Poly(L-lactic Acid)/SiO₂ Nanocomposites Prepared By In Situ Melt Polycondensation And Characterization

In this thesis, the progress and prospect in the preparation, structure and properties of PLLA and PLLA-based nanocomposites are reviewed. In situ melt polycondensation of LLA in the presence of acidic silica sol (aSS) has been proposed as a novel approach to prepare PLLA/SiO₂ nanocomposites for the first time. PLLA/SiO₂ nanocomposites with SiO₂ content in the range of 3.5%-19.1% were successfully prepared and their nanostructure, light transmittance, thermal properties and crystallization behaviors were investigated.Acidic aSS (pH 2.5) kept stable in acid LLA aqueous solution (pH 3). In melt polycondensation of LLA, the reaction mixture experiences obvious synchronous change from polarity and hydrophilic to weak-polarity and hydrophobicity both in the organic phase and on the particle surfaces due to chain growth and surface grafting, respectively. The surface grafting provided steric stabilization which made up the decreasing stability of electrical double layer. These characteristics made aSS stable during the polycondensation process. Although incompact aggregation took place during the dehydration/oligomerization process, it was facilely broken by ultrasonic and the SiO₂ nanoparticles kept stably dispersed during the dehydration/ oligomerization process thereafter and the succedent melt polycondensation process. Finally, PLLA/SiO₂ nanocomposites with nano-scale uniformly dispersed SiO₂ nanoparticles were succefully prepared, as indicated by TEM. FT-IR and TGA analysis validated qualitatively and quantitatively the grafting of OLLA on the surface of SiO₂. The grafting ratio reached 44-60% and the grafting efficiency lied in 2.8-8%.The yield of PLLA kept almost unchanged in the whole experimental range. The presence of SiO₂ nanoparticles prevented or depressed discoloration which is often encountered in LLA polycondensation and the color was clearly improved. It appears light yellow to white. The molecular weight kept almost constant when less than 6.1 wt% SiO₂ was introduced, but it decreased with higher SiO₂ content. The decrease of molecular weight is attributed to the departure of the 1/1 COOH/OH group ratio due to the existence of the silanol group on SiO₂ surface and increased diffusion resistance.The differential scan calorimetry (DSC),thermal gravimetric analysis (TGA)and Ultraviolet-Visible light spectrophotometer were employed to characterize thePLLA/SiO₂ nanocomposites. Comparing with pure PLLA, the nanocomposites have higher glass-transition, melting temperature, crystallinity (γ_c) and better heat resistant property. Furthermore, the existence of SiO₂ nanoparticles has no clear influence on the light transmittance of the nanocomposite films thanks to the nano-scale an uniform dispersion.The crystallization and melting behaviors of PLLA/SiO₂ nanocomposites by investigated with differential scanning calorimetry (DSC) and polar optical micrography (POM). The heat isothermal crystallization rate was so slow that no crystallization peak was observed in DSC curves. But the cold isothermal crystallization rate was rapid enough to be DSC-determined. Heat crystallization was used observe the spherulite growth in isothermal and non-isothermal manners. The Avrami equation was employed to analysis the cold isothermal crystallization process. Though PLLA and its nanocomposites have different nucleation manner, namely homogeneous nucleation mode for PLLA and heterogeneous nucleation for nanocomposites, the spherulites all grow in a two-mensional manner. The SiO₂ nanoparticles in the matrix behave as a good crystal nucleus and resulted in higher nucleation rate and consequent higher crystallization rate. The composites with SiO₂ content (φ_Si,c) less than 10.5% exhibited higher spherulite growth rate than pure PLLA. The spherulite growth rate increased with SiO₂ content. But composites with φ_Si,c of 19.1% exhibited lower spherulite growth rate than pure PLLA. PLLA and its nanocomposites (φ_Si,c=5.0%, 7.6%, 10.5%, 19.1%) all exhibited maximum spherulite growth rate at 125℃, and the maximum spherulite growth rate were 6.50 μ/min, 8.94 μ/min, 8.96 μ/min, 9.87 μ/min and 5.31 μ/min respectively. In terms of the total crystallization rate, the nanocomposites exhibited higher temperature (125°C) for the rapidest crystallization rate than pure PLLA due to higher nucleation rate at higher temperature. Further analysis with the Lauritzen-Hoffmann equation indicates that PLLA/SiO₂ nanocomposites have smaller nucleation parameter, namely, smaller end-surface free energy(σ_e).In addition to nano-scale dispersion and efficient surface grafting, the in situ melt-polycondensation strategy proposed in this study has several other advantages such as commercially available and cheap starting nano-material, short and environmentally friendly process and low cost. It appears a promising method for preparation of PLLA/SiO₂ nanocomposites. It may also be extended to otherpolymerization systems.