Nucleation and crystallization of polymers

The key property of interest of polypropylene in the medical and food storage applications is its transparency. The transparency of such polymers is completely dependent on the size of the spherulites formed, which in turn depends on nucleation. Studies of the nucleation mechanisms of isotactic polypropylene (iPP) were carried out with and without nucleating agents. Several nucleating agents ranging from conventional benzoates to sorbitol based nucleators (DBS) and a rosin based natural product called pine crystal were used in the investigation. Isothermal and non-isothermal DSC experiments were conducted and results were analyzed using Avrami's equation. Crystallinity measurements were done using X-Ray diffraction and crystal morphology was analyzed by optical microscopy. Theoretical studies of PP nucleation were done using molecular mechanics and molecular dynamics. Special techniques were devised to analyze the results of molecular dynamics, which compares the backbone conformation of the iPP chain to the conformation of crystalline iPP and determines the portions of the chains that are crystallized. Based on these results, the difference in the degree of crystallization between nucleated and non-nucleated iPP was determined. Results of DSC experiments indicate that the number of nuclei formed with DBS and Pine crystals are several orders of magnitude greater than formed by conventional nucleators. Molecular modeling results indicate that the ability of DBS to nucleate PP is related to the stiffness of the backbone and therefore DBS promotes nucleation on a molecular level. Molecular dynamics, when the results are properly analyzed, could be a viable tool in the investigation of nucleation mechanism of polymers and for the screening and development of new compounds for use as nucleating agents.