Impact of 2‐Methylpentamethylenediamine and Isophthalic Acid Copolymer Units on Hydrogen Bonding and Stress Relaxation in Polyamide 66

ABSTRACT The regulatory mechanisms by which hydrogen bonding interactions influence long‐term mechanical behavior have been rarely studied. This research investigates the effects of 2‐methylpentamethylenediamine (2‐MPMD) and isophthalic acid (IA) copolymer units on the hydrogen bonding, crystallization, and stress relaxation properties of polyamide 66 (PA66). Using thermal analysis, x‐ray diffraction, and variable temperature infrared spectroscopy, we demonstrate that these copolymer units within PA66 disrupt the amide sequences, hindering hydrogen bond formation and the alignment of polymer segments, resulting in reduced crystallinity and increased crystal plane spacing. Using a custom stress relaxation measurement device and an enhanced viscoelastic model, we observed that the stress relaxation time of the modified PA66 was significantly reduced, particularly the τ 1 in the early stages, with a reduction of 70.5% compared to neat PA66. Additionally, finite element simulations showed good agreement with experimentally measured force values during stress relaxation, with deviations below 5% at selected relaxation stages. This study elucidates the complex interplay between hydrogen bonding and polymer crystallization, providing a framework for predicting stress relaxation characteristics in materials.