Dielectric relaxation studies on molecular motion of poly(fluoroalkyl alpha-substituted acrylate)s and compass motion model for internal motion of the fluoroalkyl side chain

Dielectric measurements were made on poly(fluoroalkyl alpha-substituted acr ylate)s, -[-CH2CX(COO(CH2)(m)(CF2)(n)Y) -](p)- [PX-Hm-Fn-Y, X = alpha-subst ituent (F, Cl, Me, H), Y = F or H], over a wide temperature range from 90 t o 530 K at different frequencies between 100 Hz and 100 kHz. In the PF-Hm-F n-Y polymers, there were observed three relaxations, alpha above 400 K, bet a around 250 K, and gamma below 150 K, which were attributed to a reorienta tional motion of long segments above T-g, a rotational motion of carbonyl g roups in amorphous region, and an internal motion of fluoroalkyl side chain s, respectively, based on the assignments of poly(n-alkyl methacrylate)s. A s the fluoroalkyl length (N-c = m + n) increases, the gamma relaxation shif ted to higher temperatures in PF-Hm-Fn-F but was almost unchanged in PF-Hm- Fn-H, while the alpha relaxation moved to higher temperatures in both polym ers. This dielectric relaxation behavior was well explained by the chemical structures and the existence of crystallites (the ordering of main backbon e chains and fluoroalkyl chains). It is noted that the activation enthalpy for the gamma relaxation [Delta H(gamma)] increases with increasing N-c. Th e Delta H(gamma) values for PF-H1-Fn-F were well consistent with the vapori zation enthalpy (Delta H-vap) of the corresponding CnF2n+2, but those for P F-H1-Fn-H were larger than for PF-H1-Fn-F. These characteristic changes of Delta H(gamma) with N-c were well explained by compass motion model newly p roposed in this paper, in which the fluoroalkyl side chain moves on an orbi t drawn by a compass to make the gamma relaxation: It was found that Delta H(gamma) for PF-H1-Fn-F is almost consistent with Delta H-vap (the sum of t he energy in intramolecular interaction (Delta H-intra) and intermolecular interaction (Delta H-inter)), while the larger Delta H(gamma) value for PF- H1-Fn-H is related to the existence of H- - -F hydrogen bonding of the term inal -CF2-H with F whose energy (Delta H-coul) was calculated by assuming C oulomb attraction.