Controlling the Morphology of Supramolecular Nano- and Microfibers by Self-Assembly and Electrospinning

This thesis focuses on the preparation, characterization and comparison of supramolecular nano- and microfibers of 1,3,5-benzene- and 1,3,5-cyclohexane-trisamides using two routes: self-assembly from solution as bottom-up approach and melt electrospinning as top-down approach. The design and the controlled fabrication of fibrous structures from self-assembling molecules have gained increasingly interest in material and life science. Network structures (nonwovens) based on supramolecular materials exhibit intriguing properties. In addition to the high surface area to volume ratio well-defined surfaces are present. A large amount of natural materials are made out of fibrous structures with hierarchical organization. In this context the Introduction summarizes the background of supramolecular chemistry, which is the common bottom-up approach. Small molecules self-assemble on the nano- and mesoscale in well-defined structures by the formation of noncovalent bonds. Detailed information of secondary interactions necessary to build up such self-assembled constructs is described. Selected examples from nature and other supramolecular materials are presented. A prominent class of self-assembling molecules are 1,3,5-benzene- and 1,3,5-cyclohexanetrisamides. Since these organic molecules present the key structure of this thesis, a deeper insight of their self-assembly behavior in solution and the resulting solid state is provided. Trisamides self-assemble predominantly into columnar structures via the formation of strong and directed triple hydrogen bonds. In case of 1,3,5-benzenetrisamides, the columnar construct is further stabilized by π-π-interactions. The thermotropic behavior and the formation of a macrodipole within a column can be utilized to manipulate and align those supramolecular structures within an electrical field. A modern top-down approach to prepare fibers is electrospinning of polymers from solution and melt. The electrospinning process is well-studied for the fabrication of polymeric fibers. A polymer solution or melt is subjected to an electrical field, and any droplet is deformed into a cone-like shape (Taylor cone) due to electrostatic repulsion of generated charges at the droplet surface. At stronger electrical fields a jet is ejected from the cone and then accelerated towards a collector. The experimental conditions, such as applied electrical field strength, flow rate or shape of the collector, exhibit parameters to optimize and influence the fiber formation. A very recent research area focuses on electrospinning of self-assembling small molecules without the addition of a polymer. Only a few examples have been reported on solution electrospinning, and only one molecule was electrospun from melt so far. The Objective of this thesis is the fabrication, characterization and comparison of supramolecular nano- and microfibers based on 1,3,5-benzene- and 1,3,5-cyclohexane-trisamides using the two approaches: self-assembly from solution as bottom-up approach and electrospinning from melt as top-down approach. An objective was the development of structure-property relations concerning the formation of supramolecular nano- and micro fibers, and the mechanical properties of the fibers. Therefore, the chemical structures of 1,3,5-benzene- and 1,3,5-cyclohexanetrisamides was systematically varied. The morphology of obtained fibers was studied and compared in detail and the fiber stiffness was determined. In the first chapter, the influence of the molecular structure of 1,3,5-benzenetrisamides on the mechanical properties of their supramolecular fibers is investigated. Three compounds with different alkyl substituents and varying in the connectivity of the amide linkage to the core were selected. From all compounds, well-defined, self-assembled fibers were obtained by controlled recrystallization from 2,2,4,4,6,8,8-heptamethylnonane (HMN), a high boiling nonpolar solvent, which can be evaporated after fiber formation. The average diameter of the fibers varied from 200 nm to 2 µm depending on the molecular structure. In collaboration with the group of Prof. Andreas Fery, Physical Chemistry II, University of Bayreuth, the flexural rigidities of the fibers were determined using atomic force microscopy (AFM) bending experiments. The corresponding Young’s moduli E were calculated to average values of 2.1 to 3.3 GPa. Their values are comparable to semi crystalline, non-orientated polymers. The flexural rigidity of the fibers shows a difference up to three orders of magnitude, however, the Young’s modulus of each fiber type is similar. Consequently, the flexural rigidity of the fibers is a pure size effect allowing to tune the rigidity of the fibers by adjusting their diameter. In the second chapter, it was demonstrated for the first time that 1,3,5-benzene- and 1,3,5-cyclohexanetrisamides could be shaped into fibers by melt electrospinning. From the thermotropic nematic phase and, surprisingly, also from the optical isotropic phase, just above the clearing temperature, homogenous and long nano- and microfibers were collected. The fact that fibers are obtained from these low molecular weight substances is a consequence of the supramolecular structure of trisamides. The optical isotropic melt still consists of very small columnar aggregates; thus, it is assumed that their macrodipoles are acting with the electrical field and contribute to the formation of fibers. The influence of the spinning temperature and the applied electrical field strength were studied in detail. An increasing temperature resulted in an augmented sphere formation, whereas, long and thinner fibers were obtained with increasing field strength. The focus of the third chapter is on melt electrospinning of small molecules. A more detailed structure-property relation of trisamides and other self-assembling molecules was investigated. The amount and type of secondary interactions were systematically investigated to gain more information about the applicability of this new top-down approach. Different sets of compounds, including 1,3,5-benzene- and 1,3,5-cyclohexanetrisamides varying in substituents and in the connectivity of the amide linkage to the core, bisamides with benzene-, naphtyl- and cyclohexane moieties, sorbitol derivates and perylenebisimide derivates were selected. Also 1,3,5-benzenetrisamides incapable of forming hydrogen bonds were investigated. The electrospun morphology was correlated with the molecular structure. 1,3,5-benzene- and 1,3,5-cyclohexanetrisamides form most often and reliable fibers during melt electrospinning. Electrospinning of bisamides and sorbitol derivates resulted mostly in the formation of spheres due to insufficient strong secondary interactions. Due to the fast cooling of the melt during the process, supramolecular fiber formation has no time to develop. For the first time, it was shown that perylenebisimide can be shaped into fibers, since sufficient π-π-interactions are present. By melt electrospinning of tertiary 1,3,5-benzenetrismaides, it was surprisingly found that hydrogen bonds are not absolutely necessary to collect fibers. The fourth chapter connects and compares the self-assembly and melt electrospinning approach. Therefore, melt electrospun and self-assembled 1,3,5-benzene-tricarboxamide fibers were characterized on different length scales including crystal structure, fiber morphology and mechanical properties. From one compound, fibers with significantly different morphologies were obtained by the top-down and the bottom-up approach. Melt electrospun fibers offered a smooth and homogenous surface, whereas self-assembled fibers were hierarchically structured by a bundle-like structure. On the Ångström scale, X-ray diffraction revealed the same crystal structure of both fiber types. However, despite the morphological differences, nanomechanical bending experiments show that the Young’s modulus (E= 3.6-4.7 GPa) is comparable for self-assembled and melt electrospun fibers. The results are in the same range as further supramolecular 1,3,5-benzenetrisamide fibers presented in the first chapter. The results of this thesis reveal that 1,3,5-benzene- and 1,3,5-cyclohexanetrisamides can be shaped into nano- and microfibers using powerful bottom-up and top-down approaches. Their interesting morphology combined with their mechanical properties make this class of low molecular weight substances suitable for constructing new materials, such as hierarchically structured nonwovens for efficient filter applications.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Singer, Julia C.

Contributors dc:contributor

• Schmidt, Hans-Werner

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2043/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2043