(322e) Polyoxometalate Antiplasticization of An Epoxy Cured Thermoset Network

In this paper, the thermomechanical response of a phosphotungstate (PTA) loaded epoxy nanocomposite is discussed and interpreted through the methodology of antiplasticization.  PTA is a 1 nm size particle composed of a tungsten oxide shell and a central phosphate group and is part of a family of multinuclear oxometalates called polyoxometalates.  PTA is dispersed in the epoxy resin up to volume fractions of 0.1 and the resin is cured by cationic homopolymerization initiated by the PTA.  The stiffness and strength of the nanocomposite linearly increase as a function of PTA concentration.  High resolution TEM shows the PTA to be well dispersed in the polymerized network.  The PTA reduce polymer segment mobility in the glass as witnessed by a suppression of the β-relaxation peak in the loss tangent measured by dynamic mechanical analysis.  The α-relaxation peak is broadened and shifts to lower temperature signaling a reduction in the fragility of the network with an increase in the PTA concentration.  Thermal mechanical analysis shows the PTA cause an increase in the network segment density in the glass, yet a decrease in the rubber when referenced to the unfilled network.  In addition, the point of discontinuity in the thermal expansion defining the glass transition temperature shifts to lower temperature in agreement with the shift of the α-relaxation peak in the loss tangent.  The shift is non-linear with PTA concentration suggesting a greater effectiveness of PTA in lowering the glass transition temperature as the PTA concentration is increased.  These observations are consistent with PTA operating as an antiplasticizer in the polymer matrix.  Antiplasticization has been argued to only hold for molecular additives and to disappear for larger nanoparticles giving way to plasticization as the size of the additive increases.  In this polyoxometalate loaded epoxy nanocomposite, antiplasticization is shown to operate for particles of nanometer size.