Purity of double hydrophilic block copolymers revealed by capillary electrophoresis in the critical conditions.

Block copolymers enable combining properties of different polymers; double hydrophilic block copolymers are innovative examples. Size-exclusion chromatography (SEC or GPC) has a quasi-monopoly in separation-based characterization methods for polymers, including block copolymers. However, in terms of purity determination (unintended homopolymers present in the copolymers), SEC resolution proves insufficient except for the extreme compositions for which the second block is much larger than the first one. The free solution capillary electrophoresis (capillary zone electrophoresis) technique does not separate charged homopolymers by their molar mass and we thus named the corresponding method capillary electrophoresis in the critical condition (CE-CC). CE-CC provides a means to assess the purity of poly(acrylic acid-b-acrylamide) - P(AA-b-AM) - copolymers, as well as of the more challenging cationic poly(acrylamido-N-propyltrimethylammonium chloride-b-N-isopropylacrylamide)-P(APTAC-b-NIPAM). In addition it can identify that a block copolymer has been produced. It is to be noted that P(APTAC-b-NIPAM) block copolymers cannot be eluted in SEC due to their exceptional ability to adsorb onto surfaces, while some information is obtained from CE-CC. Both possible parent homopolymers can be detected and their quantity estimated in a single injection by CE-CC. In both cases, one of the parent homopolymers is neutral and comes with the electro-osmotic flow. If the electro-osmotic flow is weak (conditions used for the cationic copolymer) then pressure assisted CE-CC is used to detect this homopolymer.