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Langmuir 1993,9, 1826-1836
An Atomic Force Microscope Study of Grafted Polymers on Mica S. J. O’Shea,’J M. E. Welland,? and T. Raymenti Department of Engineering, Cambridge University, Trumpington Street, Cambridge CB2 IPZ, U.K., and Department of Chemistry, Cambridge University, Lensfield Road, Cambridge CB2 lEW, U.K. Received February 9, 1993 Atomic force microecopy is used to measure force profiea and friction forces for the block copolymer PEO/PS physisorbed on mica in xylene, 2-propanol, n-dodecane, and air. The force profiles show the distinctive repulsive forces associated with brushlike confiiations in good solvents and shorter range attractive forces in poor solvents. The brushlike profiles show that in addition to being compressed between the surfaces, the polymer chains can also bend out of the tip-surface contact region. The friction data show that the tip is beet regarded as a single asperity contact and on solid polymer surfacea there is a transition to plowing type friction as the applied force is increased which can be associated with the yielding of the polymeric material. No friction signal could be measured within the polymer brush in a good solvent. Topographicimagesof the adsorbed polymer in poor solventsare alsoshown. At submonolayer coveragesthe polymer agglomerates and during imaging the agglomerateswere either broken up or moved if the tip scanning speed was too slow. The polymer could be more eaily imaged by adding 2-propanol which further collapsed the polymer chains and thus strengthed the agglomerate structures. In good solvents the tip tends to displace the molecules along the surface and it is concluded that further studies on these systems w i l l be best undertaken with polymers chemisorbed onto the surface.
Introduction Polymersadsorb readily on most surfaces and are much used to modify the long range forces acting between particles in colloidal suspensions. Usually under good solvent conditions the force between polymer-coated surfaces becomes repulsive as the polymer chains overlap and are compressed against the surfaces, whereas in poor solvent conditions the force is attractive at separations greater than about the radius of gyration but becomes repulsive at smaller separations. This statement is of course a gross generalization and the interaction forces may be attractive or repulsive depending on the polymer used,the surface coverage,the separation distance between the surfaces, the temperature, and the nature of the solvent.’ The situation is particularly complicated for polymers which strongly adsorb onto a surface because of the dynamic nature of the segment-surface interactions. Recent reviews are given in refs 1and 2. A system which has been well studied in recent years is the physisorption of block copolymers onto mica or silica surfaces.w In this work we use the copolymer poly(ethylene oxide)/polystyrene(PEO/PS). The hydrophilic PEO block forms one end of the polymer chain and adsorbs significantly more strongly onto mica or silica than the hydrophobicPS block. In a good solvent, such as toluene + Department of Engineering. t Department of Chemistry. (1) Ieraelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Prees: New York, 1991. (2) Klein,J.InLquiduatZnterfaces,LesHouchesl988sesswn XLVIII; Charvolin, J., Joanny, J. F., Zinn-Juetin, J., Ma.; North-Holland: Amsterdam 19w), Chapter 5; p 239. (3) Motachmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991,24,3681. (4) Field, J. B.; Toprakcioglu, C.; Ball,R. C.; Stanley, H. B.; Dai, L.; Barford, W.; Penfold, J.; Smith, G.;Hamilton, W. Macromolecules 1992, 25,434.
(5) Auroy, P.; Mir, Y.; Auvray, L. Phys. Rev. Lett. 1992, 69, 93. (6) Taunton, H. J.; Toprakcioglu, C.; Fetters, L.; Klein, J. Macromolecules 1990,.2?, 571. (7) Hadziiouannou, G.; Patel, S.;Granick, S.;Tirrel,M.J. Am. Chem. SOC.1986,108, 2869. (8)Costello, B. A. de L.; Luckham, P. F.; Tadros, Th. F. Langmuir 1992, 8, 461.
0743-7463/93/2409-1826$04.00/0
or xylene, the PEO end of the polymer is effectivelybound onto the surface, but the bulk of the chain can extend into the surrounding liquid. This polymer (and similar endgrafted polymers) provides a good model system in that it allows one to investigate or use the properties of the PS block without complications due to adsorption onto the surface or bridging effects with nearby surfaces. Experimental methods which have been used to investigate the adsorption of block copolymers include ellipsometry3(to obtain the adsorption isotherm and adsorption rate kinetics), neutron reflectiviv and small angle neutron scattering6 (to determine the density profile of the adsorbed monolayer), and surface force balance measurements.- In this latter method (henceforth abbreviated as surface force apparatus or SFA) the force between two molecularly smooth mica surfaces, which can be coated with polymer, is measured directly as the distance between them is varied; that is, a “force profile” is obtained. The geometry of the experimental arrangementis well-defined which enables a strong comparison to be made between theory and experiment. In this paper we show how the atomic force microscopy (AFM) can be used to investigate the solid-liquid interface in the presence of an end-grafted polymer. In general the data obtained from the AFM are complementary to those found using the surfaceforce a p ~ a r a t u s The . ~ AFM offers, however, high spatial resolution so that in principle force profiles and the polymer configuration can be obtained simultaneously. Also the substrate need not be restricted to mica. At present the singular disadvantage of the AFM technique as distinct from the SFA data is that the shape of the AFM tip is not well-characterized. One way to circumvent this problem is to attach a colloidal particle to the end of the tip and to monitor the forces as the particle is brought toward the surface.1° This method will certainlyyield many productive results on the forces acting between particles but at the expense of poorer lateral (9) O’Shea,S. J.;Welland, M.E.; Rayment, T. Appl. Phys. Lett. 1992, 60,2356. (10) Ducker, W. A,; Senden, T. J.; Pashley, R. M.Nature 1991,363, 239.
0 1993 American Chemical Society
AFM Study of Polymers on Mica spatial resolution. The aim of this work is to comparethe equilibrium force profile results obtained from the AFM and SFA experiments while maintaining the high lateral resolution capability of the AFM. In this regard it is of importance to note that the study of relatively simple synthetic polymers such as PEO/PS should provide a useful benchmark for the deeper understanding of more complex biopolymers (e.g. proteins) which are extensively studied using scanning probe microscopies. In addition to obtaining topographic images and force profiles, the frictional force acting on the AFM tip was also measured in air and in liquids. Another area of current interest of AFM studies is the measurement of the dynamic properties of adsorbed macromolecules since the time scales involved (milliseconds) are easily resolved with current technology. Some work has already been undertaken in this direction using scanning tunneling microscopy,most notably for adsorbed monolayers of alkanes on graphite.l’ We present some data on the time dependence of the compressibility of PEO/PS monolayers and the effect of scan speed on the imaging of the polymers.
Experimental Section
Langmuir, Vol. 9, No. 7, 1993 1827 sample distance is defied as zero at the point where the net force begins to increase monotonically as the sample is driven toward the tip. Positive valuer, of distance correspond to the tip being off the surface. In a friction experiment the sample is dithered laterally with respect to the normal direction and the resultant square wave signal is meaeured with a lock-in amplifier. The output of the lock-in amplifier is therefore ameaeureof the static friction force since the amplitude of the output square wave is a measure of the force required to startthe tip sliding over the surface.’6 If the dither amplitude is too large, then topography effecta will complicatethe interpretation ofthe friction signal and therefore the dither amplitude in these experimentsis alwaya leaa thau 6 nm. The dither frequency was varied from 20 to 300 Hz with no observed changes in the friction data. The friction force data presented here is calibrated by assuming that the coefficient of friction for Si& on mice is 0.2.’’ The friction data should therefore be considered as eemiquantitative. For work in liquid media the entire cantilever and sample are immersed in the solvent so that there are no surface tension effecta acting on the tip. Evaporation is minimal. The only components of the liquid cell which come into contact with the solvent are made from s t a i n l e ~steel and quartz. Prior to an experimentthe liquid cell, cantilever, and sample me ultrasonically cleaued in acetonefollowed by 2-propanol. The components are then rineed and soaked in dry xylene for a day. We saw no indication of any contaminants during a 6-h test experimenton a freshly cleaved mica sample in xylene. We used the block copolymer PEO/PS-184 (Polymer Laboratories, U K MW 184 OOO, w t % PEO 4%). About 2 mg of polymer was dissolved in 100mL of xylene,which is agood solvent for theae materials, to form a bulk solution for general use. The xylene was singly distilled and dried over sodium. Teflon tubing was used for all transfers of the solutions. Results were also obtained in two poor solvents (n-dodecane, 98% pure, and 2-propanol, 99.8% pure). Three special samples were prepared (henceforth called ‘monolayer” samples) in which a piece of cleaved mica was left in the bulk solution of PEO/PS-l&i/xylene for 2 days. After removal from the solution the sampler, were immediately rineed in xylene. Previous studies indicate that this procedureshould produce a monolayercoverageof polymer.18 Prior to each experiment (excepting experiments on the monolayersample4 the freshly cleaved mica substrate was imaged and shown to be atomically flat. Increasing quantities of PEO/ PS-184 were then added. The xylene was left to evaporateafter each addition and the appropriate solvent was then added to the cell. The system was then left for at least 2 hours to equilibrate. The tip was withdrawn from the solution during this time to ensure that it was not covered with polymer. Polymer may adsorb on the tip once monolayer coveragehas been reacted on the other surfacesof the liquid cell. It is estimated that monolayer coverage will occur once -0.3 l g of polymer hae been added to the mica surface. All the experiments were performed at -22 O C .
In this atudy an optical deflection type AFM12is used. Alaser beam is focused on the free end of a cantilever and deflections of the laser beam caused by the movement of the lever are measured with a quadrant photodetector. Typically lever movements of -0.2 A can be measured. We have used wedgeshaped microfabricated Si3N4 levers13with a spring constant of 0.58 N m-l. The radius of curvature of the tip protruding from the free end of the cantilever is -100 nm. In normal imaging operation the sample is rastered underneath the tip and the feedback loop maintains a constant force on the tip during scanning. We always operate the AFM in the repulsive force mode. That is, the tip is always touching the surface when the feedback control loop is on. The control electronics for the feedback loop is an all digital system which allowe for the input of a variable number of signals during scanning or while taking force The piezoelectrictubescanner has been calibrated by imaging clusters of 91-nm latex spheres. To simultaneouslymeasure the applied force and friction forces acting on the tip, we adopt the method of Meyer and AmeP in which the quadrant photodiode is used to separate the measured deflection of the laser beam into two orthogonal components. One component measures the movement of the lever normal to the surface from which we can calculate the applied (or normal) force aa the displacement multiplied by the bending moment spring constant (0.68 N m-l). The other component measures the twisting of the lever caused by friction forces acting on the tip as the sample is moved. One can calculate the effectivespring constant for the torsional motion of the lever which gives rise to a given friction force from the dimension of the leverl6and for the cantilevers used this was determined to be 320 N m-l. The net applied force acting on the tip can be measured as a function of the tipsample distance by vertically moving the samplewith respect to the tip and simultaneously measuring the cantilever deflection signal. To convert the deflection signal to a force, it is noted that when the tip is in contact with the surface, there is a linear relationship between the lever deflection and the sample displacement, with the force acting on the tip simply given by the displacement multiplied by the lever spring constant. This is an approximation because it wumes that the lever compliance is much smaller than any of the other system compliances, which may not always be the case for ‘soft” systems. The net force is taken to be zero when the tip is far from the surface. The tipsample distance is calculated by subtracting the lever deflection from the sample displacement. The tip-
Results. Imaging (a) Imaging in Xylene. We could not image submonolayer or monolayer coverages of the polymer under good solvent conditions. This was a consequence of the weak adhesion in xylene between the substrate and the polymer in comparison to the lateral forces exerted by the’ tip during scanning. In general when the tip encounters an adsorbed molecule during scanning with the AFM, the image will only show a topographic feature if a force acta to deflect the cantilever. Weakly adhering molecules offer little resistance to the movement of the tip and, consequently, the resultant force acting to deflect the tip is very small. In effect the adsorbate tends to be simply pushed along by the tip.lg Thus in our experiments at submonolayer coverages only the mica substrate is observed as the
(11) Rabe, J. P.; Buchholz, S. Phys. Rev. Lett. 1991,66,2096. (12) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1988,53,1054. (13) Digital Inatrumenta, Santa Barbara, CA. (14) Wong,T.M. H. Ph.D. Thais, University of Cambridge,Cambridge CB2 lPZ, UK. (15) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1990,57, 2089.
(16) Mate, C. M.; McClelland, G. M.; Erlandeaon, R.; Chiang, 5.Phya. Rev. Lett. 1987, 69, 1942. (17) Erlandsaon, R.; Hadziiounnou, G.; Mate, C. M.; McClelland, G. M.; Chiang, S. J. Chem. Phys. 1988,89,5190. (18) Bo&, D.; Tripp, C. P.; Guzonas, D.; Hair, M. L. Langmuir 1992, 8, 2070.
O'Shea et al.
1828 Langmuir, Vol. 9,No. 7, 1993
after the initial addition of 0.02 pg of polymer (Figure la) shows that the moleculeshave adsorbed on the surface in small isolated structures. The estimated volume of the features ranges from about 500 to 32000 nm3. These volumes are comparable to the volume of about 2-110 molecules, respectively, if we take the volume of a single moleculeto be 294 nm3as calculated from the bulk density of polystyrene (1.05 g / ~ m ~ )It . ~isl apparent that even at this low surface coverage the molecules tend to aggregate. We could not zoom-in on individual features as the tip tended to sweep the molecules across the surface if the scanningspeed was tooslow. This alsoprecluded obtaining force profiles over an individual polymer molecule. Figure lb-d shows images of the surface in air after the addition of a further 0.03 pg of polymer. A t these polymer concentrations the molecules have aggregated to form almostcircular-shapedislands. The volume of the features is -lo6 nm3 (i.e. there are -3500 molecules per feature). The islands are flat on top to within 1nm and are about -13 nm high. This height is greater than that for the smallest features seen in Figure l a (-6 nm) and suggests that the polymer chains are constrained laterally as they are compressed within an island so that their effective length normal to the surfaceincreases. The sizeand shape of the islands will depend both on the local drying conditions and on the local surface concentration. In the preparation of the samples the polymer adsorbs rapidly onto the mica at submonolayer coverages3and the bulk solvent (xylene) is allowed to evaporate. On evaporation the liquid xylene film over the surfacethins and at a critical thickness ruptures to form smaller regions of wetted surface. As evaporation continues the surface tension acting at the edge of the retreating film will drag any polymer within the wetted region across the surface to form the observed island structures. To produce a nonaggregated film the molecules would have to be significantlymore stronglybound to the surface. It should be noted that the data have not been corrected for a significantskewingof the images approximatelyalong the 45O direction arising from the piezoelectric tubescanner so that the islands are more circular in shape than is apparent from Figure 1. Parts c and d of Figure 1show the progressivebreak-up of the islands marked A and B in Figure l b after these islands had been imaged over smaller scan areas. The breaking up of the islands was independent of the applied force on the tip for both large and small scan areas. This indicatesthat the tip speed is the important factor involved in effecting island break-up when this system is imaged in constant force mode. It is difficult to preciselyidentify the origin of the speed-dependent displacement of the polymer islands. It is clear that the effect is related to the weak adhesion of the polymer to the substrate rather than deformation of the polymer itself.22A possiblemodel relies on the strong adhesive contact (asshown below) existing between the tip and the polymer in air. As the tip moves off a polymer island, this adhesive force will tend to drag material alongwith it for some distance. At high tip speeds the adhesive contact would be broken more readily and material transport would be reduced. After the data of Figure 1were obtained in air, the liquid cell was filled with 2-propanol. We again observed the polymer islands similar to those of Figure lb. The significant differencehoweverbetween these data and that of Figure 1is that it is now possible to zoom-in on small features without the polymer moving or breaking up.
-
Figure 1. Submonolayer coveragesof PEO/PS-184taken in air. The images were taken in constant force mode although the deflection (error) signal images are shown for clarity. (a) 2.0 X 2.0 pm2 area after the initial addition of 0.02 pg. (b) Island formation after the addition of 0.05 pg. Height of islands 13.5 nm. Scan area 2.8 X 2.8 pm2. (c) Image showing the break up of the polymer islands (A and B)during scanning. Scan area 2.0 X 2.0 pm2. (d) Image showing further breakup of the islands. Scan area 2.8 X 2.8 pm2.
tip sweeps away the adsorbed polymer. At coverages around a monolayerthe tip may control either on the mica surface or within the extended polymer brush. However the control point corresponding to the tip being within the polymer brush is not stable in the sense that the tip tends to snap into the mica surface during scanning. If control was maintained within the polymer brush, the image contrast was extremely poor with a "smeared out" appearance indicating that the material was too soft to obtain any useful spatial information. To obtain useful images of soft or weakly adhering material with the AFM, one often minimizes the applied load. This has the effect of decreasing the deformation of the material and of reducing the lateral forces acting on the material during scanning so that it is less likely to be displaced by the tip. However for the mica-xylenetip system the adhesiveforcesare strong and, as will be shown, the friction force is not negligible at zero applied load. This is consistent with the tip-mica contact region being dominated by adhesive forces so that the friction force depends mainly on the tip-surface contact area rather than the applied load.20 A t zero load there is still a finite tip-surface contact area and hence a finite lateral force during scanning. In such cases reducing the applied force is therefore not sufficientto effect imaging. For example, in the present study no images of the polymer could be observed even at very small loads. The above results indicate that to obtain useful images of end-grafted polymers in good solvents will require the molecules to be much more strongly bound (i.e. chemisorbed) to the substrate. (b) Submonolayer Coverages in Air and Poor Solvents. Figure 1shows images at submonolayer coverages of PEO/PS-184 in air. The image of the surface (19) Lea, A. S.; Pungor, A.; Hlady, V.; Andrade, J. D.; Herron, J. N.; Voss, E. W. Langmuir 1992,8,68. (20) Tabor, D. In Microscopic Aspects of Adhesion and Lubrication, Tribology Series 7; Georges, J. M., Ed.; Elsevier: Amsterdam, 1982; p 651.
(21) Brandrup, J.; Immergut, E. H. Polymer Hadbook; John Wiley & Sons: New York, 1989. (22) We have seen similar tip speed effects for other weakly bound molecules, for example chrome or small proteins on graphite.
AFM Study of Polymers on Mica
Figure 2. Topography image of one of the polymer islands of Figure 1 taken in 2-propanol: height of island, 7.5 nm; scan area, 0.7 X 0.7 pm2.
Figure 2, for example, shows a higher resolution image of one of the polymer islands. The increased stability of the structures arises because of the increased segmentsegment interaction of the hydrophobic PS chain in the polar solvent. This leads to a reduction in the polymer volume and hence island height, the height of the islands now being -8 nm. The rigidity of the polymer structure caused by the stronger intersegment interactions could also be enhanced by the further entanglement of the polymer chains. Thus it is more difficult for the tip to break up the islands with a given shear stress or move the islands by a plowing action. The increased rigidity of the polymer structures was maintained after the 2-propanol was allowed to evaporate away and the samples again imaged in air. This was evident both from the height of the islands, which remained the same as that found in 2-propanol, and in the stability of the islands during imaging. (c) Monolayer Samples. Figure 3 shows images of the monolayer samples. Parts a-c of Figure 3 are taken in air immediately after the samples have been prepared. The drying patterns shown that at these higher concentrations the polymer not only aggregatesbut tends to form a well-defined network structure. This has also been observed in a recent AFM study of polystyrene spin-coated onto silicon23where it was shown that, over a small range of polymer concentrations, the patterns were consistent with Vorroni tessalation. This is evidence that as the xylene solvent evaporates and the liquid film thins, the film ruptures simultaneously over large areas of the substrate. The differences in the observed patterns of Figure 3 for different samples may be attributed to variation in the local drying of the film and also to the poor reproducibilityof the sample preparation technique. It was calculated that the surface coverage is higher in Figure 3a than in Figure 3b, even though the sample preparation is identical. This differencein polymer surface density also manifests itself in the force profile data where it will be shown that data taken over the sample shown in Figure 3a has a more extended brush structure and exhibitsstrongerrepulsive forcesthan force profiles taken over Figure 3b. As a further example it is instructive to compare Figure 3a with Figure 3c, which is taken on the (23) Stange, T. G.; Mathew, R.; Evans,D. F.; Hendrickson, W. A. Langmuir 1992,8,920.
Langmuir, Vol. 9, No. 7,1993 1829
Figure 3. Images taken on monolayer samples. Parts a, b, and c are deflection signal images taken in air immediantly after preparation. (a) Typical feature height is 9 nm. Scan area 2.8 X 2.8 pm2. (b) Typical feature height is 16 nm. Scan area 2.5 X 2.5 pm2. (c) Image of the uncleaved surface of (a). Typical feature height is -21 nm. Scan area 2.8 X 2.8 pm2. (d) Topography image taken in xylene showing a strongly adhering region (A) of polymer surrounded by flat mica regions (B). Scan area 1.5 X 1.5 pm2.
uncleaved side of the same sample. While this example is rather extreme, it is evident that the film has dried much more uniformly on the cleaner side of the sample. The uncleaved side has relatively larger clumps of material which have formed as the xylene film will tend to form larger wetting patches prior to complete evaporation. It is apparent from both Figures 1and 3 that the sample preparation methods do not ensure a uniform distribution of material on the mica surface. The lack of reproducibility of the surface coverage is a problem in the study of polymer-surface systems and is of significance because the surface coverage is an important parameter in fitting any scalingtheory to the experimentaldata. It is not trivial to find the surface coverage from SFA or reflectometry type experiments, both of which require a good estimate of either the optical or neutron refractive index of the polymer layer. In a recent comparison of PEO/PS block copolymer on mica,18 prepared in essentially the same manner as the monolayer samples of this study, it was found that the calculated monolayer coverage from the SFA experiments was -50% higher from that calculated using an infrared spectroscopictechnique. This difference was attributed to the uptake of solvent in the polymer layer for the SFA experiment whereas the infrared data were taken while the polymer was dry. In principle, the AFM could circumventthis general difficultybecause the surface coverage for each sample can be measured from the topographicimagesprovided that the surfacecoverage appears uniform. For example, using a polymer density of 1.05 g/cm3,we can calculate that the surface coverage in Figure 3a is 4.7 f 0.5 mg/m3and in Figure 3b is 3.1 f 0.4 mg/m3. These monolayercoveragesare in satisfactory agreement with previous results for this polymer (2.6 f 0.4 mg/m3) although our data tend to be higher. This is probably because the polymer density we have assumed is toohigh since as alreadynoted abovethe polymer islands can collapse to form denser structures in the presence of a poor solvent.
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1830 Langmuir, Vol. 9, No. 7, 1993
Figure 4. (a) Topography image in air after the addition of 5 pg of PEO/PS-184. The height of the large flat feature is 27 nm.Scan area 1.5 X 1.5 pm2. (b) Image taken on top of the large flat feature of (a). The typical height of the periodic features is 8 nm. Scan area 0.5 X 0.5 pm2.
Figure 3d is unique in that it is the only image of polymeric material obtained in xylene at monolayer coverages. The force profile data indicah that the entire surface is covered in a polymer brush in both regions A and B. Region B shows the flat mica substrate which is usually observed when trying to image the monolayer samples. In contrast however region A shows some structure and remained stable for the duration of the experiment. The height difference between regionsA and B is -3 nm. It is not clear why the material in region A does not move to form a more uniform layer on the entire surface or, if the local polymer density is higher than monolayer density,why the excesspolymer does not desorb into the solvent. One possible reason is that the polymer chains are highly entangled in this region, but this seems unlikelygiven that the features were stable for many hours. A more plausible explanation is that the PEO chain is bound tightly to region A because of some defect in the substrate. (d) Multilayer Coverages. If large amounts of polymer (5 pg) were added and allowed to dry, the material tended to form large clumps on the mica surface, as shown in Figure 4a. There was no appreciable change in the appearance of the polymer clumps after the addition of xylene to the liquid cell. This shows that many of the polymer chains are considerably entangled within the clumps. The clumps tended to have an ill-defined surface profile, with surfaceroughnessof -3 nm. However certain regions on top of the clumps showed definite structure, as typified by the ridges shown in Figure 4b. The height of the ridges is -8 nm, which is commensurate with the size of the features shown in Figure 1. It seems unlikely however that the features arise from some local molecular ordering of the polymer since the spacing between the ridges is large. The ridges are very similar to those observed in a recent AFM study of 1pm thick polystyrene films24where it was found that the structures were formed over time by the movement of the tip dragging weakly entangled molecules into oriented aggregates. Results. Force Profiles (a) Monolayer Samples in Xylene. A t coverages greater than one monolayer the force profiles in xylene show a repulsive force region over the entire surface as the tip pushes against the polymer brush. However, the interpretation of the data is complicated because the (24)h u n g , 0.M.;Goh, M.C. Science 1992,255,64.
0
20
-
40
60
80
TIP SAMPLE DISTANCE (nm)
Figure 5. Typical force profiles over a monolayer sample in xylene at two different approach speeds.
softness of the underlying polymer material makes it difficult to assign a tipaample distance zero with any certainty. At submonolayercoveragesin xylene,repulsive forces due to the polymer are seen in the force profiles, but only intermittently because the molecules are easily moved aside by the tip. Therefore our detailed discussion of force profiles in xylenewill concentrate on the monolayer samples since these yield the most useful data. Figure 5 shows a typical force profile for a PEO/PS-184 monolayersample in xylene. The tip begins to experience a repulsive force at a tip-mica separation distance of 92 f 12 nm which is consistent with brush lengths obtained from SFA6(75 f 5 nm) and neutron experiments4 (80 f 5 nm). The repulsive force increases approximately exponentiallyuntil the tip strikes the micasubstrate. These profiles are different from the data obtained using the SFA where it was found that (i) the two surfaces could not be brought into contact with the spring constants being used; the distance of closest approach being 15nm and (ii) the repulsive forces are much larger than those measured with the AFM, with F J R 0.1 N m-l at 15 nm separation, whereFais the applied forceand R is the radius of curvature of the surfaces. The maximum force reached in the AFM data is typically 4 nN, which gives the maximum value of F J R as -0.04 N m-l using a value of R = 100 nm. We have estimated R by measuring the adhesive force (Fade) required to pull the tip off the mica surface in xylene (-20 nN). HenceR is obtained by noting that Fa&, = 3.lrA.yR for our geometry, where AT is the interfacial surface energy (-20 mJ m-2).1
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AFM Study of Polymers on Mica
Langmuir, Vol. 9, No. 7, 1993 1831 2.0-
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-I 200 400 APPROACH SPEED (rids)
Figure 7. (a) The maximum repulsive force observed within the polymer brush as a function of the tip approach speed. The force profiles were taken over the regions marked A and B in
monolayer samples in xylene: (a) taken over the raised region A of Figure 3d, where the polymer ie strongly attached to the mica;(b) taken over the flat region B of Figure 3d; (c) taken over a different monolayer sample.
Figure 3d. (b) The estimated distance the tip jumps into the surface during approach for the force profiles taken over region B in Figure 3d.
The differences between the AFM and SFA data noted above arise because the tip radius is small enough so that only relatively few molecules are directly influenced by the tip. It is easy to displace a single polymer chain with a sharp tip in comparison to the many polymers that must be moved laterally as the mica surfaces approach in the SFA. The data suggest that either (i) the PS chain is being displaced laterally but the PEO chain is stationary or (ii) the PEO chain, and therefore the entire molecule, is being moved laterally along the substrate. We have performed two types of experiments (measuring the adhesive forces and varying the tipsample approach speed) which indicate that both mechanism are possible with (ii) being observed in most situations and (i) being favored as the local polymer concentration increases and the PEO chain mobility decreases. Figure 6 shows the general types of force profiles observed. Parta a and b of Figure 6 were taken over the monolayer sample shown in Figure 3d. Figure 6c is on a different monolayer sample which appears to have a smaller packing density of polymer. Data are shown for the tip approaching and retreating from the surface, with the tip retreat data taken immediatelyafter the approach data. We see that in Figure 6b,c there is significant hysteresis in the approachhetreat force profiles arising from adhesive forces. The adhesive force can be attributed to contact between the tip and the mica substrate. In this case the entire polymer molecule is forced aside by the tip as it jumps into the surface on approach. Force profiles showing strong adhesive behavior are the most common and were observed over the entire surface of two monolayer samples and over m a t of the surface of the third monolayer sample. Figure 6a is exceptional in that it is representative of data taken over the raised region (A) of Figure 3d, i.e. over a region where the tip is not sweeping away the PEO chains on the surface. In this case there is negligible
adhesion, no tip instability jump during the approach, and the force profile is essentially reversible, even &r the tip comes into strong repulsive contact with the surface. It is apparent that in this case (as has been notad in the imaging of the sample) the polymer chain is not being displaced from the mica and the PEO chain remains strongly bound to the substrate. The very low adhesive force appears to be a characteristic of tippolymer contact in xylene and presumably the compressed polymer chaine around the contact also assist in supporting the applied load. If the tip is scanned across the edge of the feature (from A to B in Figure 3d) the force profiles show a gradually increasing adhesive force as more of the tipsurface contact area occurs on the mica. The lateral distance over which the force profiles change from no adhesion to strong (mica) adhesion is -60 nm, which is of the same order as the estimated tip radius. The movement of the polymer chain under the tip is ale0 seen in the variability of the force profiles as the tip approach speed is changed. This is seen in Figure 6 where we note that at the slower approach speed the maximum repulsive force is smaller and, if the profiles show adhesive behavior, the instability jump into the surface is larger, i.e. at slower speeds the tip must jump a larger distance to reach the surface. Figure 7 shows data taken over a larger number of force profiles on two monolayer samples at various approach speeds. The definition of the jump distance is slightly ambiguous because the distance zero is ill defined, particularly if strong adhesive forces are present. Nevertheless, changes in the jump distance are meaningful. The scatter in the data of Figure 7 is high but the trends noted above are clear. It is apparent that at slower epeede the PS chains have more time to move out of the tipsample gap and this reduces the maximum repulsive force exerted on the tip. That is, the effective compliance between the tip and the sample decreases as
O’Shea et al.
1832 Langmuir, Vol. 9, No. 7, 1993
The Alexander-de Gennes expression for the force acting between two surfaces bearing polymer brushes in good solvents has been shown to give a good fit to SFA force profile data?’ and it is instructive to also apply the expression to our AFM data. The Alexander-de Gennes equation can be written, to within a scaling factor of order unity, a s 2 g 2 6 forD12L < 1 (1)
0
40 60 80 100 TIP SAMPLE DISTANCE (nm)
20
-
Figure 8. Logarithmicplots of the force profiles shown in Figure 6. Curves 1, 2, and 3 correspond to parts a, b, and c of Figure 6, respectively.
less polymer material remains in the tip-sample gap. Similarly the decrease in local polymer density allows the tip to jump into contact with the surface at larger tipsampledistances. For profiles showingadhesive behavior, slower approach speeds may also allow the PEO chain to be moved further from the center of the tip-surface contact area prior to a tip jump and this will also facilitate a longer tip jump distance. It was found that the force profiles were quantitatively similar over entire 2 pm2 areas for a given monolayer sample. However, there is considerable variation between the three monolayer samples which reflects the different polymer surface coverage arising from the sample preparation technique. This is shown in Figure 8 which is a logarithmic plot of the approach force profiles of Figure 6. If little adhesion is present (curve 1)then (i) the log plot of the profile is linear over the entire interaction range of the force and (ii) the first indication of the repulsive interaction is seen at a much longer tip-sample distance than if adhesive forces are acting (curve 2). Even greater contrast is provided by comparingthe data for a different monolayer sample (curve 3). In this case both the interaction distance and the magnitude of the force are significantly smaller and we can conclude that the polymer is less densely packed between the tip and the sample and that the brush does not extend as fully into the solvent. The simplest explanation for these differences is that the surface coverage is lower for this sample. The above results indicate clearlythat if the local surface coverage is low or if the molecules can move laterally dong the surface, then the polymer brush and the PEO anchor are less constricted in their movement and hence the measured repulsive forces are weaker both in magnitude and in range. The force profile data elucidates the reasons why it is difficult to image the polymer in a good solvent at monolayer or submonolayer coverages. If the lateral forces acting on the polymer are sufficiently high, then the tip will displace the entire molecule because the PEO chain is not bound strongly enough to the surface. Also, the friction data presented below show that the polymer brush itself offers little resistance to the lateral movement of the tip. It is important to note that the approach and retreat force profiles are identical provided that the tip does not push through the polymer to the substrate, i.e. provided that the tip remains within the polymer brush. This is consistent with SFA results for nonadsorbing polymers on mica.2 Moreover, the absence of hysteresis within the polymer brush was observed at all approach speeds. That is, the compliance of the system can readily respond to changes in the applied force on the most rapid time scales of the experiment (- 1 ms).
where s is the mean distance between the grafting points of the chains (Le. the surface coverage is Us2),k is Boltzmann’s constant, T is the temperature, D is the distance between the surfaces, L is the equilibrium thickness of the brush layer, and P is the pressure acting on the surfaces. The first bracketed term represents the osmotic repulsion due to increasing polymer concentration as the surfaces are pushed together. The second bracketed term accounts for the decrease in elastic energy of the chains as they are compressed. The derivation assumes that the density of the polymer brush is uniform, but recent experiments have shown that a parabolic profile is more realistic.s This leads to a more complicated expreeeion for the force profile26but the numerical results for P(D) are essentially the same as those calculated from eq 1. The Alexander-de Gennes expreasionassumesthat there is no interpenetration of the polymer brushes as they are brought together, which is in general true. We can therefore rewrite eq 1 for the compression of a polymer brush on a single wall by substituting 20 for D and dividing the right-hand side by 2. Equation 1 can then be approximatedover the restricted distance range 0.2
