Influence of the third monomer component on the X-ray-analyzed crystal structure of ethylene–tetrafluoroethylene copolymer

Abstract

Influence of the third monomer component on the crystal structure of the low-temperature phase of ethylene (E)–tetrafluoroethylene (TFE) alternating copolymer (CH2CH2CF2CF2)n was investigated by the measurement of 2-dimensional wide-angle X-ray diffraction (WAXD) diagrams. Nonafluoro-1-hexene (NFH) and hexafluoropropylene were chosen as the third monomer component (termonomer) with the side chains of C4F9 (ET-C4F9) and CF3 (ET-CF3), respectively. Although the termonomer content was low, 1.5 and 2.7mol%, respectively, the third monomer component has been found to affect more or less the crystal structure of the original 2-component ET copolymer. The crystal structure of these three types of ET copolymer is commonly of the triclinic type. In the case of ET-C4F9 terpolymer, the unit cell size and the setting angle of the planar-zigzag chains in the unit cell are almost the same as those of the 2-component ET copolymer. On the other hand, in the case of ET-CF3 terpolymer, the unit cell becomes larger and the chain setting angle is appreciably different from the 2-component ET copolymer’s case. As reported in the previous paper (Macromolecules, 44, 1540 (2011)), the long C4F9 side chains are not included in the crystal lattice but they are excluded on the lamellar surfaces, not giving significant effect on the inner structure of the unit cell. The short CF3 groups are included in the crystal lattice and expand the unit cell, resulting in the significant modification of the chain setting angle. These copolymers are known to show the phase transition between the low-temperature-phase and high-temperature phase in a certain temperature region. The transition region is not very different between the 2-component copolymer and ET-C4F9 terpolymer, while ET-CF3 terpolymer shows the transition in an appreciably low temperature region below the room temperature. The difference in such a characteristic phase transition behavior originates from the above-mentioned different situation of the long and short side groups.