Study On Synthesis And Crystallization Behavior Of Polylactic Acid

The linear, three-and four-arm poly(L-lactide)(PLLA) and three-arm stereocomplex block polylactide (TPLLA-b-PDLA) were synthesized using water, trimethylolethane, pentaerythritol and three-arm PLLA as a initiator respectively. The effects of the molecular weight and chain architecture on the crystallization behavior of PLLA, TPLLA-b-PDLA and PLLA/Cotton were investigated. The main research contents are as follows:The three-and four-arm PLLA were synthesized by manipulating the reaction temperature. The TPLLA-b-PDLA polymers were prepared by end group activation technique. The chain structures were examined by the digital automatic polarimeter, gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR). The results showed that the composition and chain structures of the products, which were consistent with the results of experiment design, could be controlled by adjusting the monomer and initiator feed ratio.The influences of molecular weight and chain architecture on the isothermal crystallization behavior were investigated. The results indicated that the crystal growth rate increased, and then decreased with the increasing of the molecular weight of PLLA. The maximum crystal growth rate was almost constant at4.0um/min for four-arm PLLA when its molecular weight was higher than4.4x104. The value of the end surface free energies of the folded chain lamellae of LPLLA, TPLLA and FPLL A were all about60.72Jm-2, while that of the extended chain lamellae of TPLLA obtained were about93.27Jm-2by using Lauritzen-Hoffman kinetic theory.The effects of molecular weight and chain architecture on the non-isothermal crystallization of the PLLA from melt were also investigated. The non-isothermal crystallization rate was evaluated by the increment of crystallinity per minute. The results showed that the non-isothermal crystallization rate of LPLLA was faster than those of TPLLA and FPLLA in a lower molecular weight region. As the molecular weight increased, the non-isothermal crystallization rate of LPLLA decreased, while those of TPLLA and FPLLA increased, then decreased, and then increased. This phenomenon indicated that the effect of the branch point on the segment movement decreased while the influence of it on the nucleation increased. The non-isothermal crystallization activation energies of LPLLA, TPLLA and FPLLA were calculated by OWF, KAS, L-T, Friedman and Kissinger methods. The results showed that the non-isothermal crystallization activation energy of the PLLA with similar molecular weight increased with the number of branched chains, which was basically consistent with the results of non-isothermal crystallization rates.Based on our previous discussion, the crystallization behavior of the TPLLA-b-PDLA was investigated. The results indicated that both thicknesses of the lamellar layer and the random layer increased with crystallization temperature. The hydrogen bond between CH and C=O in the crystal of the TPLLA-b-PDLA was stronger than that in the crystal of the PLLA. It was found that the molecular weight did not significantly affect crystal growth rate as the ratio of L-lactide and D-lactide blocks was1/1, but the crystal growth rate increased with molecular weight as the ratio of them deviated from1/1. The end surface free energy of TPLLA-b-PDLA (100.4-132.4Jm1) was higher than those of homo-crystallites of PLLA.Moreover, the influence of the molecular weight and chain architecture on the crystallization of the PLLA/Cotton from melt was investigated. The polarizing optical microscopy (POM) photographs revealed that the cotton had a nucleating effect on the crystallization process of LPLLA, but no significant effect on those of TPLLA and FPLLA when the molecular weight of them were lower than1.0×104, while it had nucleating effects on the crystallization process of TPLLA and FPLLA but no significant effect on that of LPLLA when the molecular weight of them were higher than2.0×104. The activation energy of non-isothermal crystallization of FPLLA110/Cotton obtained from the DSC data was lower than that of FPLLA110. The results showed that the nucleating effect of the cotton firstly depended on the formation of hydrogen bond between the terminal hydroxyl group of PLLA and the free hydroxyl groups of the cellulose and secondly on the fact that the chain absorbed on the surface of cotton could form a folded chain nucleus.