E. V. Khozina

Studies of domain morphology in segmented polyurethanes by pulsed NMR

A new technique based on the Goldman-Shen pulse sequence with varying preparation interval is proposed for the study of domain morphology of segmented polyurethanes. The results of numerical calculations of the magnetization recovery in different models of domain morphology show that the method provides new information about the domain morphology which is beyond the reach of the conventional Goldman-Shen experiment. A close agreement of the theoretical predictions with the experimental data on samples of segmented polyurethanes with a fixed molecular mass of the hard blocks and variable molecular mass of the soft blocks reinforces the above statement. The resulting structural parameters obtained by this new NMR technique are compared with the data from the small-angle X-ray scattering (SAXS) method.

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T1) and spin–spin (T2) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T1 and T2 on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of Water Molecules in Active Carbons. 1. Relation between the Spin-Spin Relaxation of Adsorbed Water Molecules and Structural Parameters of Microporous Active Carbons

Relation between the spin-spin nuclear magnetic relaxation time T2 of adsorbed water molecules and parameters of microporous structure of carbon adsorbents is disclosed. The pattern of dependences of T2 on the relative pressure and the number of water molecules per one primary adsorption site (PAS) is governed by the pore sizes and the number and nature of PASs. At a complete micropore filling, the T2 value depends on the volume density of PASs in active carbons. In the absence of PASs in the micropores, T2 is equal approximately to 21 ms. The larger the volume density of PASs, the smaller the number of water molecules per one PAS at the complete filling of micropores; i.e., the looser the packing of water molecules. The results of studying active carbons by the pulsed 1H NMR method agree well with the data of the adsorption method.

Features of nuclear magnetic relaxation of water and benzene molecules during absorption on activated carbons and estimation of pore size distribution in adsorbents

Nuclear magnetic relaxation in activated carbon—water and activated carbon—benzene adsorption systems was studied by pulse NMR methods. Activated carbons characterized by different porous structures and chemical state of the surface were used. The application of the three-pulse Goldman—Shen sequence to the adsorption system generates a dipole echo caused by the dipole-dipole coupling of “structural” protons, which is not averaged due to their mobility during experiment. The non-exponential character of relaxation attenuations of the transverse and longitudinal nuclear magnetizations of physically adsorbed molecules in activated carbon pores is a result of differencies in pore sizes. The pore sizes in activated carbon and the size distribution were determined from the data of nuclear magnetic relaxation with allowance for the contribution from the “structural” protons.

Energy characteristics of adsorbed water in active carbons according to the NMR relaxation data

Nuclear magnetic relaxation measurements were used to determine activation energy Eact of the motion of water molecules adsorbed in active carbons. The Eact value was found to depend on the filling of active carbon pores due to changes in the state of water molecules under adsorption. It was established that the Eact = f(p/ps) plots, where p/ps is the relative pressure of water vapor observed for microporous active carbons (FAS-1, 2, N-15, SKT-6A), are similar in form to the corresponding plots of changes in water adsorption heats. In particular, we concluded that the plateau in the Eact = f(p/ps) dependences, as in the case of adsorption heats, reflects the volumetric filling of active carbon micropores with water. We show that a linear function describes the increase in Eact values for water upon the complete filling of micropores with an increase in the volume of adsorbed water clusters per one primary adsorption center (W0/am). We establish that, for water in the FAS-3 sample, the deviation of Eact values from this linear function was due to the contribution from the vapor phase in the mesopores (x0 = 0.7−1.2 nm) that make up a considerable part of the active carbon’s porous system.

Determining the sizes of micropores in activated charcoals by the pulsed NMR method

The pulsed NMR method was used to measure the nuclear spin-spin relaxation of protons of water adsorbed in micropores of activated charcoal (AC) samples with different porous structures. A correlation was found between the spin-spin relaxation time of water protons in AC with completely filled micropores and the volume density of water primary adsorption centers in the AC samples. An equation for approximating obtained dependences is proposed that allows us to determine the volume of micropores in AC.

Adsorption-Induced Deformation of Adsorbents

Adsorption-induced deformation of AR-V and AUK carbon adsorbents and NaX zeolite has been studied upon adsorption of n-С5Н12, n-С6Н18, n-С7Н16, and CO2 at temperatures of 193−423 K. It has been shown that adsorption-induced deformation is positive upon the physical adsorption of gases and vapors on the surface of a nonporous (macroporous) solid when the excess adsorption is positive. When calculating the adsorption-induced deformation in the region of the capillary-condensation filling of mesopores, the additional pressure in capillaries, which is negative (contraction of an adsorbent), must be taken into account in the case of wetting a solid surface with a liquid adsorbate. The compressibility of AUK microporous carbon adsorbent as a porous solid is almost independent of the temperature and the properties of an adsorbate, and, for adsorption of n-C5H10 and n-C7H16 hydrocarbons and CO2, it is γа = (5.6 ± 0.6) × 10−6 bar−1. The compressibility of AUK adsorbent appears to be 87% higher than that of nonporous graphite.

THE POROUS STRUCTURE OF HARD AND DEFORMED ADSORBENTS AND MOBILITY OF ADSORBED MOLECULES

The opportunities of pulsed NMR methods for studying the mobility of adsorbed molecules and porous structure of microporous rigid and non-rigid adsorbents of different nature are considered. The distribution of nuclear magnetic relaxation times were used to determine the distribution of water and benzene molecules adsorbed in active carbons and in polymer sorbents. The dimensions of the areas corresponding to different porosities have been estimated from the self-diffusion data of water and benzene in AC. The self-diffusion study of water and benzene in the synthetic opals indicate the existence of ultramicropores. The obtained data of nuclear magnetic relaxation and self-diffusion of the adsorbed water and benzene in the different adsorbents are in accordance with the adsorption investigations and complement them.