8.2 Template Crystallization: Dynamic Mechanism …
145
shape. The ring sample can be elongated much more homogeneously upon stretching
than a dumbbell-shaped one: This is a necessary condition in order to ensure the
irradiation of the same area by X-ray throughout the time-resolved WAXD and
tensile measurements.
After the start of stretching at the onset strain, some relatively short chains are
fully extended, onto which nearby longer oriented amorphous network chains coordinate to the extended chain, i.e., it turns out to be a template, followed by template
crystallization to form a crystallite oriented to the stretching direction. This scheme
suggests no need of nucleation, since the formation of potential templates is not due
to any fluctuations but due to elongation of the sample to the onset strain, in accordance with the causality relationship as already discussed in the previous subsection.
The formation is a matter of necessity following the formal logic, and probability
is not playing any role in the formation of template upon the elongation of the NR
vulcanizates.
In this scenario, the distribution of the network chain length in NR vulcanizates
being not homogeneous is one of the necessary conditions for template crystallization. If the length is perfectly homogeneous, SIC occurs to the degree of crystallization almost 100%. In this case, however, the resultant morphology would be shish
crystallites forming the continuous phase. Thus, the remaining minor amorphous
chains, if any, are dispersed in the continuous crystalline matrix. In other words,
the crystalline phase becomes predominant, and the whole specimen would not be
elastomeric and no return to the rubber state even after the removal of the stress.
The return of the strained NR vulcanizates to the original amorphous state of the
same size is by the melting of the crystallites due to the entropic elasticity of the
amorphous region which is the continuous phase even at the strained state. Note that
the observed maximum degree of crystallizations by SIC of NR or IR vulcanizates
was found around 20% or so.
Interestingly, the scenario predicts a very surprising result. So far, many chemists
have been struggling to synthesize a model network, in which all the network chains
are of the same length, expecting that such a rubbery network should display much
higher performance of very high strength. However, in the case of NR and some
other crystallizable rubbers such as IR, CR, and high cis BR, this kind of effort for
perfectly controlled cross-linked rubber is destined to fail. The network structure
whose network chains are of the same length can be easily stretched but logically
only once. Upon the stretching, almost all network chains are crystallized to form the
continuous phase. The elongation changes the rubber to anisotropic plastic materials
of high crystallinity. In other words, they are totally of no use as an elastomer after a
single stretching. Such model networks of SBR and polydimethylsiloxane may work
as an elastomer.
The basic origin of this scenario is Fig. 8.5 reported in Ref. [64] disclosed in
2004, and this visual presentation has seemed to enjoy a popular support among SIC
researchers because of its intuitive intelligibility. However, all of their interpretations have been against our scenario explained above. For example, a review paper
reproduced Fig. 8.5 and explained [74].
145
shape. The ring sample can be elongated much more homogeneously upon stretching
than a dumbbell-shaped one: This is a necessary condition in order to ensure the
irradiation of the same area by X-ray throughout the time-resolved WAXD and
tensile measurements.
After the start of stretching at the onset strain, some relatively short chains are
fully extended, onto which nearby longer oriented amorphous network chains coordinate to the extended chain, i.e., it turns out to be a template, followed by template
crystallization to form a crystallite oriented to the stretching direction. This scheme
suggests no need of nucleation, since the formation of potential templates is not due
to any fluctuations but due to elongation of the sample to the onset strain, in accordance with the causality relationship as already discussed in the previous subsection.
The formation is a matter of necessity following the formal logic, and probability
is not playing any role in the formation of template upon the elongation of the NR
vulcanizates.
In this scenario, the distribution of the network chain length in NR vulcanizates
being not homogeneous is one of the necessary conditions for template crystallization. If the length is perfectly homogeneous, SIC occurs to the degree of crystallization almost 100%. In this case, however, the resultant morphology would be shish
crystallites forming the continuous phase. Thus, the remaining minor amorphous
chains, if any, are dispersed in the continuous crystalline matrix. In other words,
the crystalline phase becomes predominant, and the whole specimen would not be
elastomeric and no return to the rubber state even after the removal of the stress.
The return of the strained NR vulcanizates to the original amorphous state of the
same size is by the melting of the crystallites due to the entropic elasticity of the
amorphous region which is the continuous phase even at the strained state. Note that
the observed maximum degree of crystallizations by SIC of NR or IR vulcanizates
was found around 20% or so.
Interestingly, the scenario predicts a very surprising result. So far, many chemists
have been struggling to synthesize a model network, in which all the network chains
are of the same length, expecting that such a rubbery network should display much
higher performance of very high strength. However, in the case of NR and some
other crystallizable rubbers such as IR, CR, and high cis BR, this kind of effort for
perfectly controlled cross-linked rubber is destined to fail. The network structure
whose network chains are of the same length can be easily stretched but logically
only once. Upon the stretching, almost all network chains are crystallized to form the
continuous phase. The elongation changes the rubber to anisotropic plastic materials
of high crystallinity. In other words, they are totally of no use as an elastomer after a
single stretching. Such model networks of SBR and polydimethylsiloxane may work
as an elastomer.
The basic origin of this scenario is Fig. 8.5 reported in Ref. [64] disclosed in
2004, and this visual presentation has seemed to enjoy a popular support among SIC
researchers because of its intuitive intelligibility. However, all of their interpretations have been against our scenario explained above. For example, a review paper
reproduced Fig. 8.5 and explained [74].