Polymer Dynamics in Rubbery Epoxy Networks/Polyhedral Oligomeric Silsesquioxanes Nanocomposites

Dielectric techniques, including thermally stimulated depolarization currents (TSDC, -150 to 30 degrees C) and, mainly, broadband dielectric relaxation spectroscopy (DRS, 10(-2) - 10(6) Hz, -150 to 150 degrees C) were employed, next to differential scanning calorimetry (DSC), to investigate molecular dynamics in rubbery epoxy networks prepared from diglycidyl ether of Bisphenol A (DGEBA) and poly (oxypropylene)diamine (Jeffamine D2000, molecular mass 2000) and modified with polyhedral oligomeric silsesquioxanes (POSS) units covalently bound to the chains as dangling blocks. Four relaxations were detected and analyzed: in the order of increasing temperature at constant frequency, two local, secondary gamma and beta relaxations in the glassy state, the segmental alpha relaxation associated with the glass transition and the normal mode relaxation, related with the presence of a dipole moment component along the Jeffamine chain contour. Measurements on pure Jeffamine D2000 helped to clarify the molecular origin of the relaxations observed. A significant reduction of the magnitude and a slight acceleration of the alpha and of the normal mode relaxations were observed in the modified networks. These results suggest that a fraction of polymer is immobilized, probably at interfaces with POSS, due to constraints imposed by the covalently bound rigid nanoparticles, whereas the rest exhibits a slightly faster dynamics due to increaseof free volume resulting from loosened molecular packing of the chains (plasticization by the bulky TOSS units). The increase of free volume is rationalized by density measurements. (C) 2009 Wiley Periodicals, Inc. I Appl Polym Sci 113: 2569-2582, 2009.