Interaction of acrylic diblock copolymers with aluminium oxide surfaces

Abstract

Amphiphilic diblock copolymers with methacrylic acid blocks (A) for anchoring to hydrophilic surfaces and with either methyl methacrylate or n-butyl methacrylate blocks (B) as hydrophobic stabilizers are highly efficient dispersants for oxide and nitride ceramic powders in organic media. In a series of copolymers with number-average molecular weight M n ranging from 4000 to 135 000, the block length ratios (A/B) have been varied systematically from 0.07 to 2, which correspond to values for the asymmetry ratio β as defined by Marques and Joanny (Macromolecules, 22 (1989) 1454) between 4.93 and 0.66. The influence of these parameters as well as that of the solvent quality on the interaction of the copolymers with α-Al 2O 3 have been studied by adsorption experiments and surface plasmon resonance. Owing to the strong interaction between the A blocks with the alumina surface, the diblock copolymers are adsorbed spontaneously and irreversibly. Generally, the adsorbed mass increases with the molecular weight of the adsorbed polymer; however, some differences between samples with asymmetric block lengths (β > 1 ) and with symmetric or moderately asymmetric block lengths (β ⩽ 1 ) have been observed. The adsorbate layer thickness L of samples with β > 1 as well as with β < /1 increases with the degree of polymerization of the B blocks. In all cases, L can be scaled with the radius of gyration of undisturbed coils formed by the B blocks in relatively good solvents. Thus, the structure of the adsorbed diblock copolymers appears to be “mushroom-like” rather than “brush-like”. With increasing “quality” of solvent, stabilizer coil expansion and stretching of the macromolecules perpendicular to the surface have been observed.