Isorefractive matrix

Chu et al. have studied the structure and dynamics of a polymer solution composed of PS(polystyrene), PMMA(poly(methyl methacrylate)), TOL(toluene), and CNA(a-chloronaphthalene). According to the photocount intensity-intensity time correlation function measurements of the PMMA probe in the PS/MS(mixed solvent) isorefractive matrix, one surprising feature was that at least two dominant characteristic modes appeared even at small scattering angles. While the slow mode could be identified with the translational motion of the center of mass of the PMMA probe chain, it remained unconfirmed that the fast mode might be related to a coupling of PMMA motion with the cooperative motion of the isorefractive PS/MS(matrix). 

Foley and Cohen [26] analyzed concentration fluctuations in polymer polymer solvent mixtures, using an ornate Flory-Huggins form for the thermodynamic free energy of the mixture and neglecting hydrodynamic interactions. The case of a solvent isorefractive with the matrix polymer was treated. If both polymer species were nondilute, the relaxation spectrum was predicted to have three relaxation times. 

Although cannot be measured in DLS, a closely related tracer diffusion coefficient Dj can be measured. In the tracer diffusion, the motion of a labeled solute called a probe or a tracer is traced selectively. A second solute called a matrix is added to the solution and its concentration is varied, whereas the concentration of the probe molecules is held low. The matrix must be invisible, and the probe must be visible. We can give a large contrast between the matrix and probe by choosing a pair of solvent and matrix that are nearly isorefractive, i.e., having the same refractive index. Then, the light scattering will look at the probe molecules only. For instance, we can follow the tracer diffusion of polystyrene in a matrix solution of poly(dimethyl siloxane) in tetrahydrofuran. 

The cross-hnked network of polymer is called a gel. In the absence of the cross-linking, the swollen gel would form a solution. A probe polymer is added to the gel matrix, and the motion of the probe chains is traced by using DLS. The solvent needs to be isorefractive with the gel matrix to allow one to selectively detect the... 

Figure 4.41. Tracer diffusion coefficient Di of polystyrene in solutions of matrix polymer, poly(vinyl methyl ether), in o-fluorotoluene at various concentrations of the matrix polymer, plotted as a function of molecular weight Mps of polystyrene. The solvent is isorefractive with the matrix polymer. The concentration of the matrix polymer and the slope obtained in the best fitting by a power law (straight line) are indicated adjacent to each plot. (From Ref. 53.)...

At penetration of monomer-diffuser to the gel-polymer matrix along the radiuses of spherical (or cylindrical) surface the front of spreading of gradient-former is compressed, homogenous gradient layer is formed with parallel stratified isorefraction (in the view of concentrated sphere or coaxial tubes) (Figure 2). 

If the gel-polymer matrix is viewed as a spherical segment, than at the periphery of matrix will be also damage the parallelity of isorefraction, but differences in this region will be decreased the curvature radius of isorefractive layers. The analogous judgment may be spread for the matrix with concave spherical surface [61]. 

The first application of QELSS to measure Ds in the presence of an isorefiracfive matrix polymer solvent pair was due to Hadgraft, et al.(i). Figure 8.15 presents their measurements. Their seminal study introduced polymethylmethacrylate as a matrix polymer having a usable isorefractive solvent. PMMA and polystyrene are not compatible, so Rg of the polystyrene probes might well have changed with changing PMMA concentration however, for the polystyrene PMMA systems with P < M,Ds is nearly independent of c. 

Quasielashc light scattering was considered in detail in Chapter 4. The discussion here starts where Chapter 4 left off. The optical probe method refers to the special case of measurements on a ternary solvent matrix polymer rigid probe system in which the matrix polymer does not scatter significantly, while the probe species dominates scattering from the system even though it is dilute. Optical probe studies are formally similar to QELSS studies of ternary polymer polymer solvent systems containing an isorefractive polymer solvent pair. 

The probe diffusion is an attractive technique in DLS, where the diffusion of optically labeled molecules (or particles) in matrices is measured by DLS. The probe diffusion is useful for investigating matrix solutions as well as the diffusion of the probe itself Optical labeling with no chemical or special modifications on the probe is realized either by contrast matching or by using a probe with relatively strong depolarized components of scattered light. In contrast matching, an isorefractive solvent to the matrix solute is chosen. Optically aniso-... 

---