Solution-based processing of infusible polymers usually involves large quantities of undesirable solvents and transport-limited morphological evolution that is often difficult to control. The present study constitutes a rational exploration of the phase transitions in polymer solutions to form the basis for new processes that would diminish the above-mentioned problems and also identify appropriate mechanisms for controlled generation of useful morphologies. The fundamental concepts in this regard are exemplified through solutions of acrylonitrile-based homopolymer and copolymers, with important inferences drawn regarding their potential implications for the formation of fibers, films, and membranes. Thermodynamic and kinetic manipulation of transitions such as liquid-liquid phase separation and crystallization is shown to be effective in controlled generation of morphologies ranging from microporous to dense.