THE DYNAMIC NATURE OF COALS MACROMOLECULAR STRUCTURE - VISCOELASTIC ANALYSIS OF SOLVENT-SWOLLEN COALS

A simple molecular model of an entangled macromolecular network is used to parametrize the physical behavior of solvent-swollen coals strained under both constant and oscillatory uniaxial compressive stress. The model distinguishes between three dynamic regions covering a wide range of molecular mobility. These are a high-frequency dynamic mode associated with Brownian motion of the segments of the molecular strands, an intermediate-frequency region governed by intramolecular motion associated with contour length fluctuations between entanglement junctions or cross-links, and a low-frequency region governed by the intermolecular mobility of individual macromolecules. The two lower frequency dynamic modes have been characterized through their creep compliance behavior. Viscoelastic strain in response to a constant uniaxial compressive stress is parameterized with a cooperative diffusion coefficient, D(c). Results in the present paper show that D(c) range from 10(-5) to 10(-6) cm2/s. Self-diffusion of individual macromolecules is greatly restricted as evident by the low degree of fluidity of solvent swollen-coals. Coefficients of viscosity, eta, range from 10(11) to 10(13) P. The very large difference between the actual time required to reach steady-state flow and that calculated, assuming linear monodisperse chains, suggests a physical structure composed of entangled, high molecular weight, branched macromolecules with long arms. Energy dissipation in the swollen coals is significant, averaging 50% in the frequency range 10(-3) Hz, and results from a large contribution to the compliance of viscoelastic over elastic strain. The high degree of energy dissipation detected with such a low-frequency stress cycle is consistent with coal's rubbery behavior in the pyridine dilated state.