VISCOELASTIC RESPONSES OF POLYHEDRAL OLIGOSILSESQUIOXANE REINFORCED EPOXY SYSTEMS

The properties of nanostructured plastics are determined by complex re lationships between the type and size of the nanoreinforcement, the in terface, and the chemical interaction between the nanoreinforcement an d the polymeric chain, along with macroscopic processing and microstru ctural effects. Recently, families of mono- and difunctionalized polyh edral oligomeric silsesquioxane (POSS) macromers bearing epoxide group s have been developed. This paper presents an investigation sf the the rmal and viscoelastic property enhancements in commonly used model epo xy resins reinforced with monofunctional POSS-epoxy macromers. The gla ss transitions of these POSS-epoxy nanocomposites were studied using d ifferential scanning calorimetry. Small-strain stress relaxations unde r uniaxial deformation were examined to provide insight into the time- dependent viscoelastic behavior of these nanocomposites. The POSS-epox y macromers utilized in this study were monofunctional and hence occup ied chain terminus points within the network. Nevertheless, they were effective at hindering the molecular motion of the epoxy network junct ions. Thus the glass transition temperature, T-g, was observed to incr ease with increasing weight fraction of the monofunctional POSS-epoxy. The viscoelastic response at temperatures below T-g was examined and was found to correlate to a stretched exponential relaxation function. Time-aging time-superposition was found to be applicable to the data under all test conditions and for all of the materials used in this st udy. Surprisingly, the instantaneous modulus was not observed to be af fected by incorporation of the POSS nanoreinforcement. This suggests t hat while POSS cages influence polymer chain motions, including the mo tion of the molecular junctions, these nanoreinforcements did not part icipate in the overall deformation of the chains. Experiments performe d under identical thermodynamic states, revealed that the molecular le vel reinforcement provided by the POSS cages also retarded the physica l aging process in the glassy state. Therefore, the time required to r each structural equilibrium was longer for samples reinforced with POS S-epoxy than for those of the neat resins.