The viscoelasticity of actual rubber---stress relaxation

The statistical theory of rubber elasticity discusses the generality of all rubbers. The ideal molecule that has no interaction force between molecules and the molecular chain can rotate freely is the research object. The elasticity of rubber is completely generated by the change in the conformation of the crimped molecule, between stress and deformation. It is in equilibrium and has nothing to do with the chemical composition of rubber. However, the real rubber is not like this, the molecules are attractive, and the size of the attraction depends on the chemical composition and structure of the rubber. The interaction between molecules will hinder the movement of the molecular chain, which manifests as viscosity or viscosity, so that the stress and deformation are often in an imbalanced state. One part of the force acting on the rubber molecules is used to overcome the viscous resistance between the molecules, and the other part deforms the molecular chains. These two constitute the viscoelastic properties of rubber. Elasticity and viscosity are combined by many materials, but high viscosity and high elasticity are unique to rubber. The viscoelastic properties of rubber processing are discussed in Chapter 6. This chapter focuses on stress relaxation, creep, hysteresis loss, recovery and permanent deformation, and dynamic mechanical properties of rubber after being stressed, with emphasis on the effects of time and temperature.

1. Stress relaxation

1. The meaning of stress relaxation

At a constant temperature, the rubber is stretched to a certain length. As time goes by, the stress to maintain this length will gradually decrease. This phenomenon is called stress relaxation. In other words, under constant temperature and constant deformation, stress is a function of time.

The reason for the stress relaxation phenomenon is that the viscosity of the rubber is very large, and the instantaneous time when the external force acts on it cannot be uniformly distributed, and some chain segments may not be affected by the external force. Due to the uneven force distribution, the internal stress is large, and the molecules are in a state of tension. Then, the molecular chain moves and rearranges, and after a certain period of time, the internal stress can be eliminated and the equilibrium state can be reached. At this time, the stress also drops to the equilibrium value. Therefore, stress relaxation is the process of eliminating the stickiness hindrance. But for raw rubber, although there is entanglement between molecular chains, it is not a permanent horizontal bond after all. The molecules will gradually produce relative displacement between the chains to release the entanglement, and the stress will eventually relax to zero. Figure 5-20 shows the stress relaxation process of the vulcanized rubber molecular chain. If it is raw rubber, there will eventually be plastic flow, and the stress will be reduced to zero without equilibrium stress.

2. Simple stress relaxation model-Maxwall model

The use of simple models can help understand the phenomenon of stress relaxation. The earliest is the Markov model. As shown in Figure 5-21. This model consists of a spring and a piston (sticky pot) placed in a viscous liquid in series. The spring represents the curled molecular chain, and the viscous pot represents the attraction between molecules. When an external force acts on the model, the spring opens first, which means that the ideal rubber molecular chain is immediately deformed after the force is applied. The force on the spring is immediately transmitted to the piston, but the movement of the piston is hindered by the surrounding viscous liquid, and it takes a while to move. Therefore, at the moment of force application, all the deformation is reflected in the spring. With the extension of the force application time, the viscous pot moves, the elongation of the spring gradually decreases, and the force that maintains the constant deformation of the model also decreases.