Abstract: The aggregation behaviour of styrene-vinyl benzoic acid (PS$_m$-b-PVBA$_n$) amphiphilic diblock copolymers in selective solvents with different m and n was investigated by synchrotron small-angle x-ray scattering (SAXS). We have carried out a detailed analysis of scattering intensity, dimension, shape and microstructure of the diblock copolymers of narrow distribution in water, methanol, ethanol and isopropanol selective solvents, respectively. We have found that the aggregation behaviour of the copolymer depends on the nature of the solvent and the micelle forms flat disc objects with the ratio of radius $\omega$=0.4. The average radius gyration R_g of the copolymer decreases as solvents change from isopropanol to ethanol and to methanol, and increases with increasing pH in aqueous solution, but decreases with the addition of CoCl$_2$ in ethanol solvent. The scattering intensity of diblock copolymer micelle follows $I(h)\varpropto h^{-\alpha}$ in different selective solutions, suggesting that the PS$_m$-b-PVBA$_n$ coils have self-similar structure behaviour or a fractal structure in the selective solvents. All of these revealed that the aggregation behaviour of the diblock copolymer changes dramatically with experimental condition in the selective solvent. The increase of mass fractal dimension ($D_m$) from 2.12 to 2.47 indicates that the copolymer chain changes from a swollen coil to a rather compact disc in the course of changing solvents, decreasing surface fractal dimension ($D_s$) from 2.98 to 2.58 indicates that the copolymer micelle change from a rather rough surface to a smooth form in the course of increasing pH in aqueous solutions, and increasing $D_m$ and $D_s$ from 2.29 to 2.35 and 2.70 to 2.90, respectively, indicates the shrinkage of copolymer micelle to a rather compact and rough disc form by adding CoCl$_2$ in ethanol solvents.

Key words: styrene-vinyl benzoic acid amphiphilic diblock copolymers, small-angle x-ray scattering, aggregation behaviour