(657f) Structure and Nonlinear Rheology in Large Amplitude Oscillatory Shear of Polypropylene-Layered Silicate Nanocomposites

The nonlinear FT-rheology of polypropylene-layered silicate nanocomposites with a low nanoparticle loading has been investigated with large amplitude oscillatory shear (LAOS) tests over strain amplitudes ranging from 0.5% to 500% and at several frequencies from 0.5 rad/s to 5 rad/s. A sampling rate of 100 kS/s was used on each signal â?? the strain and the torque. An oversampling number of 3000 was used to collect data over sixteen cycles at each strain amplitude â?? cf. Merger and Wilhelm¹. The matrix polymer was a copolymer of propylene with 3 mole% ethylene and functionalized with maleic anhydride (PO1015 from ExxonMobil). The volume fraction of onium ion exchanged montmorillonite (I.44P from Nanocor) was 0.02. This work follows up on recent results reported²from our laboratory which related the linear viscoelastic behavior of such nanocomposites to the extent of coupling between nanolayers and the polymer chains.

The relative third harmonic content I_3/1 plotted against strain amplitude displayed quadratic dependence up to a strain amplitude of 100% at several frequencies. This would indicate the absence of a filler network in the nanocomposite melts investigated here. Hence the viscoelastic behavior can be interpreted in terms of the breakup of two different entanglement networks â?? one involving particle attached polymer chains and another involving only the free polymer chains. Comparison of the storage modulus plots against strain amplitude for the nanocomposites and the matrix showed two distinct transitions in the case of the nanocomposite. The plots of relative harmonic intensity also exhibited a plateau followed by another power law region of increasing intensity; the transition between these regions was found to vary with frequency. The relative harmonic intensity variation for filled elastomers was related by Leblanc³to a Weibull probability distribution for breakage of network links. A similar analysis was carried out in this study to extract parameters for the network formed by chains attached to particles entangling with the free polymer chains. The strength of attachment between the chains and the particles can be evaluated and compared for nanocomposites with different coupling agents in this way.

1. Wilhelm, M., Reinheimer, P., Ortseifer, M., Rheol. Acta, 38, 349-356 (1999)
2. Ren, W., Chaudhary, A.K., Jayaraman, K., Ind. Eng. Chem. Res., 54 (16), 4264-4273 (2015)
3. Leblanc, J.L., J. Appl. Polym. Sci., 109, 1271-1293 (2008)