A pulsed EPR study of surfactant layer structure in composites of a synthetic layered silicate with polystyrene and polycaprolactone

Abstract

Double electron electron resonance (DEER), deuterium electron spin–echo envelope modulation (ESEEM) spectroscopy and 31P electron nuclear double resonance (ENDOR) spectroscopy were applied to site-specifically spin-labeled surfactants in the organically modified layered silicate magadiite and its composites with polystyrene (PS) and polycaprolactone (PCL). The organomagadiite consist of stacks of silicate platelets with surfactant layers between these platelets. In PS composites the stacks are dispersed in the polymer matrix as a whole, while melt processing with PCL leads to intercalation of polymer chains into the galleries between the platelets. The DEER data prove that even in the case of the non-intercalated PS composites the density of surfactant molecules changes drastically during composite formation on length scales of a few nanometers. Deuterium ESEEM data demonstrate that spin labels attached both in the middle and at the end of the alkyl chain have contact with the headgroups of neighboring surfactant molecules. By analysis of the 31P ENDOR spectra, two characteristic distances are found between the spin labels and the headgroups of phosphonium surfactants. The shorter, proximal distance can be assigned to headgroups in the same surfactant layer. By comparison with the basal spacing between consecutive silicate platelets the longer, distal distance is assigned to a layer of surfactants that is not attached to the surface of the next platelet but rather located between platelets. Altogether the data support a picture of trilayers of disordered surfactant molecules with their alkyl chains oriented nearly parallel to the surface.