ORIGINAL CONTRIBUTION Morphological regimes of poly(ε-caprolactone)/octaisobutyl polyhedral oligosilsesquioxane composite films in relation to film composition and thickness Katie Greenman & Adam Bauer & Daniel Kool & Jianzhao Liu & Bingbing Li Received: 11 January 2014 /Revised: 29 March 2014 /Accepted: 8 May 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract The morphological evolution of poly( ε - caprolactone) (PCL)/octaisobutyl polyhedral oligomeric silsesquioxane (IBUPOSS) films was analyzed using scan- ning electron microscopy (SEM) and polarized optical mi- croscopy (POM). The morphologies of the blend films with PCL/IBUPOSS mass ratios of 95:5 to 50:50 were discussed according to decomposition mechanism in relation to film composition and thickness. In addition to the morphological regime for films with lower IBUPOSS loadings, in which the growth of PCL spherulites was nearly independent on the presence of fine IBUPOSS aggregates, two new morpholog- ical regimes were observed for the films with higher IBUPOSS loadings: (1) thicker blend films exhibited a rich dynamics, giving rise to a trilayer structure and (2) the de- composition of thinner films was induced by the kinetically controlled growth of IBUPOSS aggregates. By varying the thickness and the composition of the blend films, the current study provides important new insight into the rich phase behavior of nanoparticle-filled polymer films. Keywords Poly(ε-caprolactone) . Composite films . Morphology . Scanning electron microscopy Introduction The morphological evolution of polymer blend films highly depends on the stability of the film/substrate interface and the air/film interface, as well as the molecular interactions within a system. The stability of the film/substrate interface is deter- mined by the compatibility of surface energies between the substrate and each component in the blends. Such surface energy compatibility also determines the initial composition of the wetting layer at the film/substrate interface. The effect of the substrate surface becomes much more pronounced when the film thickness is decreased to 100 nm or below, resulting in the confinement effect that can significantly alter the thermo- dynamic and kinetic behavior of thin film blends [1–7]. On the other hand, while generally of less concern during experiments, apart from the necessity of consistent and appro- priate environmental conditions, the air/film interface plays a significant role in determining the composition of the top surface layer. The surface composition of a blend film tends to differ from its bulk blend because the components of lower solid-state surface energy can segregate onto the air/film in- terface to minimize system energy [2, 8–12]. The subsequent formation of the enriched top layer can occur either through the diffusion of the low surface energy molecules or via hydrodynamic flow through channels in the other component (the non-wet phase), leaving a contiguous or percolated de- pletion middle layer [1]. The thickness of the top layer there- fore depends on the kinetics of either (slower) diffusion or (faster) hydrodynamic flow of the surface preferred compo- nent. Thus, the morphological evolution of polymer blend films undergoing phase separation and wetting/dewetting strongly depends on the depth profile of each component [10–13] and its fluctuation induced by thermal annealing, solvent annealing, etc. [14–16]. When the surface energy of one component is significantly lower than that of another component in the blends, the phase separation can occur vertically, resulting in a bilayer structure consisting of a bottom layer and a dewetted top layer [12, 17, 18]. For instance, a study on the spin-coated polystyrene (PS)/ K. Greenman : A. Bauer : D. Kool : J. Liu : B. Li (*) Department of Chemistry, Science of Advanced Materials Doctoral Program, Central Michigan University, Mount Pleasant, MI 48859, USA e-mail: li3b@cmich.edu Colloid Polym Sci DOI 10.1007/s00396-014-3253-5