Block Copolymer Self-Assembly Under External Fields

The ability of block copolymers to autonomously assemble into regular patterns with uniform, nanometer scale features, and their straightforward integration with techniques of thin-film processing and device fabrication, have rendered them highly appealing as patterning materials for information technology, and other technological applications in energy, membranes, and fluidics. Various strategies such as thermal annealing, temperature gradients, solvent-vapor treatment, shear fields, external electric fields have resulted in improved alignment of block copolymer domains. Electric fields induce molecular polarization effect and change the conformation and dynamics of the polymer chain. Electric fields stand out as they can be readily adopted by current technologies, are lowcost, non-abrasive, and allow for instantaneous control, and mass production. The microphase separation mechanism of block copolymer microstructure in external fields still has been understanded inadequately. In this work, we report the self-assembly of block copolymer in external fields. We systematically study the effect of thermal annealing, solvent-vapor treatment and electric fields on the self-assembly of the block copolymer thin film. The results were shown as blow.The diffusion of block copolymer chain segment is very slow during thermal annealing process, the time of microphase separation is very long. In the process of thermal annealing, the phase structure of the characteristic scale increases. Annealing temperature has a great influence on the characteristics of the microphase separation structure scale of the block copolymer. The effect of temperature, time and film thickness on the phase behavior of block copolymer is larger.During solvent-vapor treatment process, due to solvent diffusion into the block copolymer film, the diffusion of chain segment enhances, but still need a rather long time to reach equilibrium structure. The block copolymer molecular chain with larger molecular weight segment motion more slowly.The orientation of the micro-separation structures in the block copolymer film can be controlled by using DC electric fields. With the increase of electric field intensity, the order of the microphase separation structure will be increased. In the early stage of the self-assembly, increasing the time of applying an electric field, can enhance the order of the film. An electric field induces the morphology transitions from a parallel cylinder block copolymer microphase orientation to a perpendicular cylinder with respect to the substrate. Salt in the block copolymer film can shield DC electric fields. However, in the present of salt, AC electric fields significantly influence the self-assembly behavior of block copolymer.