Hydrogen bonded columnar liquid crystals for nanostructured functional materials

Many functional materials rely on a well-organized internal structure for their functional properties. The length scale of this organization can vary from the atomic level (e.g., piezoelectric crystals) to a macroscopic length scale exceeding the dimensions of individual molecules (e.g., porous materials). This thesis explores the possibility to use columnar liquid crystals as a basis for nanostructured functional materials with the specific aim to develop new membrane materials with well-defined nanopores (1-5 nm). Chapter 1 introduces liquid crystals (LCs) as self-assembled soft materials that possess orientational order without three-dimensional positional order. The columnar LC phases studied in this thesis consist of discotic molecules with a benzene 1,3,5-tricarboxamide (BTA) core that form columns with an axial macrodipole due to the head-to-tail alignment of hydrogen bonded amides. As a general design approach, we aim for highly ordered, polymerizable columnar LCs in which two components are phase separated on a nanometer scale in a so-called superlattice. Such materials offer a potential route to nanostructured and, after selective removal of one of the components, nanoporous polymers that combine a superlattice structure with the dipolar properties of BTAs. In Chapter 2, it is shown that the hydrogen bonded hexagonal columnar LC phases formed by BTA derivatives are not ferroelectric but can be aligned uniformly by an electric field and display ferroelectric switching behavior with a high spontaneous polarization. The dielectric relaxations and ferroelectric switching in symmetrically substituted BTAs with alkyl chains varying in length between six and eighteen carbon atoms were investigated. Dielectric relaxation spectroscopy (DRS) revealed a columnar glass transition around 41-54 °C and a relaxation process at higher temperatures originating from the collective reorientation of amide groups along the column axis (macrodipole inversion). Electro-optical switching experiments on aligned samples showed that this reorientation gives rise to extrinsic ferroelectric switching characterized by a spontaneous polarization and coercive field of 1-2 ¿C/cm2 and 20-30 V/¿m respectively. In the absence of an external field, the polarization is lost within 1-50 s proving that the phases are not ferroelectric. The columnar glass transition might be used to freeze the orientation of the macrodipole and achieve permanent polarization. In Chapter 3, several amphiphilic discotics with a BTA core are described that form hexagonal and nematic columnar LC phases, some of which show a disordered superstructure of phase separated microdomains. The thermotropic phase behavior of two types of amphiphilic discotics was studied: (i) discotics with two alkyl chains and one incompatible moiety coupled directly to the core and (ii) essentially symmetric discotics where one of the alkyl chains is extended with a chemically dissimilar chain. The BTAs with a directly coupled tri(ethylene oxide) (EO) chain and fluorocarbon (F) chain both form hexagonal columnar LC phases. Only incorporation of EO-chains leads to a highly disordered superstructure within the hexagonal lattice. The extended discotics form nematic columnar phases as a result of the packing frustration caused by the extending F-chains. The absence of highly ordered structures can be explained by the limited conformational freedom in the molecular and supramolecular structure of the columns and insufficient phase separating ability of the dissimilar chains. In Chapter 4, a system is described in which an acid functionalized BTA and poly(propylene imine) dendrimer (PPI-dendr) self-assemble into a new type of oblique columnar LC phase that displays a well-ordered superlattice. The orthogonal combination of hydrogen bonding in the columnar direction and ionic interaction in the plane perpendicular to the columns gives rise to a structure in which the dendrimer is confined to separate columnar domains (diameter 1-2 nm). The structure of the mesophases formed in the mixed system was elucidated by infrared spectroscopy and X-ray scattering. Investigation by differential scanning calorimetry (DSC) showed that the LC phase is most stable in an 8:1 molar mixture but remains stable over a wide range of temperatures and compositions. In dendrimer enriched mixtures the lattice swells to take up more dendrimer, while discotic enriched mixtures show lamellar phases with a columnar structure that is probably closely related to the oblique superlattice. Chapter 5 shows that the principle of orthogonal self-assembly as introduced in Chapter 4 is generally applicable to create LC superlattices of amine containing polymers. The structure and phase behavior of mixtures of the acid-modified BTA with a low molecular weight amine, different generations PPI-dendr, branched and linear poly(ethylene imine), and the linear organometallic polymer poly(ferrocenylsilane) were investigated. The superlattice only forms when the phase separated structure is reinforced through supramolecular ionic cross-links provided by large molecules and polymers, while the largest PPI-dendr cannot deform into the columnar structure of the superlattice. In case of linear poly(ethylene imine), the structure and stability of the superlattice is independent of chain length up to high molecular weight (250 kDa). The calorimetric results further indicate that branching stabilizes the superlattice structure. These effects of the structure of the amine containing component are consistent with the structural model of the LC superlattice. In Chapter 6, fixation of the self-assembled LC superlattice described in Chapter 4 is reported. Cross-linking was made possible by functionalizing the system in a modular fashion by mixing in a polymerizable BTA. The phase behavior, LC structure and photopolymerization of a ternary mixture consisting of the acid functionalized BTA, a methacrylate functionalized BTA, and PPI-dendr were studied. X-ray scattering showed that the ternary 5:3:1 mixture displays an oblique disordered columnar LC phase featuring a well-ordered superlattice of PPI-dendr microdomains identical to that found for the non-polymerizable mixtures. The ternary mixture can be photopolymerized in bulk and thin films (60-100 nm) to yield cross-linked supramolecular polymers in which both the columns and the two-dimensional columnar superlattice remain intact. However, the columns are not oriented uniformly perpendicular to the surface of the films as indicated by transmission electron microscopy. Although the polymerized materials have not been tested as nanoporous membranes, the work described in this thesis illustrates that multicomponent LC superlattices obtained through orthogonal self-assembly based on BTAs have great potential to integrate permanent polarization with other functionalities in a single material (e.g., porosity, conductance).