Characterization of arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers

In this dissertation, Small-angle neutron scattering was used to characterize the structure of arborescent polystyrene-graft -poly(2-vinylpyridine) copolymers dissolved in deuterated methanol. The small-angle neutron scattering data for all generations of molecules were fit with two different models to examine the radial density profile of the molecules in solution. Although a core-shell model fit the small-angle scattering data reasonably well, discrepancies with size and molecular weight obtained were found to be significant. This discrepancy is eliminated by using a power law model which has a continuous radial density profile. The fitting results show no evidence of a distinct core-shell morphology for all generations of arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers in deuterated methanol. The radius of gyration of the polymers increases with the molecular weight according to the scaling relationship, R _g ∼ M_wν with ν = 0.24 ± 0.04.; Small-angle neutron and dynamic light scattering were performed on arborescent graft generation 0 polystyrene-graft-poly(2-vinylpyridine)(G0PS-P2VP) in acidic deuterated methanol for polymer concentrations in the range 0.5–3% (mass %). Dynamic light scattering measurement shows two diffusive relaxations on addition of HCl to the polymer in deuterated methanol. The slow mode obtained indicates that aggregates are formed from at least some of the arborescent polymers and the size of these aggregates increases as a function of polymer concentration. Small-angle neutron scattering curves show a peak upon the addition of HCl indicating liquid-like ordering of charged polymers which coexist with a small amount of aggregates containing a few molecules. Solutions of generation 0 polystyrene - poly(2-vinylpyridine) copolymers with higher molecular weight poly(2-vinylpyridine) chains (M_w ∼ 30,000 g/mole) formed a gel when HCl was added. Scattering curves from gel samples exhibit higher order reflections indicating that the gel forms an ordered structure. The gel could be transformed back to a solution by adding excess HCl which screened the electrostatic interactions. Poly(2-vinylpyridine) chains can be grafted onto larger molecular substrate, generation 1 and 2 polystyrene (G1PS and G2PS). These larger arborescent graft copolymer solutions do not form a gel or show polyelectrolyte behaviors in either SANS and DLS measurements on addition of HCl. The polyelectrolyte behavior of these arborescent graft polymers as a function of generation number is in agreement with other work which shows that sufficiently large branched polyelectrolytes retain most of their counterions in the intramolecular region and their electrostatic repulsions are strongly screened.