Synthesis of N-methylol polyamide 4: characterization, properties, and biodegradability

Polyamide 4 has attractive features: easy biodegradability in the natural environment and in vivo, potential production of monomers from biomass, excellent thermal and mechanical properties, and facile molecular design of various polymer structures utilizing a polymerization mechanism. With the aim of varying the physical properties of biodegradable polyamide 4, a series of N-methylol polyamide 4 with various degrees of methylolation were synthesized by using paraformaldehyde as a reagent. By varying the quantity of paraformaldehyde, it was possible to control the degree of methylolation. The purified products were hard or elastic solids that were white, translucent, or transparent. Depending on the degree of methylolation (mol%), the melting point of N-methylol polyamide 4 varied from 251.2 °C (4.5 mol%) to 164.1 °C (22.5 mol%) and finally disappeared (36.5 mol%). The heat of fusion decreased from 60.6 J g−1 (4.5 mol%) to 2.7 J g−1 (22.5 mol%) and eventually was not observed (36.5 mol%). There was no significant difference in thermal decomposition points, whereas polyamide 4 with a higher degree of methylolation exhibited a slightly higher thermal decomposition point. The tensile strength also depended on the degree of methylolation as in 29.6 MPa (4.5 mol%), 3.0 MPa (22.5 mol%), or 1.3 MPa (36.5 mol%). The elongation at break increased remarkably to 332% even at a low degree of methylolation such as 4.5 mol%, whereas there was no clear proportional relationship with the degree of methylolation. The presence of the methylol group on polyamide 4 main chains had a suppression effect on the biodegradation of polyamide 4, which was dependent on the degree of methylolation. The methylolation could control the high biodegradability of polyamide 4.