Phenomenological Theory of First-Order Prefreezing

Percolation transitions of confinement-induced layering and intralayer structural orders in three-dimensional Yukawa liquids

The disorder-order transitions of layering and intralayer structural orders of three-dimensional Yukawa liquids, under the enhanced confinement effect with decreasing normal distance z to the confinement boundary, is investigated numerically. The liquid between the two flat boundaries is segmented into many slabs parallel to the boundary, with the same slab width as the layer width. In each slab, particle sites are binarized into sites with layering order (LOSs)/ layering disorder (LDSs) and with intralayer structural order (SOSs)/disorder (SDSs). It is found that with decreasing z, a small fraction of LOSs starts to heterogeneously emerge in the form of small clusters in the slab, followed by the emergence of the large percolating LOS clusters spanning over the system. The smooth rapid rise of the fraction of LOSs from small values followed by their gradual saturations, and the scaling behavior of multiscale LOS clustering, are similar to those of the nonequilibrium systems governed by the percolation theory. The disorder-order transition of intraslab structural ordering also exhibits a similar generic behavior as that of layering with the same transition slab number. The spatial fluctuations of local layering order and local intralayer structural order are uncorrelated in the bulk liquid and the outmost layer next to the boundary. Approaching the percolating transition slab, their correlation gradually increases to the maximum.

Heterogeneous Crystal Nucleation from the Melt in Polyethylene Oxide Droplets on Graphite: Kinetics and Microscopic Structure

It is well known that the crystallization of liquids often initiates at interfaces to foreign solid surfaces. In this study, using polarized light optical microscopy, atomic force microscopy (AFM), and wide-angle X-ray scattering (WAXS), we investigate the effect of substrate–material interactions on nucleation in an ensemble of polyethylene oxide (PEO) droplets on graphite and on amorphous polystyrene (PS). The optical microscopy measurements during cooling with a constant rate explicitly evidenced that the graphite substrate enhances the nucleation kinetics, as crystallization occurred at approximately an 11 °C higher temperature than on PS due to changes in the interactions at the solid interface. This observation allowed us to conclude that graphite induces heterogeneous nucleation in PEO. By employing the classical nucleation theory for analysis of the data with reference to the amorphous PS substrate, the obtained results indicated that the crystal nuclei with contact angles in the range of 100–117° were formed at the graphite interface. Furthermore, we show that heterogeneous nucleation led to a preferred orientation of PEO crystals on graphite, whereas PEO crystals on PS had isotropic orientation. The difference in crystal orientations on the two substrates was also confirmed with AFM, which showed only edge-on lamellae in PEO droplets on graphite compared to unoriented lamellae on PS.

Concepts of Nucleation in Polymer Crystallization

Nucleation plays a vital role in polymer crystallization, in which chain connectivity and thus the multiple length and time scales make crystal nucleation of polymer chains an interesting but complex subject. Though the topic has been intensively studied in the past decades, there are still many open questions to answer. The final properties of semicrystalline polymer materials are affected by all of the following: the starting melt, paths of nucleation, organization of lamellar crystals and evolution of the final crystalline structures. In this viewpoint, we attempt to discuss some of the remaining open questions and corresponding concepts: non-equilibrated polymers, self-induced nucleation, microscopic kinetics of different processes, metastability of polymer lamellar crystals, hierarchical order and cooperativity involved in nucleation, etc. Addressing these open questions through a combination of novel concepts, new theories and advanced approaches provides a deeper understanding of the multifaceted process of crystal nucleation of polymers.

Solute-adsorption enhanced heterogeneous nucleation: the effect of Cu adsorption on α-Al nucleation at the sapphire substrate

Interfacial adsorption of solute atoms is a promising means to tune heterogeneous nucleation. In this study, a new method has been established to theoretically evaluate the effect of solute addition on the nucleation potency of heterogeneous nucleation interfaces. The evaluation consists of three steps: (1) analyzing the solute adsorption behavior; (2) determining the nucleation mode; and (3) evaluating the effect of solute adsorption on nucleation potency using the solute-adsorbed interface model. A combination of the ab initio and molecular dynamics methods together with the two-phase thermodynamic model was used to evaluate a prototype Al-Cu/(0001) sapphire interface. It is found that solute Cu atoms adsorb at the interface between the melt and (0001) sapphire interface. The adsorption is driven by the strengthening of the Cu-Al bonds as revealed by the Bader charge analysis which is demonstrated to reduce interfacial energy. Furthermore, it is revealed that the interfacial adoption of Cu results in the formation of an Al-Cu adsorption layer, which enhances the interfacial chemical affinity thus enlarging the nucleation driving force. Meanwhile, the lattice mismatch between the sapphire substrate and the primary Al (α-Al) nucleus is decreased by Cu addition, which lowers the barrier for nucleation. The above two effects together increase the nucleation potency of the studied interface, which is in good agreement with previous experiments. It is proposed that the effect of solute adsorption shall be considered in the search for effective substrates for tuning the nucleation.