With MultiMode™ and Dimension™
Series AFMs, the force of the tip
applies to the sample surface can be
adjusted by the user to obtain different
kinds of information. Minimization of
the tip-sample force on soft materials
provides height images which mostclosely reproduce the sample
topography - and the small tip-sample
contact area which accompanies thislow force imaging offers the best
image resolution.* At elevatedtip-forces, the sample surface may be
deformed, and if the material isheterogeneous in structure, as manypolymers are, the height image canrepresent a map of surface stiffness
which is often quite different from thesample topography. Therefore, inpractical applications it is worthwhile
to conduct imaging at different forcesin order to more rationally interpret
the AFM images and to recognize
features relevant to surface topography
and composition.

Adjusting Tip-Sample Forcesand Phase Imaging
In TappingMode, the level of the
tip-sample interaction is defined by the
relationship between the amplitude(A0) of the free-oscillating cantilever (onwhich the tip is mounted) and the setpoint amplitude (Asp)4,5. Imaging isgentler when A0 is small (2-5nm) andAsp is close to A0. The difference
between the phase of the freely
oscillating tip and the phase of the
tip as it interacts with the surface
describes the nature and magnitude of
the tip-sample interactions. Monitoringthese phase changes has become oneof the most useful characteristics of
TappingMode, allowing the* This concerns resolution for imaging of
nonperiodic structures. Atomic-scale lattice imagesof crystals can be achieved at tip-sample contact
areas substantially larger than a few atoms.

High-Resolution Surface Profiling

Figure 3. Caption to come???

compositional mapping mentioned
above. In practice, the height andphase images are recorded
simultaneously. On homogeneousmaterials, phase imaging provides
higher contrast for morphologicalfeatures. Phase images forheterogeneous materials often reflect
differences in the properties ofindividual components, and are
extremely useful for compositional
mapping in blends, copolymers,
composites, etc. Because of theincreasing importance of phaseimaging, we will discuss this mode in
more detail while describing thecompositional mapping.

To illustrate the high resolution of
AFM in studies of morphology andnanostructure, we consider aLangmuir-B lodgett film ofpoly(styrene)-bpoly(methylmethacrylate),(PSb-PMMA) copolymer. This film was
prepared on water as a subphase,and then transferred onto a mica
substrate. The height image (Figure 3)
reveals that hydrophilic PMMA blocks
form a compact layer (0.6 - 0.8nm inheight) immediately on the substrate,
whereas individual monoblocks ofhydrophobic PS are assembled on the
PMMA layer in the form of string-likeaggregates (1.5nm in height) orball-like patterns (2.5nm in height).
Individual monoblocks of PS(Mw=15K) exhibit diameters in the
8-14nm range, which is close to thesize of a polymer coil.6 This image has
been recorded with A0 = 2nm and thelargest possible Asp (1.6nm). As A0
increases, the resolution becomespoor, and mechanical deformation andfilm damage are observed.
In studies of commercial PVDF film witha biaxial orientation (Figure 4), finedetails of the nanostructure are bestvisualized in the phase image (4b).

C.
a.

b.

Figure 4. AFM height (a) and phase (b) images of biaxially oriented filmof PVDF recorded at A0 = 5nm and Asp = 3.5nm. 1µm scans.
(c) Phase image showing one of the shish-kebab structures. 0.3µm scan.
(d) Sketch of the PVDF shish-kebab structure corresponding to the structure
stretched diagonally in Figure 4c.

d.

This phase image distinctively shows
not only nanofibrils oriented inorthogonal directions, but alsoso-called ”shish-kebab” nanostructures
formed in this material during stretching
and crystallization. One such structureis shown at higher magnification in(4c). A core thread of the ”shishkebab” is about 5nm in diameter andlamellar blocks attached to the thread
are in the 15-25nm range. A sketch ofa ”shish-kebab” nanostructure, which
was found earlier in TEM micrographsof a number of polymers is shownin (4d).
Another high-resolution example istaken from studies of hot rubbed filmsof PVDF (Figure 5). The phase images
distinguish oriented nanofibrils, someof them with diameters as small as
2-3nm, demonstrating that resolutionof a few nanometers can be routinely
achieved. The AFM visualization of
nm-scale structures substantiallycomplements the results of electron
microscopy studies. In addition, AFM
allows the visualization of structuralfeatures, such as sub-lamellarorganization in polymer spherulites7,
which are not accessible byelectron microscopy.

Local Mechanical Properties
The ability of the AFM to study local
mechanical (stiffness, friction) andadhesive properties has been
demonstrated by many researchgroups. In experiments using contact
mode AFM, force induced elastic
surface corrugations were found atdifferent dimensional scales.8 These
effects can be consequences ofdifferent local stiffness of atoms inlayered materials, of chain blocks inoriented polymers, or of layers withdifferent density in microlayer samples.
More recently, researchers have

Figure 5. Height (left) and phase (right)
images of hot-rubbed film of PVDF. Scan size?

intentionally elevated the tip sample
forces to cause plastic deformationand indentation of polymer materials.
Such ”nanoindentation” experiments
using AFM include not only making an
indent, but also subsequent low-force
imaging of the indent in
TappingMode.9 Traditional commercial
nanoindentors lack this capability. In
this paper we restrict the illustration of
nanoindentation to one example(Figure 6), however more detailed
examples are available in an
application note from Veeco
Instruments entitled "Nanoindentation,
Scratching and Wear Testing Using
Atomic Force Microscopy" (AN13),
available on www.veeco.com/afm.
The figure shows an array of indents
made in the multilayer PE material,
where the central 2µm strip is a layerof high-density PE surrounded by wider
layers of low-density PE. The different
size of the indents (larger indents areon broad layers) reflects differences inthe density of these layers.
More information concerning local
studies of adhesive and mechanicalproperties using so-called force curves
can be found elsewhere.10 There is
growing interest in stretching of single
macromolecules with the AFM tip inorder to study their mechanicalproperties. In such experiments, theAFM tip is first engaged into the

Figure 6. Nanoindentations of high-density
and low-density polypthylene.

sample, upon which some of themacromolecules adhere to the tip.
Therefore, as the tip is withdrawn, theadhered molecules will be stretchedbetween the sample and the tip until
they break. This sequence of eventsis recorded by the force curves withthe ultimate goal of measuring
mechanical characteristics of single
macromolecules. Please see the Veeco
Instruments brochure for the MultiMode
PicoForce AFM.

Compositional Mapping
The ability of the AFM to distinguishcomponents in heterogeneous samples
was demonstrated using a techniquecalled force modulation imaging,which is performed in the contact
mode. With the adoption of
TappingMode and phase imaging,
this capability has been much more
broadly applied because of theelimination of damaging lateral forces.

Figure 7. Dissecting a cell membrane locally by applying too high forces.

Analysis of the mechanics of phaseimaging has shown that the difference
between the phase of free-oscillatingvs. the interacting cantilever is definedby the energy dissipated in the tip
sample contact.11 Therefore, thedifferences in adhesive or mechanicalproperties of different components ofa heterogeneous sample might be
expected to influence the energy
dissipation and, therefore, phase
contrast when such a sample isexamined with TappingMode. Indeed,in AFM measurements of manypolymer blends, the high sensitivity ofphase contrast to sample composition
has been proven.2,5 Assigning thecause of phase contrast to specificdifferences in individual propertiesof components is difficult. However,
it is reasonable to suggest that withincrease of A0 and decrease of Asp
the difference in the energy dissipating
on different components will bedefined by differences in theirmechanical properties.

which become more pronouncedwith decreasing Asp (i.e. by stronger
indentation of low density PE regions
relative to high density PE regions).
At Asp = 0.95, the phase of theinteracting cantilever almost coincides
with that of the free oscillating
cantilever, indicating low-force imaging
or ”light” tapping. As mentioned
above, at light tapping, the heightprofile most closely reproduces true
surface topography. With decreasing
Asp the phase profile exhibits step-likechanges, which differentiate regions of
low and high density PE. The largest
phase difference was found at
Asp = 0.5, where the height difference
between low-density and high-density
regions was also maximal. The
imaging conditions just described canbe defined as ”hard” tapping.

It should be noted that in this latter PE
blend, elastic moduli of low density andhigh-density components differ by atleast two orders of magnitude, making
it possible to directly correlate phase
contrast with differences in mechanicalproperties. In many other cases thedifferences in the elastic properties canbe smaller, and the changes in theheight images at different Asp might beless apparent. However, even in suchcases, the phase images exhibit higher
sensitivity to materials properties andallow easy visualization of thedifferent components.
In the following sections, we present
several examples of imaging inlight tapping (A0 = 50-100nm,
Asp = 0.9 - 0.8 A0) and compositional
mapping in hard tapping(A0 = 50 -100nm, Asp= 0.5 - 0.4 A0).
The changes in height and phase
images observed in light and hardtapping are reversible, and they do
not cause permanent damage topolymer samples.

Linear Low Density
Polyethylene (LLDPE) Film
The images recorded on the surface ofa commercial film of LLDPE are shownin Figure 8. In light tapping, the heightThis is confirmed by the chart in
Figure 7, which shows height andphase profiles recorded on a blendof polyethylenes (PE) with different
densities. These profiles were
determined by imaging with
A0 = 100 nm and Asp = 0.95 and0.5. At Asp = 0.95, the height profileshows a sequence of elevated regions
Figure 8. AFM height (left) and phase (right) images recorded on linear low-density polyethylene(LLDPE). Top images were recorded in light tapping. Bottom images were recorded in hard tapping.

image reveals slightly corrugatedsample topography, and the phase
image does not exhibit well-definedfeatures. In hard tapping, the height
image is relatively unchanged;
however, the difference in the phase
images is dramatic; i.e., aggregatesof bright ”strips” appear at thelocations shown to be elevated in theheight image.12 The ”strips” of about
20nm in width can be assigned tolamellae, which are embedded inamorphous polymer (dark contrast).
Brighter contrast of lamellae —compact arrays of folded polymer
chains — is consistent with theprevious finding that more dense PE
layers exhibit brighter phase contrast
when subjected to hard tapping.

Impact-Modified Plastics
Microscopic studies of sample
composition and component
distribution at micron and sub-micronscales can be applied to understandstructure-property relationships inpolymer materials. Transmission
electron microscopy (TEM) is often
used for such studies; however,laborious preparation of ultra-thinFigure 9. Transmission electron micrograph of
impact-modfied plastic containing threecomponents. Differences in the contrast are aconsequence of chemical staining of thesample.

Figure 10. AFM height (left) and phase (right) images of impact-modified plastics containing
three components. Top images were recorded in light tapping. Bottom images were recorded
in hard tapping.

slices and chemical staining areneeded to reveal contrast relevant todifferent components in TEM
micrographs. The TEM image in
Figure 9 shows micrograph of ultra-thinsections of an impact modified plastic,a blend of two polymers and rubber
particles. The AFM study of the same
blend — which requires neither
staining nor elaborate preparation ofultra-thin slices — provides similarmorphological information.
The height and phase images
recorded on this sample at different
Aspare shown in Figure 10. As
expected, in light tapping the height
image shows sample topography, andonly minor contrast variations arefound in the phase image. The image
recorded in hard tapping, however,reveals the composition of this sample.
Three components are well
distinguished in the phase image (b),where rubbery particles of about 1nm
in size are the brightest features. The
phase contrast is strongly different for
the two major polymer components:
one (darker contrast) is seen as amatrix and the other is presented asbrighter domains within it.

Polymer Latex Systems
Polymer latex films are formed byspreading dispersions of polymer
particles on a substrate, where they
organize into close-packed arrays
during solvent evaporation. During
thermal annealing, these arrays
develop into homogeneous films,
which are widely used as paints andcoatings for textiles and paper.13
Polymer latex systems have been
intensively examined with AFM inorder to reveal their morphology,
composition, and film formation

Figure 11. AFM height (left) and phase (right) images of a two-component latex blend consistingof spheres of about 120nm diameter. Top images were recorded in light tapping. Bottom images
were recorded in hard tapping.

characteristics. Figure 11 above shows
the height and phase images of ablend of two acrylic latex components
with glass transitions (Tg) at 37ºC
(1 part) and 11ºC (3 parts). The
diameter of latex spheres of bothpolymers is about 120nm. Under lighttapping, the height image (a) of theblend exhibits a close-packed array oflatex particles, and the phase image(b) shows contrast variations whichemphasize topographic features. The
phase image recorded with hardtapping (d), distinguishes particles ofindividual components. The smaller
number of bright spheres can be
assigned to the high Tg polymer andthe surrounding dark areas to the low
Tg polymer, consistent with the
expectation that latex spheres withlower Tg are softer than those withhigher Tg. The height images (a and c)show less changes than the phase
images when the imaging conditions
were changed.

Therefore, one can expect that at small
forces, the topmost surface layer canbe observed, whereas with increased
tip sample force, underlying structures
might be detected. This possibility isillustrated by two examples next.

Triblock Copolymer Film
Poly(styrene)-b-poly(butadiene)-bpoly(styrene) triblock copolymer has
been intensively studied bytransmission electron microscopy(TEM). For this purpose ultra-thinsections of the chemically stainedmaterial are usually prepared with anultramicrotome. TEM micrographs
revealed that the topmost layer (about

10nm in thickness) at the
copolymer/air interface consists mostly
of PB blocks, which have lower
surface energy than PS blocks. These
results are confirmed by XPS data. In
bulk, PS and PB (polybutadiene) blocks
are microphase separated, leading toa morphology with curved
nanostructures of 20 - 30nm in width.
Such morphology has also been
detected in AFM images of spin-cast
PS-PB-PS films;14 however, generationof more detailed information about thismaterial requires imaging at different
tip-sample force levels.
With light tapping, the height images(Figure 12a) reflects a smooth
copolymer film topography withcorrugations in the 0-3nm range, andthe corresponding phase image (12b)shows no pronounced contrast
variations. As Asp decreases, theseimages exhibit substantial changes:
dots of diameters in the 15-20nm
range begin to appear and increasein number.
Finally, at hard tapping the height(Figures 12d) and phase images both
exhibit ”worm-like” patterns consistent
with the microphase morphology foundin TEM micrographs of the bulk
material. It has been shown thatbrighter patterns in these height andphase images can be assigned to the

Near Surface Structure
Polymer samples are often coated witha soft, thin surface layer, which candiffer in composition from underlying
materials, for example due to different
affinity of the materials to air.

Figure 12. AFM height (left) and phase (right) images of the poly(styrene)-b-poly(butadiene)bpoly(styrene) triblock copolymer film. Top images were obtained in light tapping. Bottom images
were obtained in hard tapping.

stiffer PS blocks.15 These results can be
explained as follows: At light tapping
the height images of a PS/PB/PS film
reveal the surface topography of the
butadiene-rich topmost layer. FromTEM data it has been established that
this layer is about 10 nm thick. At
ambient conditions, PB is in a rubbery
state and, therefore, can be deformedby the AFM tip. Under hard tapping,
the tip ”feels” the more rigid underlying
structure associated with themicrophase separation of PS andPB blocks.

Mesomorphic Polysiloxanes
Some polymer materials, including
liquid crystalline polymers, exist not
only in crystalline and morphousstates but also in partially ordered(mesomorphic) phases. One of thesepolymers — polydiethylsiloxane (PDES)— was recently examined with AFM.16
The height and phase images of
PDES revealed the coexistence of
mesomorphic and amorphous phases
at room temperature and allowed
monitoring of the polymer
crystallization at -7ºC. In an AFM
study of another mesomorphic polymer— cyclolinear polydimethylsiloxane
deposited on an Si substrate byrubbing — the transition from light tohard tapping revealed different
nanostructural features of the polymer(Figure 13). With light tapping, theheight image reveals the relatively
smooth morphology of the polymer
patches and the phase imageemphasizes edges of layers, especiallyat the elevated region in the center ofthe image. With hard tapping, astriped morphology becomes evident
in the phase image, with linear
elements aligned perpendicular to therubbing direction.

Similar features are barely
distinguishable in the height image.
By analogy with imaging of PS-PB-PS
tri-block copolymer, the observedchanges reflect the presence of a
topmost layer with characteristics
different from the bulk. The height ofthe central polymer patch (c) is about
8-10nm less than that in (a). This effectis caused by the tip-force induced
deformation of the sample at hardtapping, and this difference (8-10nm)
can be used as a rough estimate of
the thickness of the topmost layer. Notethat the striped morphology of the bulk
materials is similar to the lamellar
structure found in mesomorphic PDES.
Indeed, DSC (Differential Scanning
Calorimetry) measurements ofcyclolinear polydimethylsiloxane foundthat at ambient conditions this polymer
exists in a fully mesomorphic state. The
detection by AFM of a topmost layer
with differing organization relative tothe bulk was unexpected from the DSC
measurements and clearly illustrates thehigher sensitivity of phase imaging tosample heterogeneity at micron and
sub-micron scales when compared withthe sensitivity of thermal techniques.

Summary
We have demonstrated the increasing
number of AFM capabilities useful instudies of polymer materials. In theseapplications, AFM goes far beyond
high-resolution profiling by providing
local properties, maps of sample
composition and the ability to examine
underlying surface layers. Already awidely used tool in the study ofpolymers, these results illustrate
additional potential for using AFM forrecognizing the structure/property
relationships in advanced polymer
characterization for rubber,
paints/coatings, packaging,engineering plastics, consumer goods
and other applications.

Figure 13. AFM height (lett) and phase (right) images of cyclolinear polydimethylsiloxane
patches deposited on Si substrate by rubbing. Top images were recorded in light tapping.
Bottom images recorded in hard tapping.