Agnès Rivaton

Recent advances in bisphenol-A polycarbonate photodegradation

The mechanism of the photochemical evolution of bisphenol-A polycarbonate (PC) was probed using different FTIR and UV techniques along with several post-irradiation treatments of irradiated samples, including chemical derivatization reactions, physical treatments and mass spectrometric analysis of the low molecular weight fragments. The absorption maxima that were found in the carbonyl and hydroxyl regions of the infrared were assigned, and the corresponding photoproducts were identified. These studies show that the photochemical evolution of PC mainly involves two successive photo-Fries rearrangements, a photo-induced oxidation on the side-chain and a phenyl ring oxidation. These multiple reaction pathways are shown to be largely dependent upon the spectral distribution of the excitation light source and whether oxygen is present.

Stability of polymers under ionising radiation: The many faces of radiation interactions with polymers

Ionising radiations have dramatic effects on the properties of polymers commonly used in hard radiation environments. From the early radiolysis studies, polymers have been classified into those undergoing mainly chain scissions and those being cross-linked. Real situations are however far more complex due to the semi-crystalline organisation of most polymers implying transition temperatures (glass transition, fusion temperature of the crystalline fraction), oxygen diffusion driven formation of oxidised species and effects of the polymer formulation (anti-oxidants, influence of the fillers). Progress has been made in the understanding of such phenomena and different examples have been considered illustrating the different faces of the polymer evolution under ionising radiation. Particular emphasis has been given to the changes of the final polymer properties in their conditions of use. Two examples of materials of widespread importance in nuclear technology: elastomers and epoxy resins, will be discussed.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Thermal Stabilisation of Polymer–Fullerene Bulk Heterojunction Morphology for Efficient Photovoltaic Solar Cells

A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.

Infrared analysis of the photochemical behaviour of segmented polyurethanes: 3. Aromatic diisocyanate based polymers

FTi.r. studies of the photooxidation of aromatic diisocyanate (MDI) based polyurethanes are presented. The results obtained show that the photochemical evolution of the aromatic moieties involves a dual mechanism: one is a photo-Fries type reaction whereas the other results in an induced oxidation of the central methylene group of MDI. Direct evidences that these reactions occur are obtained on the basis of the modification of the spectral features resulting from both the rearrangement of urethane functions and the formation of oxidized groups.

Photodegradation of polyethersulfone and polysulfone

Abstract The photolysis and the photooxidation of polyethersulfone (PESF) and bisphenol-A polysulfone (PSF) have been investigated under polychromatic (λ>300 nm) and monochromatic irradiations (254 and 365 nm). Under vacuum exposure, as a result of direct absorption of the incident radiation by the basic backbone structure of polyarylsulfones (PESF and PSF), the photolytic processes that occur involve chain scissions as revealed by UV–visible and high performance liquid chromatography (HPLC) identifications of the fluorescent low molecular weight fragments after methanolic extraction. In parallel, Fourier transform infrared (FTIR) spectroscopy of the irradiated films accounts for the photoreduction of sulfone moieties in benzenesulfinic acid end-groups and for the formation of phenolic extremities after photo-Claisen-type rearrangement of the ether moieties or direct chain scission and subsequent hydrogen abstraction. Phenyl radicals recombination accounts for the pronounced yellowing of the polymer. Under exposure in aerated media, the rapid photooxidation of both polymers involves chain scissions and the formation of numerous carbonylated and hydroxylated oxidative species. The experimental results indicate that most of the oxidation products arise from the photodegradation of the diphenylethersulfone moieties and the cleavage of the aromatic rings; the oxidation of the bisphenol-A moieties occurs only in secondary steps. As a consequence of the chain scissions provoked by the oxidation, the photoproducts are predominantly low molecular weight oxidation products and most of the consumed oxygen appears readily as carbon dioxide.

The photochemistry of bisphenol ― A polycarbonate reconsidered

Abstract Dual photochemistry is occurring in a bisphenol-A based polycarbonate. At short wavelengths, the photo-Fries rearrangements are not the only processes observed. The radicals primarily formed in direct photoscission lead to hydrodiphenyl units, and secondary photoreactions of the photo-Fries products are influenced by oxygen. At long wavelengths, a photo-induced oxidation is occurring and a fairly high concentration of tertiary hydroperoxides, stable at 80°C, is observed. In polychromatic light peaking at 310 nm, both mechanisms are involved, the photo-Fries photoproducts protecting only temporarily the polycarbonate against the photo-induced oxidation; hydroperoxides reached a lower stationary concentration than with 365 nm excitation.

Dual photo-chemistries in aliphatic polyamides, bisphenol A polycarbonate and aromatic polyurethanes—A short review☆

Dual photochemistries account for the photo-ageing of aliphatic polyamides, bisphenol A polycarbonate and aromatic polyurethanes. Through the excitation at short wavelength of the chromophores of the polymeric chains (amide, aromatic carbonate and aromatic urethane groups) direct photo-transformations are observed. Photo-scission of the NHCO groups in polyamides and photo-Fries in polycarbonate and polyurethane are the prevalent primary processes. Through the excitation at long wavelengths of absorbing impurities, defects or additives, a photo-initiated oxidation proceeds. Differences in stoichiometries are observed at short and long wavelengths. Hydroperoxides, imides and N-1 hydroxylated groups are the major intermediate products in polyamide photo-oxidation at long wavelengths. Aromatic alcohols and ketones are predominantly formed in polycarbonate photo-oxidation at λ > 330 nm. Photo-oxidation at long wavelengths of polyurethanes is revealed by the production of different types of hydroperoxide.

Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation

Polyvinyl alcohol (PVA) is a water-soluble polymer that is anticipated to be a good candidate for incorporation into multilayer coatings of organic solar cells due to its high transparency and ability to form a barrier to oxygen. Because a long lifetime is a prerequisite for successful applications, it was necessary to study the photochemical behavior of PVA under solar light. PVA films were exposed to UV-visible light irradiation (λ > 300 nm) in accelerated aging conditions representative of natural ageing. Modifications in the chemical structure of aged samples irradiated at ambient air were recorded. Due to the low oxygen permeability of PVA films, it was shown that the photooxidative degradation of PVA films is restricted to the surface (<5 μm) and results in a large amount of chain scissions, with a progressive erosion of the surface of the irradiated material. The oxidation products formed along the macromolecular chains, and low molecular weight species trapped in the matrix or emitted in the gas phase were also identified. An oxidation mechanism was then proposed to account for these modifications. However, irradiation in the absence of oxygen demonstrated the high photostability of PVA films, which permits the use of PVA as a sublayer in inorganic/organic multilayer encapsulation systems.

Photochemistry of poly(butyleneterephthalate): 2—Identification of the IR-absorbing photooxidation products

The photolysis of poly(butyleneterephthalate) was carried out under radiation of long wavelength (λ ≥ 300 nm) and short wavelength (λ = 254 nm) in vacuum atmosphere. Photolysis products, formed by irradiation of thin polymer films, were investigated by FTIR spectroscopy. Specific chemical and physical treatments were developed in order to identify the photoproducts. The main groups involved in the photochemical evolution have been identified to be end-group species, i.e. benzoic acid, benzaldehyde, γ-butyrolactone, formate and aliphatic alcohol, as well as chain m-biphenyl structures. It was shown that the repartition of photolytical species in the polymer was heterogeneous. On the basis of photoproducts identification, a general scheme accounting for the main routes of poly(butyleneterephthalate) photolysis was proposed.