EPR-spectroscopy 1. EPR setup

This section is to a major part adopted from ref.

The EPR spectra were recorded on a Bruker Elexsys E Bruker SHQE-W1 cavity equipped with a grid for

light source (see Figure 3) was appropriated in conjunction with 100 MHz field modulation frequency. The EPR settings: receiver gain, microwave power, sweep time, field mod

amplitude and time resolution were optimized for each system under investigation.

different detectors with specific sensitivities intensity vs. magnetic field the ADF

constant field vs. time after laser pulse initiation the

channel, TC, was used. The detection channels refer to different sensitivities and maximum time resolutions which are 1.28 ms for SC and 500 ns

typically carried out between −40 and +80

4131VT unit (Bruker) by purging the sample cavity with nitrogen.

measurements, the spectrometer is synchronized w Composers 9314 pulse generator (Scientific Instruments).

Figure 3. Scheme of the experimental setup used for the SP

sample is positioned into the EPR cavity and irradiated with UV light through a grid (shown in the expanded cavity section).

MATERIALS AND EXPERIMENTAL

dopted from ref.^[30]

The EPR spectra were recorded on a Bruker Elexsys E 500 series cw-EPR spectrometer.

equipped with a grid for irradiation of the sample by an external ) was appropriated in conjunction with 100 MHz field modulation The EPR settings: receiver gain, microwave power, sweep time, field mod

time resolution were optimized for each system under investigation.

with specific sensitivities were applied. For measurements of ADF signal channel, SC, and for monitoring EPR intensity at constant field vs. time after laser pulse initiation the fast digitizer acquisition board,

The detection channels refer to different sensitivities and maximum time resolutions which are 1.28 ms for SC and 500 ns for TC detection. Experiments were 40 and +80 °C. Temperature control was achieved via an ER 4131VT unit (Bruker) by purging the sample cavity with nitrogen. For time

measurements, the spectrometer is synchronized with the pulse laser via a Quantum Composers 9314 pulse generator (Scientific Instruments).

Scheme of the experimental setup used for the SP-PLP-EPR measurements.

sample is positioned into the EPR cavity and irradiated with UV light through a grid (shown

XPERIMENTAL SETUP

EPR spectrometer. A irradiation of the sample by an external ) was appropriated in conjunction with 100 MHz field modulation The EPR settings: receiver gain, microwave power, sweep time, field modulation time resolution were optimized for each system under investigation. Two or measurements of EPR EPR intensity at fast digitizer acquisition board, time The detection channels refer to different sensitivities and maximum Experiments were Temperature control was achieved via an ER For time-resolved ith the pulse laser via a Quantum

EPR measurements.^[86] The sample is positioned into the EPR cavity and irradiated with UV light through a grid (shown

MATERIALS AND EXPERIMENTAL SETUP 31 4.2.2. Sample tubes

Material

EPR tubes and flat-cells consist of synthetic quartz (Suprasil^®) in their critical areas. This material features high optical homogeneity, high transmission of UV irradiation in the experimentally relevant range between 200 and 1000 nm and low trace impurities which may decrease the S/N of EPR measurements by dielectric loss.^{[87, 88]}

Sample volumes and cell geometry

Polymerization kinetics restrict the concentrations of detectable propagating radical species to values typically below 5·10⁻⁵ M for instationary and 5·10⁻⁶ M for stationary UV-initiated polymerization. One the one hand, a large sample volume is generally preferred for EPR measurements, since S/N depends on the absolute number of radical species contained inside the EPR resonator cavity. On the other hand, interaction between the electric field of the irradiated microwave and dipoles in polar samples causes dielectric loss of microwave power. This generally provokes decreasing S/N, highly polar samples such as aqueous solutions may not be investigated by EPR in conventional cylindrical EPR tubes at all.

Dielectric loss can be minimized by concentrating the sample volume in a vertical plane within the rectangular EPR resonator cavity, at which the electric field density of the microwave has its local minimum. A practical strategy is reducing the inner diameter of cylindrical EPR tubes. Simple cylindrical tubes are preferable for investigations into polymerizations. They are accessible at low cost in a broad range of inner diameters between 5 and 1 mm, so that samples can be prepared in high quantities. The decreased absolute number of detectable radicals in small sample volumes may however disfavor measurements in cylindrical tubes. A second strategy for measurements in highly polar samples is using specially designed EPR flat-cells. The inner thickness of flat-cells is reduced to ca. 0.4 mm, but internal width and height of the flat region are concomitantly increased to yield sufficiently large sample volume. Flat-cells have apertures at top and bottom end, they can be filled with highly viscous materials by applying pressure from one side. Disadvantages of flat-cells for application in polymerization arise from their high cost, their tailor-made which requires a calibration procedure to be carried out for each cell, time-consuming sample preparation and complicated cleaning.

To optimize quality and productivity of the EPR investigations into radical polymerizations, advantages and disadvantages of specific EPR tubes were considered for the individual

32 MATERIALS AND EXPERIMENTAL SETUP

systems detailed in the present thesis. A selection of EPR cells used for studies into rather different systems is tabulated in Table 1.

MATERIALS AND EXPERIMENTAL SETUP 33 Table 1 Selection of EPR-cells used for EPR investigations into radical polymerization in different systems.

aExtremely high viscosity of the reaction solution after PLP requires carefull cleaning cycles of flat cells over days.

34 MATERIALS AND EXPERIMENTAL SETUP

4.2.3. Sources for UV irradiation

Polymerization is generally initiated by irradiation with UV light, which leads to decomposition of the photo-initiator, via a UV source through the grid in the resonator cavity (see Figure 3). The UV source may either be a pulsed laser (COMPex 102 excimer laser;

XeF, Lambda Physik) operating at 351 nm, with typical energies of 10-100 mJ per pulse and pulse repetition rates up to 20 Hz, or a mercury high-pressure UV-lamp (500 W, LAX 1450, Mueller Elektronik).