Light Scattering by Shock-Cooled Isotactic Polypropylene Film

Abstract

An absolute low‐angle light scattering photometer is described which is used for the quantitative evaluation of the light scattering exhibited by shock‐cooled isotactic polypropylene films. Following Stein and Wilson the supermolecular order is considered to be describable in terms of a density correlation function γ(r) and an orientation correlation function f(r), which are both dependent only upon the magnitude of the distance r between individually scattering volume elements. By means of Fourier transformations performed upon the horizontally and vertically polarized components of the scattered light under polarized incident light, these functions can be obtained. It is found that γ(r)=exp(−r²/a²), with a ≅40 000 Å and f(r)=B exp(−r²/b²)+C exp(−r²/c²), with B=0.544, b=4500 Å, C=0.456, and c=17 000 Å. The density correlation thus reaches much further than the orientation correlation. This is ascribed to the poorly developed spherulitic crystallization due to the shock cooling of the sample. This meaning of the splitting up of f(r) into two Gaussian distributions needs to be explored further. The occurrence of a Gaussian density correlation function γ(r) rather than the simple exponential function γ(r)=exp(−r/a) found for amorphous polymers by Debye and Bueche, might be ascribed to the spherical crystalline aggregates in the sample, which are visible under the polarizing microscope. Calculation shows that a collection of spheres will indeed give a scattering pattern approximately describable by γ(r)=exp(−r²/a²). Further work on samples with varying thermal history will have to reveal the connection between the correlation functions and the molecular processes leading to specific supermolecular arrangements.