MOLECULAR-DYNAMICS OF HAIRY ROD MOLECULES IN THE SOLID-STATE - POLY(GAMMA-METHYL L-GLUTAMATE)-CO-(GAMMA-N-OCTADECYL L-GLUTAMATE) IN SOLUTION-CAST FILMS

We present the thermal, structural, and dynamic properties of poly(gamma-methyl L-glutamate)-co-(gamma-octadecyl L-glutamate) with 30% octadecyl chains in solution-cast films. This substance is an example of a so-called ''hairy rod'' molecule. The peptide backbones form stiff rods which are surrounded by the side chains. The structure of the solution-cast film was measured by WAXS and SAXS. We found amorphous and crystalline side chain parts. The melting point of the crystalline side chains is ca. 293 K. The thermal properties of these ''hairy rods'' are investigated with differential scanning calorimetry, and the dynamic properties are measured between 110 and 370 K with broad-band dielectric spectroscopy (10(-2)-10(9) Hz) and variable-temperature C-13-CP/MAS solid-state NMR. Three dielectric-active relaxation processes are observed: Two processes have a temperature dependence according to the Williams-Landel-Ferry (WLF) equation. They approach each other with decreasing temperature and merge at about 238 K, i.e., the calorimetric glass transition temperature. These two processes are discussed as fluctuations of the main chain and the surrounding side chain groups. The third relaxation process has an Arrhenius-type temperature dependence with an activation energy of 43 kJ/mol. It is assigned to fluctuations of the carboxyl groups in the side chains. The variable-temperature NMR results confirm that one of the WLF-type relaxations is caused by the side chains. These NMR, experiments also show that the main chains undergo no local relaxation in the investigated temperature and frequency region and stay in the or-helix conformation.