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.
This information has been sourced, reviewed and adapted from materials provided by WITec GmbH.
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