Determination of the molar mass of polyamide block/graft copolymers by size-exclusion chromatography at room temperature

The molar mass of polyamides (PAs) can be measured by conventional size-exclusion chromatography (SEC) after they are subjected to the N-trifluoroacetylation reaction with trifluoroacetic anhydride (TFAA), leading to the corresponding N-trifluoroacetylated derivatives (PA-TFAA). This work was aimed at extending the above method to the measurement of the molar mass of polyamide block/graft copolymers and, more specifically, graft copolymers of polystyrene (PS) and polyamide 6 (PA6) with PS as the backbone and (PA6) as grafts, denoted as PS-g-PA6. Unlike a pure PA, the N-trifluoroacetylation reaction of PS-g-PA6 could not proceed in dichloromethane (CH2Cl2) alone. A co-solvent had to be used to help dissolve the PS segments of PS-g-PA6. Tetrahydrofuran (THF), toluene and chloroform, three common good solvents of PS, were tested as co-solvents. Only chloroform was suitable for the N-trifluoroacetylation reaction of PS-g-PA6. THF reacted with TFAA. The boiling point of toluene was so high that it could be removed from the copolymer only under vacuum and, at relatively high temperature, the N-N-trifluoroacetyled PS-g-PA6 could be converted back to the initial PS-g-PA6. Dual UV detection at two different wavelengths allowed determining both the molar mass and composition of PS-g-PA6.