In the uniaxial elongation process of polymer fibers, fiber deformation (or stretch magnification) is the most important factor due to stretch magnification, and the fiber orientation caused by it depends to a large extent on the nature and structure of the structural units in the system. Mechanical behavior The draw ratio has a great influence on the mechanical properties of polyimide fibers. In a certain range, the larger the draw ratio, the stronger the fiber orientation and the better the mechanical properties. But when the stretching ratio is too large, that is, the strong stretching force is greater than the force between the polymer chain and the chain in the fiber, not only the structure of the fiber is destroyed or even broken. Therefore, the stretching ratio is determined according to the different elongation at break after the fiber is imidized, and the stretching ratio increases or decreases in proportion to the elongation at break. At the same time, the appropriate draw ratio can improve the thermal stability of the polyimide fiber.
(1) The effect of draw ratio on the mechanical properties of polyimide fiber. Take the polyimide fiber PMDA-BPDA/OTOL (molar ratio (4-6)/10) system prepared by Zhang Chunling and others as an example, using dry-wet spinning, the solvent is p-chlorophenol, and the nascent one-step spinning method The mechanical properties of the polyimide fiber are obtained by stretching the fiber through a hot air rain channel at 500°C. After the nascent fiber is stretched, the breaking strength increases by 2 times from the fibril to the draw ratio of 2.8; when the draw ratio continues to increase to 3.2, the breaking strength decreases instead. When the polyimide fiber is stretched to 2.8 times, the modulus is significantly increased, which is nearly 10 times that of the fibril, and the elongation at break drops to about 1/3 of the fibril; and when the stretching ratio is 3.2, the stretching The strength and modulus are significantly higher than that of the nascent fiber, but compared with the draw ratio of 2.8, there is a significant decrease. The increase of the stretching ratio also affects the crystal morphology of the polyimide fiber. From the wide-angle X-ray diffraction pattern of polyimide fiber (PMDA-BPDA/OTOL) (Figure 2-24), it can be seen that the nascent fiber presents a fairly wide dispersion peak; when the stretching magnification is 2.8, the diffraction peak becomes narrower. The degree of crystallinity increases; when the stretching ratio reaches 3.2, the diffraction peak becomes wider and the degree of crystallinity decreases. As the stretching ratio increases, the crystal orientation of the fiber increases, that is, after the polyimide fiber is stretched, the crystal orientation of the fiber axis increases, so that the strength and modulus of the fiber axis increase. And this stretching along the fiber axis increases the crystal plane size in the fiber axis direction and reduces the radial size. When the stretching ratio continues to increase to 3.2, the crystallinity of the polyimide fiber decreases, so that the strength and modulus of the fiber are reduced, while the elongation at break is almost unchanged.
(2) The effect of draw ratio on the thermal stability of polyimide fiber. It can be seen from the thermal weight loss curve of polyimide fiber under different stretching ratios that at 800℃, the weight loss of the nascent fiber is 58%, the sample with the stretching ratio of 28 times loses 32%, and the sample with the stretching ratio of 32 times. 48% weightlessness. Appropriate stretching ratio can improve the thermal stability of polyimide fiber (PMDA-BPDAZOTOL).