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.