204
A. Nogales et al.
there the polymer is allowed to crystallize for a controlled period of time. Afterwards,
the sample is rapidly cooled down back to the temperature where the β-relaxation
was first characterized and another measurement is performed. It is worth noticing
that at T = −85
° C, the crystallization process can be considered interrupted since
the temperature is well below the polymer T g (16
° C). The process can be repeated
several times until the crystallization is considered to be finished.
Figure 5b shows the dielectric loss spectra of the β relaxation of PPT, measured at T
= −85
° C, collected by the described procedure for some characteristic crystallization
times during the crystallization experiment at T c = 30
° C. Heating and cooling rates
were 5
° C/min.
For PPT the β-relaxation, like for other aromatic polyesters, appears as a broad and
multimodal relaxation composed of two processes designated as β 1 and β 2 in order
of decreasing frequency [41, 42]. The β-relaxation of aromatic polyesters is typically
multimodal and it is composed by contribution of the three conformationally flexible
bonds of the repeating unit, namely, the aromatic ring carbon to ester carbon bond
(CA-C), the ester ether oxygen to aliphatic carbon bond (O-C) and the aliphatic
carbon-carbon bond (C-C) [43]. However, due to the small dipole moment of the
C-C bond its relaxation is not likely to be detected by BDS. For PPT the faster mode
(β 1 ) has been assigned to the relaxation of the O-C bond of the diol subunit and
the slower one (β 2 ) to the relaxation of the aromatic ring carbon to ester carbon
bond (CA-C) [35]. For PPT the β-relaxation can be described by the HN equation
(Eq. 1) considering the contribution of two symmetric processes described in Fig. 5b
by the dashed lines. In this case Eq. 1 is referred to as Cole − Cole equation. As
far as crystallization is concerned at the early stages of the crystallization process
the overall β-relaxation remains almost unaffected. As crystallization proceeds a net
decrease of the dielectric strength of both components is observed. However while
the β of β 1 decreases by 18% with respect to its initial value, the β of β 2 decreases
by 12%. This effect can be explained considering the assignment of β 1 to the O-C
bond which is a more mobile bond than the CA-C one and therefore is more affected
by the crystalline phase.
Figure 7 represents, for the experiment presented in Fig. 4, the dependence of the
total dielectric strength of the α (ε α + ε α ) and of the β (ε β1 + ε β2 ) relaxations
normalized with respect to their initial values, as a function of crystallinity degree
as evaluated by WAXS.
The dielectric strength of both relaxations, α and β, decreases linearly with crystallinity although the slopes are different being higher for the α-relaxation. In a first
approximation one may consider the dielectric strength to be related to the amount of
mobile amorphous phase and the crystallinity with the amount of material included
in the crystals. The results for the β-relaxation show an inversely proportional dependence with the crystallinity degree (Fig. 7b). This effect, observed in several polymers
[9, 44], suggests that the local dynamics of the material incorporated into the crystalline phase is significantly restricted. For the α-relaxation the decrease of the dielectric strength with crystallinity deviates from the slope -1 to be expected for a simple
amorphous/crystal two phase model. This effect has been interpreted in the literature
considering that during isothermal crystallization the restrictions to the segmental
A. Nogales et al.
there the polymer is allowed to crystallize for a controlled period of time. Afterwards,
the sample is rapidly cooled down back to the temperature where the β-relaxation
was first characterized and another measurement is performed. It is worth noticing
that at T = −85
° C, the crystallization process can be considered interrupted since
the temperature is well below the polymer T g (16
° C). The process can be repeated
several times until the crystallization is considered to be finished.
Figure 5b shows the dielectric loss spectra of the β relaxation of PPT, measured at T
= −85
° C, collected by the described procedure for some characteristic crystallization
times during the crystallization experiment at T c = 30
° C. Heating and cooling rates
were 5
° C/min.
For PPT the β-relaxation, like for other aromatic polyesters, appears as a broad and
multimodal relaxation composed of two processes designated as β 1 and β 2 in order
of decreasing frequency [41, 42]. The β-relaxation of aromatic polyesters is typically
multimodal and it is composed by contribution of the three conformationally flexible
bonds of the repeating unit, namely, the aromatic ring carbon to ester carbon bond
(CA-C), the ester ether oxygen to aliphatic carbon bond (O-C) and the aliphatic
carbon-carbon bond (C-C) [43]. However, due to the small dipole moment of the
C-C bond its relaxation is not likely to be detected by BDS. For PPT the faster mode
(β 1 ) has been assigned to the relaxation of the O-C bond of the diol subunit and
the slower one (β 2 ) to the relaxation of the aromatic ring carbon to ester carbon
bond (CA-C) [35]. For PPT the β-relaxation can be described by the HN equation
(Eq. 1) considering the contribution of two symmetric processes described in Fig. 5b
by the dashed lines. In this case Eq. 1 is referred to as Cole − Cole equation. As
far as crystallization is concerned at the early stages of the crystallization process
the overall β-relaxation remains almost unaffected. As crystallization proceeds a net
decrease of the dielectric strength of both components is observed. However while
the β of β 1 decreases by 18% with respect to its initial value, the β of β 2 decreases
by 12%. This effect can be explained considering the assignment of β 1 to the O-C
bond which is a more mobile bond than the CA-C one and therefore is more affected
by the crystalline phase.
Figure 7 represents, for the experiment presented in Fig. 4, the dependence of the
total dielectric strength of the α (ε α + ε α ) and of the β (ε β1 + ε β2 ) relaxations
normalized with respect to their initial values, as a function of crystallinity degree
as evaluated by WAXS.
The dielectric strength of both relaxations, α and β, decreases linearly with crystallinity although the slopes are different being higher for the α-relaxation. In a first
approximation one may consider the dielectric strength to be related to the amount of
mobile amorphous phase and the crystallinity with the amount of material included
in the crystals. The results for the β-relaxation show an inversely proportional dependence with the crystallinity degree (Fig. 7b). This effect, observed in several polymers
[9, 44], suggests that the local dynamics of the material incorporated into the crystalline phase is significantly restricted. For the α-relaxation the decrease of the dielectric strength with crystallinity deviates from the slope -1 to be expected for a simple
amorphous/crystal two phase model. This effect has been interpreted in the literature
considering that during isothermal crystallization the restrictions to the segmental