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Local dynamics of polymers by spin-label EPR

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Electron paramagnetic resonance (EPR), often also referred to as electron spin resonance (ESR) is a powerful spectroscopic technique for the study of structure and dynamics of paramagnetic species. Most polymeric materials, however, are diamagnetic and thus do not exhibit EPR signals. The absence of signals from the bulk material provides the opportunity to apply EPR as a selective probe technique, whereby stable paramagnetic species such as nitroxide radicals are introduced artificially. These radicals are called spin labels if they are covalently linked to the macromolecules.

Rotational dynamics of such spin labels on time scales between 10 ps and 1 µs can be characterized by the basic, fast and sensitive continuous wave (CW) EPR analyzing the line shape. The rotational dynamics of the spin labels is influenced by the restricted motion of the side group bearing the spin label and local polymer backbone motions at the point of the covalent linking of the spin label.

Fields of work

We are applying the EPR spin-label technique to study local dynamics in polyelectrolyte systems such as polyelectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). We employ the fact that the EPR line shapes are sensitive to the mobility of the spin label which informs on polymer and label dynamics. The segmental rotational mobility of the spin-labeled macromolecules is determined by simulating and fitting the line shapes of experimental EPR spectra using the EasySpin software.

Equipment

  • Continuous wave (cw) X-band (9.5 GHz) EPR spectrometer EMX-plus (Bruker BioSpin), with digital temperature control unit ER 4141VT (100-500 K)

Examples

Fig. 1: Experimental and simulated EPR spectra of SL-P(E-alt-MA) and of a complex of SL-P(E-alt-MA) and PDADMAC in aqueous solution.

Complex Coacervation of Polyelectrolytes

We have studied the formation of complexes of the weak polyanion poly(ethylene-alt-maleic acid) (P(E-alt-MA)) with the strong polycation poly(diallyldimethylammonium chloride) (PDADMAC) using the spin-labeled polyanion as reporter molecule.

It was found that, if the spin-labeled polyanion is the excess component, the spectrum of a slow-motion component is superimposed by the spectrum of a fast-motion component. This indicates that the spin labels are located both in the core and in the shell of the PEC particles. In the opposite case, if the polycation is in excess, the spectra are dominated by a slow-motion component indicating that nearly all spin labels are located in the core.

Related publications

  • U. Lappan, B. Wiesner, U. Scheler
    Rotational Dynamics of Spin-Labeled Polyacid Chain Segments in Polyelectrolyte Complexes Studied by CW EPR Spectroscopy
    Macromolecules 48 (2015) 3577-3581
  • U. Lappan, B. Wiesner, U. Scheler
    Complex coacervation of polyelectrolytes studied by spin-label EPR spectroscopy

    Macromol. Chem. Phys. 215 (2014) 1030-1035
  • U. Lappan, B. Wiesner, St. Zschoche, U. Scheler
    Polyelectrolyte Multilayers Studied by Electron Paramagnetic Resonance (EPR) Spin-Label Technique
    App. Magn. Reson. 44 (2013) 181-188