Understanding Nucleation of Mesophase Pitch Tactoids using 4D-STEM

There is ongoing interest in using pitch feeds rich in polyaromatic hydrocarbons for production of carbon fibers. For a pitch to be suitable, a significant majority of the molecules must be capable of evolving to a liquid crystalline mesophase with thermal treatment. Understanding the nucleation and growth of the mesophase from the disordered molecules (the isophase) for different compositions is critical to producing practical yields and controlling the resulting properties of the pitch. Polarized light optical microscopy (PLM) and electron diffraction have led to the commonly accepted Brooks-Taylor model for mesophase droplets: nominally spherical and comprised of lamellar planes of condensed polyaromatics organized about a single axis, with the lamella curving towards the poles near the interface [1]. Given the shape, models have assumed equivalent elastic constants. The classic model predicts a strong anchoring force with aromatics perpendicular to the isophase interface (excepting an undescribed discontinuity at each pole). While PLM is ubiquitous for the mesophase pitch studies, there have been no reports on mesophase at sub-optical resolutions in >50 years. The local ordering of the pitch can be measured directly using a nearly parallel nanoscale electron beam (α ≈ 0.6 mrad and <5 nm at 200 kV) to collect an array of electron diffraction patterns, an approach now commonly called 4D-STEM [2,3]. By measuring the orientation and strength of diffraction from the π-π stacking planes (3.5–3.8 Å) at each pixel using LiberTEM's [4] implementation of radial Fourier analysis, a map of the liquid crystal's director field can be constructed (Fig. 1). The orientation and strength of 2-fold symmetry have been mapped to color and intensity (respectively). Data were collected with doses of 20–300 e/Å2 for both -180°C and ambient to ensure results were not unduly corrupted by the beam. It is immediately clear that small mesophase droplets are not spherical, but rather somewhat spindle-shaped. Similarly shaped “tactoids” have been observed in other liquid crystalline systems [5–8], but mostly for monodisperse rod- or fibril-like nematics. For pitch tactoids, the underlying director field still resembles the Brooks-Taylor model: lamella splayed about the axis, nearly orthogonal to the axis along the axis. While it is reasonable to expect a weak interfacial energy between iso- and mesophase pitch, tactoids have never been observed—or even suggested—for an inhomogeneous molecular system. Different tactoid aspect ratios are observed for feeds with different compositions. The interfaces of tactoids are reasonably fit by circular arcs, and can be described by the radius and the angle (α) at which a radius intercepts the pole of the tactoid [5]. M-50, extracted from fluidized catalytic cracker bottoms and one of the more extensively studied mesophase-producing pitches [9], yields only slightly aspherical tactoids with α > 72°. By contrast, another source of pitch (pitch 2) results in much smaller α = 35–40°. While the average molecules in pitch 2 are smaller than M-50 before heating, they are quite similar in structure. It is not immediately obvious that the resulting tactoid shape should differ so strongly. For neither pitch are the molecules perpendicular to the interface, suggesting weak anchoring forces. The pitch-2 tactoids bend more strongly about the axis than the M-50 tactoids, perhaps suggesting that the bipolar/homogeneous transition occurs at different lengths. For both pitches, there are some indications of a crossover to a homogenous director field at length scales <100 nm. In nearly all cases the director field is slightly asymmetric with respect to the tactoid axis, in disagreement with Brooks-Taylor. A possible explanation is a small twist in addition to the more obvious bend. As both feeds evolve nominally spherical droplets at larger length scales, there must be a critical volume at which surface tension dominates. However, it might still be reasonable to treat the tactoids as the equilibrium shape at smaller length scales, as droplets caught in the midst of coalescence are more tactoid- than sphere-shaped, and shapes are similar across a range of tactoid sizes [10]. 4D-STEM has allowed a first observation of tactoid liquid crystals in an inhomogeneous molecular system, and provided insight into very early stages mesophase pitch nucleation. The details of droplet shape and molecular arrangement for different feeds have not been explored in significant detail, but related properties that could be measured with the help of 4D-STEM, e.g. the elastic constants, should have an influence on materials developed from pitch feeds. 4D-STEM maps of the strength and orientation of π-π stacking for (a) M-50 and (b) a second pitch. As these are thin cross sections, tactoids with their axis aligned closer to the viewing direction will look more circular, as may partially-sectioned tactoids (especially droplets larger than the section thickness). Similar-width tactoids from (a) and (b) have been replotted (c,d) with a line representing the local orientation of each measurement. The angle of intersect with the best-fit circular arc is shown as a means to quantify the very different aspect ratios of the tactoids formed in the two different pitches.