Polymer degradation

HALS successfully prevents polymer photoxidation.
The degradation of polymers due to light exposure leads to discoloration of the polymer and a decrease in the mechanical properties (elasticity, toughness, etc). To prevent this decomposition, hindered amine light stabilizers (HALS) are added to the polymer. By monitoring the EPR signals of these light stabilizers, their effectiveness can be evaluated using the EMXnano. Read more...

Photo initiation and radical formation upon light irradiation
The EPR signal generated in the polymer during UV irradiation (left) is completely suppressed after addition of the HALS where only the HALS EPR spectrum is observed (right)

Polymer structure

EPR studies of polyelectrolyte multilayer (PEM) films using nitroxide spin labels.
Multilayers of polyelectrolytes (polymers bearing dissociated ionic groups) are formed by the alternating adsorption of oppositely charged polyelectrolytes, so called layer-by-layer technique. PEM films composed of strong polycation and weak polyanion that is usually spin labeled with free nitroxide (4-amino-TEMPO) are studied by EPR. The growth of the PEM films is monitored and quantitative EPR analysis provides information about each double layer.

Signal intensity of polycation/TEMPO-labeled polyanion multilayer films in contact with buffer solution of pH 4 in dependence on number of double layers NDL.

Paint properties

HALS EPR signal in paint indicating deterioration after UV exposure.
The main cause of paint film deterioration is the degradation of several components, including the binder and certain pigments. This is caused by the formation of free radicals from prolonged exposure to UV light (sunlight), moisture and freeze-thaw cycles. Free radicals are highly reactive and either form or breakdown chemical bonds in substances. In the case of paint durability on exposure, free radicals actually damage the film. This process is very similar to how skin ages. Skin contains free radicals that, when exposed to years of sunlight, will show signs of aging, including wrinkling, peeling, sun spots and overall dryness.

EPR spectra of HALS (hindered amine light stabilizers) detected in paint after UV exposure

Solar cells

Defects in amorphous silicon detected by EPR.
Silicon is the most common material for the production of solar cells in the photovoltaic industry either in mono- or polycrystalline form. Specific characterization of paramagnetic defects can be done by EPR to gain insight into how paramagnetic centers induced by degradation influence the efficiency of solar cell active layers. EPR studies on amorphous silicon photovoltaics demonstrated that a strong relationship exists between the presence of paramagnetic defects and the resulting charge collection efficiency in such material.

Light-induced defect in amorphous silicon detected by EPR due to breaking of weak Si-Si bonds

Screening DNP agents

EPR spectrum and dipolar coupling determination of bis-TEMPO.
Correct concentration of DNP polarizing agents is crucial to the success of a DNP experiment. Samples can be pre-screened before DNP experiments using the patented SpinCount module, even in the MAS rotor. Relaxation times are critical for DNP efficiency therefore P1/2 measurements at low temperature to estimate the DNP efficiencies of new polarization agents are invaluable. Another characteristic of importance in DNP measurements is the electron-electron dipolar coupling that is easily measured from solution and frozen solution EPR spectra. Read more about DNP agents

EPR spectrum of DNP agent (bis-TEMPO biradical)
Data courtesy of Prof. Thomas Prisner, University of Frankfurt (Angew. Chem. Int. Ed., 2009, 48, 4996)
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