Dynamic mechanical properties of
PLLA and PLLA/PET-co-PLA blends are
examined. Figure 6(a) shows that commercial
PLLA exhibits the highest storage modulus
value. Upon blending with the copolymer,
the modulus of the blends (in glassy state)
decreases with the copolymer content.
This is likely because the medium-sized
copolymer acts as a plasticizer in PLLA
matrix, leading to a softening of the material.
This phenomenon is observed even though
the copolymer contains stiff aromatic
sequences, indicating that an effect of MW
is more significant. A large decrease in the
storage modulus is observed at 30-80°C,
reflecting the glass transition process.
In a rubbery state, however, the blends
with higher PET-co-PLA contents show
higher storage modulus, reflecting that role of
stiff aromatic sequences. Loss tangent (tan δ)
value, a ratio of loss and storage moduli, as
a function of temperature, is shown in
Figure 6(b). The placement of its maximum
value indicates a temperature where
relaxation process takes place, especially
the glass transition process. All samples
exhibit a single Tg, where commercial
PLLA has the highest value at around
65°C. Tg of the blends decreases as a
function of the copolymer composition
due to the plasticizer effect. This is in
good agreement with those observed from
DSC experiments.