Side Chain Liquid Crystalline ($$lc) Polyurethane Polymer.
Side chain $$lc polyurethane polymers are a new class of materials that are designed for optimal mechano-optical properties. The structure of a typical polymer of this kind is illustrated in Figure 3.19. Microphase segregation of the hard segments of this polymer is observed due to the favorable hydrogen bonding interaction between the urethane linkages. This extensive bonding leads to the formation of the hard domains that are dispersed in the soft matrix to anchor the polymer chains. The hard domains act as virtual cross-links that can be reversed upon heating the material to above the glass transition temperature (Tg) of the hard segment. The length of the soft segment, the degree of phase segregation, and the constrains placed in mesogen movement can be controlled.

As shown in Figure 3.20, the dynamic in-phase and quadrature spectra, and the corresponding phase rotation plot of this material indicate that different segments of the molecules have different response times to the mechanical strain as reflected by the phase delay between different regions. For the broad carbonyl band, detailed band assignment can be made. The 1730cm-1 absorption band (species I) is assigned to the “free” (non-H bonded) carbonyl of the urethane segment. This type of urethane is found embedded in the soft siloxane matrix. It is used as a marker to monitor the dynamic behavior of the soft segment. The dynamic FT-IR data show that response of this segment is approximately in-phase with the response of the mesogen (-CN stretching band) since both of them have strong intensity on the in-phase spectrum. The 1715cm-1 absorption band (species II) is assigned to the carbonyl of the urethane species that are found in the interfaces between the “hard” and the “soft” segments. Finally, the 1702cm-1 absorption band (species III) is assigned to the H-banded carbonyl of the urethane “hard” segments. Thus it can be concluded that the presence of infrared signatures in the dynamic spectra is an indication of the fact that the application of mechanical deformation induces orientation to both “hard” and “soft” segments of this type of $$lc polymer. The side chain mesogens ate free to deform in the elastic soft segment and the responses of different types of “hard” urethane segments can be examined by analyzing the carbonyl spectral region. The reorientation behavior of the “hard” and “soft” segments can be further deconvolved using 2D correlation spectroscopy, which is the subject of Chapter 7 of this book.

Polyethylene/Ethylene Vinyl Acetate Copolymer blend (PE/EVA). Figure 3.21 shows the absorbance spectrum and dynamic in-phase and quadrature spectra of a polyethylene(PE)/ethylene vinyl acetate copolymer (EVA) blend film. This film is composed of 7.6μm PE, on 29μm EVA (9% vinyl acetate), on 4.6μm methyl acrylate Sarun, on 22μm EVA (11% vinyl acetate) and low-density PE blend. The data in Figure 3.21were collected at a phase modulation frequency of 400Hz, amplitude of 3.5λHe-Ne and a sample modulation frequency of 20Hz. The sample was pre-stretched from 63.2μm to 45μm in thickness so that the intensity of bands of interest at 1371cm-1 (EVA/PE), 1044cm-1 (EVA) and 723cm-1 (PE) were all below 0.75absorbance unit, ensuring linearity of the dynamic measurement. It can be seen from the dynamic spectra that the side chain bands at 1371cm-1 (EVA/PE, C-H bending) and 1041cm-1 (EVA, C-O stretching) have stronger quadrature contributions indicating they are the major viscous components of the overall rheological properties. The very strong in-phase dynamic band at 723cm-1 (PE, C-C skeletal vibration) demonstrates strong elastic contribution of the backbone (C-C) to the rheological properties of the blend material. However, the difference in rheological contributions from the pure PE layer and from the PE component of the blend layer can hardly be distinguished using the transmission-based stretching measurement. A more sophisticated attenuated total reflectance (ATR)-based pressure/shearing modulation experiment, as mentioned earlier, might be the preferred technical choice to discriminate the subtle differences between the dynamic responses.

These examples show that SMM can be a very useful technique for understanding the chemistry behind polymeric material’s rhoelogy. The dynamic spectra can be used to infer molecular changes in response to mechanical strain and the correlation between these changes. 2D-correlation spectroscopy can be further utilizer to reveal the dynamic relationship between different segments of a material (see Chapter 7).

Chapter 4 Step-Scan Time-Resolved FT-IR Spectroscopy (S2TRS)
4.1 Introduction
Step-scan time-resolved FT-IR spectroscopy (S2TRS) is well suited to obtain spectral and kinetic information on fast, repeatable chemical reactions or physical processed with time resolutions from microseconds to nanoseconds. In S2TRS, data are collected as a explicit function of time at each mirror position when both mirrors are stopped completely, as illustrated in Figure 1.4 in Chapter 1. This time profile is repeated and recorded at all mirror positions. After the experiment finishes, the data ate sorted into individual interferograms which are then Fourier transformed into IR spectra at different times. The meaningful time resolution of step-can TRS is determined by both the speed of the digitizer and the rise time of the detector, rather than by the scanning (stepping) speed.

Multiple triggers and co-additions at each mirror step with data collection are often used to improve signal-to-noise ratio. The S2TRS spectrometer can be operated either in master (where the spectrometer triggers the excitation system of the sample) or slave operation mode (where the excitation system is used to trigger the digitizer of the spectrometer). An external perturbation-induced reversible or repeatable system can be readily studied by S2TRS.

The repeatable reactions or processes are often initiated with an external perturbation that varies in different application areas. The most common perturbations include the following:

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Electrical perturbation. Electric pulses used in studying materials such as liquid crystals and light emitting devices (LED’S).
Mechanical perturbations. Mechanical strain or pressure used in polymer stretching and shearing experiments; rapid mixing used in stop-and-flow kinetics.
Optical perturbations. Laser or flash lamp pulses used in photochemical reaction studies such as excited states of metal complexes, photochemical reactions in condensed or gaseous phase, and biophysics such as photocycles of bacteriorhodopsin;
Thermal perturbation. temperature jump used in biochemical and biophysical studies (protein conformational changes).

Irreversible systems can be also investigated by S2TRS if the process can be regenerated by repetitively providing fresh sample into the beam for each mirror position (step) with a stop-and-flow cell or motorized stage.
The dynamically aligned, DSP-controlled, highly integrated, research-grade Nicolet FT-IR spectrometers with mirror positioning control better than ±0.2nm are virtually immune to acoustic noise and environmental vibrations and are easy to interface with external devices such as lasers and pulse generators, ensuring reliable performance for highly demanding TRS applications.