Langmuir monolayers of 1,2-polybutadiene (PB) were investigated by means of surface pressure−area isotherms, Brewster angle microscopy (BAM) observations, and sumfrequency generation (SFG) spectroscopy. A homogeneous and stable monolayer is formed 1.5 h after PB spreading provided that both light and oxygen are present. This was attributed to a slight oxidation of the PB at the air−water interface. The cross-linking of PB under UV photoirradiation was then studied. SFG spectroscopy demonstrates the in situ formation of a two-dimensional network. From surface pressure−area characterizations and BAM experiments, the cross-linked PB monolayer appears significantly denser and more rigid than the non-irradiated monolayer. Atomic force microscopy images reveal an increase by a factor of three in the root-mean-square roughness of the irradiated monolayers compared with the non-irradiated ones.
Binary
blends of water-insoluble polymers are a versatile strategy
to obtain nanostructured films at the air–water interface.
However, there are few reported structural studies of such systems
in the literature. Depending on the compatibility of the polymers
and the role of the air–water interface, one can expect various
morphologies. In that context, we probed Langmuir monolayers of cellulose
acetate (CA), of deuterated and postoxidized polybutadiene (PBd) and
three mixtures of CA/PBd at various concentrations by coupling surface
pressure–area isotherms, Brewster angle microscopy (BAM), and
neutron reflectometry at the air–water interface to determine
their thermodynamic and structural properties. The homogeneity of
the films in the vertical direction, averaged laterally over the spatial
coherence length of the neutron beam (∼5 μm), was assessed
by neutron reflectometry measurements using D2O/H2O subphases contrast-matched to the mixed films. At 5 mN/m, the whole
mixed films can be described by a single slightly hydrated thin layer.
However, at 15 mN/m, the fit of the reflectivity curves requires a
two-layer model consisting of a CA/PBd blend layer in contact with
the water, interdiffused with a PBd layer at the interface with air.
At intermediate surface pressure (10 mN/m), the determined structure
was between those obtained at 5 and 15 mN/m depending on film composition.
This PBd enrichment at the air–film interface at high surface
pressure, which leads to the PBd depletion in the blend monolayer
at the water surface, is attributed to the hydrophobic character of
this polymer compared with the predominantly hydrophilic CA.
This work reports the feasibility
of polybutadiene (PB) cross-linking
under UV irradiation in the presence of a linear polymer, cellulose
acetate (CA), to form semi-interpenetrating polymer networks at the
air–water interface. The thermodynamic properties and the morphology
of two-dimensional (2D) CA/PB blends are investigated after UV irradiation
and for a wide range of CA volume fractions. A contraction of the
mixed Langmuir films is observed independent of the composition, in
agreement with that recorded for the individual PB monolayer after
cross-linking. The PB network formation is demonstrated by in situ sum-frequency generation spectroscopy on the equivolumic
CA/PB mixed film. From Brewster angle microscopy observations, the
PB network synthesis does not induce any morphology change at the
mesoscopic scale, and all of the mixed films remain homogeneous laterally. In situ neutron reflectometry is used to probe the effect
of PB cross-linking on the vertical structure of CA/PB mixed films.
For all studied compositions, significant thickening of the films
is evidenced, consistent with their contraction ratio. This thickening
is accompanied by a partial expulsion of the PB toward the film–air
interface, which is attributed to the hydrophobic character of the
PB. This phenomenon is stronger for films rich in PB. In particular,
the structure of the PB-rich film undergoes a transition from vertically
homogeneous to inhomogeneous along the depth. 2D semi-interpenetrating
polymer networks can thus be synthesized at the air–water interface
with a morphology that is strongly influenced by the polymer–polymer
and polymer–environment interactions.
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