Deformation Mechanism Of Semicrystalline Polymer During Uniaxial Stretching:from Qualitative Characterization To Quantitative Model Calculation

High-performance,functionalized films occupy a very important position in sophisticated industries,such as energy and information,and a large part of the production is through the biaxial stretching of crystalline polymers.However,in the production of various high-value-added films such as optical films and battery separators,high-end products cannot be achieved by domestic companies.The basic reason is the lack of research on the stretching process under the conditions of industry film production.Most of the existing deformation models are based on offline tests,slow speed experiments and equilibrium conditions.Hence,most of them are phenomenological descriptions,lacking of quantitative theoretical illustration.Industrial production is featured with high-speed and non-equilibrium conditions,which involves the coupling of multi-scale structures.As consequence,to establish a polymer deformation model which can be used to guide industrial production is of great significance.Uniaxial and biaxial stretching under near-industrial tensile conditions was performed in this thesis by means of a stretching device that simulates industrial production conditions.Based on synchrotron radiation X-ray scattering with high time and spatial resolution,the deformation mechanism of the stretching process was obtained.Through the micro-mechanical analysis,a quantitative evolution model was obtained for the deformation of semicrystalline polyolefin i.e.polypropylene and polyethylene.The main findings and conclusions of this paper are summarized as follows:1)With a combination of homemade temperature jump extension apparatus and in-situ synchrotron radiation small and wide angle X-ray scattering(SR-SAXS and SR-WAXS)measurements,uniaxial stretching experiments of polypropylene films at 50℃-150 ℃ are carried out to obtain the structure evolution routes.Three distinct features of the structure evolution are observed in temperature space,which can be described with three different deformation models,namely amorphization,crystalline block slip and melt recrystallization,respectively.Also,a refined morphology diagram in strain-temperature space is given to specify the morphology under two dimensional stretching parameter space.This work supplies a guidance for selecting processing conditions to obtain optimal structures after uniaxial stretching in industry.2)With a combination of in-situ synchrotron radiation X-ray scattering and uniaxial stretching,the structural evolution of polyethylene-plasticizer film is studied at temperature far below melting point of crystal.By analyzing the evolution rule of structural parameters quantitatively,stretch induced melting and recrystallization process is validated to be responsible for plastic deformation of the system.The physical essence of stretch induced melting is proved to be phase transition driven by elastic energy which can be stored by lattice deformation.On the other hand,the recrystallization process is confirmed to be controlled by temperature,furthermore,the growth of lamellae during recrystallization are in perfect accordance with kinetic theory by Lauritzen and Hoffman.This study provides a quantitative understanding to the long existing melting recrystallization model from a thermodynamics point of view.3)Structure evolution of slightly oriented polyethylene(PE)-plasticizer film is recorded with small-and wide-angle X-ray scattering(SAXS/WAXS)during stretching at temperature from 80 ℃ to 100 ℃.An interesting evolution of SAXS patterns from anisotropic ring,four-point,six-point,two point is observed,which is validated to be resulted from melting-recrystallization.With peak shift of WAXS pattern,triaxial local stresses exerted on lamellae at different azimuthal angles are acquired.As consequence,lamellae melting is revealed to be stress induced process,where stress exerted along chain direction of lamellae and stress perpendicular to chain direction are the criteria at azimuthal angle from 0° to 60° and that from 70° to 90°,respectively.Due to the heterogeneous stress distribution of the system,the appearance of four-point pattern stems from the sequence of lamellae melting at different azimuthal angles.Through thermodynamic analysis,the physical essence of stress induced melting of lamellae is validated to be elastic energy driven crystal to amorphous phase transition.4)The phase separation and crystallization during constant rate cooling of Polyethylene(PE)and Polypropylene(PP)are investigated with combination of polarized microscopy characterization and heat stage.The blend ratio of PE in PP ranges from 10%to 60%,exhibiting two distinct morphological features,i.e.isolated island PE phase dispersed in PP matrix and two interpenetrating network for 10%-30%and 40%-60%,respectively.With quantitative analysis,size of dispersed phase is the dominating factor determining the surface morphological features at PE from 10%-30%while the internal stress imbalance between interpenetrating networks is responsible for 40%-60%.The features of surface morphology keep similar by the comparison of sample at different biaxial draw ratio.Consequently,phase separation and crystallization are the most critical factor determining the surface morphology.In order to establish the relationship between microstructure formed after crystallization and haze of matt film,samples are prepared with different PE content by blending,coextrusion,film casting and biaxial stretching.The surface texture is validated to be the key factor deciding haze.Moreover,the investigation of phase separation and crystallization under static condition can be adopted to guide processing of matt film.