Chapter 6 Response of Thin Oligomer Films to Steady and Transient Shear 1
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Mark O. Robbins and ArletteR.C. Baljon
1Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 Chemistry Department, Cornell University, Ithaca, NY 14853 2
Molecular dynamics simulations are used to examine the shear response of atomically thin films of simple short-chain molecules. A wide variety of behavior is observed including steady sliding, oscillatory motion, stick-slip motion and transient ordering. The steady-state response reveals a glass transition as pressure increases or film thickness or temperature decreases. The changes in dynamics are independent of how the glass transition is approached, and all results for the shear-rate dependent viscosity collapse onto a universal curve using a generalization of time-temperature scaling. When the yield stress of glassy films is exceeded they exhibit stick-slip motion. Slip occurs through melting of the film, or interfacial sliding at the wall or within the film. Long term memory effects are observed due to ordering at the wall/film interface.
Experiments with the Surface Force Apparatus have revealed a fascinating range of dynamic behavior when oligomers are confined between two parallel surfaces separated by less than a few nanometers (1-14). As the film thickness decreases, the relaxation times and viscosities of simple fluids increase by 10 or more orders of magnitude. At small enough thicknesses, films enter a solid state that is capable of resisting static shear forces (1,3,11,12,14). When the yield stress of these films is exceeded, motion occurs through intermittent stick-slip events rather than smooth sliding (3-5,8,9). Stop/start experiments (3,5) and the response to oscillatory shear (10) reveal that films store memory of their previous sliding history for very long times. In this chapter we describe some molecular dynamics (MD) simulation studies (15-23) of this rich experimental system. After describing the simulation methods, we consider the origin of long relaxation times. Experiments and simulation results are consistent with the onset of a glass transition as the spacing between the confining
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Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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92 walls decreases. We compare results for decreasing wall spacing to results for decreasing temperature in a bulk system. The same changes in dynamic response occur in the two cases. Moreover, the results for all thicknesses and temperatures collapse onto a single universal curve when viscosities are renormalized by the low shear rate value and shear rates are scaled by the relaxation rate. This indicates that confinement does not produce a new type of glass transition. Instead it shifts the glass transition temperature in much the same way as an increase in bulk pressure. If the thickness or temperature is decreased below the glass transition, one observes very non-linear response to an applied stress. The films are solid-like at low stresses and shear when a yield stress is exceeded. In experiments, an alternating or constant displacement is applied through a system with some intrinsic elasticity. We illustrate the types of non-steady or "stick-slip" motion that results in these two cases and discuss the effect of system compliance. We also describe how molecules move when the film yields. In some cases the film transforms to a liquid-like state, and in other cases yield occurs at an interface between the film and a wall. The stick-slip events exhibit memory over extremely long times (3,5, JO). The origin of this memory must be some structure stored in the glassy film, but experiments can not determine the nature of this structure. The paper concludes with a study of the onset of sliding that shows how order in the film increases as it shears. It had been suggested, by analogy with bulk polymers under shear, that order might be stored through alignment of molecules along the shear direction. However, for our systems memory is stored through alignment of monomers in channels between lines of wall atoms that allows easy shear at the wall/film interface. This alignment allows memory of past sliding to be stored for arbitrarily long times in glassy films. Experimental tests of this mechanism are suggested. Simulation Method Potentials and Geometry. We will describe results for films containing linear shortchain molecules. These are modeled using a simple bead-spring potential that has been used extensively in studies of polymer structure and dynamics (24). Each spherical monomer within the molecule interacts with all other monomers through a LennardJones (LJ) potential V that is truncated beyond r . For monomers separated by a distance r
