Basic Aspects of Polymer Degradation

Knowledge on the mechanisms by which polymers degrade on exposure to natural or artificial ageing has considerably increased during the past few years. One reason for this is the development of novel experimental procedures and analytical techniques. The analyses of the chemical changes permit determining the main degradation routes applicable to the degradation of the properties of materials and ensures the relevance of the laboratory experiments.     

Depth Profiling by AFM Nanoindentations and Micro‐FTIR Spectroscopy for the Study of Polymer Ageing

Summary: A method of depth profiling by AFM nanoindentations is developed for the characterisation of the heterogeneity of the mechanical properties of oxidised polymers. An increase or a decrease of the sample stiffness is measured close to the surface. A comparison with micro‐FTIR profiles and a knowledge of the photooxidation mechanism permit an interpretation of the chemical and physical changes and give new insights into the understanding of the ageing behaviour.

Photooxidation profile of a TMPC film measured by AFM (•) and by micro‐FTIR (♦) after irradiation.     

Kinetic and product study of the gas‐phase reaction of sabinaketone with OH radical

Sabinaketone is one major photooxidation product of sabinene, an important biogenic volatile organic compound. This article provides the first product study and the second rate constant determination of its reaction with OH radicals. Experiments were investigated under controlled conditions for pressure and temperature in the LISA indoor simulation chamber using FTIR spectrometry. Kinetic study was carried out at 295 ± 2 K and atmospheric pressure using the relative rate technique with isoprene as the reference compound. The rate constant was found to be k_{sabinaketone + OH} = (7.1 ± 1.0) × 10^?12 molecule^?1 cm³ s^?1. Acetone and formaldehyde were detected as products of the reaction with the respective yields of R_acetone = 0.9 ± 0.2 and R_HCHO = 1.2 ± 0.3. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 415–421, 2007     

Photooxidative degradation of polyether-based polymers

The mechanisms of photooxidation of several polyether based polymers were compared in order to give general rules of orientation of the oxidative reactions. These polymers were a poly(ether-ester) and two poly(ether-urethane)s. The behavior of the polyether component as homopolymer was also studied. This study was extended to copolymers of fluorinated olefins and allyl or vinyl ethers, and the non fluorinated homologues. The elucidation of the photooxidation mechanisms was based on the identification of the photoproducts by analysis of both the solid polymeric matrix and the gaz phase formed on irradiation. It was shown that the photochemical sensitivity of the polyether component was responsible for the rapid oxidation of the copolymers. However, in the case of the fluorinated compounds, different behaviors were observed. It was shown that the presence of the fluorine atoms strongly influenced the orientation of the reaction and modified the oxidation kinetics. The reactivity of the methylene groups in α-position of the oxygen atom of the ether groups were not found to be equivalent regarding oxidation and the secondary was more oxidizable than the tertiary one.     

BENZOPHENONE PHOTOSENSITIZATION OF 2'-DEOXYGUANOSINE: CHARACTERIZATION OF THE 2R AND 2s DIASTEREOISOMERS OF 1-(2-DEOXY-β-D-erythoW-PENTOFURANOSYL)-2-METHOXY-4,5-IMIDAZOLIDINEDIONE. A MODEL SYSTEM FOR THE INVESTIGATION OF PHOTOSENSITIZED FORMATION OF DNA-PROTEIN CROSSLINKS

Abstract Benzophenone-mediated photosensitization of 2'-deoxyguanosine and its 3',5'-di-O-acetyl derivative, used as DNA model compounds, in oxygen-saturated water-methanol (1:1) solution results in the nucleophilic addition of methanol to the guanine base. The resulting modified nucleosides have been isolated by reverse-phase high-performance liquid chromatography and characterized by extensive spectroscopic measurements including ¹³C and ^IH nuclear magnetic resonance, fast atom bombardment mass spectrometry and circular dichroism as the 2R and 2s diastereoisomers of 1-(2-deoxy-˜-o-erythro-pentofuranosyl)-2-methoxy-4,5-imidazolinedione and their related 3',5'-di-O-acetyl derivates. Information concerning the absolute configuration of the two pairs of diastereoisomers was inferred from detailed nuclear Overhauser effect experiments. A reaction mechanism, involving guanine radical intermediates, is proposed to explain the generation of these new guanine photoproducts.     

Polymer surface science

Molecular level studies of the structure and mechanical properties of polymer surfaces have been carried out by sum frequency generation (SFG) surface vibrational spectroscopy and atomic force microscopy (AFM). The surfaces of different grades of polyethylene and polypropylene have been characterized-including during the glass transition and when mechanically stretched. Copolymers that have hard and soft segments with different glass transition temperatures show phase separation, an effect of hydrogen bonding between the hard and soft segments, that influences their adhesive and friction properties. AFM and SFG show that low surface energy additives migrate to the surface and alter the surface mechanical properties. Polymers, where the chemical nature of the end groups is different from the backbone, show surface segregation of the hydrophobic part of the chain in air and the hydrophilic part in water. Likewise, in miscible polymer blends, surface segregation of the more hydrophobic component in air and the more hydrophilic component in water is observed. This area of surface science requires increased attention because of the predominance of polymers as structural materials and as biomaterials.     

On the orientational dependence of resolution in 1H solid-state NMR, and its role in MAS, CRAMPS and delayed-acquisition experiments

Numerical simulations and experiments are used to show that the spin dynamics of the dipolar-coupled networks in solids is often strongly dependent on crystallite orientation. In particular, different rates of dephasing of the magnetisation mean that NMR signals obtained at longer dephasing times are dominated by orientations in which the local dipolar coupling strength is relatively weak. This often leads to a distinct improvement in spectral resolution as the dephasing time is increased. The effects are particularly noticeable under magic-angle spinning (MAS), but are also observed when homonuclear decoupling is used to reduce the rate of dipolar dephasing. Numerical simulation is seen to be a powerful and easily used tool for understanding the behaviour of solid-state NMR experiments involving dipolar-coupled networks. The implications for solid-state NMR spectra of abundant spins acquired under MAS and homonuclear decoupling are discussed, as well as insights provided into the performance of 'delayed-acquisition' and 'constant-time' experiments.     

Quadrupole splitting and isomer shifts in Te oxides investigated using nuclear forward scattering

Nuclear forward scattering by ¹²⁵Te is a viable alternative to conventional ¹²⁵Te Mössbauer spectroscopy avoiding all source related issues. Using reference compounds with known hyperfine parameters and Te oxides exhibiting stereochemically active lone pairs, we show that nuclear forward scattering by ¹²⁵Te can be reliably used to extract quadrupole splitting energy and relative isomer shift. The rough correlation between Te-Ocoordination and quadrupole splitting energy as put forward by Takeda and Greenwood (J. Chem. Soc. Dalton, 2207, 1975), is corroborated by the presented results.