140
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
8.2 Template Crystallization: Dynamic Mechanism
of Strain-Induced Crystallization of NR
8.2.1 Extended Network Chain
Details of template crystallization are to be explained in this section, which are
the recent fruits of experimental researches conducted by using the synchrotron
facilities, especially at the SPring-8 in Hyogo, Japan. The experimental results are
to be described in Sect. 8.2.2, which is to be preceded by related historical and
very simple logical considerations, on adopting template mechanism for SIC of NR
vulcanizates. In this process toward a uniquely ambitious concept of template in
crystallization, traditional idea of nucleation is further critically briefed here.
Crystallization is classified into two, homogeneous and inhomogeneous. In the
former, nucleation is accepted as a premise, i.e., a prerequisite at the present moment
[26, 35]. As shown previously, the details of nucleation mechanism have not been
elucidated yet, but it is believed that the formation process of which is dependent on
statistical fluctuation of the density, and its elucidation is still to be investigated [37].
The rate of crystallization is usually determined by temperature. On the other hand,
nucleation is, both theoretically and experimentally, dependent upon specimen temperature before the crystallization, cooling rate from the melt state, heating history
at the glassy state, and so on [43]. In other words, since nucleation is due to density
fluctuation which is fundamentally a stochastic process [11, 26, 35], such complex
dependency on experimental conditions is not irrational. Hence, the practical control
of it is very difficult, and in the plastics industries, inhomogeneous crystallization by
mixing a nucleating agent is quite common, as mentioned before. Choosing an appropriate reagent for the resin and determining the best conditions such as its amount
and timing of its addition are carefully conducted for the control of crystallization in
the manufacturing processes of plastics products.
Different from homogeneous crystallization due to nucleation, SIC of cross-linked
NR is absolutely depending on elongation of the specimen, not on any fluctuation. In
the 3D network structure of NR by vulcanization, distribution of the network chain
length is not monodispersed, since the cross-linking reaction between curing reagents
and rubber molecules is reasonably assumed to be a random process in terms of the
reaction site of the NR chains. Therefore, a relatively short network chain would be
fully extended upon elongation of a NR vulcanizate among lots of network chains
of various lengths. This imagined picture of a fully extended network chain is in
accordance with the affine assumption on deformation, which has been reasonably
assumed in the theory of rubber elasticity.
The fully extended polymer chains were already confirmed experimentally in the
so-called shish kebab crystal, reported by Pennings in 1977 [44]. Or, in the rubber
science arena, γ-filament in NR [45–51], which was first reported by Andrews [45],
may be more familiar. Figure 8.3 shows a sketch of the shish kebab crystal at its
molecular level [44]. Bassett cited this figure and gave an excellent account on
8 Self-Reinforcement in Natural Rubber (NR): Template Crystallization
8.2 Template Crystallization: Dynamic Mechanism
of Strain-Induced Crystallization of NR
8.2.1 Extended Network Chain
Details of template crystallization are to be explained in this section, which are
the recent fruits of experimental researches conducted by using the synchrotron
facilities, especially at the SPring-8 in Hyogo, Japan. The experimental results are
to be described in Sect. 8.2.2, which is to be preceded by related historical and
very simple logical considerations, on adopting template mechanism for SIC of NR
vulcanizates. In this process toward a uniquely ambitious concept of template in
crystallization, traditional idea of nucleation is further critically briefed here.
Crystallization is classified into two, homogeneous and inhomogeneous. In the
former, nucleation is accepted as a premise, i.e., a prerequisite at the present moment
[26, 35]. As shown previously, the details of nucleation mechanism have not been
elucidated yet, but it is believed that the formation process of which is dependent on
statistical fluctuation of the density, and its elucidation is still to be investigated [37].
The rate of crystallization is usually determined by temperature. On the other hand,
nucleation is, both theoretically and experimentally, dependent upon specimen temperature before the crystallization, cooling rate from the melt state, heating history
at the glassy state, and so on [43]. In other words, since nucleation is due to density
fluctuation which is fundamentally a stochastic process [11, 26, 35], such complex
dependency on experimental conditions is not irrational. Hence, the practical control
of it is very difficult, and in the plastics industries, inhomogeneous crystallization by
mixing a nucleating agent is quite common, as mentioned before. Choosing an appropriate reagent for the resin and determining the best conditions such as its amount
and timing of its addition are carefully conducted for the control of crystallization in
the manufacturing processes of plastics products.
Different from homogeneous crystallization due to nucleation, SIC of cross-linked
NR is absolutely depending on elongation of the specimen, not on any fluctuation. In
the 3D network structure of NR by vulcanization, distribution of the network chain
length is not monodispersed, since the cross-linking reaction between curing reagents
and rubber molecules is reasonably assumed to be a random process in terms of the
reaction site of the NR chains. Therefore, a relatively short network chain would be
fully extended upon elongation of a NR vulcanizate among lots of network chains
of various lengths. This imagined picture of a fully extended network chain is in
accordance with the affine assumption on deformation, which has been reasonably
assumed in the theory of rubber elasticity.
The fully extended polymer chains were already confirmed experimentally in the
so-called shish kebab crystal, reported by Pennings in 1977 [44]. Or, in the rubber
science arena, γ-filament in NR [45–51], which was first reported by Andrews [45],
may be more familiar. Figure 8.3 shows a sketch of the shish kebab crystal at its
molecular level [44]. Bassett cited this figure and gave an excellent account on