210
A. Nogales et al.
during secondary crystallization the segmental immobilization induced by the crystalline phase is not as effective as in the previous period. As a matter of fact, in this
period the amount of immobilized material runs in parallel to the amount of material
incorporated to the crystals.
5 Polymer Crystallization Induced by Strain: The Case
of Vulcanized Natural Rubber
Although temperature is the main variable involved in most of the studies in polymer
crystallization also other magnitudes like pressure [58], shear [59] and strain [60]
can play an important role in the crystallization process. A paradigmatic case for
strain induced polymer crystallization is the one occurring in rubbers [61]. Natural
rubber (NR) is a natural amorphous polymer (T g ≈ −64
° C) basically consisting of
cis-1,4-isoprene units. NR is one of the most important natural materials present in
many products of our everyday life. It possesses a unique microstructure consisting
of different type of chain connections leading to the formations of a naturally occurring network formed by branch, star and network structures [62]. For most of the
NR industrial applications it is compulsory to enhance both elasticity and tensile
strength by converting the weak naturally occurring network into a more robust threedimensional network [63]. This can be achieved by a process known as vulcanization
by which cross-linking of the polymer chains is produced by covalent bonding formation typically by chemical reaction with sulfur [64]. As far as BDS is concern NR
exhibits above its glass transition temperature two dielectric relaxation processes
which are illustrated in Fig. 11a for a NR sample (Malaysian Rubber: Berhad,
Malaysia, SMR CV60). Similarly to synthetic 1,4-cis-poly(isoprene) (PI) [65, 66],
NR presents an asymmetry in its chemical structure inducing components of the
dipolar moment parallel and perpendicular to the polymer chain. For this reason
NR exhibits a standard segmental relaxation process, associated to the perpendicular
dipole moment, and an additional slower one, referred to as normal mode, associated
to the parallel dipole moment [65].
For vulcanized NR the dielectric relaxation process associated to the normal mode
is suppressed as a consequence of the crosslinking (Fig. 11b). Therefore vulcanized
NR exhibits above T g only the α-relaxation associated to the segmental motions of
the polymer chains [64, 67].
Natural rubber, similarly to other polymers, can crystallize by thermal treatment
within the temperature window defined by its T g and its melting point [68]. However
the crystallization rate is rather slow reaching a maximum at T c ≈ −20
° C [69]. It
has been proposed that the mechanical performance of vulcanized NR originates
from partial crystallization under the action of an extensional field. The formation
of strain-induced crystals can be responsible of the tensile modulus enhancement
observed when NR is subjected to fast deformations. The crystals are embedded into
an amorphous phase forming a semicrystalline structure and orient themselves in
A. Nogales et al.
during secondary crystallization the segmental immobilization induced by the crystalline phase is not as effective as in the previous period. As a matter of fact, in this
period the amount of immobilized material runs in parallel to the amount of material
incorporated to the crystals.
5 Polymer Crystallization Induced by Strain: The Case
of Vulcanized Natural Rubber
Although temperature is the main variable involved in most of the studies in polymer
crystallization also other magnitudes like pressure [58], shear [59] and strain [60]
can play an important role in the crystallization process. A paradigmatic case for
strain induced polymer crystallization is the one occurring in rubbers [61]. Natural
rubber (NR) is a natural amorphous polymer (T g ≈ −64
° C) basically consisting of
cis-1,4-isoprene units. NR is one of the most important natural materials present in
many products of our everyday life. It possesses a unique microstructure consisting
of different type of chain connections leading to the formations of a naturally occurring network formed by branch, star and network structures [62]. For most of the
NR industrial applications it is compulsory to enhance both elasticity and tensile
strength by converting the weak naturally occurring network into a more robust threedimensional network [63]. This can be achieved by a process known as vulcanization
by which cross-linking of the polymer chains is produced by covalent bonding formation typically by chemical reaction with sulfur [64]. As far as BDS is concern NR
exhibits above its glass transition temperature two dielectric relaxation processes
which are illustrated in Fig. 11a for a NR sample (Malaysian Rubber: Berhad,
Malaysia, SMR CV60). Similarly to synthetic 1,4-cis-poly(isoprene) (PI) [65, 66],
NR presents an asymmetry in its chemical structure inducing components of the
dipolar moment parallel and perpendicular to the polymer chain. For this reason
NR exhibits a standard segmental relaxation process, associated to the perpendicular
dipole moment, and an additional slower one, referred to as normal mode, associated
to the parallel dipole moment [65].
For vulcanized NR the dielectric relaxation process associated to the normal mode
is suppressed as a consequence of the crosslinking (Fig. 11b). Therefore vulcanized
NR exhibits above T g only the α-relaxation associated to the segmental motions of
the polymer chains [64, 67].
Natural rubber, similarly to other polymers, can crystallize by thermal treatment
within the temperature window defined by its T g and its melting point [68]. However
the crystallization rate is rather slow reaching a maximum at T c ≈ −20
° C [69]. It
has been proposed that the mechanical performance of vulcanized NR originates
from partial crystallization under the action of an extensional field. The formation
of strain-induced crystals can be responsible of the tensile modulus enhancement
observed when NR is subjected to fast deformations. The crystals are embedded into
an amorphous phase forming a semicrystalline structure and orient themselves in