Viscoelastic-stress relaxation of real rubber
The statistical theory of rubber elasticity discusses the generalization of all rubbers to an ideal molecule in which there are no interaction forces between molecules and the molecular chains are free to rotate, and the elasticity of rubber is produced entirely by changes in the conformation of the crimped molecules, with stresses in equilibrium with deformations, and independently of the chemical composition of the rubber. However, this is not the case with real rubber; there are intermolecular attractions, the magnitude of which varies depending on the chemical composition and structure of the rubber. The interactions between molecules prevent the movement of the molecular chains and manifest themselves as stickiness or viscosity, so that the stresses and deformations tend to be out of equilibrium. Acting on the rubber molecules, part of the force used to overcome the intermolecular viscous resistance, the other part of the molecular chain deformation, the two constitute the viscoelastic properties of rubber. Elasticity and viscosity are common to many materials, but high viscosity and elasticity are unique to rubber. Viscoelasticity in rubber processing is discussed in Chapter 6, this chapter focuses on the stress relaxation, creep, hysteresis loss, recovery and permanent deformation and dynamic mechanical properties of rubber after stress, emphasizing the effect of time and temperature.
I. Stress relaxation
1. The significance of stress relaxation
At a constant temperature, the rubber is elongated to a certain length, and as time passes, the stress to maintain this length will gradually decrease, a phenomenon called stress relaxation. Or, constant temperature, constant deformation, the stress is a function of time.
The reason for the stress relaxation phenomenon is because the viscosity of the rubber is very large, the instantaneous time of the external force acting on it is not likely to be uniformly distributed, and some chain segments may not have been subjected to the action of the external force. Due to the uneven distribution of force, the internal stress is high and the molecules are under tension. Then, the molecular chain moves, rearrangement, after a certain period of time before the elimination of internal stress, to achieve equilibrium. At this time, the stress also drops to the equilibrium value. Therefore, stress relaxation is the process of eliminating viscous obstacles. But for the raw rubber, although there are molecular chains intertwined, but after all, is not a permanent cross-bonding, molecules will gradually produce the relative displacement of the chain and untangled, the stress will eventually relax to zero. Figure 5-20 shows the stress relaxation process of the molecular chain of vulcanized rubber, if the raw rubber, there will eventually be plastic flow, the stress is reduced to zero without equilibrium stress.
2. Simple stress relaxation model - Maxwall model
The use of simple models can help in understanding stress relaxation phenomena. The martensitic model. This is shown in Figure 5-21. This model consists of a spring in series with a piston (sticky pot) placed in a viscous liquid. The spring represents a coiled chain of molecules and the sticky pot represents the attractive forces between molecules. When an external force is applied to the model, the spring opens first, representing the deformation of the ideal rubber molecular chain immediately after the force is applied. The force applied to the spring is immediately transferred to the piston, but the piston's movement is hindered by the surrounding viscous liquid and takes some time to move. Thus, at the instant of application of force, the entire deformation is reflected in the spring, and as the time of application of force is prolonged, the viscous pot moves, the elongation of the spring decreases, and the force to keep the deformation of the model constant also decreases.