Thermally induced gelation of poly(acrylamide) grafted with poly(N-isopropylacrylamide):: A small-angle neutron scattering study

Sieving matrixes of poly(acrylamide) grafted with poly(N-isopropylacrylamide) (PNIPAM) were developed in the framework of DNA sequencing by capillary electrophoresis. As a result of their thermodynamic properties, PNIPAM side chains self-aggregate above a critical temperature, leading to strong thermo-thickening effects in semidilute solutions. We report here a study of the transient network formation, carried out in D(2)O by small-angle neutron scattering on the basis of a large set of responsive copolymers tailored with PNIPAM grafts of different sizes (M(w) = 10 000, 20 000, and 35 000 g/mol) and various graft densities (5, 7, 10, 14, and 18% w/w). For most of the aqueous solutions, a correlation peak is observed at finite scattering vectors when the temperature is increased above a critical value. The intensity of the scattering peak rises with the size of the PNIPAM stickers, the graft density, the concentration, and the temperature. Experimental data are fitted using a core-corona model, assuming that only part of PNIPAM grafts (f(PNIPAM), mass fraction) participate in the aggregation process (dry micelles). Under these assumptions, the five-parameter model allows a realistic description of the thermally induced association with core sizes ranging between 70 and 100 A and f(PNIPAM) varying between 18 and 50%, depending on the primary structure of the copolymers. Comparison between matrixes confirms that physical networks based on responsive aggregation are good candidates for DNA sequencing even if they display antagonistic properties arising from (1) an increase of the resolution due to the matrix gelation (especially for large DNA fragments) and (2) a dispersion of DNA fragments during separation due to specific interactions between hydrophobic domains and DNA, which results in a loss of resolution.