208
A. Nogales et al.
(Fig. 6) can be implemented. In this case, the polymer was held at the selected crystallization temperature, T c = 25
° C, for a controlled period of time and then rapidly
cooled down to the measuring temperature (T meas = −25
° C). After performing a
frequency sweep at the measuring temperature, lasting typically about 2 min, the
sample is immediately heated up back to T c to wait for another time period. This
process is repeated several times until the crystallization is considered terminated.
Heating and cooling rates were 5
° C/min. At the measuring temperature both relaxation processes, α and β, are well resolved. The initial amorphous state is characterized by an intense α-relaxation and a less intense β-relaxation. In the early stage
of crystallization, Fig. 9a, the α-relaxation clearly decreases in intensity with crystallization time while the β-relaxation remains nearly unaffected. Figure 9b, c show
the evolution of the dielectric relaxations with crystallization time after the initial
period until the completion of the crystallization process. Here, both α and β relaxations decrease with time. In addition, the F max of the α relaxation exhibits a shift
towards lower frequencies. The continuous lines in Fig. 9a–c correspond to the fits
of the experimental data to Eq. 1. The β-relaxation can be described as being a
symmetric process, c = 1, during the whole crystallization process. Figure 9 (right
panel) shows the changes of the characteristic parameters as a function of crystallization time. It is worth mentioning the absence of a secondary α
-relaxation during
crystallization as observed for the previously discussed PPT case (Fig. 4). This can
be explained considering that, after completion of the crystallization process, the αrelaxation of the semicrystalline PPS sample is only about half an order of magnitude
slower than the original one (Fig. 9), while for PPT the final segmental relaxation,
α
, is almost four orders of magnitude slower than the initial one (Fig. 4). This fact
provokes the α
relaxation for PPS to merged with the α one. The general trend
followed by the shape parameters of the α-relaxation are similar to that discussed in
the previous paragraph: (i) a decrease of ε α values associated to a reduction of the
amorphous mobile phase due to a transfer to the crystalline phase, (ii) a broadening
and concurrent symmetrization as reveled by evolution of the b α and c α parameters
and (iii) a shift to lower values of the frequency of maximum loss, F max , suggesting
a slowing down of the amorphous phase dynamics due to the confinement exerted by
the crystalline phase. The β-relaxation follows a similar trend although in contrast its
maximum, F max , remains essentially unaltered during crystallization. This last effect
can be well understood considering the local character of the dynamics associated
to the β-relaxation being therefore less affected by the crystalline phase [9, 40].
Considering that the dielectric strength of the β-relaxation is inversely proportional
to the amount of the crystalline phase [9, 28, 44] then a characterization of structure
development and dynamics could be possible by a single experiment. In this case
the magnitude defined as X
diel
c
= 1 − ε β //ε
0
β where ε
0
β is the initial dielectric
strength of the β-relaxation, can be used to estimate the crystallinity [28]. Figure 10
represents, for the data of PPS shown in Fig. 9, the dependence of ε α as a function
of X
diel
c .
The data reported in Fig. 10 clearly suggests the existence of three different
regimes. First, an initial period where a significant reduction of ε α takes place
without a concurrent change in X
diel
c . The existence of this first regime where the ε β
A. Nogales et al.
(Fig. 6) can be implemented. In this case, the polymer was held at the selected crystallization temperature, T c = 25
° C, for a controlled period of time and then rapidly
cooled down to the measuring temperature (T meas = −25
° C). After performing a
frequency sweep at the measuring temperature, lasting typically about 2 min, the
sample is immediately heated up back to T c to wait for another time period. This
process is repeated several times until the crystallization is considered terminated.
Heating and cooling rates were 5
° C/min. At the measuring temperature both relaxation processes, α and β, are well resolved. The initial amorphous state is characterized by an intense α-relaxation and a less intense β-relaxation. In the early stage
of crystallization, Fig. 9a, the α-relaxation clearly decreases in intensity with crystallization time while the β-relaxation remains nearly unaffected. Figure 9b, c show
the evolution of the dielectric relaxations with crystallization time after the initial
period until the completion of the crystallization process. Here, both α and β relaxations decrease with time. In addition, the F max of the α relaxation exhibits a shift
towards lower frequencies. The continuous lines in Fig. 9a–c correspond to the fits
of the experimental data to Eq. 1. The β-relaxation can be described as being a
symmetric process, c = 1, during the whole crystallization process. Figure 9 (right
panel) shows the changes of the characteristic parameters as a function of crystallization time. It is worth mentioning the absence of a secondary α
-relaxation during
crystallization as observed for the previously discussed PPT case (Fig. 4). This can
be explained considering that, after completion of the crystallization process, the αrelaxation of the semicrystalline PPS sample is only about half an order of magnitude
slower than the original one (Fig. 9), while for PPT the final segmental relaxation,
α
, is almost four orders of magnitude slower than the initial one (Fig. 4). This fact
provokes the α
relaxation for PPS to merged with the α one. The general trend
followed by the shape parameters of the α-relaxation are similar to that discussed in
the previous paragraph: (i) a decrease of ε α values associated to a reduction of the
amorphous mobile phase due to a transfer to the crystalline phase, (ii) a broadening
and concurrent symmetrization as reveled by evolution of the b α and c α parameters
and (iii) a shift to lower values of the frequency of maximum loss, F max , suggesting
a slowing down of the amorphous phase dynamics due to the confinement exerted by
the crystalline phase. The β-relaxation follows a similar trend although in contrast its
maximum, F max , remains essentially unaltered during crystallization. This last effect
can be well understood considering the local character of the dynamics associated
to the β-relaxation being therefore less affected by the crystalline phase [9, 40].
Considering that the dielectric strength of the β-relaxation is inversely proportional
to the amount of the crystalline phase [9, 28, 44] then a characterization of structure
development and dynamics could be possible by a single experiment. In this case
the magnitude defined as X
diel
c
= 1 − ε β //ε
0
β where ε
0
β is the initial dielectric
strength of the β-relaxation, can be used to estimate the crystallinity [28]. Figure 10
represents, for the data of PPS shown in Fig. 9, the dependence of ε α as a function
of X
diel
c .
The data reported in Fig. 10 clearly suggests the existence of three different
regimes. First, an initial period where a significant reduction of ε α takes place
without a concurrent change in X
diel
c . The existence of this first regime where the ε β