Solvent contribution to the stability of a physical gel characterized by quasi-elastic neutron scattering

The dynamics of a physical gel, namely the Low Molecular Mass Organic Gelator {\textit Methyl-4,6-O-benzylidene-$\alpha$ -D-mannopyranoside ($\alpha$-manno)} in water and toluene are probed by neutron scattering. Using high gelator concentrations, we were able to determine, on a timescale from a few ps to 1 ns, the number of solvent molecules that are immobilised by the rigid network formed by the gelators. We found that only few toluene molecules per gelator participate to the network which is formed by hydrogen bonding between the gelators' sugar moieties. In water, however, the interactions leading to the gel formations are weaker, involving dipolar, hydrophobic or $\pi-\pi$ interactions and hydrogen bonds are formed between the gelators and the surrounding water. Therefore, around 10 to 14 water molecules per gelator are immobilised by the presence of the network. This study shows that neutron scattering can give valuable information about the behaviour of solvent confined in a molecular gel.Comment: Langmuir (2015

Photopolymerization quantum yields in two reactive diacetylenes, 3BCMU and 4BCMU, and relation to gamma-ray induced polymerization

International audiencePhotopolymerization initiation yields from neutral excited states eta(i) are measured; for 4BCMU, eta(i)approximate to 0.075, but this value is 200 times smaller for 3BCMU, although the gamma-ray reactivities of the two materials are similar. This difference is not related to different geometries of the reaction centre. It is proposed that it is due to different strengths of hydrogen bonds between side groups. The importance of ionic initiation under gamma-ray irradiation is discussed. (c) 2007 Elsevier B.V.. All rights reserved

Stability of hydrogen bonds upon polymerization and color transition of diacetylenes: An FTIR spectroscopy study

International audienceFTIR spectra (400-7500 cm(-1)) have been recorded for polycrystalline films and single crystals of the diacetylene 4BCMU in the monomer, blue and red polymer states, and of 3BCMU monomer. Particular attention was paid to the amide bands associated to the urethanes in the side groups, which form one-dimensional H-bond linear chains quite similar to those found in peptides and proteins. The positions and strengths of these absorptions are practically unchanged by polymerization or blue to red polymer color transition. This suggests that the H-bond linear chains are the main constitutive interactions in these crystals, and that the conjugated parts must and do ``adapt'' to the unchanged H-bond lines. No evidence of side group disordering was observed at the irreversible blue to red transition in poly-4BCMU crystal. (C) 2013 Elsevier B.V. All rights reserved

Photopolymerization of Thin Polycrystalline Diacetylene Films and Quenching of the Precursor Excited State

International audienceMicrocrystalline films of the diacetylenes 4BCMU and 3BCMU were prepared by vacuum evaporation and polymerized by UV irradiation. A method for determining the absolute polymer content X in films has been developed. In 4BCMU the reaction rate is strongly decreased beyond X similar to 0.3. Existing models fail to fit the kinetics. A numerical model based on quenching of the monomer excited state, the initiation precursor, by energy transfer to a nearby existing chain is developed and quantitatively fits the kinetics. Such quenching may occur in any diacetylene solid, so that one should not assume that an apparent saturation of the absorbance of a film corresponds to complete polymerization. 3BCMU is less reactive and briefly compared to 4BCMU

Structural study of two reactive diacetylenes

International audienceThe unit cell dimensions of two diacetylenes 3BCMU and 4BCMU are measured from low temperature to 300 K. The complete crystal structures of the two phases of 3BCMU are determined at 110 K by X-ray diffraction and at 270 K by neutron diffraction helped by calculated geometrical optimization used as a starting structure. Neutron results allow to discuss the hydrogen bond strength and to determine the thermal expansion coefficients. (c) 2007 Elsevier B.V. All rights reserved

Temperature dependence of spectroscopic properties of isolated polydiacetylene chains strained by their monomer single crystal matrix

International audienceThe temperature dependence of exciton absorption and Raman frequencies of polydiacetylene (PDA) isolated chains is studied for 3BCMU and 4BCMU chains dispersed in their respective monomer crystal matrix. While isolated 3BCMU chains are practically unstrained, and behave as typical high quality bulk PDA, isolated 4BCMU chains are under a compression that increases as T decreases and show a very different behaviour. A simple model of chain deformation and its effect on the exciton energy is proposed, which qualitatively accounts for all the results. (c) 2007 Elsevier B.V. All rights reserved

Dendritic fluorene oligomers for nonlinear absorption in the visible range

International audienceWe report here the nonlinear absorption properties of dendritic molecules of oligofluorenes. Measurements were performed in chloroform between 450 and 650 nm for nanosecond time duration pulses. The nonlinear absorption is attributed to a three-photon absorption process involving first a two-photon absorption (TPA) step followed by an excited state absorption (ESA) process. The corresponding three-photon absorption coefficient α3 presents at resonance high values (α3 > 10000 cm3/GW2 for 2G1 the first dendrimer generation bearing a bifluorene at a concentration of 50 g/L). These values are strongly enhanced with respect to those obtained in the same conditions for analog linear oligofluorenes

Method for determining the polymer content in non soluble polydiacetylene films – application to pentacosadiynoic acid

International audienceThe absorptivities of polydiacetylenes (PDA) used in Langmuir films or vesicles for the development of PDA sensors films or other applications such as in non linear optics or for field-effect transistors are not known, so polymer contents cannot be deduced from experimental spectra. Here we introduce a novel method, using Nuclear Reaction Analysis (NRA) that allows quantitative determination of the polymer content X proportion of monomers that have been incorporated into PDA chains. We apply it to pentacosadiynoic acid (PCDA) evaporated microcrystalline films. A calibration curve giving X as a function of the area under an absorption spectrum normalized to the monomer areal density is obtained for blue and red PCDA. The method is applicable to all kinds of films, and to other PDA, provided films with known molecular areal density are available. An example of application to a PCDA Langmuir film is given

Unexpected Order–Disorder Transition in Diacetylene Alcohol Langmuir Films

International audienc