Miscibility of polyamide-6 and polystyrene functionalized with sulfonic acid groups

Polyamide-6 (PA-6) and sulfonated polystyrene (SPS) interact strongly in solution. Thus, solution blends of PA-6 and SPS (sulfonation levels from 0 to 12.3 mol%) in m-cresol form clear, hazy, or gelled solutions–depending on the ratio of amide (A) to sulfonic acid (S) groups (A/S). NMR studies show that the S groups interact with the nitrogen of the amides, most probably through the transfer of the sulfonic acid protons. During melt blending, the addition of SPS to the PA-6 leads to a torque rise because of strong interactions. The glass transition temperatures of the components in the melt blended samples shift towards each other as the sulfonation level of the SPS increases, indicating enhanced miscibility. The analysis of the extracted PA-6 from thee blends, however, shows that it is degraded at processing temperatures (250°C), owing to some residual water and the catalytic activity of the sulfonic acid groups. The extent of the PA-6 degradation is inversely related to the A/S ratio. Each sulfonic acid group causes between one and two chain scissions of the PA-6. In blends with high A/S ratios, the degradation of the PA-6 is minimal; yet considerable dispersion of the SPS can be achieved, as evidenced by scanning electron micrographs of the fracture surfaces.