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44157-AC7
Micro- and Nano-Scale Patterning Applied to the Study of Nucleation and Precursors to Nucleation in Polymer Crystallization
Currently there is much controversy and debate about the possibility of precursors to crystallisation in polymeric systems and the exact mechanism by which the polymer crystals nucleate. Several approaches have been proposed ranging from ‘spinodal (or spontaneous) precursors', to ‘mesomorphic precursors' to a simple continuous modification from the crystalline lamellar front to the amorphous melt over several nanometers. Our work has focused on trying to understand this transition as well as the kinetics of the formation of the initial nuclei from which the crystalline material grows. In particular, by dividing a material into many small compartments, it is possible to probe how nucleation occurs, since the growth of the crystal within the compartments must be accompanied by the formation of a nucleus. Such approaches can yield the nucleation rate which is typically difficult to de-convolve from the over crystallisation rate. Besides the fundamental interest in nucleation and the rate at which nucleation occurs in materials, the nucleation rate affects the total size of the spherulites which make up the semi-crystalline material, which in turn affects properties like the material strength, the optical clarity and also the processability of the polymer. The funding from the ACS-PRF grant has led to several research accomplishments over the last year and currently we are in the process of finalizing one of the main projects which has grown out of this funding which is described. The initial approach was to survey a diblock copolymer where one of the blocks is crystalline. The crystalline block forms tiny spheres which satisfy the requirement of the ‘small compartments' required for nucleation study. We have shown that ellipsometry, a tool not used for studies of crystallisation, can reveal great detail about the crystallisation process. In particular, we probe the thickness of a thin film of such a material and observe the contraction of the film with decreasing temperature due to the thermal contraction of the supercooled melt spheres embedded in an amorphous matrix. However, as the crystalline domains nucleate, a sudden contraction is observed in the films indicating the nucleation temperature of the domains. Upon reheating the sample the film expands, but with an expansion coefficient indicative of the crystalline spheres in the amorphous matrix. The expansion coefficients of the crystalline and melt state of the small, ~ 10 nm diameter domains, is in quantitative agreement with what might be expected from bulk materials. This in itself is a remarkable observation: as far as we can tell, both from expansion coefficients and from morphological studies carried out with atomic force microscopy, these tiny crystallites are very similar to their large bulk counterparts. We are currently investigating some differences we have observed in the lamellar spacing of the crystallites. While at this point we are not yet prepared to comment in detail on these results, it is clear that the thickness of the lamella is different and we currently suspect that this may be due to a difference in polymer entanglement which we have earlier observed in thin films of a different material. More concrete and exciting are the results that are not related directly to the intended crystallisation studies, but instead were a surprising result of our ellipsometry studies of crystallisation. When supercooling the diblock system, an anomalous transition was observed prior to crystal nucleation. After much experimentation it became clear that this transition was not related to crystalisation in any way. Rather it was the result of a not yet experimentally observed transition which has been predicted by theory. The spherical domains form small hemispherical wetting domains on the substrate, while at low temperatures the spherical domains flatten, driven by interfacial tension, and from a complete wetting layer. This sphere-to-lamellar trasntition upon cooling occurs because at high temperatures, the entropic contributions associated with chain stretching prefers a curved interface, thus forming spherical caps on the substrate, however, at low temperatures, the entropic contribution diminishes and interfacial tension dominates resulting in a wetting layer of the crystalline domains. This sphere-to-lamellar transition has not previously been observed in experiment because of the inherent challenges associated with probing a buried interface. Ellipsometry is ideally suited to such observations and is also ideal for the study of crystallisation itself. We are currently in the final stages of these studies and focused on developing the theory to complement our experimental observations.
