Internal Structure of Ultrathin Diblock Copolymer Brushes

We present neutron reflectivity (NR) and grazing incidence small angle X-ray scattering (GISAXS) measurements, scaling theory and numerical self-consistent field (SCF) calculations on the internal structure of as-deposited high grafting density (0.6 chains/nm²) ultrathin (d < 25 nm) diblock copolymer brushes (DCBs). DCBs of various thicknesses containing deuterated polystyrene (dPS) blocks and poly(methyl acrylate) (PMA) blocks with dPS (dPS-b-PMA) or with PMA (PMA-b-dPS) adjacent to the substrate were synthesized by atom transfer radical polymerization (ATRP). For the thinnest films a model of two layers with smooth interfacial gradient provides a good description of the data. For thicker dPS-b-PMA samples of sufficiently asymmetric composition a third layer must be included. This is consistent with the presence of a lateral ordering of some type in the center of the brush, as evidenced by GISAXS data.  In general, the region adjacent to the substrate is found to have a substantial composition of the “top” block in contrast to expectations from theory.  The interface widths for brushes with a PMA block tethered to the substrate are smaller than for brushes with a dPS block tethered to the substrate. Experimental interface width values are consistent with expectations from self-consistent fieldSCF theory for brushes with a dPS bottom block.

Using a scaling approach, we identify a new stretched interface regime for the interfacial width, w, in DCBs at high grafting densities or low values of the Flory interaction parameter, χ. Here, the width scales as w ~ χ^-1 as opposed to the Helfand-Tagami expression w ~ χ^-1/2 for free block copolymers and immiscible polymer blends.  The scaling theory is in qualitative agreement with experimental data and suggests directions for further work.

Acknowledgement:  The authors thank Prof. Scott Collins for use of his dry box and sector 8 staff at the APS for technical support. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under contract No. W-31-109-ENG-38.  Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund for partial support of this research (AC7-42995) and to the Ohio Board of Regents for a challenge grant. DW acknowledges partial support from NSF Award # DMR-0213918 and AFOSR Award # FA9550-08-1-0007.