Doctoral Research 

Topic: Quantification of branch content in disordered materials

The phenomenon of structural branching is ubiquitous in a wide array of materials: polymers, ceramic aggregates, polymeric networks and gels. Branching has a strong influence over structure-property relationships of these materials. Despite the generic importance across a wide spectrum of materials, our physical understanding of the scientific nature of branching and the analytic description and quantification of branching is at an early stage, though many decades of effort have been made. Existing techniques in polymers based on Size Exclusion Chromatography (SEC) and rheology to quantify branching are, at best, qualitative; and quantitative characterization techniques like Nuclear Magnetic Resonance Spectroscopy (NMR), and Transmission Electron Microscopy (TEM) (for ceramic aggregates) have limitations in providing routine quantification. For ceramic aggregates, theoretical work has dominated and only a few publications on analytic studies exist to support theory. Small-angle scattering of x-rays and neutrons can be put to use for quantifying branch content. Application of concepts native to fractal geometry facilitates acquiring such results from scattering experiments from a variety of disordered materials. A universal tool is needed, to quantify branch content in disordered materials, irrespective of the family of materials to which they belong.

Fractal dimensions for Silica nanoparticles from flame synthesis.

Masters Thesis

Phase-separation can result in the formation of self-assembling structures in networks induced by crosslinking reactions. Vinyl and hydroxyl end-functionalized poly(dimethyl siloxane) (PDMS) chains were employed for the synthesis of these networks using a tetra-functional crosslinking agent. It was found that at a very high molar fraction (approaching 1) of the low molecular weight component, approximately 50% by volume; it was possible to induce composition fluctuations in the system that resulted in phase-separation via spinodal decomposition. Time resolved small angle light scattering (SALS) and optical microscopy were used to determine the character and kinetics of phase-separation and to compare and contrast with the spinodal decomposition of simple blends. A vinyl-terminated system resulted in the formation of interconnected bicontinuous morphology reminiscent of early-stage spinodal decomposition (SD). The morphology obtained by SD was found to be “locked-in”, possibly due to network formation. Late-stage phase ripening was prevented by gelation. Surprisingly, a hydroxyl, end-terminated system was found to display different behavior with simultaneously nucleated close to monodisperse spherical domains. The difference in the mechanisms of phase-separation is believed to arise from the different reaction mechanisms of the two systems with an addition mechanism in the vinyl end-terminated system and a condensation process in the hydroxyl end-terminated system. The reaction mechanism dictates the effective quench depth in the system, which leads to different mechanisms of phase-separation in the two systems. The effect of differences in branch topology, on the phase-separation process has been investigated.

Phase-separation morphologies in PDMS.

Chan-Hilliard kinetic study of spinodal decomposition in vinyl terminated system.

Application of the maximum entropy program to scattering data and the resulting size distribution in hydroxyl-terminated PDMS

Sphere function:- fit to the scattering data for hydroxyl system