Reliable access to clean water is a major and growing societal challenge. Selective membrane technologies are expected to play a critical role in sustaining the water economy due to their energy efficient filtration of wastewater. High performing water filtration membranes require both high water permeability and high size-selectivity to ensure that purified water is produced in a cost-effective manner. Existing ultrafiltration membranes contain continuous and interconnected pores that allow for the rapid transport of water, satisfying the requirement of high water permeability. However, they typically exhibit broad pore size distributions that limit their size-selectivity and prohibit their application in highly precise separations. Block polymers represent a potentially powerful alternative class of materials for improved size-selectivity due to their self-assembly into well-defined domains of uniform size at the nanoscale. Removing one of the blocks generates the uniform pores required for precise separations. By coating a thin block polymer selective layer onto a commercially available ultrafiltration membrane, it may be possible to simultaneously obtain both high water permeability and high size-selectivity in a single membrane. However, commercialization of block polymer membranes has been impeded by technological challenges associated with producing continuous pores in an industrially scalable fabrication process from the typically observed ordered block polymer morphologies. Rather than targeting these ordered morphologies, this thesis aims to utilize the disordered state of block polymers to produce higher performing and potentially more scalable membranes. By kinetically trapping disordered state composition fluctuations, a disordered and co-continuous morphology can be obtained and subsequently converted into uniform and continuous pores without the need for challenging processing techniques. Chapter 1 introduces key concepts in block polymer self-assembly, including the order-disorder transition and composition fluctuations. Chapter 2 provides a summary of the technological requirements of an ideal water filtration membrane and discusses various strategies to integrate block polymers into these systems. Chapter 3 contains an overview of the various synthetic, processing, and characterization techniques employed throughout the thesis. Chapter 4 describes proof-of-concept results demonstrating that thermal cross-linking can be used to kinetically trap disordered state composition fluctuations. Chapter 5 details a strategy that introduces temporal control by using thermally stable photocuring strategies to arrest the disordered state. Chapter 6 describes a fundamental investigation into the temperature dependent morphological evolution of block polymers in the disordered state. Chapter 7 examines the use of large amplitude oscillatory shear to precisely control the domain structure of disordered block polymers. Chapter 8 integrates all these findings into the development of a novel co-casting technique to fabricate composite membranes with both high water permeability and high size-selectivity in a potentially scalable manner.