Dynamic FT-IR spectroscopy of liquid crystals and polymer films

Abstract

This dissertation describes the use of continuous-scan and step-scan Fourier transform infrared (FT-IR) spectroscopic techniques to study the dynamics of the response of polymer films and liquid crystals to external perturbations. The investigation of liquid crystals includes both nematic and chiral smectic C examples. In these studies the dynamic infrared absorbance is used to explore the submolecular (functional group) contributions to the reorientation dynamics of the liquid crystal director in response to both pulsed (DC) and modulated (AC) electric fields. Continuous-scan stroboscopic FT-IR and step-scan impulse-response FT-IR were used to analyze the rise dynamics of reorientation resulting from pulsed DC perturbations; whereas step-scan FT-IR was used to monitor both rise and decay processes in response to synchronously modulated (AC) electric fields. In the step-scan measurements this sub-molecular view of the dynamics of liquid crystal director reorientation was enhanced by frequency correlation analysis, to yield 2D FT-IR spectra. For the nematic liquid crystal 4-pentyl-4$\sp\prime$-cyanobiphenyl (5CB) the data suggest a different rate of response of the rigid and floppy parts of the LC molecules. The second application of dynamic step-scan FT-IR reported is the study of the response of various polymer films to sinusoidally modulated tensile strain. The main advantage of the technique is that it can provide valuable information at the molecular level that can be used to interpret the macroscopic properties of the polymeric material under investigation. Examples of application to different types of polymer films are presented. Results for several heterogeneous polymers including semicrystalline high density/low density polyethylene blends, the micro-phase separated copolymer Kraton$\sp\circler$ and a homogeneous polymer blend of polystyrene/polydimethylphenylene oxide are presented. Finally, the modification of a research-grade FT-IR spectrometer (Nicolet Instruments, System 800) for step-scan operation is described. Some applications of the instrument to photoacoustic spectroscopy (PAS) are presented.