We report the morphology evolution under tensile deformation for strictly linear polyethylenes with associating functional groups. In situ X-ray scattering measurements during elongation reveal that periodic acid group placement along the backbone is required to form hierarchical layered morphologies that lead to strain hardening. This phenomenon was observed in both semicrystalline and amorphous precise acid polyethylenes with acrylic, geminal acrylic, and phosphonic acids. Polymers with nonperiodic (pseudorandom) acid placement fail to form layered morphologies and instead retain a liquidlike distribution of acid aggregates. Acid chemistry and acid concentration influence morphological evolution in both periodic and nonperiodic polymers predominately through the modification of T-g and percent crystallinity, which subsequently impact the mechanical properties. Our results indicate that hierarchical acid-rich layered structures, commensurate with improved mechanical properties, form in polymers with strictly periodic chemical structures and sufficient chain mobility for chain alignment during elongation.