Natural rubber and synthetic rubber are typical polymer materials. Each molecule is made up of a number of structurally identical repeating units (links). For example, natural rubber is formed by the end-to-end connection of 1,4-isoprene units having the same chemical composition and spatial arrangement. However, low molecular weight materials have the same molecular weight per molecule, while rubber macromolecules do not. This is because the number of units constituting the macromolecule is repeated, and therefore, the molecular weight difference between them is also large, and the range may vary from several thousand to one million. As the molecular weight is small to large, the viscous and iconic physical property parameters (such as viscosity) are gradually increased, the fluidity is gradually reduced, the viscosity is gradually increased, and finally, the solid elastomer is at room temperature, which They are all familiar to us.

The molecular weight is a very important factor for the performance of the manufactured rubber feet. Its properties include both plasticity, fluidity, viscosity and other mechanical properties. However, the molecular weight of the rubber is not as large as possible, and the correlation between its properties and performance is mainly within the range of a certain molecular weight. When the critical value is exceeded, the change tends to shrink until it is insignificant.

As described above, the rubber molecules are formed by a plurality of links having the same composition and structure, and the number of repeating links constituting the rubber macromolecules is different, so that the molecular lengths are different and the molecular weight is large or small. Thousands of small, up to more than one million. Therefore, the concept of molecular weight of rubber is very different from that of low molecular substances. It is not based on a single molecule, but depends on the sum of the molecular weights of the chains with different molecular chain lengths, and the “average molecular weight” calculated by statistical methods is quasi. For example, natural rubber has an average molecular weight of 350,000. The molecular weight of the rubber can be expressed by the value of the average molecular weight obtained by different calculation methods, but the data are not equal. The order from small to large is the number average molecular weight, the viscosity average molecular weight, the weight average molecular weight, and the Z average molecular weight, respectively. There are various methods for determining the average molecular weight of rubber. Among them, the more mature viscosity method (for the determination of viscosity average molecular weight), the osmotic pressure method (for the determination of number average molecular weight), and the end group analysis method (for number average molecular weight) Determination) and ultracentrifugation sedimentation balance method (for weight average, determination of Z-average molecular weight) and the like. Each method requires the use of specific equipment and operating procedures, which takes a long time. Currently, the viscosity method is more commonly used.

In summary, the molecular weight and molecular weight distribution are closely related to the processing and use properties of rubber feet. In the case of natural rubber, its average molecular weight is related to tree species, cultivation, and age control. For synthetic rubber, the molecular weight is determined mainly by controlling and adjusting the polymerization process, and the specific data obtained are not equal.

If measured from the mechanical properties of rubber, as the molecular weight increases, some properties increase in a certain molecular weight range (such as strength, modulus, etc.), while others decrease (such as elongation and fluidity). From the point of view of usage, people are very comprehensive. Thus, it involves the problem of how to obtain an ideal molecular weight distribution state.

The raw rubber molecular weight distribution curve MWD (the abscissa is the molecular weight, and the ordinate is the proportion of the molecular weight segments) can reflect the overall performance of the specific green rubber. The MWD of natural rubber and synthetic rubber can exhibit a typical trend in its performance. The intermediate protrusion peak of the natural rubber MWD is relatively broad and flat, indicating that the moderately molecular weight portion dominates, while the left (low molecular weight portion) and the right (high molecular weight portion) also have a certain height on both sides. In this way, the middle and right parts provide the desired mechanical properties, while the left side (low molecular part) provides the desired processing properties (including plasticity, flow and adhesion).

Therefore, the comprehensive performance of natural rubber is ideal and comprehensive, and can take into account both processing and mechanical properties. The trend of the MWD curve of synthetic rubber represents another typical example, and its molecular weight distribution exhibits a narrow solitary peak and high protrusion, which indicates that the molecular weight is concentrated in a certain section in the middle. In this way, the performance of the rubber can be ensured to be stable and uniform, but the disadvantage is that the low molecular weight portion is lacking, and therefore, its processing property is not satisfactory, especially the fluidity and the adhesion are required to be added by processing aids or natural Rubber blending, or the introduction of certain groups in the molecular structure to solve.

As for the determination of the molecular weight distribution, the classification method is generally employed, that is, the sample is first classified into different molecular weight grades. Gel permeation chromatography, which emerged in the 1960s, is a simple, fast, and accurate method that has been widely used.