Shear rheology in a confined geometry: polysiloxane melts

Abstract

The dynamic mechanical shear response was measured of sharp fractions of molten siloxane oligomers, PDMS [poly(dimethylsiloxane) ] and PPMS [poly(phenylmethylsiloxane) ], confined between single crystals of muscovite mica, at film thicknesses <100 Á and a temperature of 23 °C. Five conclusions emerge.(1) A liquidlike mechanical response (in which the apparent dynamic viscosity was significantly enhanced over that of the bulk liquid) was clearly distinguished from a yield stress response (in which sliding over the experimental time scale occurred only after a critical yield stress was exceeded). These same features were observed previously for ultrathin films of smaller nonpolar molecules, and, despite quantitative differences in the present systems, the observation appears to be general. (2) The precise film thickness at the onset of the yield stress response, observed at film thicknesses <30-50 Á, did not depend on the molecular weight of the PDMS fractions but did depend markedly on details of the history of the experiment. (3) The yield stress increased with measurement time without a discernible change in separation. The times for the yield stress to reach a plateau increased with molecular weight and ranged from approximately 9 min (PDMS, Mn = 890) to approximately 400 min (PDMS, = 6330) and approximately 650 min (PPMS, Af" = 2240). ( 4) Enhanced viscous response was observed at larger film thickness than for liquids of smaller molecules. The distance dependence of the apparent dynamic viscosity at 0.875 Hz was quantified for one sample (PDMS, M" = 1670) by measuring the phase shift and amplitude attenuation in sinusoidal oscillation. The apparent dynamic viscosity appeared to diverge with diminishing film thickness. ( 5) After discussing how the act of shear may affect the structure of the liquid, we conclude that the yield stress rheological response may reflect a metastable, history-dependent state, in which relaxations of trapped chains have become slower than the experimental time scale of minutes to hours.

Rate-Dependent Adhesion between Opposed Perfluoropoly(alkyl ether) Layers: Dependence on Chain-End Functionality and Chain Length

“…Indeed, it is not unexpected to find indications of a complex viscoelastic response in ultrathin polymer layers. 54 We also note that the rate-dependent regime in these systems has a velocity dependence far weaker than the simple proportionality heretofore presumed: predictions for the excess adhesion to be expected from chain pullout from an opposed layer have presumed this friction to be linearly proportional to velocity. 20,21 The discrepancy might in principle arise from an inability to produce low enough crack velocity to achieve a linear viscoelastic response, but if so, the linear regime is not experimentally relevant, since effects in this regime must be overwhelmed by the larger constant term, the thermodynamic work of adhesion that was always observed when the rate was low.…”

On the nature of shear thinning in nanoscopically confined films

“…The behaviour of the whole film is the weighted average of the viscosities inside the interfacial layers and the middle part, which explains the absence of a universal law for the shear response of nanoconfined fluid lubricants. Consequently, the total viscosity is expected to scale as η eff ∼ 1/h, as observed for confined oligomers ( [9] fig. 3).…”

“…2) or three ( [7] fig. 6) confined layers, thus practically only the adsorbed layers are probed (it should be pointed out that the affinity between mica and siloxanes is very strong [9]). The same power law η eff ∼γ −0.67 was observed for short n-alkanes (dodecane) under high pressures (120 kPa at h 6σ) ([6] fig.…”

Tribology of Confined Fomblin-Z Perfluoropolyalkyl Ethers: Role of Chain-End Chemical Functionality

“…[2][3][4]56 It is generally observed that the properties of thin films go from liquidlike at large film thicknesses and low pressures toward more solidlike at higher confinement. [2][3][4]56 Results Force-Distance Measurements. Static interaction forces (normalized by the mean radius of curvature of the undeformed solid surfaces, R) are plotted against film thickness in Figure 1.…”