Preparation Of Poly(L-lactic Acid)/SiO₂ Nanocomposites Via In Situ Melt/Solid Polycondensation

Polylactic acid (PLA) is a very important bio-based and biodegradable polymer. Its raw materials come from natural resources and it can go back into the natural after use. Because of the good biodegradability and biocompatibility, it can be widely used in packaging materials, fibers, household plastics, bio-medical materials and other fields. But its crystallization rate is small, and its heat and impact resistances are poor, so it is not good enough for some demanding applications. Therefore, it is often modified by means of copolymerization, blending and nano-compositing, etc. PLLA nanocomposites often possess much better properties if the nanoparticles were well-dispersed and appropriate inter-phase interaction is introduced.This article reviewed the synthesis and modification of PLLA and PLLA nanocomposites, based on which in situ melt/solid state polycondensation of LLA in the presence of acidic silica sol(aSS) has been proposed to prepare PLLA/SiO2 nanocomposites.The stability of nanoparticles during the D/O stage was studied and improved, and dispersion stability of SiO2 nanoparticles was interpreted theoretically and experimentally.The electrical double-layer and the grafted OLLA chains provide static and steric stabilities at the early and late phases respectively. But there exists a intermediate transitional phase with weak stability when the static stability is weakened but sufficient steric stability has not yet established, leading to "soft" or "hard" aggregation depending on the SiO2 loading and agitation conditions. At low or moderate SiO2 loading (<5-10%), the "soft" aggregation can be depressed with appropriate agitation condition and re-dispersed with the aid of gradually established steric interaction energy resulting from growing grafting. The appropriate agitation energy to stabilize the nanoparticles depends on the SiO2 loading. Well dispersed PLLA/SiO2 nanocomposites with 5% and 10% SiO2 loading were successfully prepared in-situ melt polycondensation using arc stirrer at 400 rpm and 600 rpm respectively in D/O stage. But at high SiO2 loading (≥20%) or improper agitation condition, the nanoparticles are prone to form "hard" aggregation or gel which can not be re-dispersed.PLLA/SiO2 nanocomposites with different Mw(7,000-73,000g/mol) and differentφsi,p (0-7.8%) were prepared and their isothermal cold and melt crystallizations and melting behaviors were studied by DSC. (1) The isothermal cold crystallization rate is greater than pure PLLA and depends on the matrix molecular weight and SiO2 content. It increases and then decreases with increasing Mw, reaching a maximum value at Mw of 15,000 g/mol and then leveling off at Mw≥40,000. And it increases with increasingφSi,p from 0 to 8%, especially at higher temperature range. (2) The isothermal melt crystallization rate increased and the crystallinity decreased withφsi,p increasing. The crystal structure transformed at Tc of 95-100℃. (3) The isothermal cold crystallization rate was significantly higher than the melt isothermal crystallization rate, and the Tc of the greatest cold crystallization rate (120℃) is higher than the Tc of the greatest melt crystallization rate (110℃). But the isothermal crystallization methods had no significant effect on the crystallinity. (4) The melting behavior after isothermal cold crystallization and isothermal melt crystallization was similar. The melting peak experienced "double melting peaks-a single melting peak". The low-melting peak area gradually increased and the peak temperature increased. The high-melting peak area decreased until disappear, and the peak temperature was basically unchanged.The PLLA/SiO2 prepolymers (φsi,p 5.5%, Mw-40,000 g/mol) prepared by melt polycondensation were isothermally cold-or melt-crystallized and then solid state polycondensed under same conditions (particle size 0.28-0.5 mm, nitrogen flow 0.04 L-min-1·g-1, SSP program 0-5 hr/150℃,5-10 hr/155℃,10-20 hr/160℃). The isothermal cold crystallization Tc had no significant effect on the SSP. SSP after isothermal cold crystallization at various Tc gave products with almost same Mw of about 100,000 g/mol. SSP after isothermal melt crystallization at lower Tc (80℃) gave product with higher Mw of 120,000. The crystallinity and melting temperature of PLLA inceased continuously during SSP. Their evolutions was independent of the pre-crystallization temperature and the initial crystallinity. And PLLA products after SSP for 20 hr had a white color, showing no observable discoloration, and the nanoparticles were uniformly dispersed in nano-scale.