Colloids and Surfaces, 31 (1988) 211-214 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Static and Dynamic Forces across Polymer Liquid Films

ROGER HORN

Australian National University, Canberra (Australia)

(Received 7 September 1987; accepted 11 September 1987)

I would like to describe some recent results on two different polymeric liq- uids obtained using the Israelachvili surface force apparatus. In this apparatus the force between two mica surfaces is measured by the deflection of a spring on which one of the surfaces is mounted. By monitoring the transient response of the surfaces to a known displacement of the reference end of the spring, it is possible to measure the drag force which results from viscous coupling be- tween the surfaces. Thus one can probe the viscosity of very thin liquid films

[L21. The first polymeric liquid we studied was a melt, polydimethylsiloxane of

nominal M.W. 3700, or about 50 segments. The sample was obtained commer- cially (Dow Corning 200)) and was not well characterised. The results have been presented elsewhere [ 31, and so I will only describe them briefly here.

The equilibrium surface force shows a repulsion extending approximately 15 nm, which is nearly ten times the estimated radius of gyration (1.6 nm). This rather long range of the force may in part be due to the polydispersity of our sample, and further work is required to explore this, but for the point I want to make below that is immaterial. At short range the force becomes non- monotonic, “oscillating” as a function of surface separation, with the period of the oscillations corresponding to the width of the polymer chain, about 0.7 nm. Qualitatively similar forces have been observed previously in simple liquids, with a period equal to the diameter of the quasi-spherical molecules [ 41, and in linear alkanes up to hexadecane, where the period was 0.4-0.5 nm [ 51, again matching the width of the chain. They are a consequence of the inhomogeneous structure of the liquids confined to a very narrow gap between two solid walls; the segments tend to lie in layers parallel to the surfaces.

Dynamic force measurements show that the polymer melt has a viscosity equal to its bulk value (54 cP) even in very thin films, but the stick boundary conditions apply at a distance 2-3 nm from each surface. This is the hydro- dynamic layer thickness: it is only one or two radii of gyration, considerably less than the measurable range of the surface force.

0166-6622/88/$03.50 0 1988 Elsevier Science Publishers B.V.

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01 I I I I I 1 0 20 40 60

t (sec)

Fig. 1. Result of a squeeze film experiment on a Boger fluid. The separation D between the surfaces is plotted as a function of time t. Details of the method are given in Refs [ 1 ] and [ 61. Circles are experimental points and the line is a theoretical fit using an Oldroyd-B model for the fluid. If the surface force (which was measured over a time scale of minutes) is included in the theoretical curve the agreement with experiment becomes worse (dashed line).

The second system we have studied is a constant-viscosity elastic liquid (“Boger fluid”) consisting of 0.1% polyisobutylene of M.W. 1.2x106, 92% polybutene of M.W. 650, and the balance kerosene [ 61. In this liquid the “static” surface force, measured with a delay of a few minutes to allow equilibration between each point, was a monotonic repulsion extending about 150 nm.

To study the dynamics, a squeeze film experiment was performed: the sur- faces were driven towards each other and their separation measured as a func- tion of time [ 11. The measurements were compared with the results of a numerical simulation of the experiment, in which the viscoelastic liquid was represented by an Oldroyd-B model [ 61. This has been shown previously to give a reasonable description of squeeze-film flow for this type of fluid [ 71. (In fact in the measurement presented here, the shear rates were so low that no elastic effects were detectable, and modelling the fluid as a newtonian liquid gave identical results.)

The numerical simulation of the experiment balances the restoring force of the spring against the forces acting across the liquid film, namely the viscous drag plus any surface forces [ 11. Curiously, with the Boger fluid the agreement between experiment and theory is excellent when no surface force is included, and isworsened when the simulation includes the force measured quasi-stati- cally, i.e. the monotonic repulsion described above (see Fig. 1). It appears that this force is not present when the surfaces are being pushed together over a time scale of some tens of seconds, but is there when they are static and we have waited a couple of hundred seconds for equilibration.

Why does the repulsion vanish? A possible explanation is that in the quasi-

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static situation the repulsion arises from conformational restrictions on the high-molecular-weight polymer when the surfaces are brought to within a few radii of gyration. However, in the dynamic approach the liquid is sheared, which could stretch out the polymer molecule, tending to increase its dimensions parallel to the surfaces and decrease its size normal to them, thus reducing the encounters with the walls. Alternatively, the polymer (which does not adsorb) could simply be swept out by the flow.

REFERENCES

1 D.Y.C. Chan and R.G. Horn, J. Chem. Phys., 83 (1985) 5311. 2 J.N. Israelachvili, J. Colloid Interface Sci., 110 (1986) 263. 3 R.G. Horn and J.N. Israelachvili, Macromolecules, submitted. 4 R.G. Horn and J.N. Israelachvili, J. Chem. Phys., 75 (1981) 1400. 5 H.K. Christenson, D.W.R. Gruen, R.G. Horn and J.N. Israelachvili, J. Chem. Phys., 87 (1987)

1834. 6 R.G. Horn, N. Phan-Thien and D.V. Boger, in preparation. 7 N. Phan-Thien, J. Dudek, D.V. Boger and V. Tirtaatmadja, J. Non-Newtonian Fluid Mech.,

18 (1985) 227.

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DISCUSSION

S. LUBETKIN (University of Bristol, Bristol, United Kingdom) You show oscillations in the force-distance curve in the polymer melt, but a

substantial part of these oscillations are shown as ‘dotted lines’. Would you care to speculate on what is happening in these ‘dotted’ regions? As a supple- mentary question, it seems to me that the spacings between the 1st and 2nd oscillations, and between the 2nd and 3rd, are not the same. Do you have any comment on this?

R. HORN (Australian National University, Canberra, Australia) These questions refer to Fig. 2 of Ref [ 31. Because one surface is mounted

on a spring whose deflection measures the force, there is an instability when the gradient of the force exceeds the spring constant. This makes it impossible to measure the steeply rising parts of the curve; we show them as dashed lines to indicate that fact, and have no information on the true form of the force in those regions. In principle a stiffer spring would enable us to explore all parts of the curve, but in practice the spring would have to be so stiff that we would lose too much sensitivity in measuring the force.

I do not think there is any significance in the apparent variation in the spac- ings between different minima in the curve. The differences are comparable to the experimental error of about 0.2 nm.

J. MEWIS (K.U. Leuven, Leuven, Belgium) I was intrigued by your observation that in a dynamic experiment with a

Boger fluid the static effect seems to disappear. You suggested as a possible explanation a reduction in viscous resistance owing to orientation of the poly- mer molecules. However, this explanation seems unlikely. The flow is usually biaxial stretching. Molecular stretching would lead to a certain increase in viscous resistance (although less than in uniaxial stretching) in that case.

R. HORN (Australian National University, Canberra, Australia) You have made a valid point. My attempt at an explanation was speculative,

and more careful thought will need to be given to finding the reason why the force disappears. To speculate even further, perhaps the correct explanation will also be related to the phenomenon of elasticity in this fluid. The Oldroyd- B model predicts, and other experiments have confirmed, that the elasticity of the fluid causes it to support less load in a squeeze film than a newtonian liquid of the same viscosity (Ref. [ 71) . At a molecular level I do not understand this either; maybe the two effects have similar origins.