Self-Assembly in ultrahigh molecular weight sphere-forming diblock copolymer thin films under strong confinement

Wei Cao,Markus Gallei,Senlin Xia,Sebastian Grott,Michael Appold,Nian Li,Peter Müller-Buschbaum,Sigrid Bernstorff,Nitin Saxena,Lorenz Bießmann

doi:10.1038/s41598-019-54648-3

Abstract

Ultrahigh molecular weight (UHMW) diblock copolymers (DBCs) have emerged as a promising template for fabricating large-sized nanostructures. Therefore, it is of high significance to systematically study the influence of film thickness and solvent vapor annealing (SVA) on the structure evolution of UHMW DBC thin films. In this work, spin coating of an asymmetric linear UHMW polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) DBC is used to fabricate thin films, which are spherically structured with an inter-domain distance larger than 150 nm. To enhance the polymer chain mobility and facilitate approaching equilibrium nanostructures, SVA is utilized as a post-treatment of the spin coated films. With increasing film thickness, a local hexagonal packing of PMMA half-spheres on the surface can be obtained, and the order is improved at larger thickness, as determined by grazing incidence small angle X-ray scattering (GISAXS). Additionally, the films with locally hexagonal packed half-spherical morphology show a poor order-order-poor order transition upon SVA, indicating the realization of ordered structure using suitable SVA parameters.

Highlights

Ultrahigh molecular weight (UHMW) diblock copolymers (DBCs) have emerged as a promising template for fabricating large-sized nanostructures
It was reported that increasing film thickness to a thin film monolayer, the half-spherical morphology is hexagonal packing in case of a normal molecular weight BCP film[33,34,35,38]
To investigate if these findings can be extended into the regime of UHMW DBCs, films with different film thicknesses have been prepared by spin coating on pre-cleaned substrates

Summary

Introduction

Ultrahigh molecular weight (UHMW) diblock copolymers (DBCs) have emerged as a promising template for fabricating large-sized nanostructures. UHMW DBCs do not approach thermodynamic equilibrium to form ordered structures by microphase separation, due to their very low chain mobility caused by the highly entangled chain conformations[14] To address this issue, post-treatments, like solvent vapor annealing (SVA) can be employed[19,20,21,22,23,24,25]. Kim et al combined thermal annealing and SVA to achieve ordered large domain sized lamellar nanostructures of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films[32]. Stein et al studied the morphology evolution of normal molecular weight spherical-domain polystyrene-block-poly(2 vinylpyridine) (PS-b-PVP) as a function of the number of layer (film thickness)[34]. Systematic studies on UHMW sphere-forming DBC films are rarely reported, in particular at highly confined conditions

Methods

Results

Conclusion