Effect of Chain Architecture and Surface Energies on the Ordering Behavior of Lamellar and Cylinder Forming Block Copolymers

• Published in: Macromolecules 2006-12, Vol.39 (26), p.9346-9356
• Main Authors: Khanna, V, Cochran, E. W, Hexemer, A, Stein, G. E, Fredrickson, G. H, Kramer, E. J, Li, X, Wang, J, Hahn, S. F
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma0609228

We investigate the effect of surface energy and chain architecture on the orientation of microdomains in relatively thick films (600−800 nm) of lamellar and cylindrical block copolymers of poly(cyclohexylethylene) (C) and poly(ethylene) (E). The E block has 26 ethyl branches per 1000 backbone carbon atoms. Melt surface energies of the C and E blocks are 22.3 and 20.9 mJ/m2, respectively. Grazing-incidence small-angle X-ray scattering (GISAXS), scanning force microscopy (SFM), and cross-sectional transmission electron microscopy (TEM) show that cylindrical and lamellar CEC triblock copolymers orient their microdomains normal to the surface throughout the film thickness. However, a lamellar CE diblock copolymer prefers a parallel orientation of the lamellae relative to the surface with an E surface layer. Moreover, a cylindrical CEBC triblock copolymer where the EB block has 125 ethyl branches per 1000 backbone carbon atoms leads to EB cylinder domains that always orient parallel to the surface. In this case the lower surface energy EB block dominates the surface. Calculations using self-consistent-field theory allow us to interpret the experimental results in terms of the entropic cost of forming a wetting layer comprised entirely of looping blocks. Thus, in triblock copolymers, parallel orientations are only stabilized when the midblock has the lower surface energy, and the difference in surface energies of the two blocks is large enough to compensate for this conformational penalty, which is absent in diblock copolymers.