Global Solution Properties of Poly(N-vinylimidazole) in Ethanol. Macromolecules and Aggregates

Poly(N-vinylimidazole) is a weakly basic linear polymer and is soluble in aqueous media and in various alcohols. The present study deals with the global solution properties in absolute ethanol (M w, R g, R h, A 2, and [η]). Samples were prepared by precipitating free radical polymerization in toluene or benzene. Common flexible chain behavior of single macromolecules was found for samples of molar mass M w < 8 × 105 g/mol. However, large aggregates were obtained when longer chains were prepared by reducing the initiator/monomer ratio in the system. At about the same molar mass a 3-fold larger radius was found which only weakly further increased with the corresponding molar mass of the aggregates. Similar breaks in behavior at the same point were found for the hydrodynamic radius, the second virial coefficient, and the intrinsic viscosity. Power law behavior was obtained in both regimes but with different exponents before and after the break point. In the low M w regime of molecularly dissolved chains the common scaling laws among the various exponents are approximately fulfilled. Above the break point the observed exponents strongly violated these scaling relationships, which indicated the aggregates as not self-similar to each other. This conclusion was confirmed by the behavior of the ρ = R g/R h parameter and the apparent segment density and is supported by the decrease of the second virial coefficient and the intrinsic viscosity compared to those of the molecularly dispersed molecules. The apparent density exhibited the expected decrease with M w for the molecules below the break point, but conversely, it increased for the aggregates and indicated an increasingly more dense packing of segments. The findings awoke the need of considering the dissolution mechanism that led to metastable aggregates, rather than the formation of associates under thermodynamic equilibrium conditions.