Small-angle X-ray scattering (SAXS) monitors tensile and load-cycling tests of metallocene isotactic polypropylene (PP), a blend of PP and montmorillonite (MMT), and two block copolymer compatibilized PP/MMT nanocomposites. Mechanical properties of the materials are similar, but the semicrystalline nanostructure of the PP differs. This is explained by a nucleation effect of the MMT. Competitive crystal growth diminishes crystallite sizes. The reinforcing effect of the MMT filler appears consumed by weakening the PP matrix. Decays of mechanical and nanostructure response in dynamic load cycling indicate materials fatigue. Lifetimes describe the reinforcing and weakening effects. Addition of 3% MMT halves the fortifying effect of the PP nanostructure. A net gain of reinforcement (11%) is observed with the highly compatibilized composite in which the strength of the semicrystalline PP is reduced to 25%. Other results concern the evolution of Strobl's block structure and void formation during tensile loading.

Polypropylene (PP)-based polymer nanocomposites containing organically modified montmorillonite (OMMT) with and without maleic anhydride grafted PP, were compounded by twin-screw extrusion. The extrusion process was repeated various numbers of times to increase the extruder residence time (T R ) and, through that, the particle dispersion. Rheological measurements fitted to a modified Carreau-Yasuda model defining a melt yield stress were used to indicate changes in the particle dispersion with regard to T R . This analysis showed a monotonically increased dispersion of clay particles in the PP matrix with increasing extruder T R . The small-strain tensile properties were tested at both ambient (20 C) and elevated (90 C) tem-peratures, and no significant changes were observed in the tensile strength or modulus as a function of T R . Instrumented Izod impact tests showed that the nanocomposite impact strength (r i ) increased monotonically with increased T R by 70% from least dispersed to best dispersed, which was still 20% below the level for neat PP. Both the fracture initiation energy and propagation energy increased with T R , but the primary effect on r i came from the fracture propagation energy, which delivered 80% of the improvement. V C 2012Wiley Periodicals, Inc. J Appl Polym Sci 126: 620-630, 2012

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The linear viscoelastic behavior in dynamic shear and tensile creep at temperatures from À30 to 70 C is measured for an styrene-butadiene rubber (SBR) elastomer cured with dicumyl peroxide to crosslinking densities between 0 and 23.5 Â 10 À5 mol/cm 3 . The G 0 , G 00 , and tan d isotherms are analyzed by time-temperature superposition (TTS), where the tan d master curves are consistent with those of Mancke and Ferry. However, to achieve the TTS in the lightly crosslinked SBR systems, an anomalous vertical shift is required in the narrow temperature region from 10 to 30 C. The vertical shift factor in this temperature region is not the standard q 0 T 0 =qT from rubber elasticity. No anomalous behavior is detected in the equilibrium modulus, which is a linear function of temperature in accordance with the classical theory of rubber elasticity. In contrast to SBR, standard vertical shifts are required to effect TTS for uncrosslinked polybutadiene and an ethylene propylene diene monomer elastomer.