2024 Volume 7 Issues 1–3

Open Access

INEOS OPEN, 2024, 7 (1–3), 100–102  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io2441a

Influence of the Microstructure of Porous Silsesquioxanes on Spin Dynamics
According to Solid-State ¹H and ²⁹Si NMR Data

Abstract

The samples of PEGylated hyperbranched polymethylethoxysiloxanes with hierarchically organized structures were obtained. The magic angle spinning ¹H–²⁹Si variable contact time cross-polarization and static ¹H multiple-quantum NMR experiments were performed in order to reveal the influence of the microstructure on the NMR parameters.

Key words: polymethylsilsesquioxane, nanogel, aerogel, cross-polarization, MQ NMR.

 

Introduction

Lightweight aerogel-like materials with high porosity are attractive for a wide range of applications, including sound and thermal insulation, optical materials, sorbents, catalysts. Among many possible materials, organosilicon aerogels are frequently appreciated for being less brittle. The crucial step in the formation of aerogels is the drying process. The supercritical drying is well known as a superior approach, which retains the integrity of the liquid gel structure resulting in ultralight weight, high surface area and porosity. However, the complexity of the procedures performed under elevated temperature and pressure conditions stimulate the development of more cost-effective approaches such as the freeze-drying. The solvent crystallization during freezing can potentially compromise the integrity of the gel structure. One possible approach to overcome this issue involves assembling of specific micro-objects as building blocks, which form hierarchically organized structure that combines high porosity with mechanical strength. In the present study, a series of aerogel-like materials were prepared, in which the framework, comprising hollow micropores, is formed by the interconnected globular particles of hyperbranched polymethylethoxysiloxanes (PMEOSs) [1].

Results and discussion

Poly(ethylene glycol) (PEG) derivatives of PEOS exhibit enhanced surface activity at the water–oil interface, enabling the production of nanoscale hollow silica particles [2]. The aerogel-like materials were prepared based on PEGylated hyperbranched  PMEOSs in oil-in-water emulsions. The PEGylated hyperbranched PMEOS in water undergoes polycyclization [3] with the formation of a nanogel structure, and subsequently these particles assemble on the hexane–water phase boundary. During the gelation, nanogel particles form cross-linked layers around hexane droplets, encapsulating them (forming voids) and a continuous condensed framework. Five samples were obtained using 30 mL of water and 1.5 mL of ammonia with different PMEOS–PEG/hexane ratio (given in Table 1) according to the procedure described in Ref. [1].

Table 1. Parameters obtained from the NMR experiments and BET specific surface area analysis

	PMEOS-PEG/haxane	tis (ms)	T1ρ (ms)	T1ρ* (ms)	α	S (m2/g)	Sm (m2/g)
1	1.8g/0.6g	1.42	68	43.5	1.14	250	113
2	1.8g/1.2g	1.28	35	32.0	1.10	131	59
3	1.2g/1.2g	1.63	50	36.8	1.06	137	62
4	1.2g/0.6g	1.13	33	33.3	1.06	10	–
5	1.8g/1.8g	1.21	50	36.3	1.03	20	–

NMR spectroscopy is an indispensable tool for studying chemical bonding, specifically for compositionally complex disordered materials. The most significant internal interaction for nuclear spins in solids is the dipole–dipole interaction, the intensity of which is inversely proportional to the cube of the distance between nuclei. Thus, the exploration of dipolar coupling in material potentially contains information beyond the nearest chemical environment, which typically determines the chemical shift. We used variable contact time cross-polarization (CP) and multiple-quantum (MQ) NMR experiments to investigate the features of dipolar coupling between ¹H and ²⁹Si spins and its relation with the microstructure of the porous PMEOSs.

The ²⁹Si spectra for five investigated samples, obtained under 6 kHz magic angle spinning (MAS), demonstrate two lines with the relative contents of 78.0 and 22.0 ± 1.5%: –65.7 ppm, corresponding to fully condensed methylsiloxane unit, and –58.0 ppm, which belongs to the unit with one substituent (ethyl or ethylene glycol). The dependence of the signal amplitude as a function of contact time, t, in ¹H–²⁹Si CP experiment is shown in Fig. 1 for different samples (shifted along y-axis for clarity). The curves follow a thermodynamic model [4] described by the following formula

(1),

where A is the signal amplitude, t_is is the cross-relaxation time, T_1ρ is the proton rotating-frame spin-lattice relaxation time. The solid lines in Fig. 1 demonstrate the approximation with (1). The experimental data for samples 2 and 4 are well described by the theoretical expression. The other curves demonstrate the flat region near the maxima indicating the presence of several t_is. In general, the initial parts of the curves are quite similar, giving t_is = 1.3 ± 0.2 μs. The curves also deviate from the experimental data at long times. A more reliable fit to the data for long times (>12 ms) can be obtained using the exponential decay. These data are denoted as T_1ρ* in Table 1 and are shown with dashed lines in Fig. 1. T_1ρ* for sample 1 is significantly higher than the corresponding values for other samples, which correlates with the largest specific surface area (S and Sm for micropores in Table 1) determined in BET method.

Figure 1. Intensity of ²⁹Si NMR signal as a function of contact time in the CP MAS experiment. Numbers near the curves indicate different samples and coincide with the notations in Table 1 and Fig. 2.

For all the samples, the oscillatory behavior near the maxima of the curves is observed in Fig. 1, indicative of slow spin diffusion, which is probably associated with high porosity of the materials.

Another NMR technique applied in the present study is ¹H MQ spectroscopy for static solids. The high order MQ coherences develop under the action of a special sequence of radiofrequency pulses. The rate of the growth of MQ coherences increase with increasing dipolar coupling strength and the local spin density [5]. The MQ NMR spectra is described in terms of the Gaussian model:                   

(2)

where A is the amplitude of the coherence of order n and N is the effective number of spins in the cluster at a given MQ excitation time τ. The growth rate of MQ coherences is usually characterized by exponent α in the power law N~τ^α. Previous studies revealed the correlation between α and the porosity of the methylsilsesquioxane gels [5, 6]. For ambient dried dense gel, α was found to be 1.65 ± 0.02, while for the supercritical dried aerogel, a significantly smaller value α = 1.27 ± 0.03 was observed.

Results of the MQ experiments for the PEGylated PMEOS aerogel-like samples

Different samples demonstrate steady growth of the effective number of correlated spins with no statistically significant difference in the growth rate, which can be described by α = 1.08 ± 0.01. This value is smaller than that observed for methylsilsesquioxane gels, showing a high porosity of the materials. However, the method cannot discriminate between different samples, which have different specific surface areas according to the results of BET analysis. We noted, that the usual static ¹H NMR spectra of methylsilsesquioxane gels and PEGylated PMEOS aerogel-like samples are practically indistinguishable, nevertheless the MQ dynamics for these samples differs.

The α value smaller than that for the aerogel indicates that the growth rate is sensitive to the presence of organic substituents (ethoxy and ethylene glycol), which are absent in the aerogel. Another difference from methylsilsesquioxane gels is the faster onset of the relaxation (decrease of N at long times in Fig. 2), which restricts the maximum observable number of correlated spins to about 30 at τ ≈ 400 μs and prevents more accurate discrimination of the samples.

Figure 2. Effective number of spins in the cluster as a function of preparation time τ in the ¹H MQ NMR experiment for 5 investigated samples.

Conclusions

The results obtained suggest that the porous microstructure of PEGylated PMEOS aerogel-like affects the spin dynamics in CP MAS and MQ NMR experiments due to the different dipole–dipole coupling network, though no simple correlation with solid matrix properties, such as surface area, is evident.

Acknowledgements

This work was supported by the Russian Science Foundation (project no. 22-43-04439).

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