Structure Formation of Blend and Sheath/Core Conjugated Fibers in High-Speed Spinning of PET,Including a Small Amount of PMMA

The blend of poly(ethylene terephthalate) (PET) and a small amount of polymer that has higher T _g than PET, such as polymethylmethacrylate (PMMA)—and is dispersed finely as immiscible particles in PET—exhibits lower molecular orientation than pure PET under high-speed fiber spinning. To obtain insight into the mechanism of the lower molecular orientation of the blend fiber, the sheath/core structure of PET (sheath)/PMMA (core) conjugated fiber (the same PET/PMMA weight ratio as in the blend fiber), was produced as a model. The thinning profile of the fiber diameter along the spinning line and the birefringence distribution of the cross-section were examined and compared among pure PET fiber, the conjugated fiber, and the blend fiber. The conjugated fiber had the lowest molecular orientation of PET in the sheath part, and its thinning process was accelerated similar to the blend fiber. As a result of the distribution of molecular orientation across the diameter of the conjugated fiber, it is considered that PMMA, having the high T _g , tends to solidify at a higher temperature (upstream) than PET in the thinning process, making the flow of PET accelerate as if it was pushed by PMMA. This causes the maximum dv/dx just before the solidification point to be smaller; therefore, the lower spinning stress, resulting in smaller birefringence of PET, can be considerable. This acceleration was generated at the interface of PET and PMMA, and spread toward the fiber surface as both polymers were thinning in elongational flow (in melt). On the other hand, close to the interface, molecules of PET were stretched by PMMA, resulting in an increase of birefringence. Such discussion is also considered to apply to the blend fiber. However, because the blend fiber had a significantly larger interface area compared with the conjugated fiber, it is considered that the increase of birefringence of PET by the interface drag force cannot be neglected. The larger particles of PMMA dispersed in PET results in the lower birefringence of PET that is supported by the elongation effect (i.e., the interface drag force).