Topics Covered
Background
Introduction
Nanoscale Material Properties of
Two Rubber Compounds
Topography and Pulsed Force Curves
of the Polymer Thin Films
Calculating the Differences in Adhesion
Forces of the Two Polymer Blends
Calculating the Stiffness of the
Two Polymer Blends
Background
WITec is a
manufacturer of high-performance instrumentation for scientific and
industrial applications focused on new solutions for Optical and Scanning
Probe Microscopy.
Introduction
The Digital Pulsed
Force Mode (DPFM) provides new perspectives for materials research
on the nanometer scale. Its ability to store the full tip-sample interaction
during an AFM imaging process allows the user to record, along with
the topography, mechanical and energetic properties.
Nevertheless, it is often difficult
to extract real physical properties from AFM measurements, due to
the continuous changing of the cantilever tip shape, changes in the
environment during a measurement, etc. All these uncontrollable parameters
can be compensated, if a reference is included in the imaging area
for material characterization.
Nanoscale
Material Properties of Two Rubber Compounds
In this study, material properties
on the nanometer scale of two rubber compounds are presented:
SBR (Styrene - Butadiene - Rubber) is a statistical copolymer with rubber properties.
It contains 30% Styrene, with a bulk elasticity modulus of 4.1 MPa, and 70%
Butadiene, with a bulk elasticity modulus of 0.25 Mpa.
SBS (Styrene - Butadiene - Styrene) is a block-copolymer, known as thermoplastic
rubber. It has the same percentage of Styrene and Butadiene as SBR, the difference
lies in the succession of the various chemical groups.
Both rubber compounds were studied
as thin films of polymer blends in combination with PMMA (Poly-Methyl-Met-Acrylate).The
thin films were produced by the spin-coating of polymer solutions
on glass substrates.
Topography and Pulsed Force Curves of the Polymer Thin Films
The images in Fig. 1 show the topography of thin films of SBR-PMMA and SBS-PMMA
on glass substrates. Confocal Raman Microscopy measurements have shown that
the higher topographical features correspond in both images to PMMA. Typical
Pulsed Force curves obtained from PMMA and SBR in the SBR-PMMA blend are shown
in Fig. 2 (top) and from PMMA and SBS in the SBS-PMMA blend in Fig. 2 (bottom).
Figure 1. Topography
of polymer blends: Top: SBR-PMMA (image size 7x7x0.03 µm3 ) Bottom:
SBS-PMMA (image size 10x10x0.08 µm3).
Figure
2. Typical Pulsed Force Curves obtained
from the two polymer films.
The adhesion maps recorded simultaneously with the topography images shown
in Fig. 1 are represented in Fig. 3.
Figure
3. Adhesion maps of the thin film of SBR-PMMA
(left) and SBS-PMMA (right).
Calculating the Differences in Adhesion Forces of the Two Polymer Blends
The difference in adhesion forces
of the two polymer blends was calculated as shown below:
Δ A1 = ASBR
- APMMA = kS(Vad(SBR) - Vad(PMMA))
= 10 ± 3 nN
(1)
Δ A2 = ASBS - APMMA = kS(Vad(SBS)
- Vad(PMMA)) = 11 ± 3 nN
(2)
with cantilever spring constant k = 2.8 N/m, sensitivity S = 200 nm/V, and
Vad(SBS) , Vad(SBS), Vad(PMMA) the measured
voltages on the adhesion outputs of the DPFM electronics (Peter Spizig, PhD
Thesis, Univ. Ulm 2002). These results do not show clear evidence for two different
polymers blended with PMMA.
Figure
4. Stiffness maps of the thin film of
SBR-PMMA (left) and SBS-PMMA (right).
Calculating the Stiffness of the Two Polymer Blends
The stiffness maps for the two thin
films are shown in Fig. 4. The difference in stiffness of the two
polymer blends was calculated as shown below:
Δ S1 = SPMMA - SSBR = kS(Vstiff(PMMA)
- Vstiff(SBR)) / Δ z = 2.5 ± 0.3 N/m
(3)
Δ S2 = SPMMA - SSBS = kS(Vstiff(PMMA)
- Vstiff(SBS)) / Δ z = 0.6 ± 0.3 N/m
(4)
with z representing the tip penetration
depth, Vstiff(PMMA) , Vstiff(SBR), and Vstiff(SBS)
the measured voltage on the stiffness output of the DPFM electronics
(Peter Spizig, PhD Thesis, Univ. Ulm 2002).
Due
to the fact that both thin films contain the reference PMMA, the stiffness
difference calculated in equation (3) and (4) is related to different
stiffness properties of the SBR and SBS. The stiffness ratio: S /
S = 4 indicates, that SBR is a factor of 4 softer then SBS.
Source: Digital Pulsed Force Mode Polymer Differentiation by
Witec
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