Self-diagnostic polymers through host-guest interactions

Abstract

This thesis focuses on the the use of supramolecular host-guest interactions to impart new properties to polymeric systems.

Part I shows the development of self-diagnostic polymers and composites, i.e., polymers with the intrinsic ability to highlight areas where the material experienced high strain or where defects are present. Properly designed host-guest probes allow the detection of failures in polymeric systems in the initial state through strain sensing by fluorescence turn-on triggered by the mechanical dissociation of the complex and hence help to prevent the effects of fatal failure in polymeric materials. Three different systems, two for epoxy resins and one for poly(dimethylsiloxane), have been developed for this purpose. The first system was developed to sense strain in rubber-like, elastomeric poly(dimethylsiloxane), using a tetraphosphonate cavitand with a suitable N-methyl pyridinium guest as the sensing host-guest system. In this case, the N-methyl pyridinium guest is highly fluorescent on its own, but this fluorescence disappears upon complexation with the cavitand. Dissociation of this comple therefore turns on the fluorescence signal, and, by incorporation of the intact complex into the polymer system, as probe for strain inside the material. The system was thoroughly investigated using a three point bending setup and subsequent observation of the fluorescence. In addition, the obtained results were correlated to a theoretical model of the strain present in the samples during breaking. The second system developed is employing the non-covalent interactions between carbon fibres, which are commonly used to prepare epoxy composite materials, and polycyclic aromatic hydrocarbons. Interactions of the carbon fibres and fluorescent polycyclic aromatic hydrocarbons lead to fluorescence quenching of the latter, and the fluorescence of the pristine material can be restored by separation. The formation of nanohybrids between those two components and the preparation of composite materials thereof has been investigated and optimised. Samples of this material were pulled using a tensile strength test setup and the resulting fluorescence investigated. The third system, based on a ternary cucurbit[8]uril complex, and can be used for epoxy thermosets. In this case, two guest are forced into spacial proximity by complexation inside the cucurbit[8]uril cavity, which leads to a change in the fluorescence of the guests. By mechanical separation of the components of the complex, the fluorescence of the pristine guests is restored. Also in this case, the optimized system has been investigated using a tensile strength test setup and by observation of the resulting fluorescence.

Part II shows the use of supramolecular host-guest interactions for the preparation of homogeneous blends consisting of otherwise immiscible polymers. This has been demonstrated for a poly(styrene) and poly(butyl methacrylate) system, using covalently incorporated phosphonate cavitand hosts and N-methyl pyridinium guests respectively as compatibilisers. Furthermore, it has been demonstrated that an electrochemical stimulus can be used to turn off the host-guest interactions, leading to decompatibilisation in the solid phase, and subsequently to separation of the polymer blend upon annealing above the glass transition temperature of the components.