Crystal Deformation And Crystallization Behavior Of Poly(Vinylidene Fluoride) And Its Copolymer Under Uniaxial Deformation

Polymer is widely used in many fields,such as farming,industry,daily life and so on.As we know,two thirds of them are crystalline.Thus,polymer crystallization plays a very important role in polymer physics.Flow induced crystallization can help control crystal forms,accelerate nucleation and control morphology.Besides,stretch of semicrystalline polymer will induce crystal orientation,phase transformation,crystal destruction and so on.Flow induced crystallization and crystal deformation can both decide final crystal morphology of polymer and product performance.Study on flow induced crystallization and crystal deformation will help industrial processing.Poly(vinylidene fluoride)(PVDF)has varied polymorphisms and can be used as piezoelectric polymer.Besides,copolymer based on PVDF can be used widely in industry and daily life.Copolymer of vinylidene fluoride(VDF)and chlorotrifluoroethylene(CTFE)such as F2311 and F2314 can be used as binder in polymer bonded expolosives.In our work we study flow induced crystallization and crystal deformation of PVDF and its copolymer with small and wide angle x-ray scattering(SAXS and WAXS).In order to obtain direct result about crystal deformation during necking,we also study plastic deformation process during neck propagation of isotactic polypropylene(iPP)with help of Fourier transform infrared spectroscopy(FTIR).Our main results are as following.(1)Flow induced crystallization of lightly crosslinked PVDF melt is studied with in-situ SAXS and WAXS technology.Polar crystal,β crystal,can be induced in flow induced crystallization.Crystal form and morphology are distributed differently with temperature.Temperature can be divided into three regions.Region I:shish-kebab morphology&α-β crystal,Region II:shish-kebab&β crystal,Region Ⅲ:shish&β crystal.(2)Relationship between crystal deformation and phase transformation and its mechanical behavior is systematically studied with in-situ WAXS technology.Results show that phase transformation of PVDF has close relationship with stretching temperature.When temperature is lower than 110 ℃,β crystal nucleates and orients in tensile direction after yield.Meanwhile,content of a crystal decreases with strain.When temperature is higher than 110 ℃,β crystal doesn’t nucleate after yield strain and a crystal shows highly orientation in tensile direction.It is due to decrease of external work at yield at high temperature,which can help overcome energy barrier of phase transformation from α to βcrystal.(3)Structural origin of fast yield-strain hardening is studied with in-situ SAXS and WAXS.Solution cast F2314 films has soft rubbery and hard crystalline/microphase separated domains.Upon stretching,crystals and CTFE domains are destructed first which induces obvious yielding behavior.After that,CTFE is stretching induced crystallization with a much smaller long period which gives rise to an increase of stress.Similarity between VDF and CTFE can also help crystallization of CTFE and induce defects with VDF chains.Fast transition from yielding to strain hardening during extension may inhibit instability or failure of material,which is wanted in material design.(4)Crystallization behavior of F2311 is studied with different stretching rate with in-situ SAXS and WAXS technology.Solution cast F2311 cannot easily crystalize compared with F2314.When stretching rate is small as 10 μm/s,CTFE chains have enough time to adjust their conformation and can crystallize at large strain.It also shows that when stress is unloaded,crystals will be melted.However,when stretching rate is increased to 50 μm/s,CTFE chains don’t have enough time to adjust its conformation and it is hard to crystallize.When stress is unloaded,oriented network can retract to its original state.It indicates that it is quite difficult to crystalize for fluoroelastomer F2311.(5)Plastic deformation of isotactic polypropylene(iPP)is studied with Fourier transform infrared spectroscopy(FTIR).It can help give information of spatial distributions of crystallinity,orientations of crystal and amorphous chains in necking region during tensile deformation of isotactic iPP.As we know slip occurs at yield.Decrease of crystallinity during neck propagation is supposed to be related with crystal slip,and the recovery is related with melting-recrystallization upon necking.Transition from necking region to non-necking region,fragile crystals are melted and fragmented.Highly oriented amorphous chains can be resource of recrystallization and shows decrease of orientation in necking region.Innovation point:(1)Polar crystal(β crystal)can be induced during stretching of lightly crosslinked PVDF melt and we establish a phase diagram of crystal form and its morphology with temperature and strain.(2)Establish relationship between α-β crystal transformation and stretching parameters.(3)Disclose structure origin of fast yielding strain hardening transition during stretching of F2314.Crystallization of CTFE can help prevent instability or failure of sample.(4)It is difficult for F2311 to crystallize even when using solution cast method.F2311 sample needs enough time to adjust its conformation and then it can crystallize.Thus,F2311 can only crystallize at slow stretching rate.(5)During plastic deformation of iPP,stretching induced melting recrystallization occurs during necking propagation.