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Density and confinement effects of glass forming m-toluidine in nanoporous Vycor investigated by depolarized dynamic light scattering
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View: Figures


Image of FIG. 1.
FIG. 1.

Sketch of a resealable cylindrical sample cell for the cold finger optical cryostat. The cell allows for optical access in the range of 0°–180° and establishes good thermal contact between the sample and the cold finger, while at the same time being of comparatively large diameter to reduce the lens effect. Indicated components: (1) copper shell; (2) quartz tube; (3) carbon-filled PTFE layer; (4) + (5) brass screws; and (6) PTFE seals.

Image of FIG. 2.
FIG. 2.

The degree of heterodyning in a mixed-signal light scattering experiment can be estimated by comparing the scattered intensity of the mixed signal ⟨I⟩ with the scattered intensity ⟨I s ⟩ in the corresponding homodyne experiment. In that case the intercept of the mixed-signal intensity correlation function , is related to the ratio ⟨I s ⟩/⟨I⟩ as given by Eq. (3) shown as solid line. The data points are from measurements of bulk m-toluidine in the small cuvette, where a certain degree of heterodyning occurs due to reflexes because of the strong surface curvature. Different ratios C are related to the laser beam passing through the cuvette at slightly different positions. Estimates for ⟨I s ⟩ ≈ 1 kHz and g 1(0) ≈ 0.6 are derived from homodyne scattering experiments in the large sample cell.

Image of FIG. 3.
FIG. 3.

Raw data of the intensity autocorrelation function of bulk m-toluidine and m-toluidine in Vycor. For better visibility these data are normalized to an initial value of 0.5. Due to partially heterodyne detection a second, temperature independent correlation decay is observed in the millisecond regime, which allows to describe the data via Eq. (2) and to unambiguously extract the field autocorrelation function g 1(t) from the data as shown in the upper part.

Image of FIG. 4.
FIG. 4.

Autocorrelation function of the electric field for bulk m-toluidine measured in a fully homodyne light scattering experiment. Temperature decreases in steps of 2 K from left to right. (Solid lines) Fits with Eq. (5) , which includes an excess wing contribution to account for the deviations of g 1(t) from KWW behavior at low temperatures. For comparison a KWW-fit in the range from 0.1 to 104 s is shown for the lowest temperature (dashed red line). In the upper part of the figure the short-time decay of the lowest temperature correlation function is again shown separately for clarity.

Image of FIG. 5.
FIG. 5.

The laser beam passing through the cuvette with a rod of nanoporous Vycor immersed in m-toludine. Both pictures show the same sample mounted on the cold finger in the vacuum chamber of the optical cryostat at 196 K after cooling from room temperature. (Left) Cooling performed at 1 K/min and (right) at 0.02 K/min, respectively. At standard cooling rates the sample becomes turbid (left), while it stays clear at very low cooling rates (right) even below the bulk glass transition temperature.

Image of FIG. 6.
FIG. 6.

The field autocorrelation function extracted via Eq. (2) from partially heterodyne intensity correlation functions for bulk (full symbols) and fully heterodyne correlation functions for confined (open symbols) m-toluidine at different temperatures. Solid lines represent fits with a KWW function. Residual oscillations are due to an imperfect description of the phase shift decay by Eq. (4) , as discussed in Sec. II .

Image of FIG. 7.
FIG. 7.

Average correlation times and stretching parameter βKWW for m-toluidine in bulk (squares) and in confinement of 6.8 nm Vycor pores (diamonds). Full squares: τ and βKWW extracted from fully homodyne intensity correlation functions; open squares: τ and βKWW extracted via Eq. (2) from the partially heterodyne . Open diamonds show the correlation times and stretching of confined m-toluidine extracted in the latter way. For comparison a calorimetric time constant is calculated for bulk m-toluidine via Eq. (6) (black pentagon).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Density and confinement effects of glass forming m-toluidine in nanoporous Vycor investigated by depolarized dynamic light scattering