neat PLA since the polymer structure spontaneously progressed to
a lower free-energy state. Due to the smaller difference between Tg
and the storage temperature (driving force), this increase in
relaxation enthalpy was lower for 15 wt% plasticized films and it
mainly occurred in the early times of storage, remaining practically
constant from 3 to 90 days (Table 3). In both cases an increase in Tg
values with time was observed due to the reduction in free volume
and consequently in the mobility of macromolecular segments. The
presence of the relaxation endotherms makes an accurate determination
of Tg difficult when using conventional DSC. However, in
the case of PLAe15 wt% OLA an increase of around 3 C could be
clearly observed after 3 days of storage. On the other hand, films
with 20 and 25 wt% of OLA did not show relaxation enthalpy during
the ageing study because their Tg values were below or near the
storage temperature, in agreement with the results reported by
other authors for plasticized PLA [18e20].