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A. Nogales et al.
dynamics of the α-relaxation mainly by three effects: i) a reduction of the dielectric
strength, ii) a slowing down of the dynamics and iii) a modification of the shape of
the relaxation [9, 14, 20]. Accordingly, the α-relaxation has been used extensively as
a probe to follow in real-time the crystallization of polymer materials [17, 21–23],
polymer thin films [24] and polymer liquid crystals [25, 26] among other polymer
systems. Moreover, different ordered stages previous to crystallization have been
discussed on the basis of dielectric experiments [27–30]. In the frequency domain,
the α-relaxation appears as a maximum in the dielectric loss, ε
, and as a step in
the dielectric constant, ε
[18]. As an example, Fig. 2a shows the variation with
temperature of the dielectric loss spectra for initially amorphous poly(trimethylene
terephthalate) [16]. The initial dielectric spectrum shows the α-relaxation associated with the segmental motions of chains in the amorphous phase above the T g .
As expected, the position in frequency of the relaxation maximum shifts to higher
frequencies with increasing temperature. As the temperature rises there is a dramatic
reduction of the intensity of the dielectric relaxation which is associated to the onset
of crystallization. This effect is accompanied by a decrease of the area underneath
the relaxation curve as a consequence of the reduction of the dielectric strength and
by a significant shift of the maximum loss towards lower frequencies. At higher
temperatures, the maximum further moves toward higher frequencies according to
the temperature activated character of the α-relaxation. At even higher temperatures
the contribution of the dc (direct current) electrical conductivity is observed by the
increase of ε
at low frequencies.
Scattering and diffraction techniques either with X-rays or with neutrons can
provide structural information about the ordered regions in semicrystalline polymers
at different length scales [4, 31]. Wide angle X-ray scattering (WAXS) allows the
possibility of obtaining information about molecular order in the range of tenths of
nanometers. Hence, WAXS can be useful to deal with crystalline structure determination and to estimate the crystallinity degree, X c , defined as the amount of crystalline
fraction in a semicrystalline polymer [3, 4, 7, 32]. Therefore, WAXS experiments
can be used to visualize a crystallization process by monitoring the evolution with
temperature of the WAXS patterns as illustrated in Fig. 2b. The two initial WAXS
patterns show the typical broad maximum characteristic of an amorphous material. As
temperature increases, crystallization is revealed by the appearance of Bragg peaks
concurrently with the dramatic reduction of the intensity of the dielectric relaxation
(Fig. 2a). In addition to WAXS, Small angle X-ray scattering (SAXS) enables to characterize structural features of semicrystalline polymers at length scales of the order of
nanometers [31]. Figure 2c shows that crystallization is revealed by an increase of the
scattered intensity at low angles in the SAXS patterns further developing into a welldefined maximum, commonly referred to as Long-spacing, revealing the presence of
a periodicity between crystalline lamella and disordered domains. The experiments
illustrated in Fig. 2 highlight that an improvement in the characterization of a crystallization process can be achieved by inter-correlating structural information, obtained
by scattering about the crystalline phase, and molecular dynamics results about the
amorphous phase obtained by BDS.
A. Nogales et al.
dynamics of the α-relaxation mainly by three effects: i) a reduction of the dielectric
strength, ii) a slowing down of the dynamics and iii) a modification of the shape of
the relaxation [9, 14, 20]. Accordingly, the α-relaxation has been used extensively as
a probe to follow in real-time the crystallization of polymer materials [17, 21–23],
polymer thin films [24] and polymer liquid crystals [25, 26] among other polymer
systems. Moreover, different ordered stages previous to crystallization have been
discussed on the basis of dielectric experiments [27–30]. In the frequency domain,
the α-relaxation appears as a maximum in the dielectric loss, ε
, and as a step in
the dielectric constant, ε
[18]. As an example, Fig. 2a shows the variation with
temperature of the dielectric loss spectra for initially amorphous poly(trimethylene
terephthalate) [16]. The initial dielectric spectrum shows the α-relaxation associated with the segmental motions of chains in the amorphous phase above the T g .
As expected, the position in frequency of the relaxation maximum shifts to higher
frequencies with increasing temperature. As the temperature rises there is a dramatic
reduction of the intensity of the dielectric relaxation which is associated to the onset
of crystallization. This effect is accompanied by a decrease of the area underneath
the relaxation curve as a consequence of the reduction of the dielectric strength and
by a significant shift of the maximum loss towards lower frequencies. At higher
temperatures, the maximum further moves toward higher frequencies according to
the temperature activated character of the α-relaxation. At even higher temperatures
the contribution of the dc (direct current) electrical conductivity is observed by the
increase of ε
at low frequencies.
Scattering and diffraction techniques either with X-rays or with neutrons can
provide structural information about the ordered regions in semicrystalline polymers
at different length scales [4, 31]. Wide angle X-ray scattering (WAXS) allows the
possibility of obtaining information about molecular order in the range of tenths of
nanometers. Hence, WAXS can be useful to deal with crystalline structure determination and to estimate the crystallinity degree, X c , defined as the amount of crystalline
fraction in a semicrystalline polymer [3, 4, 7, 32]. Therefore, WAXS experiments
can be used to visualize a crystallization process by monitoring the evolution with
temperature of the WAXS patterns as illustrated in Fig. 2b. The two initial WAXS
patterns show the typical broad maximum characteristic of an amorphous material. As
temperature increases, crystallization is revealed by the appearance of Bragg peaks
concurrently with the dramatic reduction of the intensity of the dielectric relaxation
(Fig. 2a). In addition to WAXS, Small angle X-ray scattering (SAXS) enables to characterize structural features of semicrystalline polymers at length scales of the order of
nanometers [31]. Figure 2c shows that crystallization is revealed by an increase of the
scattered intensity at low angles in the SAXS patterns further developing into a welldefined maximum, commonly referred to as Long-spacing, revealing the presence of
a periodicity between crystalline lamella and disordered domains. The experiments
illustrated in Fig. 2 highlight that an improvement in the characterization of a crystallization process can be achieved by inter-correlating structural information, obtained
by scattering about the crystalline phase, and molecular dynamics results about the
amorphous phase obtained by BDS.