Size Effect of Form I Nanocrystals in Heterogeneous Nucleation of Polybutene-1

The effect of Form I nanocrystal size on the induction time (ti) of isotactic polybutene-1 (iPB-1) heterogeneous nucleation has been investigated using small-/wide-angle X-ray scattering (SAXS/WAXS). By controlling the crystallization temperature (Tc1) of starting Form II and the transition time from Form II to Form I, respectively, we tune the height (h) and radius (r) of cylindrical Form I nanocrystals. A faster rate (i.e., a shorter ti) of heterogeneous nucleation on the larger nanocrystals (h ∼ 57 nm, r ∼ 23 nm) can be observed at the same mass fraction, even if the number density (N) of the smaller nanocrystals (h ∼ 37 nm, r ∼ 13 nm) is approximately five times that of the larger ones. In addition, we observed a dramatic increase (nearly 600%) in (1/ti)/N, representing the nucleation ability of individual nanocrystal, when h increases from ∼37 to ∼57 nm (by ∼54%) at a constant r of ∼15 nm, whereas (1/ti)/N only exhibits a ∼110% increase when the nanocrystal increases its radius by ∼120% at a constant h (e.g., ∼37 nm). These results indicate that h, rather than r, plays a central role in heterogeneous nucleation. Further, in a size range of h = 27–57 nm, we found the ti scales with h according to a power law ti ∼ h–α, where the exponent α increases with secondary crystallization temperature (Tc2). This reveals that the impact of h on the heterogeneous nucleation rate intensifies as the critical nucleus size increases. Our findings demonstrate that such an experimental model system could shed light on investigating the size effect of polymer nanocrystals in heterogeneous nucleation.