Order and Dielectric Relaxation During Polymer Crystallization
197
or tacticity since its free energy can be higher than that of the crystalline state. If
this is the case, the supercooled liquid spontaneously suffers a first order liquid-tocrystal phase transition [3, 4]. Semicrystalline polymers usually adopt a characteristic folded chain crystalline lamellar morphology of nanometer dimensions when
located in the supercooling temperature range. The lamellar morphology typically
consists of stacks of laminar crystals with amorphous regions intercalated between
them. While the crystalline phase provides strength to the material, the amorphous
one is responsible of its toughness. Although extended chain crystals would be the
thermodynamically most stable state, kinetic factors favor that polymer chains may
fold several times forming crystalline lamellae [4–7]. From the point of view of
the potential energy landscape [8], Fig. 1b, when the temperature decreases the crystalline phase is the one offering a minimum in energy. However, depending on cooling
rate, the whole system cannot reach this state for kinetic reasons and fractions of the
material become eventually trapped in local minima as an amorphous phase. While
molecular dynamics is severely arrested in the crystalline phase, that of the fraction
in local minima can exhibit both segmental and local dynamics above T g [9].
If the polymer is quenched fast enough below T g a fully amorphous state can be
obtained where only local dynamics is allowed. In this case a semicrystalline state
can be achieved by a thermal treatment process referred to as “cold crystallization”
consisting in heating the glassy polymer above its T g .
Experimentally in the past years Broadband Dielectric Spectroscopy (BDS) has
been shown to be a powerful technique to investigate crystallization in complex
fluids including liquids and polymers. Particularly attractive is when scattering and
diffraction experiments are performed simultaneously with dielectric spectroscopy
during crystallization in real-time [10–13]. The study of the isothermal polymer
crystallization by dielectrics can be complemented with diffraction experiments using
either X-rays[14–16] or neutrons [17].
In this chapter we will review the application of BDS to the study of polymer
crystallization emphasizing those aspects related to the combination of X-ray scattering techniques, rendering information about the crystalline phase, with dielectrics
providing information about the amorphous phase.
2 Probing Polymer Crystallization in Real-Time
by Dielectric Spectroscopy
As far as polymers are concerned, BDS is very suitable to investigate molecular
dynamics below and above T g [18]. In particular, the segmental motions of the polymeric chains giving rise to the α-relaxation can be observed by BDS provided the
polymer possesses dipolar moieties in its chemical structure. Pioneering work by
Williams et al. showed that the dielectric α-relaxation can be used as a probe for
crystallization since it is strongly sensitive to development of the crystalline phase
[19]. The existence of a crystalline phase in a polymer material is reflected in the
197
or tacticity since its free energy can be higher than that of the crystalline state. If
this is the case, the supercooled liquid spontaneously suffers a first order liquid-tocrystal phase transition [3, 4]. Semicrystalline polymers usually adopt a characteristic folded chain crystalline lamellar morphology of nanometer dimensions when
located in the supercooling temperature range. The lamellar morphology typically
consists of stacks of laminar crystals with amorphous regions intercalated between
them. While the crystalline phase provides strength to the material, the amorphous
one is responsible of its toughness. Although extended chain crystals would be the
thermodynamically most stable state, kinetic factors favor that polymer chains may
fold several times forming crystalline lamellae [4–7]. From the point of view of
the potential energy landscape [8], Fig. 1b, when the temperature decreases the crystalline phase is the one offering a minimum in energy. However, depending on cooling
rate, the whole system cannot reach this state for kinetic reasons and fractions of the
material become eventually trapped in local minima as an amorphous phase. While
molecular dynamics is severely arrested in the crystalline phase, that of the fraction
in local minima can exhibit both segmental and local dynamics above T g [9].
If the polymer is quenched fast enough below T g a fully amorphous state can be
obtained where only local dynamics is allowed. In this case a semicrystalline state
can be achieved by a thermal treatment process referred to as “cold crystallization”
consisting in heating the glassy polymer above its T g .
Experimentally in the past years Broadband Dielectric Spectroscopy (BDS) has
been shown to be a powerful technique to investigate crystallization in complex
fluids including liquids and polymers. Particularly attractive is when scattering and
diffraction experiments are performed simultaneously with dielectric spectroscopy
during crystallization in real-time [10–13]. The study of the isothermal polymer
crystallization by dielectrics can be complemented with diffraction experiments using
either X-rays[14–16] or neutrons [17].
In this chapter we will review the application of BDS to the study of polymer
crystallization emphasizing those aspects related to the combination of X-ray scattering techniques, rendering information about the crystalline phase, with dielectrics
providing information about the amorphous phase.
2 Probing Polymer Crystallization in Real-Time
by Dielectric Spectroscopy
As far as polymers are concerned, BDS is very suitable to investigate molecular
dynamics below and above T g [18]. In particular, the segmental motions of the polymeric chains giving rise to the α-relaxation can be observed by BDS provided the
polymer possesses dipolar moieties in its chemical structure. Pioneering work by
Williams et al. showed that the dielectric α-relaxation can be used as a probe for
crystallization since it is strongly sensitive to development of the crystalline phase
[19]. The existence of a crystalline phase in a polymer material is reflected in the