Dielectric absorption at sub-millimetre wavelengths in poly(pentafluorostyrene) and poly-(αββ-trifluorostyrene)

Abstract

Measurements have been made of the sub-millimetre microwave (very far infrared) absorption in poly(pentafluorostyrene) and poly(αββ-trifluorostyrene) and the results compared with earlier studies of polystyrene. The results are consistent with a collisional loss mechanism arising from the interaction of electromagnetic radiation with a fluctuating dipole-induced dipole or quadrupole-induced dipole formed when the librating aromatic side groups come into proximity with their nearest neighbours.

Poly(pentafluorostyrene) exhibits a higher loss (“effective” induced dipole moment 0.40 D) and lower collision frequency (ca. 5 cm^–1 at room temperature) than polystyrene as expected for the greater mass and quadrupole moments of the perfluoro nucleus. Poly-(αββ-trifluorostyrene) also exhibits a greater “effective” induced dipole moment (0.40 D) and slightly lower collision frequency (26 cm^–1 at room temperature) than polystyrene, a phenomenon ascribed to the larger permanent dipole moment of the backbone-aromatic ring moiety. The “effective” dipole moment increases with increasing temperature, but the collision frequency is almost independent of temperature.

Observations of the effect of solvent suggest that where strong intermolecular interactions between solvent and side group exist, the associated polarization causes an enhanced loss and lowered “sticky” collision frequency.