Study On The Physics Of Biaxial Oriented Polyethylene Terephthalate Film Processing By In-situ Synchrotron Radiation

The biaxial oriented polyethylene terephthalate(BOPET)film has the characteristics of high light transmittance,high mechanical strength,excellent oxygen and moisture resistance,stable physical and chemical properties,etc.The BOPET film has extremely wide applications in packaging,printing,photovoltaic,optical display and other special fields.With the rapid development of photovoltaic,electronic appliances and new display industries,the demand for BOPET films with special properties is becoming more and more urgent,such as the need for films with extremely low or extremely high orientation,high temperature resistance,fatigue resistance and low creep performance,etc.However,the lack of basic theory on the structural evolution process of each section in the actual production and processing of BOPET makes the industry lack underlying supporting principles and technologies when improving product performance and developing functional products,which brings difficulties to the industry breakthrough.The processing of BOPET is a complex physical process coupled with multiple process steps and processing fields.The processing technology mainly includes dehumidification and drying,melt extrusion,cast casting,longitudinal stretching,transverse stretching,and heat setting,etc.Among them,dehumidification and drying are the annealing and crystallization process;melt extrusion and cast casting are typical polymer processing rheological processes;stretching induced chain orientation(SIO)and crystallization(SIC)behavior happens during machine direction(MD)and transverse direction(TD)stretching;heat treatment process involves the relaxation of molecular chains and crystal growth and perfection.Physical changes such as SIC and crystallization during chain relaxation are challenging problems in polymer processing.Due to the many process steps,the structures and properties of BOPET products are the result of multi-step process coupling.Decoupling the coupling effect is also a challenge faced by academia and industry.Based on the above background,from the perspective of BOPET production process,this dissertation systematically studies the non-equilibrium structural evolution of PET during MD and TD stretching and heat treatment by decoupling the multi-stage process conditions.Due to the advantages of high brightness,collimation and good coherence,synchrotron radiation X ray has obvious advantages of high time resolution and high space resolution in structural characterization.In current work,the technology of combining synchrotron radiation X-ray wade angle scattering(WAXS)and stretching device is used to in situ track the crystallization and structural evopution of PET in the simulated industrial processing.The specific research contents and results are as follows:(1)Taking water as the plasticizer,the effect of chain mobility on the SIC of PET at temperatures slightly above the glass transition temperature(Tg)was investigated.By analyzing the mechanical response behavior,the relationship between crystallization behavior and water content,and the dependence of crystallization behavior on the total work done by the external system during the stretching process of samples with different plasticization degrees,the necessity of stretching-induced chain activation in SIC is demonstrated.The improvement of chain mobility in the plasticizing system reduces the stress during stretching,and the improvement of chain mobility does not promote crystallization,confirming that chain stretching and orientation play important roles in SIC near Tg.(2)The effect of deformation rate on stretch-induced orientation and crystallization of PET was investigated.The orientation and crystallization behavior of amorphous chains were compared and analyzed,and it was found that the deformation of the molecular chain network had a linear dependence on the natural logarithm of the deformation rate.The material exhibits significant orientation at low deformation rates.At high deformation rates,the strain corresponding to the onset of crystallization is also linearly related to the natural logarithm of the stretching rate,which demonstrates that chain relaxation plays a key role in SIC.(3)The asynchronous stretching process in BOPET production was simulated by stretching in the MD and TD),and the effect of MD pre-stretching on the structural evolution during TD stretching was investigated.The structural parameters of the microscopic crystallinity,crystal plane orientation and amorphous chain orientation in the two-dimensional space of MD and TD are constructed,and a molecular model of MD pre-stretching to transverse stretching to induce crystallization and form a crystal network is proposed.(4)The effects of pre-orientation structure on the crystallization kinetics and crystal morphology of PET films during heat treatment were investigated by subjecting the samples with different initial orientation structures to elevated temperature.By analyzing the crystallization starting point and crystallinity growth trend of different samples,it is found that the initial orientation structure acts as a nucleation point during the heating process,which significantly shortens the crystallization induction period.At the same time,the crystal morphology after heat treatment was analyzed,and it was found that the crystal always grew along the orientation direction of the initial structure during the heat treatment process,and the stress relaxation would cause a slight change in the crystal morphology.Different roles of pre-orientation and stress relaxation in regulating crystal morphology are demonstrated.