High field EPR spectroscopy studies in polymers and small molecule glass-formers

In recent years, an increased interest in the Electron Paramagnetic Resonance (EPR) spectroscopy technique has been observed. As in Nuclear Magnetic Resonance (NMR), there is a tendency in EPR to go to higher microwave frequencies (95 GHz and higher) to obtain enhanced spectral resolution and sensitivity. In this thesis, the potentialities of high field high frequency EPR (HF2 EPR) are exploited in the study of the dynamics of a paramagnetic probe guest molecules (spin probe) in disordered matrices such as polymers and molecular glass formers. The high magnetic fields involved, offer a unique angular sensitivity to the reorientation motion, while by increasing the microwave frequency the dynamics appear more and more in the slow motion regime. The information from the slow motion HF2 EPR spectra analysis, are obtained from the line shifts rather than the lines widths. Moreover a multi-frequency approach is adopted (95 GHz, 190 GHz and 285 GHz) which offers great advantages in the dynamics studies. In fact the motional model chosen in order to describe the reorientation of the spin probe molecules must satisfactorily fit the three sets of spectra. The focus of this work was to investigate the characteristics temperatures of the polystyrene (PS) well below the glass transition temperature Tg down to cryogenic temperatures as well as the characteristics temperatures in the molecular liquid ortho-terphenyl (OTP), and in the polymer polybutadiene (PB) both above Tg. The slow motion spectra of a small, stiff, spherical spin probe in glassy polystyrene (PS) were obtained. A fully analytical and numerical simulation analysis was carried out. Two different regimes separated by a crossover region were evidenced. Below 180 K the spin probe is trapped, the rotational times are nearly temperature independent with no apparent distribution. In the temperature range, 180-220 K a large increase of the rotational mobility is observed with a widening of the distribution of correlation times which exhibits two components: i) a delta-like temperature-independent component representing the fraction of spin probe w still trapped; ii) a strongly temperature-dependent component representing the fraction of un-trapped spin probe 1-w undergoing activated motion over an exponential distribution of barriers heights. Above 180 K a steep decrease of w is evidenced. The de-trapping of spin probe and the onset of its large increase of the rotational mobility at 180 K are interpreted as signature of the onset of the fast motion detected by neutron scattering in PS at 175 ± 25 K. By the analytical evaluation of the frequency shift an alternative approach to characterize the spin probe dynamics is found that confirms the results from full numerical simulation. In the temperature range T>Tg, an optimal choice of the spin probes allowed the investigation of the molecular glass former ortho-therphenyl (OTP) and polymer polybutadiene (PB) by studying the slow motion regime of HF2 EPR spectra. As a function of temperature, the frequency shift of the HF2 EPR spectra exhibit several well distinct regimes with a characteristic cusp-like behaviour. The cusp is found to be very close to the so-called critical temperature Tc which is predicted by the mode coupling theory developed by Götze and co-workers

High-Field Electron Paramagnetic Resonance Reveals a Stable Glassy Fraction up to Melting in Semicrystalline Poly(dimethylsiloxane)

The reorientation of the guest 4-methoxy-TEMPO (spin probe) in the disordered fraction of semicrystalline poly(dimethylsiloxane) (PDMS) is investigated by high-field electron paramagnetic resonance (HF-EPR) at 190 and 285 GHz. Accurate numerical simulations of the HF-EPR lineshapes evidence that the reorientation times of the spin probes are distributed between the melting temperature Tm and Tm-30 K. The distribution exhibits, in addition to a broad component, a narrow component with low mobility up to the PDMS melting point. It is shown that the temperature dependence of the reorientation time of the spin probes with low mobility is the same of the spin probes in glassy PDMS. The result suggests that the low-mobility fraction is localized in the so-called rigid amorphous fraction

Local Reversible Melting in Semicrystalline Poly(dimethylsiloxane): A High-Field Electron Paramagnetic Resonance Study

The reorientation of the paramagnetic guest 4-methoxy-TEMPO (spin probe) in the disordered fraction of semicrystalline poly(dimethylsiloxane) (PDMS) is investigated by high-field electron paramagnetic resonance (HF-EPR) at 190 and 285 GHz. The distribution of reorientation times is evidenced by accurate numerical simulations of the HF-EPR line shapes above 200 K. The distribution exhibits a bimodal structure with (i) a broad component corresponding to spin probes with fast and intermediate mobility located in the disordered fraction far from the crystallites and (ii) a narrow component corresponding to spin probes with extremely low mobility trapped close to the crystallites in a glassy environment persisting up to the PDMS melting. The spin probe undergoes an exchange process between the trapped and the more mobile fractions which is accounted for by an equilibrium reversible process with standard Gibbs free energy of reaction per spin probe mole Î\u94Gr0 â\u89\u83 4(Î\u94Hm - TÎ\u94Sm), where Î\u94Sm is the equilibrium melting entropy per monomer mole following the absorption of the heat Î\u94Hm. The process is interpreted as signature of reversible tertiary nucleation, occurring at the intersection of crystalline surfaces, thus suggesting surface roughness of the crystal-amorphous interface. It becomes thermodynamically favored at temperatures higher than T â\u88¼ 209 K where the onset of PDMS melting is located according to differential scanning calorimetry

A New Method for Tungsten Oxide Nanopowder Deposition on Carbon-Fiber-Reinforced Polymer Composites for X-ray Attenuation

A new method for the synthesis and deposition of tungsten oxide nanopowders directly on the surface of a carbon-fiber-reinforced polymer composite (CFRP) is presented. The CFRP was chosen because this material has very good thermal and mechanical properties and chemical resistance. Also, CFRPs have low melting points and are transparent under ionized radiation. The synthesis is based on the direct interaction between high-power-density microwaves and metallic wires to generate a high-temperature plasma in an oxygen-containing atmosphere, which afterward condenses as metallic oxide nanoparticles on the CFRP. During microwave discharge, the value of the electronic temperature of the plasma, estimated from Boltzmann plots, reached up to 4 eV, and tungsten oxide crystals with a size between 5 nm and 100 nm were obtained. Transmission electron microscopy (TEM) analysis of the tungsten oxide nanoparticles showed they were single crystals without any extended defects. Scanning electron microscopy (SEM) analysis showed that the surface of the CFRP sample does not degrade during microwave plasma deposition. The X-ray attenuation of CFRP samples covered with tungsten oxide nanopowder layers of 2 µm and 21 µm thickness was measured. The X-ray attenuation analysis indicated that the thin film with 2 µm thickness attenuated 10% of the photon flux with 20 to 29 KeV of energy, while the sample with 21 µm thickness attenuated 60% of the photon flux