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 detailedlybased 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 absolutely based on Stewart’s work. The results arelisted below.1) FEPs possess quite narrow stable flow region (<86s-1at335°C)and very poor processability; special strong interchain interaction of FEP originatedfrom the bigger size of F atom than H atom and stronger electronic polarity of C-Fbond than C-H bond is the intrinsic driving force of its poor processability. Thebigger size of F atom and stronger electronic polarity of C-F bond both cause strongchain stiffness of FEP. During shear flow, stress is cumulated quickly due todifficulty in energy dissipation within high viscosity melt, and thus exceeds criticalstress of melt fracture and melt distortion occurs.2) Compared to FEP, ETFE holdswider stable region and superber processability. Researching on the high shear rateextrusion of FEP, ETFE and PE, it can be concluded that the more in fluorinesubstitution the narrower steady flow region, the lower critical shear stress in meltfracture and the higher flow activation energy polymers appear.3) Process rheologyis adept enough in evaluation the processability of PFSFs and reflecting prompt theeffect of microscopic structures of PFSFs on their process performance. It appearthat the PFSF with narrow molecular weight distribution (MWD)(nPFSF) possesseshigh shear viscosity and the viscosity sharply shifts down when shear rate increases.While the PFSF with wide MWD (wPFSF) holds low shear viscosity and shearviscosity decreases slowly at the first and then the decrease accelerates with slowrate. At the same time, the nPFSF holds high melt elasticity and very low meltfracture critical shear stress (~0.07MPa within260~280°C), far lower than that ofcommon polymers and other fluoropolymers, melt distortion appears extremely early.Contrary, the wPFSF, even its molecular weight (MW) is twice than the nPFSF, bearslow melt elasticity and wide steady flow region.2. Dynamic oscillatory shear rheological characteristics of FEP, ETFE and PFSFwere 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 nonlinear regularization algorithm. Plateau modulus and criticalentanglement molecular weight (Mc) of the fluoropolymers were calculated. Resultsconfirm that relaxation time spectra can characterize the molecular motion andrelaxation of the fluoropolymers accurately, and also map molecular structuresdependence of the fluoropolymers melt flow.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. Via Whilhelm model onnonlinear dynamic rheology, critical strain, which nonlinear viscoelasticity ofpolymer melts initiates, of ETFE is0.17and correlating critical shear rate is0.76s-1.Critical strains of PFSF-A,-B and-C are0.28,0.32and0.42, respectively. Largercritical strain means better processability and stronger resistance to deformation andstructure breakage, which means that nonlinear dynamic rheology is capable of inestimation on 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 polyolefinsshifts 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, or degradation of PFSF during measuring which needsfurther experiments.