Solvent Dependence of
the Morphology of Spin-Coated
Thin Films of Polydimethylsiloxane-Rich Polystyrene-<i>block</i>-Polydimethylsiloxane Copolymers
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Abstract
The as-spun, thin film morphologies of a series polydimethylsiloxane-rich
cylinder and lamellar-forming polystyrene-<i>block</i>-polydimethylsiloxane
(PS<i>-<i>b</i>-</i>PDMS) copolymers with constant
PDMS molecular weight and varying PS volume fraction were studied
with a range of solvents of varying solubility parameter. It was found
that PDMS occupies the surface of the thin films regardless of the
choice of solvent used in spin-coating due to its extremely low surface
tension. The morphology shifted from parallel cylinders to hexagonally
perforated lamellar to parallel lamellar as the solvent was varied
from PDMS to PS selective solvents (increasing solvent solubility
parameter). The transition points between each morphology were also
dependent on the volume fraction of the block copolymer where the
transitions were observed at lower solubility parameter with increasing
PS volume fraction of the polymer. The morphology variations are attributed
to selective swelling effects of the individual blocks even under
good solvent conditions. These results are discussed in the context
of current theories of solvent evaporation induced ordering of block
copolymer thin films