Polymer Characterization with the Atomic Force Microscope

TLDR: The universal character of repulsive forces between the tip and the sample, which are employed for surface analysis in AFM, enables examination of even single polymer molecules without disturbance of their integrity as discussed by the authors.

Abstract: Atomic force microscopy is a powerful characterization tool for polymer science, capable of revealing surface structures with superior spatial resolution [1]. The universal character of repulsive forces between the tip and the sample, which are employed for surface analysis in AFM, enables examination of even single polymer molecules without disturbance of their integrity [2]. Being initially developed as the analogue of scanning tunneling microscopy (STM) for the high-resolution profiling of non-conducting surfaces, AFM has developed into a multifunctional technique suitable for characterization of topography, adhesion, mechanical, and other properties on scales from tens of microns to nanometers [3].

Figures

Figure 9. Progress from non-annealed gold-coated mica to the actual sample preparation and examination.

Figure 5. Schematical representation of the amplitude modulation mode. Parts of the image were reproduced with permission by C. Roduit [5].

Figure 10. Schematical depiction of the AFM tips functionalization procedures: a) functionalization of silicon-based tips and b) functionalization of gold-coated tips [45].

Figure 14. LEFT: typical force curve (black – approach curve, red – retract curve); RIGHT: typical plot of the extracted parameters as a function of measurement duration (approach labels were removed to increase plainness of the scheme). Reproduced with permission of the Royal Society of Chemistry from [57].

Figure 6. A typical force curve. When approaching the surface, the cantilever is in an equilibrium position (1) and the curve is flat. As the tip approaches the surface (2), the cantilever is pushed up to the surface – being deflected upwards, which is seen as a sharp increase in the measured force (3). Once the tip starts retracting, the deflection starts to decrease and passes its equilibrium position at (4). As we start moving away from the surface the tip snaps in due to interaction with the surface, and the cantilever is deflected downwards (5). Once the tip-sample interactions are terminated due to increased distance, the tip snaps out, and returns to its equilibrium position (6). The image was reproduced with permission by C. Roduit [5].

Figure 13. Force spectroscopy results for the measurements in two solutions with different pHs. TOP: retract force curves for amylose at two different measurement durations with two pHs, BOTTOM: retract force curves for carboxymethyl cellulose at two different measurement durations with two pHs. The results are reproduced from article [48].

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Chapter 4
© 2013 Maver et al., licensee InTech. This is an open access chapter distributed under the terms of the
Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits
unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Polymer Characterization
with the Atomic Force Microscope
U. Maver, T. Maver, Z. Peršin, M. Mozetič,
A. Vesel, M. Gaberšček and K. Stana-Kleinschek
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/51060
1. Introduction
1.1. Atomic force microscopy
Atomic force microscopy is a powerful characterization tool for polymer science, capable of
revealing surface structures with superior spatial resolution [1]. The universal character of
repulsive forces between the tip and the sample, which are employed for surface analysis in
AFM, enables examination of even single polymer molecules without disturbance of their
integrity [2]. Being initially developed as the analogue of scanning tunneling microscopy
(STM) for the high-resolution profiling of non-conducting surfaces, AFM has developed into
a multifunctional technique suitable for characterization of topography, adhesion,
mechanical, and other properties on scales from tens of microns to nanometers [3].
1.2. The technique
A schematic representation of the basic AFM setup is shown in Figure 1. Using atomic force
microscopy (AFM), a tip attached to a flexible cantilever will move across the sample surface
to measure the surface morphology on the atomic scale. The forces between the tip and the
sample are measured during scanning, by monitoring the deflection of the cantilever [1].
This force is a function of tip sample separation and the material properties of the tip and
the sample. Further interactions arising between the tip and the sample can be used to
investigate other characteristics of the sample, the tip, or the medium in-between [4].
1.2.1. Force between the sample and the tip
To understand the mechanisms behind the interacting components in multi-component
formulations, we have to take into account all the contributing forces. This is especially

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important if a quantitative analysis of the interaction is required, like in the case of
interactions between polymers and biological macromolecules [6]. The forces between
the tip and the substrate have short- and long-range contributions. When measurements
are performed, it is crucial that we can separate the contributions of various forces and
eliminate the undesired ones. This ensures the measurement of desired sample
properties only and makes further quantitative analysis possible [7]. In vacuum,
chemical forces of very short range (less than 1 nm), electrostatic, magnetic and Van der
Waals forces can be determined, while in air forces with longer range, which can be up to
100 nm, cover them, making the measurements mostly qualitative [8]. At room
conditions water moisture can condense on the tip, which is a source of capillary force.
Capillary forces are relatively big and can cover the contributions of other forces;
therefore they have to be avoided if possible. The latter is possible by measuring in
special, water free conditions, like in a N
2
or Ar atmosphere or in liquid environments.
To represent forces on the atomic level, different potentials corresponding to changes of
potential energy at various particle positions, are used. Known empirical models used to
illustrate chemical bonds are the Lennard-Jones and Morse potential [9]. These models quite
satisfactory fit the force regime curve shown in Figure 2, which represents the course of tip-
sample interaction.
Figure 1. Schematical representation of the AFM. The image was reproduced with permission of C.
Roduit [5].

Polymer Characterization with the Atomic Force Microscope
115
Figure 2. Force regimes governing the AFM measurement.
1.2.2. AFM modes for polymer examination
Many different variations of the basic AFM setup have been developed through the years of
its use. Although most of them are applicable to all types of samples, not all yield the same
amount and quality results. Proper use of these versatile measurement variations enables
one to study and understand processes even at the fundamental, namely molecular level
[10]. Considering various different samples, several modes have been developed and
adapted to cope with the demand of field specific research [11]. In the scope of the next few
paragraphs only some of the most popular will be presented.
1.2.3. Contact mode
Contact mode was the first developed mode of atomic force microscopy. In this mode, the
tip is moving across the surface and deflects according to its profile (Figure 3). Two types of
contact mode measurements are known, the constant force and the constant height mode. In
the constant force type, a feedback loop is used to move the sample or the tip up and down
and keep its deflection constant. The value of z-movement is equal to the height changes of
the sample’s surface. The result of such measurement is the information about the surface
topography. Since the tip is in constant contact with the surface, significant friction forces,
which can destroy or sweep soft samples like polymers or biological macromolecules on the
surface, appear [12].
Figure 3. Schematic representation of the contact mode. The image was reproduced with permission by
C. Roduit [5].

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116
The other type of contact mode AFM measurement is based on the constant height, while
the forces are changing. In this case, the cantilever deflection is measured directly and the
deflection force on the tip is used to calculate the distance from the surface. Since no
feedback loop is required for this type of measurement, it is appropriate for quick scans of
samples with small height differences (if height differences are big, the tip will very likely
crash into the surface, by which it gets destroyed or damages the samples’ surface). With
this type of measurements atomic resolution was achieved at low temperatures and in high
vacuum. Such measurements are often used for quick examination of fast changes in
biological structures [13].
1.2.4. Noncontact mode
In noncontact mode, the sample’s surface is investigated using big spring constant
cantilevers. The tip attached to the cantilever is hovering very close to the surface (at a
distance of approximately 5-10 nm), but never gets into contact with it, hence the name
noncontact mode (Figure 4). A major advantage of this mode is negligible friction forces,
making this mode capable for measurements of biological and polymeric samples without
alteration of their surface. The biggest drawbacks of this mode are low lateral and z-
resolution when compared to the contact mode. Recently it was used for characterization of
single polymer chains [14].
Figure 4. Schematical depiction of the non-contact AFM mode.
1.2.5.Amplitude. modulation mode or dynamic force mode
This mode is often called the intermittent-contact or tapping mode and it eliminates major
weaknesses of the noncontact mode (such as the low lateral and z-resolution). Instead of
hovering above the sample, the cantilever vibrates above the surface and moves through the
force gradient above the surface, during which it might momentarily touch the surface [15].
Due to interactions of the AFM tip with the sample surface, the amplitude of vibrations
decreases and a phase shift occurs (Figure 5). We can choose either of these parameters
(amplitude or phase shift) and keep it constant through the feedback loop by moving either
the sample or the tip in z-direction. This gives us information about the surface topography
similar to the contact mode. To measure in the amplitude modulation mode we need much
stiffer cantilevers, which exhibit the smallest possible damping factors (this factor is

Frequently asked questions

Q1. What is the way to use CFM?

Quantitative assessment of such interactions can be used for identification purposes, for determination of compatibility between different materials to be put into one single final product, and to predict interactions with the target site in drug delivery systems [40].CFM is best used with a defined experimental setup, comprising model surfaces, a controlled environment during measurements and materials of high purity. 

Q2. Why did the authors try to extend their understanding of their behavior in different environments?

Because the authors are also focused on the development of different products made of cellulose derivatives, the authors tried to extend their understanding of their behavior in different environments, to better predict and more efficiently choose the right derivate for the desired purpose. 

Q3. What are the main research fields which gained the from this technique over the past years?

The main research fields, which gained the most from this technique over the past years, are pharmaceutical technology (measurements in simulated body fluids and in vitro detection of interactions between different components in complex formulations [3]), supramolecular chemistry (real time follow up of formation of self-assembled monolayers [46]), biochemistry (simulating the binding of drugs to their targets [6]), and microbiology (measurements of interactions between materials and bacteria [47]). 

Q4. What is the way to evaluate a sample?

If their morphology has to be evaluated, the authors have to use flat surfaces, which do not temper their actual properties, measured on the nanoscale. 

Q5. What was the main interest of the study?

Their main interest in this field was the evaluation of materials performance after different exposure times in simulated physiological environments. 

Q6. What is the role of AFM in the development of medical implants?

The mechanisms behind appearing interactions between surfaces are of utter importance for many research areas, ranging from the development of polymers for protective films to preparation of implants for medical use [52]. 

Q7. What is the importance of examining chemical functional groups?

Mapping chemical functional groups and examining their interactions with different materials is of significant importance for problems ranging from lubrication and adhesion, to the recognition of biological systems and pharmacy [22]. 

Q8. What is the way to measure the distance from the surface of a sample?

Since no feedback loop is required for this type of measurement, it is appropriate for quick scans of samples with small height differences (if height differences are big, the tip will very likely crash into the surface, by which it gets destroyed or damages the samples’ surface). 

Q9. What is the effect of the noncontact mode on the sample?

Due to interactions of the AFM tip with the sample surface, the amplitude of vibrations decreases and a phase shift occurs (Figure 5). 

Q10. How was the preparation of the sample?

Prior to any preparation steps, all used laboratory accessories were cleaned in a multi-step procedure, combining different chemicals, to assure extreme cleanliness. 

Q11. What is the goal of the study?

Their goal is to define methods, which enable effective correlation of easy obtainable laboratory data with final products characteristics even in the development stages. 

Q12. What is the importance of the latter?

The latter is especially important in testing of polymeric materials for medical use, where the bond type between material and tissue is of high importance for the actual outcome of the healing process. 

Q13. What did the researchers find in the results of the AFM tips?

All used procedures resulted in mainly decorating the edge of tips, leaving the surroundings almost as clean as before the functionalization. 

Q14. What are the different potentials used to represent forces on the atomic level?

To represent forces on the atomic level, different potentials corresponding to changes of potential energy at various particle positions, are used. 

Q15. What is the main difference between the AFM and STM?

The universal character of repulsive forces between the tip and the sample, which are employed for surface analysis in AFM, enables examination of even single polymer molecules without disturbance of their integrity [2]. 

Q16. What is the procedure of an AFM force measurement?

The procedure of an AFM force measurement is schematically depicted in Figure 6 and goes as follows: the tip attached to a cantilever spring is moved towards the sample in a normal direction, during this movement the vertical position of the tip and the deflection of the cantilever are recorded and converted to force-versus-distance curves, briefly called force curves [1].