The molecular weight of a polymer has a significant impact on the flow behavior of its melt under shear stress. This is because the molecular weight is directly related to the size, structure, and entanglement of the polymer chains, which in turn influence the viscosity, elasticity, and overall rheological properties of the polymer melt.1. Viscosity: As the molecular weight of a polymer increases, the viscosity of its melt also increases. This is because higher molecular weight polymers have longer chains, which can become more entangled with each other. These entanglements create resistance to flow, resulting in a higher viscosity. Consequently, higher molecular weight polymers require more force shear stress to achieve the same flow rate as lower molecular weight polymers.2. Shear thinning: Polymers generally exhibit shear thinning behavior, which means that their viscosity decreases with increasing shear stress. This is due to the alignment of polymer chains in the direction of flow, which reduces the number of entanglements and allows for easier flow. Higher molecular weight polymers have more pronounced shear thinning behavior because they have more entanglements that can be reduced by the applied shear stress.3. Elasticity: The elasticity of a polymer melt is related to its ability to recover its original shape after deformation. Higher molecular weight polymers have greater elasticity due to the increased number of entanglements that can store and release energy during deformation. This increased elasticity can lead to more complex flow behavior, such as strain hardening where the viscosity increases with increasing shear stress and the formation of elastic instabilities such as die swell or extrudate distortion .4. Relaxation time: The relaxation time of a polymer melt is a measure of how long it takes for the polymer chains to return to their equilibrium state after being deformed. Higher molecular weight polymers have longer relaxation times because it takes more time for the longer chains to disentangle and return to their equilibrium state. This longer relaxation time can lead to a more delayed response to changes in shear stress, which can affect the flow behavior of the polymer melt.In summary, the molecular weight of a polymer significantly affects the flow behavior of its melt under shear stress by influencing its viscosity, shear thinning behavior, elasticity, and relaxation time. Higher molecular weight polymers generally have higher viscosities, more pronounced shear thinning behavior, greater elasticity, and longer relaxation times, which can lead to more complex flow behavior and processing challenges.