Nanorheology of Confined Polymer Melts. 2. Nonlinear Shear Response at Strongly Adsorbing Surfaces

Abstract

The large-amplitude nonlinear shear rheology of polymer melts confined between strongly adsorbing surfaces (parallel plates of mica) was studied as a function of strain, frequency, and thickness of the polymer films. The shear strains varied from less than 0.1 (linear response) to over 30 (at which the film structure was strongly modified by the imposed shear). The measurements employed a surface forces apparatus modified for dynamic mechanical shear. The polymers were atactic poly(phenylmethylsiloxane) (PPMS), with chain lengths from 31 to 153 skeletal bonds. The nonlinear shear forces, decomposed into a Fourier series of harmonic frequencies, were always odd in the excitation frequency, as required by symmetry considerations. The in-phase and out-of-phase oscillatory shear responses at the same frequency as the excitation (the nonlinear storage and loss moduli G\ and Gi", respectively) were analyzed. Four principal conclusions emerged. First, from the frequency dependence of G\ and Gi" at constant strain, we conclude that relaxations were accelerated by large strain. Second, a marked decrease of both Gf and Gi" was observed with increasing strain at constant frequency, except at the smallest film thickness, «s40 Á, where Gi" passed through a maximum with increasing strain but Gi continued to display shearthinning. Third, the critical strain for onset of nonlinear response increased with the excitation frequency. Fourth, at sufficiently large strains (larger than 10), the shear moduli were independent of polymer molecular weight (comparisons made at fixed film thickness) and appeared to reach limiting strain-independent levels at sufficiently large strains. This final observation contrasts sharply with the linear response and is consistent with shear-induced loss of inter digitation between opposed adsorbed polymer layers, consistent with the tendency toward slippage of adsorbed polymer layers over one another.

On the nature of shear thinning in nanoscopically confined films

“…For the h = 6σ, w = 1 films, the power law decrease of this η eff in the shear thinning region across the whole film is a η eff ∼γ −1/2 ( fig. 2), this dependence coincides with SFA experiments of wide enough films [4]- [7] and constant-volume NEMD simulations of flow in the bulk [17]- [19] and under confinement [15], but not with constant-pressure MD simulations [15] which find η ∼γ −2/3 . Recent NEMD studies comparing constant-volume and constantpressure simulations [20] also find this deviation in the exponents for bulk oligomers.…”

“…This quantity has dimensions of viscosity -this is also the way that η eff is measured in SFA experiments [1]- [4], [6]- [8]-and is named effective viscosity; it characterizes the response of the whole film. A local effective viscosity can also be defined as the τ xz stress divided by the local shear rate.…”

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

“…Detailed considerations described previously [28] show that the energy dissipated over one complete cycle of oscillation is determined by G eff (ω), where this quantity is measured at the drive frequency, i.e., a cancellation of higher order terms signifies that they fail to contribute. With these considerations in mind, in this study we report values of elastic responses only for experiments in which the deformation amplitude was comparable to or smaller than the film thickness.…”

Rheology and Microstructure of Entangled Polymer Nanocomposite Melts

“…[1][2][3][5][6][7] The origin of these phenomena is poorly understood. Explanations for mechanical enhancement include confinement enhanced entanglements, 8,9 trapped entanglements, 7 polymer bridging, 4 and particle phase separation leading to gelation. 10 In the study of nanocomposites, the state of the dispersion is not always documented, adding to difficulties in understanding what controls nanocomposite mechanical properties.…”