Self-Assembly of Amphiphilic Statistical Copolymers and Their Aqueous Rheological Properties

A range of poly(n-butyl methacrylate-stat-methacrylic acid) [P(BMA-stat-MAA)] statistical copolymers of various compositions and molecular weights ranging from 5 to 30 kDa were prepared using either reversible addition-fragmentation chain transfer (RAFT) solution copolymerization or conventional free radical polymerization in isopropanol (IPA). On dilution with water, these amphiphilic copolymers self-assembled to form spherical nano-objects as confirmed by small-angle X-ray scattering (SAXS) and transmission electron microscopy. Various structural models were examined to extract information regarding the mean nano-object size and morphology. It is found that nano-object radii are independent of copolymer molecular weight but depend on the copolymer composition: the smaller the amount of MAA units in the copolymer chains, the larger the nano-objects that are formed. Combined SAXS and aqueous electrophoretic measurements indicated that most of the MAA units are located at the nano-object surface. Furthermore, SAXS and rheology measurements were used to monitor the effect of solvent composition on the copolymer morphology both at a fixed copolymer concentration (either 1 or 25 wt %) and also for a gradual variation in copolymer concentrations (from 1 to 40 wt %) when adding water to the initial copolymer solution in IPA. These studies revealed that the copolymers are present in solution as molecularly dissolved Gaussian chains when the solvent composition is IPA-rich. However, the copolymer chains self-assemble into spherical nano-objects when the solvent composition is water-rich. At intermediate solvent compositions, SAXS analysis confirmed the formation of an interconnected nano-object network, which accounts for the apparently anomalous increase in solution viscosity on dilution indicated by rheology measurements.