Effect of Rubbing-Induced Polymer Chain Alignment onAdhesion and Friction of Glassy Polystyrene Surfaces

Melvina Leolukman and Seong H. Kim*

Department of Chemical Engineering, The Pennsylvania State University, University Park,Pennsylvania 16802

Received August 29, 2004. In Final Form: October 28, 2004

The friction and adhesion properties of polystyrene surfaces are studied below the glass transitiontemperature by means of atomic force microscopy in argon. Even at a temperature far below the glasstransition, the repeated sliding of a polystyrene bead tip on the non-cross-linked polystyrene surfacecauses significant reduction of friction and adhesion forces. There is no measurable wear of the polystyrenesurface due to repeated sliding. These decreases are associated with the alignment of the outermost polymersegments induced by repeated rubbing. There are only little changes in friction and adhesion on thecross-linked polystyrene surface in which the covalent cross-linking prevents chain realignment.

Introduction

The fundamental understanding of the friction behaviorof polymer surfaces at the molecular level is becoming anincreasingly crucial factor in applications such as polymer-polymer microbearings, micromechanical devices, andnanotribology.1-3 The molecular-level studies of frictionof polymeric materials have been focused on lubricationbehaviors of low-Tg polymers, low molecular weightpolymeric surfactants, and polymer melts.4,5 These studiesfound that the friction of polymer-on-mica systems issignificantly affected by the relaxation dynamics of theshear stress at the polymer interface6,7 and the friction ofpolymer-on-polymer systems is strongly dependent onchain interdigitation at the interface.8-11 Compared tothese rubbery or fluidic polymer systems, the molecular-level studies for adhesion and friction between glassypolymers are found much less in the literature. Recently,Maeda et al. reported that the glassy polymer surfacesalso undergo significant chain interdigitations uponcontact, which increases adhesion hysteresis and friction.12

However, how these properties of the glassy polymerchange upon repeated cycles of sliding action is not wellstudied yet. Understanding these changes will be criticalto predicting the tribological properties of polymericmaterials in extended operation.

This paper reports the effect of repeated rubbing onpolystyrene (PS) friction and adhesion at room temper-ature, which is far below its glass transition temperature

(∼100 °C). Atomic force microscopy (AFM) was used tomeasure friction and adhesion forces between high mo-lecular weight polystyrene surfaces. The repeated scan-ning of a PS bead attached to an AFM cantilever causesthe decrease of adhesion and friction of the non-cross-linked PS surface without any wear, while it does notalter adhesion and friction of the cross-linked PS surface.The rubbing-induced decrease of the adhesion and frictionforce of the non-cross-linked surface implies that restruc-turing of polymer chain segments occurs even in the glassystate.

Experimental MethodThe PS films were prepared by spin coating a solution of 2.54

wt % atactic polystyrene (MW ) 223 200 g/mol, polydispersity) 1.11) dissolved in high purity toluene on a silicon wafer at3000 rpm. The resulting polymer films were dried and heatedat 100 °C in vacuum for 20 h. The typical thickness of the spin-cast films was ∼150 nm. The cross-linking of the PS film wascarried out by exposing the PS thin film to UV irradiation froma mercury arc lamp in an argon environment. The volume fractionof the cross-linked PS chains was calculated by comparing thethickness of the cross-linked PS film left after toluene washingwith that of the initial PS film before UV irradiation.13 It wasfound that about 60% of the PS film was cross-linked under ourUV exposure conditions.

Polystyrene particle tips (BioForce Nanoscience Inc.) were usedfor measurements of adhesion and friction between PS surfaces.The PS particle tip consisted of a commercially available latexbead of a diameter of 5 µm attached to a V-shaped cantileverwith a spring constant of 0.58 N/m. The initial adhesion andfriction forces of the as-purchased tip were too large to bemeasured probably due to low molecular weight polymer chainsloosely entangled at the latex bead surface or surfactant molecules(used in the emulsion polymerization making latex beads)remaining at the bead surface. The PS particle tips werepreconditioned by repeated rubbing on a PS film surface in anargon environment until there were no changes in adhesion andfriction. The preconditioning of the PS particle tip was neededto make sure that the adhesion and friction changes measuredwith the tip on a freshly prepared thin film substrate are due tothe changes in the substrate surface, not mostly due to thechanges in the tip surface.

The friction and adhesion changes of non-cross-linked andcross-linked PS film surfaces were studied in contact mode withapplied normal forces from 14 to 54 nN in an argon environmentat room temperature. At these conditions, the Johnson-Kendall-

* Corresponding author. E-mail: shkim@engr.psu.edu.(1) Rymuza, Z.; Kusznierewicz, Z.; Solarski, T.; Kwacz, M.; Chizhik,

S. A.; Goldale, A. V. Wear 2000, 238, 56.(2) Nanotribology, Critical Assessment and Research Needs; Hsu, S.

M., Ying, Z. C., Eds.; Kluwer Academic Publishers: Boston, 2003.(3) Microstructure and Microtribology of Polymer Surfaces; Tsukruk,

V. V., Wahl, K. J., Eds.; ACS Symposium Series 741; American ChemicalSociety: Washington, DC, 2000.

(4) Klein, J. Annu. Rev. Mater. Sci. 1996, 26, 581.(5) Kumacheva, E. Prog. Surf. Sci. 1998, 58, 75.(6) Heuberger, M.; Luengo, G.; Israelachvili, J. N. J. Phys. Chem. B

1999, 103, 10127.(7) Luengo, G.; Heuberger, M.; Israelachvili, J. N. J. Phys. Chem. B

2000, 104, 7944.(8) Luengo, G.; Pan, J.; Heuberger, M.; Israelachvili, J. N. Langmuir

1998, 14, 3873.(9) Yamada, S.; Israelachivili, J. N. J. Phys. Chem. B 1998, 102, 234.(10) Ruths, M.; Granick, S. J. Phys. Chem. B 1998, 102, 6056.(11) Dhinojwala, A.; Cai, L.; Granick, S. Langmuir 1996, 12, 4537.(12) Maeda, N.; Chen, N.; Tirrell, M.; Israelachvili, J. N. Science

2002, 297, 379. (13) Yan, M.; Harnish, B. Adv. Mater. 2003, 15, 244.

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10.1021/la0478503 CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 12/14/2004

Roberts (JKR) theory predicts that the contact diameter is 0.9-1µm and the contact pressure is 5-18 kPa.14 One line of the polymerfilm surface was scanned repeatedly for 3072 times with a line-scan mode, and the lateral force signal was recorded as a functionof scan cycles. Here, one line scan is defined as a cycle of traceand retrace scans. The force-distance curve was taken every1024 line scans. A typical scan speed was 8-30 µm/s. The frictiondata of the polymer films were obtained from the lateral signaldifference, and the adhesion data were obtained from the pull-off force of the force-distance curve. The friction and adhesionforces of each substrate were normalized with the initial databefore repeated scanning to highlight the rubbing-inducedchanges.

Results and Discussion

There are recurring features that are observed in theadhesion and friction measurements of polymer surfaceswith newly purchased PS bead AFM tips. When a freshlyprepared PS thin film surface was scanned with an as-purchased PS tip, the adhesion and friction force werealways too high to be measured with AFM. When the tipis continuously used for contact mode imaging, theadhesion and friction forces decrease to the region thatcan be measured with a reasonable reproducibility. Onecould simply consider that these decreases are caused byrubbing-induced cleaning of the scanned area, wear ofthe polymer tip, or restructuring of surface segments atthe interface. An optical microscope imaging did not findany visible wear of the PS tip, but optical microscopycannot detect surface wear of a few nanometers. The useof scanning electron microscopy (SEM) to image the usedtip did not reveal any discernible wear of the 5 µm PS tipbecause the nonconducting polymer surface is coated witha thin gold film to avoid charging problems in SEM.Because of these uncertainties, one could not understandexactly what caused the friction and adhesion behavior

of the rubbed polymer surface. This can be resolved withthe AFM results shown here, which rule out the first twocases and indicate the restructuring of the surfacesegments as the main source for the rubbing-inducedadhesion and friction decreases.

Figure 1 shows the three-dimensional images of to-pography and lateral force signals of the non-cross-linkedPS surface. The top part of the image was repeatedly line-scanned for 3072 times. The topography image does notshow any hint of line scan history or wear marks, whilethe lateral force image clearly reveals significantly lowerfriction at the line-scanned region compared to the regionwhich was not scanned previously. Most of the smallasperities shown in both images appear to be noises. Theirsize is much smaller than the tip-surface contact diam-eter. Therefore, it can be said that the rubbing-inducedfriction decrease is not related to the polymer surface wearat all. Since the tip is made of the same material, it is notunreasonable to assume that the same changes occur onthe PS tip during the scanning, especially during thepreconditioning. The cross-linked PS film also shows nowear in the topographic image, but it makes only minorchanges in the friction over the same repeated line scans.

The friction and adhesion force changes as a functionof the scanning cycles for the non-cross-linked and cross-linked PS substrate are shown in Figure 2. The initialadhesion and friction forces of the non-cross-linkedsamples were about 30% higher than those of the cross-linked samples. During the repeated line-scan test, thefriction and adhesion between the preconditioned PS tipand the non-cross-linked PS film gradually decrease, whilethose between the preconditioned PS tip and the cross-linked film remain fairly constant. The fact that theadhesion force does not increase after the repeated rubbingalso supports the absence of the polystyrene tip wear. Ifthere was any tip wear due to rubbing, the adhesion forcewould be increased after the line-scan experiment.14 The

(14) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London1971, A324, 301.

Figure 1. (a) Topography and (b) friction force images of a 10 × 10 µm2 region of the polystyrene film after 3072 line scans ofthe top region. The applied normal force is 27 nN.

Polymer Chain Alignment on PS Surfaces Langmuir, Vol. 21, No. 2, 2005 683

exact amount of the decrease varies from sample to sample,but the decrease over the 3072 line scans is typically30-45% for the non-cross-linked sample and <10% forthe cross-linked sample. The magnitude and rate of thedecrease do not depend on the applied load and sampleaging within our experimental conditions. Both freshlyprepared and 1-month-old samples showed the samemagnitude of decrease. The polystyrene is stable in air.The adsorption of airborne molecules cannot be avoidedsince the sample was exposed to air during the sampletransfer to the AFM stage; however, the surface con-tamination cannot explain the rubbing-induced changeobserved for the non-cross-linked polystyrene. The air-borne molecule adsorption is a physisorption processdriven by van der Waal interactions and is not affectedby the degree of polymer cross-linking as long as thechemical functional groups at the polymer surface are thesame. If the rubbing-induced surface cleaning was thedominating factor, one should observe the same magnitudeof changes upon rubbing for both cross-linked and non-cross-linked samples. But this is not the case.

Ruling out the rubbing-induced wear and surfacecleaning effects,one can attribute the decrease of the frictionand adhesion forces during the repeated scanning to therestructuring of the polymer segments at the interface. Infact, it is known from optical property measurements thatpolymer surface rubbing can change the segment align-ment at the surface. The non-cross-linked polystyrene hasrandomly oriented phenyl groups at the pristine sur-face.15-17 When rubbed with other polymers, polystyrene

chains at the surface are aligned such that phenyl groupsare oriented perpendicular to the rubbing direction.18-20

In contrast, the cross-linked polymer surface cannotundergo the same alignment process because simplemechanical rubbing in the nonwear condition cannotprovide enough energy to break chemical bonds thatconnectpolymerchains. Inotherwords, thepolymerchainsat the surface are structurally locked through chemicalbonding to neighboring polymer chains and do not havethe mobility needed for chain alignment. Therefore, it canbe concluded that the propensity of polymer chain segmentalignment at the interface governs the decrease of frictionand adhesion forces between polymer surfaces uponrepeated rubbing under zero-wear conditions.

The decrease of polymer surface friction upon repeatedrubbing has been reported previously; the importantaspect of our AFM data is the fact that this change isobserved for polymers in the glassy state. In previousinvestigations using surface force apparatus,6,7 a micasurface is slid against a poly-n-butyl methacrylate(PnBMA) surface at temperatures near its glass-rubbertransition temperature (Tg ∼ 25 °C). The kinetic frictionof the mica-on-PnBMA system decreases fast by 90% uponrubbing and levels off after only 3-4 sliding cycles at roomtemperature. In contrast, the polystyrene surfaces showa very slow decrease and have not reached the end valueeven after >3000 sliding cycles at room temperature. Inthe case of polystyrene, the glass-rubber transition occursat ∼100 °C. This means that the polystyrene chains areessentially frozen into their entangled conformation inthe experiment time scale at room temperature. The factthat the rubbing-induced segment alignment at theinterface occurs without any measurable surface wearmight indicate the presence of loose loops and danglingchain ends at the polymer surface that are more mobilethan chains in the bulk.21,22 These mobile segments willbe more susceptible to mechanical force or transienttemperature rise caused by rubbing and can be alignedeven at a temperature below the bulk Tg.

Since the experiment temperature is much lower thanTg (∼100 °C), the polystyrene chains in the bulk cannotmove or reorient with simple rubbing at the interface.The temperature rise due to friction is negligible. Evenif all frictional energy is assumed to be used to heat thetop 5 nm thick region (which is close to the radius ofgyration of the polystyrene molecule), the calculatedmaximum temperature rise is only ∼5 K under ourexperiment conditions. Without the displacement of thebulk polymer chains (at least the polymer chains in thesubsurface), the degree of rubbing-induced segmentalignments at the interface would not be near completion.Instead, there will be a distribution of small domains inwhich all or some fraction of polymer chain segments areoriented to a certain direction.23 Once the surface densityof these domains increases to a certain level, there seem

(15) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L.J. J. Phys. Chem. B 2001, 105, 2785.

(16) Gautam, K. S.; Schwab, A. D.; Dhnojwala, A.; Zhang, D.; Dougal,S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854.

(17) Clancy, T. C.; Jang, J. H.; Dhinojwala, A.; Matiice, W. L. J. Phys.Chem. B 2001, 105, 11493.

(18) Greary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl.Phys. 1987, 62, 4100.

(19) Oh, M.; Hong, S.-C.; Shen, Y. R. Appl. Phys. Lett. 2002, 80, 784.(20) Lee, S. W.; Chae, B.; Kim, H. C.; Lee, B.; Choi, W.; Kin, S. B.;

Chang, T.; Ree, M. Langmuir 2003, 19, 8735.(21) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2000,

33, 7588.(22) Sasaki, T.; Shimizu, A.; Mourey, T. H.; Thurau, C. T.; Ediger,

M. D. J. Chem. Phys. 2003, 119, 8730.(23) Kim, J.-H.; Rosenblatta, C. J. Appl. Phys. 2000, 87, 155.

Figure 2. Changes of friction (lateral signal difference) andadhesion (pull-off force) forces of (a) non-cross-linked and (b)cross-linked polystyrene films. The friction signal is shown withcontinuous (scattered) lines, and the adhesion signal is shownwith open circles. The dashed line is drawn to guide the eye.The applied normal force is 14 nN, and the scan speed is 16µm/s. The error bar of the adhesion force, calculated from 5measurements per location, is smaller than the symbol size.

684 Langmuir, Vol. 21, No. 2, 2005 Leolukman and Kim

to be no more measurable changes in the friction andadhesion forces for the polymer-on-polymer interface atroom temperature. This might be due to depletion of looseloops and dangling chain ends or interlocking of thesealigned domains. The difference in the magnitude of thefriction force decrease for PnBMA and PS can be explainedwith the degree of chain segment alignment.23 The PnBMAsurface shows a decrease by more than 90%, while the PSsurface shows only 30-45% reduction. This could beinterpreted as almost complete alignment in the PnBMAcase and only partial alignment in the PS case.

If the distribution of the aligned segment domainscannot be measured, it is difficult to make a quantitativecorrelation of the observed friction and adhesion changesto a specific origin that governs these changes. However,one can still make a qualitative interpretation of the effectof the chain segment alignment at the interface. Whenthe polymer chains at the interface get aligned to someextent, (a) the entanglement of loops and chain ends atthe interface can be reduced and (b) the dynamic relaxationof shear stress caused by sliding can be enhanced. Thesetwo factors will be discussed in the following paragraphs.

The important fact discovered from the AFM resultsreported here is that the friction reduction of the glassypolymer is accompanied by the decrease of adhesion force.According to the JKR theory, the reduction of adhesionforce implies the reduction of the contact area. Since thefriction in microscale depends on the contact area, thereduced adhesion will lead to a lower friction.24-27 It isintuitively expected that when the polymer segments atthe surface are oriented, the degree of chain packingincreases and the surface density of loose loops anddangling chain ends decreases. This will result in lessentanglement and thus lower adhesion force upon contactwith other polymer surfaces.28-30 The effects of the reducedloops and chain ends on friction have been demonstratedby Maeda et al. on cross-linked poly(vinylbenzyl chloride)and polystyrene.12

Let us turn the focus to dynamic energy transfer andshear stress relaxation during the mechanical contact oftwo surfaces. One can assume that the bending vibrationof the pendent group with respect to the polymer backbonealigned along the rubbing direction can reduce energytransfer upon collisions of two polymer chains movingopposite directions.31 The collective motion of the alignedpendent group can also effectively dissipate the energytransferred to the polymer chain.31 If these dynamic energytransfer and relaxation processes are dominant factors,

one can expect that there should be friction anisotropy forthe aligned polystyrene surface, as in crystalline poly-ethylene32 and ordered liquid crystals.33,34 In other words,the scanning orthogonal to the alignment direction wouldgive a higher friction value than the scanning along thealignment direction. This can be studied best by takingcontact mode images of the aligned segment domains witha sharp AFM tip made of silicon or silicon nitride. In thisway, one could avoid the complication from the inter-penetration effect of polymer chains at the interface.However, finding the repeatedly scanned location afterchanging the tip in AFM is practically impossible sincethere is no specific wear feature on the spin-cast polymerfilm that can be used as a guide. An alternative approachis to investigate the crystalline domains of isotropicpolystyrene, which will be the subject of future study. Orit can be studied more thoroughly by using computationalmethods.

The experiment results described in this paper cautionthat the adhesion and friction force measurement ofpolymer surfaces can be dynamically changing even attemperatures far below the glass-rubber transition. Sinceone image scanning in AFM typically consists of 512 linescans, the adhesion and friction forces can be changing upto 5-20% during the one full image scan. If one tries tomeasure the friction coefficient of polymer surfaces byrepeated scanning of one area at varying loads, there couldbe a significant error due to dynamic changes of thepolymer surface during the measurement.

ConclusionThe adhesion and friction of the non-cross-linked

polystyrene surface decrease upon repeated rubbing. Therubbing history dependence of adhesion and friction isattributed to alignments of loose-packed chain segmentsat the surface. What is interesting is that these alignmentsoccur in the glassy state and at a very low load sliding andlower the adhesion and friction behavior of the glasspolymer surface. Compared to other polymers studied attemperatures near their glass-rubber transition, thepolystyrene surface shows a much slower change and themagnitude of the decrease is much smaller, indicatingonly partial alignment of chain segments at the surface.The origin of the lower adhesion and friction of the partiallyaligned polymer surface could be attributed to the reducedinterpenetration of the more closely packed chains at theinterface and/or the reduced energy transfer and enhancedshear stress dissipation of the aligned polymer chains.

Acknowledgment. This work was supported by PennState Start-up funds and, in part, by the National ScienceFoundation (Grant No. DMI-0210229).

Supporting Information Available: Topography andlateral force images of the pristine PS film surface. This materialis available free of charge via the Internet at http://pubs.acs.org.

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