Order and Dielectric Relaxation During Polymer Crystallization
205
Fig. 7 Evolution of the
dielectric strength of the a α
and b β relaxations with
crystallinity, X c for the cold
crystallization of PPT(•) at
T c = 30°C. The data have
been normalized to the initial
value and correspond to
ε norm
α
= ε a + ε a (top
panel) and
ε norm
β
= ε β1 + ε β2
(bottom panel). Additional
data for PET [15] () and
PTT [16] (◯) have been
included. The dashed and
doted lines are a guide for
the eye. “Adapted with
permission from ref. [35].
Copyright (2020) Elsevier.”
mobility not only affect to polymer segments included in the crystalline phase but
also to other non-relaxing and non-crystallized segments which form a rigid amorphous phase (RAP) [45]. The RAP is a common structural feature of polymers with
a medium degree of crystallinity and it is formed by polymer segments which relax
at a significant slower rate than those of the amorphous phase. BDS measurements
have evidenced the existence of a RAP in different aromatic polyesters [15, 16, 37,
45–49]. For the sake of comparison similar results for two aromatic polyesters PET
[15] and PTT [16] have been included in Fig. 7a. As shown, the dielectric strength for
PPT decreases lineally with X c with the same slope for the whole investigate range
suggesting that the rate of reduction of relaxing species is almost constant during
crystallization. A qualitatively similar behavior is followed by PTT. However, for
PET clearly two regimes with different slopes are observed in Fig. 7a. A similar
behavior has been found for poly(butylene isophthalate) [37] and for poly(lactide)
[49]. It has been proposed in the literature that for some polymers, like PET, cold
crystallization proceeds by filling the space with a heterogeneous distribution of
lamellar stacks separated by larger amorphous regions referred to as liquid pockets
[15]. A schematic ideal picture of such a model is described in Fig. 8. In this case,
it was proposed the RAP to be associated to the intra-lamellar amorphous regions,
being the material in the inter-lamellar stacks amorphous regions (liquid pockets)
the phase giving rise to the dielectric segmental relaxation. Accordingly, the initial
strong decrease of the dielectric strength with crystallinity observed in PET (See
Fig. 7a) can be associated to the immobilization of segments in the intra-lamellar
stacks amorphous regions while the second weaker dependence can be associated
to the formation of secondary lamellae in the liquid pockets. For PTT, and PPT the
205
Fig. 7 Evolution of the
dielectric strength of the a α
and b β relaxations with
crystallinity, X c for the cold
crystallization of PPT(•) at
T c = 30°C. The data have
been normalized to the initial
value and correspond to
ε norm
α
= ε a + ε a (top
panel) and
ε norm
β
= ε β1 + ε β2
(bottom panel). Additional
data for PET [15] () and
PTT [16] (◯) have been
included. The dashed and
doted lines are a guide for
the eye. “Adapted with
permission from ref. [35].
Copyright (2020) Elsevier.”
mobility not only affect to polymer segments included in the crystalline phase but
also to other non-relaxing and non-crystallized segments which form a rigid amorphous phase (RAP) [45]. The RAP is a common structural feature of polymers with
a medium degree of crystallinity and it is formed by polymer segments which relax
at a significant slower rate than those of the amorphous phase. BDS measurements
have evidenced the existence of a RAP in different aromatic polyesters [15, 16, 37,
45–49]. For the sake of comparison similar results for two aromatic polyesters PET
[15] and PTT [16] have been included in Fig. 7a. As shown, the dielectric strength for
PPT decreases lineally with X c with the same slope for the whole investigate range
suggesting that the rate of reduction of relaxing species is almost constant during
crystallization. A qualitatively similar behavior is followed by PTT. However, for
PET clearly two regimes with different slopes are observed in Fig. 7a. A similar
behavior has been found for poly(butylene isophthalate) [37] and for poly(lactide)
[49]. It has been proposed in the literature that for some polymers, like PET, cold
crystallization proceeds by filling the space with a heterogeneous distribution of
lamellar stacks separated by larger amorphous regions referred to as liquid pockets
[15]. A schematic ideal picture of such a model is described in Fig. 8. In this case,
it was proposed the RAP to be associated to the intra-lamellar amorphous regions,
being the material in the inter-lamellar stacks amorphous regions (liquid pockets)
the phase giving rise to the dielectric segmental relaxation. Accordingly, the initial
strong decrease of the dielectric strength with crystallinity observed in PET (See
Fig. 7a) can be associated to the immobilization of segments in the intra-lamellar
stacks amorphous regions while the second weaker dependence can be associated
to the formation of secondary lamellae in the liquid pockets. For PTT, and PPT the