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Dynamic 2D IR dichroism spectroscopy study of polymers based on the correlation analysis of the time dependence of localized reorientational motions of various submolecular moieties constituting a system has proven to be a very powerful tool for a broad range of applications, especially in the study of complex macromolecules, such as polymers and proteins. It was noted from the early stage that the peak resolution of 2D IR correlation spectra is substantially enhanced by spreading the overlapped IR bands along the second dimension. The presence or lack of strong chemical interactions or connectivity among submolecular groups located in various parts of the polymer system is readily detected by the appearance of correlation cross peaks at appropriate spectral coordinates. Furthermore, the relative reorientation directions of intensity changes and the sequential order of realignment of submolecular structural units are also conveniently provided by the signs of the cross peaks appearing in 2D IR spectra.
Information obtained from specific applications of 2D IR dichroism spectroscopy include the local dynamics of an amorphous polymer, where side groups reorient independently of the polymer main chain and abrupt changes in the side-group realignment mechanism above and below the glass transition temperature. By using dynamic 2D IR, it is possible to probe the microscopic spatial
Repetitive Perturbations Beyond DIRLD
distribution of molecular components, especially those found in phase-separated systems. The degree of specific interactions between components in mixtures is effectively identified. The application certainly is not limited to the characterization of traditional synthetic polymers. Complex macromolecules of biological origin can be readily studied by this technique. 2D IR spectroscopy thus greatly expands the scope of possible spectroscopic studies of materials.
8.3 REPETITIVE PERTURBATIONS BEYOND DIRLD
Dynamic 2D IR analysis based on rheo-optical DIRLD measurement using sinusoidal mechanical perturbation has been enormously successful in the study of complex polymer systems. The basic scheme for obtaining 2D correlation spectra using a form of repetitive perturbation can be readily expanded to encompass a much broader range of spectroscopic measurements. The excitation of system constituents induced by an arbitrary external stimulus may be monitored by any type of electromagnetic probe. The universal applicability of the 2D correlation approach based on the perturbation-induced spectral variation is demonstrated in this section.
8.3.1 TIME-RESOLVED SMALL ANGLE X-RAY SCATTERING (SAXS)
Small angle X-ray scattering (SAXS) analysis is a powerful technique to probe the spatial heterogeneity of systems in the range of nano-to micrometer scales. This technique has been especially useful in the morphological characterization of microphase-segregated domain structures of block polymers. We now extend 2D correlation analysis to the dynamic SAXS studies.
In SAXS studies, the X-ray scattering intensity I(q) is measured as a function of scattering vector q defined as
q =----------------------------------------- (8.8)
where 0B is the Bragg diffraction angle (i.e., half of the scattering angle), and X is the wavelength of the X-rays.
If the system is placed under a time-dependent perturbation such as a sinusoidal strain with a fixed angular velocity m, the scattering intensity also becomes a
function of time, reflecting the dynamic changes in the spatial distributions of
the constituents of the system.
I(q, t) = l(q) + I(q, t)
= I(q) + I'(q) sin cot + I"(q) cos cot (8.9)
Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
Thus, the synchronous and asynchronous 2D SAXS correlation spectra should be obtained as
<*>(ft,ft) = /'(ft) ? /'(ft) +/"(ft) ' /"(ft)] (8.10)
*(ft, ft) = \U\qi) • /'(ft) ~ /'(ft) • /"(ft)] (8.11)
Unlike the 2D IR spectrum, the correlation intensity of a 2D SAXS spectrum is plotted as a function of two independent scattering vector axes, q1 and q2.
Figure 8.31 shows the schematic diagram of the dynamic SAXS instrument
developed by Professor Takeji Hashimoto’s group at Kyoto University.30 All
the measurements were carried out with this instrument at room temperature. Dynamic tensile strain was applied continuously to the sample at a rate of 0.47 Hz and an amplitude of 1.1%. The dynamic strain was superposed onto a static strain of 10%. The dynamic scattering intensity was measured by dividing the deformation period into 64 sectors and coadding the intensity to the histogram memory of the multichannel analyzer. Fourier analysis was applied to the results, and the first-order terms were used to calculate the static and dynamic scattering intensities.
The material studied was a styrene-butadiene-styrene (SBS) triblock copolymer with 23 % styrene content. This system is known to form microphase-separated regularly spaced domain structures with hexagonally packed cylindrical morphology. The sample was prepared by melt pressing at a temperature above the glass transition temperature of polystyrene (ca 100 °C) but below their order-disorder transition temperature (ca 180 °C). Under such a condition, the microphase-separated cylindrical domains are oriented in a preferential direction.