The AFM-IR Instrument
Analysis of the Rubbery Polymer
About Anasys Instruments
The rubber products used commercially is mostly composed of natural rubber that has been vulcanized (heat treated with sulfur) to provide its resilient physical and impact resistant properties at a wide range of temperatures.
Rubber is often tailored for specific applications, and its structure is highly complicated. Material properties are governed by tightly controlling the distribution of fillers and/or the extent of phase separation in the sub-micrometer regime.
Infrared (IR) spectroscopy is a most frequently practiced analytical measurement techniques in industrial and academic research laboratories. Diffraction physics restricts its spatial resolution to a few microns to 10's of microns, depending on the optics involved and the wavelength of the radiation.
The AFM-IR technique overcomes this major limitation. The enhancement of spatial resolution is obtained by using a microfabricated tip as an IR absorbance detector in an atomic force microscope (AFM).
Anasys Instruments nanoIR - AFM-IR Platform
The nanoIR from Anasys Instruments has integrated nanoscale thermal property mapping as well as AFM-IR capabilities, resulting in a multifunctional tool that provides nanoscale structure, chemical, mechanical and thermal properties in a single platform.
The nanoIR platform, shown in Figure 1, is a patented technology based on photothermal induced resonance (PTIR), which is a natural phenomenon first detected with an AFM tip by Dazzi et. al.
Figure 1. The nanoIR™ instrument
Figure 2. Close up view of the prism and AFM measurement head
The nanoIR AFM-IR system uses a pulsed, tunable IR source to excite molecular vibrations in a sample that has been mounted on an IR transparent (ZnSe) prism. This causes an illumination configuration similar to attenuated-total-reflectance infrared (ATR FT-IR) spectroscopy.
Proprietary technology is used to design the system’s IR source, which is continuously tunable from 900 to 3600 cm-1 covering a broad range of the mid-IR spectrum.
The sample heats up as it absorbs radiation leading to rapid thermal expansion that excites resonant oscillations of the cantilever. The induced oscillations decay in a characteristic ringdown as shown in Figure 3.
Figure 3. Schematic of the AFM-IR technique
The AFM-IR instrument can quickly survey regions of a sample via AFM imaging and then rapidly acquire high resolution chemical spectra at selected regions on the sample. As shown in Figure 4,polymer spectra acquired with the AFM-IR system have demonstrated good correlation with bulk Fourier transform infrared (FT-IR) spectra.
The operator can import an individual AFM-IR spectrum into commercial IR databases where they can be digitally searched in an attempt to chemically identify the materials at specific sample locations.
Figure 4. A comparison of the spectrum generated by AFM-IR (red) and FT-IR (blue) of a polystyrene sample
Analysis of the Rubber Polymer
Chemical information is readily obtained below the diffraction limit. First, an AFM image is collected to locate regions of analytical interest as demonstrated in Figure 5.
Two phase-separated domains are identified, the light-colored regions has a smooth complexion while the darker regions appear rough. Such a rough pattern is observed in an AFM image is collected on soft and rubbery domains while the tip is in direct contact with the sample surface.
This AFM tip may exhibit a stick-slip mechanical behavior as it scans over the region in contact mode, leaving a characteristic pattern in the topography image. Then the AFM tip rests at locations where the corresponding AFM-IR spectra are recorded.
Very distinctive IR spectra are collected in spectral ranges of 1200 to 1800 cm-1 as shown in Figure 5 and 2800 to 3400 cm-1 as shown in Figure 6 from each type of surface topology. The natural rubber region is significantly different than the smooth nylon counterparts.
Using this AFM-IR technique, the chemical difference can be directly linked to their respective mechanical make-up in a simple-to-understand manner.
Figure 5. Point-and-click spectral acquisition over a large area of a thin microtomed section of a rubbery polymer blend on a ZnSe prism
Figure 6. AFM-IR spectra collected in the high wavenumber range at locations as indicated in the above AFM image
Distribution of the two phases is mapped based on their specific absorption bands using the AFM-IR imaging capability. After tuning the IR laser to 3300 cm-1 and subsequently to 2956 cm-1, ring-down amplitudes of the scanning AFM tip are recorded simultaneously with topographic data. In each image of Figure 7, there are 512 pixels in x-direction and 64 lines in y-direction.
This leads to approximately 20nm per x-pixel and 230nm per y-pixel. A majority of the sampled region absorbs at 2956 cm-1 as strong ringdown amplitudes are detected by the AFM tip across most of the area (light colored regions in Figure 7D).
However, the AFM tip could only detect comprehensible ring-downs at specific regions (Figure 7C). Upon ratioeing the ringdown amplitudes in the images of 3300 cm-1 to 2956 cm-1, sharp interfaces within one or two pixels can be seen in Fig. 7B with the exception of the circled region.
This calculation suggests that each phase consists of the entire vertical volume of the thin microtomed sample for the most part of this sampled region. The circled region would likely have a substantial amount of nylon beneath the rubbery surface. Such sensitivity is crucial for identifying hidden domains
Figure 7. A set of AFM and AFM-IR images; A) AFM image; B) AFM-IR image intensity ratio of 3300 cm-1 to 2956 cm-1 ; C) AFM-IR image at 3300 cm-1; and D) AFM-IR image at 2956 cm-1
The results obtained show that the AFM-IR technique is capable in obtaining chemical information at below the diffraction limit. By coupling AFM and AFM-IR, good correlation between topographical, chemical and mechanical observations can be made simultaneously about the sample.
It is possible to positively identify sub-micron wide domains as natural rubber and nylon by AFM-IR spectra and mapped with AFM-IR imaging. This combined analysis brings light to identifying relevant chemistries in a complex polymer blend.
About Anasys Instruments
Anasys Instruments provides innovative AFM and related accessories which offer chemical, mechanical, and thermal analysis at the sub-100nm scale. The Company’s technology and products are being used to address metrology and analysis challenges in the polymers, pharmaceuticals, data-storage, and advanced-materials markets.
Anasys has been awarded numerous awards which establish Anasys as leaders in innovative technology, including the inaugural MICRO/NANO 25 Award in 2007, the R&D 100 Award in 2010 for the nanoIR and Microscopy Today’s 2011 Innovation Award for their breakthrough AFM-IR platform.
This information has been sourced, reviewed and adapted from materials provided by Anasys Instruments.
For more information on this source, please visit Anasys Instruments.