Multiscale Structural Features of Multi-Acrylate Networks Produced by Radiation-Initiated Polymerization
Institut de Chimie Moléculaire de Reim
Université de Reims Champagne Ardenne
The radiation-initiated crosslinking polymerization of multifunctional monomers is a very attractive method for the fabrication of coatings and high performance composite materials. The method offers many advantages compared to conventional energy- and time-consuming thermal curing processes.1) However, the fast polymerization of multiacrylates is known to generate micro-heterogeneous networks. In order to gain an insight into the polymer microstructure, a combination of analytic methods was used to quantify polymer segment mobility in the different domains.
Solid state proton T2 NMR relaxation experiments were performed on radiation-cured materials prepared from model difunctional monomers. This method allowed us to distinguish two phases inside the materials: one consisting in rigid domains, and a second one with higher local mobility and distinct relaxation features.2) The decay of transverse magnetization was fitted with two components, (short or long T2), which can be assigned to the highly cross-linked and the loosely cross-linked phase, respectively. The influence of acrylate conversion on the relaxation behavior of cured samples was examined to describe the gradual evolution of the different domains, in terms of local mobility and associated fraction of material, as the radiation-induced polymerization proceeds. Network analysis by atomic force microscopy in the phase imaging mode provides a complementary picture of the network with indications on the actual dimensions of the soft and rigid domains.3) Temperature-modulated DSC thermograms can be further analyzed in the light of these results.4) Comparing the NMR relaxation data as well as the calorimetric features of networks prepared by UV- or by EB-induced polymerization did not reveal noticeable differences to be related to the initiation mechanism and/or curing conditions. Various structural and kinetic data will be discussed for interpreting the observed polymerization behavior of the model diacrylates.
Improving fibre-matrix adhesion and upgrading polymer network toughness are currently the two major challenges in this area. The efforts to overcome the brittleness associated with the heterogeneities described above are focused on new formulations including a high Tg thermoplastic toughener.5,6) Particular attention is also paid on the functional groups present at the surface of carbon fibres, as identified and quantified by XPS analysis. Free radical reactions are suspected to be less efficient in the vicinity of the carbon materials. We are evaluating the influence transfer agents attached to the carbon materials for forcing the formation of covalent links with the matrix. Significant improvements are achieved on transverse strain at break by applying original surface treatments to the fibres so as to induce covalent coupling with the matrix.7,8)
(1) Litvinov V. M., Dias A. A., Macromolecules, 2001, 34, 4051-4060.
(2) Coqueret X., Krzeminski M., Ponsaud P., Defoort B., Radiat. Phys. Chem. 2009, 78, 557-56.
(3) Krzeminski M., Molinari M., Troyon M., Coqueret X., Macromolecules 2010, 43, 8121-8127.
(4) Krzeminski M., Molinari M., Troyon M., Coqueret X., Macromolecules 2010, 43, 3757-3763.
(5) Krzeminski M., Molinari M., Defoort B., Coqueret X., Radiat. Phys. Chem. 2013, 84, 79-84.
(6) Krzeminski M., Defoort B., Coqueret X., PCT Int. Appl. WO 2011-042554, 2011./span>
(7) Defoort B., Ponsaud Ph., Coqueret X., PCT Int. Patent PCT/EP2007-0545, 2007.
(8) Krzeminski M., Ponsaud. Ph., Coqueret X., Defoort B., Larnac G., Avila R., SAMPE 2011 Long Beach, May 23-26, 2011.
About the Speaker
Xavier Coqueret, born in 1956, received his doctorate in Organic Chemistry from the University of Reims in 1984. He joined at that time the Polymer group at the University of Lille, as an associate scientist of the CNRS (Centre National de la Recherche Scientifique). After a one-year leave at the Max-Planck-Institut fur Polymerforschung in Mainz (Germany) as an Alexander-von-Humboldt Foundation fellow (1989-1990), Xavier Coqueret was appointed as Professor of Polymer Chemistry at the Ecole Nationale Supérieure de Chimie de Lille (1991). His major area of research since then is polymer photochemistry with emphasis on the synthesis and reactivity of functionalized macromolecules. His activities were extended to the electron beam-induced modification of polymers by cross-linking or by grafting, and to radiation-initiated polymerizations.
In September 2005, Xavier Coqueret moved to the University of Reims Champagne Ardenne to initiate a research activity on Polymer chemistry, with emphasis on clean chemical processes involving radiation treatment as well as on new products and materials derived from biomass. He teaches analytical chemistry, polymer chemistry and chemical valorization of bio-sourced raw materials. Since January 1st 2008, he is heading the Institute of Molecular Chemistry, a research Institute under contract with CNRS (UMR 7312). He is the author or co-author of about 150 scientific papers and 12 patents.
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