Polyethylene nanoparticle dispersions studied by X-band CW-ESR spectroscopy

Abstract

Electron spin resonance spectroscopy (ESR) has been widely used to study properties of polymers, e.g. microviscosity or polarity, on a local scale as sensed by spin probes. The underlying principle is the very high sensitivity of rotational and lateral diffusion of a spin probe to its environment. Nitroxide free radical spin-probes have been studied most intensely for this purpose because of their rather high chemical stabilities. In general, three lines are observed in a continuous-wave (CW) ESR nitroxide spectrum due to hyperfine coupling between the electron spin and the nuclear spin of 14N, which is characterized by the splitting between the outermost extrema in the spectra (2Azz ). In addition, the shape of the ESR spectra can be strongly altered by interactions between spin probes and the surrounding medium. The film formation process from dispersions, and phase transitions, particularly glass transitions, in polymers have been studied by ESR probe or label techniques.We have recently reported aqueous dispersions of nanoscale polyethylene (PE) crystals, consisting of a single lamella of only 6 nm thickness and with a lateral pseudo-diameter of ca. 12 nm. In addition to these dispersions (I) composed of linear PE, dispersions (II) with similar overall particle size but composed of branched (50 methyl branches per 1000 carbon atoms), low crystalline PE are accessible. The uptake of fluorescence probes as guest molecules by the dispersed particles was observed by optical spectroscopy. The environment experienced by these guests can be switched by the melting/crystallization of the PE nanoparticles, and the temperature at which this occurs can be controlled by the degree of branching of the polymer. Here we present preliminary results on spin probes as a reporter for the interactions of guest molecules with the PE particles.