Polymer dynamics in rubbery epoxy networks/polyhedral oligomeric silsesquioxanes nanocomposites

Abstract

Dielectric techniques, including thermally stimulated depolarization currents (TSDC, À150 to 30 C) and, mainly, broadband dielectric relaxation spectroscopy (DRS, 10 À2 -10 6 Hz, À150 to 150 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 c and b relaxations in the glassy state, the segmental a 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 a 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 POSS units).The increase of free volume is rationalized by density measurements.

Fluorinated networks dynamics studied by means of broadband dielectric spectroscopy

“…Therefore, it is reasonable to assume that the same group is involved in these relaxations, i.e., epoxy group. [20][21][22] A similar behavior is observed for the c relaxation of both Jeff and JMF (where the relaxation times and activation energies also coincide). In addition, FTIR spectroscopy was employed to help in the identification of these dielectric signals through the analysis of the chemical species within the material.…”

Segmental dynamics in hybrid polymer/POSS nanomaterials

“…A later study with thermal and dielectric methods confirmed slight plasticization on addition of the DGEBA-P monomers [170]. The results are discussed in terms of two contradictory effects, namely the loosened packing of chains and constraints to their mobility due to tethering on the rigid cages.…”

“…All techniques suggest a broadening of the distribution of relaxation times related to increased inhomogeneity. The dielectric study [170] revealed that α-relaxation is followed by the slower normal mode (NM) relaxation that reflects the orientation of the end-to-end vector of the macromolecular contour of POPD. This relaxation follows here the trends of α with respect to timescale.…”

Epoxy/PCL nanocomposites: Effect of layered silicate on structure and behavior

“…At the same time, Table 1 clearly shows that the clay also decreases the T g of neat epoxy. Generally, nanofillers influence the T g of polymer matrices by two contradictory effects [38,39]: reduction of the chain mobility by interaction with the rigid nanofiller and (b) increase of the free volume because of the presence of the nanofiller and the loosened molecular packing of the chains. In the case of the rigid DGEBA-DDS network, the effect of the increased free volume dominates, leading to a decrease in the T g .…”