Synthesis,Structure And Properties Of Perfluoro-2,2-dimethyl-1,3-dioxole/Tetrafluoroethylene Copolymers

Perfluoro-2,2-dimethyl-1,3-dioxole/tetrafluoroethylene(PDD-TFE)copolymer is an amorphous polymer,which exhibits not only similar heat resistance and chemical resistance to other fluoroplastics,but also the characteristics of high light transmittance,low refractive index and dielectric constant and great gas permeability.It has broad application prospects in optical fibers,instruments,military devices and so on.Till now,few attentions have been paid to the synthesis,structure and properties of PDD-TFE copolymers.In this thesis,methods for PDD-TFE copolymerization and structure characterization of PDD-TFE copolymer were established,the copolymerization kinetics,effects of polymerization conditions on the intrinsic viscosity([?])and composition of PDD-TFE copolymers,and the relationship between structure and properties of PDD-TFE copolymers were investigated.Firstly,PDD-TFE copolymers were synthesized using peroxide A as an initiator and fluorinated solvent B as a reaction medium.Effects of temperature,initiator concentration and monomer feed ratio on the copolymerization kinetics were investigated.It was found that the monomer conversion was increased linearly with reaction time at the initial stage of copolymerization,and the relationship between the initial polymerization rate and monomer concentration obeyed a first-order reaction characteristic.With the increase of polymerization temperature and initiator concentration,the copolymerization rate was greatly increased.The copolymerization rate was decreased initially and then increased as PDD/TFE ratio in feed increased,due to the fact that the cross-termination rate of free radicals is greater than the termination rate of same free radical and the monomer distribution characteristics of the reaction system.With the increase of polymerization temperature and initiator concentration,[?]of PDD-TFE copolymer was decreased.Secondly,the method to calculate the composition of PDD-TFE copolymers using the peak area of CF2 and CF groups determined by 19F NMR was established.The properties of PDD-TFE copolymers with different compositions and[?]were characterized to obtain the relationship between the structure and properties of the copolymers.It was found that PDD-TFE copolymers exhibited excellent thermal stability,and was hardly decomposed within 400?.The thermal stability was decreased with the increase of PDD content and decrease of[?]of PDD-TFE copolymer.DSC and WAXD results showed that the crystallinity of the copolymers was decreased as PDD content increased,and only weak crystallization occurred when PDD content in the copolymer was 35%.When PDD content in the copolymer exceeded 35%,amorphous copolymers were formed.The glass transition temperature(Tg)of the copolymer was increased as the PDD content increased,and the relationship between Tg and[?]was obtained as follows:Tg=167.85-308.15/[?].The complex viscosity |?*| of PDD-TFE copolymers was decreased with the increase of frequency,showing the characteristic of a pseudoplastic fluid.It was found that the existence of crystallization zone would lead to the great increase of |?*",storage modulus G'and loss modulus G".The copolymer with lower[?](40 mL/g)had smaller |?*",G' and G",and the |?*|,G" and G" of the copolymers were significantly increased when[?]of copolymer exceeded 78 mL/g.PDD-TFE copolymers had a low dielectric constant(1.90?2.05),which was decreased as PDD content and[?]of the copolymer increased.The copolymer with greater PDD content exhibited a higher optical transparency,a wider band gap and a wider wavelength range of transmissible light.In addition,PDD-TFE copolymer exhibited good hydrophobicity,and the water contact angle of the copolymer was decreased with the increase of PDD content and the decreased of[?]of the copolymer within a certain range.Finally,the composition and lamellar thickness distributions of tetrafluoroethylene-perfluoropropyl vinyl ether random copolymer(PFA)were characterized by successive self-nucleation annealing thermal classification(SSA)and step crystallization(SC)methods.SSA method was also applied to characterize the composition and lamella thickness distributions of PDD-TFE copolymers with low PDD content.The results showed that for PFA with greater Mw,SSA was more effective to achieve fractionation than SC method.The initial thermal fractionation temperature Ts0 of SSA was determined by the self-nucleation melting behavior of PFA,and key parameters affecting the SSA fractionation result were optimized as follows:heating/cooling rate=10?/min,temperature step of fractionation ?T=5? and isothermal time=5 min.The SSA results showed that the composition heterogeneity was decreased and the lamellar thickness distribution became wider as the content of perfluoropropyl vinyl ether(PPVE)in PFA increased.Additionally,for PFA samples with closed composition,the lamellar thickness distribution,crystallinity and melting temperature were also varied with the composition heterogeneities of PFA.The composition distribution of the crystallizable PDD-TFE copolymers could also be characterized by the SSA method.The PDD-TFE copolymers used in the experiment exhibited slightly wide composition distributions,low composition heterogeneities,and wide lamellar thickness distributions(13 nm?30 nm).In conclusion,PDD-TFE copolymers with different structures were successfully synthesized,and the relationship between the structure and properties of PDD-TFE copolymer were obtained.It would provide some guidance for synthesis and application of PDD-TFE copolymers in industry through this research.