Influence of the third monomer component on the temperature-dependent crystallite modulus and tie chain fraction evaluated for ethylene-tetrafluoroethylene copolymers

Temperature dependence of apparent crystallite modulus along the chain axis Ecapp has been measured on the basis of X-ray diffraction measurement under the assumption of homogeneous tensile stress distribution for the uniaxially-oriented ethylene-tetrafluoroethylene (ETFE) alternating copolymers with and without the third monomer component of short or long side groups. The Ecapp of the 2-components alternating ETFE copolymer was found to decrease remarkably in the phase transition temperature region between the low- and high-temperature phases at around 80 °C. For the ETFE copolymer sample with the third comonomer component of long C4F9 side groups the Ecapp was found to show the similar temperature dependence to that of the pure alternating copolymer although the Ecapp value itself was slightly lower than the latter. The ETFE copolymer with short CF3 side groups was found to show remarkably low Ecapp value in the temperature region from −50 to 50 °C since the crystal structure transferred already to the high-temperature disordered phase in this temperature region. The bulk Young's moduli of these copolymer samples, estimated by the dynamic viscoelastic measurements, were analyzed on the basis of a complex mechanical model consisting of parallel and series sequences of Ectrue and Ea, where Ectrue and Ea were the true moduli of the crystalline and amorphous regions, respectively. The temperature dependences of the bulk moduli observed for these three samples were successfully reproduced by estimating the Ectrue as a function of temperature. The Ectrue estimated at a low-temperature, was 215 GPa for ETFE 2-components sample, which was consistent with the value 230 GPa calculated theoretically for the planar-zigzag chain conformation. The parallel crystalline component, which was assumed to be a kind of taut tie chain to support the mechanical toughness of the bulk sample, was found to be lower in population for ETFE 2-components sample and higher for the copolymer samples containing the third monomer component. This tendency is consistent with that of the actually-observed stress crack resistance or the mechanical toughness in a high-temperature region near the melting point: ETFE < ETFE-CF3 < ETFE-C4F9. In this way, the terpolymer sample has an important role to create the higher content of taut tie chains, resulting in the higher stress crack resistance.