The Dynamic Properties of 5Cb Filled with Aerosil Particles Investigated by Pcs
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This paper focuses on the dynamic behavior of the nematic liquid crystal (5CB) filled with Aerosil particles investigated by the photon correlation spectroscopy (PCs). The dynamic and static light scattering experiments show significant changes in the physical propertxies of filled LC and suggest that there is some evidence for glass-like dynamical behavior, although bulk liquid crystal does not have glassy properties in both anisotropic and isotropic phases. EXPERIMENT We performed photon correlation and static light scattering measurements using a A N 0.6328gm He-Ne laser and the ALV-5000/Fast Digital Multiple Tau Correlator (real time) operating over delay times from 12.5 ns up to 103 s with the Thorn EMI 9130/100B03 photomultiplier and the ALV preamplifier. In addition, we used ALV/LSE unit which allows, simultaneously with photon correlation measurements, monitoring of the laser beam intensity and stability of its space position as well as monitoring of the intensity of scattered light integrated over all frequencies (static light scattering). The depolarized component of scattered light was investigated. All dynamic light scattering data that we discuss below were obtained at 6=30'. In the dynamic light scattering experiment, one measures the intensity-intensity autocorrelation function 92 (t) = (!(t)/(0))/(!(0))2. The intensity-intensity autocorrelation function g2(t) is related to the dynamic structure factor f(q,t) of the sample by 2 g2 (t) = 1 + kf (q,t),
where k is a contrast factor and q = 47rnsin(EJ/2)/A, (n is the refractive index, O - the scattering angle and A is the wavelength of laser radiation). In our experiments the nematic liquid crystal 5CB filled with 2.3 volume percent of Aerosil particles was placed between two untreated glass plates separated by 50 gm teflon spacers. The spherical primary particle of amorphous silicon dioxide with an average diameter 10 nm (Aerosil from Degussa) form aggregates of irregularly branched strings moieties. Hydroxyl OH groups on the surface of Aerosil particles form hydrogen bonds between different particles leading to the formation of agglomerates [13]. 5CB filled with two types of particles was investigated: (1) hydrophilic Aerosil A200 anid (2) hydrophobic Aerosil R974. In hydrophobic Aerosil, about 70 %of the surface hydrophilic groups are replaced by hydrophobic groups reducing the interaction with liquid crystal and between particles. RESULTS AND DISCUSSION From static light scattering experiments we obtained that the temperature of the nernatic - isotropic (N-I) phase transition in filled 5CB is depressed compared to that in the bulk liquid crystal. The transition is not as sharp as in the bulk LC, it is smeared out and occupies a finite temperature region. The phase transition temperatures were found to be 307.65 K (hydrophobic sample) and 307.60 K (hydrophilic sample). These temperatures are lower with respect to the phase transition temperature in bulk 5CB (308.1 K). In both systems the temperature hysteresis of the phase transition on cooling and
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