Page 29 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
26-27
Figure 2
a) Reflectivity (R) data as a function of wavevector Q for the direct emulsion case. Satisfactory fit involving two layers of different polymer densities (full curve) contrary to the dotted curve (one polymer layer). Extensions of layers are on the water side LWS= 183 Å and on the oil side LOS= 74 Å.
b) Sketch of the models with different polymer conformations in agreement with values obtained from the fits. From left to right are presented the direct emulsion case with two extended, charged and neutral brushes on the lower (upper) water (oil) side, the multiple emulsion case where both extensions of chains are roughly similar on each side maintaining a low molecular area, and the inverse emulsion case where the water side is more contracted than the oil side in a larger molecular area. Red balls are used to represent some of the PS monomers included in the hydrophilic (PS-stat-PDMAEMA) block: in the direct emulsion case, these PS monomers are assumed to have little interaction between themselves; in the multiple and inverse emulsion cases they act as stickers to make intra- or inter-molecular bridges.
Neutron reflectivity measurement at the liquid/liquid interface is an invaluable tool, despite requiring careful calibration procedures, for exploring complex polymer conformations at the oil-water interface. Such an
advance was possible thanks to the optimisation of a sample cell involving a macroscopic oil phase suited to measurements at the water-oil interface in a ‘reflection down’ configuration, and exploiting the high flux FIGARO reflectometer at the ILL. Interpretation of the data is another complex procedure that requires careful consistency checks. Such a procedure was possible here because a model was constructed for refining the parameters. The next experimental challenges will involve better mastering of
the wetting of the substrates in order to reach a controlled water thickness.
Three cases, corresponding to conditions in which
direct, multiple and inverse emulsions are formed,
were investigated by varying salt concentration. The reflectivities found corresponding to the conditions described were markedly different, indicating—before
any quantitative interpretation of the data—that different chain conformations occur in each case [4]. Data analysis revealed that the observed differences were related to changes in the extensions of the polymer on the water side (LWS) and the oil side (LOS). These results can be understood in terms of changes in conformation, as salt addition screens the charged moieties more and more to the benefit of hydrophobic interactions induced by the PS monomers (figure 2).
The exceptional stability and very high reproducibility
of formation—a rare feature for multiple emulsions—
are consistent with a specific polymer conformation corresponding to a well-defined kind of emulsion. The kinetics and pathway of formation leading to multiple architectures in a single step has still to be explored. The absence of observed multiple emulsions of the O/W/O kind is probably more indicative of a slight asymmetry between bulk phases than a noticeable asymmetry in the polymer interfacial conformation.
www.ill.eu