The dynamic mechanical properties of both the neat UPR and
the nanocomposites are shown in Fig. 6. Fig. 6(a) shows that
the neat UPR has a storage modulus of ∼4560 MPa at −100 ◦C.
At this temperature, the polymer is in the glassy state and the
modulus remained approximately constant with temperature. At
around 50 ◦C, the modulus began dropping sharply, corresponding
to the relaxation of the polyester. In contrast, both the LENR–UPR
blends and the nanocomposites displayed a two-step drop in the
modulus (Fig. 6(b)). The low temperature modulus drop, which
increased with increasing of rubber content, is associated with
the glass transition temperature (Tg) of the LENR (Kargarzadeh
et al., 2015). The results revealed that the UPR’s storage modulus
increased to 4819 MPa and 4619 MPa after modification with 1.5
and 4.5 wt% LENR, due to the high density of crosslinking within the
LENR–UPR blend. However, the blend with the 4.5 wt% LENR displayed
a lower value than that with the 1.5 wt%. A similar trend was
reported for a blend of epoxy and carboxyl-terminated butadieneco-acrylonitrile
copolymer (CTBN) (Thomas et al., 2004).