Multi-scale structure and dynamics of macroporous thermosensitive gels
(Françoise EHRBURGER-DOLLE, Roger CASALEGNO, Isabelle MORFIN in coll. with M. Aguilar de Armas (CSIC Madrid))
Macroporous polymer gels present a significant interest from fundamental and application point of view, particularly for biotechnological and biomedical applications. In the case of intelligent polymeric materials, which exhibit response to external stimuli, the response rate is significantly increased in macroporous gels as compared to bulk gels. Poly(N-isopropylacrylamide) (PNIPAM) based macroporous gels are temperature sensitive materials. They exhibit a volume phase transition at a critical temperature Tc of about 34°C in aqueous media. The aim of the work was to investigate the change of structure occurring at the macroscale and at the meso/nano scale in the vicinity of the volume transition in macroporous gels elaborated with different chemical additives (PhD thesis of M. Chalal).
Two-photon fluorescence microscopy (TPFM) has been applied for the first time to macroporous hydrogels (collaboration with MOTIV). This method led to the macropore size distribution in the swollen state and to its change with temperature. The temperature induced structural modifications of the hydrogel at the meso/nano scale was investigated by small-angle X-ray scattering (SAXS) over a broad range of q values (0.03 to 12 nm -1 ) at temperatures ranging between 18 and 37°C. The SAXS curves obtained at each temperature were fitted by a sum of four equations describing respectively the scattering resulting from the gel surface, from the solid-like and liquid-like heterogeneities and from the chain-chain correlation yielding a broad peak in the high q domain. The analysis of the temperature dependence of the parameters obtained from the fit gave new insights in the origin of the isoscattering point observed up to Tc.
Structure, dynamics and mechanical properties of filled elastomers
(Françoise EHRBURGER-DOLLE and Isabelle MORFIN in coll. with F. Livet and F. Bley (SiMAP-Grenoble-INP), M. Sutton (Univ. McGill, Montreal), Y. Chushkin (ESRF, Grenoble) and G. Heinrich (Polymer Institute, Dresden))
The research work performed in the field of polymer nanocomposites was aiming at relating the dynamics of the filler particles with the mechanical stress relaxation in strained materials. To this end, X-ray Photon Correlation Spectroscopy (XPCS) experiments have been performed on strained samples. The stress relaxation was measured simultaneously. The mechanical protocol was established in order to compare UP and DOWN jumps as well as first and second tensile stress relaxation. The matrix was an ethylene propylene diene monomer (EPDM) cross-linked rubber. The parameters involved were the matrix-filler and the filler-filler interaction, the filler volume fraction, the strain intensity and the mechanical history of the sample. At a given strain, the velocity of the filler particles deduced from XPCS performed with heterodyne detection (HD-XPCS) was shown to become smaller and its decrease with time faster as the fillermatrix became stronger. For all materials, the tensile relaxation modulus was shown to vary as a power law of time for strains above a given threshold. Pertinence of the soft glassy rheology (SGR) model proposed by Sollich and coll. to the description of the stress relaxation modulus was discussed. In particular, the noise temperature x defined in the SGR model could be related to the mobility of the polymer chains influenced by the interaction with the surface of the filler and the strain. The dynamical behavior of the filler particles deduced from HD-XPCS experiments appears to be consistent with this interpretation.
Physical molecular gels
(Marie PLAZANET, Isabelle MORFIN and Sylvie SPAGNOLI)
Physicals gels formed by small organic gelators find applications in diverse domains, although the understanding of their formation and stability severely limits their practical use. We therefore undertook the study of a small amphiphilic gelator in various solvents as model system, we the aim of characterising both structural and dynamical aspects involved in the gel phase stability. Using quasielastic neutron scattering, we have been able to quantify the number of solvent molecules that are immobilised with the solid network, as well as the fraction of slow-down molecules on the picosecond timescale. These results show a direct correlation with the stability of the gel.
Structure and reactivity in polydiacetylenes
(Sylvie SPAGNOLI, in coll with INSP and ITODYS labs in Paris)
Diacetylenes (DA) R-C-C-C-C-R’ can be polymerized by UV or gamma irradiation of self assembled DA (crystal, Langmuir-Blodgett film) through a topochemical reaction. Microcrystalline films of the DA 4BCMU (R=R’=(CH2)4-OCONH-CH2-COOC4H9) were polymerized by UV irradiation. A method for determining the absolute polymer content X in films has been developed. In this DA the reaction rate is strongly decreased beyond X 0.3. Existing models fail to fit the kinetics. A model based on quenching of the monomer excited state precursor to initiation by energy transfer to a nearby existing chain is developed and quantitatively fits the kinetics. Such quenching may occur in any diacetylene film. One should not assume that an apparent saturation of the absorbance of a film coresponds to complete polymerization. Most often the polymerization leads to long conjugated blue PDA which can undergo a transition to red PDA under various stimuli (temperature, pressure...). In 4BCMU the amide bands associated to the urethanes in the side groups, which form one-dimensional H-bond linear chains are quite similar to those found in peptides and proteins. The positions and strengths of these absorptions, studied by FTIR, are practically unchanged by polymerization or blue to red 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 polymer crystal as often stated.
(Isabelle MORFIN in coll. with I. Grillo (ILL, Grenoble), J. Combet (ICS, Strasbourg) and LLB (Saclay))
Hyaluronan complexes have been studied for a few years at the LIPhy. The hyaluronan, HA, is a negatively charged polysaccharide which has many biological functions that can even be opposite depending on the chain length. In spite of a large use for pharmaceutical applications, almost no research investigates the multiscale HA structure in complex, possibly in relation with these biological processes. Our goal is a precise knowledge of the full structures and dynamics of HA complex systems in aqueous solutions, the effect of the surrounding medium (salt for example), the role played by the chain stiffness or the HA chain length. Large instrument facilities, in particular small angle neutron or X-ray scattering (SANS and SAXS), are essential for such studies involving HA/protein, HA/ionic micellar or HA/non ionic micellar systems. A first study on HA/lysozyme (a globular protein mainly positively charged) has shown a particular rod like structure revealed by SANS.
Such structure has not been observed with other polyelectrolytes/proteins mixtures (HA/Human Serum Albumin or sulphonated polystyrene (PSS)/lyso) for which the polymer chains seem to wrap more closely the proteins. In addition, in contrast with the PSS/lyso complexes, no experimental condition leading to protein unfolding has been obtained with HA/Lyso. The effect of the chain stiffness is also considered in other systems, HA/Dotab, a positively charged surfactant forming micelles and the PSS/Dotab are investigated in parallel.