A simple method to prepare highly ordered PS-b-P4VP block copolymer template

Abstract

A control strategy for tuning the film morphology of asymmetric polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers (BCPs) is reported. After preparation of the film by spin-coating method, the as-cast films were annealed in different solvent vapor. It is found that chloroform is a wonderful solvent for forming PS-b-P4VP regular pattern. Otherwise, with changing the concentration of PS-b-P4VP, cylindrical or parallel nanostructures could be attained. The PS-b-P4VP films with cylindrical structure are used as template to deposit FePt nanoparticles into the pores. Nanoparticles reaching the bottom of the holes form a disordered magnetic array.

Introduction

Spontaneous self-assembly of block copolymer-based systems is an incredibly powerful area of current research since it provides the opportunity for nanoscale patterning otherwise inaccessible by lithographic procedures [1], [2], [3]. The self-assembling nature of block copolymers (BCPs) provides the potential to fabricate high-density arrays for use in data storage, electronics, and molecular separation, etc. Since most of the applications of BCPs rely on their structure formation at surfaces, hence, the self-assembly of block copolymers in thin films is being widely investigated [4], [5], [6]. Controlling the orientation of cylindrical and parallel microdomains in BCP thin films is crucial and has received significant attention [4], [5]. To achieve an orientation of cylindrical microdomains normal to a surface, many approaches have been developed, including using electric fields [7], graphoepitaxy [8], epitaxial crystallization [9], solvent annealing [10], and by varying film thickness [11], [12], etc.

The solvent annealing technique where the films are cast on the substrates and subsequently exposed to solvent vapor turns out to be an efficient way to improve BCP film nanostructures [10], [11], [13], [14]. The process of solvent annealing can improve both the kinetics of phase separation and the order of the structure. Besides the solvent evaporation, thickness is another important parameter in controlling order in BCPs [11]. Though polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) copolymers have been extensively investigated in thin film, their detailed behavior in thin films have so far not been fully elucidated. Here, we have studied the time development of the microdomain structure in asymmetric PS-b-P4VP via different vapor treatments by atomic force microscopy (AFM). By precisely controlling the film thickness, carefully choosing solvent and vapor exposure time, different morphologies of thin film are produced and characterized. In the process of our experiment, we found a simply method to generate highly ordered BCPs template. We also deposited pre-synthesized FePt nanoparticles into the cylindrical templates, followed by removal of the polymer, led to disordered magnetic patterns.