Study On Rheological Behaviors And Processability Of Tetrafluoroethylene-Based Thermoplastic Fluoropolymers

Fluoropolymers, especially perfluorinated ionomers, are essential highperformance materials employed in fundamental basic industries and Hi-tech fieldssuch as aerospace exploration, energy utilization, and defense and securityapplications etc, and therefore attract continually intense and enormous attentionfrom all over the world. Moreover, with growing anxieties in resources exhaustingand insufficiency in energy support, fluoropolymers are a growing rapid hot focus ininternational scientists and engineers’ community and thus driving them to getinsight into the intrinsic relationship between microscopic structures andmacroscopic properties (the structures-properties relationship) of fluoropolymers.Understanding on such relationship not only guides synthesis, production, andprocess of fluoropolymers, and thus qualifies better application in the fundamentalbasic industries and Hi-tech fields above mentioned, but also set partially atheoretical basis for the further improvement and optimization of fluoropolymersperformance.Among a great variety of techniques in exploring the structures-propertiesrelationship of polymers, rheology is one of such techniques with huge importance,applicability and easiness in operation. Rheology holds macroscopic statisticalcharacteristics, and rheological behaviors of materials are the mirror of their internalstructures, processability and other macroscopic performances, and herein therelationship between the processability, flow behaviors and molecular structures ofthree tetrafluoroethylene-based thermoplastic fluoropolymers, fluorinated ethylenepropylene copolymer (FEP), ethylene tetrafluoroethylene alternative copolymer(ETFE) and perfluorosulfonic acid ion exchange resin precursor (orperfluorosulfonyl fluoride resin, PFSF), was researched systematically and detailedly based on rheology, with aim to1) model, also predict, the processing and flowcharacteristics,2) control and optimize fluoropolymers forming and production, and3) tailor fluoropolymer products end performances. The outlines of this thesis are asfollowing:1. Extrusion flow behaviors with high shear rates of FEP, ETFE and PFSF werepresented. The stable flow region and the temperature, shear rate dependence ofshear viscosity of the three fluoropolymers were determined, and modeling towardthe shear viscosities was also performed. Extensional viscosities, and flowperformance and melt fracture beyond stable flow region of the three fluoropolymerswere reported. Effect of fluorine substitution on processability of linearfluoropolymers was estimated. The results are listed below.1) FEPs possess quitenarrow stable flow region (<86s-1at335°C) and very poor processability; specialstrong interchain interaction of FEP originated from the bigger size of F atom than Hatom and stronger electronic polarity of C-F bond than C-H bond is the intrinsicdriving force of its poor processability. The bigger size of F atom and strongerelectronic polarity of C-F bond both cause strong chain stiffness of FEP. Duringshear flow, stress is cumulated quickly due to difficulty in energy dissipation withinhigh viscosity melt, and thus exceeds easily critical stress of melt fracture and thenmelt distortion occurs.2) Compared to FEP, ETFE holds wider stable region andsuperber processability. Researching on the high shear rate extrusion of FEP, ETFEand PE, it can be concluded tentatively that the more in fluorine substitution thenarrower steady flow region, the lower critical shear stress in melt fracture.3)Process rheology is adept enough in evaluation the processability of PFSFs andreflecting prompt the effect of microscopic structures of PFSFs on their processperformance. It appears that the PFSF with narrow molecular weight distribution(MWD)(nPFSF) possesses high shear viscosity and the viscosity sharply shifts downwhen shear rate increases. While the PFSF with wide MWD (wPFSF) holds lowshear viscosity and shear viscosity decreases slowly at the first and then the decreaseaccelerates with slower rate. At the same time, the nPFSF holds high melt elasticityand very low melt fracture critical shear stress (~0.07MPa within260~280°C), farlower than that of common polymers and other fluoropolymers, melt distortionappears extremely early. Contrary, the wPFSF, bears lower melt elasticity and widersteady flow region.2. Dynamic oscillatory shear rheological characteristics of FEP, ETFE and PFSF were evaluated. Experiments justify that the three fluoropolymers havethermorheological simplicity. Corresponding dynamic modulus master curves ofeach the fluoropolymer were constructed and relaxation time spectra extracted fromdynamic modulus were obtained based on a more robust numeric analysis technique,Generalized regularization algorithm. Results confirm that relaxation time spectracan characterize the molecular motion and relaxation of the fluoropolymersaccurately, and also map molecular structure dependence of the fluoropolymers meltflow.3. Nonlinear dynamic rheological properties of FEP, ETFE and PFSF wereexplored for the first time. It is found that nonlinear dynamic rheology can mapsensitively effect of oscillatory shear force field on the rheological behaviors of thefluoropolymers and differentiate their molecular structures. Compared withpolyolefins, some novel nonlinear dynamic rheological phenomena occur influoropolymers. It is the first discovery and report that I31(the third harmoniccomponent) value of fluoropolymers scale up in proportion with strain acted, ratherthan with the square of strain like non-fluoropolymers reported by Hyun et al. ViaWhilhelm model on nonlinear dynamic rheology, critical strain, which nonlinearviscoelasticity of polymer melts initiates, of ETFE is0.17and critical strains ofPFSF-A, PFSF-B and PFSF-C are0.28,0.32and0.42, respectively. Larger criticalstrain of the same resin means better processability and stronger resistance todeformation and structure breakage, which means that nonlinear dynamic rheologycan estimate processability of fluoropolymers. It is found that MW and MWDdependence of I31value of PFSF is not the same as that of polyolefins. When MWdecreases and MWD narrowens, I31of PFSF increases, while that of polyolefinsreported shifts down.There exists a local maximum in the frequency function of Q0coefficient (vanishing strain Q coefficient) of PFSF, which means have molecularrelaxation or network disentanglement corresponding to this maximum. Thefrequency corresponding to this local maximum is0.5Hz, not equal to the crossfrequency of equal modulus (G’=G’’) in the dynamic modulus master curves ofPFSF. Consequently, the maximum may correlate with sulfonyl fluoride branchrelaxation or crosslinking (and/or degradation) of PFSF during measuring whichneeds further experiments.