Platypus Untangles the Structure of Microphase Separated Diblock Copolymer Thin Films

Authors

Chiara Neto, Wilasinee Sriprom, Andew Telford, and Sebastien Perrier (School of Chemistry and Key Centre for Polymers and Colloids, University of Sydney), and Michael James and Andrew Nelson (Bragg Institute, ANSTO).

Introduction

The first published results from Platypus are based on structural studies of microphase separated diblock copolymers in combination with Atomic Force Microscopy (AFM). The effect of block polydispersity on microphase separation was examined for PMMA / PBA thin films synthesized by Reversible Addition-Fragmentation chain Transfer (RAFT) (W. Sriprom, et al. 42(8), 3138-3146, Macromolecules, 2009).

Four distinct morphologies were observed, depending on composition and film thickness: parallel lamellae, hexagonally packed perforated lamellae, parallel cylinders, and hexagonally packed spheres (Figure 1 below). The main differences in domain formation with respect to monodisperse systems was the shift of the domain boundaries to more asymmetric volume fractions, the ability to accommodate domains in layers different from exact multiples of a layer period, and the stabilization of hexagonally perforated lamellae.

Platypus data collected an annealed PMMA77-PBA23 diblock copolymer thin film of 57 nm revealed a complete (L1) lamellar structure

(Figure 2 below) as opposed to a cylindrical structure expected for monodisperse copolymers of this volume fraction. This effect is not usually observed in monodisperse block copolymers and is probably due to the relatively large polydispersity of the synthesized PBA block.

The second published article from Platypus (C. Neto et al., Macromolecules, In Press 2009) examines the structure of microphase separated PS-PEO thin films, and in particular the composition of the uppermost layer. Block copolymers containing polyethylene oxide (PEO) are often proposed as ideal biomaterials, as PEO is very effective in the prevention of protein adsorption and platelet adhesion. 

The combination of patterning ability and biological compatibility has made block copolymers of PEO among the most interesting and promising materials for the design of bio-interfaces.  Platypus data in concert with AFM, XPS and optical microscopy reveal that the copolymer films separate into lamellar structures oriented parallel to the silicon substrate (Figure 3 below), and that bicontinuous or island/hole morphologies appear depending on composition. 

In addition to internal details of the lamellar structures (having ~16 nm bilayer sub-units), the neutron reflectivity data pointed unequivocally to the presence of (and in contrast to a 30 year belief to the contrary) a thin (~6 nm) surface layer of polystyrene at the air/film interface.