Polyester Fiber: Orientation Effects on Properties

High-speed spinning technology significantly influences the structural properties of polyester (PET) fibers. This process involves drawing the fiber at high speeds, which aligns the polymer chains.

Understanding Fiber Structure and Birefringence

• Crystalline Orientation: In PET fibers spun at high speeds, the polymer chains within the crystalline regions become highly aligned along the fiber axis. This contributes to a high degree of crystalline orientation.
• Amorphous Orientation: However, the amorphous regions (regions where polymer chains are less ordered) may not achieve the same level of orientation as the crystalline regions under these conditions. They can remain relatively unoriented or randomly arranged.
• Birefringence: Birefringence ($\Delta n$) measures the difference between the refractive indices for light polarized parallel ($n_e$) and perpendicular ($n_o$) to the fiber axis (${\Delta n = n_e - n_o}$). It is directly related to the overall molecular orientation. While oriented crystalline regions increase birefringence, unoriented amorphous regions possess random refractive indices, which do not contribute effectively to the overall anisotropy.

Reason for Low Birefringence

Therefore, the combination of high crystalline orientation and relatively unoriented amorphous regions in high-speed spun PET fibers leads to a high degree of crystalline alignment but results in lower-than-expected birefringence compared to fibers where both crystalline and amorphous regions are well-oriented.

Conclusion

The phenomenon observed is due to oriented crystalline but unoriented amorphous regions.