Water Is a Poor Solvent
for Densely Grafted Poly(ethylene
oxide) Chains: A Conclusion Drawn from a Self-Consistent Field Theory-Based
Analysis of Neutron Reflectivity and Surface Pressure–Area
Isotherm Data
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Abstract
By use of a combined experimental and theoretical approach,
a model
poly(ethylene oxide) (PEO) brush system, prepared by spreading a poly(ethylene
oxide)–poly(<i>n</i>-butyl acrylate) (PEO–PnBA)
amphiphilic diblock copolymer onto an air–water interface,
was investigated. The polymer segment density profiles of the PEO
brush in the direction normal to the air–water interface under
various grafting density conditions were determined by using the neutron
reflectivity (NR) measurement technique. To achieve a theoretically
sound analysis of the reflectivity data, we used a data analysis method
that utilizes the self-consistent field (SCF) theoretical modeling
as a tool for predicting expected reflectivity results for comparison
with the experimental data. Using this data analysis technique, we
discovered that the effective Flory–Huggins interaction parameter
of the PEO brush chains is significantly greater than that corresponding
to the θ condition in Flory–Huggins solutions (i.e.,
χ<sub>PEO–water</sub>(brush chains)/χ<sub>PEO–water</sub>(θ condition) ≈ 1.2), suggesting that contrary to what
is more commonly observed for PEO in normal situations (χ<sub>PEO–water</sub>(free chains)/χ<sub>PEO–water</sub>(θ condition) ≈ 0.92), the PEO chains are actually not
“hydrophilic” when they exist as polymer brush chains,
because of the many body interactions that are forced to be effective
in the brush situation. This result is further supported by the fact
that the surface pressures of the PEO brush calculated on the basis
of the measured χ<sub>PEO–water</sub> value are in close
agreement with the experimental surface pressure–area isotherm
data. The SCF theoretical analysis of the surface pressure behavior
of the PEO brush also suggests that even though the grafted PEO chains
experience a poor solvent environment, the PEO brush layer exhibits
positive surface pressures, because the hydrophobicity of the PEO
brush chains (which favors compression) is insufficient to overcome
the opposing effect of the chain conformational entropy (which resists
compression)