Poly(L-lactic acid)/silicone dioxide nanocomposites prepared via in situ melt polycondensation of L-lactic acid in the presence of acidic silica sol: Dispersion stability of nanoparticles during dehydration/oligomerization

In this work, the stability of nanoparticles during the dehydration/oligomerization (D/O) stage in situ melt polycondensation of L-lactic acid to prepare poly(L-lactic acid) (PLLA)/SiO2 nanocomposite was studied. The change in the appearance of the reaction mixture was tracked, and the resultant oligo(L-lactic acid) (OLLA)/SiO2 and PLLA/SiO2 nanocomposites were characterized by transmission electron microscopy, 1H-NMR, and light transmittance. The electric double layer and the grafted OLLA chains provided static and steric stabilities during the early and late phases, respectively. However, there existed an intermediate transitional phase with weak stability when the static stability was weakened, but sufficient steric stability had not yet been established; this led to soft or hard aggregation, depending on the SiO2 loading and agitation conditions. At low or moderate SiO2 loading (<510%), the soft aggregation could be depressed with appropriate agitation conditions and redispersed with the aid of gradually established steric interaction energy. Consequently, well-dispersed PLLA/SiO2 nanocomposites with SiO2 loadings of 5 and 10% were successfully prepared by in situ melt polycondensation with an arc stirrer at 400 and 600 rpm, respectively, during the D/O stage. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012