MSE Seminar Series: Xavier Coqueret
Friday, September 20, 2013
1:00 p.m.-2:00 p.m.
Room 2110, Chemical and Nuclear Engineering Bldg
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)