Sustainable polyamide elastomers from a bio-based dimer diamine for fabricating highly expanded and facilely recyclable microcellular foams via supercritical CO₂ foaming

In this study, we developed a sustainable polyamide foam system. A new series of polyamide elastomers was synthesized using a bio-based dimer diamine monomer as a building block to copolymerize with aminocaproic acid and adipic acid through step-growth polymerization, followed by supercritical CO₂ foaming for the fabrication of microcellular foams. The introduction of the branched-structured dimer diamine monomer effectively restricted the molecular packing and hydrogen bonding density of the polyamide system, leading to numerous small crystals acting as nucleation sites for heterogeneous cell nucleation. Simultaneously, the branched long chains of the dimer diamine formed chain entanglement networks, which increased the material melt strength against the elongational stresses of cell growth. The synergistic effect provided by dimer diamine led to the successful manufacture of foamed samples with a high expansion ratio and robust dimensional stability, effectively overcoming the foaming difficulties of high crystallinity and insufficient melt strength in traditional polyamide 6 (PA6). In addition, this polyamide elastomer foam system could be recycled and reprocessed due to the advantages of non-crosslinking and additive-free. Therefore, the study's findings constitute a promising direction for the usage of renewable sources in the development of foamable polyamide materials for sustainable applications in the plastics industry.