ABSTRACT
A highly dense polymer brush was previously fabricated by the spontaneous segregation of amphiphilic diblock copolymers in an elastomer matrix into water and a hydrophobic polymer interface and named a 'dynamic polymer brush'. We fabricated a lower critical solution temperature (LCST)-type thermoresponsive dynamic polymer brush by mixing polyisoprene-b-poly[tri(ethylene glycol)methyl ether methacrylate] (PI-b-PME3MA) into a polystyrene-b-polyisoprene-b-polystyrene (SIS) elastomer. The LCST of PME3MA in water is 52 °C. The structure of the polymer brush was determined at several different temperatures using neutron reflectivity. With increasing temperature, the brush thickness of the LCST-type thermoresponsive dynamic polymer brush decreases, similar to the conventional fixed brush with the LCST-type thermoresponse. However, the graft density of the dynamic polymer brush surprisingly increases with increasing temperature. The change of the brush density of the conventional fixed polymer brush is not allowed. However, we observed for the first time that dynamic polymer brushes uniquely respond to increasing temperature with increasing brush densities.
ABSTRACT
Polymer brush formation kinetics was measured by quartz crystal microbalance (QCM). In the QCM measurement anomalous complex frequency shift as a function of graft density was observed: dissipation rate shift ΔΓ first increased, showed a peak, and then decreased despite the graft density of the polymer brush was increasing monotonically. We calculated the shear modulus of the brush layer from the measured complex frequency shift and revealed that the peak of ΔΓ described the crossover from a viscous brush layer to elastic brush layer. The crossover for the poly(ethylene glycol) with a molecular weight of 2000 occurs at around the characteristic graft density of 0.17 chains/nm2 which was revealed from the structure analysis by surface plasmon resonance (SPR) and neutron reflectivity (NR).
