| Abstract
| - The properties of nanostructured plastics are determined by complex relationships betweenthe type and size of the nanoreinforcement, the interface, and the chemical interaction between thenanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects.Recently, families of mono- and difunctionalized polyhedral oligomeric silsesquioxane (POSS) macromersbearing epoxide groups have been developed. This paper presents an investigation of the thermal andviscoelastic property enhancements in commonly used model epoxy resins reinforced with monofunctionalPOSS-epoxy macromers. The glass transitions of these POSS-epoxy nanocomposites were studied usingdifferential scanning calorimetry. Small-strain stress relaxations under uniaxial deformation wereexamined to provide insight into the time-dependent viscoelastic behavior of these nanocomposites. ThePOSS-epoxy macromers utilized in this study were monofunctional and hence occupied chain terminuspoints within the network. Nevertheless, they were effective at hindering the molecular motion of theepoxy network junctions. Thus the glass transition temperature, Tg, was observed to increase withincreasing weight fraction of the monofunctional POSS-epoxy. The viscoelastic response at temperaturesbelow 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 ofthe materials used in this study. Surprisingly, the instantaneous modulus was not observed to be affectedby incorporation of the POSS nanoreinforcement. This suggests that while POSS cages influence polymerchain motions, including the motion of the molecular junctions, these nanoreinforcements did notparticipate 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 retardedthe physical aging process in the glassy state. Therefore, the time required to reach structural equilibriumwas longer for samples reinforced with POSS-epoxy than for those of the neat resins.
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