TY - THES ID - 137921675 TI - Structural aspects and crystallization kinetics of polyamide 12 as revealed by dielectric spectroscopy AU - Clinckemalie, Lotte AU - Goderis, Bart AU - Verkinderen, Olivier AU - KU Leuven. Faculteit Wetenschappen. Opleiding Master in de chemie (Leuven) PY - 2018 PB - Leuven KU Leuven. Faculteit Wetenschappen DB - UniCat UR - https://www.unicat.be/uniCat?func=search&query=sysid:137921675 AB - 3D printing is a very promising technology with high application potential in several industries. However, few polymeric materials compatible with this technique are known and further research is needed to identify new ones. This requires an understanding of the currently used materials. PA 12 is the most used polymer material for selective laser sintering, which is the 3D printing technique of interest to this thesis. It is not yet fully understood and for this reason, its structural aspects and crystallization kinetics are investigated in this thesis. The crystallization behaviour of PA 12 was studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD) and broadband dielectric spectroscopy (BDS). By combining and comparing the results of the different measurement techniques, a better understanding of the lamellar and spherulitic growth of PA 12 is obtained. By using the Avrami theory, it is concluded that with a lower degree of undercooling, and thus a higher crystallization temperature, the crystallization rate is smaller and spherulites grow more three dimensional. Since crystals have more time to grow during a smaller undercooling, the question arose if a smaller undercooling would result in a larger amount of crystallinity. The amount of crystalline material was therefore calculated using the results obtained with DSC and XRD and even though a slightly decreasing trend could be seen for larger undercooling, the measurements resulted in approximately the same values of crystallinity. Furthermore, the lamellar thickness was calculated using the Scherrer equation and it was deduced that a smaller degree of undercooling resulted in a larger lamellar thickness. Besides this, using the results of BDS, it was concluded that crystal growth follows the Strobl model. It was deduced that lamellae grow only as discrete lamellar thicknesses, which is visualised as a step function of the growth velocity as a function of the degree of undercooling. This step function is a consequence of the transition of growing crystals into thicker crystals if a higher crystallization temperature is used. In addition, the experiments using BDS allowed concluding that a pre-ordered structure is present during the crystallization process of PA 12. This pre-ordered structure might be the so-called mesophase, which is a mobile, but slightly structured layer. It functions as a precursor of lamellar growth by forming thin layers, which behave as lamellar growth fronts. A next topic of interest is the behaviour and the presence of the rigid amorphous fraction (RAF), since the RAF can decrease the volume drop of the semi-crystalline polymer during crystallization. This fraction is the third fraction of a semi-crystalline polymer, present next to the amorphous and crystalline fraction. When the RAF fraction is increased, the density of the crystallized material will be lower than a material with the same amount of rigid material. If this fraction is controlled and maximized, the unwanted shrinkage of the material during printing can be minimized. During BDS measurements, a possible splitting of the α-relaxation was observed, which may be an indication of the presence of the rigid amorphous fraction. No conclusion about the RAF could be made and further research of the α-relaxation is needed. ER -