Polymer-induced transient networks in water-in-oil microemulsions studied by small-angle x-ray and dynamic light scattering

We study water-in-oil microemulsions, in particular dispersions of water droplets coated with a monolayer of the anionic surfactant AOT in a continuous phase of $n$-decane. Upon addition of the amphiphilic triblock copolymer PEO(polyethylenoxide)-PI(polyisoprene)-PEO, a transient network is formed. At constant droplet size we vary the polymer concentration and there is clear evidence for an increasing crosslinking of the droplets from structural investigations with small-angle x-ray scattering. The dynamics of concentration fluctuations consisting of the translational diffusion of the droplets and the relaxation of the network are monitored with photon correlation spectroscopy. We mainly focus on the variation of the dynamic behavior as a function of the number of polymer molecules per droplet and the droplet volume fraction, which may be taken as a measure for the interdroplet distance. With increasing polymer content the dynamics of the system slows down and three different relaxation processes may be distinguished. We discuss the origin of the different relaxation modes. In particular, it turns out that the intermediate relaxation mode may be suppressed by index matching the oil matrix and the PI block and that it is effectively slowed down by an additional loading of the emulsion droplets with polyethylene glycol of increasing molecular weight.

Figure 1

(Color online) Schematic representation of the water-in-oil microemulsion with triblock copolymer PEO-PI-PEO both in decoration and bridging configuration.

Figure 2

(Color online) Polymer concentration vs droplet volume fraction of the systems investigated in the present work. The applied methods are photon correlation spectroscopy and small angle x-ray scattering as indicated.

Figure 3

(Color online) SAXS intensity profile of the pure microemulsion system water-AOT-decane for a droplet size of $\omega =40$ and selected droplet volume fractions $\varphi$. Each data set is vertically shifted by a factor of 10. The solid lines show fits of the hard-sphere model described by Eq. (4).

Figure 4

(Color online) Water core radius $R_w$ of the pure microemulsion as a function of droplet volume fraction. Up to $\varphi \approx 0.12$ size and volume fraction (i.e., average distance of the droplets) are independent quantities.

Figure 5

(Color online) Collective diffusion coefficient $D_c$ of the pure microemulsion system as a function of droplet volume fraction $\varphi$. Out of the linear range at low $\varphi$ the free diffusion constant $D_0$ is determined. Inset: Comparable plot for an oil-in-water microemulsion with nonionic surfactant ${\text{C}}_{12}{\text{E}}_5$ taken from Ref. [17].

Figure 6

(Color online) SAXS intensity profiles $I\left(q\right)$ of a microemulsion with $\omega =40$ and $\varphi =0.1$ at increasing numbers of polymer chains per droplet $Z$.

Figure 7

(Color online) Structure factor part $S\left(q\right)$ from the fits to the SAXS intensity profiles in Fig. 6 reflects increasing order in the system with increasing polymer content.

Figure 8

(Color online) Squared electric field autocorrelation function $g_1\left(t\right)$ for a microemulsion with varying polymer content $Z$ at $\varphi =0.1$ and $\omega =40$ measured at $T=298\text{K}$ and at a scattering angle of $2\theta =90°$. For clarity the curves are vertically shifted. Solid lines are fits with up to three stretched exponential relaxation functions.

Figure 9

(Color online) Diffusion constants of the fast (squares), intermediate (circles), and slow (triangles) relaxation process as a function of the number of polymer chains per droplet $Z$ at constant droplet volume fraction $\varphi =0.1$. Dashed lines: Guide to the eye.

Figure 10

(Color online) Diffusion constants of the fast (squares), intermediate (circles), and slow (triangles) relaxation process as a function of the droplet volume fraction at a constant number of polymer chains per droplet of $Z=5.1$. Dashed lines: Guide to the eye.

Figure 11

(Color online) Correlation functions of a microemulsion with triblock copolymer ($Z=5.1$ and $\varphi =0.1$) with decane and dipentene as respective continuous oil phase. Spectra were taken at $T=298\text{K}$ at a scattering angle of $2\theta =130°$.

Figure 12

(Color online) Diffusion constants of the first and the second relaxation process in a network of micelles $\left(\omega =40,\varphi =0.07,Z=5\right)$ loaded with a mixture of water and polyethylene glycol of varying molecular weight at a concentration of $c_{\text{PEO}}=0.1\text{g}∕\text{ml}$. Multiple points at the same molecular weight represent repeated measurements of the same sample.