Greffage radiochimique de l'acrylonitrile dans la masse de films de PTFE

Acrylonitrole can be grafted in the bulk of PTFE films provided the monomer is allowed to diffuse into the grafted zones. For a given radiation dose, the amount of grafting is higher the lower the dose-rate. Even higher grafting efficiencies are reached under conditions of discontinuous irradiations, the reacting mixture being stored in the dark in a thermostat between successive irradiations. The grafting yield is higher the higher the storage temperature. The bulk grafting involves a progressive swelling of the grafted polyacrylonitrile branches by acrylonitrile monomer via a dipole-dipole association of the -CN groups of the monomer with the -CN groups in the polymer.     

Free radical polymerization of acrylonitrile in dimethylsulphoxide solutions

The gamma-ray initiated polymerization of acrylonitrile in DMSO solutions was investigated at various monomer concentrations and temperatures. In a narrow range of monomer concentrations (70–80%), the “auto-acceleration index” of the reaction is higher than in the bulk polymerization. Some auto-acceleration persists at high DMSO contents when the reaction proceeds in a homogeneous medium. The results are interpreted by a “matrix effect” which is enhanced in those mixtures in which the precipitated polymer particles are swollen to a larger extent by the reacting mixture.     

Polymérisation en milieu précipitant du styrène en solution dans différents alcools

Earlier studies from this laboratory on the polymerizations of acrylic acid and acrylonitrile under precipitating conditions have shown that the auto-acceleration is not caused by non-stationary conditions resulting from the precipitation of growing chains (“occlusion effect”) but by a “matrix effect”, an oriented association complex between the monomer and the polymer formed in the early stages of the reaction leading to assisted propagation. In the present work, a non-polar monomer-polymer system was selected in which molecular associations are unlikely. It was found that when polystyrene precipitates as a fine powder (in diluted monomer solutions in alcohols) auto-acceleration is observed but its extent drops with increasing rate of initiation and increasing temperature. Such situations do not arise in polymerizing systems in which a “matrix effect” operates. The study of the post-polymerization and of the swelling of polystyrene in styrene (10-propanol (90) mixtures) led to the conclusion that polystyrene in equilibrium with this mixture exhibits a glass transition temperature at ca 50°. The various results obtained in this study conform with the assumption of an occlusion effect. The growing chains being buried in the precipitated polymer, chain termination is severely restricted and becomes the determining step in the polymerization.     

Influence des associations moleculaires sur la copolymerisation de l'acide acrylique avec l'acide methacrylique et avec l'acrylate de methyle

The copolymerization of acrylic acid with methacrylic acid in bulk is investigated at 40 and 60°. It is confirmed that a “matrix effect” occurs only for high contents of acrylic acid. The critical concentration beyond which the matrix effect disappears is shifted towards lower acrylic acid contents for higher temperatures. The copolymer composition is independent of temperature. The copolymerization of acrylic acid with methyl acrylate is investigated in a mixture which determines an “exaltation of the matrix effect” in the homopolymerization of acrylic acid (molar fractions: m_Monomers = 0.34; m_n-Hexane = 0.52; m_Methanol = 0.14). The resulting copolymers are found to contain a much larger fraction of acrylic acid residues than the copolymers formed in bulk or in toluene or DMF solutions.     

Polymerisation purement thermique de l'acide acrylique en masse et en solution

The thermal polymerization of acrylic acid in bulk is faster than that of styrene. The conversion curves exhibit auto-acceleration and the product contains a significant fraction of syndiotactic polymer. The overall activation energy is 14 kcal/mol. The rate of the thermal polymerization decreases sharply when the monomer is diluted with toluene. In 50% monomer solutions, the conversion curves are linear and the overall activation energy is 29.8 kcal/mol. With 75 and 90% monomer solutions, the Arrhenius diagrams showed breaks caused by a change in the type of auto-association of the monomer. A comparison of these results with earlier findings obtained in the radiation polymerization of acrylic acid makes it possible to estimate the activation energies of the thermal initiation. It is found that E_i is 14.1 kcal/mol in systems where the monomer forms linear oligomeric association complexes and 34.4 kcal/mol if only cyclic dimers are present in the system.     

Unusual swelling behavior of films of polyvinyl- and polyvinylidene/fluorides in various solvents

The swelling of PVF and PVDF films in various solvents was investigated. It was found that in the swollen state both polymers show little tendency to retain the solvent that leaks out under a mild pressure and evaporates quickly when the specimen is exposed to air. The equilibrium swelling ratios were measured in numerous solvents. From the results it is concluded that the solubility parameters are δ_PVDF = 12.0–12.3, δ_PVF = 12.0–12.1. The unusual swelling behavior is explained by strong polymer–polymer interactions via dipoles which tend to replace polymer–solvent interactions once the sample is withdrawn from the solvent.     

Sur les causes de l'auto-accélération dans la polymérisation de l'acrylonitrile en masse et en solution

A study of the polymerization of acrylonitrile in bulk and in DMSO solutions has shown that the auto-acceleration in these systems cannot be explained by non-steady-state conditions as usually assumed. It is suggested that the polyacrylonitrile formed in the initial stages of the reaction acts as a matrix onto which the monomer associates to form an oriented molecular complex in which chain propagation is favoured. This assumption conforms with the various results of this investigation.     

Density functional theoretical study of Cun, Aln (n = 4-31) and copper doped aluminum clusters: Electronic properties and reactivity with atomic oxygen

A DFT study of the electronic properties of copper doped aluminum clusters and their reactivity with atomic oxygen is reported. Firstly we performed calculations for the pure Cu_n and Al_n (n = 4, 9, 10, 13, 25 and 31) clusters and we determined their atomization energy for some frozen conformations at the B3PW91 level. The calculated work functions and M-M (M = Cu, Al) bond energies of the largest clusters are comparable with experimental data. Secondly, we focused our attention on the change of the electronic properties of the systems upon the substitution of an Al atom by a Cu one. This latter stabilizes the system as the atomization energy of the 31-atoms cluster increases of 0.31 eV when the substitution is done on the surface and of 1.18 eV when it is done inside the cluster. We show that the electronic transfer from the Al cluster to the Cu atom located at the surface is large (equal to 0.7 e⁻) while it is negligible when Cu is inserted in the Al_n cluster. Moreover, the DOS of the Al₃₁ and Al₃₀Cu systems are compared. Finally, the chemisorption energies of atomic oxygen in threefold sites of the Al₃₁, Cu₃₁ and Al₃₀Cu clusters are calculated and discussed. We show that the chemisorption energy of O is decreasing on the bimetallic systems compared to the pure aluminum cluster.     

Sur une exaltation de l'“effet de matrice” lors de la polymerisation de l'acide acrylique dans certains melanges ternaires

The polymerization of acrylic acid proceeds with an extremely high degree of auto-acceleration in certain ternary mixtures. The most drastic effects are observed when small amounts of methanol are added to a dilute solution of acrylic acid in n-hexane. In such systems the auto-acceleration index “β” may exceed 10. β Was found to exhibit a maximum over a fairly narrow range of concentrations. Moreover, the values of β are highest at temperatures between ?5 and + 10°, where a maximum of 16 is reached. The swelling of poly(acrylic acid) in the various reacting mixtures was measured and the molecular associations of the monomer with itself and with methanol were investigated. It was concluded that the “exaltation of the matrix effect” in some of the systems is caused by the complex [(Acrylic acid)₂, McOH]. This complex associates very rapidly with the polymer formed in the early stages of the reaction to produce a structure in which ultrafast propagation occurs. The swelling of the polymer favours the accessibility of the monomer to the polymeric chains during polymerization, leading to auto-acceleration which progresses with conversion and gives rise to a reaction with explosive character.     

Kinetic evidence of a “matrix effect” in the bulk polymerization of acrylonitrile

The kinetics of the γ-ray-initiated polymerization of acrylonitrile in bulk are reexamined in broad ranges of temperatures and radiation dose rates. The discussion of the results coupled with an analysis of earlier data indicate that the polymerization of acrylonitrile proceeds by different mechanisms depending on the reaction temperature. Above 60°C the precipitated growing chains recombine readily; therefore, the autoaccelerated conversion curves cannot be accounted for by an “occlusion effect.” It is suggested that autoacceleration is caused by a fast propagation taking place in oriented monomer aggregates which result from dipole-dipole association of the monomer with the polymer chains formed in the early stages of the reaction (“matrix effect”). Below 10°C the precipitated growing chains are buried in the dead polymer and monomer diffusion toward the occluded chain ends is very limited (“occlusion effect”). Between 10 and 60°C the system gradually changes from one dominated by “occlusion” to one where the “matrix effect” determines the kinetic behavior. The conclusion based on kinetic data is in agreement with results obtained from studies of the postpolymerization in these various systems.