Mechanical strength of polymer increases with increase in molecular weight upto certain limit beyond that property is nearly independent of chain length of polymer.

Dear Bademaw Abate,

Molecular weight has an important effect on the mechanical properties of polymer, more especially above the glass transition temperature. Indeed, glass transition increases with the molecular weight as it does the storage modulus.

At large deformations (above Tg), the mechanical strength and deform ability are higher. Additionally, when comparing the same polymer at the same conditions (temperature, strain rate), the polymer with higher molecular weight will display a mechanical behaviour tending to be more hyperelastic (less hysteresis).

For further information, this article addresses this topic on PMMA:

Article Large strain/time dependent mechanical behaviour of PMMAs of...

Hi,

As Mr. Federico said, molecular weight play an important roll in the mechanical properties of polymers. Moreover, the most important effect of that relates to the glass transition temperature, so that increasing the molecular weight increases the glass transition temperature. Due to glass transition temperature modification, the main properties of a polymer are changed, namely: density and specific volume, etc.

Bests,

Aali,

A higher molecular weight also means that there is more increased chain entanglement. This bring about an increase in tensile strength and elastic modulus due to the fact that more energy is required to break or loosen the entangled chains. This is the situation at ambient temperature, but with increase in temperature, the mechanical properties decreases.

The mechanical properties of polymeric fibers represented by stress, strain, and Young’s modulus increase by increasing the molecular weight. The positive relationship between the molecular weight and the mechanical properties of polymeric fibers can be attributed to the increase in the length of the molecular chains (increasing the viscosity). More details can be found in this paper:

“Controlling the surface structure, mechanical properties, crystallinity, and piezoelectric properties of electrospun PVDF nanofibers by maneuvering molecular weight” DOI: 10.1080/1539445X.2019.1582542.

Yes the mechanical properties like strength, modulus and viscosity increase with MW. But the variations in properties depend on the chain characteristics i.e. stiff, semiflexible or highly flexible chains. The variation of viscosity gives transition between extent of rise in viscosity with MW giving a critical MW below which viscosity varies linearly and above as a power of 3.5 to MW. The temperature has pronounce effect on these properties.

Yes, I agree with the previous arguments. However, why from the molecular theories (Rouse, Zimm, Tube model) elastic and viscous modulus are inversely proportional to molecular weight? I am trying to figure that out and so far it has been impossible.

When molecules are closely packed/ densely packed they provides a more crystalline structure of the macromolecule, that is a polymer , hence better the mechanical properties & strength to the polymer is good enough. it means more the molecular weight better the strength to the polymer.