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All of the measurements were performed on a Bio-Rad FTS-60A FT-IR spectrometer operating in step-scan mode. A phase modulation frequency of 400 Hz with an amplitude of 1.0 HeNe laser wavelength was applied through the dynamic alignment piezoelectric devices of the spectrometer. The 400 Hz phase modulation signal was demodulated using a lock-in amplifier supplied by Bio-Rad with the FTS-60A spectrometer. The PAS measurements were carried out by aligning an MTEC 200 PAS cell in the sample compartment of the spectrometer. The instrument response phase was determined using a standard carbon black sample equipped with an optical screen supplied by MTEC. The PAS cell was purged with helium gas. Full double-sided interferograms were collected at a spectral resolution of 8 cm-1. Interferograms were under sampled by a factor of two (i.e., every other HeNe laser zero crossing) and zero filled two times. Approximately 16 000 data points in each channel were integrated, and the average value was stored. Sample interferograms were computed using the carbon black in-phase stored phase array which contains only instrument response function contributions.
The first sample studied was a model laminate made of approximately 25 Rm of low-density polyethylene (PE) on top of a 25 Rm thick layer of polystyrene (PS) with a thin (ca 3 Rm) coating of polydimethylsiloxane (PDMS) spread over the PE surface. The second sample was a surface hydrophilic elastomer latex (SHEL) film. SHEL is a novel polymeric alloy consisting of styrene-butadiene rubber (SBR) and an amphiphilic block oligomer containing a short polyethyleneoxide segment.32 SHEL is prepared by the emulsion copolymerization of styrene and butadiene in the presence of the amphiphilic block oligomer. The latex spontaneously forms a film having a surface which has exceptionally high surface energy and consequently water wettability. It is believed that the ethoxylate block chain segments are rejected from the bulk SBR phase and accumulate at the surface
Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
to render it hydrophilic. The enthalpic penalty of forming such a high energy surface is most likely compensated by the unfavorable entropic energetics of creating high-curvature inclusions of polymeric moieties within the bulk phase.
The depth-correlated synchronous 2D photoacoustic FT-IR spectrum of the model laminate sample is shown in Figure 8.39. Each set of cross peaks and corresponding autopeaks corresponds exclusively to a particular layer component. For example, peaks arising from the contributions of PDMS layers are synchronously correlated with other PDMS bands. However, no cross peaks are observed between PDMS bands and PS or PE bands. Clear differentiation between PAS signals from different depth layers is demonstrated.
The close up view of the asynchronous spectrum (Figure 8.40) reveals very useful information. By examining the signs of the cross peak intensities, it is possible to determine the depth sequence of the laminate. Thus, the positive asynchronous peaks on the far right of the 2D PAS spectrum correlate the small PDMS band at 1405 cm-1 with a PE band at 1460 cm-1 and with PS bands at
Synchronous 2D IR Correlation Spectrum
------------------ 1----1-----1------1---1------1----1-----!-----1-----f 1/3U
Figure 8.39 Synchronous 2D PAS correlation spectrum of a DMPS/PS/PE laminate film
Repetitive Perturbations Beyond DIRLD
Figure 8.40 Asynchronous 2D PAS correlation spectrum of a DMPS/PS/PE laminate film
1450 and 1490 cm-1. This result indicates that the PDMS photo-acoustic signals detected at the microphone precedes those of PS and PE. In other words, the PDMS signal originates from the layer nearer to the surface of the sample than the other two layers. The other two signals below the diagonal line (one positive and one negative) correlate PE and PS bands and indicate that the PS signal originates from a deeper layer in the sample. By combining the above information, one can readily deduce that the sample consists of layers in the depth order of PDMS < PE < PS.
Figure 8.41 shows the depth-correlated synchronous 2D photo-acoustic spectrum of the SHEL film. The presence of cross peaks clearly indicates that the ethoxylate moieties of the block oligomer are located at some depth substantially different from that of SBR. The synchronous spectrum correlates all bands assignable to SBR components with each other. However, no synchronous correlation is observed between ethoxylate and SBR bands. The signs of the asynchronous peaks (not shown) indicate that ethoxylate moieties are located
164 Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
1600 600 Wavenumber, v,
Figure 8.41 Synchronous 2D PAS correlation spectrum of a SHEL film
at the surface, while SBR is buried at a much deeper position. This fact can be independently verified by comparing the regular transmission spectrum in Figure 8.42(A) with the attenuated total reflectance (ATR) spectrum of a SHEL film in Figure 8.42(B). While the transmission spectrum has strong peaks of SBR copolymer bands, the corresponding ATR spectrum is dominated by the signals assignable to the C-O stretching vibrations of ethoxylate moieties of the amphiphilic block oligomer. Thus, the ATR result of this system agrees well with the conclusion drawn from the depth-correlated 2D IR PAS spectroscopy.