Similarity of Polymer Packing in Melts Is Dictated by N̅

The literature usage of the invariant degree of polymerization N̅ is surveyed and categorized according to the physical arguments involved. By providing rationales based on the universal packing geometry and the structure of the free energy in polymer melts, it is argued that the packing similarity of flexible polymers is controlled by the value of N̅. The historical introduction of N̅ and the connection of N̅ with the other dimensional properties are reviewed, followed by discussion of factors constraining the application of N̅.