Nanomechanical Properties of Switchable Binary Polymer Brush Films

Introduction The conformation of a polymer chain is very sensitive to changes in the surrounding environment. When polymer chains are chemically tethered at one end to a stiff solid substrate or interface with a sufficiently high grafting density, the chains avoid overlapping and stretch along the surface normal to form a polymer brush layer. Thus, substrates and interfaces can be modified with thin (5 – 100 nm) highly dense polymer layers that have the ability to adapt, or exhibit switching of surface morphologies according to changes in their surrounding medium, such as solvent quality or pH. Such a surface is of great interest because versatile microsystems are now being designed that integrate mechanical, optical, fluidic, chemical gating, and biological devices to perform complex sensing functions. Such systems require “smart surfaces”, or surfaces that have the ability to respond to external stimulus. In the case of two-component or binary polymer brush layers, the variety of surface morphologies possible greatly increases depending upon the chemistry used. If the two polymers are incompatible, each chain will interact very differently with the surroundings, and switching of phase segregations and morphologies can result. Surface composition and hence properties such as surface energy, adhesion, friction, and wettability have the possibility of being “tuned” to the necessary state. Here, we report results of fabricating dense homopolymer brush layers as well as adaptive binary brushes that demonstrate complete reversible switching of states upon exposure to different solvents. Furthermore, very little experimental work has been carried out to determine physical properties of brushes. We address this issue by obtaining direct experimental results of the elastic and thermal properties, and measuring the thermo-elastic response of thin polymer brush layers.