Diblock copolymers (A-B chain) microphase-separate due to competing forces: A-B repulsion (χ parameter) drives segregation, but chain connectivity prevents macrophase separation. The Ohta-Kawasaki free energy yields periodic morphologies: spheres (f≈0.1), cylinders (f≈0.3), lamellae (f≈0.5), and mirror images for larger f. The order-disorder transition occurs at χN ≈ 10.5 for symmetric diblocks.