Table of ContentsIntroductionnanoIR Platform The nanoIR System Setup Measurement Methods Features of the nanoIR SystemMeasurements of Polymeric Samples Requirements for Sample Preparation Multilayer Films Polymer Blends Polystyrene-Epoxy Composite Degradable PolymersConclusionAbout Anasys Instruments
Infrared (IR) spectroscopy is commonly used for analytical measurements in industrial and academic R&D laboratories. The spatial resolution has been limited to ~5 microns. To overcome this major limitation, Anasys Instruments has collaborated with the University of Paris-Sud, Stanford University and the University of Illinois at Urbana-Champaign, as well as with The Dow Chemical Company to develop the nanoIR. The spatial resolution breakthrough is obtained through a novel method that uses a nanoscale probe from an atomic force microscope (AFM) that acts as the IR absorbance detector. Based on the nature of the IR absorbance detection, simultaneous measurements of nanoscale mechanical properties and nanoscale morphology, along with chemical composition can be conducted. The nanoIR also has an integrated nanoscale thermal property mapping capability resulting in a multifunctional tool that provides nanoscale structure, chemical, mechanical and thermal properties.
Award-winning researcher, Dr. Alexandre Dazzi from the Laboratoire de Chimie Physique, CLIO, Université Paris-Sud, Orsay, France pioneered a patent-pending technology based on photothermal induced resonance (PTIR) which lies at the basis of the design of the nanoIR platform as shown in Figure 1.
Figure 1. The nanoIR platform
Figure 2. Close up view of the prism and AFM measurement head
The nanoIR System Setup
The nanoIR system uses a pulsed, adjustable IR source to induce molecular vibrations in a sample mounted on an IR-transparent prism. An illumination configuration is created which is similar to previous attenuated-total-reflectance (ATR) spectroscopy. The system’s IR source is designed using the company’s own technology and is adjustable continuously between 1200 to 3600 cm-1 covering a broad range of the mid-IR spectrum. The absorbance of radiation results in sample heating that leads to quick thermal expansion that activates resonant oscillations of the cantilever. The induced oscillations result in a characteristic ringdown as shown in Figure 3.
Figure 3. Schematic showing the technique behind the nanoIR
Fourier techniques are used to analyze the ringdown to enable extraction of the frequencies and amplitudes. The cantilever oscillation amplitudes are measured as a function of the source wavelength and local absorption spectra are created. The oscillation frequencies of the ringdown are related to the mechanical stiffness of the sample. It is possible to rapidly survey sample regions using AFM and then acquire high-resolution chemical spectra at specific regions on the sample. Polymer spectra acquired with the nanoIR system show good correlation with bulk Fourier transform infrared (FT-IR) spectra as shown in Figure 4
Figure 4. A comparison of the spectrum generated by the nanoIR (red) and conventional FT-IR (blue) of a polystyrene sample.
Individual nanoIR spectra can be imported into commercial IR databases where they can be digitally searched so as to chemically identify the materials at the specific measured sample locations. Optionally, the IR source can be made into to a single wavelength to map compositional variations across the sample surface.
Features of the nanoIR System
The salient features of the nanoIR system are listed below:
- The nanoIR system provides data on the mechanical characteristics of the sample by monitoring the frequency of the basic or higher resonant modes of the cantilever.
- The contact resonant frequency of the cantilever correlates to the stiffness of the sample and can be used to map the modulus of the sample qualitatively.
- The nanoIR platform can also perform nanoscale thermal analysis utilizing novel AFM cantilevers that deploy a resistive heating element at the cantilever tip.
- The combination of cantilevers with the system results in local measurement of the transition temperature of materials at a single point or at multiple points across the sample.
- Detection or mapping of the extent of cure, amorphous/crystalline content, stress, or other material properties is determined by the transition temperature of the material.
- Integration of measurement capabilities results in a multifunctional tool, which provides chemical, mechanical, thermal properties and a nanoscale structure.
Measurements of Polymeric Samples
The nanoIR technique is perfect for measurement of polymeric samples in which there are local material variations. This includes materials such as polymer blends, multilayer films, nanocomposites and micro and nanoscale defects in materials.
Requirements for Sample Preparation
The preliminary requirements for sample preparation are listed below:
- The sample should be a thin film and needs to be deposited on the surface of the prism
- Ultramicrotomy is used to cut sections with thickness from 100nm to 1000nm
- Sections are transferred to the prism surface or can be deposited out of solution by either spin-coating or drop-casting.
A multilayer film example is shown in Figure 5 and it demonstrates the multifunctional measurement capability of the nanoIR system. The film has a central nylon layer sandwiched between two ethylene acrylic acetate (EAA) layers.
Figure 5. Illustration of the multi-functional ability of nanoIR on a multilayer film of EAA-Nylon-EAA
Figure 5A shows the topographic image of the surface of the sample created by embedding and microtoming the film. Figure 5B shows the array of spectra collected across the sample surface. Figure 5C shows the direct correlation between mechanical stiffness and chemical composition data. Figure D shows nanothermal analysis on the sample identifying softening at different temperatures for EAA and nylon layers.
The usage of the chemical identification capability in the nanoIR to identify domains in a blend is shown by the example demonstrated in Figure 6. A polycarbonate - poly(methyl methacrylate) (PC-PMMA) blend sample is used, which shows domain structure at the micron and submicron scale. The domain structure helps in differentiating the components viewed in the AFM image with one material showing smooth domains after having been microtomed and the other a rough surface. These domains can then be identified as either PC or PMMA based on the strength of the characteristic PC absorptions at 1770 and 1496 cm-1. Six spectra were observed across an interface between the two components with a separation of 100 nm. There is a significant change in spectra between the two components at that spatial resolution.
Figure 6. PC-PMMA blend: 4 x 6 micron AFM image (bottom) and spectra (top) corresponding to 6 pts spaced 100 nm apart
An AFM image with spatially resolved IR absorption spectra observed on a thin section of a model composite of polystyrene (PS) and epoxy is shown in Figure 7.It is important to understand that the IR spectrum at the center of the PS circular domain is an excellent match with spectra recorded at 100 nm of the PS-epoxy boundary. Spectra on the lower left and right of Fig 7 collected between 2500 cm-1 and 3700 cm-1 within 100 nm of the PS-epoxy boundary show negligible evidence of the polystyrene aromatic CH-stretching absorption bands above 3000 cm-1.
Figure 7. An AFM image and the spectra of a polystyrene-epoxy composite sample
Figure 8. Spectral mapping of a degradable polymer blend
AFM measurements allow mapping of the structure of the polymer matrix and their additives. The nanoIR can then spatially map variations in chemical components. In the line spectral map shown in Figure 8 the spatially varying intensities of the C=O carbonyl band (1740 cm-1) and the single bond C-O peak at around 1100 cm-1 are recorded. This is an indication of the location of both the components in this material.
The nanoIR system enables IR Spectroscopy with 100 nm spatial resolution. It also provides high resolution topographic, mechanical, chemical, and thermal mapping. Applications in polymer blends and multilayer films have been shown and applications have been demonstrated applications in a range of other materials from photovoltaics to sub-cellular spectroscopy.
About Anasys Instruments
Anasys Instruments Corporation is the pioneer in the field of sub-100nm thermal property information. 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. In 2007, Anasys was named as winner of two prestigious industry awards, the R&D 100 Award and the inaugural MICRO/NANO 25 Award, both of which recognise Anasys as leaders in innovative technology.
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