Experimental evidence on the effect of substrate roughness on segmental dynamics of confined polymer films

Understanding the properties of thin films remains an unresolved issue even after decades of scientific research. In this article, we investigate the role of surface roughness, one of the various factors that affect the dynamics of polymer films. The tested sample is poly-(4-chlorostyrene) coated onto silicon substrates treated with plasma for different amounts of time. By using dielectric spectroscopy, we provide experimental evidence that as substrate roughness increases (i) the segmental mobility slows down, and (ii) the distribution of the α-relaxation time becomes narrower. Ultimately, the confinement effect vanishes, and the polymer film recovers its bulk-like behavior for sufficiently rough surfaces. Further studies that employ AFM, ellipsometry, and contact angle measurements allowed us to conclude that the deviation in segmental dynamics as a function of surface roughness is due to the changes in interfacial energy that affects the number of chains irreversibly absorbed to the substrate which in turn improve packing density near the substrate. Therefore, the surface roughness is an inevitable factor to consider when designing numerous devices which rely on supported polymer films.