| The present study has focused on in-depth probing of the interfacial molecular structures of two related systems: self-assembled monolayers (SAMs) and thin polymeric films. We have studied the growth mechanism of octadecyltrichlorosilane (OTS) SAMs by probing the alkyl chain conformation using surface-sensitive techniques. Results show that the alkyl chain conformation of OTS monolayers evolves in three stages as monolayer formation progresses. Initially, OTS molecules adsorb from the solution and the surface coverages increase rapidly, yet the alkyl chains within the OTS film still contain disordered gauche conformations. The monolayers then experience a dramatic change from disordered to well-ordered with the alkyl chains adopting an all-trans conformation. Finally, a closely packed OTS monolayer is formed. By correlating sum-frequency (SF) spectroscopy, contact angles, and lateral force microscopy, we found that the adsorption of OTS on fused silica surfaces has two competing growth mechanisms, island growth and uniform growth, and a small amount of water in the deposition solution favors island growth.; We have also studied the molecular orientation of polymer functional groups at the polymer-air, polymer-liquid and polymer-polymer interface. For the first time, we detected SF signals from a polymer-polymer interface. The ester methyl groups on poly(methyl methacrylate) (PMMA) were found to be oriented at the PMMA-polystyrene (PS) interface. In addition to the molecular orientation information that is usually obtained in SF spectroscopy, we have also demonstrated that SF spectroscopy can provide information about the interfacial structure of the polymer blends. From the SF spectra of annealed PS/PMMA blends, their interfacial structure is shown to be laterally discontinuous. Our work in this field opens up a new way to study multicomponent polymeric systems such as polymer blends, copolymers and other binary systems. |
