Lawrence Frezet

Direct fluorination applied to wood flour used as a reinforcement for polymers

Direct fluorination was applied to wood flour in order to improve its compatibility with polymers and thus enhance the properties of wood-polymer composites. Fourier-transform infrared spectra and (19)F solid-state nuclear magnetic resonance results underlined a successful covalent grafting of fluorine atoms onto the wood chemical structure. No physical damage of the wood particles was observed during scanning electron microscopy analysis. The thermal behaviour of the wood flour was also studied by thermogravimetric analysis. The hydrophilic property changes of wood flour were examined by evaluating the water content and the rate of water uptake of samples under different relative humidity conditions. A decrease in the wood flour water content was noted after fluorination. All these studies tend to prove the efficiency of this treatment on wood hydrophilia.

Hologram formation reconsidered in dichromated polyvinylalcohol: polymer cross-linking around chromium (V)

The photochemical behavior of dichromated polyvinylalcohol (DCPVA) films was analysed upon exposure at 365 nm in connection with the hologram quality recorded in such a photosensitive material. The evolution of both involved species, chromium and polyvinylalcohol, were quantified by implementing an innovative approach. This approach combines the monitoring of the structural modification of the polymeric matrix and the fate of the various chromium species ((VI), (V) and (III)). For the first time, it was established that chromium (V) was at the origin of the cross-linking implied in the hologram formation by acting as a bridge between hydroxyl groups of the polymeric chains. A second unanswered question was also elucidated. The improvement brought by ammonium dichromate with respect to potassium dichromate involves amide groups as additional chelating sites for chromium (V) resulting in the increase of the matrix cross-linking.

Origin of the photo-cross-linking process in dichromated polyacrylamide under conventional and laser irradiation.

This work is devoted to determining the contribution of amide groups in the photoredox and cross-linking process of dichromated polyacrylamide based on the fate of the photoactive species and of the polymer under conventional and laser irradiation. It was shown that, in parallel to the reduction of chromium(VI) into chromium(V), the cross-linking of the matrix occurred through a complexation reaction around chromium(V) and through formation of covalent bonds between macromolecular chains. A comparison with dichromated poly(vinyl alcohol) was also reported to highlight the role of the chemical structure of the polymeric matrix in the mechanism of hologram formation. Moreover, for the first time it was demonstrated by in situ infrared spectroscopy that the physicochemical modifications undergone by the photosensitive materials were similar for the two modes of irradiation.

Enhanced concentration of dispersed carbon nanofibres in organic solvents through their functionalization by fluorination.

Covalent functionalization through pure molecular gaseous fluorination has been applied on carbon nanofibres. Nuclear magnetic resonance and thermal gravimetric analysis investigations have been performed on fluorinated carbon nanofibres in order to determine the chemical and thermal stability of the C-F bonding. The high covalency obtained allows no significant modification of the physicochemical nanostructure of fluorinated carbon nanofibres after sonification. Such modification of surface chemistry leads to a high increase in the limit concentration of dispersed carbon nanofibres in organic solvents without surfactant. An exciting maximum of 570 mg L(-1) of fluorinated nanofibres can be homogeneously dispersed in N-methylpyrrolidone, whereas 310 mg L(-1) is the maximum for non-fluorinated carbon nanofibres. In order to understand such dispersibility differences, Hildebrand and Hansen solubility theory has been used.

Photochemical Processes Involved in a Biopolymer Doped by Chromium(VI) during Hologram Recording

This paper is devoted to the study of the photochemical processes occurring during hologram formation in a biopolymer, agar doped by chromium(VI). The evolution of both the absorbing chromium species and the polymer allowed elucidating the reactions taking place during irradiation in conditions representative of hologram formation. As previously observed with dichromated poly(vinyl alcohol) (PVA), polymer containing hydroxyl groups as agar, irradiation of dichromated agar provoked a two step reduction of chromium species and the crosslinking of the host polymer. But agar has a rather complex chemical structure and the photoproducts formed along the polymeric chains throughout the photoredox process were different in agar and in PVA. The photostability of the matrix, that has been proven, coupled with the possibility to record good quality holograms, allowed us to propose dichromated agar as a new natural photosensitive biomaterial.

Dichromated photosensitive materials: involvement of the polymeric matrix

Irradiation of dichromated polyvinyl alcohol and dichromated polyacrylic acid DC(PVA-PAA) at 365 nm was investigated in order to understand the involvement of the polymeric matrix in the reduction process of the Cr(VI) leading to the formation of the hologram. The photochemical evolution of the matrix was directly correlated to the disappearance of the absorbing species, chromium(VI). A special attention was paid to the absorption profile of the films. Due to the impermeability of the polymer, no oxidation proceeded. The reduction of Cr(VI) into Cr(V) induced the formation of carboxylate species perfectly correlated with the consumption of carboxylic groups. Besides the acido-basic reaction undergone by the carboxylic groups, the reduction of Cr(VI) provoked the cross-linking of the polymer. Each polymer appears to play a specific role in the mechanism.

Chemical photostructuration in iodoform-doped carbazolic copolymers for holographic recording

Carbazol-containing polymers were shown to be good reliefographic photosensitive substrates for holographic registration. Holographic images with satisfactory properties were obtained with carbazolic copolymers containing carbazolylethylmethacrylate and octylmethacrylate units and iodoform as sensitizer. The final goal of this study is to find the best copolymer formulation ensuring large amount of data stored with a high transfer rate and an optimized lifetime of the hologram and of the host polymer. It is then crucial to elucidate the involved photochemical mechanisms. We first carried out irradiations at 405 nm (wavelength of the laser diode used for holographic recording) in order to follow the photostructuration of the doped polymer. Spectroscopic analyses showed that the formation of a charge transfer complex is observed and its increase of concentration in the polymer could be followed versus exposure time. We also focused our attention on the durability of the hologram and of the polymeric matrix itself. Under irradiations representative of natural ageing (λ>300 nm), we have shown that the complex is successively formed and destroyed and that the photodegradation of the polymer matrix was also observed.