Spectroscopic characterization of the structure and motion of polymer liquid crystals and polymer dispersed liquid crystals
In Chapter I, the applications of Nuclear Magnetic Resonance (NMR) spectroscopy to solid polymers are reviewed. In Chapter II, a side-chain polymer liquid crystal (PLC) based on the 4-hydroxy-4?-methoxy-?-methylstilbene mesogen attached through a flexible spacer of eight methylenic units to a poly(methyl acrylate) backbone is studied via solid-state ^13C NMR spectroscopy. Molecular dynamics in the MHz frequency regime are characterized as a function of temperature by the spin-lattice relaxation time constant T1. Rotational correlation times (? c) and activation energies (E a) are calculated for motions at various local sites in the glass and nematic states. Rapid spinning of the methyl carbons occurs on the fast side of the T1 minimum and the motional activation energy does not change at the phase transition. The activation energies of all three sites in the mesogen are nearly equal, indicating a single collective motion. The activation energy of the ? spacer carbon is nearly equal to that of the mesogen, and the ? spacer carbon is thirty times more mobile. Further flexibility is introduced at the ? and ? positions. In Chapter III, NMR spectroscopy and Fo urier transform infrared (FT-IR) spectroscopy are used to characterize trans-gauche isomerization of methylene groups in a new class of main chain PLCS. The new series of liquid crystalline copolymers is based on the 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane mesogen where R is F, Cl or CH3 and flexible spacers containing an odd number of methylene units. Trans-gauche isomerization is characterized by FT-IR spectroscopy through measurements of the absorbance of characteristically trans bands, and characteristically gauche bands. Trans-gauche isomerization is characterized by ^13C NMR spectroscopy through measurements of the ^13C chemical shifts in the solid-state. FT-IR spectroscopy shows that an increase in temperature results in an increase in the percentage of gauche isomers for methylene units in both the spacer and the mesogen. In Chapter IV, two-dimensional NMR spectroscopy is used to characterize polymer dispersed liquid crystals (PDLCS) in the solid-state. The wideline separation (WISE) pulse sequence is used to correlate structure and mobility in a PDLC. The PDLC is 40% E7 as 0.4? droplets in an Epon 828 matrix. The WISE pulse sequence generates ^1H wideline spectra in one dimension for each high resolution solid state ^13C resonance in the other dimension. The width of the ^1H line shape reflects the mobility of the protons in the vicinity of the respective ^13C. The aliphatic portion of E7 exhibits extremely narrow ^1H line widths of a few hundred hertz. Therefore, the aliphatic pendant group of E7 undergoes random isotropic motions with rates above 1MHz. The aromatic portion of E7 exhibits pronounced spinning sidebands in the ^1H dimension. Therefore, the aromatic core of E7 undergoes anisotropic motions. (Abstract shortened by UMI.
School:Case Western Reserve University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:spectroscopic characterization polymer liquid crystals dispersed
Date of Publication:01/01/1994