Photo-responsive microgels

• Licht-empfindliche Mikrogele

Phua, Dazril I.; Pich, Andrij (Thesis advisor); Böker, Alexander (Thesis advisor)

Aachen (2016, 2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

In the field of smart materials, employing light as a stimulus is advantageous. Light can be manipulated with high degrees of spatial and temporal control, and applied remotely, without direct contact to the system under study. It is therefore noninvasive, does not contaminate the system or produce unwanted side reactions. The extent or rate of photo-reaction from light stimulation can additionally be tuned by varying the intensity, wavelength, and direction of polarization of the light.Microgels as a class of smart material, endowed with a photo-responsive property is the focus of this work. Novel microgel systems were synthesized via precipitation polymerization to include photo-sensitive moieties. The changes in the particle size due to light and/or temperature stimuli were detected via dynamic light scattering(DLS). Light stimulus in the form of UV irradiation or visible light was applied in situ during DLS measurements by means of optical fiber-coupled LEDs.In Chapter 2, dual thermo- and photo-responsive microgels are described. N-Vinylcaprolactam (VCL) was used as the functional monomer to impart the thermosensitive property to the microgels. 4-[(4-Methacryloyloxy)phenylazo]benzenesulfonic acid (ABSA) was used as a water-soluble, photo-switchable comonomer. N,N’ Methylenebisacrylamide (BIS) cross-linked the polymeric chains to produce the microgel particles. In a low regime of ABSA incorporation (< 10 wt. %) as pendant groups, the P(VCL-BIS-ABSA) microgels displayed volume phasetransition temperatures that were shifted to lower temperatures relative to P(VCLBIS) microgels. Under UV irradiation (λ = 365 nm), the P(VCL-BIS-ABSA) microgels deswelled by up to 28% of their sizes in the native, dark-adapted state. The photoresponse was more pronounced in the microgel with higher ABSA content. The UV-induced deswelling is found to be reversible, through irradiation with visible light (λ = 450 nm). A mechanism for volume collapse caused by UV stimulus in these microgels is proposed and deduced to be analogous to that caused solely by a temperature stimulus.Microgels similar to those in Chapter 2 but with higher ABSA incorporation (20-30 wt. %) are described in Chapter 3. These microgels, in their native state, were detected by DLS, cryo-TEM and AFM to exist as pairs of particles in dimeric assemblies. These assemblies could not be disrupted by temperature and/or photo stimuli. However, they were separated by the addition of a small volume of dilute KCl solution as a chaotropic agent followed by a short period of mechanical agitation by ultra-sonication. The abundance of ABSA pendant groups bearing sulfonic acidterminal ends is believed to lead to extensive complexation of the sulfonic acid groups of ABSA to the C=O groups of the VCL units (strong charged-assisted hydrogen bonding) and to the formation of H-aggregates (π-π stacking interactions) of the azobenzene moieties. This resulted in the formation of inter- and intra-particle cross-links, which act in concert to suppress the thermo- and photo-responses of the particles.Azobenzene-based cross-linkers and VCL were used to produce dual thermo- and photo-responsive microgels in Chapter 4. Low feed amounts of azo cross-linkers were used for polymerization (~ 1.0 mol %). The UV-induced deswelling of these microgels was found to be less significant that in the microgels described in Chapter2.Non-temperature responsive microgels in Chapter 5 were synthesized with 2- acetoacetoxyethyl methacrylate (AAEM) as the main monomer. AAEM contains the β-dicarbonyl moiety which exhibits both enol and diketo structures that should interconvert through a tautomeric equilibrium. In their native states, the P(AAEM)chains in the microgels exist predominantly in the enol form and is tautomerized to the diketo form with UV (λ = 254 nm) irradiation. The diketo tautomer preferentially forms hydrogen bonds with water molecules, thereby causing an increased swelling and larger hydrodynamic radii of the particles that were detected by DLS. Theaddition of hydrophilic reagents during polymerization alters the topology and consequently the hydrophilicity of the internal micro-environment and porosity of the microgel network. By introducing greater hydrophilicity in the network, we obtained particles with less restricted topology that swelled more rapidly and isotropically.Increased hydrophilicity of the network, however, mitigates the extent of photoinduced swelling and increases the possibility of aggregation. We did not achieve the back-isomerization of the diketo form to the enol form, and hence, the increased swelling was irreversible.