Polymer enrichment decelerates surfactant membranes near interfaces

Close to a planar surface, lamellar structures are imposed upon otherwise bulk bicontinuous microemulsions. Thermally induced membrane undulations are modified by the presence of the rigid interface. While it has been shown that a pure membrane's dynamics are accelerated close to the interface, we observed nearly unchanged relaxation rates for membranes spiked with large amphiphilic diblock copolymers. An increase of the polymer concentration by a factor of 2–3 for the first and second surfactant membrane layers was observed. We interpret the reduced relaxation times as the result of an interplay between the bending rigidity and the characteristic distance of the first surfactant membrane to the rigid interface, which causes the hydrodynamic and steric interface effects described in Seifert's theory. The influence of these effects on decorated membranes yields a reduction of the frequencies and an amplification of the amplitudes of long-wavelength undulations, which are in accordance to our experimental findings.

Figure 1

GISANS intensities for different incident angles leading to different scattering depths $\Lambda$. For $\Lambda =54$ nm, the lamellar peak at $(Q_y,Q_z)=(0,0.32){\text{nm}}^{-1}$ starts to be well distinguished from the background. For $\Lambda =58$ nm, the isotropic contribution appears weakly at $\left|Q\right|=0.32{\text{nm}}^{-1}$. Finally, at $\Lambda =66$ nm the bicontinuous bulk structure prevails.

Figure 2

Intensity ratio of the bicontinuous and lamellar signals in a GISANS experiment on the microemulsion with polymer additive.

Figure 3

Intensity plots of film contrast GISANS measurements with logarithmic color scaling. The scattering depth was $\Lambda =21$ nm. The passing primary beam and the specular reflex are colored white due to the high intensities. The right graph was taken for the same conditions with polymer added. The white boxes indicate the range of intensity integration used to obtain the excess polymer scattering.

Figure 4

Excess polymer, diffuse GISANS intensities. The solid line interpolates the near-surface and bulk concentration with a sharp near-surface layer of $17\pm 1$ nm. Deep in the bulk, the initial polymer weight is reached, while the near-surface concentration is about three times higher.

Figure 5

Dispersion relation of the Seifert theory compared to the Zilman-Granek theory. The classical Zilman-Granek theory limited the integration $k$ range by the patch size $\xi$, which is eliminated for the Seifert theory. The additional long-wavelength modes are responsible for effectively faster relaxations of the lamellar structure close to the surface. The membrane-wall distance is denoted by $\overline{l}$.

Figure 6

Intermediate scattering function $S(Q,t)$ for the boosted and pure microemulsion at the interface (top curves) and the respective bulk measurements (shifted by $-0.2$, right axis). The fits are obtained for the Seifert dispersion relation in the Zilman-Granek theory (thin black lines). For the near-surface relaxations, phenomenological fits with a stretching exponent $\beta \approx 0.5$ are included (thick grey lines).

Figure 7

Relaxation times for different scattering depths and the bulk microemulsion with polymer (black: measured at J-NSE, green: measured at SNS-NSE) and without polymer (orange: J-NSE) obtained from stretched exponential fits.