English Abstract

Strain-Induced Crystallization and Mechanical Properties of Rubber.
5. Network, Entanglement, Strain Induced Crystal (SIC) and Mechanical Properties.

Shigeyuki TOKICPh. D
Retired from Mahidol University, State University of New York at Stony Brook, Tonen Chemical and Bridgestone.
Nippon Gomu Kyokaishi,(2018),91(9),344-350 General Review in Japanese

1. Hysteresis of stress-strain, SIC-strain, temperature-strain in un-vulcanized rubber and vulcanized rubber have been observed. The elucidation of the SIC phenomena should be applicable on these hysteresis. Temperature increase is a result of heat producing process of phase transition from amorphous to crystal, that is, latent crystallization heat. Therefore, temperature change is directly connected to SIC.
2. A new constitutive equation of stress-strain relation with inhomogeneous distribution of network chain, SIC and temperature change was proposed by Khiêm and Itskov. They succeeded to simulate a stress-strain relation of vulcanized natural rubber during loading and un-loading. They addressed that a creation of bundle-like SIC decreases the increasing stress during loading process and SIC may transform from bundle-like crystal to folded-chain crystal during un-loading process.
3. Rubber is composed of not only polymer chain but also solid particles and crystals such as zinc oxide, stearic acids, non-rubber components of NR and others. Therefore, rubber is very inhomogeneous material. During deformation, polymer matrix is deformed in-homogeneously, therefore, stress and strain distribute and concentrate at certain area. At these stress and strain concentrated area, void or crack might be occurred. SIC are created easily at such area because of high stress or strain. In such case, rubber is no more non-compressible material.
4. Shape memory phenomenon. After stretching 800% and keeping the sample for a few minutes, then release the sample, the sample shrink a little bit to 750% and keep the stretched state without any force. The phenomenon is a kind of shape memory. It must be a special relationship between SIC and networks. It may suggest us to try other deformation technique in order to characterize SIC phenomena.

Summary of the five special reviews on “strain-induced crystallization (SIC) and mechanical properties of rubber” in the first review as “uniaxial deformation and SIC of rubbers” at 90 (8) 401 (2017), the second review as“ SIC of vulcanized and un-vulcanized Rubber, reinforcement filled rubber, thermoplastic elastomer” at 90 (11) 531 (2017), the third review as “SIC under static and dynamic condition” at 91 (3) 78 (2018), the fourth review as “crystal and crystallites structure” at 91 (4) 129 (2018) and this fifth review.
1. Strain-induced crystallization (SIC) of un-vulcanized rubber and vulcanized rubber is one of crystallization processes of polymer. During deformation at temperature beyond Tg, crystalline polymer shows SIC as polymer melt, that is, entangled chain molecule in liquid state. Therefore, no special ingredients is necessary for SIC, but entanglement (and crosslink) is vital for SIC. The temperature of rubber sample increases during deformation due to latent crystallization heat.
2. Unit cell of SIC is composed of 4 chains and is the same unit cell of TIC (temperature induced crystallization). The crystallites of SIC is not folded-chain but bundle-like crystal. The size of SIC crystallites depends on temperature and is around 13 nm × 6 nm × 6 nm at 25 °C. One crystallites is composed of 280 chains at 25 °C.
3. SIC is created by not only decrease of entropy of amorphous chain but also decrease of free energy of crystal in oriented amorphous molecules during deformation.
4. Stress-downward is observed first, then stress-upturn starts during uniaxial deformation. Stress-downward accompany with temperature rise due to SIC. On the other hand, stress-upturn is created by not only limited extensibility of polymer chain between entanglements and crosslinks, but also alignment of amorphous chains, inhomogeneous distribution of crosslinks and existence of SIC as a filler.
5. Theoretical approach with concepts of network, SIC, temperature change, in-homogeneous matrix and compressibility during deformation is necessary to understand the effects of SIC phenomena. A new constitutive equation has been proposed.

Keywords: Crystallization, Mechanical Properties, Rubber