The rubbery epoxy network, based on diglycidyl ether of Bisphenol A (DGEBA) and poly-(oxypropylene)diamine (Jeffamine D2000), was reinforced with a nanometer-sized inorganic building blocksspolyhedral oligomeric silsesquioxanes (POSS). The organic-inorganic networks contained POSS as pendant units of a network chain or as network cross-links of various functionality. Thermomechanical properties and thermal stability of the POSS-containing networks were determined by DMA and TGA. The strongest reinforcement was achieved in the networks with pendant POSS forming ordered crystalline domains, which act as physical cross-links. The POSS skeleton with "soft" flexible substituents, such as octyl, shows formation of weak aggregates only, which do not contribute to reinforcement. The rubbery modulus of the networks with POSS in a junction grows with increasing POSS functionality due to enhanced network cross-link density. These networks are more homogeneous, and the modulus of the network with the octafunctional POSS junction well agrees with theoretical prediction. The mechanical properties are affected mainly by POSS-POSS interactions while the POSS-network chain interactions are of minor importance.

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.

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Three epoxy-amine thermoset systems were cured at a low ambient temperature. Evolution of the reaction kinetics and molecular structure during cure at the sub-glass transition temperature was followed by DSC and chemorheology experiments. The effect of vitrification and the reaction exotherm on curing and final mechanical properties of the epoxy thermosets was determined. Thermomechanical properties of the low-temperature cured systems depend on the reaction kinetics and volume of the reaction mixture. Curing of the fast-reacting system in a large volume (12-mm thick layer) resulted in the material with T g exceeding the cure temperature by 70 -80°C because of an exothermal temperature rise. However, the reaction in a too large volume (50-mm layer) led to thermal degradation of the network. In contrast, thin layers (1.5 mm) were severely undercured. Well-cured epoxy thermosets could be prepared at sub-T g temperatures by optimizing reaction conditions.