Chain-extension And Characterization Of Terminated Poly (Lactic Acid)

This thesis primarily studied terminated modification and chain extention of polylactic acid (PLA) with terminated modifiers and diisocyanate, and explored the synthesis of three-block copolymers of PLA.The hydroxyl-terminated modified polylactic acid copolymers of Poly (Lactic Acid/ε-Caprolactone)[P(LA/CL)], Poly (Lactic Acid/1, 4-Butanediol) [P(LA/BDO)] and Poly (Lactic Acid/Diethanolamine) [P(LA/DEA)] were synthesized via directly melt polycondensation using D, L-racemic lactic acid (D,L-LA) as raw material andε-caprolactone (ε-CL), 1, 4-butanediol (BDO) and diethanolamine (DEA) as Modifier. And,the carboxyl-terminated modified polylactic acid copolymers of Poly (Lactic Acid/ succinic anhydride)[P(LA/SA)], Poly (Lactic Acid/ adipic acid) [P(LA/AA)] and Poly (Lactic Acid/citric acid) [P(LA/CA)] were synthesized with D, L-racemic lactic acid (D,L-LA) as raw material and succinic anhydride(SA) , adipic acid (AA) and citric acid (CA) as Modifier. The copolymerization reaction was performed at the pressure of 0.098 MPa using stannous octoate as catalyst and by linear heating method. The copolymers were characterized by viscosity-average molecular weight, FT-IR, ¹H-NMR, DSC, TGA and XRD. The results demonstrate that Tg of the hydroxyl-terminated copolymers is lower than that of PLA, and reduced the rigidity of PLA with certain plasticization; The copolymers have good thermal stability for one-step decomposition; Compared with PLA, the crystallinity of P(LA/CL) and P(LA/BDO) decreased, and the flexibility increased. While the P(LA/DEA) is amorphous polymer, this amorphous structure is conducive to the degradation. After chain extension with IPDI, the molecular weight all increase; And, the Tg of carboxyl-terminated copolymers is lower than that of PLA, and reduced the rigidity of PLA; The copolymers have good thermal stability for one-step decomposition; Compared with PLA, the crystallinity of P(LA /SA) and P(LA/AA) decreased, While the P(LA/CA) slightly decreased, so that, the mechanical properties of polymers were adjusted to the ratio of lactic acid and modifiers. The method of synthesizing end-groups PLA copolymers is simple and with low cost, and it provides effective oligomers for the subsequent chain extension reactions.The optimum conditions of P(LA/CL) were that pressure of 0.098 MPa, n(D, L-LA)∶n(ε-CL)=8∶2, Sn(Oct)₂ content of 0.8 %, temperature of 170℃and reaction time of 7 h. The largest Mηof the series of P(LA/CL) is up to 20, 785. The triblock copolymer was synthesized via directly melt polycondensation with Lactic acid,ε-caprolactone and succinic anhydride. The copolymerization reaction was performed at the pressure of 0.098 MPa using stannous octoate as catalyst and by linear heating method. The copolymers were characterized by viscosity-average molecular weight, FT-IR, ¹H-NMR, DSC, TGA and XRD. The results showed that the diblock oligomers P(LA/CL) were synthesized first, and then plusing a third monomer to synthesize triblock copolymer [P(LA/CL-SA)], which had the highest molecular weight. Molecular weight of P(LA/CL-SA) was little increased after chain extension, probably because IPDI was hydroxyl-terminated chain extender, the chain extension effect of carboxyl endofthechain was not obvious. FT-IR and ¹H-NMR results obtained that the triblock copolymers of polylactic acid were synthesized successfully; TGA results showed that the thermal stability of copolymer P(LA/CL-SA) was lower than that of PLA, it was one step decomposition; X-ray diffraction showed that it was the amorphous material,and it was readily biodegradable, had lower crystallinity than that of PLA, so that the flexibility of PLA was enhanced.