Dielectric Relaxation Studies on Molecular Motion of Poly(fluoroalkyl α-substituted acrylate)s and Compass Motion Model for Internal Motion of the Fluoroalkyl Side Chain

• Published in: Macromolecules 1999-03, Vol.32 (5), p.1651-1660
• Main Authors: Tadano, Kenji, Tanaka, Yoshito, Shimizu, Tetsuo, Yano, Shinichi
• Format: Article
• Language: English
• Subjects: Applied sciences; Dielectric relaxation; Electrical, magnetic and optical properties; Enthalpy; Exact sciences and technology; Fluorine containing polymers; Hydrogen bonds; Molecular dynamics; Molecular structure; Organic polymers; Physicochemistry of polymers; Properties and characterization
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma981048o

Dielectric measurements were made on poly(fluoroalkyl α-substituted acrylate)s, −[−CH2CX(COO(CH2) m (CF2) n Y)−] p − [PX−Hm−Fn−Y, X = α-substituent (F, Cl, Me, H), Y = F or H], over a wide temperature range from 90 to 530 K at different frequencies between 100 Hz and 100 kHz. In the PF−Hm−Fn−Y polymers, there were observed three relaxations, α above 400 K, β around 250 K, and γ below 150 K, which were attributed to a reorientational motion of long segments above T g, a rotational motion of carbonyl groups in amorphous region, and an internal motion of fluoroalkyl side chains, respectively, based on the assignments of poly(n-alkyl methacrylate)s. As the fluoroalkyl length (N c = m + n) increases, the γ relaxation shifted to higher temperatures in PF−Hm−Fn−F but was almost unchanged in PF−Hm−Fn−H, while the α relaxation moved to higher temperatures in both polymers. This dielectric relaxation behavior was well explained by the chemical structures and the existence of crystallites (the ordering of main backbone chains and fluoroalkyl chains). It is noted that the activation enthalpy for the γ relaxation [ΔH(γ)] increases with increasing N c. The ΔH(γ) values for PF−H1−Fn−F were well consistent with the vaporization enthalpy (ΔH vap) of the corresponding C n F2 n +2, but those for PF−H1−Fn−H were larger than for PF−H1−Fn−F. These characteristic changes of ΔH(γ) with N c were well explained by compass motion model newly proposed in this paper, in which the fluoroalkyl side chain moves on an orbit drawn by a compass to make the γ relaxation:  It was found that ΔH(γ) for PF−H1−Fn−F is almost consistent with ΔH vap (the sum of the energy in intramolecular interaction (ΔH intra) and intermolecular interaction (ΔH inter)), while the larger ΔH(γ) value for PF−H1−Fn−H is related to the existence of H- - -F hydrogen bonding of the terminal −CF2−H with F whose energy (ΔH coul) was calculated by assuming Coulomb attraction.