Jaroslav Holoubek

Light scattering and reflectance of optically heterogeneous polymers in multiple scattering regime

Abstract A simple approach is used to measure the reduced scattering coefficient μ s ′ of a thick turbid medium having a small absorption coefficient μ a ′⪡ μ s ′. Based on the light reflectance measurement of semi-infinite media by means of a CCD camera, the value of shift Δ x between the centre of the incident laser bears and the centre of the diffuse background is determined, which is inversely proportional to the μ s ′. The experimental values of μ s ′ determined for EPDM particles in a polypropylene matrix are compared with model calculations using the Lorenz-Mie theory. A comparison of theoretical data with parameters from image analysis of SEM pictures gives good accord.

Light scattering by optically heterogeneous polymer systems: stress whitening in polypropylene materials

SummaryA theoretical background is presented which allows the assessment of the physical nature of turbidity in heterogeneous polymeric materials. The theory (based on the diffusion approximation of the transfer theory and Kubelka-Munk theory) predicts a decreasing spectral dependence of turbidity with increasing wavelength for a matrix with embedded particles of slightly different refractive indices, but a flat dependence of this quantity for a matrix material with microvoids. It is demonstrated that the diffuse reflectance displays the same type of wavelength dependence for the thick layer approximation. Indeed, diffuse light reflectance experiments on bulk specimens using an integrating sphere accessory reveal the first type of behaviour for nondeformed neat and rubber-modified polypropylenes. On the other hand, the second type of behaviour was observed with stress-whitened neat and rubber-modified polypropylenes after solid-state drawing.

Multiphase Polymer Systems: Morphology and Optical Properties by Light Scattering Methods

Time-resolved light scattering, diffuse reflectance measurements and image analysis were conducted to investigate the morphology and supermolecular structure of two polymer systems: (i) polymer composites with dispersed particles in the polymer matrix and (ii) co-continuous structure of polymer blends. We used a video reflectometer to measure the reduced scattering coefficient from a series of non-oriented samples made of polypropylene composites (polypropylene matrix-EPDM particles) and polycarbonate-poly(methyl methacrylate) (PC-PMMA) blends with various amounts of anisometric PMMA minor phase. The time evolution of the co-continuous structure in polystyrene-PMMA blends was studied during an annealing process. The spinodal peak position, qmax(t,T), (where qmax is the wavenumber of the periodicity in the co-continuous structure) and the corresponding intensity Imax(t,T) was compared with theoretically predicted values of exponents for distinct time scales of the phase dissolution under various temperature regimes. The influence of diblock copolymers on the kinetics of phase separation and dissolution of polymer blends with a co-continuous structure was studied.

Small-angle light scattering calculations and evaluation: application to phase separation and phase dissolution processes

The model calculations and corresponding experiments (time-resolved light scattering and image analysis) have been conducted to investigate the kinetics of phase separation and phase dissolution of polymer blends. The blends studied were polystyrene-d8/polybutadiene (PSD/PB) and polystyrene/poly (methyl methacrylate) (PS/PMMA), both with or without diblock copolymers composed of the corresponding homopolymers, and a PS/poly(methyl vinyl ether) (PS/PMVE) blend. The time evolution of the spinodal peak position qm(t,T) and the scattered intensity maximum Im(t,T) at qm have been compared with theoretically predicted values of exponents for distinct time scales of the phase dissolution in various temperature regimes.

Note on light attenuation by scattering: comparison of coherent and incoherent (diffusion) approximations

A simple procedure characterizing light attenuation by transmission (based on the diffusion approximation of the transfer theory, the asymmetry factor, and the effective refractive index of the scattering medium) is compared with the coherent approaches based on (1) published Monte Carlo simulations, (2) scattering from a system of closely packed spherical particles with interparticle correlation characterized by the Percus-Yevick approximation, and (3) experimental data. Use of the procedure in the analysis of phase separation in polymer blends is presented. It is shown that the results obtained by this procedure are in reasonable accord with theoretical and experimental data so far published for systems with a volume fraction of minor phase less than ~0.2 and without any specific interaction in the system.