Typically, pure substances may be found with only one gaseous or liquid state, while their solid state may exist in various polymorphic states. The existence of two distinct liquid forms in a single component substance is more unusual since liquids lack the long-range order common to crystals. Yet, the existence of multiple amorphous states in a single component substance, a phenomenon known as "liquid polyamorphism," has been observed or predicted in a wide variety of substances. In contrast to standard phase transitions, it has been suggested that polyamorphic liquid-liquid transitions are caused by the interconversion of molecular or supramolecular states. To investigate this phenomenon, a nonequilibrium thermodynamic model was developed to quantitatively describe the interplay between the dynamics of molecular interconversion and fluid-phase separation. The theory has been compared to a variety of interconverting systems, and has demonstrated a quantitative agreement with the results of Monte Carlo and Molecular Dynamics simulations.

In this thesis, it is shown that there are two major effects of molecular interconversion on the thermodynamics and the kinetics of fluid-phase separation: if the system evolves to an equilibrium state, then the growth of one of the alternative phases may result in the destruction of phase coexistence - a phenomenon referred to as "phase amplification." It is demonstrated that depending on the experimental or simulation conditions, either phase separation or phase amplification would be observed. Previous studies of polyamorphic substances report conflicting observations of phase formation, which may be explained by the possibility of phase amplification occurring. Alternatively, if the system evolves to a nonequilibrium steady state, the phase domain growth could be restricted at a mesoscopic length scale. This phenomenon (referred to as "microphase separation") is one of the simplest examples of steady-state dissipative structures, and may be applicable to active matter systems, hydrodynamic instabilities, and bifurcations in chemical reactions, in which the nonequilibrium conditions could be imposed by an external flux of matter or energy.