Microheterogeneous solutions of amphiphilic copolymers of N-isopropylacrylamide. An investigation via fluorescence methods

Abstract

Four copolymers of N-isopropylacrylamide (NIPAAM) with N-hexadecylacrylamide (HDAM) were prepared by radical copolymerization. Incorporation of more than 1.5 mol 5% of HDAM renders the copolymers insoluble in water at room temperature. The solution properties of NIPAAM copolymers containing 0.4-1.1 mol 5% HDAM were compared with those of the NIPAAM homopolymer (PNIPAAM). Cloud point and microcalorimetric measurements report lower critical solution temperatures (LCST) for the copolymers that are slightly depressed in comparison with the LCST of PNIPAAM. Fluorescence emission spectra were recorded for four probes (pyrene (11, 1-pyrenecarboxaldehyde (PyCHO, 2), 8-anilino-1-naphthalenesulfonic acid (ANS, 3), and 2-(N-dodecylamino)naphthalene-6-sulfonic acid (C_(12)NS, 4)) dissolved in aqueous solutions of PNIPAAM and of the NIPAAMIHDAM copolymers. Pyrene reports a decrease in polarity (inferred from an abrupt decrease in I_1/I_3) at the LCST of PNIPAAM but reports either no change or a modest increase in polarity in the copolymer solutions. Similar inferences may be drawn from the behavior of C_(12)NS. In contrast, PyCHO reports large polarity losses at the LCST in each of the polymer solutions. The behavior of ANS is intermediate, in that small decreases in polarity are reported at the LCST of each of the copolymers. These results are interpreted in terms of a micellar model for the amphiphilic copolymers in which a relatively nonpolar HDAM core is segregated at room temperature from a hydrated NIPAAM corona. Collapse of the corona at the LCST is accompanied by increased mixing of NIPAAM and HDAM units. The consequences of such mixing in terms of fluorescence emission spectra are strikingly dependent upon the micellar site of probe solubilization.