Two-Dimensional Radial or Ring-Banded Nonbirefringent Spherulites of Semifluorinated Alkanes Coexistent with Close-Packed Self-Assembled Surface Nanodomains.

Abstract

A series of semifluorinated alkanes (C nF2 n+1C mH2 m+1 diblocks, F n H m, n = 6, 8, 10; m = 16, 18, 20), when cast as films onto solid substrates, were found to form ring-banded or radial spherulites when heated above their isotropic temperature and subsequently cooled down to room temperature, demonstrating that the formation of two-dimensional (2D) spherulites is a general feature of molecular fluorocarbon-hydrocarbon diblocks. These spherulites are not birefringent, a seldom encountered feature for such structures (never, so far, for spherulites made of small molecules). They also provide examples of fluorinated 2D spherulites. Film morphology was analyzed by optical microscopy, interferometric profilometry, atomic force microscopy (AFM), and scanning electron microscopy. Increasing the length of the Fn segment favors the formation of ring-banded spherulites, whereas short Fn segments tend to favor extended radial stripes. Variation of the cooling rate provides control over the size and morphology of the spherulites: slow cooling promotes fibers and radial spherulites, whereas fast cooling fosters ring-banded spherulites. The AFM studies of F10 H16 films revealed that the latter consist of stacks of regularly spaced lamellae. We also observed that, remarkably, stacked lamellae (repeating distance ∼6 nm) can coexist with a layer of close-packed monodisperse circular self-assembled surface nanodomains of Fn Hm diblocks (∼30 nm in diameter); the latter are known to form from such diblocks at interfaces at room temperature. Substrates partially covered with F10 H16 contain incomplete ring-banded spherulites and smaller objects in which the lamellae and circular nanodomains coexist.