Origin of transcrystallinity and nucleation kinetics in polybutene-1/fiber composites

This work presents an in-depth study of fiber-induced nucleation and crystalline morphology in polybutene-1/single-fiber composites. The nucleation ability of various fibers, including carbon, glass, polypropylene (PP), poly(L-lactide) (PLLA), and stereocomplex (SC)-poly(lactide), is investigated via polarized optical microscopy. A polybutene-1 (PB-1) fiber made of the trigonal Form I polymorph is also adopted in an all-PB-1 composite. Two different types of morphologies of PB-1 Form II developed on the surface of the different fibers during isothermal crystallization. They are a transcrystalline layer (TCL) on the Form I fiber only and a hybrid shish-calabash (HSC) on all of the other fibers. Upon nonisothermal crystallization, a transition from the HSC to TCL morphology for the sole PP fiber is found. Quantitative studies of nucleation kinetics showed that the lowest nucleation free-energy barrier is exhibited by the PB-1 Form I fiber, due to the occurrence of cross-nucleation with Form II, possibly via epitaxial matching. On the other hand, the highest nucleation energy barrier is that of the PP fiber, despite the very similar chemical constitution. Moreover, a large variation in the pre-exponential factor of the nucleation rate among the various fibers suggests major differences in the available nucleation sites per unit area. The number of such sites is found to inversely correlate with surface roughness and, for the first time, is used to understand the obtainable morphology. In fact, we show that, notwithstanding the height of the free-energy barrier for nucleation, TCL can only be obtained when a sufficiently high number of nucleation sites are provided.