Tiny polymer droplets that crystallize on a surface are a shrewd expedient to study the birth of a polymer crystal by the elusive homogeneous nucleation mechanism. In most cases, take for example the dust particle in a snowflake, nucleation starts from a heterogenous defect. Homogenous nucleation is difficult to study because of the prevalence of defects in any bulk sample. Crystallization in small droplets alleviates this difficulty in a manner that is conceptually simple: subdivide the system into more domains than the number of defects. If the domains greatly outnumber the defects then only the homogenous mechanism can induce nucleation in a defect free compartment.
Such an approach, reported in EPJ E, has been used to investigate nucleation in polyethylene (PE) droplets. A polystyrene layer forms an unfavorable surface for a thin PE film. On heating, the unstable PE film dewets to form tiny droplets, much like water beading up on a waxy leaf. The result is a sample made of thousands of droplets that can be monitored simultaneously through optical microscopy. This is an ideal setting in which a nucleation event becomes easily distinguishable by the rapid growth of the crystal. Each droplet becomes an isolated independent nucleation experiment and studying homogenous nucleation becomes straightforward.
Relating the probability of homogenous nucleation to the size of the droplet, the authors show that nucleation is surface activated. Stated most simply, a droplet with twice the surface area is twice as likely to nucleate, indicating that the perturbation induced by the interface favours crystal nucleation.
To read the full paper “Surface nucleation in the crystallization of polyethylene droplets” J.L. Carvalho and K.Dalnoki-Veress, Eur. Phys. J. E (2011) 34: 6, click here.
