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Reversible and dynamical control of aggregation and soft adhesion of T-responsive polymer-coated colloids

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=200 nm and 1 mu m) 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 > T-c = 32 +/- 1 degrees C, reversible aggregation and/or adsorption on glass plates. Dynamic light scattering following a T-jump from 25 degrees C to 40 degrees 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. Belowf approximate to 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 Tramp of 10 degrees C/min from below to above T-c. 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 alpha(f) presents a crossover from constant to linearly increasing at f approximate to 20%. Altogether, the characteristic coverage f* approximate to 20% is interpreted as a crossover from discrete to continuous coverage pattern within the soft contact domain.

soft adhesion colloids

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