Micromechanics and Microtribology of Polymer Films - American

Chapter 7 Micromechanics and Microtribology of Polymer Films 1

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F. Oulevey , D. Gourdon ,E.Dupas , M. Liley , C. Duschl , A. J. Kulik , G. Gremaud , and N. A. Burnham 1

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Department of Physics, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland Advanced Microsystems, CSEM SA, CH-2007, Neuchâtel, Switzerland Department of Chemistry, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland Nanomechanics LLC, P.O. Box 700, Pittsford, NY 14534-0700

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Downloaded by UNIV LAVAL on May 12, 2016 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch007

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Mechanical properties of a polymer blend change as a function of temperature, particularly near phase transitions. Normally, the modulus and damping of a blend are recorded for the entire sample. Here, we present atomic force microscopy results from local measurements of the individual components of a polymer blend. The data are collected either as images at a set temperature, or spectra of a small area on the surface as a function of temperature. Using lateral force microscopy on Langmuir-Blodgett films, we found unexpected friction anisotropics and asymmetries in the frictional behavior. The data were found not to be related to the hexagonal packing of the molecules, but rather related to a small molecular tilt.

New instrumentation allows access to new length scales. Here, we summarize results obtained using two techniques based on atomic force microscopy. The first [1] is a nanoscale equivalent of DMA, dynamic mechanical analysis; the second is a small-scale version of a tribometer [2]. Micromechanics In the nano-DMA, a sinusoidal mechanical oscillation is applied to the sample by the transducer shown in Fig.l. The tip of an atomic force microscope is placed in contact with the sample, and the amplitude and phase of the cantilever's response is recorded as a function of position on the sample's surface, forming images related to the sample's elastic modulus and damping. Lateral resolution is of the order of a few tens of nanometers. The microscope may also be operated in spectroscopy mode, where the tip remains nominally on the same point of the surface, and the temperature of the sample is ramped.

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© 2000 American Chemical Society

Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Fig. 1. Schematic diagram of the nano-DMA. Micromechanical measurements are far from easy. There are two technical hurdles that must be overcome. The first is to understand and control the cantilever response [3]. For example, if the stiffness of the cantilever is 0.1 N/m, and that of the interaction 10 N/m, the cantilever bends 99% of the applied displacement. If the interaction stiffens to 100 N/m, then the cantilever effectively moves 100% of the applied displacement. Not many detection systems are sensitive enough to resolve a difference of less than one percent of an applied displacement of a few Angstoms. Naturally, many researchers increase the excitation amplitude in order to work above the detection limits of their microscopes. This induces sliding of the cantilever along the surface of the sample because cantilevers are typically placed at an angle of 15 degrees to the surface plane. A modulation amplitude of 10 nm causes 2.2 nm of sliding. This incorporates frictional effects into the modulation images [4].



120 One way to avoid the problems of frictional effects and low sensitivity to stiffness is to drive the cantilever into the inertial regime usingfrequenciesabove the contact resonance, instead of exciting the system at a few kilohertz-the usual approach. This stemsfromthe inherent response time and thus inherent response frequency of the cantilever-sample system. If the modulation frequency is higher than the response frequency, the tip of the cantilever cannot move quickly enough to keep up with the motion of the sample and transducer, and the sample is compressed underneath the mass of the tip. The relationship between applied displacement and cantilever response now becomes, to first order, linearly dependent on stiffness [3,5]. Large modulation amplitudes are no longer necessary in order to discern changes in contact stiffness. This in turn reduces the coupling between friction and the modulation images. The second technical problem is thermal drift. Materials can have thermal expansion coefficients ranging from negative values up to, typically, 10" /°C. A change of 100 °C causes a 1 cm bar to expand 10 |im-many force microscopes' scan range. The instrument should be constructed with low-expansion materials in a design as concentric as possible, with the tip centered. A corollary to the mechanics of thermal drift is the detection sensitivity to temperature, which should be compensated for temperature. Figure 2 shows a blend of polyvinylchloride (PVC) and polybutadiene (PB) at various temperatures above and below the glass transition of the PVC. The images in the upper row are atomic force microscope images representing topography. (Bright means high.) In the lower row are the rigidity images (cantilever modulation amplitude), related to the elastic modulus, where bright means stiff. We have also succeeded in collecting data at a single point (nominally) as a function of temperature, and the plots show good correlation with global DMA-style measurements [1]. 5

Fig.2. Force microscope (top row) and rigidity images ( = cantilever response amplitude, bottom row) as a function of temperature. The gray scale remains the same for each row. The excitation frequency was 820 kHz. PVC (T = 95°) forms the continuous matrix of this 60/40% compound. g


121 Microtribology


The tip of an atomic force microscope makes a single point contact with a surface. Consequently, understanding tribological behavior becomes intellectually simpler. Friction has been shown to be correlated not with the magnitude of the adhesion between two bodies, but rather with the adhesion hysteresis [6]. Our recent study of a model system, a Langmuir-Blodgett film on mica [7], shows no dependence on adhesion hysteresis, but rather a dependence on molecular orientation [2].

Fig.3, A series of lateral force images. Dark means more torsion, that is, higher friction. For each image, the scan direction was from left to right, a), b), and c) show the same flower-shaped domain in different orientations after rotation of the sample underneath the cantilever. In d), the cantilever torsion is plotted as a function of position along a line through the bottom half of the flower in c) for both the left-to-right and right-to-left scans. The use of 'A' and 'B', indicated in d), is explained in the text.



122 We observed the effect presented in Fig.3. A Langmuir-Blodgett monolayer deposited on mica formsflower-shapeddomains. The molecules within aflowerare in the condensed state. The matrix surrounding the flowers is a disordered phase. The condensed domains are comprised of subdomains. Upon rotation underneath the cantilever, the contrast of the subdomains changes in a systematic way. The cantilever torsions marked ' A ' and ' B ' in Fig.3d were added to ascertain the total friction A+B and subtracted for the friction difference A-B. Upon 360° rotation of the sample, the total friction behaved with C2 symmetry, whereas Ci symmetry was discerned for the friction difference A-B. We denote the C2 behavior by the term "anisotropy" and the Ci behavior by "asymmetry". The anisotropy provides most of the contrast between the subdomains (petals). The small contribution from the asymmetry can be seen in the horizontal petals of Fig.3c. There, the petals have similar orientations with respect to the scan direction, yet exhibit slightly different contrast. Are the anisotropy and asymmetry related to molecular orientation? Electron diffraction demonstrated that the condensed molecules are hexagonally packed, and the upper limit for the tilt from the normal was fifteen degrees. Brewster angle microscopy determined that the tilt from the normal was approximately ten degrees, and the azimuthal tilt direction was along the subdomain boundary, as sketched in Fig.4.

Fig.4. Schematic of the frictional and structural results.


123 The anisotropy (totalfriction)and asymmetry maxima and minima are depicted with the double-ended arrows. The hexagonal packing of the molecules does not correlate with the frictional behavior, but the molecular tilt does. The anisotropy corresponds to moving across or along the molecular tilt direction, whereas the asymmetry corresponds to moving against or with the nap of the molecules. Interestingly, the friction scanning against the nap was lower than with the nap. Adhesion hysteresis is an isotropic phenomenon and cannot account for the effects summarized here and elaborated in Ref. [2]. The orientation of molecules tilted less than 15 degrees strongly contributed to the friction behavior.


Closing words Technical advances in force microscopy permit investigations of the micromechanical and microtribological properties of polymers. Insights into the fundamental processes of friction have already been gleaned, and soon phase transitions in polymer blends will be systematically studied at the nanoscale. Acknowledgement We are indebted to H. Pollock and M. Song of Lancaster University for the PVC/PB sample, and to the British Council/Swiss National Fund, who supported our work. References 1. Oulevey, F.; Gremaud, G.; Semoroz, A.; Kulik, A.J.; Burnham, N.A.; Dupas, E.; Gourdon, D.; Rev. Sci. Instruments 1998, 69, 2085. 2. Liley, M.; Gourdon, D.; Stamou, D.; Meseth, U.; Fisher, T.M.; Lautz, C.; Stahlberg, H.; Vogel, H.; Burnham, N.A.; Duschl,C.;Science 1998, 280, 273. 3. Burnham, N.A.; Kulik, A.J.; Gremaud, G.; Gallo, P.-J.; Oulevey, F.; J. Vac. Sci. Technol. 1996, 14, 794. 4. Mazeran, P.E.; Loubet, J.-L.; Tribol. Lett. 1997, 3, 125. 5. U. Rabe and W. Arnold; Appl. Phys. Lett. 1994, 64, 1493. 6. J.N. Israelachvili, Y.-L. Chen and H. Yoshizawa; J. Adhesion Sci. Technol. 1994, 8, 1231. 7. We used the amphiphile bis[8-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)-3,6dioxaoctyl] disulfide that consists of two chiral phospholipids linked by a hydrophilic spacer.


Micromechanics and Microtribology of Polymer Films - American

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