Porous polyurethanes based on hyperbranched amino ethers of boric acid

Abstract

Novel polyurethanes with hierarchical supramolecular structure were synthesized via polyaddition reaction of amino ethers of boric acid and polyisocyanate. The structural features of amino ethers of boric acid were investigated using ¹¹B NMR spectroscopy and dynamic light scattering. The changes of surface topology and pore size which were dependent on the hyperbranched structure of amino ethers of boric acid for synthesized polymers were characterized by atomic force microscopy and capillary flow porometry. Macromolecular packing in polyurethanes was analyzed from the swelling, water sorption, thermogravimetry, mechanical loss tangent, contact angle measurements, and tensile test results. According to the results of gas transport and water vapour testing, the synthesized polyurethanes exhibit the non-additive character of permeability changing.

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Introduction

Nowadays, intense interest in advanced materials with a controlled pore size and chemical structure of their inner cavities is observed^1–4 due to the possibility to influence the polymer architecture and its macrostructure. Significant progress has been made in the synthesis of porous supramolecular architectures utilizing the self-assembly technique, and among them the unique and fascinating combination of physicochemical properties is inherent for boron-containing polymers having potential applications as self-healing materials, non-linear optics, drug delivery systems, and sensors.^5–15 The specific feature of boronic acid moieties within the polymeric structures is a reversible bond formation leading to covalent macromolecular assemblies in the case of boronic acids and diols¹⁶ interaction.

Promising results have been reported in the works of Lavigne et al., in which self-repairing advanced macromolecular structures were synthesized on the basis of multifunctional boronic acid and diol monomers.^17–21 Mikami^22,23 and Shimizu²⁴ have prepared the boronate esters via the polymerization of diboronic acids and multisaccharide-containing species. Polymers containing pendant²⁵ or terminal boronic acid groups have been synthesized by living cationic,^26,27 methatesis²⁸ or metallocene²⁹ polymerization. The reversible addition–fragmentation chain transfer polymerization has been carried out to obtain polymers with boronic acid-functional terminal groups and controlled molecular weight distribution.³⁰

In this manner, three dimensional porous materials have been synthesized by dynamic covalent formation in a one step condensation reaction³¹ or, alternatively, a two-component self-assembly and subsequent post-modification technique.³² Telodendrimers composed of linear polyethylene glycol and dendritic cholic acid block copolymers, and cross-linked using boronate ester on the core–shell interface, have proved to be promising materials for the chemical stimulus-triggered switching of supramolecular systems in water.^33,34

Ordered nanoporous structures called covalent-organic frameworks (COFs) containing boron acid moieties are currently a central issue of sustainable chemistry.³⁵ The ordered 2D architecture of boronate COFs accompanied by tunable porosity and nanocavities, is based on the homo- and heteropolymerisation involving the reversible boronate ester bonds. It should be pointed out, that the heteropolymerization of boron acids and diol components may result in versatile COFs with different functionalities.³⁶

The addition of boron-containing moieties to B–O, B–S, B–N and B–P bonds within the polymer backbone is the main challenge to develop semi-inorganic polymers^37–43 with enhanced thermal stability, mechanical, electrical, antibacterial and antifungal properties.⁴⁴ One of the most promising approaches in the given field is associated with the synthesis of hyperbranched macromolecular structures. In the literature, there are numerous reports of such polymers⁴⁵ including hyperbranched polyurethanes.^46,47 According to literature data, branching is not a sufficient condition for the formation of free volume in polymer. The creation of steric hindrances and structural elements is of great importance here due to impossible dense packing of the macromolecules.

A promising approach in this respect is the synthesis of polyurethanes based on hyperbranched amino ethers of boric acid (AEBA). In our previous work,⁴⁸ it was shown that the polyurethanes obtained with the use of 2,2-bis-(4-hydroxyphenyl)propane as a glycol component exhibited high values of gas permeability. However, relatively large dimensions of interconnected voids within the polymer was the reason for the low selectivity of gas mixtures containing ammonia and carbon dioxide.

Bearing in mind these aspects, the aim of this study is the further modification of the hyperbranched macromolecular structure of amino ethers of boric acid in order to create the regular arrangement of topology within the polymer, spatially separated ion pairs and the three-dimensional spatial elements.

Results and discussion

A Synthesis and characterization of amino ethers of boric acid

The hyperbranched AEBA structure is obtained by polycondensation, involving triethanolamine (TEA) as a branching center and boric acid as a main building block (Scheme 1). Boric acid actively reacts with hydroxyl groups of TEA. Reactions with glycols–diethylene glycol (DEG), triethylene glycol (TEG), and poly(ethylene glycol) (PEG) – proceed mainly with less activity at the elevated temperatures and, in some cases, require the use of chlorides of divalent metals as a catalyst.

Scheme 1 Synthesis of AEBA-3.

In our previous work,⁴⁸ the possibility of the intermolecular complex formation in amino ethers of boric acid based on the low molecular weight glycols was established by the determination of the content of terminal hydroxyl groups, conductometry, viscosmetry, electron and IR spectroscopy (Scheme 1). In this work, the additional data about the structure and reactivity of AEBA are obtained by NMR spectroscopy (Fig. 1).

Fig. 1 ¹¹B NMR spectrum of AEBA-6 and AEBA-3.

Amino ethers are synthesized at molar ratios of [TEA] : [H₃BO₃] : [DEG] = 1 : 3 : 6 (AEBA-3) and [TEA] : [H₃BO₃] : [TEG] = 1 : 6 : 12 (AEBA-6) (Scheme 2). For aforementioned reaction conditions, in both cases the ratio of [H₃BO₃] : [TEG] and [H₃BO₃] : [DEG] was equal to 1 : 3. Amino ethers AEBA-3 and AEBA-6 differ in the molar content of TEA.

Scheme 2 Synthesis of AEBA-6.

It was found, that ¹¹B NMR spectrum of AEBA-3 and AEBA-6 samples in methanol (T = 30 °C) contained four signals. Among them, the signal (1) at 19.15 ppm corresponds to boron atoms of boric acid, nearby signal (2) at 18.48 ppm belongs to boron atoms in partially etherified boric acid, in which the part of B–OH groups remains free. The signal (3) near 14.07 ppm points to the presence of boron atoms in borates, and the signal (4) at 10.51 ppm corresponds to borate intermolecular complexes. In the case of AEBA-6, the distribution of signals is (1 + 2) : 3 : 4 = 18.13 : 1.83 : 1.00. The signals 1 and 2 reduces more than 2-fold as compared with AEBA-3.

AEBA may undergo partial hydrolysis as ¹¹B NMR spectra is measured in the methanol medium. At the same time, borates are present in both AEBA obtained at molar ratio of [TEA] : [H₃BO₃] : [DEG] = 1 : 3 : 6 and [TEA] : [H₃BO₃] : [TEG] = 1 : 6 : 12, respectively. This fact indicates that the formed complexes are stable structural elements. Boric acid ethers are relatively stable in methanol medium as well.

The NMR data point to the occurrance of the reactions involving OH-groups due to the presence of free B–OH bonds. In this case, before the reaction of urethane formation, the organic compounds with active functional groups may be introduced into AEBA structure, such as ionic liquids or tetraethoxysilane as the basis for gel–sol transformations. A spatial accessibility of boron atoms in B–OH bond also suggests their complex formation with nitrogen atoms in low molecular weight nitrogen-containing compounds.

Fig. 2 represents the particle size distribution of AEBA-3 and AEBA-6 compounds measured by dynamic light scattering in water. It turned out that amino ethers of boric acid form relatively large associates with an average size of 600 nm for AEBA-3 and 1000 nm for AEBA-6. As the measurements of AEBA dimensions have been carried out in dilute aqueous solutions, we can predict strong interactions occurring in amino ethers of boric acid. Additionally, the existence of the B–O–C bond within the hindered AEBA associates prevents the hydrolysis reaction, which rapidly proceeds in “unprotected” boric acid ethers.

Fig. 2 Particle size distribution by intensity of AEBA-3(1) and AEBA-6(2).

We used two approaches to create steric hindrances and spatially separated ion pairs. In the first case, we prepared difunctional and trifunctional adducts (EM) based on the reactions of the epoxy resin (DIR331) with monoethanolamine (MEA) as shown in Scheme 3. Next, the amino ethers of boron acid (AEBA–EM) were obtained on the basis of EM adduct in one of the branches and AEBA-PEG. The presence of free B–OH groups (established by NMR spectroscopy ¹¹B) and a secondary amine in AEBA–EM leads to the formation of secondary ammonium borate.

Scheme 3 Synthesis of EM adducts on the basis of DIR331 and MEA.

Significant steric hindrances in AEBA–EM could initiate the spatial separation of the anion and proton (Scheme 4 and 1 ESI†).

Scheme 4 Formation of intramolecular interactions in AEBA–EM.

In another case, the boric acid ethers (EMB) were pre-synthesized using DIR331, MEA and PEG as a branch within the structure of AEBA–EMB polymers (Scheme 5 and 2 ESI†). Further, AEBA–EM and AEBA–EMB were used for the synthesis of organoboron polyurethanes.

Scheme 5 Formation of EMB on the basis of DIR331, MEA and PEG.

For this purpose AEBA–EM, AEBA–EMB or AEBA–PEG reacted with polyisocyanate (PIC), which was a mixture of methylene diisocyanate (MDI) with its multifunctional derivatives. The polyurethanes derived from amino ethers of boric acid were designated as AEBA–EM–PU, AEBA–EMB–PU and AEBA–PEG–PU, respectively.

According to IR spectroscopy data (Fig. 3), the isocyanate groups are completely involved in the urethane formation reaction. We observed the absence of characteristic peak at 2275 cm⁻¹ corresponding to stretching vibrations of N=C bond of isocyanate group. Absorption bands at 1730 and 1710 cm⁻¹ correspond to the stretching vibrations of C=O bond in urethane group and C=O bond involved in hydrogen bonding, respectively.

Fig. 3 IR spectra of AEBA–EM–PU obtained at the content of EM (wt%): 0(1), 0.001(2), 0.005(3), 0.01(4), 0.05(5), 0.1(6), 0.25(7), 0.3(8), 0.5(9), 0.8(10).

B Surface topography and porosity of polyurethanes based on amino ethers of boric acid

The surface morphology of samples was investigated using atomic force microscopy (AFM). AFM images confirm the formation of surface pores in polyurethane samples (Fig. 4). AEBA–EMB–PU have the largest pore size.

Fig. 4 AFM images of AEBA–EM–PU obtained at the content of EM (wt%): 0.01 (a), 0.03 (b) 0.35 (c); and AEBA–EMB–PU (d).

The results of water absorption and AFM correlate with the porometry (Table 1) and water vapor permeability (Fig. 5a and b) data. Thus, the largest pore size is observed for AEBA–EMB–PU, as for AEBA–EM–PU the relation of the pore size and EM content is non-additive. Such non-additive changes in AEBA–EM–PU speak for the ambiguous way of EM impact on macromolecular and supramolecular structure.

Table 1 The pore size, determined by capillary flow porometry for AEBA–EM–PU and AEBA–EMB–PU

Content of EM, wt%	Average pore size, nm	Pressure, bar
AEBA–EM–PU
0	277	1.65
0.005	451	1.015
0.15	112	4.099
0.5	117	3.925
0.8	323	1.416
0.9	300	1.532
1	413	1.107
AEBA–EMB–PU
—	1753	0.261

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Fig. 5 The time dependence of water vapor permeability coefficient for AEBA–EM–PU at various EM content (wt%): 0(1), 0.04(2), 0.05(3), 0.06(4), 0.15(5), 0.23(6), 0.25(7), 0.3(8), 0.5(9), 0.85(10), 0.9(11), 1.0(12).

In contrast to AEBA–EM–PU containing less than 1.1% of aromatic branches, the basic elements of hyperbranched structure in AEBA–EMB–PU are composed of aromatic units. Therefore, a consequence of the specific macromolecular structure of AEBA–EMB–PU is the increase of pore size as AFM and porometry studies have shown.

Pores in AEBA–EMB–PU are a few times larger as compared to AEBA–EM–PU and have the uniform size distribution (Table 1). Since an average pore size of AEBA–EMB–PU is 1753 nm, this polyurethane belongs to the class of the macroporous polymers. Features of supramolecular organization and large free volume within the AEBA–EMB–PU result in the reduction of thermal stability compared with AEBA–PEG–PU and AEBA–EM–PU.

Summarizing the results of physicochemical methods and pore size distribution data, the present work represents the developed experimental approach to directional changes in free volume, pore size and chemical structure of the surface of hyperbranched polyurethanes based on amino ethers of boric acid.

C The sorption, mechanical and thermal properties of porous polyurethanes

Water absorption and swelling of samples in acetone have been measured to determine free volume of polymers and relatively changes in polymer network formation. It was established that varying the EM content in AEBA–EM led to changes in both the shape of the water absorption curve as a function of time, and the water absorption values for AEBA–EM–PU. Particularly, a notable decrease in water absorption was observed at the low content of EM in AEBA–EM–PU. In this content range (EM content is more than 0.001 wt%), the maximum swelling of samples in acetone decreased significantly. At such a low content, despite its bulky structure, EM fails to create steric hindrances within the polymer matrix.

Thus, tests of water absorption and swelling in acetone revealed the significant modifying effect of EM on AEBA–EM–PU.

At EM content of 0.9% in AEBA–EM–PU the quantity turns into quality. Presumably, at this concentration and above, the EM moieties turn into the bulky fragments in AEBA–PU–EM structure.

Amino ethers of boric acid stop their flow at the concentrations of EM higher than 1.2% and, consequently, are not desirable to be used in the synthesis of the polyurethanes. The discontinuous changes in AEBA–EM properties indicate the significant intra- and intermolecular interactions involving also the secondary ammonium–borate interactions.

The structures of AEBA–EMB–PU and AEBA–EM–PU are significantly different. When considering AEBA–EMB–PU, the steric hindrances are the part of the macromolecular architecture of the amino ether of boric acid. In this case, the free volume of polyurethane increases markedly up to 17%. The macromolecular structure becomes more rigid and, therefore, AEBA–EMB–PU swelling rate becomes low and does not exceed 11% (Fig. 6).

Fig. 6 The curves of swelling in acetone (a) and water absorption (b) of AEBA–PEG–PU (1), AEBA–EMB–PU (2).

TGA analysis (Fig. 7 and 8) reflects that high thermal stability in AEBA-based polyurethanes occurs owing to the macromolecular packing density, nor to the presence of boron atoms. Here, the correlation between the thermal stability and water absorption is observed. It is worth note that AEBA-based EM-free polymers absorb the water, but are impermeable to gases.

Fig. 7 TGA curves of AEBA–EM–PU at the content of EM (wt%): 0(1), 0.25(2), 0.8(3).

Fig. 8 TGA curves of AEBA–EMB–PU (1) and AEBA–PEG–PU (2).

The lowest thermal stability value is observed for AEBA–EMB–PU. This circumstance is consistent with the water absorption behavior and the largest pore size of the polymer. Polyurethane AEBA–EM–PU exhibits a single-stage water absorption behavior, and the pore sizes from AFM images have the maximal values.

It is well known, that urethane groups exhibit a relatively low thermal stability (thermal degradation generally starts in the range of 250–270 °C), which depends on the macromolecular packing density of polyurethanes. The thermal stability of polyurethanes increases with rising macromolecular packing density due to the screening of urethane groups. On the contrary, loosening the macromolecular packing diminishes the thermal stability of polyurethanes. Thus, the thermal stability of polyurethanes derived from hyperbranched AEBA indicates the presence (and the increase) of the free volume.

Mechanical tests of the polyurethanes reveal the presence of spatially separated ion pairs, which have been formed during the synthesis of AEBA in a presence of EM adduct. The introduction of ions with uncompensated charges into polymers is a conventional approach of the ionomers preparation. Ionomers are generally characterized by the higher tensile strength compared to the ordinary polymers, and if ionomers are porous, it results in an increase of sorption properties. The tensile strength non-additively varies with the adduct content, but with the areas of decrease and the subsequent increase of mechanical properties.

This phenomenon can be associated with the hierarchy of supramolecular structure, where the macromolecules attract to each other and create the additional intermolecular bonds due to the existence of spatially separated cations and anions. Increasing of the EM adduct content changes the packing type of such structures causing the nonmonotonic dependence of mechanical properties on the content of adduct.

Mechanical properties changes are consistent with the results of measurements of the temperature dependence of the mechanical loss tangent. According to Fig. 9, EM content has a significant effect on the supramolecular organization of AEBA–EM–PU. Thus, for samples containing [EM] = 0.001, 0.005, 0.05 wt%, the α-transition temperature is found near 15 °C, while for others it exceeds 50 °C.

Fig. 9 Temperature dependence of the mechanical loss tangent (tg σ) for AEBA–EM–PU at various EM content (wt%): 0(1), 0.001(2), 0.005(3), 0.01(4), 0.05(5), 0.1(6), 0.3(7), 0.5(8), 0.8(9).

While the samples with relatively low temperature of the initial segmental motion ([EM] = 0.001, 0.005, 0.05%) exhibit the elastic deformation, the mechanical behavior of samples with the α-transition temperature exceeding 50 °C is typical for glassy polymers.

The analysis of physical and mechanical properties along with temperature dependences of the mechanical loss tangent led us to the conclusion that the formation of AEBA–EM–PU was accompanied by supramolecular organization with flexible chain component PEG.

This is proved by the relatively low temperature of the initial segmental motion for the crosslinked polymers obtained using low molecular weight PEG as a spacer. Due to the peculiarities of the hierarchical supramolecular organization within the AEBA–EM–PU the temperature dependence of α-transition on the EM content exhibits non-additive character.

Along with the hyperbranched structure of AEBA–EM, the most probable reason for the pores formation in AEBA–EM–PU is the association of polyoxyethylene chains. So, the microphase separation involving PEG chains has an impact on the arrangement of AEBA–EM fragments within the polymer matrix of AEBA–EM–PU. The result is the formation of voids and specific chemical structure of their surface containing polyoxyethylene glycol moieties.

To confirm this hypothesis, the contact angles of water droplets on the samples with different content of EM were measured (Fig. 10). It was found that the contact angles change follows to the regularities of the pore size variation (Table 1). Thus, the diminishing of pore size values leads to the decrease of the contact angles and vice versa.

Fig. 10 Water contact angles of AEBA–EM–PU at various EM content (wt%).

D Gas transport properties of polyurethanes based on amino ethers of boric acid

Transport properties of pure gasses as helium, methane, nitrogen, ammonia, carbon dioxide and hydrogen sulfide in AEBA–EM–PU-based polymeric films have been investigated and are summarized in Table 2. According to the test results, the permeability values of various gases non-additively depend on the content of EM.

Table 2 The permeability of AEBA–EM–PU^a

Gas	[EM], wt%
	0.0	0.001	0.05	0.1	0.25	0.5	0.8
a 1 Barrer = 3.348 × 10^−19 kmol m m^−2 s^−1 Pa^−1.b n/d – not detected.
He	17 ± 0.2	25 ± 0.3	17 ± 0.2	45.7 ± 0.5	12 ± 0.1	32 ± 0.3	6.5 ± 0.1
N2	9.7 ± 0.1	14 ± 0.2	9.4 ± 0.1	36 ± 0.4	5.5 ± 0.1	15.9 ± 0.2	2.9 ± 0.1
CH4	8.4 ± 0.1	6.1 ± 0.1	3 ± 0.1	26.3 ± 0.3	17.6 ± 0.2	7.8 ± 0.1	4.7 ± 0.1
CO2	5.8 ± 0.1	4.8 ± 0.1	3.8 ± 0.1	16.5 ± 0.2	6.3 ± 0.1	8.7 ± 0.1	2.7 ± 0.1
NH3	6.3 ± 0.1	7.6 ± 0.1	12.5 ± 0.1	19.8 ± 0.2	6.9 ± 0.1	10 ± 0.1	3.3 ± 0.1
H2S	n/d^b	n/d	n/d	n/d	5.2 ± 0.1	n/d	n/d

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Here, it is worth note that all tested gases have relatively low permeability values, and this observation can be explained by the absence of through-pores in polymers. However, the water vapour transport through polymeric membranes have high values, indicating that the pores surface has the hydrophilic nature. Furthermore, this observation confirms the microphase separation of polyoxyethylene component creating the conditions for the formation of pores and specific supramolecular structures within the polyurethanes. Understanding of pores formation mechanisms within the studied polyurethanes is relevant for further investigations focused on the improving of gas transport properties of aforementioned polymers.

Experimental

A Materials

Glycols i.e. diethylene glycol (DEG), triethylene glycol (TEG), and poly(ethylene glycol) (PEG) were purchased from PJSC Nizhnekamskneftekhim (Nizhnekamsk, Russia). Triethanolamine (TEA) was obtained from OJSC Kazanorgsintez (Kazan, Russia). Boric acid (99.99%) was purchased from Sigma-Aldrich. The epoxy resin DIR331 was obtained from the Dow Chemical Company. Polyisocyanate “Cosmonate-200” (PIC) was purchased from Kumho Mitsui Chemicals, Inc.

Glycols were additionally dried at low pressure at 5 mmHg and 353 K down to 0.01% of moisture concentration. Crystal hydrates of CuCl₂·6H₂O were dehydrated at 393 K for 48 hours.

We used gases (carbon dioxide, hydrogen sulphide, helium, nitrogen, and methane) with purity not less than 99.995% (NII KM, Russia) when analyzing the gas separation of polymers. Furthermore, we used high purity ammonia 99.99999% (Firm HORST, Russia).

Other reagents and solvents were used without further purification.

B Synthetic procedures

General procedure for synthesis of amino ethers of boric acid (AEBA). The synthesis of AEBA was based on the usage of different glycols. To obtain AEBA-3, triethanolamine (1 mol), boric acid (3 mol) and DEG (6 mol) were added into a 250 mL three-necked round-bottom flask equipped with a magnetic stirrer. The mixture was heated to 353 K for 2 hours under low pressure (0.2–2.0 mmHg) in a vacuum oven. The synthesized liquid AEBA–DEG was collected into a stoppered flask.

For the synthesis of AEBA-6, TEA (1 mol), boric acid (6 mol), TEG (12 mol), and CuCl₂ as a catalyst (0.1 wt%) were added into 250 mL three-necked round-bottom flask equipped with the magnetic stirrer. The mixture was heated to 363 K for 2 hours under the low pressure (0.2–2.0 mmHg) in the vacuum oven. The synthesized liquid AEBA–TEG was collected into the stoppered flask.

AEBA–PEG was obtained in two stages. In the first stage, boric acid (6 mol) and PEG (3 mol) were added to 250 mL three-necked round-bottom flask equipped with the magnetic stirrer. The mixture was heated to 363 K for 2 hours under the low pressure (0.2–2.0 mmHg) in the vacuum oven. In the second stage, the remaining PEG (6–12 mol) and TEA (1 mol) were added to the first mixture, followed by heating under similar conditions. The synthesized liquid AEBA–PEG was collected into the stoppered flask.

The composition and structure of AEBAs are determined by the mole ratio of TEA, boric acid, and glycols. The reaction was quenched after the desired amount of hydroxylation toward the target product. Reaction progress is monitored by titration to determine hydroxyl group concentration.

General procedure for synthesis of polyurethanes based on amino ethers of boric acid. Polymeric films were obtained by bulk polymerization. The synthesized AEBA was mixed with polyisocyanate (PIC) in a ratio of 1 : 1 to obtain the polyurethanes.^49–54 The polymerization was carried out on a glass surface. Cast film was dried in the vacuum oven at 353 K for two hours. The polymers were identified according to the molar ratio of AEBA and PIC, (e.g. sample [[TEA] : [H₃BO₃] : [PEG] = 1 : 3 : 6] : [PIC] = 1 : 1 contains 1 mol of AEBA–PEG per 1 mol of PIC). The flat-sheet polymeric films with a diameter equal to 80 cm were obtained and used for further investigation in gas separation. The thickness of the polymeric films was approximately 140 microns.

C Light-scattering of AEBA water solution

Dynamic light scattering experiments were carried out on Zetasizer Nano ZS (Malvern, Great Britain). This instrument has 4 mV He–Ne laser, which is working on 632.8 nm wavelength. Measurements were carried out at 173° detection angle.⁵⁴ The experiments were carried out at 25 °C in the disposable plastic cuvettes of 1 cm path length.

D Thermal gravimetric analysis

Thermal gravimetric analysis (TGA) was performed using thermal analyzer STA-6000 (Perkin Elmer). The samples (0.1 g) were loaded in alumina pans and ramped to 773 K at a heating rate of 5 K min⁻¹ in a nitrogen atmosphere.

E NMR spectroscopy

¹¹B NMR spectra of samples were recorded on an Avance II-500 (Bruker) spectrometer. Chemical shifts (δ) are reported in ppm for the solution of compound in methanol. Operating frequency was 160.46 MHz. Sample temperature was 303 K.

F Dynamic mechanical analysis

DMA curves of the films were obtained using DMA 242 D dynamic mechanical analyzer. All the samples were studied in the range 273–473 K. The fixed frequency used was 10 Hz with the heating rate of 3 K min⁻¹.

G Capillary flow porometry

The average pore size of samples was measured using the capillary flow porometer (Porolux 100™). Porefil (Benelux Scientific) wetting fluid with a surface tension of 16 dyn cm⁻¹ was used to wet the samples prior to the measurements. The average pore diameter was calculated from the flow pressure, which corresponds to the intersection of the wet curve with the calculated half-dry curve (dry and wet run).

H Fourier transform infrared spectroscopy analysis

The IR spectra of the products were recorded on an InfraLUM FT 08 Fourier transform spectrometer using the attenuated total reflection technique. The spectral resolution was 2 cm⁻¹, and the number of scans was 60.

I Contact angle measurements

The static contact angle was measured using an optical contact angle measurement system. The droplet of water was placed on the sample surface and an image of drop shape was obtained. The optical system apparatus consisted of a light source, an adjustable stage, and a USB optical microscope. The microscope TS-H (Chuo Seiki, Japan) was fixed on an adjustable microscope mount.⁵⁵ A digital image of the drop shape was made using CCD camera of a microscope. The Image J® software with Dropsnake plugin was used for calculation of contact angle value. The contact angle value for each testing liquid was calculated as an average of 5 measurements of different positions for each sample.⁵⁸

J Atomic force microscopy topology analysis

The topography of the membrane surface was determined by atomic force microscopy (AFM). The topography of the membrane surface was determined by atomic force microscopy (AFM). Atomic force microscope SPM-9700 (Shimadzu, Japan) with scanner 30 mkm was used. Because the test polymer films have a weakly bound surface structure, scanning was performed in the tapping mode with POINTPROBE FMR-20 silicon vibration cantilevers (Nano World Innovative Technologies, USA) with a spring constant of 1.3 N m⁻¹ and a typical tip curvature radius of no greater than 8 nm (definitely no more than 12 nm), the tip height varied from 10 to 15 μm. The cantilever was chosen with taking into account the structural softness of the polymer surfaces. The cantilever with a curvature radius of tip not more than 8 nm was used for the observation of a topographic map to minimize the error introduced by the cantilever due to the narrowing of profile recesses.^56,57 The surface characterization was carried out at ambient temperature. The digital visualization of the results of measurements consisted in the representation of a relief in the form of a topographic map (heights are reflected by colours) and three dimensional images. The treatment of the resulting AFM images and measurement of pores diameters were performed with the aid of the SPM Manager® ver. 4.02 software (Shimadzu, Japan) using a correction for natural inclination. Correction filters were not used in the measurement of the size of pores for preserving the true surface features. The samples with size 10 × 10 mm were cleaned of dust with ethanol before measurement, and then affixed to the centre of the sample holder using a two-sided carbon tape (SPI Supplies Division of STRUCTURE PROBE Inc., USA). In the course of scanning, the automatic correction of linear interferences was applied.

K Water sorption measurements

The effect of water absorption on porous polymers was investigated in accordance with National Standard 2678-94. The method of water absorption is based on the gravimetric determination of the amount of water which is entrapped in an elemental sample.

L Gases permeability measurements

The single gas permeability coefficients of N₂, CH₄, CO₂, NH₃ and H₂S through the obtained membranes were measured at the room temperature using the constant-volume variable-pressure apparatus^48,59–61 according to Daynes–Barrer technique.^62,63

Permeability coefficients (P) (1 Barrer = 3.348 × 10⁻¹⁶ mol m m⁻² s⁻¹ Pa⁻¹) were calculated according to the method proposed in works,^59–61 using the experimental set-up with the automatization of the permeability measurement process, which was calculated through the use of programming logic controller (Unitronix, Airport City, Israel). The increase of separation efficiency can be achieved using novel membrane cascades.^64–69

M Water vapour permeability measurements

The water vapour permeability of polymeric films was measured via standard ASTM E 398-03-9237 method, using the Textest FX 3100 permeability tester. The measurements were performed for 3 samples at a constant pressure drop of 100 Pa (20 cm² test area). All tests were performed under standard atmospheric conditions.

Conclusions

Hydrolytically stable hyperbranched amino ethers of boric acid (AEBA) with the terminal hydroxyl groups were synthesized on the basis of boric acid, triethanolamine and hydroxyl-containing compounds. The structure features and reactivity of those compounds were investigated using NMR spectroscopy and dynamic light scattering. It was established that amino ethers of boric acid form intermolecular complexes and have both reactive hydroxyl and B–OH groups.

The adduct (EM) and the ether of boric acid (EMB) were synthesized on the basis of diglycidyl ether of 4,4′-dihydroxy-2,2-diphenylpropane and monoethanolamine. Synthesized compounds were used to develop the method of preparation of hyperbranched amino ethers of boric acid containing EM or EMB in one of the branches of AEBA.

Porous polymeric materials were obtained on the basis of AEBA and polyisocyanates. EM modified the polyurethane structure, while EMB was a component of a macromolecular architecture of polymers. It was observed that the thermal stability of polyurethanes derived from AEBA depends on the pore sizes, rather than the boron content.

The permeability and gas transport properties of polyurethanes based on amino ethers of boric acid were investigated. It was found the novel polyurethanes were characterized by the high water steam permeability and, in contrary, the poor gas transport properties. This phenomenon was explained by the hydrophilic nature of the pores' surface as a result of microphase separation of polyoxyethylene component and the absence of through-pores in polymers. Understanding of pores formation mechanisms within the studied polyurethanes is relevant for further investigations focused on the improving of gas transport properties of aforementioned polymers.

Abbreviation

COFs	Covalent-organic frameworks
AEBA	Amino ethers of boric acid
TEA	Triethanolamine
DEG	Diethylene glycol
TEG	Triethylene glycol
PEG	Poly(ethylene glycol)
AEBA-3	Polymer synthesized on the basis of [TEA] : [H3BO3] : [DEG] = 1 : 3 : 6
AEBA-6	Polymer synthesized on the basis of [TEA] : [H3BO3] : [TEG] = 1 : 6 : 12
EM	Trifunctional adducts based on the reactions of the epoxy resin DIR331 with monoethanolamine
EMB	Boron acid ethers synthesized using epoxy resin DIR331, monoethanolamine and poly(ethylene glycol)
PU	Polyurethane
AFM	Atomic force microscopy
PIC	Polyisocyanate “Cosmonate-200”

Acknowledgements

This work was supported by the Russian Science Foundation (grant no. 15-19-10057).

References

1. M. Ulbricht, Polymer, 2006, 47, 2217.
2. P. Escalé, L. Rubatat, L. Billon and M. Save, Eur. Polym. J., 1999, 48, 1001.
3. F. S. Bates, M. A. Hillmyer, T. P. Lodge, C. M. Bates, K. T. Delaney and G. H. Fredrickson, Science, 2012, 336, 434.
4. B. H. Jones and T. P. Lodge, Nature, 2012, 44, 131.
5. J. N. Cambre and B. S. Sumerlin, Polymer, 2011, 52, 4631.
6. W. Yang, X. Gao and B. Wang, Med. Res. Rev., 2003, 43, 346.
7. W. L. A. Brooks and B. S. Sumerlin, Chem. Rev., 2016, 116, 1375.
8. W. Niu, C. O'Sullivan, B. M. Rambo, M. D. Smith and J. J. Lavigne, Chem. Commun., 2005, 34, 4342.
9. F. Jäkle, J. Inorg. Organomet. Polym. Mater., 2005, 15, 293.
10. J. H. Lee, Y. Kim, M. Y. Ha, E. K. Lee and J. Choo, J. Am. Soc. Mass Spectrom., 2005, 16, 1456.
11. M. D. Phillips and T. D. James, J. Fluoresc., 2004, 14, 549.
12. E. Uguzdogan, H. Kayi, E. B. Denkbas, S. Patir and A. Tuncel, Polym. Int., 2003, 52, 649.
13. A. Özdemir and A. Tuncel, J. Appl. Polym. Sci., 2000, 78, 268.
14. B. Elmas, M. A. Onur, S. Şenel and A. Tuncel, Colloids Surf., A, 2004, 232, 253.
15. E. Uguzdogan, E. B. Denkbas and A. Tuncel, Macromol. Biosci., 2002, 2, 2014.
16. J. P. Lorand and J. O. Edwards, J. Org. Chem., 1959, 24, 769.
17. J. J. Lavigne, B. M. Rambo and W. Niu, Polym. Mater. Sci. Eng., 2004, 90, 816.
18. W. Niu, C. O'Sullivan, B. M. Rambo, M. D. Smith and J. J. Lavigne, Chem. Commun., 2005, 34, 4342.
19. W. Niu, M. D. Smith and J. J. Lavigne, J. Am. Chem. Soc., 2006, 128, 16466.
20. R. W. Tilford, W. R. Gemmill, H.-C. zur Loye and J. J. Lavigne, Chem. Mater., 2006, 18, 5296.
21. R. W. Tilford, S. J. Mugavero, P. J. Pellechia and J. J. Lavigne, Adv. Mater., 2008, 20, 2741.
22. M. Mikami and S. Shinkai, Chem. Commun., 1995, 2, 153.
23. M. Mikami and S. Shinkai, Chem. Lett., 1995, 7, 603.
24. I. Nakazawa, S. Suda, M. Masuda, M. Asai and T. Shimizu, Chem. Commun., 2000, 10, 881.
25. Y. Qin, V. Sukul, D. Pagakos, C. Cui and F. Jäkle, Macromolecules, 2005, 38, 8987.
26. B. Koroskenyi, L. Wang and R. Faust, Macromolecules, 1997, 30, 7667.
27. L. Balogh, Z. Fodor, T. Kelen and R. Faust, Macromolecules, 1994, 27, 4648.
28. T. C. Chung and M. Chasmawala, Macromolecules, 1991, 24, 3718.
29. T. C. Chung, Prog. Polym. Sci., 2002, 27, 39.
30. P. De, S. R. Gondi, D. Roy and B. S. Sumerlin, Macromolecules, 2009, 42, 5614.
31. G. Zhang, O. Presly, F. White, I. M. Oppel and M. Mastalerz, Angew. Chem., Int. Ed., 2014, 53, 1516.
32. K. Ono, R. Aizawa, T. Yamano, S. Ito, N. Yasuda, K. Johmoto, H. Uekusa and N. Iwasawa, Chem. Commun., 2014, 50, 13683.
33. Y. Li, W. Xiao, K. Xiao, L. Berti, J. Luo, H. P. Tseng, G. Fung and K. S. Lam, Angew. Chem., Int. Ed., 2012, 51, 2864.
34. M. Arzt, C. Seidler, D. Y. W. Ng and T. Weil, Chem.–Asian J., 2014, 9, 1994.
35. Z. Xiang and D. Cao, J. Mater. Chem. A, 2013, 1, 2691.
36. A. P. Côté, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J. Matzger and O. M. Yaghi, Science, 2005, 310, 1166.
37. G. Jungang, S. Xiaohui and X. Liya, Int. J. Polym. Mater., 2005, 54, 949.
38. J. Gao, Y. Liu and F. Wang, Eur. Polym. J., 2001, 37, 207.
39. Y. Liu, J. Gao and R. Zhang, Polym. Degrad. Stab., 2002, 77, 495.
40. W. Tu and S. Wei, China Plast. Ind., 1981, 4, 16.
41. G. Jungang, L. Yanfang and W. Fengli, Eur. Polym. J., 2001, 37, 207.
42. F. Jakle, J. Inorg. Organomet. Polym. Mater., 2005, 15, 293.
43. C. Martin, B. J. Hunt, J. R. Ebdon, J. C. Ronda and V. Cadiz, React. Funct. Polym., 2006, 66, 1047.
44. J. K. Adewole, J. Appl. Polym. Sci., 2016, 133, 43606.
45. K. Zhang, A. M. Nelson, S. J. Talley, M. Chen, E. Margaretta, A. G. Hudson, R. B. Moore and T. E. Long, Green Chem., 2016, 18, 4667.
46. R. Duarah, Y. Pratap Singh, B. B. Mandal and N. Karak, New J. Chem., 2016, 40, 5152.
47. M. Vanjinathan, A. Shanavas, A. Raghavan and A. S. Nasar, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 3877.
48. I. M. Davletbaeva, O. Y. Emelina, I. V. Vorotyntsev, R. S. Davletbaev, E. S. Grebennikova, A. N. Petukhov, A. I. Akhmetshina, T. S. Sazanova and V. V. Loskutov, RSC Adv., 2015, 5, 65674.
49. I. M. Davletbaeva, I. I. Zaripov, R. S. Davletbaev and F. B. Balabanova, Russ. J. Appl. Chem., 2014, 87, 468.
50. I. M. Davletbaeva, A. I. Akhmetshina, R. S. Davletbaev, I. I. Zaripov, A. M. Gumerov and R. R. Sharifullin, Polym. Sci., Ser. B, 2014, 56, 781.
51. R. Davletbaev, A. Akhmetshina, A. Gumerov, I. Davletbaeva and V. Parfenov, Compos. Interfaces, 2014, 21, 611.
52. I. M. Davletbaeva, V. F. Shkodich, A. M. Gumerov and G. I. Vasil'ev, Polym. Sci., Ser. A, 2010, 52, 914.
53. I. M. Davletbaeva, V. F. Shkodich, A. M. Gumerov, O. Y. Emelina and A. V. Naumov, Polym. Sci., Ser. A, 2010, 52, 392.
54. V. V. Gorbachuk, L. S. Yakimova, O. A. Mostovaya, D. A. Bizyaev, A. A. Bukharaev, I. S. Antipin, A. I. Konovalov, I. Zharov and I. Stoikov, Silicon, 2011, 3, 5.
55. A. A. Akhmetshina, I. M. Davletbaeva, E. S. Grebeschikova, T. S. Sazanova, A. N. Petukhov, A. A. Atlaskin, E. N. Razov, I. I. Zaripov, C. F. Martins, L. A. Neves and I. V. Vorotyntsev, Membranes, 2016, 6(1), 4.
56. T. S. Sazanova, I. V. Vorotyntsev, V. B. Kulikov, I. M. Davletbaeva and I. I. Zaripov, Pet. Chem., 2016, 56(5), 427.
57. K. V. Otvagina, A. E. Mochalova, T. S. Sazanova, A. N. Petukhov, A. A. Moskvichev, A. V. Vorotyntsev, C. A. M. Afonso and I. V. Vorotyntsev, Membranes, 2016, 6(2), 31.
58. L. Mochalov, R. Kornev, A. Nezhdanov, A. Mashin and A. Lobanov, et al., Plasma Chem. Plasma Process., 2016, 36(3), 849.
59. I. V. Vorotyntsev, P. N. Drozdov and N. V. Karaykin, Inorg. Mater., 2006, 42(3), 231.
60. V. M. Vorotyntsev, P. N. Drozdov, I. V. Vorotynysev and K. Y. Smirnov, Desalination, 2006, 200(1–3), 232.
61. V. M. Vorotyntsev, I. V. Vorotyntsev and S. Y. Smirnov, Inorg. Mater., 2009, 41(11), 1263.
62. H. A. Daynes, Proc. R. Soc. A, 1920, 97, 286.
63. C. Eric and K. Rideal, Trans. Faraday Soc., 1939, 35, 628.
64. A. Kadomtseva, A. Vorotyntsev, V. Vorotyntsev, A. Petukhov, A. Ob'edkov, K. Kremlev and B. Kaverin, Russ. J. Appl. Chem., 2015, 88(4), 595.
65. I. M. Davletbaeva, A. M. Gumerov and A. F. Galyautdinova, et al., Russ. J. Appl. Chem., 2011, 84, 1587.
66. V. M. Vorotyntsev, P. N. Drozdov, I. V. Vorotyntsev and E. S. Belyaev, Pet. Chem., 2011, 51, 595.
67. V. M. Vorotyntsev, P. N. Drozdov, I. V. Vorotyntsev and O. A. Pimenov, Pet. Chem., 2012, 52, 631.
68. A. Vorotyntsev, S. Zelentsov and V. Vorotyntsev, Russ. Chem. Bull., 2011, 60(8), 1531.
69. V. M. Vorotyntsev, P. N. Drozdov, I. V. Vorotyntsev and D. V. Murav'ev, Dokl. Chem., 2006, 411, 243.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21638b

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