Two mechanisms of polymer chain crystallization within nanoglobule

In molecular dynamics (MD) simulation, the crystallization behavior of a polymer nanoglobule was studied as a function of temperature. The crystallinity in the nanoglobule was determined by using the site order parameter method [Macromolecules, 2008, 41: 6733]. The isothermal crystallization kinetics was determined at different temperatures and the rate of crystallization, Kc, and the Avrami exponents, n, were determined as a function of temperature. The typical bell-shape curve of the crystallization rate vs. temperature was observed to be asymmetric and very different from the symmetric bell-shaped behavior seen in bulk crystallization. This can be attributed to a lower chain diffusion activation free energy near the surface of the nanoglobule than in bulk. The obtained Avrami exponents n are non-integer and vary with temperature, ranging from close to n = 1 at the lowest investigated temperatures and reaching n = 4 for the highest investigated crystallization temperatures. A detailed morphological and statistical analysis of the MD snapshots and trajectories suggests that at least two kinds of crystallization environments and corresponding mechanisms are active: one in the (∼3 nm) surface layer and one in the inner part of the nanoglobule. The two crystallization mechanisms were used in an approximate binary model providing a satisfactory prediction of the observed overall Avrami exponents.