Three-dimensional textures and defects of soft material layering revealed by thermal sublimation

Abstract

Layering is found and exploited in a variety of soft material systems, ranging from complex macromolecular self-assemblies to block copolymer and small-molecule liquid crystals. Because the control of layer structure is required for applications and characterization, and because defects reveal key features of the symmetries of layered phases, a variety of techniques have been developed for the study of soft-layer structure and defects, including X-ray diffraction and visualization using optical transmission and fluorescence confocal polarizing microscopy, atomic force microscopy, and SEM and transmission electron microscopy, including freezefracture transmission electron microscopy. Here, it is shown that thermal sublimation can be usefully combined with such techniques to enable visualization of the 3D structure of soft materials. Sequential sublimation removes material in a stepwise fashion, leaving a remnant layer structure largely unchanged and viewable using SEM, as demonstrated here using a lamellar smectic liquid crystal.sublime | direct visualization T he study of the structure of layered soft materials began in the early 1900s with the legendary inference of Grandjean and Friedel of the fluid layer structure of smectic liquid crystals (LCs) based on their observation of the elliptical core lines of focal conic defects and their interpretation of these as the singularities that appear when space is filled with equally spaced curved surfaces generated from the cyclides of Dupin (1-4). Since that time, the techniques of X-ray diffraction, and visualization, using optical transmission and fluorescence confocal polarizing microscopy; atomic force microscopy (AFM); and SEM and transmission electron microscopy (TEM), including freeze fracture TEM (FFTEM), have been widely applied to characterize soft layering and defects (5-10), but the ability to visualize layering directly throughout a 3D sample (e.g., in particular confinement conditions) has been limited. Here, we apply thermal sublimation, a technique that is widely applicable due to the relatively low vapor pressure of many soft material components, as a method for removing material from a 3D sample that maintains its internal structure (i.e., moves the material surface into the bulk, creating a deeper interface that exhibits a topography that reflects the local structure).We demonstrate this method using the thermal sublimation of a fluid smectic LC phase, wherein the LC material was removed layer by layer. We show that even in small-molecule, hightemperature fluid smectic A (SmA) phases, the underlying layer structure successfully resists surface tension (which tends to smooth out the advancing interface) to yield a surface topography that exhibits features of the underlying layer structure. Local features, such as defects, can advance or retard sublimation, leading to distinct visualization and control effects. The full 3D structure of a layered sample was exposed by successive sublimation steps and maintained during visualization by cooling. The remn...

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