Free volume characteristics of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene copolymers: Effect of composition and molecular weight |
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Authors: | Dayang Yu Guoliang Mao Huaixun Cai Shuhua Wang Jiandang Liu Pengju Pan Yongzhong Bao |
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Affiliation: | 1. State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou, China;2. State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou, China Process Safety Evaluation Laboratory, Institute of Zhejiang University-Quzhou, Quzhou, China;3. Institute of Polymer Materials and Engineering, Zhejiang Juhua Technology Center Co., Ltd, Quzhou, China;4. State Key Laboratory of Particle Detection & Electronics, Chinese Academy of Sciences, University of Science & Technology of China, Hefei, China |
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Abstract: | 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. |
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Keywords: | 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole composition copolymer free volume molecular dynamics simulation positron annihilation lifetime tetrafluoroethylene |
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