Mariia Ivanova

In this work, we investigated the dynamics of water in the pores of ultrathin BPH zeolite (Linde Q) nanosheets at different temperatures using ¹H time-domain NMR. Relaxation times and pore size distribution were determined by fitting the curves obtained using (i) Hahn and (ii) Carr-Purcell-Meiboom-Gill (CPMG) echo trains. Two distinct relaxation components and pore sizes were identified and assigned to water molecules in micro- and mesopores, respectively. The relaxation behavior within each individual pore adheres to a fast exchange regime, while an intermediate one governs the exchange between water molecules in different pores. The ratio of longitudinal and transversal relaxation times (T₁₁/T₂₁) of ∼2 reflects a strong surface interaction for water in zeolite pores featuring a surface relaxivity of 0.15 nm/ms. The confinement of water in the pores delays the rates of solid-liquid phase transitions, giving rise to a hysteresis loop reflecting a higher stability for confined liquid water or amorphous ice. These results highlight the possibility of tuning the separation capacities of zeolites in wet environments at different temperatures.

The sample of GdFeO₃ nanoparticles obtained by ultrasound-assisted co-precipitation synthesis may be classified as a T₁-contrast agent for MRI at 0.47 and 1.5 T, as a T₁–T₂ dual-modal contrast agent at 3 T and as a T₂-contrast agent at 4.7 T.

The influence of the hard segment nature on the crystallization kinetics of multi-block thermoplastic polyurethanes containing poly(butylene adipate) (PBA) as a soft segment was investigated. Using a combination of FTIR spectroscopy, time-domain ¹H nuclear magnetic resonance (TD-NMR), differential scanning calorimetry (DSC), fast-scanning calorimetry (FSC) and wide-angle X-ray diffraction (WAXS), it was shown that aliphatic, cycloaliphatic and aromatic diisocyanates affect the phase separation efficiency of soft and hard segments. The best phase separation efficiency was observed for a sample containing aliphatic diisocyanate due to the development of a hydrogen bond network. The thermal history, phase separation and the degree of ordering of the polyurethane determine the polymorphic behavior of melt-crystallized PBA. The formation of a partially-ordered mesophase of linear aliphatic polyurethane leads to an increase in the crystallization rate of PBA at room temperature and the formation of thermodynamically stable α-crystals. The presence of bulk cycloaliphatic and aromatic diol-urethane fragments prevents the phase separation of PBA, which crystallizes after slow cooling in a mixture of α- and β-crystalline forms. The new nanocalorimetry technique allows the identification of a direct correlation between the phase separation and crystallization kinetics of the melt-crystallized PBA in a wide range of cooling rates – from 2 to 30 000 K s⁻¹. Particularly, ultra-fast cooling suppresses the nucleation of the β-phase of PBA resulting in slow crystallization of only α-modification at room temperature. The role of the polyurethane mesophase in the crystallization of the soft segment was discussed for the first time.

Nanocrystals of gadolinium orthoferrite (GdFeO₃) with morphology close to isometric and superparamagnetic behavior were successfully synthesized using direct, reverse and microreactor co-precipitation of gadolinium and iron(III) hydroxides with their subsequent heat treatment in the air. The obtained samples were investigated by PXRD, FTIR, low-temperature nitrogen adsorption-desorption measurements, HRTEM, SAED, DRS and vibration magnetometry. According to the X-ray diffraction patterns, the GdFeO₃ nanocrystals obtained using direct co-precipitation have the smallest average size, while the GdFeO₃ nanocrystals obtained using reverse and microreactor co-precipitation have approximately the same average size. It was shown that the characteristic particle size values are much larger than the corresponding values of the average crystallite size, which indicates the aggregation of the obtained GdFeO₃ nanocrystals. The GdFeO₃ nanocrystals obtained using direct co-precipitation aggregate more than the GdFeO₃ nanocrystals obtained using reverse co-precipitation, which, in turn, tend to aggregate more strongly than the GdFeO₃ nanocrystals obtained using microreactor co-precipitation. The bandgap of the obtained GdFeO₃ nanocrystals decreases with decreasing crystallite size, which is apparently due to their aggregation. The colloidal solutions of the obtained GdFeO₃ nanocrystals with different concentrations were investigated by ¹H NMR to measure the T₁ and T₂ relaxation times. Based on the obtained r₂/r₁ ratios, the GdFeO₃ nanocrystals obtained using microreactor, direct and reverse co-precipitation may be classified as T₁, T₂ and T₁–T₂ dual-modal MRI contrast agents, respectively.

Water sorption on cellulose was studied in detail. The mechanism of the adsorption–desoprtion hysteresis loop was considered using NMR relaxation methods and sorption phenomena. The degree of crystallinity was found to decrease in cellulose during water sorption, while the specific surface area of cellulose increased because a wedging pressure arose as a new adsorbent–adsorbate interface formed. The most significant changes in these parameters were observed during desorption and were accompanied by the formation of an additional capillary-porous system in cellulose. Correlations were observed between the surface tension coefficient at the solid–liquid interface, the adsorption equilibrium constant, and the net heat of adsorption.