Effects of segregation of marked and normal species in SANS from crystalline polymers

Abstract

The use of deuterium marking has proved to be very valuable in studying single-chain behavior in amorphous polymer systems. However, it has been difficult to use the technique for crystalline polymers because of anomalous forward scattering. This scattering has been attributed to segregation of the normal and deuterated species resulting from the small differences between them. Computations of the effects of segregtion have all assumed the absence of correlation between concentration and density fluctuations. This assumption precludes the possibility of different concentrations of marked polymer in the amorphous and crystalline regions of the sample. To explain the magnitudes of the anomalous scattering the previous computations required what seems to be extreme segregation of the two species. Here we compute the effects of concentration variations between the amorphous and crystalline regions and show that even modest differences in concentration can produce the observed magnitudes of the anomalous, forward scattering. Unfortunately the results seem to show that single chain SANS experiments in segregated, crystaline polymers are fundamentally problematical.