(28a) Experimental and Theoretical Nucleation Barriers for Polyethylene and Polypropylene

Classic experiments on homogeneous nucleation in polyethylene (PE) and polypropylene (PP) using the emulsion-drop technique provide values for the nucleation barrier. Nucleation theory predicts the barrier, in terms of the bulk free energy difference between undercooled melt and ordered phase, and the interfacial tensions of the ordered phase with adjacent melt. Nuclei for polymers are roughly cylindrical, with distinct side-face and end-face tensions. End-face tensions can be inferred from melting point suppression; recent simulations predict the side-face tensions. Evidence suggests nucleation in PE and PP proceeds via mesophase, rather than directly to the crystal. End-face tensions for the mesophase can be inferred from the crystallization line, while recent simulations can predict side-face tensions of mesophases, as well as their bulk free energy difference from the undercooled melt. Thus all ingredients are in place to compare experiments to theory, for nucleation via crystal and via mesophase. We predict at experimental undercoolings that nucleation via mesophase indeed has a lower barrier, consistent with experiment. However, the predicted barrier for PE is about 3 times too big, suggesting tensions for nanoscale nuclei are smaller by about 30 percent than inferred from experiments on fully developed lamellae and simulations with periodic boundary conditions.