Study On Physics Of Polymer Film Stretching Processing

With the development of industry,polymer f-ilms have become very important products in agriculture,light industry,aerospace,energy and other fields.Due to the long-chain nature of polymer,imposing stretch on polymer can effectively improve the film property.Agricultural films,packaging films,optical films and microporous films are all produced by such methods.These stretching processing methods include film blowing,casting and uniaxial/biaxial stretching,during which polymer films are uniaxially or biaxially stretched and cooled quickly by air ring.For semicrystalline polymers;,stretch-induced nonlinear rheology,crystallization and deformation or phase transition of crystals occur during these stretching processing,which are still the unsolved scientific problems.Considering film stretching is a complex external field driven fast multi-scale structural evolution process,it is challengeable to experimentally study the fundamental non-equilibrium scientific questions during film stretching processing.With high time resolution and environmental adaptability,in-situ technique based synchrotron radiation has become a very powerful tool in studying polymer physics during film stretching processing.Aiming to solve physical problems mentioned above,polyethylene(PE),polyethylene terephthalate(PET),polyvinyl alcohol(PVA)and Polycaprolactam(PA6)are chosen as research objects.Combining high time-and space-resolved synchrotron radiation X-ray,the structural evolutions of PE,PET,PVA and PA6 are tracked during the stretching processing,which are important to understand the non-equilibrium physics during processing and give a guideline to improve the properties of polymer films.The main results and conclusions of these works are summarized as follows:(1)Combining a homemade film blowing machine and in-situ synchrotron radiation small-and wide-angle X-ray scattering(SAXS and WAXS),an investigation on film blowing of polyethylene(PE)is studied.From the die exit to the positions above the frost line,four zones are defined with different structural features as observed with SAXS and WAXS measurements.In Zone ?,precursor and crystal emerge from the polymer entanglement network during cooling and extension,which lead to the formation of a deformable crystal-crosslinked network at the boundary between Zones I and II.The occurrence of the crystal-crosslinked network enhances the effective chain stretch during further deformation in Zone ?.Crystallization is largely accelerated,which generates crystal with high orientation.Further increasing crystallinity results in the deformable crystal-crosslinked network transforming into a non-deformable crystal scaffold at the frost line(boundary of Zones ? and ?),which stabilizes the bubble and prevent further deformation.Zones ? and IV are crystal filling processes of the scaffold and the entire sample.Interestingly,increasing take-up ratio(TUR)does not influence the critical crystallinity(2.5%)for the formation of the deformable crystal-crosslinked network,while the crystallinity at the frost line or for the formation of non-deformable scaffold does vary with TUR,suggesting that the former is mainly controlled by molecular parameters while the later is determined by both processing and molecular parameters of PE material(2)Strain-induced crystallization(SIC)of poly(ethylene terephthalate)(PET)slightly above glass transition temperature(Tg)is investigated by adopting a sequential biaxial stretching mode in this work.Small angle X-ray scattering(SAXS)and wide angle X-ray diffraction(WAXD)are employed to study variation of orientation parameter and crystallinity of stretch films in machine and transverse draw ratio space.Our results reveal that improving chain mobility by stretching is necessary before crystallization occurs slightly above Tg,during which the total external work of sequential biaxial stretching is not dissipated but "stored" in polymer chains with high mobility.The total crystallinity is precisely correlated with the total external work,which does not relate to stretch direction and demonstrate that enhanced chain mobility has no directionality.Considering the structures that restrict the polymer chains in the initial sample,a molecular model is proposed to interpret the structure changes during SIC of PET above Tg.(3)The structural evolution(not only intra-fibrillar but also inter-fibrillar structures)during extension of poly(vinyl alcohol)(PVA)in water at different temperatures are investigated by in-situ small-and wide-angle X-ray scattering(SAXS and WAXS).The stress-strain curves can be defined into four zones with the onset and the end of stress plateau,and the strain hardening point as the zone boundaries.WAXS results show that stretch induces melting of PVA crystal from zones ? to?,while SAXS measurements reveal stretch-induced transformation from initial lamella to nanofibrils starts at the onset of zone ?,which proceeds via a melting-reconstruction process instead of direct transformation.In zone ?,the nanofibrils increase in content and pack periodically with an average inter-fibrillar spacing of about 14-18 nm.Increasing temperature leads to larger inter-fibrillar spacing and more perfect packing of nanofibrils,which may benefit the production of polarizer with homogeneously structural and optical properties.(4)Stretch-induced phase transitions of polycaprolactam(PA6)have been studied with in-situ synchrotron radiation wide-angle X-ray scattering(WAXS)technique during uniaxial tensile deformation at different temperatures.Two crystal forms,a and? with different chain conformations and hydrogen-bond distributions are observed,which can transform from one to another in strain-temperature space.Below the Brill transition temperature(115 ?),stretch induces a a to ? transition,while unexpectedly a stretch-induced pseudo-reentrance phase transition ?-a-? is observed above 115 ?,which may be attributed to the coupled effect of stretch and temperature.It is found that,with the help of thermal fluctuation above 115 ?,a phase transition from ? to a is induced by the extension of polymer chains,while shearing among intra-sheet chains further drives a form transforming to ? form.A non-equilibrium phase diagram in strain-temperature space and schematic images of structural evolution during stretching at different temperatures are constructed,which may give a guideline to improve the properties of PA6 products.