Synthesis,Structure And Membrane Separation Properties Of 4,5-difluoro-2,2-Bis(Trifluoromethyl)-1,3-dioxole-co-tetrafluo-Roethylene Copolymers

2,2-Bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene(PDD-TFE)copolymers are new and advanced fluoropolymers with high light transmission,low refractive index,low dielectric dissipation and good gas permeability,and have wide application prospects in the field of optical devices,separation membranes,integrated circuits,and so on.Establishing synthesis technologies and relationships between the molecular structure,condensed matter structure and properties of structurally controllable PDD-TFE copolymers is key issues to achieve the industrial production of PDD-TFE copolymers and to extend their applications.Due to technical secrecy and the specific characteristics of monomers,PDD-TFE copolymerization laws were seldom reported,and researches on the structure and properties of copolymers are mainly limited to the commercial PDD-TFE copolymers with two compositions(Teflon(?) AF1600 and 2400).Aiming to construct relationships between molecular structure,condensed matter structure and properties of PDD-TFE copolymers in a wide composition range,PDD-TFE copolymers with narrow composition distributions were synthesized,and effects of the copolymer composition and monomer sequence distribution on the crystalline ability of copolymers,the copolymer composition and the molecular weight on free volume characteristics,the copolymer composition on the optical and gas separation properties of copolymer membranes,and introducing of vinylimidazole group and complexation with Co2+on gas separation properties of copolymer membranes,were investigated in this thesis.Firstly,a series of PDD-TFE copolymers with low and high conversions were synthesized by dispersion polymerization using fluorinated solvents as the polymerization medium,the reactivity ratios,variations of copolymer composition with conversions and the monomer sequence distributions of copolymers with different compositions were investigated.By correlating the relationship between the monomer compositions at the reaction site and compositions of corresponding copolymers,the copolymerization reactivity ratios at 45℃ and using 1,1,2-trichlorotrifluoroethane as the polymerization medium were obtained as follows:rPDD=0.59±0.06 and rrFE=0.87±0.11.After considering the diffusion of TFE monomer from the gas phase to the solvent phase during the copolymerization process,it was found that the calculated compositions of copolymers was slightly decreased with the increase of conversions when the conversions were lower than about 50%,and slightly increased with the increase of conversions when the conversion exceeded 50%.Generally,high conversion copolymers prepared in a 600 mL reactor and in a 5 L scaled-up autoclave exhibited narrow compositions and the measured average compositions were in good agreement with the calculated values.As FPDD in the copolymer increased,the ratios of crystallizable(PTFE)n sequences with n>5 were rapidly decreased.Secondly,effects of PDD-TFE copolymers composition on the crystalline/amorphous change,crystallinity and microcrystalline parameters were investigated by wide-angle X-ray diffraction and differential scanning calorimetric analysis,and the free volume characteristics of copolymers with different compositions and molecular weights were studied by the positron annihilation lifetime spectroscopy and a molecular dynamics simulation.It was found that the crystallization ability of copolymers was rapidly decreased with the increase of FPDD,and the crystallinity of copolymer was only 0.07%when FPDD=30.72 mol%,and the PDD-TFE copolymers were completely amorphous when FPDD≥52.22 mol%.The average size of free volume(cavities)inside the copolymers was increased with the increase of FPDD,and the size distributions of cavities were gradually evolved from a single peak distribution to a doublepeak distribution.The copolymer composition had a strong influence on the free volume characteristics of copolymers,while the intrinsic viscosity([η]>68 mL g-1)had no significant effect on the average size and distribution of the free volume of the copolymers.For copolymers with greater FPDD(i,e,70.42 and 85.11 mol%),the cavities corresponding to the third lifetime component(τ3)were caused by the stacking of short(PTFE)n chain segments,and the cavities corresponding to the fourth lifetime component(τ4)were formed by the stacking of(PPDD)m chain segments.Thirdly,the thermal stability,glass transition behavior,refractive index and reflectivity of PDD-TFE copolymers with different compositions,and the morphology,transmittance and gas permeability of the copolymer cast membranes were investigated.As FPDD increased,the PDDTFE copolymer showed a slight decrease of the 5%weight loss temperature,a rapid increase of the glass transition temperature,and a linear decrease of refractive index.PDD-TFE copolymers could be dissolved in hexafluorobenzene and perfluoroheptane,and homogeneous membranes with few defects could be obtained by the casting method.The transmittance of copolymer was gradually increased with the increase of FPDD,and the transmittance of membranes cast from perfluoroheptane solution was greater than that of membranes prepared using hexafluorobenzene as the solvent.When FPDD≥ 30.72 mol%,the transmittance of membranes cast from perfluoroheptane with a thickness of 18±0.5 μm was higher than 95%in the wavelength of 340～780 nm.The permeation coefficients of pure H2,O2,N2 and CH4 gases in the PDD-TFE copolymer membranes were gradually increased with increase of FPDD,and the gas permeation coefficients of the membranes cast from perfluoroheptane solution were greater than that of membranes prepared using hexafluorobenzene as the solvent.The selectivities of PDD-TFE copolymers for O2/N2 and O2/CH4 gas pairs were higher than that for O2/H2 gas pair at the same FPDD,and the selectivities of copolymers for O2/N2 and O2/CH4 gas pairs were gradually decreased,while that for O2/H2 were gradually increased with the increase of FPDD.Finally,PDD-TFE terpolymers containing vinyl imidazole and amine groups were synthesized,and specific group-modified PDD-TFE copolymer membranes were prepared by complexation of imidazole groups with cobalt tetraphenylporphyrin(Co2+)and in situ generation of covalent organic framework(COF)structures by amine groups on the copolymers,respectively.The formation of Co2+ complexation structure and COF structure was confirmed by FT-IR spectroscopy and WAXD analysis,and the gas separation characteristics of the modified copolymer membranes were investigated.It was found that the permeation coefficient of O2 of the copolymer membranes complexed with cobalt tetraphenylporphyrin was increased to 1.31 times and the selective separation coefficients of O2/H2,O2/N2 and O2/CH4 were increased to 1.30 times when the molar addition of vinylimidazole was 0.5 mol%.In conclusion,policies to synthesize PDD-TFE copolymers with narrow distributions,and relationships between the molecular structure,sequence distribution,condensed matter structure and properties of the copolymers were established,which would be helpful for the precise synthesis of PDD-TFE copolymers with different compositions and their applications in optical devices and separation membranes.