NANORHEOLOGY OF CONFINED POLYMER MELTS .2. NONLINEAR SHEAR RESPONSE AT STRONGLY ADSORBING SURFACES

The large-amplitude nonlinear shear theology of polymer melts confined between strongly adsorbing surfaces (parallel plates of mica) was stu died as a function of strain, frequency, and thickness of the polymer films. The shear strains varied from less than 0.1 (linear response) t o over 30 (at which the film structure was strongly modified by the im posed shear). The measurements employed a surface forces apparatus mod ified for dynamic mechanical shear. The polymers were atactic poly( ph enylmethylsiloxane) (PPMS), with chain lengths from 31 to 153 skeletal bonds. The nonlinear shear forces, decomposed into a Fourier series o f harmonic frequencies, were always odd in the excitation frequency, a s required by symmetry considerations. The in-phase and out-of-phase o scillatory shear responses at the same frequency as the excitation (th e nonlinear storage and loss moduli G(1)' and G(1)'', respectively) we re analyzed. Four principal conclusions emerged. First, from the frequ ency dependence of G(1)' and G(1)'' at constant strain, we conclude th at relaxations were accelerated by large strain. Second, a marked decr ease of both G(1)' and G(1)'' was observed with increasing strain at c onstant frequency, except at the smallest film thickness, approximate to 40 Angstrom where G(1)'' passed through a maximum with increasing s train but G(1)' continued to display shear-thinning. Third, the critic al strain for onset of nonlinear response increased with the excitatio n frequency. Fourth, at sufficiently large strains (larger than 10), t he shear moduli were independent of polymer molecular weight (comparis ons made at fixed film thickness) and appeared to reach limiting strai n-independent levels at sufficiently large strains. This final observa tion contrasts sharply with the linear response and is consistent with shear-induced loss of interdigitation between opposed adsorbed polyme r layers, consistent with the tendency toward slippage of adsorbed pol ymer layers over one another.