AbstractThe properties of molecular weight fractions of linear polyethylene, encompassing the range 3 × 103to 1.5 × 106, which were isothermally crystallised at low undercooling at. atmospheric pressure, were studied. Marked changes in various thermodynamic and mechanical properties are observed with molecular weight, which can be attributed to the finite sine of the crystallites in the chain direction and to the ratio of the crystallite size to the extended‐chain length. Electron microscopic examination of fracture surfaces indicates the formation of striated, lamella crystallites over the complete molecular weight range although the ratio of crystallite size to extended‐chain length varies from approximately unity at the lower molecular weights to extremely small values for the higher molecular weights. Concomitant with the changes in properties and sizes, the interfacial free energy of a mature crystallite increases significantly with molecular weight. Based upon these experimental results, a crystallization mechanism is developed which has as its basic tenents the relation between the critical size initiating nucleus and the extended‐chain length and the spatial requirements of chain units in random conformation in the interzonal regions. For molecular weights above which extended‐chain crystals are formed, the analysis and experiment indicate that the interfarnal region associated with the basal plane is of a nonregular structure, and that a significant number of chain units, which connect different crystallites, must exist, in a non‐ordered conformation. It is concluded that lamella‐type crystallites in bulk‐crystallized polymers are not to be associated with an interface comprised of regularly folded chains. The assumptions that either an initiating or growth nucleus is comprised of regularly folded chains or that such a structure is an equilibrium requirement, are not substantiate