Block copolymers (BCPs) are a group of fascinating materials that self-assemble into highly uniform nanoscale structures. With precise control of interfacial properties on both interfaces, these nanostructures can be directed to form user-defined periodic patterns. The directed selfassembly (DSA) of BCPs offers a cost-effective solution to complement the conventional lithography with the capability of density multiplication and pattern rectification. This dissertation mainly focuses on the chemoepitaxial DSA of symmetric BCP into line patterns. The chemoexpitaxy approach of DSA is developed by Liu and Nealey, in which the geometrical and chemical boundary conditions of the chemical pattern could be individually controlled. As the foundation to build up all the nanostructures, the guiding stripe pattern is fully characterized in both real and reciprocal spaces, and we locally and statistically analyze the trapezoid shape and line roughness. With optimized pattern conditions to minimize the interfacial energy, we investigate the DSA of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) in the context of 3X density multiplication and reveal a different assembly mechanism in thicker film regime. Grain growth is decoupled from both interfaces at early assembly stage, and a faster defect annihilation process is visualized by X-ray scattering on the free surface compared to the film bulk. With the current trend of device miniaturization, further evolution of DSA technology now faces challenges to realized sub-10 nm features. As the leading material for DSA applications for past decades, PS-b-PMMA can only form lamellas above ~12 nm in half pitch as limited by a low Flory-huggins interaction parameter, χ. We illustrate a simple strategy to scale down its intrinsic resolution limit by blending in selective ionic liquid additive to enhance the segregation strength in between blocks. By carefully controlling the volume fraction of ionic liquid additive, we maintain the favorable attributes of PS-b-PMMA and demonstrate successful sub-10 nm DSA with thermal annealing and a free surface. More importantly, this PS-b-PMMA based high-χ material facilitates us to explore the fundamental and practical impacts of χ increase on interfacial width between polymer blocks, line edge roughness of DSA patterns as well as the kinetics of assembly. The scope of this work is to gain fundamental understanding in the assembly mechanism of density multiplication based on the maturing platform of first generation material. Then the material research is extended to next-generation high-χ BCP. On the one hand, it provides a potential solution to access smaller dimension. On the other hand, it also brings about new challenges of DSA technology for current and future generations of low-cost, high-resolution nanopatterning applications.