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Photo-induced Force Microscopy (PiFM)  combines infrared (IR) absorption spectroscopy and atomic force microscopy (AFM) to achieve nanoscale chemical analysis via localized IR absorption spectra and mapping of heterogeneous materials on the surface of a sample (with sub-10nm spatial resolution).
The exceptional spatial resolution is due to the tip-enhanced near-field profile, which extends only ~20 nm into the sample surface. However, there is a PiFM configuration where the technique becomes more bulk sensitive (sampling down to ~1 mm), which when combined with the surface-sensitive PiFM mode, allows a nanoscale “3-dimensional” analysis of a heterogeneous material system.
In this note, we will demonstrate the elegance and utility of the technique by analyzing amphiphilic siloxane-polyurethane (AmSi-PU) coatings, which have shown excellent fouling release properties due to the lateral phase separation and vertical stratification of the different constituent polymers .
Amphiphilic surfaces with mixed hydrophobic and hydrophilic character are candidates for durable marine coatings that minimize adhesion of marine organisms. In this study, we will analyze two amphiphilic siloxane-polyurethane (AmSiPU) fouling release formulations via PiFM. The two formulations were derived from two polyisocyanate prepolymers with different types of polyethylene glycol (PEG) reacted with polydimethyl siloxane (PDMS) and an isophorone diisocyanate (IPDI) trimer. Sample #9 and #12 used PEG-550 and PEG-750 respectively; the sample designation is the same as used in reference 2.
Figure 1 shows the topography and PiFM spectra acquired at two different locations in surface-sensitive and bulk PiFM modes on both samples (FTIR spectra are also presented for comparison). Looking at the surface-sensitive spectra for sample #9, the peaks associated with PDMS are observed at both locations. However, we don’t see any evidence of PU in either (lack of the representative peak at 1730 cm−1) even though the laser power was increased for wavenumbers greater than 1331 cm−1. On the other hand, the bulk-sensitive spectra exhibit stronger PU (PDMS) absorption peaks in the matrix (circular island). Therefore, on sample #9, there is a top layer of PDMS (thicker than ~20 nm since PU peak is not observed) covering the whole surface. Underneath the PDMS layer, there is a phase separation into PU-rich matrix and PDMS-rich island regions. Note that the FTIR spectrum is a combined representation of the PiFM spectra from all four regions (from two laterally spaced and two vertically spaced regions).
For sample #12, there is less stratification along the z-direction since PU absorption peaks are also visible in the surface-sensitive spectra. Like sample #9, there is a phase separation into PU-rich matrix and PDMS-rich island regions; however, unlike sample #9, this phase separation is seen throughout the sample depth, including the surface. To better analyze the distribution of pure components, we took a hyPIR (hyperspectral PiFM IR) image where PiFM spectrum is acquired at each pixel of the image (256 x 256 pixels, consisting of ~65,000 spectra). We then performed principal component analysis (PCA) followed by multivariate curve resolution (MCR) to produce pure component maps and spectra. The results are shown in Figure 2.
On sample #9, the spectrum for pure component 3 (green in the composite image) matches very well with the PDMS FTIR spectrum. Similarly, the spectrum for component 1 (blue in the image) and PU FTIR spectrum agree quite well. The spectrum for component 0 (red in the image) contains features from PEG and PDMS primarily with minor contribution from PU. The color image shows the relative distribution of these components. For sample #12, PCA-MCR analysis revealed only two major pure components, component 2 with similar spectral features to PU and component 0 with mostly PDMS and minor PU and PEG (based on the strength of the peak at 1100 cm−1) contributions. The composite image clearly shows the simpler phase separation compared to sample #9. For sample #12 where we observed phase separation on the surface, higher resolution, surface-sensitive PiFM images were acquired and shown in figure 3.
In conclusion, we presented the capability of PiFM to analyze nanoscale phase separation of polymer constituents both laterally and vertically.
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