96
A. Sanz
during crystallization above T g , with particular interest on the role played by the HB
network on the crystallization process [27].
It is worthy to remind that neutron diffraction is a type of coherent scattering and,
consequently, the sample to be measured must have a high coherent cross-section. For
this reason, deuterated isopropanol (2-propanol d8, 99% of deuterium) was used. The
sample was quenched in the glassy state at 75 K from room temperature and different
crystallization temperatures above T g were reached on heating. The evolution of the
ND-DS data during the crystallization of supercooled isopropanol at 134 K is shown
in Fig. 5. The formation of a crystalline lattice and the concomitant destruction of
the liquid mobile phase have a strong impact on both neutron diffraction and dielectric spectroscopy data. Regarding diffraction, as crystallization proceeds, several
Bragg peaks stem at the expense of the typical amorphous halo observed in disordered matter that decreases progressively [27]. In parallel, the dielectric dispersion
decreases in intensity as the fraction of mobile dipoles in the system becomes smaller
in accordance to the theory of dielectric relaxation that relates directly the dielectric
strength to the density of relaxing entities. As expected for a true crystalline phase
with rotational and translational order, the area of the dielectric dispersion totally
vanishes when the crystallization kinetics is over.
Dielectric loss data were described by two Cole–Davidson functions: (i) a primary
relaxation at low frequencies (Debye peak) attributed to the hydrogen-bond network
dynamics, and (ii) a secondary relaxation which is assigned to the alpha process.
The dielectric dispersion curves are then fitted to the following expression, ε
=
Im[ε∗] = Im
ε ∞ +
x=I, II ε x (1 + (iωτ x ))
−c x
, with ε the dielectric strength, c
the shape parameter which describes the asymmetric broadening of the relaxation
time distribution function, and τ the central relaxation time [32].
A major advantage of performing simultaneous DS and ND measurements is the
possibility of correlating dynamic properties with the fraction of crystalline phase in
an unambiguous way, that is, both quantities being generated under exactly the same
conditions, precluding unwanted external effects associated with different sample
environments. It was shown that the dependence of the dielectric strength with the
degree of crystallinity for the primary and alpha relaxations was quite different as
indicated in Fig. 6. From the very early stages of crystallization, a dramatic decrease
of ε I is observed which is interpreted as a depletion of the HB network.
Provided that the relaxing species being lost from the network are directly transferred to the crystalline phase in a linear fashion, a two-phase scenario for the dielectric strength denoted in Fig. 6 by a continuous line, should be expected. The strong
deviation from the two-phase model for the Debye peak is interpreted as due to the
existence of an intermediate step between the situation in which the molecule is in
the network and that in which it occupies a position in the crystal. This indicates that
the breakage of the HB network is a necessary but not sufficient step for nucleation
[27]. On the other hand, the alpha relaxation shows an almost linear dependence of
ε with crystallinity, much closer to the two-phase behaviour indicating that the
relaxing species contributing to the alpha process which are lost in the amorphous
phase are almost completely transferred to the crystalline phase. It is important to
remark that the fraction of crystallinity is calculated by describing the diffraction
A. Sanz
during crystallization above T g , with particular interest on the role played by the HB
network on the crystallization process [27].
It is worthy to remind that neutron diffraction is a type of coherent scattering and,
consequently, the sample to be measured must have a high coherent cross-section. For
this reason, deuterated isopropanol (2-propanol d8, 99% of deuterium) was used. The
sample was quenched in the glassy state at 75 K from room temperature and different
crystallization temperatures above T g were reached on heating. The evolution of the
ND-DS data during the crystallization of supercooled isopropanol at 134 K is shown
in Fig. 5. The formation of a crystalline lattice and the concomitant destruction of
the liquid mobile phase have a strong impact on both neutron diffraction and dielectric spectroscopy data. Regarding diffraction, as crystallization proceeds, several
Bragg peaks stem at the expense of the typical amorphous halo observed in disordered matter that decreases progressively [27]. In parallel, the dielectric dispersion
decreases in intensity as the fraction of mobile dipoles in the system becomes smaller
in accordance to the theory of dielectric relaxation that relates directly the dielectric
strength to the density of relaxing entities. As expected for a true crystalline phase
with rotational and translational order, the area of the dielectric dispersion totally
vanishes when the crystallization kinetics is over.
Dielectric loss data were described by two Cole–Davidson functions: (i) a primary
relaxation at low frequencies (Debye peak) attributed to the hydrogen-bond network
dynamics, and (ii) a secondary relaxation which is assigned to the alpha process.
The dielectric dispersion curves are then fitted to the following expression, ε
=
Im[ε∗] = Im
ε ∞ +
x=I, II ε x (1 + (iωτ x ))
−c x
, with ε the dielectric strength, c
the shape parameter which describes the asymmetric broadening of the relaxation
time distribution function, and τ the central relaxation time [32].
A major advantage of performing simultaneous DS and ND measurements is the
possibility of correlating dynamic properties with the fraction of crystalline phase in
an unambiguous way, that is, both quantities being generated under exactly the same
conditions, precluding unwanted external effects associated with different sample
environments. It was shown that the dependence of the dielectric strength with the
degree of crystallinity for the primary and alpha relaxations was quite different as
indicated in Fig. 6. From the very early stages of crystallization, a dramatic decrease
of ε I is observed which is interpreted as a depletion of the HB network.
Provided that the relaxing species being lost from the network are directly transferred to the crystalline phase in a linear fashion, a two-phase scenario for the dielectric strength denoted in Fig. 6 by a continuous line, should be expected. The strong
deviation from the two-phase model for the Debye peak is interpreted as due to the
existence of an intermediate step between the situation in which the molecule is in
the network and that in which it occupies a position in the crystal. This indicates that
the breakage of the HB network is a necessary but not sufficient step for nucleation
[27]. On the other hand, the alpha relaxation shows an almost linear dependence of
ε with crystallinity, much closer to the two-phase behaviour indicating that the
relaxing species contributing to the alpha process which are lost in the amorphous
phase are almost completely transferred to the crystalline phase. It is important to
remark that the fraction of crystallinity is calculated by describing the diffraction