Addition of 30 wt% PEG to PLA of low stereoregularity
decreased Tg from above ambient temperature to below
ambient temperature and thereby decreased the modulus
and increased the ductility of this relatively rigid,
brittle thermoplastic. Immediately after cooling from
the melt, blends with up to 30 wt% PEG were amorphous
and homogeneous with a single Tg that depended on
composition in accordance with the Fox relationship.
However, the blends with 20 wt% or more PEG were not
stable at ambient temperature. Over time the modulus
increased and the elongation at break decreased. The origin
of the aging phenomenon was crystallization of PEG.
Preferential crystallization of PEG gradually enriched the
amorphous phase in PLA and increased Tg of the amorphous
phase. When Tg reached ambient temperature aging
essentially ceased even though PEG was not completely
crystallized. Reduced molecular diffusivity slowed the
crystallization rate dramatically. Aging resumed if the
temperature was raised above Tg: As a generalization,
blends exhibited aging under ambient conditions if the PEG
content was high enough to put Tg below ambient
temperature. Conversely, the blend with 15 wt% PEG and
Tg slightly above ambient temperature appeared to be stable
over time under ambient conditions.