Rubber Science by Yuko Ikeda Atsushi Kato Shinzo Kohjiya & Yukio Nakajima

Author:Yuko Ikeda, Atsushi Kato, Shinzo Kohjiya & Yukio Nakajima
Language: eng
Format: epub
Publisher: Springer Singapore, Singapore

Without the addition of any nucleating agent, homogeneous nucleation is generally a precondition of crystallization for polymers, which is depending on statistical fluctuation at molecular level [129, 131, 186, 189, 190]. While the crystal growth rate is dependent only on temperature, nucleation rate is strongly dependent on various experimental conditions such as previous temperature of melting, cooling process from the molten state, heating process from the glassy state, etc. [190]. The nucleation is therefore expressed as being sporadic, and further mechanistic studies on the puzzle solving are recently in active research. For example, molecular dynamics simulation is extended up to 100 ns to investigate the evolution of cluster size, intrachain vs. interchain potential energies and pair correlations of PE solution at 300 K: The embryonic development begins with the aggregation of trans-rich sequences of characteristic length of 2 nm, forming clusters of short stems which reorganize to the embryonic clusters, and these clusters may give rise to nucleation [191]. Consequently, scientific considerations are to include statistical treatments [126, 130]. The fluctuation may exactly be treated in rheology as a viscoelastic behavior of a polymeric chain using the reputation theory. No such attempts for explaining the nucleation process have been reported as far as we know. When a nucleating agent is added to the crystallizing system, it is named inhomogeneous or heterogeneous crystallization, which has been a popular technique to reduce the crystal size by producing as many nucleates hence crystallites as possible.

The detailed crystal structure of NR, which is the final crystal both via SIC and via TIC, has been studied by Mayer and Mark (see Sect. 2.​4.​3), Bunn [192], Nyburg [150], Natta et al. [193], Takahashi et al. [194], Immirizi et al. [195], and Rajkuma et al. [196]. However, the final conclusion on a few details is still to be investigated.

On low-temperature crystallization of NR, time to reach half the degree of crystallization to the equilibrium one ( t 1/2) is from several hours to a few days at −25 °C, at which t 1/2 shows its maximum value. At room temperature, it is between a few weeks and several months or even years. Wood and Bekkedahl [186] wrote at 14 °C the time required for crystallization was of the order of years (see Fig. 3.24). It was reported that the removal of non-rubber components by solvent extraction delays the crystallization , and the addition of long-chain aliphatic acids, e.g., stearic acid , has been known to accelerate it [197, 198]. These results indicate that the low-temperature crystallization (or TIC) of NR is usual homogeneous or heterogeneous nucleating crystallization, different from SIC of NR. Additionally, the maximal degree of crystallinity attained so far was 40% [199], not much higher than that by SIC. Relatively low degree of crystallization is a feature of polymers in general, but in the case of NR, higher chain mobility due to the micro-Brownian motion around room temperature, which is leading to rubber elasticity, is a reason in spite of its very high stereo-regularity .

In the cross-linked NR, i.

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