This manuscript describes the microstructure of a series of nearly monodisperse poly(n-butyl) acrylate (PnBA) chains center-functionalized with a triurea interacting moiety, able to self-associate by six hydrogen bonds. Different molecular weights have been investigated, from 5000 g.mol(-1) up to 80 000 g.mol(-1). For molecular weights (M-n) below 40 000 g.mol(-1), X-ray scattering experiments and atomic force microscopy at ambient temperature clearly show that the systems organize as nanofibers hexagonally packed in oriented domains. This supramolecular structure explains the solid-like gel behavior of these polymers, which is suppressed at high temperature (at an order disorder transition temperature). For higher molecular weights, nanofibers still form at ambient temperature but their concentration is too low to self-assemble in oriented domains. This is consistent with the reported viscoelastic behavior of these systems described in the companion paper.