Adsorbed layer thickness of polycarboxylate and polyphosphate superplasticizers on polystyrene nanoparticles measured...

Common superplasticizers (dispersants) for cementitious materials such as concrete include carboxylated comb polymers which strongly chelate Ca²⁺ ions. Thus, a substrate holding Ca²⁺ as docking site on its surface and of a narrow particle size distribution was expected to produce reliable values for the adsorbed layer thickness (ALT) of polycarboxylate comb polymers, as measured via dynamic light scattering. ALT presents a key parameter which controls the dispersion power of these polymers and provides the steric hindrance effect between cement particles.

Using octadecyl phosphate as emulsifier, phosphate modified polystyrene nanoparticles (d_H(z)~86 nm) of narrow size distribution were successfully synthesized and characterized via scanning electron microscopy, dynamic light scattering (DLS) and zeta potential measurements. The particles allowed to measure concentration dependent adsorbed layer thicknesses of a series of structurally different, self-synthesized polycarboxylate superplasticizers.

The phosphate-modified polystyrene nanoparticles were found to present a suitable substrate for ALT measurements of adsorbed polycarboxylate comb polymers. DLS measurements revealed that with progressing adsorption, these polymers develop a concentration-dependent isothermal ALT. At saturation point, layer thicknesses of 3 to 8 nm were observed. The values correlate well with molecular properties of the polymers whereby increasing side-chain length, side-chain density and molecular weight provoke higher ALTs.