Glass transition and polymer dynamics in silver/poly(methyl methacrylate) nanocomposites

Dynamic mechanical-thermal analysis (DMTA), differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) were employed to investigate in detail glass transition and polymer dynamics in silver/poly(methyl methacrylate) (Ag/PMMA) nanocomposites. The nanocomposites were prepared by radical polymerization of MMA in the presence of surface modified Ag nanoparticles with a mean diameter of 5.6 nm dispersed in chloroform. The fraction of Ag nanoparticles in the final materials was varied between 0 and 0.5 wt%, the latter corresponding to 0.055 vol%. The results show that the nanoparticles have practically no effect on the time scale of the secondary ß and ¿ relaxations, whereas the magnitude of both increases slightly but systematically with increasing filler content. The segmental ¿ relaxation, associated with the glass transition, becomes systematically faster and stronger in the nanocomposites. The glass transition temperature T g decreases with increasing filler content of the nanocomposites up to about 10 °C, in good correlation by the four techniques employed. Finally, the elastic modulus decreases slightly but systematically in the nanocomposites, both in the glassy and in the rubbery state. The results are explained in terms of plasticization of the PMMA matrix, due to constraints imposed to packing of the chains by the Ag nanoparticles, and at the same time, of the absence of strong polymer-filler interactions, due to the surface modification of the Ag nanoparticles by oleylamine at the stage of preparation. © 2011 Elsevier Ltd. All rights reserved.JLGR acknowledges the support of the Spanish Ministry of Science and Innovation through project No. EUI2008-00126 and funding in the Centro de Investigacion Principe Felipe in the field of Regenerative Medicine through the collaboration agreement from the Conselleria de Sanidad (Generalitat Valenciana), and the Instituto de Salud Carlos III (Ministry of Science and Innovation).Pandis, C.; Logakis, E.; Kyritsis, A.; Pissis, P.; Vodnik, VV.; Dzunuzovic, E.; Nedeljkovic, JM.... (2011). Glass transition and polymer dynamics in silver/poly(methyl methacrylate) nanocomposites. EUROPEAN POLYMER JOURNAL. 47(8):1514-1525. https://doi.org/10.1016/j.eurpolymj.2011.06.001S1514152547

Phenomenological theory of structural relaxation based on a thermorheologically complex relaxation time distribution

he aim of this work is to explore the consequences on the kinetics of structural relaxation of considering a glass-forming system to consist of a series of small but macroscopic relaxing regions that evolve independently from each other towards equilibrium in the glassy state. The result of this assumption is a thermorheologically complex model. In this approach each relaxing zone has been assumed to follow the Scherer-Hodge model for structural relaxation (with the small modification of taking a linear dependence of configurational heat capacity with temperature). The model thus developed contains four fitting parameters. A least-squares search routine has been used to find the set of model parameters that fit simultaneously four DSC thermograms in PVAc after different thermal histories. The computersimulated curves are compared with those obtained with Scherer-Hodge model and the model proposed by Gómez and Monleón. The evolution of the relaxation times during cooling or heating scans and also during isothermal annealing below the glass transition has been analysed. It has been shown that the relaxation times distribution narrows in the glassy state with respect to equilibrium. Isothermal annealing causes this distribution to broaden during the process to finally attain in equilibrium the shape defined at temperatures above T<sub>g</sub>

Cooperative Segmental Motions in Ethyl Acrylate/Triethylene Glycol Dimethacrylate Copolymer Networks Studied by Dielectric Techniques

The molecular dynamics of ethyl acrylate/triethylene glycol dimethacrylate (EA/TrEGDMA) copolymer networks on the overall composition range were investigated thoroughly by employing dielectric techniques: thermally stimulated depolarization currents (TSDC) and dielectric relaxation spectroscopy (DRS). Furthermore, di¿erential scanning calorimetry (DSC) was performed in order to investigate the thermal glass transition of these copolymers. The results show that, for low content of the TrEGDMA component, the overall dielectric behavior of the copolymers is dictated by the dynamics of PEA component, exhibiting the ¿cop relaxation process which is strongly a¿ected by the TrEGDMA moieties becoming broader and shifting to higher temperatures with increasing cross-linker (TrEGDMA) content. For the copolymer networks rich in TrEGDMA (w TE g 40 wt %), the overall dielectric behavior indicates that the dynamics of the dimethacrylate component dominates, resembling that of poly(n-alkyl methacrylates) in the sense that the temperature dependence of molecular relaxation processes presents a merging region where the local relaxation process of TrEGDMA,ßPTE, merges with the more cooperative¿cop process of the copolymers into an¿ßcop locally cooperative relaxation process. By increasing the EA content, the strength of ¿cop relaxation process enhances and simultaneously the merging region shifts to lower temperatures. The cooperative motions are signi¿cantly suppressed in the copolymers with TrEGDMA contents higher that 80 wt %. The copolymers present enhanced spatial heterogeneity the more TrEGDMA content, whereas no dynamic heterogeneity was detected.A.T.S. thanks the Department of Physics, National Technical University of Athens, for the financial support of this work.Stathopoulos, AT.; Kyritsis, A.; Gallego Ferrer, G.; Gómez Ribelles, JL.; Christodoulides, C.; Pissis, P. (2011). Cooperative Segmental Motions in Ethyl Acrylate/Triethylene Glycol Dimethacrylate Copolymer Networks Studied by Dielectric Techniques. Macromolecules. 44:8233-8244. doi:10.1021/ma201755yS823382444