The
cooperative assembly of functional precursors with block copolymers
(BCPs) is a powerful, general route to fabricate ordered mesoporous
materials, but the precise tuning of the mesopore size generally requires
trial and error to obtain the correct BCP template or appropriate
swelling agent. Here, we demonstrate the ability to effectively modulate
both expansion and contraction of the ordered cylindrical mesopores
relative to those obtained from cooperatively assembled Pluronic F127,
resol, and tetraethylorthosilicate. The two key physical parameters
for the swelling agents are their hydrophobicity, as quantified by
the octanol–water partition coefficient (K
ow), and Hansen solubility parameters that describe the
interactions of the solvent with the different components of the BCP
template. Four low volatility solvents are examined that span a wide K
ow with up to 90 wt % solvent relative to the
Pluronic F127. Glycerol triacetate (K
ow < 1) can decrease the average mesopore size from 5.9 to 4.8 nm
due to segmental screening of the interactions in the Pluronic F127
to decrease chain stretching at intermediate loadings. A modest increase
in mesopore size to 8.1 nm can be achieved with trimethylbenzene (TMB, K
ow = 3.42). Dioctyl phthalate (DOP), which is
slightly more hydrophobic (K
ow = 8.1),
is more effective than TMB at expanding the pore size (maximum: 13.5
nm) without loss of ordered structure. A more hydrophobic solvent,
tris (2-ethylhexyl) trimellitate (K
ow =
12.5), is less effective at increasing the pore size (maximum: 8.2
nm). The Hansen solubility parameters for DOP most closely match those
of the hydrophobic segment in the Pluronic F217 template. We attribute
this similarity, which is related to the solvent quality, to the improved
efficacy of DOP in increasing the pore size. These results illustrate
that both the Hansen solubility parameters (relative to the hydrophobic
segment of the template) and relative hydrophobicity of the swelling
agent determine the obtainable pore sizes in cooperatively assembled
ordered mesoporous materials.
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