Heterogeneous nucleation in polymer crystallization
The effect of talc, sodium benzoate, boron nitride, saccharin and sorbitol on the nucleation and crystallisation of PP, PE, PB, POM, PHB, PET and Nylon 66 has been investigated by DSC, light microscopy and electron microscopy. The effectiveness of the nucleating agent has been evaluated by the study of the interfacial layer between the polymer and the substrate of densely packed particles using light microscopy and by quantitative analysis of the DSC cooling exotherms, obtained from polymers with homogeneously dispersed nucleating agent. It has been found that the crystallisation temperature of the nucleated polymer is exponentially dependent on the concentration of the nucleating particles. This has been explained solely by the decrease in the average distance between the nuclei, with no changes in the crystallization kinetics. The extent of the shift in the crystallisation temperature of a heterogeneously nucleated polymer has been found to be related to the initial number of spherulites present in the un-nucleated polymer. The highest shift in the crystallization temperature has been obtained from partially molten polymers (self nucleation); the shift which can be obtained with the best nucleating agents is approaching this level. Electron microscopy studies, especially the diffraction patterns obtained from the nucleating interface, have shown a good crystallographic correlation between the polymer matrix and the nucleant. Thus, the correlation between the growth direction of the spherulite lamellae and the nucleating facet is good for all active nucleants, but a perfect match of the molecular spacing in this plane (epitaxy) has been observed only for very strong nucleants.