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Adhesion Force Analysis of Dynamic Polymer Brushes.

Spontaneous surface segregation of amphiphilic diblock copolymers at the water interface from the elastomeric portion was utilized for fabrication of hydrophilic brushes, and named a "dynamic polymer brush." Observation of dynamic polymer brushes appears only when immersed in water and demands advanced experimental techniques for embedded interfaces such as neutron reflectivity. Measurement of the hydrophobic interaction at the polymer/water interface is not only an alternative method to monitor the brush but also reveals its unique surface properties. We carried out adhesion force measurements using atomic force microscopy with a hydrophobic probe for measuring the hydrophobic interactions of dynamic polymer brushes in water. Dynamic polymer brushes showed reduced hydrophobic interaction, which becomes more significant with higher graft density. Moreover, unique transitional response to the applied pressure was observed for the dynamic polymer brush: the adhesion force was almost zero at low applied pressure and increased by further increasing applied pressure. This phenomenon may indicate reallocation or retraction of the block copolymer chains from the contact area by applied pressure, which are the unique characteristics of non-bound dynamic polymer brush chains. We also conducted adhesion force imaging and proved that dynamic polymer brushes form uniform layers without any defects, irrespective of brush density, which suggest the interaction between the dynamic polymer brush chains is repulsive.