“Entanglement” concept and chain conformations in bulk amorphous polymers

It is shown that the presently available information on the structure of bulk amorphous polymers is consistent with the concept of the existence of a substantial amount of short‐range segmental order. It is suggested that the sharp change of molecular‐weight dependence of NEWTONian viscosity of polymer melts with the approach to the critical “entanglement” molecular weight M c is brought about mainly by the transition from the extended‐chain conformation of the macromolecules into the statistically folded‐chain conformation. Finally, empirical relationships are obtained which relate the values of polymer packing coefficients in the crystalline state as well as the “best” numerical values of M c with molecular parameters of a polymer.