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 (<5–10%), 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.