Viscoelastic Responses of Polyhedral Oligosilsesquioxane Reinforced Epoxy Systems

The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface, and the chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. Recently, families of mono- and difunctionalized polyhedral oligomeric silsesquioxane (POSS) macromers bearing epoxide groups have been developed. This paper presents an investigation of the thermal and viscoelastic property enhancements in commonly used model epoxy resins reinforced with monofunctional POSS-epoxy macromers. The glass transitions of these POSS-epoxy nanocomposites were studied using differential scanning calorimetry. Small-strain stress relaxations under uniaxial deformation were examined to provide insight into the time-dependent viscoelastic behavior of these nanocomposites. The POSS-epoxy macromers utilized in this study were monofunctional and hence occupied chain terminus points within the network. Nevertheless, they were effective at hindering the molecular motion of the epoxy network junctions. Thus the glass transition temperature, Tg, was observed to increase with increasing weight fraction of the monofunctional POSS-epoxy. The viscoelastic response at temperatures below Tg 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 study. Surprisingly, the instantaneous modulus was not observed to be affected by incorporation of the POSS nanoreinforcement. This suggests that while POSS cages influence polymer chain motions, including the motion of the molecular junctions, these nanoreinforcements did not participate in the overall deformation of the chains. Experiments performed under identical thermodynamic states, revealed that the molecular level reinforcement provided by the POSS cages also retarded the physical aging process in the glassy state. Therefore, the time required to reach structural equilibrium was longer for samples reinforced with POSS-epoxy than for those of the neat resins.