How does the polydispersity index (PDI) affect the mechanical properties of a polymer?

The polydispersity index (PDI) is a measure of the distribution of molecular weights within a polymer sample. It is calculated as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn). A lower PDI indicates a more uniform molecular weight distribution, meaning the polymer chains are closer in size. A higher PDI signifies a broader distribution with significant variations in chain lengths. A polymer with a low PDI generally exhibits improved mechanical properties such as higher tensile strength and ductility because the more uniform chain lengths allow for more efficient packing and stronger intermolecular forces. These stronger forces lead to greater resistance to deformation and fracture. Conversely, a polymer with a high PDI tends to have lower tensile strength and increased brittleness. The presence of shorter chains can act as defects, disrupting the packing of longer chains and reducing the overall strength. The varying chain lengths also lead to less efficient stress transfer throughout the material, resulting in premature failure under stress. For example, a PLA sample with a PDI close to 1 will be stronger and more flexible than a PLA sample with a PDI of 3 or higher.