Stretching the fiber is an important link in the process of preparing high-performance fibers. Undrawn fibers often show low strength, high irreversible (plastic) deformation and low modulus. Stretching stretches the nascent fiber irreversibly to 20%-200% of its original length under solid conditions. This elongation is often accompanied by the stretching and orientation of macromolecules and crystallites along the fiber axis, and the orientation often has phases. Structural changes (crystallization or partial destruction of crystalline regions) and changes in other structural characteristics.
The stretching of polyimide fiber refers to the imidization of the fiber before its microstructure is completely fixed, under a specific tension and temperature (>7), the curled and disordered macromolecules are arranged in the axial direction. And the process of stretching. The excellent performance of polyimide fiber is not only due to its special chemical structure, but also from the high orientation of molecular chains along the fiber axis and the two-dimensional orderly arrangement in the transverse direction. Polyimide fibers are generally semi-crystalline polymer materials. Through stretching, the amorphous and crystalline regions will be oriented along the fiber axis. However, to obtain high-performance polyimide fibers, high Crystallinity and high degree of orientation. In this process, disordered macromolecules develop in an orderly direction, the contact points between macromolecules increase, the intermolecular force increases, and the aggregation area expands, which provides conditions for the crystallization of fibers. At this time, the density of the fiber increases, and the tensile breaking strength increases; the fiber fineness decreases, and the elongation at break decreases; the surface gloss and thermal conductivity of the fiber present anisotropy. In short, the comprehensive mechanical properties of the fiber after stretching are improved, and the practical value is increased.
The stretching process of polyimide fiber is carried out on a stretching device with two or three sets of guide rollers or guide discs with different rotation speeds. The stretched fiber or tow passes between the spacing of the guide rollers or guide discs. The speed ratio of the guide rollers or guide discs at both ends is called the draw ratio (indicated by DR), and the speed difference causes the fiber to elongate.
Under normal circumstances, under the action of the tensile external force, the originally formed crystalline units are not simply arranged in the direction of the external force, and are regularly and neatly arranged, which often include the slip and rotation of the original wafer, resulting in rupture, and some The folded chain is stretched to become a straight chain, so that the original structure is partially or completely destroyed, and then under the new equilibrium conditions, a new crystalline structure is formed, that is, the folded chain lamella and penetrates in the orientation direction. A silk crystal structure formed by the stretched segments between the lamellae. Under normal circumstances, stretching a crystalline polymer often increases the straight chain part of the crystalline structure and reduces the proportion of folded chains. With the increase in the number of bound molecular chains between the wafer and the wafer and the improvement of their order, the degree of orientation of the fiber is improved, and the strength of the fiber is also improved. Harries et al. believe that in order to obtain polyimide fibers with excellent mechanical properties, the crystallization rate of the fiber during the stretching process must be controlled.The crystallization rate is too fast, which is not conducive to the stretching of the fiber and thus is not conducive to the orientation of the crystallites; At the same time, due to the difference of the process, the crystallinity of the fiber will directly change during the stretching process, resulting in the change of the crystal size and morphology. Therefore, the suitable drawing frame rate, drawing tension and drawing temperature together constitute the drawing process parameters of polyimide fiber.