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Langmuir 1992,8, 2330-2335
Is the Molecular Surface of Polystyrene Really Glassy? Gregory F. Meyeis,+Benjamin M. DeKoven,*pj and Jerry T. Seitzj The Dow Chemical Company, Michigan Research and Development, 1897 Building, Midland, Michigan 48667, and The Dow Chemical Company, Central Research Materials Science and Development Laboratory, 1702 Building, Midland, Michigan 48674 Received March 5, 1992. In Final Form: April 22, 1992 The atomic force microscope (AFM)has been used to probe the surface of a glassy polymer, polystyrene (PS). We have found for the first time that the AFM t i p P S surface interaction produces a morphology on the nanometer scale dependent on the PS molecular weight (MW). In the range from 13k MW to lOOOk MW, two distinct types of oriented patterns on the PS surface occur in the top 7 nm of the surface. The marked transitions in the patterning and the evolutionof the patterning as a function of time are indications of the elastic nature of the PS surface. Two distinct patterns are observed an abrasion'pattern for MW's less than 24k and a fully developed oriented pattern for MW'sgreater than1OOk. The onset of the oriented pattern occurs very near the entanglement MW (Me)for PS, -19k. Between molecular weighta of 24k and lOOk the oriented pattern does not fully develop. The "wear" of the PS surface decreaseswith increasing M W ,although the exact functional dependence is unknown. The proposed scaling of the oriented wave spacings compared to those observed for macroscopic spacings for rubbers suggest that the near surface of PS behaves more like a material exhibiting rubber elasticity than one in the glassy state. The oriented patterns using the AFM are postulated to be due to the stretching of polymer chains which possess an extra degree of freedom at the PS surface. With further refmementa in fabricating tips with smaller radii, it may be possible to disassemble a polymer surface in a macromolecule by macromolecule fashion. Also, by investigating other glaasy polymers above and below Me, it should be possible to contribute to the development of polymer surface structural models.
Introduction The properties of a polymer surface are difficult to probe. It is possible to obtain information about the surface composition and phenomena such as surface segregation present on polymer surfaces using conventional surface probe techniques.'P2 Surface structural information is much more difficult to obtain since the polymers exist in an amorphous state. The macromolecules present at the polymer surface experience an extra degree of freedom of motion. A priori, it is not expected that the physical properties of these surfaces will resemble the bulk. For glassy polymers the molecular motions are virtually suppressed and the molecules are spatially confined in the bulk due to reduced free ~0lume.3 However, in the study of friction and wear of surfaces of glassy polymers, there has been much speculation about the elastic nature One technique of the surface compared to the well suited to the study of both structure and processes (e.g. adhesion and friction79a t surfaces is the atomic force microscope (AFM). Others have recently examined polymer surfaces using the AFM."ll
* Author to whom correspondence should be addressed.
+ The Dow Chemical Co., Michigan Research and Development. The Dow Chemical Co., Central Research Materials Scienceand Development Laboratory. (1)Clark, D. T., Feast, W. J., Eda. Polymer Surfaces; John Wiley & Sons, Ltd.: Chichester, UK, 1978. (2)Feast, W.J., Munro, H. S.,Eds. Polymer Surfaces and Interfaces; John Wiley & Sons, Ltd.: Chichester, UK, 1987. (3)Munk, P. Introduction to Macromolecular Science; John Wiley & Sons,Inc.: New York, 1989. (4)Tabor, D.J. Lubr. Technol. 1981,103, 169. (5)Briscoe, B. J.; Tabor, D. In Polymer Surfaces; Clark, D. T., Feast, W. J., Eds.; Wiley: Chichester, UK, 1978; Chapter 1. (6)Archard, J. F. Proc. R. SOC.London, A 1957,243,190. (7)Burnham, N. A.; Dominguez, D. D.; Mowery, R. L.; Colton, R. J. Phys. Reu. Lett. 1990,64,1931. (8)Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987,59,1942. (9)Rugar,D.;Hansma, P. Phys. Today 1990 (October), 23. (lO)Mizes,H.A.;Loh,K.-G.;Miller,R.J.D.;Ahuja,S.K.;Grabowski, E. F. Appl. Phys. Lett. 1991,59,2901.
During the course our investigations, Leung and Goh12 reported orientational ordering of a glaae polymer, polystyrene (PS),resulting from tipsurface interactions during AFM imagingusing N loads. The patterns observed were oriented perpendicular to the scan direction. The oriented bundles were 50-7Onm wide. They also reported no changes in the dimensions and patterns of the structures for molecular weights (MW)ranging from 32k (32 OOO) to 573k. YangetaL~3reportedtheobservationoflargesurface fluctuations (bumps) using noncontact scanning force microscopy (SFM)on PS and poly(pheny1eneoxide) films that were stretched to elongations of 343% at room temperature. The bumps on theee deformed PS surfaces are 5-15 nm in height and 250 nm in diameter. No mention of the influence of molecular weight on the "bumps" is made in this work. In this paper we describe morphology produced as a result of the interaction of the atomic force microscope (AFM) tip with the PS surface. W e have found for the first time that the morphology produced on the nanometer scale is dependent upon the molecular weight of the polymer. This is discussed below.
-
Experimental Methods Films were made using anionically polymerized polystyrenes havingthefollowing molecular weights: lSk, 24k, 35k, 66k, look, 215k, and lOOOk having polydispersityvalues (MJM.)equal to 1.06,1.04, 1.25,1.03,1.18, 1.27, and 3.16, respectively. The 13k and 24kpolymers were obtained from Aldrich ChemicalCo. while allotherswere obtainedfrom the Dow Chemical Co. Ultravioletozone treated silicon wafers were used aa substrates for polystyrene f i b deposition. Films were depositedusing solutionaof the polymer dissolved in toluene having concentrations of 2-12 w t % of the polymer. Films having a thicknesses 1 2 pm (11)Unertl, W.N.;Jin, X.In Thin Film: Stresses and Mechanical Properties IZ& Nix,W. D., Bravmen, J. C.,Artz, E.,Freund, L. B., Eds.; Materials &search Society: Pittsburgh, PA, 1992; Vol. 239. (12)Leung, M.; Goh, M. C. Science 1992,255,64. (13)Yang, A. C.-M.; Terris, B. D.; Kinz, M. Macromolecules lW1,24, 6800.
0743-7463/92/2408-2330~03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 9,1992 2331
Molecular Surface of Polystyrene F
m WFl
-1
II
I
t
ii
i
I
13k
24k
35k
215k
c
Figure 1. AFM top view images for a 1pm by 1pm area using PS surfaces prepared with different molecular weights after 15 min (A-D) and 45 min (E-H) of scanning using a 1.5 X lo-* N load. The vertical scale for these images does not exceed 20 nm. All surfaces
show the development of an oriented pattern, but the manifestation of these patterns is markedly different. See text for discussion. (measured using profilometry) were prepared by spin coating and air-drying a t room temperature. The AFM used was a Nanoscope I1 (Digital Instruments, Inc.) using the ’G” head which is capable of scanning a 90 pm X 90 pm lateral region and 5 pm vertically. Unlessotherwisereported, a feedback load of 1.5 X N was used during the scanning of the AFM over the PS surfaces in air. All measurements were recorded a t room temperature and normal laboratory humidity levels (4040% relative). All tips used were commercially available microfabricatedSiSNd fastened to cantilevers having a nominal force constant of 0.58 N/m. Before imaging, the load on the surfacewas set by moving the polystyrene surface into the tip while applying a small ac modulation. In this way a forcedistance curve was generated which plots the lever deflection force against the sample m0~ement.l~ During this procedure the feedback control was disabled. The sample position piezo was calibrated using an interfer~meter.’~The lever deflection force was calculated by using the deflection signal obtained from contactingacleaned and polished sapphire surface and assuming the nominal value of the lever spring constant. Once the load was determined, the tip was raster scanned over a 1pm by 1 pm area centered about the contact point used for setting the load. During scanning the feedback was enabled to keep the load (as determined above) constant. The tip was scanned a t a speed of 4.8 pm/s and data were collected over a 200 lines X 200 points/line array. Images were captured after the first raster scan (time = 0 min) and then a t selected time intervals rangingfromminutestohoursduringwhich thechangesinsurface morphology were monitored. After the scanning, the feedback was again disabled and the load was rechecked by generating another force-distance curve. The load drifted by no more than 3~20%for the duration of a 2-h measurement. Although we do not know the exact contact area in these experiments,we can estimate an upper limit of 375 nm2.16 While the general appearance of the images did not change from tipto-tip, the dimension of the features did. We believe that this is due to variability in the sharpness of the tips. For all the experiments described below the same tip was used and was cleaned between samplesby immersion in toluene (1min) followed by air-drying. This cleaning method was found to give images (14) DeKoven, B. M.; Meyere, G. F. Unpublished material. (15) Calibrated by Digital Instruments, Inc. (16) The contact area during loading estimate is based on electron microscopyof the AFM tip (sphericalradius -40 nm) using the measured
upper limit of 1.5 nm for the tip penetration into the PS surface.
with dimensionallyreproducible features for the same load and scan times on the same PS surface.
Results: Patterns on Polystyrene Surfaces Resulting from Tip-Surface Interactions as a Function of Molecular Weight
The AFM tip can be raster scanned over the surface for imaging or, with the x and y17 scan disabled, the tip can be positioned over the surface and can be held stationary but in repeated contact at a single location for measuring the load. In these experiments the tip was rastered over a 1pm by 1pm region (left to right starting at the top of each area and then incrementing downward). The images were recorded as a function of time for varying molecular weights. AFM images after 15 min (Figure 1A-D) and 45 min (Figure 1E-H) for four different molecular weights show marked difference upon comparison. Although not shown, the “zero”time (I1min of rastering) interval images for each MW series are similar and essentially featureless. There are two distinct patterns which develop fully on the PS surfaces after 45 min: an “abrasion”pattern which is best typified by the 24k MW (Figure 1F) and an “oriented” pattern1* which is formed most distinctly on the 215k MW surface (Figure 1H). Note the complete absence of any signs of the oriented pattern for the 13k MW. There are, however, signs of the oriented pattern forming in the early stages on films made with MW’s greater than 24k (Figure lC,D). However, after 45 min of rastering the oriented pattern is only manifested strongly in the 215k MW surface. The vertical dimension of the patterning can be described more quantitatively by measuring the geometric roughness (root-mean-squared or rms) of the various MW polystyrenes as a function of time (Figure 2). These (17) The x , y, and z refer to AFM raster directions in the plane ( x and y) and perpendicular to the plane ( z ) of the images shown in Figure 1. (18) The oriented patterns (in x and y) shown in Figure 1 are very similar to those described in ref 12. At times patterns of bands of cones have been observed on PS. We are not certain at this time if this is due to the solvent used to cast the film or the mode of AFM imaging (load w pull-off force scanning).
Meyers et al.
2332 Langmuir, Vol. 8, No.9,1992
period of bands within the pattern is approximately 70100 nm. The spectral density maximum is now 6.8 nm2. As the molecular weight increases the patterning becomes more periodic and well defied, For the 36k MW (Figure 40,the maximum density occurs generally perpendicular to the scan direction with a radial period of about 50 nm between bands and is consistent with Figure 1C. The maximum spectral intensity has increased dramatically to 16.5 nm2, indicating improved definition of the patterning. Further increase in molecular weight to 66k MW (Figure 4D) produces an even more well defied spot pattern with a radial period of slightly less than 50 nm. The maximum spectral density of Figure 4D is equivalent to Figure 4C. 0
5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0
time (min)
Figure 2. Roughness (rms) for PS surfaces as a function of time and molecular weight from 0.4 pm by 0.4 pm regions. See text for discussion.
measurements were taken from 0.4 pm by 0.4 pm areas within the larger fields of Figure 1in order to avoid the central contacted area created duringthe initial AFM t i p surface interaction. The initial rms roughness measured for all PS surfaces is very low, less than 0.4 nm.19 Note that the roughness over the first 15 min of scanning increases under constant load and that the relative values generally increase monotonically with MW at a particular time. On longer scanning the relative roughness order practically inverta, i.e. the higher MWPS surfacesroughen less and the lower molecular weight materials roughen more. Differences in morphology of the oriented pattern obtained using MWs greater than l00k can be observed at the same loads if the surfacesare scanned for long enough time. For example, changes in the oriented pattern are observed for l00k MW (parts B and D of Figure 3) compared to lOOOk MW (parts A and C of Figure 3) for longer scanning times of 60 and 120 min. The images shown in Figure 3 are for a 0.4 pm by 0.4 pm area selected from a 1p m by 1pm region and tilted 60° from z (30 nm full scale). After 60 min of rastering there is essentially no difference in the appearance of the oriented bands for MW greater than 100k. But after 120 min, the lOOk MW surface shows weak doubling up of the initial oriented bands present and an increase in the vertical height ("white" in Figure 3D)compared to the 215k (not shown) and 1000k. The PS surfacesmade using65k MWPS shows evidence for the breaking up of the oriented pattern after 60 min of rastering (image not shown). In order to better appreciate the periodic features of the spacings within the patterns, two-dimensional Fourier transforms (FT)of 0.4 pm by 0.4 pm regions from selected surfaces after 15 min of scanning are shown in Figure 4. For comparison the transform of the initial surface is also shown. The spectral densities are not normalized relative to each other. The FT of the initial surface (Figure 4A) exhibits features which are artifacts of the zoomingproceaa indicativeof the edges of the zoomed regions at 400 nm in I: and y. The maximum spectral intensity is 5.8 nm2. The F'T of the 24k MW surface after 15 min of scanning (Figure 4B) shows four intense spots which are off-axis indicating components which are -30° off perpendicular. The radial component suggests the (19) The initial rms roughness differs remarkably from the initial surfacee in ref 12 (Figure 1) and may be related to the manner in which the films are prepared, spinning (this work) vn dropping (ref 12).
Discussion Abrasion patterning in rubbers was first described 40 years ago by A. Schallamach" who studied the wear in rubber tires by abrasion with sandpaper where parallel ridgeswere formed perpendicularto the rubbing direction. In another study using a phonograph needle abrader, Schallamach found that when pure gum vulcanizates were scratched under 0.025-0.08 kg loads, the traces of the normal force duringsliding versustime were discontinuous, indicating periodic stick-slip behavior.2I Optical images of the rubber tracks showed alternating raised areas and pits on the rubber surface which corresponded to sticking and slippingin addition to much tearing and deformation. The interaction of the sharp asperity (a needle) stretches the rubber in its vicinity leading to tensile failure and severe damage of the surface. The wear of the rubber surface can be described as abraeion. The pile up of rubber on the surfacehas been qualitativelydescribed as buckling or detachment of rubber near the surface under the tangential stress during ~liding.~ The concentration of stress was found to be greatest at right angles to the sliding direction and compressive in nature.21 In the early 1970s Schallamachdescribed the movement of detached rubber under load when a hard, optically transparent slider was contacted to various rubber surfaces(wavesof detachment or 'Schallamach waves").22 The wave and abrasion spacingsobserved by Schallamach were truly macroscopic ( 1Oa m spacings) compared to the spacings seen due to the AFM tip-PS surface interactions. The patterning due to the interaction of the AFM tip with the PS surfaces,in particular the low molecular weight PS (1!24k),is strikinglyreminiscentof Schallamach's early work on rubber abrasion. The AFM abrasion patterns appear similar to the rubber abrasion; however, the spacings are 10 OOO times smaller. The AFM patterns for the low molecular weight surfaces could be considered a plastic analog of the rubber abrasion patterns. The AFM oriented pattern for MWs greater than 24k are much different in their appearance and spacing. The transition between the abrasion and oriented patterns occurs near the entanglement MW, Me,for PS (19k2% As the MW increases beyond Me,there is greater resistance to plastic flow because of increasing chain entanglements. The observation of oriented patterns for all MW's above Me and the dimension scaling of the abrasion pattem suggest that the elastic modulus of the PS surface is much lower than the bulk. Although the exact mechanism of the AFM tip-surface interaction is not precisely known, some comments about N
~~
(20) Schallamach, A. Rubber Chem. Technol. 19S2,26, 230. (21) Schallamach, A. J. Polym. Sci. 19S2,9, 385. (22) Schallamach, A. Wear 1971,17, 301. (23) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley Q Sons: New York, 1980.
Molecular Surface of Polystyrene
Langmuir, Vol. 8, No. 9, 1992 2333
60 min
1OOOk
1OOk
Figure 3. Perspective views of two higher MW PS surfaces exhibiting an "oriented" pattern (see text). The images are 0.4 pm X 0.4 pm in lateral dimension and 30 nm in vertical dimension. The loading w8s 1.5 X N. The pattern in the lOOk MW surface breaks up more quickly than for the lOOOk MW PS (compare A to B and C to D)by scanning for long times.
the mechanism can be made. The physical penetration of the AFM tip into the PS surface is less than 1nm under a load of 1.5 X lo-* N. This penetration depth is well below the vertical height of the fully developed oriented wave pattern ( 2 5 nm). A mechanism involving compressive loadingwhich leads to squeezing of PS out from under the tip is, therefore, not likely. Some of the 15-min AFM topviews shown in Figure 1 (A and B) have a "melted" appearance. If the tip interaction results in a surface temperature rise to at least Tg(-380 K for PS24),then the polymer strength would greatly diminish and perhaps become much lower than the interfacial shear strength. This interaction near Tgwould produce a surface that would be the result of the removal of large clumps of p01ymer.~However, using a model proposed by Lim and A ~ h b to y estimate ~~ the temperature of the surface during sliding, we calculate a temperature rise much less than 1 K. This is due to the very low load and sliding speeds present in the AFM tip-surface interaction. The statistical size of the polymer chain (MW = 100k) (24) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989. (25)Lim, S. C.; Ashby, M. F. Acta Metall. 1987,35, 1.
for the fully developed oriented pattern can be estimated. The radius of gyration, R,, is calculated to be -8 nm.26 From these data the random coil length27of a lOOk PS chain, [ ( / ~ ~ ) ] lis/ ~-20 , nm and composed of five entanglements. The stretched end to end distance of the random chain is estimated to be -240 nm.28 From these size predictions the AFM-tip PS surface interaction may involve just several single polymer coils. The contact diameter under load is -22 nm as discussed above. The interaction could be thought of as a plastic analogue of the real Schallamach wave on the molecular scale which evolves under load via irreversible deformation. Qualitatively, a mechanism which involves pulling (due to elastic nature of the PS surface) combined with stretching of the entanglements at the PS surface is most (26) Moore, E. R.,Ed. Encyclopedia of Polymer Science and Engineering; John Wiley & Sons, Inc.: New York, 1989; Vol. 16. (27) Van Krevelen, D. W. Properties of Polymers: Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions,3rd ed.; Elsevier ScientificPubliehing Co.: Amsterdam. 1990. (28) Estimated using 962 repeat units in the lOOk chain, a C-C bond length of 0.154 nm, and a C-C-C tetrahedral bond angle of 109.5' from Hendrickson, J. B.; Cram, D. J.; Hammond, G. S. Organic Chemistry; McGraw-Hill Book Co.: New York, 1970.
2334 Langmuir, Vol. 8,No. 9,1992
n
Meyers et al.
n
n
n
Figure 4. Two-dimensional Fourier transforms from 0.4 pm by 0.4 pm regions after 15 min scanning of the PS surfaces using a 1.5 X N load. The spectral intensities are not normalized. Note the difference in the appearance of the pattern for the 24k (B) compared to the higher MW surfaces (C and D). The increasing separation of the spots within each pattern is indicative of the decreasing period of the patterning as a function of MW.
likely. This is consistent with our measured force-distance profiles using the AFM for the loading and unloading process.29 The pull-off force required to break the tip-PS contact is greater than 10 times the force measured upon forming the contact. From these observations it is likely that the description of the nature of the tip-surface interaction must take into account the elastic properties of the polymer surface. In scanning the PS surface in a pull-off mode instead of a loading mode, we observed distinct oriented cones instead of bands (same spacing, however) for a 300k MW PS.30 The AFM-tip surface interaction during sliding is quite complicated. There are surfaceforces3' which exist in the absence of load a t this interface. They are estimated to be as high as 3 X 1W8N in our studies3l and could certainly lead to unexpected surface deformation. On the contrary, in the absence of interfacial adhesion, the force required to maintain tip sliding due to elastic recovery is 2 orders (29)Although not shown, the force-distanceprofiles for the AFM t i p PS surface interaction during loading and unloading show a very large hysteresissimilar to those shown for polycarbonate surfacesdescribed in ref 10. (30) We have found in our studies of pull-off force measurements on other polymer and inorganic surfaces that this is the usual observation and that the pull-off forces scale somewhat with the surface energies.We are in the process of examining the influence of rate, environment (humidity), and pull-off forces on the patterns produced on PS and other 'glassy" polymer surfaces. (31) Derjaguin,B. V.; Muller, V. M.;Toporov, Y. P. J . Colloidlnterface where R Sci. 1975,53,31. The surface force is equal to 4~R(yti~yps)1/~, = 40 nm, ytip= 0.10 J/m2, and yps = 0.04 J/m2. The surface energies, yps and ytip,are obtained from ref 27 and ref 7, respectively.
of magnitude lower for the glassy state (1X 10-loN).32If the PS surfaceis treated as "rubberlike" in the deformation zone, then the calculated sliding force becomescomparable to the surface forces present (5X N).32In the absence of interfacial adhesion the PS surface would have to be "rubberlike" in order to account for bond breaking necessary to rearrange the surface into the oriented patterns. In reality the energy losses will be a convolution of adhesion and viscoelastic energy losses coupled with a complex strain path experienced by the polymer in the near surface r e g i ~ n . ~ Recently Yang and Wu studied the abrasive wear and %aze" (a precrack defect zone within the glassy polymer) breakdown in PS.33 They found that the PS surface wear rate shows a 2-fold decrease for increasing MW over the range of 13k to 150k. For MW above 150k the wear rate is relatively flat. The results described here using the AFM as a "nanoabrader" are consistent with the notion of decreased wear for increasing MW. Although initial roughening of the polystyrene surfacesincreases with MW during the first 15min of scanning, the roughening order reverses at long scanningtimes of 45 minor greater (Figure 2). In the series of images shown in Figure 1following 45 min of scanning (Figure 1E-H)there is a high ridge of material to the right side of each image. This material is (32) Calculated using eq 1 of ref 5 where R is 40 nm and W ia 1.5 X 10-8 N. For the glassy case the elastic modulus (E)is 3.1 X 108 N/m2, Poisson's ratio ( Y ) is 0.35, and tan a is 0.02. For the rubber case, E is 1.75 X 105 N/m2, Y is 0.5, and tan a is 0.3. (33) Yang, A. C.-M.; Wu, T. W. IBM Res. J. 1991, no.8340, and to be published in J . Mater. Sci.
Langmuir, Vol. 8, No. 9,1992 2335 under contact the molecules are being partially disentangled due to adhesion during sliding of the tip. When the shear force of the contacted tip exceeds the adhesive and viscoelastic forces, the stretched macromolecule partially recoils into a new frozen glassy state (plastically deformed). At this time we do not precisely know whether the patterning develops due to molecular bond scission or chain disentanglement.% Further work to elucidate the mechanism is underway.
10"
1 io4
I
%
1111111
10"
J I
I 1111(
I
10"
11111111
I
10'~ wave spacing (m)
I
1111111
I
1111111
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I
104
Figure 6. log (load) va log (wave spacing) for rubbers and PS surfacea compared with the spacing measured due to the AFM tip-PS surface interaction. See text for discussion.
piled up PS as a result of wear and the height and breadth of this feature decrease in size with increasing MW. The break-up of the oriented patterning in the higher MW materials (comparethe 1OOksurfacewiththe 1OOOksurface after 120 min in Figure 3) is also a signature of less "wear" resistance of the lOOk PS surface. Is there a connection between macroscopic Schallamach abrasion spacingson rubber and n a n d e patterning observed on PS surfaces?6 Is there a physical property, e.g. hardness of the surface, which would allow us to scale with lower load the observed spacings down the molecular level? Figure 5 [log (load) vs log (spacing)] attempta to show that scaling to the molecular size is within reach. Included in Figure 5 are data from Schallamach on rubbersmand spacings from scratch testa on PS surfaces.33 If we linearly extrapolate the abrasion wave spacings on rubbers down to the loadings with the AFM, the spacings measured by nanoforce loading do fall within the range for rubbers. We can suggest that the observation of waves and patterning on rubber and a glassy polymer surface (PS) implies that the physical properties of the surface may be much different than the bulk. These oriented patterns and their evolution in time allow one to speculate that the macromolecules at the surface which have this extra degree of freedom of motion just need a little pulling to be separated from the bulk. Perhaps properties such as Tgand elastic modulus are also lower for the PS surface. From the observation of the oriented bands such as those shown in Figure 3, we speculate that (34)Willett, J. L.;O'Connor, K.M.;Wool, R. P.J. Polym. Sci., Part B 1986,24, 2583.
Conclusions In conclusion we have examined the apparent plastic deformation of a glassy polymer (PS)due to the interaction with an AFM tip. Two distinct patterns are observed: an abrarrion pattern for MW's less than 24k and a fully developed oriented pattern for MW's greater than 1OOk. The onset of the oriented pattern occurs very near the entanglement MW for PS, -19k. Between 24k and lOOk the oriented pattern does not fully develop. The "wear" of the PS surface decreaseswith increasing MW, although the exact functional dependence is unknown. As MW increases there exists a greater resistance to plastic flow due to molecular entanglements. The proposed scaling of the oriented wave spacings to those observed for macroscopic spacings for rubbers suggests that the near surface of PS behaves more like a material exhibiting rubber elasticity than one in the glassy state. The oriented patterns using the AFM are due to the stretching of chains which possessan extra degree of freedom at the PS surface. With further refinementa in fabricating tips with smaller radii, it may be possible to disassemble a polymer surface in a molecule by molecule fashion. Also, by investigating it should be other glassy polymers above and below Me, possible to contribute to the development of polymer surface structural models.
Acknowledgment. B. Dalke and S. Hahn of Dow Chemicalare thanked for providing the anionicpolystyrene materials and providing molecular weight analysis using gel-permeation chromatography. Professor E. Andrews is acknowledged for many helpful discussions regarding surface deformation mechanisms. Dr. M. Chaudhury (Dow Corning Co.) is also acknowledged for pointing out the references to Schallamach waves. We recognize the many helpful discussions with Drs. C. Berglund, M. Bernius, R. Bubeck, S. Martin, G. Mitchell, and M. Radler all with the Dow Chemical Co. Mr. M. Keinath and Drs. M. Newsham and P. Townsend are thanked for their assistance in preparing the polystyrene films. We appreciate the critical review of this manuscript by Dr. G. Potter. Drs. R. Chrisman, D. Dix, and W. Knox are also thanked for their support of this work. -@try NO. PS,9003-53-6.
