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For melt-compounded poly(lactic acid) composites containing graphene nanoplatelets (GNPs) at low volume fractions, ΦGNP≤0.34 vol.%, this study focused on correlation between the composite properties and a large-scale structure of GNPs formed by a strong alternating current electric field (with the frequency and intensity of 60Hz and 1.75kV/mm, respectively). The electric field was applied to the composite in a rheometer through the parallel plate fixture, so that the parallel plates served as the rheological measurement fixture as well as the electrodes. Optical microscopy, transmission electron microscopy, and two-dimensional wide-angle X-ray diffraction measurements revealed that almost randomly oriented, thin stacks of GNPs in the as-fabricated composites were aligned by the electric field in the field direction and further organized into an extended chain (or column) of stacks. This chained-stack structure bridged two parallel plate electrodes when ΦGNP was above a threshold value, ΦGNP*≅0.17vol.%. Corresponding to this structural change, the liquidlike rheological response of the as-fabricated composites, characterized by the storage modulus G' almost proportional to ω2 at intermediate-to-low angular frequencies ω, became the solidlike response, G'~ω0 at low ω (for the composites having ΦGNP≥ΦGNP*), after application of the electric field. In contrast, the loss modulus G" was insensitive to the electric field and remained almost proportional to ω at low ω, suggesting that the chained-stack structure formed by the electric field was an elastic structure bridging the parallel plates. This bridge also served as an electrically conductive path between the plates so that an insulator-conductor transition occurred when the composites with ΦGNP≥ΦGNP* were subjected to the electric field. Quantitative analysis of the equilibrium modulus and conductivity of the composites suggested that the chained-stack structure formed by the electric field was not a rigid single slab but included junctions being much softer and less conductive compared to the body of GNP (each graphene nanoplate). Moreover, analysis of the ΦGNP dependence on the equilibrium modulus and static electrical conductance after the transition (ΦGNP≥0.17vol.%) suggested that an elastically inert secondary structure was formed by the GNP stacks remaining out of the chained-stack structure (primary structure). This secondary structure appeared to be in soft contact with the primary structure to provide an extra conducting path. [ABSTRACT FROM AUTHOR]

The structural anisotropy and shear rheology of colloidal gels under startup of shear flow are calculated by Brownian dynamics simulations modified to include particle surface-mediated attractions, which is a model for thermoreversible colloidal gels. Shear-induced structural changes are analyzed in both real and reciprocal space through computation of the pair distribution function and structure factor. A distinct structural anisotropy is evident as alignment along the compressional and vorticity axes. A spherical harmonics expansion of pair distribution function is calculated to analyze structural anisotropy. In reciprocal space, structural anisotropy is quantified through an alignment factor, which shows overshoot behavior similar to the stresses. Based on the microstructure analysis, the evolution of the structural anisotropy is explained by the anisotropic rupture of the colloidal gel microstructure. This result provides evidence for how shearing creates structural anisotropy. The structural anisotropy during flow startup is quantitatively related to rheological behavior via a stress-small angle neutron scattering (SANS) rule that is modified to consider the influence of the rupture of the colloidal gel structure. A simple structure kinetic equation accounting for bond creation and rupture provides a unifying view of the results. This new, modified stress-SANS rule quantitatively links the shear-induced structural anisotropy to the nonlinear rheology. [ABSTRACT FROM AUTHOR]

The first normal stress difference (N1) in large amplitude oscillatory shear (LAOS) consists of two contributions: one from non-oscillating nonzero mean value and the other from oscillating even harmonics, while the nonlinear shear stress can be expressed as a sum of odd harmonics. In this study, the two nonlinear contributions of N1 in entangled polymer solutions have been analyzed by systematical comparison with their shear stress counterparts (storage modulus and odd harmonics, respectively) as a function of strain amplitude and of Deborah number (De). The contributions of N1 were greater than those of the shear stress in the LAOS region. The nonzero mean value of N1 decreased with an increase in Deborah number, but conversely, the second harmonic of N1 increased. The relative intensity (the second harmonic over the fourth harmonic of N1) as an indicator of the nonlinearity of N1 decreased with the Deborah number. We also compared experimental results to model predictions from the eight-mode Giesekus constitutive equation. For the model, we obtained two sets of nonlinear parameters from the best fit of model predictions with both steady shear viscosity and the first normal stress difference coefficient. The model could not quantitatively predict the first normal stress difference behavior in LAOS even with these nonlinear parameter sets, which fit the steady data very well. This study shows that the contribution of the first normal stress difference in LAOS is as vital as that of shear stress, and it can be used as a good estimate of the performance of the constitutive equation. [ABSTRACT FROM AUTHOR]

Nonlinear response of linear and branched polymers has been investigated under medium strain amplitude oscillatory shear (strain amplitude range from 10% to 100%) with Fourier-transform rheology. A power law relationship was found between the relative third intensity (I3/I1), which is an indicator of nonlinearity, and the strain amplitude at low and medium strain amplitudes. On a log-log plot, the intercept and slope of I3/I1 were investigated at different excitation frequencies and temperatures. Simulation results with three different constitutive equations [Giesekus, exponential Phan-Thien Tanner (E-PTT), pom-pom model] were also compared. Experimental results show that the intercept was affected by the excitation frequency and temperature, and the slope of I3/I1 for linear polymer remained constant regardless of molecular weight, molecular weight distribution, and excitation frequency in accordance with the predictions of the constitutive equations (Giesekus and E-PTT). It should be noted that the slope of I3/I1 for branched polymer was lower than that of linear polymer, unlike the prediction of the pom-pom model. Among the molecular architecture and processing parameters (e.g., molecular weight, molecular weight distribution, frequency, and temperature), the slope of I3/I1 under medium amplitude oscillatory shear was found to depend only on the long chain branching, which means that it can be used as a measure of the degree of branching. The failure of the pom-pom model in predicting the nonlinear shear behavior was also pointed out. [ABSTRACT FROM AUTHOR]