150
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
affords much higher green strength to NR than non-crystallizing synthetic rubbers
like SBR. Even crystallizable IR, the SIC tendency of which is less than NR, is much
inferior to NR in terms of green strength.
Regrettably, a full-scale kinetics on the basis of this model is still to be developed.
Crystallization kinetics using the Avrami equation is a standard [26, 35, 81], which
has enabled the discussions on crystallization mechanism from lamella to sphere
crystal by the Avrami index. SIC is a crystallization process from the random coil, and
the Avrami’s treatment is not applicable, which is somewhat related to the Markov
chain approach applied to physical adsorption of a coiled polymer onto a surface
[82, 83].
Among several kinetic studies on SIC conducted before our proposal of template
crystallization, those by Brüning et al. are notable [75, 76]. They reported some
semiquantitative discussions on the results of a few 10 ms time scale, while taking
SIC’s ultrahigh rate into account in planning the experiments. They also suggested
that P-jump method was worth to be tried for the more quantitative kinetic studies
on SIC of NR. Anyway, it is certain that the quantitative elucidation of template
crystallization mechanism is an urgent issue for understanding the self-reinforcement
of NR vulcanizates.
Here again, significance of vulcanization of rubber, i.e., Goodyear’s invention
in 1839, is reconfirmed: It not only arrested LTC of NR, which surely made NR a
material of use, but also afforded NR’s capability of SIC. Both crystallizabilities are
intrinsic to NR, a natural polymer, and the vulcanization dissolved a negative potential
of LTC by arresting it (see the last paragraph in Sect. 8.1.2). It also made potential of
SIC ability to come in sight, by providing the NR vulcanizates with polydispersity
in their network chain length distribution. Due to the presence of network chains
of various lengths, SIC occurs automatically upon stretching to result in excellent
performance of NR. Without this heterogeneity of NR vulcanizates, SIC would have
given highly crystalline materials upon stretching only once, remote from elastomeric
materials of utmost value (see Prefactory Remark). In vulcanization’s technological
significance, particularly in its contribution to the people and the modern societies
by means of rubber tires, few inventions have been comparable so far in terms of
social significance.
Here, additional comments should be given: The behavior of the NR vulcanizate
is really unique, but this unique performance may also be observed in synthetic NR
(IR), though much less or weaker in their performance. Uncross-linked NR (raw NR)
shows similar behaviors, but the details are more complex [53], because its performance is not due to the chemical cross-links, but due to the physical entanglement
(a movable pseudo cross-link).
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
affords much higher green strength to NR than non-crystallizing synthetic rubbers
like SBR. Even crystallizable IR, the SIC tendency of which is less than NR, is much
inferior to NR in terms of green strength.
Regrettably, a full-scale kinetics on the basis of this model is still to be developed.
Crystallization kinetics using the Avrami equation is a standard [26, 35, 81], which
has enabled the discussions on crystallization mechanism from lamella to sphere
crystal by the Avrami index. SIC is a crystallization process from the random coil, and
the Avrami’s treatment is not applicable, which is somewhat related to the Markov
chain approach applied to physical adsorption of a coiled polymer onto a surface
[82, 83].
Among several kinetic studies on SIC conducted before our proposal of template
crystallization, those by Brüning et al. are notable [75, 76]. They reported some
semiquantitative discussions on the results of a few 10 ms time scale, while taking
SIC’s ultrahigh rate into account in planning the experiments. They also suggested
that P-jump method was worth to be tried for the more quantitative kinetic studies
on SIC of NR. Anyway, it is certain that the quantitative elucidation of template
crystallization mechanism is an urgent issue for understanding the self-reinforcement
of NR vulcanizates.
Here again, significance of vulcanization of rubber, i.e., Goodyear’s invention
in 1839, is reconfirmed: It not only arrested LTC of NR, which surely made NR a
material of use, but also afforded NR’s capability of SIC. Both crystallizabilities are
intrinsic to NR, a natural polymer, and the vulcanization dissolved a negative potential
of LTC by arresting it (see the last paragraph in Sect. 8.1.2). It also made potential of
SIC ability to come in sight, by providing the NR vulcanizates with polydispersity
in their network chain length distribution. Due to the presence of network chains
of various lengths, SIC occurs automatically upon stretching to result in excellent
performance of NR. Without this heterogeneity of NR vulcanizates, SIC would have
given highly crystalline materials upon stretching only once, remote from elastomeric
materials of utmost value (see Prefactory Remark). In vulcanization’s technological
significance, particularly in its contribution to the people and the modern societies
by means of rubber tires, few inventions have been comparable so far in terms of
social significance.
Here, additional comments should be given: The behavior of the NR vulcanizate
is really unique, but this unique performance may also be observed in synthetic NR
(IR), though much less or weaker in their performance. Uncross-linked NR (raw NR)
shows similar behaviors, but the details are more complex [53], because its performance is not due to the chemical cross-links, but due to the physical entanglement
(a movable pseudo cross-link).