Light scattering measurement systems

Fig. 8. Time-resolved light scattering measuring system combined with a short laser pulse...

Figure 2.30. Schematic of the geometry around a sample ceU in a light-scattering measurement system. A photodetector detects the light scattered by a polymer solution in the beam path into a direction at angle 6 from the forward direction. The vat is filled with an indexmatching liquid.

Figure 3.1. Dynamic light scattering measurement system. The pulse-amphfier discriminator converts the analog signal of the photodetector, I t), into a digital signal, which is further converted by the autocorrelator into the autocorrelation function of the signal.

This stipulation of the interaction parameter to be equal to 0.5 at the theta temperature is found to hold with values of Xh and Xs equal to 0.5 - x < 2.7 x lO-s, and this value tends to decrease with increasing temperature. The values of = 308.6 K were found from the temperature dependence of the interaction parameter for gelatin B. Naturally, determination of the correct theta temperature of a chosen polymer/solvent system has a great physic-chemical importance for polymer solutions thermodynamically. It is quite well known that the second viiial coefficient can also be evaluated from osmometry and light scattering measurements which consequently exhibits temperature dependence, finally yielding the theta temperature for the system under study. However, the evaluation of second virial... 

For accuracy in light-scattering measurement the proper choice of solvent is necessary. The difference in refractive index between polymer and solvent should be as large as possible. Moreover, the solvent should itself have relatively low scattering and the polymer-solvent system must not have too high a second virial coefficient as the extrapolation to zero polymer concentration becomes less certain for high A2. Mixed solvent should be avoided unless both components have the same refractive index. 

Three molecules of the host-guest complex can associate in three-molecular associate (Figure 9.10). This structure was confirmed by light scattering measurements. When free matrix was added to the associated system, dissociation occurs (Figure 9.11). 

The observation of crossover has later been substantiated by several other studies. In particular, Jacob et al. [165] performed light scattering measurements on the system 3-MP + water + NaBr. The data indicate comparatively sharp crossover in the range 10-4 salt concentration. It is intriguing to characterize this crossover by a suitably defined crossover temperature Tx, defined here by the point of inflection in the T-dependence of the effective exponent yeff. Figure 8 shows fx as a function of the amount of added NaBr. Eventually, plain mean-field behavior is obtained in a solution containing about 16.8 mass% NaBr. 

All experimental measurements were carried out with a Coulter EPICS V System. This instrument is a laser-based flow cytometer which, as one of its simpler analytical functions, utilizes light scatter measurements to accurately size cells or similar particles (e.g., artificial surfactant-stabilized microbubbles) suspended in aqueous media. The light scatter measurements are sensitive to particle sizes as small as 0.3 pm in diameter. 

As we shall see, the intensity, polarisation and angular distribution of the light scattered from a colloidal system depend on the size and shape of the scattering particles, the interactions between them, and the difference between the refractive indices of the particles and the dispersion medium. Light-scattering measurements are, therefore, of great value for estimating particle size, shape and interactions, and have found wide application in the study of colloidal dispersions, association colloids, and solutions of natural and synthetic macro-molecules. 

In contrast to osmotic pressure, light-scattering measurements become easier as the particle size increases. For spherical particles the upper limit of applicability of the Debye equation is a particle diameter of c. A/20 (i.e. 20-25 nm for A0 600 nm or Awater 450 nm or a relative molecular mass of the order of 10 ). For asymmetric particles this upper limit is lower. However, by modification of the theory, much larger particles can also be studied by light scattering methods. For polydispersed systems a mass-average relative molecular mass is given. 

K and a in the expression, [17] = KM , are 3.7 x 10-5 m3 kg-1 and 0.62, respectively, for this polymer-solvent system. Assuming a constant density for the solutions, calculate an average relative molecular mass for the polystyrene sample. How would this relative molecular mass be expected to compare with the relative molecular mass of the same sample of polystyrene in toluene determined from a) osmotic pressure and (b) light-scattering measurements ... 

Figure 32. The a-relaxation times for the glass formers studied in the present work (cf. Fig. 27). In addition data of diglycidyl ether of bisphenol A (DGEBA) and phenyl glycidyl ether (PGE) are included time constants as obtained from DS data sets of m-TCP and 2-picoline were combined with xrl from conductivity and light scattering measurements, respectively, (a) Relaxation times as a function of T Ts. The systems differ by the slope of Ta at Tg. (b) By plotting xr, as a function of the rescaled temperature z = m(T/Tg — 1) the effect of an individual fragility is removed and a master curve is obtained for systems with similar To. Solid line represents Eq. (41) with Kf) — 17. (c) Upper part master curve for xa according to Eq. (42). Deviations of the data from Eq. (42) (solid line) indicate break-down of the VFT equation. Lower part The ratio lg(ra/rvft) shows deviations from a VFT behavior most clearly. Dashed vertical lines indicate shortest and fastest tx, respectively, observed. All the figures taken from Ref. [275].

Refractive index and specific refractive index increments - (k = dn/dc) of polymers in solution have been studied extensively in connection with light scattering measurements and size exclusion chromatography applications to polymer characterization for which refractometers are used as standard concentration detectors. Contrary to the observations made in the infrared region (12), refractive index increments have been shown to be a function of the molecular weight of the polymers (2) and, in some cases, of the copolymer composition (17). Therefore, the assumptions of linearity and additivity (Eq. 1 to 4) have to be verified for each particular polymer system. In the case of styrene/acrylonitrile copolymers, there is an additional uncertainty due to the... 

Light scattering measurements and theoretical treatment strongly support the idea that attractive interactions between inverse micelles play an important role in the stability of oil rich microemulsions. In the system containing pentanol, attractions between (i)/o micelles can be sufficient to give rise to a phase separation between two microemulsion phases. 

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