SYMMETRY-BREAKING IN NANOSTRUCTURE DEVELOPMENT OF CARBOGENIC MOLECULAR-SIEVES - EFFECTS OF MORPHOLOGICAL PATTERN-FORMATION ON OXYGEN AND NITROGEN TRANSPORT

A comprehensive study has been undertaken to establish the primary fac tors that control transport of oxygen and nitrogen in polymer-derived carbogenic molecular sieves (CMS). Characterization of the nanostructu re of CMS derived from poly(furfuryl alcohol) (PFA) indicates that sig nificant physical and chemical reorganization occurs as a function of synthesis temperature. Spectroscopic measurements show a drastic decre ase in oxygen and hydrogen functionality with increasing pyrolysis tem perature. Structural reorganization and elimination of these heteroato ms lead to a measurable increase in the unpaired electron density in t hese materials. High-resolution transmission electron microscopy and p owder neutron diffraction are used to probe the corresponding changes in the physical structural features in the CMS. These indicate that as the pyrolysis temperature is increased, the structure of the CMS tran sforms from one that is disordered and therefore highly symmetric to o ne that is more ordered on a length scale of 15 Angstrom and hence les s symmetric. This structural transformation process, one of symmetry b reaking and pattern formation, is often observed in other nonlinear di ssipative systems, but not in solids. Symmetry breaking provides the d riving force for these high-temperature reorganizations, but unlike mo st dissipative systems, these less-symmetric structures remain frozen in place when energy is no longer applied. The impact of these nanostr uctural reorganizations on the molecular sieving character of the CMS is studied in terms of the physical separation of oxygen and nitrogen. These results show that the effective diffusivities of oxygen and nit rogen in the CMS vary by more than an order of magnitude across the ra nge of synthesis temperatures studied. Although the electronic nature of the CMS leads to higher equilibrium capacity for oxygen, it is the physical nanostructure which governs the separation of these two molec ules. It is concluded that the primary separation mechanism is steric and configurational in nature, a conclusion in good agreement with the general features of the kinetic hypothesis conjectured by earlier wor kers.