142
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
behavior has been assumed the basis of higher green strength of NR than that of
synthetic rubbers. Green strength is a tensile strength of raw rubber and is important in rubber processing, e.g., at shaping step (see Fig. 2.1 in Sect. 2.1). However,
entanglements are movable with the micro-Brownian motion of the rubber chains,
and hence, the SIC behaviors of raw NR may be much more complex than those of
NR vulcanizates. Namely, LTC behaviors of the unvulcanized NR is both theoretically and experimentally coherent, but SIC of it is much more complicated since its
network structure is not due to chemical bonding but due to physical entanglement.
For instance, the classic racking method on NR, a kind of application of SIC toward
higher modulus, failed to produce a high modulus rubber thread [53]. Ultrahigh molar
mass polyethylene (PE), on the other hand, has been successfully processed to a high
modulus and high strength PE by the gel spinning method. In Refs. [48–51], benzene
solution of the uncross-linked NR was casted on water surface to obtain a thin NR
film for TEM observations. The elongation of the thin NR film was conducted on
the water surface, and the elongated film was scooped up onto a specimen holder
for TEM. In this handling, shish might be formed at the stretching step on the water
surface.
It is interesting to note that WAXD measurement of SIC of NR was reported as
early as in 1925 by Katz [54–57], which was well before the establishment of polymer science at the middle of the 1930s or early in the 1940s [1, 58]. In the 1920s,
the scientific significance of WAXD began to be appreciated by many physicists
and chemists, and many polymeric materials were subjected to the measurement.
However, rubber (i.e., NR, since no synthetic rubber was available then) was not
highlighted at all: It gave only an isotropic ring-shaped pattern due to its amorphous and isotropic nature, while fiber materials afforded various spotted patterns,
which much stimulated the curiosity of scientists. Under this situation, the reports by
Katz disclosed the ‘fibering’ of rubber upon stretching the rubber specimen, which
attracted much interest to be given the name the ‘Katz effect.’ Later, H. Mark testified
it in the Preface of the book [59] that lots of scientists read the paper and repeated the
experiment. This popularity did not continue long, probably because of the difficulty
in analyzing the phenomenon: The crystallization rate upon elongation was too high
to measure, and the studies remained simply reporting the final fiber-like WAXD
pattern of the stretched NR. For the further development, to conduct quantitative
studies by time-resolved WAXD measurements is necessary, for which we have had
to wait for the usage of synchrotron radiation in the 1990s.
8.2.2 Formation of Template Followed by Instantaneous
Strain-Induced Crystallization
Length distribution of the network chains in NR vulcanizate is approximated by
the random distribution, and relatively short network chains are fully extended
upon stretching. These extended chains are a potential template, on which nearby
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
behavior has been assumed the basis of higher green strength of NR than that of
synthetic rubbers. Green strength is a tensile strength of raw rubber and is important in rubber processing, e.g., at shaping step (see Fig. 2.1 in Sect. 2.1). However,
entanglements are movable with the micro-Brownian motion of the rubber chains,
and hence, the SIC behaviors of raw NR may be much more complex than those of
NR vulcanizates. Namely, LTC behaviors of the unvulcanized NR is both theoretically and experimentally coherent, but SIC of it is much more complicated since its
network structure is not due to chemical bonding but due to physical entanglement.
For instance, the classic racking method on NR, a kind of application of SIC toward
higher modulus, failed to produce a high modulus rubber thread [53]. Ultrahigh molar
mass polyethylene (PE), on the other hand, has been successfully processed to a high
modulus and high strength PE by the gel spinning method. In Refs. [48–51], benzene
solution of the uncross-linked NR was casted on water surface to obtain a thin NR
film for TEM observations. The elongation of the thin NR film was conducted on
the water surface, and the elongated film was scooped up onto a specimen holder
for TEM. In this handling, shish might be formed at the stretching step on the water
surface.
It is interesting to note that WAXD measurement of SIC of NR was reported as
early as in 1925 by Katz [54–57], which was well before the establishment of polymer science at the middle of the 1930s or early in the 1940s [1, 58]. In the 1920s,
the scientific significance of WAXD began to be appreciated by many physicists
and chemists, and many polymeric materials were subjected to the measurement.
However, rubber (i.e., NR, since no synthetic rubber was available then) was not
highlighted at all: It gave only an isotropic ring-shaped pattern due to its amorphous and isotropic nature, while fiber materials afforded various spotted patterns,
which much stimulated the curiosity of scientists. Under this situation, the reports by
Katz disclosed the ‘fibering’ of rubber upon stretching the rubber specimen, which
attracted much interest to be given the name the ‘Katz effect.’ Later, H. Mark testified
it in the Preface of the book [59] that lots of scientists read the paper and repeated the
experiment. This popularity did not continue long, probably because of the difficulty
in analyzing the phenomenon: The crystallization rate upon elongation was too high
to measure, and the studies remained simply reporting the final fiber-like WAXD
pattern of the stretched NR. For the further development, to conduct quantitative
studies by time-resolved WAXD measurements is necessary, for which we have had
to wait for the usage of synchrotron radiation in the 1990s.
8.2.2 Formation of Template Followed by Instantaneous
Strain-Induced Crystallization
Length distribution of the network chains in NR vulcanizate is approximated by
the random distribution, and relatively short network chains are fully extended
upon stretching. These extended chains are a potential template, on which nearby