A. S. Abhiraman

Synthesis and characterization of spinnable sol-gel derived polyborates

Abstract Polyborate sol-gel routes have been studied in the context of molecular structure development and spinnability of the sols obtained through the sol-gel process. The similar chemical structures of the lithium polyborates derived from lithium methoxide and tri-n-butyl borate or from lithium methoxide and trimethoxyboroxine suggest mechanistic similarities in the two sol-gel reactions. Identical mixed oxide compositions could be formed by heat treatment of the lithium polyborate gels obtained in the two reactions. A controlled hydrolysis and polycondensation of trimethoxyboroxine has also been investigated to form polyborates without the presence of a counterion. The porous gel filaments that can be drawn from these sols may be suitable as precursors for the formation of other ceramic structures such as boron nitride.

Two-dimensional 13C n.m.r. studies of the morphology and orientational order in gel-spun ultrahigh molecular weight polyethylene fibres

Abstract In this communication we demonstrate the feasibility of determining the orientational distribution functions of various morphological components in ultrahigh molecular weight polyethylene fibres by two-dimensional rotor-synchronized 13C cross-polarization/magic angle spinning nuclear magnetic resonance (n.m.r.) techniques. For comparative purposes we have also included a drawn polyethylene fibre of normal molecular weight. The results showed that these fibres contain two crystalline components, designated C1 and C2, and an amorphous (non-crystalline) component, A. Order parameters and orientational distribution functions of these components can be obtained through analysis of the two-dimensional 13C n.m.r. spectra.

Synthesis and characterization of processable polyborate precursors

Polyborates have been formed by disproportionation and polycondensation of trimethoxyboroxine and also of trimethoxyboroxine and boric acid. The crtiical role of stoichiometry in the formation of polyborates without requiring incorporation of a metal counter ion is revealed. The emphasis is on integration of this chemistry with fabrication processes. Incorporation of an appropriate linear organic polymer is shown to yield the required rheological and thermal characteristics in this regard. These polyborates are inferred to be suitable as precursors for boron nitride in geometrical forms that can be processed as supported structures. However, they are also determined to be unsuitable as precursors for boron nitride fibers, oriented or isotropic, primarily due to their low glass transition temperature.