We present a study of homogeneous and heterogeneous nucleation in polymer crystallisation. In bulk samples the crystallization is typically dominated by nucleation from defects (heterogeneous nucleation), and consequently studies must rely on sample preparation to minimize this effect. We present a study of nucleation within discrete droplets of poly(ethylene oxide) that are formed by the dewetting of a thin film on an unfavourable substrate. The samples provide an ensemble of impurity-free droplets, with length scales that can easily be measured. We show that the data for heterogeneous and homogeneous nucleation is qualitatively different, and that the data mirrors the fundamental differences in the underlying mechanisms for the two nucleation processes. The experiments presented here provide a simple method that can be used to study heterogeneous and homogeneous nucleation in great detail.

The birth of a crystal is initiated by a nucleus from which the crystal grows--a dust grain in a snowflake is a familiar example. These nuclei can be heterogeneous defects, like the dust grain, or homogeneous nuclei which are intrinsic to the material. Here we study homogeneous nucleation in nanoscale polymer droplets on a substrate which itself can be crystalline or amorphous. We observe a large difference in the nucleating ability of the substrate. Furthermore, the scaling dependence of nucleation on the size of the droplets proves that the birth of the crystalline state can be directed to originate predominantly within the bulk, at the substrate surface, or at the droplets' edge, depending on how we tune the substrate.

We have studied crystallization in poly(ethylene oxide) droplets with volumes ranging over several orders of magnitude. In all samples, homogeneous nucleation is observed, scaling with the volume of the droplet, down to systems with as few as approximately 10 polymer chains. Surprisingly, nucleation is unaffected by the high degree of confinement, despite a large surface-to-volume ratio and the restriction of chains to length scales much smaller than the radius of gyration. Nucleation was also found to be independent of chain length for two molecular weights studied, differing by an order of magnitude. The results suggest that, for these highly supercooled systems, the formation of a nucleus is influenced by its immediate surroundings and does not depend on the entire length of the constituent chains.