Adsorption of poly(ethylene oxide)—poly(propylene oxide) ABA block copolymers on carbon black and the rheology of the resulting dispersions

Abstract

The adsorption of ABA block copolymers, where A is poly(ethylene oxide) (PEO) and B is poly(propylene oxide) (PPO), on carbon black was investigated at room temperature (20 ± 2°C). Two series of block copolymers were studied: series A containing 56 PO units and 16–148 EO units per chain; series B containing 30 PO units and 30 or 77 EO units per chain. The results showed a decrease in the amount of adsorption Γ (μmol m −2) with increase in the ethylene oxide (EO) chain length. Adsorption increased as the number of propylene oxide (PO) units in the chain decreased. The area per molecule σ showed a linear increase with n 1 2 , where n is the number of EO units, indicating that adsorption is governed by the size of the PEO chain. Concentrated dispersions of carbon black that were stabilised using the block copolymers were studied using steady state (shear stress—shear rate) and oscillatory measurements. The relative viscosity—volume fraction φ curves were compared with the theoretical curves for hard-sphere dispersions (calculated according to the Dougherty—Krieger equation) to obtain the adsorbed layer thickness as a function of φ. Storage modulus G′-φ curves showed a rapid increase above a critical volume fraction φ cr for the carbon dispersions. With the ABA block copolymer containing the shortest EO chain (16 EO units per chain) φ cr was significantly lower than the values obtained with the other block copolymers, indicating weak flocculation of the suspension. All other block copolymers stabilised the carbon dispersions, and φ cr decreased with increase in the EO chain length, as expected. The critical flocculation temperature (CFT) of carbon dispersions, stabilised using a block copolymer containing 56 PO units and 37 and 148 EO units per chain, was measured as a function of K 2SO 4 concentration using rheological measurements. The results showed that for both polymers the CFT is lower than the θ temperature of the PEO chain at the same electrolyte concentration. This indicated that the block copolymers were not as effective for stabilisation of carbon black dispersions as the previously studied ABC surfactants containing the nonylphenyl group in addition to the PPO chain, indicating that the nonylphenyl group plays a major role in anchoring the chain to the carbon surface.