ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 2, с. 335-341
^^^^^^^^^^^^^^^^ ФИЗИЧЕСКАЯ
ОПТИКА
УДК 535.36
RELATIONSHIP BETWEEN RELAXATION PROCESSES OF LIGHT SCATTERING IN NETWORK OF DROPLETS
© 2015 г. Soheil Sharifi
Department of Physics, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran
E-mail: ssharifi@ferdowsi.um.ac.ir Received July 14, 2014
This work presents a study of the association behavior of different length scale of tri-block polymers in aqueous solution, in the presence of oil in water microemulsion nano-droplets. We have investigated various types of multiply bridging tri-block polymers and their effect on the structure and dynamics of droplets. A detailed structural form was obtained by X-ray scattering measurements, especially with respect to the effects on the droplet sizes and even more on the interactions in the microemulsion systems induced by the bridging tri-block polymer. The results show that the size of droplets is little affected by the addition of the polymer while the interactions are modified by the presence of the polymer. The dynamic response of the systems becomes much more complex with increasing number of arms and slow relaxation processes become very pronounced due to a much more efficient network formation. The distance between diffusion coefficient of slow and fast motion of droplets is increasing with increase of length scale of bridging tri-block polymer.
DOI: 10.7868/S0030403415020191
INTRODUCTION Microemulsions are isotropic, thermodynamically
stable mixtures of two fluids forming nano-scale domains separated by an interfacial fluid film [12, 13]. In general, these fluids are water and oil and the film is made of surfactants. The surfactant at the oil-water interface lowers the surface tension. Their structure can be oil-in-water (O/W) or water-in-oil (W/O) droplets but also bicontinuous microemulsions are known. TDMAO/1-hexanol/water is a oil to water microemulsion that it is known to form short rodlike micelles that are transformed into spherical microemulsion droplets upon solubilization of hydrocarbon [14]. The addition of hexanol to the TDMAO solutions leads to a growth of rodlike micelles [15]. Parallel to this growth the amount of decane required to induce the rod-to-sphere transition increases. The phase behavior of the system TDMAO/l-hexanol/decane/water was investigated for a constant surfactant concentration of 100 mM as a function of the amount of added hexanol and decane [16].
Due to their compartmental structure oil-in-water microemulsions are considered to be complex solvents for ABA triblock polymers containing hydrophilic PEO chain and hydrophobic PDMS tails. One can imagine that such polymers can form mesophases in microemulsions by incorporating their hydrophobic PDMS blocks into the oil cores of the nano-droplets and by bridging the oil domains via their water-soluble PEO midblocks. Above a certain polymer concentration a three-dimensional polymer-microemulsion network having the typical physical properties of transient networks is formed.
Light scattered by an obstacle is related to its physical properties and, hence in principal it is possible to obtain information about scatter from analysis of the scattered light [1]. Thus, for many years, the light scattering technique has been used for inferring the size, shape and refractive index of particles in various scientific disciplines [2—4]. Areas of interest include bio-particles, macromolecules, colloidal, aerosols particles [5-8].
In light scattering experiments, the electric field of light is sufficiently weak that the system can be respond linearly to it. The theory can be explained this effect, is called linear response theory [9, 10]. Time-dependent correlation function is used before in the theory of noise and stochastic processes.
Correlation function provides a method to explain how dynamical in the system are correlated during the time. In the scattering of light, the intensity (I) depends on the positions and momenta of particles in the systems. By changing their thermal motion the position and momenta are changing in time. For the very small time t, the value of I(t+ t) is very close to I(t). Thus we can say that the value of intensity is correlated in the small time. The function that shows correlation time is autocorrelation function, which is defined by [11]
* "(0 = . (1) {I (t f
The dynamic properties of microemulsion mixed with polymer are not completely understood. So far, most of the study on dynamics was done by photon correlation spectroscopy (PCS) experiments, which for the case of networks of droplets showed two or three relaxation modes [17—21]. The fastest motion that we called Alpha relaxation was associated with the concentration fluctuations of the microemulsion droplets, the slowest motions that we called Beta and Gamma relaxation is come from the droplets in the surrounding network.
One of interesting topic in this work is the study of the relationship between the different relaxations in the mixture systems. There is not clear study in this field was done before. The mechanist of each relaxation in the mixture systems and relationship between the relaxations is important question in this study. The PDMS-PEO-PDMS tri-block polymer mixed with TDMAO/l-hexanol/decane/water can induce a network between droplets. So, several relaxations we can find in this system. We used PCS and X-ray scattering for study the dynamic and structure of droplets in the systems.
EXPERIMENTAL Materials
Tetradecyl dimethyl amine oxide (TDMAO, C14H29N(CH3)2O; Aromox 4D-W970, 24-26%) was obtained from Hoechst Co. (Gendorf, Germany). N-decane (~98%) and hexanol were obtained from Sigma Aldrich and used as supplied. Water was either of MilliQ water is used for providing all samples and Chemicals which are necessary. The tri-block PDMS(1000)-PEO(2000)-PDMS(1000) and PDMS(1000)-PEO(4000)-PDMS(1000) was obtained from polysciences company, Germany.
The samples were prepared by taking the required amount of a stock solution of 200 mM of surfactant. The appropriate amount of oil and water was added to achieve the final composition of the microemulsion (100 mM TDMAO/35 mM decane /water). The volume ratio [volume ratio = (surfactant + hexanol + de-cane)/(surfactant)] is constant for all the samples 1.75.
The polymer containing microemulsions were prepared by mixing weighted amounts of microemulsions with varying amounts of polymer and mixing with a vortex mixer under heat to ensure complete dissolution of the polymer. The polymer addition thereby led to a variation of the droplet volume fraction, which however is rather small. The concentration of polymer was explained with polymer volume fraction and it shows with O that is volume of droplet to total volume. The samples of microemulsions with polymer are prepared based on volume in terms of the volume fraction of polymer (O^ = volume of polymer/(total volume)).
Methods
Photon correlation spectroscopy (PCS): PCS pediments were performed at 25.0°C using a setup consisting of an ALV-5000/60X0 correlator, an ALV CGS-3 goniometer and a He-Ne Laser with a wavelength of 600 nm. Cylindrical glass sample cells were inculcate in an index matching toluene vat.
In the case of scatters, the intensity correlation function g®(t) measured in an experiment is related to the field correlation function g(1)(t) by the Siegert relation [22]:
g (2)(t) = 1 + B\g (1)(t)|2, (2)
where B is an instrumental constant that reflects the deviations from ideal correlation. The correlation function g(1)(t) can be written as the Laplace transform of the distribution of relaxation rates G(F):
g (1)(t) = J G(Z)exp(-r t )d r, (3)
0
where r is the decay rate and G(r) was obtained by a regularized inverse Laplace transformation of the PCS data using the CONTIN algorithm [23] implemented in the ALV software. An alternative way of analyzing multimodal relaxation processes is by fitting g(1)(x) to a stretched exponential function. For the cases discussed here a very suitable functional form was found to be a stretched exponential decay for the fast relaxation process that we called alpha relaxation together with the slower relaxation (Beta and Gama relaxation), which is given by:
g(1)(t) = At ■ exp(-r,-t)p, (4)
where the amplitudes Af, the relaxation times Ti, and the stretching parameter |3 characterize the relaxation process (stretched parameter).The collective diffusion coefficient was calculated with equation 5.
r = q 2D, (5)
where D is translational diffusion coefficient and q = = 4jxn/Asin(6/2) where n, X and 6 are refractive index of dispersant, wavelength of the laser and scattering angle.
Small angle X-Ray Scattering: Small-angle X-ray scattering (SAXS) measurements were performed using the pinholes SAXS instrument at Iran Polymer and Petrochemical Institute. A Hecus (Austria) small angle X-ray spectrophotometer S3-micropix model was employed to obtain the SAXS patterns. The experiments were done at a fixed wavelength of X = 1.54 A and two different sample-detector distances. The samples are contained in sealed Mark capillaries with a wall thickness of 1 mm. They are placed in an oven allowing temperature control with stability better than 0.5°.
g2(t)
Fig. 1. Autocorrelation function of TDMAO/1-Hexa-nol/Decane/H2O (always 100 mM TDMAO, Volume Ratio 1.75 (surfactant + hexanol + decane)/(surfactant) and 50 mM Hexanol with 80 mM decane) mixed with (a)PDMS(1000)-PEO(2000)-PDMS(1000) at Op = = 0.014, 0.044, 0.083 and (b) PDMS(1000)-PEO(4000)-PDMS(1000) at Op = 0.0165, 0.0319, 0.088.
RESULTS AND DISCUSSIONS
In order to study the dynamic behavior of the polymer bridged microemulsions, we performed PCs
measurements. Figure 1 shows the intensity autocorrelation functions g2)(0 for various concentrations of PDMS( 1000)—PEO(2000)—PDMS( 1000) and PDMS(1000)-PE0(4000)-PDMS(1000) added to the microemulsion. The pure microemulsion and microemu
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