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A. Nogales et al.
the sample can be obtained. During cold crystallization not only the α-relaxation but
also the β-relaxation can be significantly affected by crystallization. The dielectric
strength of β-relaxation linearly decreases with crystallinity suggesting that local
dynamics is essentially arrested in the crystalline phase.
BDS can be used to follow in a straightforward manner the isothermal polymer
crystallization of some low T g polymers exhibiting at the same temperature both
the α and the β relaxations in the measured frequency window. In these cases, by
simultaneous measurement of the α and the β relaxations a characterization in a single
experiment of the structure development, by the magnitude X
diel
c
= 1 − ε β //ε
0
β
where ε
0
β is the initial dielectric strength of the β-relaxation, and of the dynamics
evolution, by ε α , can be accomplished during the crystallization process. The results
obtained suggest the existence of three differentiated regimes for crystallization as
revealed by BDS. Initially there is an induction period where ε α decreases without
significant increase of the crystallinity. These results point towards the existence of
precursors of crystals in the induction period of crystallization. After the induction
time a primary crystallization period follows, in which ε α decreases strongly with
crystallinity. This effect supports the existence of a Rigid Amorphous Phase (RAP)
formed by amorphous but immobile material in addition to the crystalline phase
immobilized within the crystals and to the ordinary mobile amorphous phase. Finally
there is secondary crystallization regime where ε α decreases inversely proportional
to crystallinity. Here, the segmental immobilization induced by the crystalline phase
is not as effective as in the previous period and the amount of immobilized material
runs in parallel to the amount of material incorporated to the crystals.
BDS can by also used to investigate strain induced polymer crystallization as
occurring in vulcanized natural rubber whose segmental dynamics is significantly
affected by uniaxial stretching. By combining BDS and X-ray scattering techniques
three regimes have been characterized upon stretching. For low strains, stretching
takes place without crystallization but with a dramatic increase of the dielectric
strength which cannot be accounted for by considering an increase in density. The
dielectric experiments suggest an increase of the effective dipole moment induced by
upon stretching as the responsible for the increment in ε α . For intermediate strain
ratios, crystallization takes place and the inclusion of segments into the crystalline
phase counteracts the increment in ε α provoked by stretching. The experiments
support a model in which by stretching, only the molecules of small chain length
between the densely packed network regions can be oriented and form crystallites,
whereas longer molecules remain in the random coil state. In summary, BDS has
contributed significantly to the understanding of crystallization. However, the continuous engagement of scattering techniques to investigate novel in situ and in-operando
processes [79, 80] will require of complementary techniques. For sure BDS can be
one of them.
Acknowledgements Part of this work has been supported by Spanish Ministry of Science, Innovation and Universities (MSIU) by the projects MAT2014-59187-R, MAT2015-66443-C02-1-R.
A. Nogales et al.
the sample can be obtained. During cold crystallization not only the α-relaxation but
also the β-relaxation can be significantly affected by crystallization. The dielectric
strength of β-relaxation linearly decreases with crystallinity suggesting that local
dynamics is essentially arrested in the crystalline phase.
BDS can be used to follow in a straightforward manner the isothermal polymer
crystallization of some low T g polymers exhibiting at the same temperature both
the α and the β relaxations in the measured frequency window. In these cases, by
simultaneous measurement of the α and the β relaxations a characterization in a single
experiment of the structure development, by the magnitude X
diel
c
= 1 − ε β //ε
0
β
where ε
0
β is the initial dielectric strength of the β-relaxation, and of the dynamics
evolution, by ε α , can be accomplished during the crystallization process. The results
obtained suggest the existence of three differentiated regimes for crystallization as
revealed by BDS. Initially there is an induction period where ε α decreases without
significant increase of the crystallinity. These results point towards the existence of
precursors of crystals in the induction period of crystallization. After the induction
time a primary crystallization period follows, in which ε α decreases strongly with
crystallinity. This effect supports the existence of a Rigid Amorphous Phase (RAP)
formed by amorphous but immobile material in addition to the crystalline phase
immobilized within the crystals and to the ordinary mobile amorphous phase. Finally
there is secondary crystallization regime where ε α decreases inversely proportional
to crystallinity. Here, the segmental immobilization induced by the crystalline phase
is not as effective as in the previous period and the amount of immobilized material
runs in parallel to the amount of material incorporated to the crystals.
BDS can by also used to investigate strain induced polymer crystallization as
occurring in vulcanized natural rubber whose segmental dynamics is significantly
affected by uniaxial stretching. By combining BDS and X-ray scattering techniques
three regimes have been characterized upon stretching. For low strains, stretching
takes place without crystallization but with a dramatic increase of the dielectric
strength which cannot be accounted for by considering an increase in density. The
dielectric experiments suggest an increase of the effective dipole moment induced by
upon stretching as the responsible for the increment in ε α . For intermediate strain
ratios, crystallization takes place and the inclusion of segments into the crystalline
phase counteracts the increment in ε α provoked by stretching. The experiments
support a model in which by stretching, only the molecules of small chain length
between the densely packed network regions can be oriented and form crystallites,
whereas longer molecules remain in the random coil state. In summary, BDS has
contributed significantly to the understanding of crystallization. However, the continuous engagement of scattering techniques to investigate novel in situ and in-operando
processes [79, 80] will require of complementary techniques. For sure BDS can be
one of them.
Acknowledgements Part of this work has been supported by Spanish Ministry of Science, Innovation and Universities (MSIU) by the projects MAT2014-59187-R, MAT2015-66443-C02-1-R.