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 years 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 order to understand the polystyrene substrates – ultrathin films, supported or free standing – we have carried out several studies. In particular work was completed on the effects of various atmospheres on the glass transition and degradation. This was of importance because previous work by others had suggested that degradation might be responsible for the large reductions in the glass transition that have been observed. We found, by comparing results obtained in air and contrasting these to inert dry environments as well as vacuum, that degradation was not an issue. In carrying out our work on thin films of polystyrene we have learned much about the entanglement properties of these films – an ongoing study, which we cannot report on further at this time. The studies of crystallization in thin films have switched somewhat from our work on diblock copolymer crystallization to growth rates in films. The latter is a well studied subject and while we expected the measurements to result merely in a better understanding of the system, we have instead found some anomalies that have not been previously reported. This work will be presented in the near future when the details are a little better understood. Most exciting is that fact that the work provides new insight into nucleation of crystallization and specifically molecular nucleation.
