Formation of Tubular Scrolls with Controlled Internal Cavity

Self-rolled nanotubes with controlled hollow interiors by patterned grafts

By patterning surface grafts, we propose a simple and systematic method to form tubular structures for which two-dimensional grafted sheets are programmed to self-roll into hollow tubes with a desired size of the internal cavity. The repeating pattern of grafts utilizing defect sites causes anisotropy in the surface-grafted nanosheet, which spontaneously transforms into a curved secondary architecture and, thus, becomes a potential tool with which to form and control the curvature of nanotubes. In fact, the degree and the type of graft defect allow control of the internal cavity size and shape of the resulting nanotubes. By performing dissipative particle dynamics simulations on coarse-grained sheets, we found that the inner cavity size is inversely proportional to the graft-defect density, the difference in the graft densities between the two surface sides of the layer, regardless of whether the defects are patterned or random. While a random distribution of defects gives rise to a non-uniform local curvature and often leads to twisted tubes, regular patterns of graft defects ensure uniform local curvature throughout the sheet, which is important to generate monodisperse nanotubes. At a low graft-defect density, the sheet-to-tube transformation is governed by the layer anisotropy, which induces spontaneous scrolling along the long edge of the sheet, resulting in short tubes. Thus, the curve formation rate and the cavity diameter are independent of the pattern of the graft defects. At a high graft-defect density, however, the scroll direction owing to the graft pattern may conflict with that due to the layer anisotropy. To produce monodisperse nanotubes, two factors are important: (1) a graft-defect pattern parallel to the short edge of the layer, and (2) a graft-defect area wider than half of the graft coil length.

Surface graft configuration dependency of the morphologies of heterosurface sheet polymers

Using dissipative particle dynamics, we investigated the graft configuration-dependent scroll formation of sheet polymers and their morphologies. Two types of coarse-grained graft disorder models were considered at various displaced tether fractions. Although tether coils were identical, sheet anisotropy arose from discrepancies in graft configurations on the two opposite-side surfaces and resulted in spontaneous scroll formation. An anisotropy parameter based on the relative free volumes of tether coils was introduced and shown to be linearly related to the radius of gyration. This demonstrates that sheet anisotropy, and consequently internal cavity diameters of tubular scrolls, can be regulated by surface grafting. We also examined a coassembly of laterally grafted rod-coil amphiphiles as an alternative way to form sheet polymers with heterosurfaces. The coassembly of conformation mismatching rod-coil molecules is expected to form anisotropic bilayers, as each layer is assembled independently with different degrees of graft disorder. We believe this work provides a framework for further research regarding morphology control by surface grafts of sheet polymers.