| The crystallization of polymer melts under the influence of flow is a means of affecting polymer morphology. Therefore, it is a subject of special importance to the engineer, since virtually all of today's processing methods involve flow of polymer melt either during or immediately after flow. This manuscript reports on one aspect of this subject, i.e., changes in the crystal nucleation of melts due to flow. In this work, a series of specially prepared model polymers were made to crystallize while undergoing elongational flow. These experiments were made possible through use of a specially designed extensional rheometer. Changes in the nucleation process due to flow are described. In addition, these changes are compared to changes predicted by a molecular model.;These studies show that elongational flow fields cause a dramatic acceleration in the crystal nucleation process. It was also found that melts composed of linear molecules nucleate at essentially the same level of tensile strain, at least when made to crystallize at equivalent levels of tensile stress.;Systems containing branched molecules are found to nucleate at larger levels of tensile strain. This is apparently due to the fact that branched molecules are less mobile in a flow field than linear molecules. Nucleation induction times predicted from a molecular model assembled in this research show good agreement with those experimentally determined for the systems composed of linear molecules. Induction times predicted for systems containing branched molecules fall short of those determined by approximately a factor of 2. This discrepancy appears to reflect the fact that the value for activation energy of chain transport across the melt/solid interface used in the prediction is too small. Nevertheless, the fact that the model predicts most induction time changes lends validity to the assumptions involved in the development of the model. |
