Effect of swelling degree on structural evolution and orientation behavior of Poly(vinyl alcohol) films during stretching

For the preparation of polarizing film, the initial condensed structure of the PVA film is the key factor to determine the subsequent process conditions such as dyeing and stretching of PVA film. In the present work, the difference of the initial condensed structure of Poly (vinyl alcohol) (PVA) films with different swelling degrees and their influence on the orientation behavior and morphological structure evolution during stretching were investigated in detail. The results showed that the PVA film with a higher swelling degree possessed a lower content of microfibril structure at the same strain and the higher critical strain εs, revealing that the undissolved lamellae could act as the precursor for the formation of oriented microfibrillar structures. Furthermore, in order to investigate the influence of different swelling degrees on the lamellar orientation and the formation of microfiber structure during the stretching of PVA films, the long period of lamellae (Lp), the period of nanofibrils (Lf), and the Full Width at Half Maxima (FWHM) of Lf were calculated. It was found that the Lp and the Lf of the microfibrillar structure in the equatorial and meridional directions were larger and the FWHM of Lf was smaller for PVA films with higher swelling degree, indicating that the PVA film with a higher swelling degree could form a more relaxed and uniform periodically folded microfibrillar network, resulting in a higher light transmission in the visible region. Therefore, on the premise of ensuring the mechanical strength, the PVA film with a relatively higher swelling degree can be used to obtain commercial polarizing films with excellent light transmittance.