(43b) 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.