Molecular Weight and Density - Thermoplastic polymers, elastomers and additives

It seems that logic induces us to consider that density and molecular weight are directly proportional, when in reality they can vary independently.

The polymers are constituted by functional groups that form polymer chains each with a different molecular weight. To obtain the molecular weight of the polymer, the weight of each of the chains present must be calculated. Due to the molecular weight distribution, it is impossible to characterize the polymer with a single molecular weight, so it is necessary to use various averages: Viscosity average molecular weight Mv (in solution), average molecular weight in Mn number and average molecular weight en masse, Mw. In this case we will analyze the last two molecular weights that are the most used. Mn represents the total weight of all polymer molecules contained in a sample, divided by the total number of polymer molecules in said sample. Mw which is based on the fact that a larger molecule contains more of the total mass of the polymer sample than the small molecules (it is the parameter that is used to characterize the molecular weight of the polymer). These weights are distributed like a Gaussian bell.

PE is a semicrystalline material, so its structure is formed by both amorphous and crystalline zones.

PE is a semicrystalline material, so its structure is formed by both amorphous and crystalline zones. Different degrees of crystallinity can be obtained due to their ability to form linear or branched structures. In this way densities are obtained from 0.857 g / cm3 to 0.975 g / cm3.

An example can be the comparison between HDPE (PE-500) and UHMWPE (PE-1000). Both have a similar density value, but not equal: ≈0.944g / cm3 and 0.93g / cm3 respectively; when its molecular weight (HDPE 200,000-500,000 and UHMWPE 3-6,000,000) has nothing to do with it. HDPE has very small branches that make its chains practically linear, being relatively easy to order its chains and achieve a high degree of crystallinity (≈90%). However, in the case of UHMWPE, their long molecular chains do not allow free movement of them, obtaining a less ordered structure (degree of crystallinity 58% -75%). This degree of crystallinity directly influences the density of the materials. Remember first that density is the relationship between mass and volume. For a same volume, in a crystalline structure the weight will be greater (there are more chains) than in an amorphous structure, the density being also greater. This is the reason why, in the case of the 2 types of polyethylene indicated above, PE with a higher molecular weight (but an amorphous structure) has a lower density.

The branches in a polymer cause that the main chains that compose it remain distant, reducing therefore the density and the capacity of load of the material.

Crystallinity also affects other parameters. For example, a lower degree of crystallinity (more amorphous) implies a gradual tendency of greater deformation. On the other hand, a higher degree of crystallinity results in a lower elongation at break. This is the reason why, for example between the PE-500 and PE-1000, some of its mechanical properties, differ significantly.

The parameter related to molecular weight is the flow index (the capacity to flow of the material). The lower the molecular weight, the higher the fluidity index (they are inversely proportional). If the main chains are long, the fluidity index will be lower, obtaining higher viscosity and giving the material greater impact resistance than those materials with a comparable density and smaller chains.

Creep resistance can be improved by increasing density and molecular weight. On the contrary, to make changes in the tenacity, one must either increase the molecular weight or reduce the density.