Georges Geuskens

Photo-oxidation of polymers—Part V: A new chain scission mechanism in polyolefins☆

Abstract The photo-oxidation of an ethylene-propylene copolymer has been studied at 310 and 365 nm. The results indicate that the Norrish type II reaction occurs but that another chain scission mechanism is also operative. This involves the reaction of free radicals produced by the photo-decomposition of hydroperoxide groups with neighbouring ketones.

Energy transfer in polymers—V. Singlet and triplet energy transfer in polyacenaphthylene☆

Abstract The emission spectra of polyacenaphthylene (PAcN) and polyvinylnaphthalene (PVN) films and solutions have been compared at room temperature and 77°K. In organic glasses at 77°K, normal fluorescence and phosphorescence are observed for both polymers. Quenching of PAcN phosphorescence by piperylene in a glass obeys a Stem-Volmer type equation; the slope of the straight line is identical to that reported for PVN in the same conditions. In solution at room temperature, the ratio of excimer to normal fluorescence yields is 1 · 3 for PAcN whereas it is 12 · 6 for PVN. No luminescence other than excimer fluorescence is emitted by polymer films at room temperature and 77°K. The intensity of that emission decreases in the presence of benzophenone because of singlet energy transfer to the additive. The efficiency of the transfer is identical for PAcN and PVN (Forsters R o = 15 A ). The results are discussed according to a general kinetic treatment and in terms of the mobility of the chromophore side groups.

Influence of temperature on the permeability of polyacrylamide hydrogels and semi-IPNs with poly(N-isopropylacrylamide)

Abstract Several polyacrylamide hydrogels and semi-interpenetrated polymer networks with poly(N-isopropylacrylamide) included were synthesised and characterised by their swelling ratio. Their permeability to Orange II was studied as a function of temperature in the range 25–45°C. In polyacrylamide hydrogels the permeability increases with temperature according to an Arrhenius relationship and the apparent activation energy for permeation decreases linearly as the swelling ratio of the hydrogel increases. In semi-interpenetrated polymer networks a more or less pronounced increase of the permeability to Orange II is observed above 32°C, the lower critical solution temperature of poly(N-isopropylacrylamide). Different values of the apparent activation energy for permeation were obtained below and above 32°C, the last one being always lower than the former one. The permeability of semi-interpenetrated polymer networks increases above 32°C though a slight shrinking of the gels is observed. This can be explained by the collapse of poly(N-isopropylacrylamide) chains resulting in an increase of the pore volume that facilitates the diffusion of the dye but simultaneously water is expelled and the swelling ratio of semi-interpenetrated polymer networks decreases.

Energy transfer in polymers—I. Triplet-triplet transfer in solid polyvinylbenzophenone-naphthalene system

Abstract Energy transfer has been shown to occur from triplet polyvinylbenzophenone to naphthalene by an exchange mechanism at liquid nitrogen temperature. The radius of the Terenin “critical sphere” is much larger for the polymer containing naphthalene (29A) than for mixtures benzophenonenaphthalene in organic glasses (11 A). The results are interpreted in terms of energy migration in the polymer according to Voltz theory.

THE PHOTO-OXIDATION OF POLYMERS. A COMPARISON WITH LOW MOLECULAR WEIGHT COMPOUNDS

Abstract The photo-oxidation of polymers involves different steps in which their reactivity is different from that of low molecular weight compounds. This is mainly due to the close vicinity of reactive groups in polymers and to the rigidity of the matrix. As a consequence, transfer of energy has a major role in the initiation of the photo-oxidation of most polymers on exposure to sunlight. It results in the sensitized decomposition of neighbouring hydroperoxide groups which, however, produce free radicals and thus initiate the oxidation less efficiently than model compounds in fluid solution. Scission of the polymer backbone, responsible for the alteration of the physical and mechanical properties, involves the decomposition of isolated hydroperoxide groups by two different mechanisms the relative importance of which changes with temperature.

Luminescence studies in polymers-I. Temperature effect on polyvinylnaphthalene and polyacenaphthylene fluorescence

Abstract The fluorescence spectra of polyacenaphthylene and poly-1-vinylnaphthalene films and solutions have been studied between 77 and 450°K. Excimer fluorescence only is observed in films. The ratio of monomer to excimer fluorescence intensities is a function of temperature for solutions. The influence of temperature on the intensity of monomer and excimer fluorescence has been analysed according to a general kinetic treatment. Activation energies for excimer formation and dissociation have been measured.

Luminescence studies in polymers—II: Temperature effect on polyvinylcarbazole fluorescence☆

Abstract Polyvinylcarbazole (PVCa) films and solutions emit normal and excimer fluorescence between 77 and 425 K. The absolute (IM and ID) and relative (IM/ID) intensities emitted strongly depend on temperature. The usual U-shaped curve is obtained for log IM/ID as a function of 1/T in the case of polymer films. In solution, two minima corresponding to two different excimers are observed. The formation and dissociation of PVCa excimers in films and solutions have been interpreted according to the usual kinetic scheme. The binding energies for the excimer in films and for the high temperature excimer in solutions are respectively 4 and 2·8 kcal mole−1.

Polyacrylamide hydrogels and semi-interpenetrating networks (IPNs) with poly(N-isopropylacrylamide): Mechanical properties by measure of compressive elastic modulus

Semi-IPN hydrogels (based on cross-linked polyacrylamide having poly(N-isopropylacrylamide) (PN1PAAm) inside) were synthesized and their properties, such as swelling ratio and compressive elastic moduli, were studied at several temperatures. Equilibrium swelling ratios of semi-IPN markedly decreased due to the presence of less hydrophilic PNIPAAm chains. The semi-IPN presented greater elastic modulus when compared to the cross-linked PAAm hydrogel. The effect was explained as being an additional contribution of the PNIPAAm chains, which collapsed around the PAAm networks, to the elastic modulus. It was pointed out that the PAAm networks support the collapsed chains. According to the results presented in this work, semi-IPN hydrogels present better mechanical properties than the PAAm hydrogel, mainly when the PNIPAAm chains are in a collapsed state.© 2001 Kluwer Academic Publishers

Surface modification of polymers VII.: Photochemical grafting of acrylamide and N-isopropylacrylamide onto polyethylene initiated by anthraquinone-2-sulfonate adsorbed at the surface of the polymer

Photoinitiators anthraquinone-2-sulfonate sodium salt and (4-benzoyl benzyl) trimethylammonium chloride both adsorb from aqueous solutions at the surface of polyethylene and desorb partly in the presence of water. After adsorption of either photoinitiator on polyethylene, surface grafting occurs easily when the polymer, in contact with a thin layer of acrylamide solution, is irradiated at 350 nm. As a consequence of the grafting mechanism, homopolymer is always formed as by-product. Of both photoinitiators anthraquinone-2-sulfonate is the most suitable because it adsorb more strongly and has a higher absorption coefficient in the near UV. Grafting of acrylamide results in a decrease of the contact angle of the polymer surface with a water droplet from 80 to 25°. After grafting of N-isopropyl acrylamide the contact angle measured at room temperature also drops to 25° but remains 80° when measured at 40°C, i.e. above the L.C.S.T. of the grafted polymer.

Luminescence studies in polymers—III. Copolymers styrene-methylmethacrylate and styrene-methylacrylate

Abstract The emission spectra of copolymers styrene-methylmethacrylate and styrene-methylacrylate with various compositions have been analyzed. Films and solutions have been studied at 77 and 300 K. With the exception of glassy solutions at 77 K, normal and excimer fluorescence are emitted. Their relative and absolute intensities vary with temperature and copolymer composition. An interpretation of the results, which takes into account the statistical composition of the copolymers, is proposed. It shows that energy migration can occur from isolated to non-isolated styrene units.