Barrier Capacity of Hydrophilic Polymer Brushes To Prevent Hydrophobic Interactions: Effect of Graft Density and Hydrophilicity

Abstract

The performance of biomaterials in contact with biological systems can be greatly affected by hydrophobic interactions at the interface between the biomaterial surface and surrounding biomolecules. Polymer brushes can function as a protective layer, preventing such interfacial hydrophobic interactions. In this paper, a systematic study of the barrier properties of a hydrophilic polymer brush is made by investigating the influence of graft density and its chemical nature (hydrophilicity/hydrophobicity) on hydrophobic interactions with the surface. To achieve this, a series of novel thermoresponsive poly-N-[(2,2-dimethyl-1,3-dioxolane)methyl]acrylamide (PDMDOMA) polymer brushes were grown from silicon wafers via surface-initiated atom transfer radical polymerization. Without changing graft density or degree of polymerization, the hydrophilicity of the PDMDOMA brushes was manipulated by partial or complete hydrolysis of the pendent dioxolane moieties. A lower critical solution temperature (LCST) was observed at 22−24 °C, below which the PDMDOMA brush was found to be in a hydrated state (amphiphilic), while at temperatures above the LCST, the PDMDOMA brush formed a collapsed, more hydrophobic structure. A physical method was developed to analyze the ability of these brushes to act as a barrier against hydrophobic interactions based on AFM force−distance measurements. The adhesive forces between the Si3N4 tip and the silicon wafer surface upon (a) modification with ATRP initiator, (b) grafting of PDMDOMA brushes, and (c) partial and complete hydrolysis of PDMDOMA were investigated. Hydrophobic interactions decreased after each modification, while graft density and the degree of hydrolysis increased the barrier function of the surface layer. In particular, when graft density was above 0.22 chains/nm2, the barrier capacity completely counteracted the hydrophobic interactions, as evidenced from the disappearance of the adhesive force in AFM measurements. Further studies revealed that the barrier property as assessed by AFM correlated well with the wettability of the surfaces.