| We present the results of four projects investigating the nucleation of polymer crystals from a supercooled melt. In the first study, an ensemble of droplets was prepared by dewetting a thin film on an unfavourable substrate. Crystallisation was directly monitored with optical microscopy, enabling the identification of each individual droplet over successive experiments. It was shown that homogeneous and heterogeneous nucleation could be distinguished based upon the behaviour of each droplet over multiple crystallisation experiments.;In the third study, crystal-memory of the melt was investigated by controlling the thermal history of the droplets prior to crystallisation. Memory effects were identified by 'self-nucleation' within droplets: where crystallisation occurred at elevated temperatures, beyond their observed crystallisation temperature when the sample was fully-annealed. For one system, polyethylene, pronounced melt memory effects were observed when samples were annealed (at temperature Ts) well above the observed melting temperature. As Ts decreased, there was an increase in the number of droplets which experienced the memory-effect. By comparing the crystallisation over multiple experiments it was shown that, while the number of droplets experiencing memory effects depended on Ts, self-nucleation was a randomly occurring process throughout the droplet ensemble. Memory effects in a second system, poly(ethylene oxide), were much less pronounce; self-nucleation was observed only if the samples were annealed 1--2 degrees above the observed melting temperature.;The fourth project investigated the effects of system size on homogeneous nucleation. Ellipsometry was used to study crystallisation, and the mean droplet sizes were varied over several orders of magnitude in volume. The results showed that homogeneous nucleation scaled with the volume of the droplet over the entire range of droplet sizes. Even droplets which contained only ∼20 polymer chains behaved the same as one composed of 107 molecules. Diblock copolymers were also investigated, where the polymer was ordered into spherical domains, and the behaviour was consistent with that of the homopolymer droplets. The nucleation rate was found to be independent of molecular weight, over two orders of magnitude in chain length for two homopolymers plus the diblock copolymer systems. Additionally, no confinement effects were observed, arising from either the large change in surface to volume ratio, or from chain-confinement effects. The results suggest that the formation of a nucleus involves only a finite region of the melt, which, in the current study, does not necessarily extend as far as the lengthscale of a single polymer chain.;The second study focussed on homogeneous nucleation within dewetted droplets. Direct visualisation made it possible to measure the volume of each droplet. The nucleation rate was determined as a function of droplet size and it was shown for the first time that the homogeneous nucleation rate scaled with the volume of the droplet. The temperature dependence was also measured, and found to be consistent with behaviour expected from classical nucleation theory. |
