Boulos Youssef

Photoinitiated polymerization of a dimethacrylate oligomer : 1. Influence of photoinitiator concentration, temperature and light intensity

Abstract The photoinitiated polymerization of dimethacrylate oligomer with 2,2-dimethyl-2-hydroxy acetophenone (Darocur 1173) as radical photoinitiator was studied by using isothermal photocalorimetry. The effect of temperature, light intensity and photoinitiator concentration on reaction was investigated. The maximum conversion was obtained at temperature near 90°C. This temperature is above the glass transition temperature of the resulting material and below the thermal polymerization temperature of the reacting system. Above 90°C, thermal polymerization interferes on photocalorimetric measures. Assuming that glass transition temperature of the final polymer and conversion are connected, we have estimated thermal conversion. A maximum conversion was obtained for a photoinitiator concentration of 1% (w/w) and for the highest light intensity studied (2.7 mW cm −2 ).

Photoinitiated cross-linking of a thiol-methacrylate system

Abstract The photoinitiated thiol–ene cross-linking of a dimethacrylate polyether of Bisphenol A and pentaerythritol tetrakis 2-mercaptopropionate was studied in the presence of 2,2-dimethyl-2-hydroxy acetophenone (Darocur 1173). Two complementary techniques were used: photocalorimetry and real-time infrared spectroscopy. In the first part, the kinetic reaction was characterized by a stoichiometric mixture in reactive functions. The influence of temperature, photoinitiator concentration and ultraviolet (UV) light intensity was investigated. The results mainly show that the methacrylate homopolymerization is faster than the thiol–ene addition so that the reaction is usually stopped because of the complete consumption of the methacrylate double bonds. A theoretical approach has allowed us to determine a transfer constant of 0.26. Moreover, an increase in the autoacceleration rate was observed in the presence of the thiol monomer. In the second part, the molar fraction of methacrylate double bonds and thiol functions was changed to determine the effect on the kinetic reaction and the glass transition temperature of the final material.

Photoinitiated polymerization of a dimethacrylate oligomer: 2. Kinetic studies

The kinetic of photoinitiated polymerization of a dimethacrylate oligomer was studied by using isothermal photocalorimetry. The reaction was realized with 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) as radical photoinitiator. Two kinetic models were applied. First, it was shown that an autocatalytic model can describe this reaction in a satisfying way. The reaction temperature does not influence the m and n orders of the reaction which were found to be constant and respectively equal to 0.8 and 2. The phenomenological rate constant k varies with temperature according to the Arrhenius law up to 80°C. Above this temperature, this law can again be checked if the initial variation of double bond concentration due to thermal polymerization is taken into account. In addition, by means of a mechanistic model, the kp and kt rate constants were calculated. Their evolution with conversion was studied at 50°C and well illustrates the importance of the reactive diffusion mechanism.

Photoinitiated polymerization of a dimethacrylate oligomer: Part 3. Postpolymerization study

Abstract The effects of the presence of trapped radicals after UV irradiation were studied on a dimethacrylate oligomer. The postpolymerization reaction was characterized by differential scanning calorimetry and dynamic mechanical analysis. The photopolymerization was realized with 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) as radical photoinitiator by using isothermal photocalorimetry. The postpolymerization was clearly shown in the isothermal mode and under N 2 atmosphere by following the variation of the glass transition temperature of the samples. The influence of crosslinking density, O 2 and thermal postcure were investigated.

Synthesis of a new phosphonated dimethacrylate : Photocuring kinetics in homo- and copolymerization, determination of thermal and flame-retardant properties

In this work, we describe the synthesis of a new dimethacrylate phosphonate monomer by condensation of methacryloyl chloride with a phosphonate diol resulting from a radical addition of allyldiethyl phosphonate with 3-mercapto propane-1,2-diol. The photochemical polymerization of this new monomer was first studied as a function of reaction temperature. The optimal conversion of the photopolymerization was 63% at 70°C. This new monomer is less reactive than the commercial dimethacrylate polyether of Bisphenol A, that we have used for copolymerization. The mechanical and thermal properties of the final material were studied as a function of dimethacrylate phosphonate monomer content. The ultimate conversion slightly decreases (from 72 to 55%) and the glass transition temperature drops from 44 to –26°C when the phosphorus content increases. The addition of phosphorus leads to an improvement of the thermal and flame-retardant properties, i. e. a significant decrease in the degradation rate (factor 4.5) and a high increase in the amount of residue that remained after combustion (15% for the 2.5% phosphorus copolymer). Moreover, the Limited Oxygen Index (LOI) is increased from 16.9 (0% P) to 21.5 (2.5% P).

Molecular motions in functional self-assembled nanostructures.

The construction of “smart” materials able to perform specific functions at the molecular scale through the application of various stimuli is highly attractive but still challenging. The most recent applications indicate that the outstanding flexibility of self-assembled architectures can be employed as a powerful tool for the development of innovative molecular devices, functional surfaces and smart nanomaterials. Structural flexibility of these materials is known to be conferred by weak intermolecular forces involved in self-assembly strategies. However, some fundamental mechanisms responsible for conformational lability remain unexplored. Furthermore, the role played by stronger bonds, such as coordination, ionic and covalent bonding, is sometimes neglected while they can be employed readily to produce mechanically robust but also chemically reversible structures. In this review, recent applications of structural flexibility and molecular motions in self-assembled nanostructures are discussed. Special focus is given to advanced materials exhibiting significant performance changes after an external stimulus is applied, such as light exposure, pH variation, heat treatment or electromagnetic field. The crucial role played by strong intra- and weak intermolecular interactions on structural lability and responsiveness is highlighted.

Molecularly Intercalated Nanoflakes: A Supramolecular Composite for Strong Energy Absorption

Energy absorption or dissipation from thin fi lms is increasingly demanded by civil and military applications. [ 1 ] And aluminum is probably the most well-known material that can absorb signifi cant amount of mechanic energy as thin fi lms. [ 2–4 ] Staircaselike loading and sharp unloading curves were revealed by nanoindentation, which were attributed to nucleation and glide of large number of dislocations. [ 5 ] Certainly, when great amount of these nanometer scale dislocations are made available in organic thin fi lms, they offer unmatched advantages including unlimited variability and processing ease. [ 3 ] While few studies were steered toward molecularly engineering an aluminum-like material, a simple mixing of fi brous(1D) or laminar-like (2D) fi llers within a bulk material could deliver a composite with signifi cant energy absorption. [ 6 ] To name a few, these include carbon fi ber/PEEK, [ 7–9 ] carbon fi ber/epoxy, [ 10 , 11 ] and glass cloth/ epoxy. [ 12 ] Central to the success of these polymer-based composites is their much increased fl exibility. Under external impact, the composites dissipate energies via mechanisms, such as structure buckling, interface cracking, delamination, or even laminar fragmentation. [ 6 ] Unfortunately, when such a macroscopic structure is condensed into a thin fi lm, the absence of bulk deformation in a confi ned space cannot generate enough response. Hence, it is desirable to have a solid, lightweight counterpart to aluminum thin fi lm and engineer dislocations therein for superior energy absorption. In this communication, we address this need by varying interfaces in supramolecules. [ 13 , 14 ] Over the past years, extensive studies on these well-ordered nanomaterials focused on modifying building

New polysiloxanes bearing heterocyclic groups—synthesis and curability

Abstract The synthesis of new polysiloxanes bearing thioether and heterocyclic functions (epoxy or oxetane) was performed. The reaction is a radical addition between a poly(dimethyl - co-methyl mercaptopropyl) siloxane and allyl or vinyl heterocyclic compounds. The corresponding sulfones were obtained by oxidation of thioether functions by using m -chloroperbenzoic acid. All the products were characterized by 1 H NMR, SEC and viscosimetry. The cross-linking of these products with isophorone diamine was studied at different temperatures. Thermal and mechanical properties were examined by dynamical mechanical and thermogravimetric analysis.

Kinetic studies of photoinitiated polymerization of a dimethacrylate oligomer

Abstract The kinetics of photoinitiated polymerization of a dimethacrylate oligomer with 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) was studied by using isothermal photocalorimetry. First, an autocatalytic model was used and the influence of the reaction temperature on the m and n orders of the reaction and on the phenomenological rate constant k was investigated. Then, a mechanistic model was applied to calculate the k p and k t rate constants. Their evolution with conversion was studied at 50°C.

Influence of UV radiation wavelength on conversion and temperature distribution profiles within dimethacrylate thick material during photopolymerization

UV-light initiation is now commonly used to induce polymerization of multifunctional monomers. The highly crosslinked networks obtained have a wide variety of applications. The thermal effects which take place during polymerization can be the cause of non-homogeneity and defects in the final material. These defects greatly alter the physical properties of the final products, particularly the optical ones, which causes problems in the design of thick and optically perfect materials. To better control the homogeneity of photocured materials and to determine the influence of different experimental parameters on them, conversion and temperature distribution profiles within a material during photopolymerization were simulated numerically, using the general heat equation applied to one-dimensional system. To describe the true conditions of kinetic experiments, some necessary parameters were measured, like conversion, reaction rate, spectral irradiance of the Hg vapor lamp and dimethacrylate spectral absorbance. We focused our attention more particularly on the influence of the irradiation wavelength. Indeed, the high values of the spectral absorbance coefficient cause a great decrease in light intensity in the depth of the material and lead in turn to a sharp drop in conversion.