title: Self-assembly, dimensional control and application of supramolecular 1D- and 2D- materials
Creators: name: Steinlein, Christoph
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Chemistry
subject: Supramolecular Chemistry
subject: Polymer Foam
subject: Self-Assembly
subject: 540

description: Supramolecular chemistry is supposed to become one of the significant research fields in material science of the 21st century. This is attributed to the manifold self-assembly processes resulting in distinct supramolecular architectures with specific functionalities. However, several issues are still not easy to solve. For example, tailoring supramolecular architectures with precise dimensions via bottom-up approaches remains challenging. Therefore, this work is dedicated to self-assembly, dimensional control and application of supramolecular 1D- and 2D- nanomaterials based on 1,4-bisamides and 1,3,5-trisamides. The first part addresses the self-assembly of 1,4-benzene- or 1,4-cyclohexanebisamides into 2D-nanoobjects. These novel 1,4-bisamides are designed with different fluorocarbon or tertbutyl substituents resulting in symmetrical or asymmetrical substitution patterns. Within each substitution pattern, the length of the fluorocarbons was varied from C3F7 over C5F11 to C7F15. A symmetric 1,4-bisamide with tert-butyl groups was used as reference. All bisamides proved to feature sufficient thermal stability allowing self-assembly experiments at elevated temperatures. In this context, an important aspect was the structural elucidation by X-ray diffraction, solid-state NMR and IR spectroscopy, as these methods reveal the H-bonding pattern, which typically reflects the shape of nano-objects on the mesoscale. In cooperation with the department of Inorganic Chemistry III at the University of Bayreuth, it was shown that bisamides without tert-butyl substituents form rows of molecules connected by H-bonds. These rows align into layers, which stack to form platelets. By contrast, bisamides comprising at least one tert-butyl substituent connect to four neighbors to form layers, which also stack into platelets. Based on this finding, a reference bisamide was used to evaluate different self-assembly processes and to tune self-assembly conditions to obtain thinner nano-platelets. Transferring and further optimizing these results to the symmetric and asymmetric 1,4-bisamides with fluorocarbon substituents it was found that an asymmetric bisamide formed the thinnest platelets, featuring an average thickness of around 32 nm, which equals 15 layers. In addition, a reduction of platelet thickness with longer fluorocarbon chains was revealed. Moreover, contact angle measurements of two fluorocarbon substituted bisamides revealed that the surfaces of their 2D-objects are highly hydrophobic. The second part focuses on dimensional control of supramolecular fibers of 1,3,5-benzene-trisamides via a top-down approach. In particular, length control of such fibers was addressed via ultrasound. For this, supramolecular submicron fibers of 1,3,5-benzene-trisamide were produced in large amounts by self-assembly upon cooling of solutions in high-boiling hydrocarbons. For later systematic sonication experiments, several dispersion media for supramolecular submicron fibers such as n-hexane, methyl cyclohexane and anisole were explored. A systematic screening of sonication parameters such as sonication time, ultrasonic power amplitude, medium, cooling bath temperature and concentration of BTA fibers revealed their influence on the final fiber dimensions. For instance, the applied ultrasonic energy is the major factor for the length of the obtained fibers. By contrast, raising the concentration or lowering the temperature gave only slightly shorter submicron fibers. Remarkably, the used medium during sonication altered fibers’ length as well as their aspect ratio. This way, it was possible to vary the aspect ratio from 3.7 to 6.8 and the fiber length from 0.66 to 0.98 μm. In this context, the fiber lengths were successfully correlated with the viscosity of the media revealing shorter fibers in more viscous media even after a long sonication time demonstrating the control over the fibers’ dimensions. In the third part, supramolecular nanofibers were applied to improve the foam morphology and mechanical properties of extruded polypropylene foams. This work was realized in cooperation with the department of Polymer Engineering at the University of Bayreuth. Conceptually, the homogeneously dissolved BTAs self-assemble during cooling in the extrusion process into solid nanofibers, which act as finely dispersed nucleation sites for the foam cells and consequently control the foam morphology. To realize this, three different BTAs at different concentrations were compounded into an isotactic polypropylene (i-PP) grade and injection molded. The specimens were thoroughly investigated and, based on these results, compounds comprising different concentrations of the three BTAs were chosen for foam extrusion. Talc at different concentrations was used as reference. Foam extrusion was realized in a tandem extrusion line using CO2 as physical blowing agent. It was found that the density and the morphology of extruded foams can be significantly altered by the presence of BTAs. With BTAs, the foam density is strongly reduced by more than 40% to 0.09 g/cm3 compared to neat i-PP. Also, the average foam cell diameter was reduced by more than 40%, reaching an optimum diameter of 27 μm. Such homogenous foams with small cell sizes could not be achieved with the talc reference foams. Moreover, it was demonstrated that the specific compression moduli of foams with BTA could lead to an improvement of more than 100% compared to neat i-PP and more than 65% compared to the talc reference foam. This finding is attributed to a reinforcing effect of BTA fibers.