Mikrostruktuursete ränioksiidsete sool-geel materjalide arendamine ja funktsionaalsete omaduste hindamine

Väitekirja elektrooniline versioon ei sisalda publikatsiooneKäesolev doktoritöö tutvustab kahte uudset erineva mikrostruktuursusega ränioksiidset pinnakatet – ümarate struktuuridega pindeid ja makropoorset vahtu. Töö eesmärk oli uurida nende materjalide sünteesiprotsesse ja valmistamist ning võimalikke rakendusi. Töös tutvustatav sool-geel-faasieralduse meetod on perspektiivikas alt-üles-meetod, millega saab valmistada funktsionaalseid struktuurseid ränioksiidseid pindeid ning pihustamine võimaldab selle meetodi kasutamist suurte pindade katmiseks tööstuslikus skaalas. Doktoritöös näidasime, et ümaraid pinnastruktuure saab valmistada erineva erineva suuruse, kuju ja pindtihedusega, kui valida järgmised sobivad parameetrid: alkoksiidi kontsentratsioon, lahusti tüüp, vee ja alkoksiidi moolsuhe, katalüsaatori ja alkoksiidi moolsuhe, suhteline õhuniiskus. Erinevas suurusjärgus struktuursusega pinded, mille elementide diameeter on alla mikromeetri ja pindtihedus võrdlemisi suur, omasid märkimisväärseid peegeldumisvastaseid ja valguse pinnalt hajumist muutvaid omadusi nähtavas ja lähi-infrapuna piirkonnas. Mõnedel valmistatud struktuursetel pindadel oli veekontaktnurk üle 130° ja see tähendab, et täiendava pinna funktsionaliseerimisega on võimalik saavutada ka superhüdrofoobsus (kontaktnurk üle 150°). Lisaks näitasime doktoritöös, et need mikro- ja nanostruktuursed pinnad on biosobivad ja et rakkude kasv ja paljunemine sõltuvad substraadi struktuuride läbimõõdust ja pindtihedusest. Kasutades sool-geel meetodit ja katalüütilist vesinikperoksiidi lagunemist, valmistati ränioksiidsed makropoorsed vahud, millel on hästi defineeritud suletud poorsus. Valmistatud vahu makropoorne struktuur on sarnane aerogeelidega ning väikseimaks soojusjuhtivuseks mõõdeti 0.018 W/(m∙K). Seega on seda võimalik kasutada kõrgetemperatuurilise soojusisolatsioonimaterjalina.In the present thesis two novel differently microstructured silica coatings were introduced. Our aim was to explore two distinct preparation methods for obtaining silica domes and a silica foam with sphere-like morphology, and to characterize and evaluate the applicability of these structured materials. Sol-gel phase separation method together with spray-coating is a promising bottom-up technique for preparing structured functional silica surfaces and is potentially suitable for covering large surfaces. We demonstrated that by choosing suitable alkoxide concentration, solvent type, water- and catalyst-alkoxide molar ratios and also relative humidity, it is possible to vary the size, shape and surface density of the domes. Sufficiently dense packing of silica domes in sub-µm range and multilayered coating were leading to a notable antireflection and light scattering effects in the Vis-NIR spectral range. As some produced silica surfaces had water contact angles exceeding 130°, simultaneous superhydrophobicity (contact angle > 150°) can be achieved by further surface functionalization. These surfaces are also found to be biocompatible and it is shown that growth characteristics morphology of fibroblasts is influenced by the morphology of the substrate. Sol-gel process together with catalytic decomposition of hydrogen peroxide is a novel method for the preparation of thick silica foam film, where well-defined closed-cell porosity appears. Macroporous characteristics and the lowest measured thermal conductivity (0.018 W/(m∙K)) of the prepared foams were similar to silica aerogels. These silica foam materials are potentially applicable as efficient thermal insulation materials

Surface-Active Thermally Responsive Hydrogels by Emulsion Sedimentation for Smart Window Applications

Thermally responsive polymers are a subject of increasing interest in research and development as a basis for a potential smart window technology. Here, we present a concept of preparing thermally responsive hydrogels with a thin active surface layer exhibiting rapid and reversible switching of light scattering in the visible and near-infrared spectral ranges. The process relies on the forced emulsion formation and sedimentation from the aqueous prepolymer solution by using a crosslinker that is engineered to serve as an antisolvent for the prepolymer and at the same time exhibit a suitable solubility profile in the sedimented hydrogel layer with respect to the supernatant aqueous phase. While the method can be employed for different polymer and crosslinker systems, as an example, here, we employ this concept for preparing thermally responsive hydrogels based on ethoxylated trimethylolpropane tri(3-mercaptopropionate) (ETTMP) and glycerol-derived crosslinkers with a dimaleate functionality, enabling crosslinking by the thiol-Michael click reaction. The material exhibits a luminous transmittance of over 95% and solar energy modulation of 59.91%. Moreover, we show that the pH and additives in the aqueous operating solution of the hydrogel enable the choice of the transition temperature in a wide range. The unique thin layer on the surface of the hydrogel, scalability to large surface areas, and robust and fast response at the practically relevant temperature range give this material a strong potential for smart window technology applications

Fabrication of Gelatin-ZnO Nanofibers for Antibacterial Applications

In this study, GNF@ZnO composites (gelatin nanofibers (GNF) with zinc oxide (ZnO) nanoparticles (NPs)) as a novel antibacterial agent were obtained using a wet chemistry approach. The physicochemical characterization of ZnO nanoparticles (NPs) and GNF@ZnO composites, as well as the evaluation of their antibacterial activity toward Gram-positive (Staphyloccocus aureus and Bacillus pumilus) and Gram-negative (Escherichia coli and Pseudomonas fluorescens) bacteria were performed. ZnO NPs were synthesized using a facile sol-gel approach. Gelatin nanofibers (GNF) were obtained by an electrospinning technique. GNF@ZnO composites were obtained by adding previously produced GNF into a Zn2+ methanol solution during ZnO NPs synthesis. Crystal structure, phase, and elemental compositions, morphology, as well as photoluminescent properties of pristine ZnO NPs, pristine GNF, and GNF@ZnO composites were characterized using powder X-ray diffraction (XRD), FTIR analysis, transmission and scanning electron microscopies (TEM/SEM), and photoluminescence spectroscopy. SEM, EDX, as well as FTIR analyses, confirmed the adsorption of ZnO NPs on the GNF surface. The pristine ZnO NPs were highly crystalline and monodispersed with a size of approximately 7 nm and had a high surface area (83 m2/g). The thickness of the pristine gelatin nanofiber was around 1 µm. The antibacterial properties of GNF@ZnO composites were investigated by a disk diffusion assay on agar plates. Results show that both pristine ZnO NPs and their GNF-based composites have the strongest antibacterial properties against Pseudomonas fluorescence and Staphylococcus aureus, with the zone of inhibition above 10 mm. Right behind them is Escherichia coli with slightly less inhibition of bacterial growth. These properties of GNF@ZnO composites suggest their suitability for a range of antimicrobial uses, such as in the food industry or in biomedical applications

Surface-Active Thermally Responsive Hydrogels by Emulsion Sedimentation for Smart Window Applications

Thermally responsive polymers are a subject of increasing interest in research and development as a basis for a potential smart window technology. Here, we present a concept of preparing thermally responsive hydrogels with a thin active surface layer exhibiting rapid and reversible switching of light scattering in the visible and near-infrared spectral ranges. The process relies on the forced emulsion formation and sedimentation from the aqueous prepolymer solution by using a crosslinker that is engineered to serve as an antisolvent for the prepolymer and at the same time exhibit a suitable solubility profile in the sedimented hydrogel layer with respect to the supernatant aqueous phase. While the method can be employed for different polymer and crosslinker systems, as an example, here, we employ this concept for preparing thermally responsive hydrogels based on ethoxylated trimethylolpropane tri(3-mercaptopropionate) (ETTMP) and glycerol-derived crosslinkers with a dimaleate functionality, enabling crosslinking by the thiol-Michael click reaction. The material exhibits a luminous transmittance of over 95% and solar energy modulation of 59.91%. Moreover, we show that the pH and additives in the aqueous operating solution of the hydrogel enable the choice of the transition temperature in a wide range. The unique thin layer on the surface of the hydrogel, scalability to large surface areas, and robust and fast response at the practically relevant temperature range give this material a strong potential for smart window technology applications

Long-term drivers of forest composition in a boreonemoral region: the relative importance of climate and human impact

Aim To assess statistically the relative importance of climate and human impact on forest composition in the late Holocene. Location Estonia, boreonemoral Europe. Methods Data on forest composition (10 most abundant tree and shrub taxa) for the late Holocene (5100-50 calibrated years before 1950) were derived from 18 pollen records and then transformed into land-cover estimates using the REVEALS vegetation reconstruction model. Human impact was quantified with palaeoecological estimates of openness, frequencies of hemerophilous pollen types (taxa growing in habitats influenced by human activities) and microscopic charcoal particles. Climate data generated with the ECBilt-CLIO-VECODE climate model provided summer and winter temperature data. The modelled data were supported by sedimentary stable oxygen isotope (O-18) records. Redundancy analysis (RDA), variation partitioning and linear mixed effects (LME) models were applied for statistical analyses. Results Both climate and human impact were statistically significant predictors of forest compositional change during the late Holocene. While climate exerted a dominant influence on forest composition in the beginning of the study period, human impact was the strongest driver of forest composition change in the middle of the study period, c.4000-2000years ago, when permanent agriculture became established and expanded. The late Holocene cooling negatively affected populations of nemoral deciduous taxa (Tilia, Corylus, Ulmus, Quercus, Alnus and Fraxinus), allowing boreal taxa (Betula, Salix, Picea and Pinus) to succeed. Whereas human impact has favoured populations of early-successional taxa that colonize abandoned agricultural fields (Betula, Salix, Alnus) or that can grow on less fertile soils (Pinus), it has limited taxa such as Picea that tend to grow on more mesic and fertile soils. Main conclusions Combining palaeoecological and palaeoclimatological data from multiple sources facilitates quantitative characterization of factors driving forest composition dynamics on millennial time-scales. Our results suggest that in addition to the climatic influence on forest composition, the relative abundance of individual forest taxa has been significantly influenced by human impact over the last four millennia