Finally, the bleaching could shift the glass transition temperature
to lower temperatures.
As is reported,53 PLLA has a Tg in the
range of 60–65C.
It is observed that starch lowers Tg as it facilitates
the mobility of chains.
According to the results from DSC,
the bleaching process could enhance the mobility of polymer
chains due to removal of lignin which had a steric hindrance
effect on the polymer chains leading to reduction of Tg in LASR6.
We expect the addition of RH into the PLLA/starch (LASR7)
would depress the mobility of polymer chains leading to a higher
Tg but it can be observed that Tg exhibited no significant change
for LASR3.
We postulate that there could be an opposing effect.
On one hand, as we expect, the presence of lignin could depress
the mobility of polymer chains which might lead to larger Tg.
Conversely, lignin can act as obstacle for mobility of the PLLA
chains to fold and join the crystallization growth front, finally
leading to a reduction in the extent of crystallinity.
This reduction
in degree of crystallization could results in lower amount of Tg
due to effect on amorphous phase.
In conclusion, these two
effects are the main dominating factors affecting the transition
process and as could be seen these two effects counteract each
other leading to no significant change in Tg.