Research Summary

Polymers are known to exhibit strong time-dependent mechanical behavior. In different temperatures or frequency regimes, the rate sensitivities of polymers change as various primary and secondary molecular mobility mechanisms are accessed. The incorporation of nanoparticles into the polymer matrix can potentially alter the local molecular level structure and thus offers an opportunity to tailor the rate-dependent mechanical deformation and failure behavior of the polymer.

Our study focuses on polyhedral oligomeric silsesquioxanes (POSS) enhanced polymeric systems. POSS has received much attention due to its hybrid organic-inorganic structure, which consists of a silica cage with organic functional groups attached to the cage corners. Methacryl-POSS (C56H88O28Si8) is incorporated into poly(vinyl chloride) (PVC) through melt blending and uniform polymer compounds with different POSS contents are produced. Dioctyl phthalate (DOP) plasticized PVC is prepared using the same method as a comparison to POSS-filled PVC. Dynamic Mechanical Analysis (DMA) revealed that the incorporation of POSS in PVC introduced reductions in both the primary (α) and secondary (β) transition temperatures over a range of strain rates (10-3 to 10-1/s). As for the PVC/DOP blends, while the α-transition temperatures were reduced, β-relaxation motion vanishes when DOP content was increased. The rate dependent yield and post-yield behavior is characterized in compression testing over a wide range of strain rate. Zwick Mechanical Tester is used for low to moderate strain rate (10-4 to 10-1/s) and Split Hopkinson Pressure Bar is used for high strain rate (500 to 3000/s). It was found that PVC with POSS shows a delay in yield strength rate-sensitivity transition, which is consistent with the observation in the β-transition reduction. For PVC with higher DOP concentration which shows restricted β-motions, the rate-sensitivity transition in yield strength faded away.