Fabrication and Application of Nanopatterns Based on Baroplastic Block Copolymer

Abstract

Block copolymers are composed of two or more chemically distinct, and frequently immiscible, polymer blocks covalently bound together. The segregation of the block components arising from thermodynamic incompatibility and connectivity produces various microdomains such as, lamellar, cylindrical, spherical, and gyroid structure. Polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) exhibited baroplasticity properties enabling processing polymers at low temperatures with a relatively low pressure. This interesting property was related to the closed-loop phase behavior of PS-b-PnPMA. Furthermore, since the transitions of PS-b-PnPMA are very sensitive to hydrostatic pressure, the microdomain PS-b-PnPMA disappears and becomes disordered state at relatively low pressure of 62 bar.Here, we introduce the fabrication of the ultrahigh density array of nanopatterns on polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) films by atomic force microscope (AFM) tip for next-generation data storage media. This block copolymer exhibits the baroplasticity which enables one to process at a relatively lower pressure (~ 62 bar) and room temperature by transforming microphase transition of the block copolymer. We found that this pressure range could be easily achieved by the indentation of the AFM tip on a polymer film at room temperature. This is because the contact area by AFM tip is very small (< 15 nm in radius) even though the contact force of AFM is very small (nN scale). We clearly demonstrated via cross-sectional transmission electron microscopy that the phase transition from microdomain structures to disordered state indeed occurred even in the very small sized nanopatterns during the indentation of AFM tip. We showed that the generated nanopatterns were transformed into electric signals by piezoelectric sensing method. We also found that the erasing process was easily achieved at a slight heating (120 oC) for a short time (~ 10 s). During the erasing process, the disordered state was completely transformed into the well-ordered lamellar microdomains, essentially the same morphology of the initial film. We consider that the generated ultrahigh density array of nanopatterns on baroplastic PS-b-PnPMA film can be used as next-generation storage media.Furthermore, we succeed in monitoring in-situ the phase of the morphology on the PS-b-PnPMA film during the entire nanoindentation process (from loading to unloading of indenter tip on the thin film) by using special TEM holder accommodating special indenter. For the first time, we directly observed that the disordered region from lamellar microdomains was gradually expanded during loading, while it was slightly decreased during the unloading. On the other hand, we do not observe any disordered region for non-baroplastic polystyrene-block-polyisoprene (PS-b-PI) film. In-situ TEM technique during the nanoindentation could be used to understand the deformation process of polymer by indentation force and the relaxation process.Finally, incorporation of nanoparticles into self-assembled block copolymers allows improvement of the mechanical strength, conductivity, permeability, catalytic activity, and optical and magnetic properties. We have synthesized thiol-terminated polystyrene-block-(polystyrene-random-(azido-polystyrene)) (PS-b-(PS-r-P(S-N3))-SH) block copolymer with a number-average molecular weight (Mn) of ∼2900 g/mol by RAFT polymerization. Then, thiol-terminated polymer capped gold nanoparticles were synthesized using two-phase method as described by Brust et al. These gold nanoparticles were thermally stable by introducing photo-cross-linkable azide groups (-N3) into the polymeric ligands attached to the Au core. We prepared PS-b-(PS-r-P(S-N3))-coated gold nanoparticles /PS-b-PnPMA composite where gold nanoparticles were selectively incorporated in PS microdomain of the block copolymer. One of possible applications is nanocircuit arrays which are fabricated with PS-b-PnPMA/gold nanoparticle composite.