134
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
styrene-butadiene rubber (SBR) on a table in summer (supposedly at a temperature
above 25 °C), you may recognize a change of its shape in months or so, and ultimately, the lump would be spread over the table exactly like a spilled water. On the
one hand, SBR is highly amorphous due to the randomized distribution of styrene
unit and butadiene unit in the chain.
On the other hand, natural rubber (NR) is uniquely stereo-regular at its molecular
level: Only 2 (or 3 depending on the plant species from which NR is obtained) of
the monomeric units at a chain end are of trans-1,4-configuration, and all the others
are of cis-1,4 [5]. If we assume a NR molecule whose degree of polymerization
(DP) is 1000, the 998 units are of cis-1,4 configuration except two trans-1,4 units
at a chain end. That is, the stereo-regularity of NR is 99.8%. (Often, this value has
practically been understood as 100%.) So far, the effort by the synthetic polymer
chemists has not attained so high a stereo-regularity in industrial manufacturing
of cis-1,4-polyisoprene. From the viewpoint of crystallization, more important is
the presence of the isomeric trans units only at one chain end. In other words, the
rests of monomeric units are all cis-1,4 to result in the sequence length of cis-1,4
being 998 which constitute one cis-1,4 sequence [6, 7]. Due to this sequence length,
NR shows not only low-temperature crystallization (LTC) but also a unique straininduced crystallization (SIC). The former is explained in this section, and SIC in the
next section. SIC has been assumed to explain the superior performance of NR over
lots of synthetic rubbers [6–10], and the essence of this specialty of NR is called
‘self-reinforcement,’ which is the main topic of this chapter.
Even if a synthetic organic chemist is successful in attaining 99.8% stereoregularity by chemical polymerization of isoprene, the two trans units are distributed statistically (in accordance with the Markov statistics or most probably simple Markov, i.e., randomly) as usual for many chemical (non-biochemical) reactions
[6, 7, 11]. Therefore, the average length of cis-1,4 sequences in the synthetic cis-1,4polyisoprene whose DP is 1000 is to be 998/3 = 333, which means the decrease of
cis-1,4 sequence length to one-third. The drastic decrease of sequence length is less
favorable to its crystallization. Due to the high stereo-regularity with a long stereoregular sequence length, NR is unique in its molecular character as a crystalloid
among rubbers which are basically amorphous.
Crystallization was earlier observed on raw NR and NR products by their turning
hard and brittle during the wintertime in North America and in Europe. This phenomenon is now understood as LTC of NR, and it had been the main obstacle for the
popularization of the NR utilization before the invention of rubber vulcanization in
1839 by C. Goodyear (see Prefactory Remark at the last section in this chapter). To
tell an essence of vulcanization, it was an unintentionally found method to prevent
NR from LTC in winter [9]. Incidentally, it is remarkably an eye-opener that the
potential crystallizing ability of NR makes its obvious appearance again as SIC by
the vulcanization. (See Prefactory Remark, again.) The resultant crystal structures
by LTC and by SIC are identical, but the two crystallizations observed on NR are
much different in terms of their crystallization rate, orientation of the crystallites,
and technical significance from the viewpoint of materials science.
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
styrene-butadiene rubber (SBR) on a table in summer (supposedly at a temperature
above 25 °C), you may recognize a change of its shape in months or so, and ultimately, the lump would be spread over the table exactly like a spilled water. On the
one hand, SBR is highly amorphous due to the randomized distribution of styrene
unit and butadiene unit in the chain.
On the other hand, natural rubber (NR) is uniquely stereo-regular at its molecular
level: Only 2 (or 3 depending on the plant species from which NR is obtained) of
the monomeric units at a chain end are of trans-1,4-configuration, and all the others
are of cis-1,4 [5]. If we assume a NR molecule whose degree of polymerization
(DP) is 1000, the 998 units are of cis-1,4 configuration except two trans-1,4 units
at a chain end. That is, the stereo-regularity of NR is 99.8%. (Often, this value has
practically been understood as 100%.) So far, the effort by the synthetic polymer
chemists has not attained so high a stereo-regularity in industrial manufacturing
of cis-1,4-polyisoprene. From the viewpoint of crystallization, more important is
the presence of the isomeric trans units only at one chain end. In other words, the
rests of monomeric units are all cis-1,4 to result in the sequence length of cis-1,4
being 998 which constitute one cis-1,4 sequence [6, 7]. Due to this sequence length,
NR shows not only low-temperature crystallization (LTC) but also a unique straininduced crystallization (SIC). The former is explained in this section, and SIC in the
next section. SIC has been assumed to explain the superior performance of NR over
lots of synthetic rubbers [6–10], and the essence of this specialty of NR is called
‘self-reinforcement,’ which is the main topic of this chapter.
Even if a synthetic organic chemist is successful in attaining 99.8% stereoregularity by chemical polymerization of isoprene, the two trans units are distributed statistically (in accordance with the Markov statistics or most probably simple Markov, i.e., randomly) as usual for many chemical (non-biochemical) reactions
[6, 7, 11]. Therefore, the average length of cis-1,4 sequences in the synthetic cis-1,4polyisoprene whose DP is 1000 is to be 998/3 = 333, which means the decrease of
cis-1,4 sequence length to one-third. The drastic decrease of sequence length is less
favorable to its crystallization. Due to the high stereo-regularity with a long stereoregular sequence length, NR is unique in its molecular character as a crystalloid
among rubbers which are basically amorphous.
Crystallization was earlier observed on raw NR and NR products by their turning
hard and brittle during the wintertime in North America and in Europe. This phenomenon is now understood as LTC of NR, and it had been the main obstacle for the
popularization of the NR utilization before the invention of rubber vulcanization in
1839 by C. Goodyear (see Prefactory Remark at the last section in this chapter). To
tell an essence of vulcanization, it was an unintentionally found method to prevent
NR from LTC in winter [9]. Incidentally, it is remarkably an eye-opener that the
potential crystallizing ability of NR makes its obvious appearance again as SIC by
the vulcanization. (See Prefactory Remark, again.) The resultant crystal structures
by LTC and by SIC are identical, but the two crystallizations observed on NR are
much different in terms of their crystallization rate, orientation of the crystallites,
and technical significance from the viewpoint of materials science.