1.Molecular weight and molecular weight distribution of rubber.

Rubber is a highly elastic polymer compound (with a molecular weight generally above 100,000), and thus possesses high elasticity that other materials do not have. Therefore, it is also called an elastomer.

Many properties of rubber change with the increase of molecular weight

（1）Rubber containing a large amount of low-molecular-weight components has a relatively low softening point and high plasticity in the softened state.

(2) Rubber with a majority of high-molecular-weight components has higher strength, toughness and elasticity, and a higher softening point, but its plasticity is relatively low.

(3) Rubber with a high molecular weight and a narrow distribution, although it has higher strength and other properties, is difficult to process during rubber compounding and molding, and has high processing energy consumption.

2. Characteristics of rubber.

（1）High elasticity: The elastic modulus of rubber is small, generally ranging from 1 to 9. 8MPa. It has a large elongation deformation, with an elongation rate as high as 1000%, still showing the characteristic of recovery, and can maintain elasticity within a wide temperature range (-50 to 150℃).

(2) Viscoelasticity: Rubber is a viscoelastic body. Due to the existence of intermolecular forces, rubber is subjected to external forces. When deformation occurs, it is affected by conditions such as time and temperature, showing obvious stress relaxation and creep phenomena.

(3) Buffering and shock absorption effect: Rubber has a mitigating effect on sound, vibration and its transmission. This feature can be utilized to prevent noise and vibration.

(4) Electrical insulation: Rubber, like plastic, is an electrical insulating material. The volume resistivity of natural rubber and butyl rubber can reach over 1015Ωcm.

(5) Temperature dependence: Polymer materials are generally affected by temperature. Rubber becomes hard and brittle in a glassy state at low temperatures, but at high temperatures, it softens, melts, undergoes thermal oxidation, thermal decomposition and even burns. (6) It shows signs of aging: Just like metal corrosion, wood decay, and rock weathering, rubber can also age due to changes in environmental conditions, deteriorating its performance and shortening its lifespan.

(7) Vulcanization is mandatory: Rubber must be added with sulfur or other substances that can vulcanize (or cross-link) rubber, so that the rubber macromolecules are cross-linked into a spatial network structure, in order to obtain rubber products with practical value.