New responsive polymers and supramolecular systems - synthesis and characterization

  • Neue reaktionsfähige Polymere und supramolekulare Systeme - Synthese und Charakterisierung

Dolgopolov, Andrey; Möller, Martin (Thesis advisor); Pich, Andrij (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020


This thesis deals with the synthesis and properties of new photochromic polymers, low-molecular compounds and their supramolecular complexes. The first chapter describes the goal and motivation of this work. The second chapter presents an overview of isomerization of photochromic groups, e.g. spiropyrans and azobenzenes, and their application in the design and synthesis of photochromic materials. The third chapter describes the preparation and properties of microgel-surfactant complexes, in which the surfactant molecules contain a photoisomerizable azo-group. For this purpose, microgels based on copolymers of vinylcaprolactam and vinylimidazole were neutralized with sulfonic acid containing surfactant molecules. In the structure of the wedge-shaped surfactant molecules three long-chain alkyl groups are connected with a sulfonated azobenzene unit via an ester group. The alkyl chain contains 8 or 12 carbon atoms. The ionic binding of the surfactants to the microgels has a mutual influence. On the one hand, the micelle formation of the microgel-bound surfactants is altered compared to freely dissolved surfactants. On the other hand, the solubility behavior of the surfactants changes. These association and aggregate formation processes can in turn be influenced by the switchability of the surfactants under irradiation. The results show that the increase of the degree of neutralization of the vinylimidazole groups leads to the increase of the activation energy of the cis-trans-transition of the azobenzene groups. The activation energy also depends on the chain length. The longer the chain, the higher the activation energy. These effects result from 1) aggregation of wedge-shaped groups 2) low degree of neutralization of the vinylimidazole groups (no possibility to form microdomains). It has also been shown that the modification with the surfactant molecules has an influence on volume phase transitions of the microgels. While the thermal shrinkage behavior at low degrees of neutralization (25%) was only slightly less pronounced than that of the unmodified microgels, an increased degree of neutralization significantly affected the temperature dependence and led to the reduction in the hydrodynamic radius. The samples with the degree of neutralization of 100% further showed the decrease of the hydrodynamic radius: with the surfactant with dodecyl groups, the microgels largely lose their thermal sensitivity, while the increase of the transition temperature was observed for the surfactants with shorter alkyl chains. Chapter 4 deals with the domain formation of pure surfactant molecules. Under dry conditions and with low water content, the surfactant molecules form inverse micelles, which can also adopt a cylindrical structure. In this chapter the dependence of the structure on the water content was systematically investigated. With increasing relative air humidity (RH), the amount of absorbed water increases significantly for the surfactant with dodecyl chains, unlike in comparison with one with octyl groups. In the previous case, water sorption is accompanied by a significant increase in ion conductivity and a phase transition. In particular, an increase in RH leads to a transition from a lamellar to a columnar phase; this results in the formation of one-dimensional water channels along the axis of the supramolecular columns. In the case of the molecules with shorter alkyl chains, the lamellar phase exists over the entire RH range with prominent swelling at high RH values, which then also lead to a two-dimensional water channel structure. NMR diffusometry was used to address the molecular mobility in the lyotropic mesophases of the wedge-shaped amphiphilic sulfonate molecules. Chapter 5 focuses on the design, synthesis, and investigation of core-shell nanoparticles that contain spiropyran groups. The particles were synthesized using a two-stage polymerization procedure: 1) preparation of a spiropyran-containing polystyrene-based core via emulsion polymerization and 2) seed polymerization of N-isopropylacrylamide to build up a thermosensitive shell. This synthetic approach provides a simple method for the preparation of monodisperse particles with up to 4 mol.-% of spiropyran groups. The incorporation of such groups renders the polystyrene core pH-sensitive, that is expressed in the blue shift of the absorbance by adding acid, and UV-responsive: irradiation with light of 365 nm wavelength leads to isomerization from the spiropyran isomer to the merocyanine form, which is stable for up to 12 hours at room temperature. In addition, the formation of the merocyanine group is accompanied by both a significantly increasing absorbance in the range of 450-510 nm and an increase of the hydrodynamic radius of the particles through the formation of a solvation envelope. The presence of the poly-N-isopropylacrylamide shell makes the nanoparticles thermosensitive: their temperature of the volume-phase transition is 33° C. This chapter clearly shows the new approach to the synthesis of nanoparticles with multisensory properties. Overall, the work presented here describes a couple of interesting examples, where the sensitivity of different types to stimuli of different origins (temperature, pH and light) can be combined in multifunctional materials, therefore, there is great potential for their use in nanotechnology