Chapter 8
Self-Reinforcement in Natural Rubber
(NR): Template Crystallization
Abstract Natural rubber (NR) is unique in its characteristics. The most notable
among them is its self-reinforcing ability. Namely, it is not necessary to mix carbon
black (CB) onto NR simply for mechanical strength. The use of CB on tire rubber
is for controlling tack, wear or abrasion, and dynamic friction-related properties
such as grip and traction. To explain the self-reinforcement of NR, strain-induced
crystallization (SIC) has been reasonably assumed, and now, it is accepted widely.
However, most researchers have adopted the traditional nucleation mechanism for
SIC, even though SIC is due to fully extended network rubber chains. The extended
chain formation upon elongation is a matter of necessity. In other words, nucleation
is a stochastic process, while SIC is a deterministic process upon straining of NR
vulcanizates. Thus, a new concept, template crystallization mechanism, is introduced
here in order to rationally explain the SIC behaviors of NR vulcanizates. Template
crystallization may be an important concept in ‘elastocaloric’ contractile process,
which has been proposed for some functional applications of NR vulcanizates such
as a shape memory device.
Keywords Self-reinforcement of natural rubber · Strain-induced crystallization
(SIC) · Extended network chain · Template crystallization · Elastocaloric effect
8.1 Low-Temperature Crystallization of NR
8.1.1 Amorphous and Crystal
Rubber is mostly amorphous and is not destined to crystalize in general. Rubber is
constantly in random movement at room temperature, or more precisely, a rubber
molecule is incessantly moving at microscopic level due to the segmental microBrownian motion of its chains above glass-transition temperature (T g ), though not in
motion at macroscopic level (i.e., standing still). Accordingly, rubber is fundamentally amorphous, and at an ambient temperature, it is liquid-like and hence may be
treated as a polymeric solvent [1–4], whose viscosity is very high. The high viscosity affords seemingly solid-like appearance to a raw rubber, and ‘cold flow’ displays
the hidden nature of rubber as liquid visibly: By standing a lump of raw rubber of
© Springer Nature Singapore Pte Ltd. 2020
S. Kohjiya et al., Reinforcement of Rubber, Springer Series on Polymer
and Composite Materials, https://doi.org/10.1007/978-981-15-3789-9_8
133
Self-Reinforcement in Natural Rubber
(NR): Template Crystallization
Abstract Natural rubber (NR) is unique in its characteristics. The most notable
among them is its self-reinforcing ability. Namely, it is not necessary to mix carbon
black (CB) onto NR simply for mechanical strength. The use of CB on tire rubber
is for controlling tack, wear or abrasion, and dynamic friction-related properties
such as grip and traction. To explain the self-reinforcement of NR, strain-induced
crystallization (SIC) has been reasonably assumed, and now, it is accepted widely.
However, most researchers have adopted the traditional nucleation mechanism for
SIC, even though SIC is due to fully extended network rubber chains. The extended
chain formation upon elongation is a matter of necessity. In other words, nucleation
is a stochastic process, while SIC is a deterministic process upon straining of NR
vulcanizates. Thus, a new concept, template crystallization mechanism, is introduced
here in order to rationally explain the SIC behaviors of NR vulcanizates. Template
crystallization may be an important concept in ‘elastocaloric’ contractile process,
which has been proposed for some functional applications of NR vulcanizates such
as a shape memory device.
Keywords Self-reinforcement of natural rubber · Strain-induced crystallization
(SIC) · Extended network chain · Template crystallization · Elastocaloric effect
8.1 Low-Temperature Crystallization of NR
8.1.1 Amorphous and Crystal
Rubber is mostly amorphous and is not destined to crystalize in general. Rubber is
constantly in random movement at room temperature, or more precisely, a rubber
molecule is incessantly moving at microscopic level due to the segmental microBrownian motion of its chains above glass-transition temperature (T g ), though not in
motion at macroscopic level (i.e., standing still). Accordingly, rubber is fundamentally amorphous, and at an ambient temperature, it is liquid-like and hence may be
treated as a polymeric solvent [1–4], whose viscosity is very high. The high viscosity affords seemingly solid-like appearance to a raw rubber, and ‘cold flow’ displays
the hidden nature of rubber as liquid visibly: By standing a lump of raw rubber of
© Springer Nature Singapore Pte Ltd. 2020
S. Kohjiya et al., Reinforcement of Rubber, Springer Series on Polymer
and Composite Materials, https://doi.org/10.1007/978-981-15-3789-9_8
133