Reorganization of an Amphiphilic Glassy Polymer Surface in Contact with Water Probed by Contact Angle and Sum Frequency Generation Spectroscopy

We address the question of how a surface of a glassy polymer reorganizes after coming in contact with water. Because contact angle hysteresis measurements are also affected by surface roughness and chemical heterogeneity, we have used surface-sensitive sum frequency generation spectroscopy (SFG) in conjunction with water contact angles to answer this question. To increase the magnitude of the surface reorganization, we have designed an amphiphilic polymer, poly(alpha-hydroxymethyl-n-butyl acrylate) (PHNB), to study the changes in the structure of polar hydroxy groups and nonpolar (methyl and methylene) groups at the interface. The SFG and the water contact angles show that reorganization does occur for PHNB below T-g. However, complete reorganization requires heating the sample above the bulk T-g. These heating experiments were conducted by first heating the sample in the presence of water and then followed by cooling the sample to room temperature in the presence of water to lock the changes in the surface structure (we refer to this treatment as water annealing). The polar contribution to the total surface energy of PHNB, determined by Owens-Wendt-Rabel-Kaelble (OWRK) method at room temperature, increases after water annealing above T-g. This is consistent with our SFG results that show an increase in concentration of polar hydroxy groups at room temperature after water annealing the PHNB film above T-g. For PHNB, the contact angle hysteresis is higher for samples that are water annealed above T-g. This is consistent with the surface energy and SFG results. For a low-T-g polymer, poly(n-butyl acrylate), which has the same nonpolar side group but lacks the hydroxyl group, surface reorganization takes place immediately after contact with water, and these changes are reversible.