Chemical Structure Dependence of Surface Layer Thickness on Polymer Films

The thickness of the surface layer on both poly(methyl methacrylate) (PMMA) and polystyrene (PS) films was investigated by monitoring the surface reorganization of fluorinated tracer-labeled PMMA and PS. Three different surface-sensitive techniques with various analytical depth capabilities were used, including contact angle (CA) measurements, sum-frequency generation (SFG) spectroscopy, and angle-resolved X-ray photo-electron spectroscopy (XPS). The results indicated that the surface layer consisted of a surface isoactive layer and a gradient layer and its thickness was related to the chemical structure of the polymers. The surface isoactive layer thickness (h(si)) was approximately 0.44 times the radius of gyration (R-g) for PMMA and approximately 0.55 R-g for PS. The gradient layer (h(g)) for PMMA was lower than that for the PS when the radius of gyration was the same. The relative thinner surface layer of PMMA may be attributed to its side-chain flexibility associated with chemical structure features. This should be the main reason why PMMA and PS showed dissimilar T-g -confinement effects although their bulk layers have similar physical properties.