AbstractA theory is presented explaining the crystallization of polymers into thin and flat crystals or lamellae. Following the physical cluster theory, the internal and surface free energy of the “amorphous” macromolecular domain is described. Crystallization proceeds through an internal ordering of the domain, the attachment of the partially ordered domain onto a growth face, and, finally, regularization and domain deformation leading to minimization of both internal and surface free energy of the attached domain and its immediate neighborhood. The resultant fold surfaces comprise adjoining intact or deformed domes composed from proximal‐reentry loops. Although arising from an adaptation of the physical cluster theory to describe the macromolecular domain, the free‐energy equations controlling the crystallization process are essentially the same as those describing the primary nucleation in the prevailing crystallization theories. Segregation according to molecular weight is shown to arise from the size‐dependent ability of the domains to improve their internal ordering. Lamellar thickening is shown to follow a critical‐exponent equation dependent on surface free energy. Experimental data from the literature, corroborating the model, ar
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