Characterizing and Mapping Polymers and Thin Films Using NanoIR Spectroscopy

IR spectroscopy is one of the most popular analytical measurement methods used for polymeric material characterization. However, this is mostly resticted to macroscopic analysis as Abbe diffraction laws limit the spatial resolution of traditional bulk IR spectroscopy to between 3 and 10 µm, based on the type of method employed.

PS/PMMA blend chemical contrast image using Tapping AFM-IR™

Atomic Force Microscopy (AFM) is a widely-used nanoscale imaging method, which provides a topographic map of a surface sample high spatial resolution. Until recently, the inability to chemically characterize the material beneath the tip has been the major disadvantage of using AFM. However, this barrier can be overcome by using an AFM cantilever as a detection point for incident IR radiation.

The result is an AFM-IR technique that overcomes the diffraction limit of traditional IR spectroscopy by orders of magnitude, while still possessing the high-resolution imaging capabilities of AFM.¹

This article discusses the application of AFM-IR to perform nanoscale chemical characterization on a range of polymeric samples.

Characterization of Multilayer Films²

Multilayer films play a key role in many products, especially in packaging materials. Since decreasing the thickness of multilayer layers increases the number of individual layers, characterization is necessary.

FT-IR spectroscopy is a widely-used characterization technique for cross sections of multilayer films, providing data on the chemical nature of the individual polymers layers as long as their thickness is at least a few micrometers. Earlier, it was required to delaminate the films for analysis of individual layers that are thinner than the diffraction limit of conventional FT-IR. These limitations can be overcome using AFM-IR, which is capable of providing true nanoscale chemical characterization.

To reverse engineer multilayer films, microtome is used to cross section the samples and the cross sectioned samples are then placed on IR transparent substrate (ZnS) for subsequent analysis (Figure 1a). The resulting AFM-IR spectra are represented by the respectively colored markers. Here, the film contents were successfully identified as polyethylene and polyamide (Figure 1b and 1c).

Figure 1. (a). AFM height image showing cross sectioned multilayer film. AFM-IR spectra at 2800-3600 cm^-1 (b) and 1200-1800 cm^-1 (c); spectra collected from layers C, E, and G are consistent with a polyamide. The IR spectrum recorded from layer D, which can be considered as the barrier layer in the film, is consistent with polyethylene-co-(vinyl alcohol) (EVOH).

Each layer within multilayer films performs a specific function, such as moisture or oxygen barrier layers. Incompatibility is prevalent among these layer materials and therefore, it is necessary to use “tie” layers to bind these layer materials together during film formation. Until now, characterizing these tie layers using FT-IR spectroscopy has been unfeasible due to their thickness of less than 500 nm.

One example is the relationship between polyethylene (PE) and polyamide (PA). Due to their incompatibility within films, they require a tie layer to lie between them.

An AFM height image and AFM-IR spectra from the boundary region between the PE and PA layer of a cling film cross section are shown in Figure 2. The nine colored marker locations on the AFM image correspond with the AFM-IR spectra of the same color (100 nm spacing).

Figure 2. AFM height image and AFM-IR spectra across PE/PA layer interface.

The spectra obtained within 200 nm of the boundary consist of significantly sharper CH₂-stretching bands than the regions of the PE further away from the boundary. Moreover, the center-of-mass peak wavenumber of the CH₂ antisymmetric stretching band also shifts to lower wavenumber (2916 cm^-1) at the boundary, indicating the presence of more ordered hydrocarbon and less branched chains. As a result, accurate chemical characterization of the tie layers between polymers is now possible.

Measuring Monolayers³

Self-assembled monolayers (SAMs) of 4-nitrothiophenol (NTP) and a monolayer island sample of poly(ethylene glycol) methyl ether thiol (PEG) were deposited on template-stripped gold substrates. Using AFM topography measurements, the thickness of the NTP monolayer films was confirmed to be less than 1 nm (Figure 3a).

AFM-IR spectra and molecular structures of NTP SAMs on gold (in blue) are shown in Figure 3. Each AFM-IR spectrum originates from an approximate sample surface area of 25 nm × 25 nm, limited only by the contact area of the AFM probe with the sample.

Figure 3c shows the corresponding IR reflection absorption spectra acquired over a substantially larger area of NTP SAMs in red.

A strong NTP absorption peak around 1339 cm^-1 is associated with the symmetric NO₂ stretching mode, while the weaker absorption band around 1175 cm^-1 correlates to an aromatic CH-bending mode. Figure 3c shows an array of AFM-IR spectra recorded across a gap in the monolayer. For these measurements, the spatial resolution achieved was ~20 nm.

Figure 3. (a) AFM deflection image showing SAM’s of NTP on gold substrate. (b) Comparison of AFM-IR spectrum (blue) and ATR spectrum (red) of NTP SAM’s. (c) Array of AFM-IR spectra collected across red line shown in (a) demonstrating spatial resolutions of ~20 nm.

The AFM topography image is shown in the top left of Figure 4 and an IR absorption image with the IR laser tuned to the fixed wavenumber of 1340 cm^-1 of a monolayer island film of PEG on gold is shown in the top right of Figure 4.

From the AFM image, the thickness of the PEG islands is found to be 4 nm. The IR absorption band at 1340 cm^-1 correlates to a CH₂-wagging mode and the location of the PEG island regions is confirmed by the image. PEG monolayer island regions as small as 25 nm × 25 nm are easily resolved in the IR absorption image.

The broad IR band centered at 1102 cm^-1 is assigned to the C-O-C antisymmetric stretching mode.

Figure 4. AFM topography image (top left), and IR absorption image at 1340 cm^-1 (top right) of a monolayer island film of PEG on gold. An AFM-IR spectrum of one of the PEG islands is shown below.

Quantifying Chemical Content in Nanoscale Polymer Domains⁴

Desirable properties such as processability, heat resistance and tensile strength makes polypropylene (PP) one of the most important and widely used polymers. However, the poor impact resistance of this polymer especially at low temperatures limits its widespread adoption.

To address this problem, a new copolymerization method had been developed, which improves the impact resistance and overall performance of PP by blending of other polymers with it.

Figure 5 shows the structure of these high impact polypropylene (HIPP) materials. Three distinct regions are present in these materials: the matrix, intermediate layer, and core. It is necessary to have the ability to investigate the chemical composition of each region to refine the performance of HIPP.

Figure 5. Schematic diagram and AFM phase image showing different regions within high impact polypropylene sample.

In general, IR spectroscopy is a powerful technique to evaluate chemical composition, but domains within HIPP are not large enough to be analyzed by traditional FTIR. Based on the strong relationship between traditional FTIR and AFM-IR, FTIR was used to perform a calibration from standards with known material composition, in order to correlate the ethylene content in the copolymers with the peak area ratio of the CH₂ and CH₃ bending bands at ~ 1456 cm^-1 and ~ 1378 cm^-1, respectively.

A zoomed in AFM image of the nanoscale domains within HIPP is shown in Figure 6, with markers on the image corresponding to the location where AFM-IR spectra were recorded from each region within the material, the core (red), intermediate layer (blue) and matrix (black).

Figure 6. (a) AFM height image of nanoscale core domains and (b) AFM-IR image of domains at 1378 cm^-1.

Normalization of the collected spectra shown in Figure 7 to 1378 cm^-1 allowed the comparison of the AFM-IR peak ratio data to the FTIR calibration curve (See Table 1 in the application note).

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The comparison of the peak ratios to the calibration curve generated by FTIR allows determining the chemical content of each region (See Table 2 in the application note).

Figure 7. AFM-IR spectra acquired from each region within the HIPP. Data has been normalized to 1378 cm^-1.

Conclusions

AFM-IR is a powerful characterization technique for a range of polymer materials.

The combination of the capabilities of AFM and IR spectroscopy enables the nanoIR2-FS system to collect topographical images of the sample surface and characterize the chemical composition of these materials with a spatial resolution of <20 nm.

This article has discussed the characterization or reverse engineering of polymer multilayer films using the AFM-IR technique. This breakthrough technique has also allowed the characterization of the tie material between film layers for the first time. Furthermore, polymer crystallinity and chemical content have also been quantified at a high spatial resolution.

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References

1. Dazzi, A.; Prazeres, R.; Glotin, F.; Ortega, J.M.; Opt. Lett. 2005, 30, 2388-2390.

2. Kelchtermans, M; Lo, M; Dillon, E; Kjoller, K; Marcott, C; Vib. Spec.2016, 82, 10-15.

3. Gong, L; Chase, B; Noda, I; Liu, J; Martin, D; Ni, C; Rabolt, J; Marcomolecules. 48, 6197 – 6205.

4. Tang, F.; Bao, P.; Su, Z., Analytical Chemistry 2016, 88 (9), 4926-4930.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces. For more information on this source, please visit Bruker Nano Surfaces.