2,2‐bistrifluoromethyl‐4,5‐difluoro‐1,3‐dioxole‐co‐tetrafluoroethylene (PDD‐TFE) copolymer is a good candidate to prepare gas separation membranes with excellent permeability due to its free volume characteristics. However, the influence of PDD‐TFE copolymer structure on its free volume characteristics is less studied. In this paper, PDD‐TFE copolymers with different compositions and molecular weights were synthesized, and their free volume characteristics were analyzed by positron annihilation lifetime spectroscopy and a molecular dynamics simulation. It indicated that the molar fraction of PDD in copolymers had a significant effect on free volume characteristics, while the molecular weight of copolymers exerted a slight influence on free volume when the molecular weight exceeded a critical region (intrinsic viscosity [η] > 68 ml g−1). PDD‐TFE copolymers with greater PDD molar fractions (i.e., 72% and 84%) showed bimodal distributions in positron lifetime and free volume size distributions, while PDD‐TFE copolymers with lower PDD molar fractions (i.e., 27% and 35%) exhibited a single peak. The long‐lifetime parameter τ3 was assigned to micro‐cavities formed by [‐(TFE)y‐PDD‐] segments and τ4 was attributed to micro‐cavities formed by [‐(PDD)x‐TFE‐] segments. The cis and trans transitions of PDD led to a local multilayer spiral structure with a 2.6–4.3 Å layer spacing, which would also increase the free volume of copolymers.

Synthesis of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene (PDD-TFE) copolymers with precise chemical structures and understanding effects of the structures on properties are essential for their application. Herein, PDD-TFE copolymers with different average compositions and narrow composition distributions (NCDs) were synthesized and applied to study the influences of chemical structures on condensed matter structures and optical properties of copolymers. The average length of (P TFE ) n sequences were decreased from 7.62 to 1.63, and percentage of TFE in long (P TFE ) n sequences (n > 5) decreased from approximately 50% to less than 1%, as PDD molar fraction in the copolymer (F PDD ) increased from 13.24% to 52.22%. Consequently, the copolymer with F PDD = 13.24% was semi-crystalline with a crystallinity degree of 11.57% (determined by WAXD),