The Crystallization Behavior And Properties Of Biaxially Stretched PA612 Films

Polyamide 612(PA612),called Nylon 612 as well,it exhibit s advantages in low moisture sensitivity,high flexibility and ductility over short-chain nylons and superior mechanical strength compared with long-chain nylons.Therefore,PA612 can be applied in various fields,including automobile manufacturing,aerospace engineering and electronic appliances.PA612 films exhibits great potential as packaging material in virtue of its low moisture absorption,good dimensional stability and high mechanical strength.Stretching has a significant effect on the microstruct ure and ultimate performance of semi-crystalline polymers.In order to investigate the effect of stretching on structure and mechanical properties of stretched PA612 films,PA612 films were subjected to unrestricted uniaxial stretching,restricted uniaxial stretching and biaxial stretching.Wide-angle X-ray diffractometer(WAXD),Two-dimensional wide-angle X-ray Scattering(2D-WAXS),and two-dimensional small-angle X-ray Scattering(2D-SAXS)were employed to analysis the microstructure,the crystal orientation,and the crystal lamellae orientation.The results show that the crystal forms and crystal structure of PA612 in the three stretching modes have not changed obviously.In the unrestricted uniaxial stretching and restricted uniaxial stretching procedure,crystals are oriented along the stretching direction and the c axis was parallel to the equatorial direction after stretching.In the biaxial stretching procedure,the orientation direction of crystals is mainly affected by the stretching direction.The crystallization property of stretched PA612 films was analyzed by differential scanning calorimeter(DSC).During restricted uniaxial stretching,stress-induced crystallization is observed,but the stress-induced crystallization isn't obvious during restricted uniaxial stretching and biaxial stretching.After stretching,due to the orientation of molecular chains in the amorphous region and crystal orientation,the mechanical strength of the film increased significantly,but the elongation at break decreased.After DMA testing,the glass transition temperature and storage modulus are significantly improved,and the peak of loss factor appeared to decrease,indicating that the material elasticity was enhanced and the viscosity decreased.The transmittance of the films subjected to the restricted uniaxial stretching and biaxial stretching has a weak increase,the change of haze is not obvious,and the barrier coefficient of oxygen and water vapor in the films is obviously decreased,which is more conducive to t he practical application of the films.The influence of the structure of amorphous region on the film transmission coefficient was studied by characterization of the structure of amorphous region.After stretching,its ? relaxation temperature increases,the free energy of chain motion increases,and the free volume fraction decreases.With the increase of stretching ratio,the higher the free energy of chain motion,the lower the free volume fraction,and the lower the transmittance coefficient of the film.