Molecular Weight Distribution and Mechanical Properties

Abstract

After the chemical structure, the polymer chain length and its distribution are undoubtedly the next most important molecular parameters controlling the physical, mechanical and processing properties of plastic materials. Change in material properties with increasing molecular weight (MW) without a change in chemical composition is well-exemplified by the series of n-alcanes. For low MW paraffins, the load bearing capacities are nearly zero since short chains in the bulk material may easily slip past each other when subjected to mechanical stress. In higher MW polyethylene (PE), the increase in the number of weak van der Waals interactions per chain can effectively immobilize the macromolecule in an entanglement network. Depending on MW and its distribution (MWD), PE can exist under a variety of formulations, each one with tailored properties for specific applications. The different commercial grades include low MW resins (~ 10³ daltons) used as hot melt adhesives ultra-high MW polymers (~10⁶ daltons) aimed at demanding applications in which high draw ratio (gel-spinning) or high wear and fatigue resistance (hip protheses) are required, and bimodal MWD products with unique mechanical and processing properties, employed in the fabrication of large diameter PE pipes.