Study On Polylactides And Its Nanocomposites

Poly(L-lactide) (PLLA) is a biodegradable polymer with good mechanical properties, elastic modulus and thermal plasticity. Howerver, PLLA still has some defects, for examples, poor thermal stability, slow crystallization rate, lower impact resistance, and long synthesis cycle of PLLA. The objective of this dissertation is to develop a new kind of PLLA materials with high performance, including PLLA-ESO(epoxy soybean oil), PLLA/g-SiO2, PLLA-ESO/g-SiO2, and PLLA/PLLA-POSS materials, by chemical and physical modification. The properties of the materials has been systematically studied.Firstly, PLLA-ESO copolymer has been synthesized by ring-opening polymerization. The reaction mechanism between L-lactide(LLA) and ESO has been investigated by Fourier Transform Infrared (FT-IR) and1H NMR. Furthermore, the effect of ESO, SnOct2 and antioxidants on the polymerizaion of LLA has also been studied. The thermal behavior and mechanical properties are characterized by Differential Scanning Calorimeter (DSC), Thermogravimetric Analysis (TGA) and Universal testing machine. The results show that the star-shaped PLLA-ESO copolymer has been synthesized by the ring-opening copolymerziation of LLA with ESO. Gel permeation chromatography(GPC) analysis indicates that ESO could increase the molecular weight (Mw) of PLLA, while the content of SnOct2 has no effect on the Mw of PLLA-ESO copolymer, but could affect the polymerization rate and molecular weight distribution. The glass transition temperature (Tg), the melting temperature (Tm), crystallinity (Xc) and thermal stability of PLLA-ESO copolymer decrease, while the thermal oxidation stability and mechanical properties improve, as compared to those of pure PLLA.Secondly, surface-grafted silica (g-SiO2) has been prepared by ring-opening polymerization of LLA and nano-SiO2, and father, PLLA/g-SiO2 and PLLA-ESO/g-SiO2 nanocomposites have been prepared by solution blending. FT-IR, DSC, TGA, TEM and POM are utilized to characterize the reaction mechanism between LLA and nano-SiO2, structure and properties of the nanocomposites. The results show that chemical reaction occurs between the low molecular weight (LMW) PLLA and surface of silica nanoparticles. The amount of grafted LMW PLLA investgated by thermal gravimetric analysis (TGA) is about 14.9-28.2% in weight. g-SiO2 nanoparticles could be easily dispersed into PLLA-ESO matrix to form a uniform PLLA-ESO/g-SiO2 composite, while un-grafted SiO2 nanoparticles have the tendency to aggregate. DSC analyses indicate that g-SiO2 nanoparticles can serve as a nucleating agent for the crystallization of PLLA-ESO in the nanocomposites. The Tm and Tg of PLLA-ESO/g-SiO2 nanocomposites seem to be independent of loading of g-SiO2 particles. The DSC cures of PLLA/g-SiO2 nanocomposite obviously shows double melting peaks, while that of PLLA-ESO/g-SiO2 nanocomposites only a single melting peak. PLLA-ESO/g-SiO2 composites exhibit higher tensile strength and elongation than those of PLLA-ESO/SiO2 composites.Finally, PLLA-POSS hybrids were successfully prepared by solution ring-opening polymerization of L-lactide with with octaglycidylether POSS (OPOSS), aminopropylheptakis(isobutyl) POSS (AMPOSS) and 1,2-propanediolisobutyl POSS (AMPOSS) in the presence of Sn(Oct)2 catalyst, and then different kind of PLLA/PLLA-POSS nanocomposites with various PLLA-POSS content of 1-30 wt%were prepared by solution mixing of the neat PLLA homopolymer with PLLA-POSS hybrids with 0.50 mol% POSS. FT-IR,1H NMR, DSC and TGA are utilized to characterize the PLLA-POSS hybrids and PLLA/PLLA-POSS nanocomposites. The results showed that the carboxyl of PLLA has reacted with epoxy group of OPOSS, amino group of AMPOSS and hydroxyl group of ALPOSS. The Tg and Tm of PLLA/PLLA-POSS increased and then decreased with the PLLA-POSS nanohybrids increasing, and three kinds of PLLA/PLLA-POSS nanocomposites have high thermal oxidation stability. Furthermore, PLLA-POSS can can serve as effective nucleating agents for the crystallization of PLLA in the nanocomposites. PLLA shows homogeneous nucleation process followed by a disc-type two-dimensional crystal growth, while PLLA/PLLA-POSS nanocomposites hint heterogeneous nucleation process followed by a three-dimensional crystal growth with spherulites, in which PLLA-POSS hybrids alter the nucleation mechanism and crystal growth of PLLA. PLLA-ALPOSS has the best nucleation effect. PLLA/PLLA-POSS nanocomposites had the higher crystallization rate than PLLA at the same crystallization temperature, in which PLLA/PLLA-OPOSS nanocomposites had the highest crystallization rate with 10%content of PLLA-OPOSS, while PLLA/PLLA-AMPOSS and PLLA/PLLA-ALPOSS nanocomposites had the highest crystallization rate with 30% content of PLLA-POSS nanohybrids. PLLA-OPOSS, PLLA-AMPOSS and PLLA-ALPOSS nanohybrids can strengthen and toughen PLLA, separately.