Reversible and dynamical control of aggregation and soft adhesion of T-responsive polymer-coated colloids A Physicochemical and engineering aspects

Colloids with aggregation and adhesion properties reversibly tunable by shift of pH, T, light etc. can be designed by deposition of stimuli-responsive polymer chains on the particle surface. The aim of this work was to investigate how to control the strength of temperature-triggered attraction by analysing self-aggregation kinetics and soft adhesion of colloids to a flat substrate. In order to endow the colloids with reversible and temperature-controlled interactions, silica or polystyrene microbeads (d=200nm and 1μm) were coated by mixed solutions of poly(lysine)-grafted-polyethylenoxide (PLL-g-PEG, for steric repulsion) and PLL-g-PNIPAM (i.e. PLL with poly-N-isopropylacrylamide T-responsive side chains). PEG-coated particles were stable in suspension, while the presence of PNIPAM provoked, at T>Tc=32±1°C, reversible aggregation and/or adsorption on glass plates. Dynamic light scattering following a T-jump from 25°C to 40°C was used to measure the aggregation rate and corresponding stability ratio W. For a molar fraction of PLL-g-PNIPAM, f, ranging from 100% down to about 20%, particles aggregate rapidly with slowly increasing W. Below f≈20%, W increases by 3 orders of magnitude. The real-time 2D tracking method was used to monitor (x, y) positions of particles in suspension above microscope glass slides during a T-triggered adsorption. In order to capture transitory dynamics near PNIPAM collapse transition, particles tracks were recorded within a T-ramp of 10°C/min from below to above Tc. The particle-substrate interaction was found to hinder the near-wall diffusion and provoke the soft adhesion, as revealed by observation of characteristic confined Brownian motion. Resulting confinement potential stiffness profile α(f) presents a crossover from constant to linearly increasing at f≈20%. Altogether, the characteristic coverage f*≈20% is interpreted as a crossover from discrete to continuous coverage pattern within the soft contact domain.