Abstract
Block copolymers (BCPs) have been extensively studied because of their ability to self-assemble into various ordered nanostructures, such as lamellae, body-center cubic spheres, hexagonally packed cylinders, double gyroid, or even ordered bicontinuous double diamond, resulting from the incompatibility of their constituted blocks and the constrain of chemical junction. Among those nanostructured phases, network phases always receive intensive attention because of their unique geometry and continuous texture. For such a nanostructured material to prove useful in applications, thin-film samples with oriented nanostructures must be formed. Different approaches to control the ordering of BCP thin films have been reported. One of the most effective ways is to integrate top-down and bottom-up methods. In this study, we aim to systematically examine the phase behavior of a chiral block copolymer system, polystyrene-b-poly(L-lactide) (PS-PLLA), in particular with the volume fraction of PLLA ranging from 0.35 to 0.40 at which stable network phases are expected to be found. Interestingly, in contrast to conventional three-fold double gyroid, a four-fold network phase OBDD (ordered bicontinuous double diamond) can be found, as evidenced by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The formation of the OBDD reflects the releasing of packing frustration from the chiral segment with higher persistent length. For practical applications, we aim to examine the feasibility to control orientation of forming network phases using topographic patterns fabricated by a distinctive lithographic approach. With well development of the lithographic approach, topographic nanopatterns with hundreds nanometer trenches in different textures possessing sharp sidewall and uniform bottom surface can be fabricated. Subsequently, directed self-assembly (DSA) of gyroid-structured PS-PLLA is carried out using the fabricated topographic nanopatterns as graphoepitaxial pattern to create gyroid-structured thin films with controlled orientation. Most interestingly, the ordering process by DSA follows a nucleation and growth mechanism at which the (211)G plane of the double gyroid texture will be the preferred growth plane to be initiated from the sidewall of the trench, eventually giving the well-ordered double gyroid texture with controlled orientation at which the (211)G plane is the surface plane from the air and the (111)G plane will be on the sidewall of the trench. In the end, the nanostructured thin films can be developed as topographic nanopatterns by removal of the PLLA segments through dry and wet etching process, providing a unique patterns for lithographic applications.