Paper Technology International 2020 - Journal - Page 81

PAPERTECHNOLOGYINTERNATIONAL
Mechanical characterization of single cellulosic fibers. Julia Auernhammer, Robert W. Stark,
Technical University of Darmstadt, Institute of Materials Science, Physics of Surfaces
INTRODUCTION:
An excellent tool to study and characterise the physical properties of solid samples is the atomic force microscope (AFM). The AFM
is not only useful for imaging the topography, but also to quantify the mechanical properties. One operation mode of the AFM is the quasistatic “PeakForce-Tapping mode”(B. Pittenger 2012), which provides access to mechanical properties such as elastic modulus, adhesion,
dissipation and deformation of the sample at the nanoscale. Furthermore, from mapping with static force vs. distance curves it is possible
to reconstruct the topography or mechanical properties such as the Young’s modulus and the maximum indentation depth into the sample.
Thus, a three-dimensional nanomechanical characterisation can be obtained. AFM measurements are not only limited to the use in dry
conditions, also measurements in liquid can be accomplished at high resolution.
Paper, as a high-tech material made from cellulose,
has promising applications in areas such as electronics, sensor
technology, microfluidics and medicine (Bump et al. 2015; Delaney
et al. 2011; Gurnagul and Page 1989; Hayes and Feenstra 2003;
Liana et al. 2012; Ruettiger et al. 2016). Cellulose is a natural
material, is abundant and renewable, and is the most important
raw material in the papermaking industry. But, the use of paper
as substrate material in technical applications is raising some
challenges. For example, the production from natural fibres or the
loss of the mechanical stability of fibres in wet state.
As cellulose has a hygroscopic character and is insoluble
in water, the fibres begin to swell, become more flexible and thus,
the strength of the paper sheet is decreasing (Cabrera et al. 2011;
Dunlop-Jones 1996; Fidale et al. 2008; Gumuskaya et al. 2003;
Janko et al. 2015; John and Thomas 2008; Lindman et al. 2010;
Rogovin and Gal’brajch 1983). Hence, the understanding of the loss
of mechanical properties in wet state has to be tackled. As paper is
consisting of single fibres, the knowledge of the loss of mechanical
stability has to be built from bottom up. Therefore, this work is
focussing on single fibres and their mechanical behaviour at dry
state and increasing relative humidity (RH).
Furthermore, the challenge to improve the wet-strength has
to be tackled. So far, Polyamidoarminepichlorohydrin (PAAE) resins
are the most commonly used wet strength agents (H. Pahl 1988;
Yano et al. 1991). In addition to all the advantages of increasing
strength in dry and wet conditions, PAAE resins in particular have
some disadvantages e.g. reusability is greatly reduced, or halogen
by-products can cause environmental pollution. Thus, alternative
polymers have to be investigated to increase the wet-strength of
paper.
1. The Atomic Force Microscope (AFM)
The atomic force microscope (AFM) was firstly presented
in 1968 by Binning, G. et al. [67]. It belongs to the category of the
scanning probe microscopes (SPM). In general, a SPM consist of
a sharp tip that scans the sample line by line while the interactions
between the surface and the tip are detected and a feedback system
that controls the scanning process.
The set-up of an atomic force microscope in figure 1 shows
a laser diode, a sample, a cantilever with a sharp tip, a foursegmented photodiode, a feedback system, and a piezo element for
an x-, y-, z-movement of the sample and the cantilever. The working
principle behind the AFM is to detect the interactions between the
tip and the surface atoms of the sample. For this purpose,
the laser beam is focused on the reflectively coated back
of the cantilever, which is directed onto the four-segmented
photodiode. In the rest position, the laser beam hits the
centre of the photodiode, which induces the same proportion
of current in each segment due to the photoelectric effect.
When interactions between tip and surface are detected, the
cantilever bends and the laser beam changes its position on
the photodiode. Therefore, a different proportion of current is
now induced in each segment. The difference in the induced
current is detected. The difference between the upper and
lower segment is an indicator of the vertical bending of the
cantilever and is used to record the topography. The difference
between left and right is described as an indicator of the
lateral forces acting on the tip. The evaluated signal from the
photodiode is processed further in the feedback system.
Figure 1: Schematic set up of an Atomic Force Microscope (AFM).
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