Precise Synthesis and Physicochemical Properties of High-Density Polymer Brushes designed with Poly(N-isopropylacrylamide)

A high-density polymer brush of poly(N-isopropylacrylamide) (PNIPA) was precisely prepared following carefully selected procedures, which included selecting the underlying substrate, preparing its surface, and grafting PNIPA on the substrate. As a result, the graft density and the dried thickness of the brush reached more than 0.5 chains/nm(2) and 200 nm, respectively, for the best combination of each procedure. This high-density polymer brush showed gradual collapse with increasing temperature in water, which must be attributed to both the low swelling and the low shrinking abilities of the brush that result from the physically constrained state of the polymers. The contact angle of the air bubbles underneath the high-density polymer brush also gradually decreased up to around 25 degrees C in water with increasing temperature, which indicates that the hydrophilicity of the surface decreases as it does in typical PNIPA-grafted membranes and gels. Starting at the lower critical solution temperature of free PNIPA in water, approximately 32 degrees C, the value of the contact angle started to increase dramatically, and it became constant when the solution temperature exceeded 40 degrees C. Ultimately, the surface exhibited a mostly hydrophilic nature at higher temperatures. The temperature-dependent contact angles can be interpreted by assuming that the terminally chlorinated alkyl groups of the elongated PNIPAs can be positioned on the surfaces or hidden in the vicinity of the membranes, depending on the temperature of the solution.