Dear Alejandro Serrano, the main reasons are essentially first the increase in the enteglements knots with increase in MW After a critical threshold MWc, and second the increase in the cohesive energy density. All these contribute to storing input energy and release it upon relaxation. My Regards

Thank you Abdelkader, I understand that. My doubt is relatated to the fact that why from the Rouse, Zimm and Tube equations it is observed that G' and G'' are inversely proportional to MW? That is not in agreement with what is found experimentally. Perhaps I am not understanding the equations.

If I remember well, these equations where derived under special assumptions, among which the chains are assumed linear. I will check further and reach you back sooner. My Regards

There are many texts available via simple search. Thé following RG document deals with the answer. It is in accordance with my first contribution. My Regards

https://www.researchgate.net/publication/226820232_Why_does_the_rouse_model_fairly_describe_the_dynamic_characteristics_of_polymer_melts_at_molecular_masses_below_critical_mass

A High molecular weight increases the chemical resistance - to a point. It takes more damage to the main chains of the molecules before it will affect the strength of the material. A High molecular weight increases the viscosity of the material – makes it harder to process the material using conventional methods.

The molecular weight of a polymer is directly related to its properties. As the molecular weight increases, mechanical properties generally increase. Every polymer has an ideal molecular weight at which the balance of different properties (such as processability, strength, brittleness etc.) is optimized.

Plz check out this link below

https://www.nature.com/articles/pj197322