NotesSPM METHODS FOR POLYMER INVESTIGATIONS

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Alexeev A.M.(State Lukin Institute for Physical Problems, NT-MDT, Moscow)Buzin A.I. (Institute of Synthetic Polymer Materials Russian Academy of Sciences, Moscow)

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The polymers investigations by SPM have anumber of advantages.

High resolution, which allows visualizing theobjects of several nanometers size.

Minimal requirements to sample preparation.Wide range of different methods to investigate

the polymer’s properties.All of the results described below are obtained

on commercial SPM SOLVER P47.When we use the contact mode the tip-sample

interaction is strong and soft materials like poly-mers can be either deformed or damaged. Usuallygood quality topography measurements of the poly-mers with contact mode are difficult in air. Theusing of soft cantilevers and measurements in liq-uids reduce the tip-sample interaction, but contactwith liquid can change the sample structure. Despitethese disadvantages in some cases contact regimecan be used for nanotribology experiments and alsofor revealing the different components in compos-ites [1,2]. This is achieved by measurements of thelateral deflection of the cantilever (lateral forcemicroscopy (LFM)) that depends on the frictionforce. The rough surfaces make difficulties for theinterpretation of the results, because lateral deflec-tion of the cantilever depends not only on frictionbut also on topography. Fig.1 shows topography (a)and LFM image (b) of Langmuir-Blodgett film ofblend of two components (p-octadecylcarboxya-zobenzene-p’-sulphonamide and copolymer octa-fluoroamilacrylate and metacrylicacid). Despite vis-ible damages on the topography the areas with dif-ferent friction are clearly seen on LFM image. Thebright areas (the higher friction force) correspondto polymer.

The using of semicontact mode allows elimi-nating the destructive action of the lateral forces,which exist in contact mode. The tip-sample inter-action in semicontact mode is greatly reduced incomparison with contact mode. This is especiallyimportant for easily injured polymer surfaces.Fig.2a shows the topography of the same LB filmacquired in semicontact mode. Increasing of thequality of the image is clearly seen from comparisonFig1a and Fig.2a.

Fig.1. Topography (a) and lateral force (friction)distribution (b) for two-component LB-film.

Fig.2. Topography (a) and phase image (b) for two-component LB-film.

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a b

Strong contrast is the typical feature of theimaging of the oscillating cantilever phase shift. Thephase image is acquired by simultaneous measuringof topography and cantilever phase shift duringscanning in semicontact mode. This strong contrastcan be explained by the dependence of the phaseshift on the surface properties. The phase shiftdepends not only on topography but also on adhe-sion, elasticity, and damping. Because of it phaseimage enable to use it for revealing the surface areaswith different properties. For example it is possibleto reveal the distribution of the components in com-

It is convenient to introduce coefficientk=mag/mag0, where mag0 is amplitude of freeoscillations of cantilever, mag is amplitude of thecantilever oscillations during scanning (set-point).The minimum possible mag0 and maximum possi-ble mag are desirable in semicontact mode fortopography (height) imaging. Force acting on thesample is minimal when k close to 1. As a result thequality of the topography image will be better. Forexample the topography of Langmuir-Blodgett filmof cyclolinear polyorganosiloxane is shown on Fig.3.Collapsed film was deposited on mica by Langmuirmethod. The islands of the second and third layerslay on the monolayer. The height of the every layer isabout 1nm. The cantilever amplitude of free oscilla-tions is 7nm for Fig.3a and 60nm for Fig.3b. In bothcases k=0.97. It is clear seen that not only quality ofthe image reduce after increasing of the amplitudebut also the destruction of the film is observed(lower part of the Fig.3b).

In some cases the amplitude-image (feedbackerror) helps to distinguish the small features on thetopography. The amplitude-image is signal of error,

which exists due to non-ideal feed back loop. Onlysmall irregularities are visible in this case becausesmooth topography is tracked by feedback. TheFig.4 presents the spherulite organization ofpolypropylene. The height-image (Fig.4a) and theamplitude-image (Fig.4b) were obtained simultane-ously. It is seen that lamellae structure of spherulitesis more visible on Fig.4b. The contrast ofthe ampli-tude-image will increase after either the scanningvelocity increasing or feedback gain decreasing. Theoptimal scanning parameters for amplitude-imageshould be particularly selected for every sample.

Fig.3. Topography of the LB film obtained with differentcantilever amplitude: a) 7nm, b) 60nm.

Fig.4. Topography (a) and feedback error image (b) ofthe polypropylene spherulites.

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a b

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posite material, the distribution of the amorphousand crystalline phases. Fig.2b demonstrate thephase-image for two-component LB film thatobtained simultaneously with Fig.2a. The dark areason Fig.2b correspond to areas with greater phaseshift. This phase contrast is explained by both dif-ferent hardness and adhesion of the different com-ponents. Fig.5 shows the topography (Fig.5a) andvalue of mag*sin(phase) (Fig.5b) obtained simulta-neously for poly(diethylsiloxane). The sample isthin film of polymer deposited on silicon surface byrubbing. The direction of rubbing is from left lowercorner to right upper corner. Dark background onFig.3b is the amorphous polymer. The lamellaeplaced perpendicular to the rubbing direction areembedded into the amorphous polymer and consistof folded molecular chains. The lamellae are meso-morphic (partially-ordered) state of PDES. The mol-ecules in lamellae are stretched along rubbing direc-tion. The strips in lamellae stretched along rubbingdirection, which are clearly seen on Fig.3b, can becompared with molecular structure of the lamellarblocks.

Fig.6 demonstrates the crystalline structure ofthe polyethyleneoxide sample that was obtained bycooling of the melt. The lamellae are tilted at differ-ent angles with respect to surface. The grains of var-ious sizes are visible on the edges and on the sides ofthe lamellae.

The phase image strongly depends on set-point, i.e. on strength of the tip-sample interaction.The influence of the material properties on phase-image usually increases with decreasing k (bydecreasing mag) [3,4]. The numerical simulation in[3] demonstrated that in general the influence of thesample properties on the cantilever response isintricate when k is small. But in the some cases it is

possible to connect the phase changes with thechanges of the some of surface properties. Fig.7shows the boundary of the two spherulites ofpoly(buthyleneterephtalate)-poly(tetramethyle-neoxide) (PBT-PTMO) block-copolymer. Crystallinephase of copolymer consist only PBT, amorphousphase is combination of amorphous PTMO anduncrystallized PBT. The topography and phaseimage obtained simultaneously are shown on Fig.7aand Fig.7b, k=0.95. The phase image for the samearea for k=0.7 imaged on Fig.7c. The increasing ofcontrast on Fig.7c is explained by influence of thelocal stiffness. The dark areas on Fig.7c can be asso-ciated with softer amorphous phase.

The force modulation imaging is also used tostudy the local stiffness. In this case the modulationof the probe-sample spacing during contact modeby scanner oscillations leads to cantilever oscilla-tions. The different response of the cantilever fromareas with different stiffness can be observed. Fig.8aand Fig.8b show topography and force-modulationimage (cantilever amplitude) obtained simultane-ously for the sample of two-component LB film. Thelight areas on Fig.8b correspond to higher cantileveramplitude, i.e. higher hardness. These areas arepolymer. The substrate can influence on the resultsof the LFM, force-modulation mode etc. when thinfilm are studied [1,2].

Acknowledgments:

Authors are thankful to Prof. Y.K. Godovsky(Karpov Institute of Physical Chemistry, Moscow)and his colleagues for the samples and helpful dis-cussions. Authors are also thankful to Dr. I.V.Myagkov and Dr. E.A. Mazurina (Lukin StateInstitute for Physical Problems, Moscow) for thesample of Langmuir-Blodgett film of blend of twocomponents.

Adhesion force microscopy (AdFM) allowsmapping the differences in adhesion. The depend-ence of the cantilever deflection on the piezotube

location is obtained in each point of the scan. Thedistribution of the minimum of this dependence forfull scan gives information about differences inadhesion. Fig.9 shows the topography (a) and theAdFM image (b) for the sample of two-componentLB film. The dark areas (polymer) on Fig.9b corre-spond to higher adhesion. This different adhesionsas well as contrast of the force-modulation imageexplain the phase contrast on the Fig.2b.

The sample of two-component LB film consistsof polar materials. NT-MDT SPMs are useful tools tostudying the electric properties of this sample.

The using of the SMENA for high temperaturemeasurements (up to 300°) allows investigating var-ious processes such as crystallization, melting etc.The growth of the polypropylene spherulite in themelt observing with this type of SPM demonstratesthis possibility

References:

1. S.N. Magonov, Scanning force microscopy ofpolymers and related materials, VMS B, 1996, 38(1), 143-182, in Russian.

2. S.N. Magonov, M.-H. Whangbo, Surface analysiswith STM and AFM, VCH, Weinheim, 1996, 323p.

3. N.A. Burnham et al. How does a tip tap?,Nanotechnology, 8, 1997, 67-75.

4. M.-H. Whangbo, S. N. Magonov, H. Bengel, Tip-sample force interactions and surface stiffness inscanning probe microscopy, Probe Microscopy,1997, 1, 23.

Fig.5. Topography (a) and amplitude*sin(phase) image(b) for the poly(diethylsiloxane).v

Fig.7. Topography (a) and phase images (b, c) obtainedwith different set-point.

Fig.6. Topography (a) and phase image (b) ofpolyethyleneoxide.

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a

b

Fig.8. Topography (a) and force-modulation image (b)for two-component LB-film.

a b

Fig.9. Topography (a) and adhesion force distribution (b)for two-component LB-film.

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b c

a b