGA1K in improving the mechanical properties of PLLA.In conclusion,PTGA1K and PTGA14K with number-average molecular weights of 1000 and 14000 g/mol,respectively,were synthesized by melt polycondensation method and then blended with PLLA by solution casting.The effects of composition and molecular weight of PTGA on the PLLA/PTGA blends were studied in depth.It was found that PLLA/PTGA blends changed from completely miscible to partially miscible with the increase of PTGA content.The glass transition temperature of PLLA in the blends decreased and the chain mobility increased gradually with the PTGA content increasing,thereby accelerating the isothermal crystallization rate and improving the elongation at break of PLLA.Compared with PTGA1K,PTGA14K with high number-molecular weight was less miscible with PLLA.The reduction of glass transition temperature and the increase of isothermal crystallization rate of PLLA caused by PTGA14K were smaller than those caused by PTGA1K.However,PTGA14K was better than PTGA1K in improving the elongation at break of PLLA because of the crystallization occurred in PLLA/PTGA1K blends at room temperature.