Polycarbonate and a Polycarbonate-POSS Nanocomposite at High Rates of Deformation

Polymeric materials are known to exhibit strong time-dependent mechanical behavior, as evidenced by rate-dependent elastic moduli, yield strength, and post-yield behavior. The nature of the rate sensitivity is found to change between different temperature regimes as various primary $(α)$ and secondary (⁠$β$⁠, $γ$⁠, etc.) molecular mobility mechanisms are accessed. The ability to tailor these molecular-level mechanics through the incorporation of nanoscale particles offers new opportunities to design polymer-based material systems with different behaviors (elastic, yield, post-yield) in different frequency∕rate regimes. In this study, the macroscopic rate-dependent mechanical behavior of one particular polymer nanocomposite—polycarbonate compounded with TriSilanolPhenyl-POSS® particles—is compared with that of its homopolymer counterpart. The experimental and theoretical techniques follow those established in previous research into the rate-dependent mechanical behavior of amorphous homopolymers over a wide range of strain rates. On the experimental side, dynamic mechanical analysis tension tests were used to characterize the viscoelastic behavior of these materials, with focus on the rate-dependent shift of material transition temperatures. Uniaxial compression tests on a servohydraulic machine $(10−3s−1to0.3s−1)$ and an aluminum split-Hopkinson pressure bar $(1000s−1to3000s−1)$ were used to characterize the rate-dependent yield and post-yield behavior. The behaviors observed in these experiments were then interpreted within the theoretical framework introduced in previous work. It is concluded that, for this particular material system, the POSS has little influence on the polycarbonate $α$ regime. However, the POSS clearly enhances the mobility of the $β$ motions, significantly reducing the resistance to high rate elastic and plastic deformation. Furthermore, it is shown that the continuum-level constitutive model framework developed for amorphous homopolymers may be extended to this polymer nanocomposite material system, simply by accounting for the reduced deformation resistance in the $β$ process.