Structure, Dynamics, and HostâGuest Interactions in POSS

Article pubs.acs.org/JPCC

Structure, Dynamics, and Host−Guest Interactions in POSS Functionalized Cross-Linked Nanoporous Hybrid Organic−Inorganic Polymers Tomasz Pawlak,† Anna Kowalewska,† Bozė na Zgardzińska,‡ and Marek J. Potrzebowski*,† †

Polish Academy of Sciences, Centre of Molecular and Macromolecular Studies, Sienkiewicza 112, 90-363 Lodz, Poland Institute of Physics, Maria Curie-Sklodowska University, Pl. M. Curie-Sklodowskiej 1, 20-031 Lublin, Poland

‡

S Supporting Information *

ABSTRACT: We report the synthesis and structural studies of new hybrid organic−inorganic porous materials with flexible siloxane spacers with different lengths bonded to polyhedral oligomeric silsesquioxane (POSS) cages. Positron annihilation lifetime spectroscopy (PALS) was used to measure the pore sizes of two polymers. For sample 1, which has a shorter linker, the free volume diameters were found to be 5.32 ± 0.06 Å, 8.28 ± 0.05 Å, and 10.42 ± 0.05 Å at 25 °C. The first value represents the POSS cage, while the others describe the space between linkers. For polymer 2, the appropriate diameters are 4.96 ± 0.04 Å and 7.96 ± 0.06 Å for the POSS cage and linker spacing, respectively. This sample is extremely well organized and homogeneous. The evolution of the free volume sizes as a function of temperature in the range from −20 °C to +60 °C was investigated. Solid-state NMR spectroscopy (SS NMR) was employed to study the structure and molecular dynamics of the polymers. The signal assignment was performed using 1H MAS (magic angle spinning), 13C CP/MAS (cross-polarization magic angle spinning), and 29Si CP/MAS experiments. The dynamic processes were investigated by inspection of the relaxation times, analysis of the 29Si principal elements of the chemical shift tensors (CST), and the center-band only detection of exchange (CODEX) technique, which is specific for studies on slow molecular motions. It was found that the POSS cage in sample 2 undergoes a complex reorientation in the cross-linked polymer lattice. Both evaluated samples are capable of hosting small organic molecules in the pores formed by the spacers and the POSS after cross-linking. The host−guest interactions were investigated by SS NMR and PALS. A strong influence of the guest molecules on the dynamics of the POSS cages for sample 2 was observed. The formation of host−guest complexes is a reversible process, and guest molecules can be replaced by other species; thus, the polymers under investigation can be used as molecular containers.

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INTRODUCTION Porous silica materials (PSMs), created by nature or by synthetic design, have attracted considerable attention due to their potential for use in a number of applications, including adsorption, catalysis, separation, delivery, and sensing.1 An important ability of such structures is the transport of chosen guest molecules to a target site and the release of said cargo in a controlled process.2 Silica materials offer several unique features, such as structural stability, large surface areas, tunable pore sizes and volumes, and well-defined surface properties, for the site-specific delivery and hosting of molecules with various sizes, shapes, and functionalities. Many potential applications require specific pore size, and precise control of pore dimensions is crucial for success. According to IUPAC classification, microporous materials have pore diameters up to 2.0 nm; mesoporous materials have an intermediate pore size between 2.0 and 50.0 nm; and materials with pore sizes exceeding 50.0 nm are classified as macroporous materials.3−9 Tailor-made PSM systems with specific pore sizes and specific surface areas are still in high demand. In this work, we present polymeric materials that belong to the group of © XXXX American Chemical Society

nanoporous organic−inorganic hybrids with polyhedral oligomeric silsesquioxanes (POSS) as structural knots. Cubic (RSiO1.5)8 octamers are nanocage structures comprised of a well-defined silica-like inorganic core surrounded by eight reactive peripheral organic groups. They provide an excellent platform for the synthesis of nanocomposite hybrid materials.10 The cube interior diameter is approximately 0.5 nm, and POSS can thus be regarded as structural equivalents of double fourring units found in zeolites.11,12 POSS can act as nanocrosslinkers and were used for the preparation of high-surfacearea micro- and mesoporous materials that are of immense interest because of their utility in potential applications such as hydrogen storage, separation techniques, catalysis, and materials for electronics and sensors.13−15 Mesoporous POSSbased materials are most frequently prepared via hydrosilylation reaction using vinyl- or hydrido-functionalized POSS species.16 Received: September 11, 2015 Revised: October 29, 2015

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DOI: 10.1021/acs.jpcc.5b08868 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Models of POSS Functionalized Cross-Linked Hybrid Organic−Inorganic Polymers

The cubic topology contributes to the geometrical constraint that hinders the filling of all available space upon cross-linking of POSS molecules and results in a diminished density of packing. Not all reactive functional groups are accessible beyond the gelation point of the polymeric product, and thus the resulting materials may contain some unreacted functional groups. The degree of porosity also depends on the efficiency of the polycondensation reaction,17,18 as well as the length and the flexibility of linking moieties.16 Highly porous polyhedral silsesquioxane polymers have attracted much attention during the last two decades. Zhang et al. were the first to report preliminary structural studies of new hybrid microporous compounds by employing a multitechnique approach, including advanced analytical methods (Solid-State Nuclear Magnetic Resonance (NMR), Differential Scanning Calorimetry (DSC), Thermal Gravimetry Analysis (TGA), Positron Annihilation Lifetime Spectroscopy (PALS)) and theoretical calculations.18 In this work, we analyze the structures and dynamics of new models that belong to the class of porous hybrid materials (Scheme 1). Usually, porous materials of this type are considered as efficient gas carriers. We found that such systems, which were constructed in a manner typical for host−guest complexes, can also be used as containers of small organic molecules. The size and shape of pores in porous materials determine the practical applications of this class of compounds. If PSMs are used as molecular containers, the size and shape have influence on the rates at which guest molecules are adsorbed into and diffuse through the frameworks. Moreover the preferred locations of the guest within the host lattice are controlled by the geometry of the pores. It is apparent that dynamic processes of the host framework can greatly modify both shape and size. Hence, knowledge about such processes is crucial if we wish to have reliable structural constraints. Host−guest interactions were investigated by employing solid-state NMR spectroscopy. Guest molecules in host−guest complexes are commonly accepted to be very mobile, while the host matrix is rigid. As we reveal for the polymers under investigation, the host template can also undergo complex dynamic processes in the solid state. Thus, we can monitor interaction between host and guest molecules. Dynamic processes can also suggest how much free space is contained in investigated structures. Our structural NMR studies are

supported by molecular modeling and porosity analysis via PALS.

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EXPERIMENTAL PROCEDURES Reagents. Polyhedral silsesquioxane−(octavinyl)octasilsesquioxane (T8Vi) was purchased from Hybrid Plastics and used as received. Linear siloxanes [1,1,3,3-tetramethyldisiloxane (99%) and 1,1,3,3,5,5,7,7-octamethyltetrasiloxane (97%)] and platinum catalyst [Karstedt’s catalyst: platinumdivinyltetramethyldisiloxane complex in xylenes (3%)] were bought from ABCR. Solvents were purified according to literature procedures.19 Preparation of Cross-Linked POSS Networks. Silsesquioxane and siloxane compounds with vinyl and hydride reactive groups were cross-linked by means of hydrosilylative addition of Si−H moieties to CC double bonds. Suitable reagents were placed under dry argon in a carefully dried threenecked flask equipped with a condenser (equimolar ratio [Vi]0/ [SiH]0) and dissolved in dry toluene (20 mL, [Vi]0 = 0.435 mol/dm3]. The platinum catalyst (10 μL, [Pt]/[Vi]0 = 10−4]) was added with a microsyringe to the stirred solution of reagents at room temperature. The reaction mixture was then stirred at room temperature for 10 min to 1 h and then heated at 70 °C for 24 h. The viscosity of the reaction mixture gradually increased until finally reaching the gel point. Toluene was removed under reduced pressure, and the cross-linked, insoluble product was washed for 16 h with toluene in a Soxhlet apparatus. Then, the product was dried for 8 h under high vacuum (at 60 °C/1 Torr) to constant weight. Finally we obtained two hybrid models containing a POSS cage in the knot positions with different length of cross-linkers (−(CH2)2SiMe2OSiMe2(CH2)2− model 1 and −(CH2)2SiMe2(OSiMe2)3(CH2)2− model 2) which were shown on Scheme 1. The scheme of the synthetic procedure is attached as Supporting Information. NMR Spectroscopy. Solid-state cross-polarization magic angle spinning (CP/MAS) NMR and one-pulse 1H MAS experiments were performed on a 400 MHz Avance III spectrometer (operating at 400.15, 100.62, and 79.50 MHz for 1 H, 13C, and 29Si) equipped with a MAS probe head using 4 mm ZrO2 rotors. Adamantane (resonances at 38.48 and 29.46 ppm) was used as a secondary 13C chemical-shift reference separate from external tetramethylsilane (TMS) in all experiB


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The Journal of Physical Chemistry C ments.20 The precise setup of the Hartmann−Hahn conditions was performed using 13C-labeled histidine hydrochloride and sample 1 as reference compounds for 13C and 29Si, respectively. The conventional 13C and 29Si CP/MAS spectra were obtained with a proton 90° pulse length of 4 μs and contact times of 2 and 5 ms for 13C and 29Si, respectively. The repetition delay was 5 s, and the spectral width was 40 kHz. The RAMP shape pulse21 was used during the cross-polarization and TPPM decoupling.22 The temperature in the sample was carefully calibrated by the temperature dependence of the 207Pb chemical shift for Pb(NO3)2.23 In the CODEX experiments,24−26 the MAS frequencies were set to (5.000 ± 3) Hz for the τm and N·TR dependencies. The 1 H r.f. field strength for CW decoupling during the 13C and 29Si 180° pulses, with a pulse length of 4 μs, was set to 83 kHz. Before and after the mixing time, the CSA orientation was encoded via a REDOR-type 180° pulse train that recoupled the CSA. Changes in the CSA orientation during τm (i.e., molecular reorientations) lead to a specific loss in the acquired NMR signal, called the exchange signal intensity S. To account for the signal decay due to T1 relaxation during τm and T2-type relaxation during the recoupling periods N·TR, a reference intensity S0 was recorded, for which τm and a short final z filter (tz = 10 μs) were simply interchanged. Exchange and reference spectra were measured for CSA recoupling durations N·TR between 0.2 and 2.0 ms and mixing times τm between 0.1 and 1000 ms. For normalization, the exchange signal intensity S was simply divided by the reference signal intensity S0.24 A spindiffusion correction of the decays was included for τm longer than 100 ms by measuring a CODEX experiment at −10 °C, which was far outside of the dynamic window of the method; thus, only spin exchange was detected. All other r.f. parameters were the same as those used in the 13C CP/MAS experiments. The number of scans was varied between 1024 and 5120 to ensure a sufficiently high signal-to-noise ratio. The total experimental time of an N·TR, as well as τm, experiment was approximately 1.5−8 h. Positron Annihilation Lifetime Spectroscopy (PALS). The positron annihilation lifetime spectra were measured using a standard fast−slow delayed coincidence spectrometer,27 with a time resolution fwhm of ∼220 ps. The positron source was 0.8 MBq of 22Na in a Kapton envelope. The samples were placed inside the measuring chamber (sample 1 in powder form and sample 2 in nugget form) and affixed at the tip of a copper rod, which could be heated or cooled. The temperature selection, stabilization, and measurement, as well as spectra collecting, were software controlled. The PALS spectra with the statistics of 106 coincidences/h were analyzed using the LT 9.2 program,28 while the spectra with high statistics (more than 107/spectrum) were analyzed by the MELT program.29,30 For analysis with the LT 9.2 program, four or five exponential components were convoluted with the instrumental resolution curve, and a constant random coincidence background was assumed. In the MELT program, only the resolution fwhm and source component were assumed a priori. The shortest-lived component, with a lifetime τ1 ≈ 0.12 ns, was ascribed to the decay of singlet para-Ps and the second oneτ2 ≈ 0.38 nsto the annihilation of free positrons, and the other two or three longest-lived components, with τi ≈ 1−7 ns, were assigned to the decay of ortho-Ps (o-Ps). The lifetime of o-Ps is very sensitive to the size of free volume (FV, electron-free regions in the sample) in which a Ps atom is trapped, and this size can be estimated using the Tao−

Eldrup (TE) model.31,32 The relation between the o-Ps lifetime and FV size described by the TE model is valid if no other pickoff processes occur. The intensity of the o-Ps component is often assumed to be proportional to the number of FVs per unit volume;33 however, the preference of Ps to occupy a larger size FV is also possible.34 It is usually assumed that once positronium is trapped then it cannot escape to another void;35 on the other hand, such transitions are observed in plastic crystals.36 If there is a possibility of a transition from one type of FV to another, the decay rate of o-Ps, λ3 = 1/τ3, is increased, λ3 = λ03 + K, where λ03 is the decay constant via the pick-off process and K is the transition rate. Structural changes in the medium, e.g., phase transition or reorganization, are usually accompanied by changes in the FV sizes or their population; thus, the PALS technique can serve as a convenient tool to observe such changes. To observe how fast the dynamic process of a guest evacuation from a matrix takes place, the spectrum collection time was reduced to 10 min. Instead of traditionally analyzing spectra with such low statistics by decomposition into a set of exponentials, only the area under the tail of lifetime distributions (2.8−9 ns) was calculated. The number of counts in this area depends on both the lifetimes and intensities of the longer-lived components, and a change in the counting rate reflects rapid changes of τ and I, which allows for the determination of the time constant of this process. MM Calculations. Molecular mechanics calculations were performed using Discover Materials Studio 7.0, Accelrys Inc. A well-validated condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS) force field was used for the simulation.37 The starting geometries were constructed de novo and carried out within the Nosé thermostat scheme38 of the NVT canonical ensemble39 at 25 °C with thermostat. The simulation time step was set to 0.2 fs with a dynamic time of 1 ns. Finally, each of the models was optimized using a combination of the following methods: steepest descent, conjugative gradient with Fletcher−Reeves algorithm, and Newton with BFGS algorithm.40−47

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RESULTS AND DISCUSSION 1. Theoretical Model of Polymers. PALS Measurement of Porosity. We began this study by constructing the molecular models of polymers with linkers, as shown in Scheme 1. In the first approach, we assumed the ideal case where all available functional groups of POSS are bonded and a regular network is formed in a single plane. Such bonding will lead to the formation of a two-dimensional polymer structure. The calculations were carried out by employing Material Studio 7.0 Accelrys software. As shown in Figure 1, the computed pores have an ellipsoidal shape with their sizes depending on the length of linkers. The pore sizes for models 1 (8−12 Å) and 2 (8−15 Å) are similar; their sizes are related to the lengths of the cross-linkers, which suggests slightly more empty space for the model with the longer linker. Among the experimental techniques available for analysis of the porosity of polymers and the validation of theoretical models, positron annihilation lifetime spectroscopy (PALS) was found to be the most suitable. Typical nitrogen sorption analysis, in the case of networks cross linked with flexible siloxane spacers, is rather useless because it gives reasonable results only for structures with pore sizes larger than 20 Å.18 We also recorded the transmission electron microscopy (TEM) images (Figure S1) and FT-IR spectra (Figure S2). C


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The Journal of Physical Chemistry C

Figure 2 shows the PALS results for models 1 and 2 at 25 °C, performed on spectra with similar high statistics using the MELT program.29,30 Important information about the porosity of the material is carried by the ortho-positronium (o-Ps) component (or components, as in this case). The mean lifetimes of o-Ps are shown in Figure 2, and each lifetime depends on the radius R of a free volume (FV). Assuming that the FVs have a spherical geometry, R can be determined by the Tao−Eldrup model.31,32 For ellipsoidal FV of eccentricity ε < 0.1, the o-Ps lifetime is essentially the same as that for a spherical FV of the same volume.50 In sample 1, three types of FV were identified. The presence of three FVs is not expected for this type of sample. However, due to the method of analysis, one cannot doubt the results. The MELT program does not require a declaration of the number of expected components and defines this parameter itself based on the best fit to the experimental spectrum; moreover, the longest-lived component in the spectrum is easiest to identify. The smaller FV value (D = 5.32 ± 0.06 Å, D = 2R − diameter) represents the POSS cage, while the intermediate (D = 8.28 ± 0.05 Å) and the larger ones (D = 10.42 ± 0.05 Å) fit the theoretical model estimates of linker pores. It is noteworthy that the PALS technique distinguishes two types of larger FVs, while the model suggests the presence of one type of FV, whose size varies in the range from 8 to 12 Å. The intensities of o-Ps components correspond roughly to the relative amount of FV: 24% of all FV is the POSS cage, the majority of components (64%) belong to intermediate-size FVs, while the largest FV represents only 14%. The spectrum for 2 is different, and the existence of only two o-Ps components was established. A uniquely narrow distribution of lifetimes (peak width on the lifetime scale) suggests that the test material is unusually homogeneous in the size of free volumes; hence, the resulting material has a high degree of structural organization. Two clear o-Ps peaks belong to pores with diameters of 4.96 ± 0.04 Å and 7.96 ± 0.06 Å. On the basis of the results of PALS measurements as a function of temperature, which are described in Section 6, one can suppose that, in 2, the process of Ps transition from smaller to larger FV occurs. If this was the case, then the lifetime of the shorter component is reduced further. Taking into account such a possibility, one obtains a diameter of FV ∼ 5.9 Å (the lifetime of the longest component and its respective diameter are unchanged). The intensity of the shorter-lived o-Ps component is similar to that determined in 1, which, together with the R value, confirms the structural similarity of cages in samples 1 and 2.

Figure 1. Theoretical model of 1 (A) and 2 (B) with a pore size of 8− 12 Å and 8−15 Å, respectively.

Unfortunately the TEM technique does not provide conclusive information because traces of platinum catalyst found in polymer after synthesis dominate the images.48,49

Figure 2. Distribution of mean lifetimes in the PAL spectra obtained from MELT processing for 1 (a) and 2 (b) at 25 °C. The values of D correspond to the diameter of the spherical free volumes in the materials. D


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Figure 3. 29Si CP/MAS for sample (A) 1, (B) 1t, (C) 2, and (D) 2t under 5 kHz spin rate.

The small difference between the values shown in Figure 1 and that measured by PALS is because in the theoretical model we labeled distances between atom centers, while PALS shows the volume of negligible electron density (the dimensions reduced by the size of atoms). An additional argument is the fact that the theoretical model is considered in only the in-plane system, while experimental results obviously consider full threedimensional space. The results are consistent with similar systems in the literature. For example: (i) the estimated intracube spherical pores have D ∼ 5.28 Å, and intercube pores have 10.90 Å at 30 °C;18 (ii) the cage pore diameter is ∼2.1−5.5 Å, and intercage pores are ∼6.8−8.6 Å in diameter, depending on the pH during sample synthesis;51 and (iii) a cage pore diameter ∼5.8 Å and for an intercage ∼13.3 Å in diameter (calculated based on o-Ps lifetimes reported in the paper).52 2. Solid-State NMR. Preliminary Studies of Host−Guest Interactions. Figure 3 shows 29Si CP/MAS spectra for samples 1 and 2. Sharp signals near δ = −70 ppm represent the POSS cage. The linker silicon resonances are found at δ = 5 ppm (−CH2−Si(CH3)2−O−) and δ = −25 ppm (−O−Si(CH3)2− O−). In the next step, polymer samples were soaked with organic solvents: toluene (t), chloroform (c), and ethyl acetate (ea). The 29Si spectra of host−guest species are shown as bottom traces (Figures 3B and 3D). Inspection of toluenetreated samples revealed a small influence of solvent on the 29Si chemical shift of the host. A similar conclusion is valid for other solvents (c and ea). The most interesting result was observed for sample 1, where the addition of each solvent increased the resolution of the spectra and two clearly separated resonances for the −CH2−Si(CH3)2−O− position were recorded. This result means that added solvents closely interact with linkers. The more convincing information showing the interactions between host and guest was obtained by analyzing the 13C CP/ MAS spectra. Here, reduced line broadening of the 13C signals (Figure 4A,B compared with C,D) is shown upon the addition of a solvent, indicating an interaction between the solvent and the inorganic material in the cross-linked organic part of the polymer (−CH2− and −CH3 groups). After the addition of toluene, significantly narrower lines are noted for sample 2 than for 1 suggesting a stronger interaction between solvent molecules and linkers. Further results confirming the short contacts between the host and guest molecules were derived from inspection of the 1 H spectra. The addition of solvents dramatically changes the proton line shape. The challenge remains the determination of

Figure 4. 13C CP/MAS for sample (A) 1, (B) 2, (C) 1t, and (D) 2t under 5 kHz spin rate. * indicates residual solvent signal.

guest locations within the polymer network and the penetration depth of pores. A significant reduction of the line widths of the linker signals from thousands to tens of hertz is observed (Figure 5). Narrow lines are recorded for the −CH2 and −CH3 groups. (Figure 5 A,B compared to C,D). We assume that the narrowing of the resonance lines is due to local dynamic processes that averaged homonuclear proton−proton dipolar interactions. 3. 29Si Chemical Shift Anisotropy (CSA) as a Tool for Analysis of Dynamics Processes. It is well-known that, in the solid state, local molecular dynamics will average NMR tensor parameters, such as the CSA, dipolar interactions, and quadrupolar interactions. Although most CSA data for mobile systems is reported for species containing 13C and/or 15N nuclei, the 29Si CSA was also used for the study of fast regime exchange. Bonhomme and co-workers have investigated 29Si δii chemical shift tensor (CST) parameters for a series of silsesquioxane derivatives by employing experimental and theoretical methods.53 As they have shown, molecular dynamics can lead to significant discrepancies between computed and experimental data because quantum chemical calculations are typically performed using static structures, i.e., at 0 K, where zero-point motion is neglected. The same authors have also concluded that 29Si CSA is averaged due to the fast reorientation of the R groups around the Si−C bonds. Such a process, with a large amplitude motion of linkers, is rather excluded in the case of cross-linked polymers, but small amplitude wobbling can be considered. In the course of our studies, we have found that 29Si chemical shift tensor (CST) values for untreated samples (host polymer) and host−guest complexes are significantly different. The E


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Figure 5. 1H single-pulse experiment for samples (A) 1, (B) 1t, (C) 2, and (D) 2t under 5 kHz spin rate.

Figure 6. Experimental 1D 29Si CP/MAS NMR spectra of native samples 1 (A), 2 (B), and their host−guest modification 1t (C) and 2t (D) collected at a spinning rate of 800 Hz. Red dotted lines represent the best fitted spectra during CSA parameters determination. Numerical fitting results are presented in Table S1.

Figure 7. Simulated static 29Si CSA line shapes for the POSS cage signals from 1 (A) and 2 (B). The black lines indicate the native sample; violet, toluene; red, chloroform; and green, ethyl acetate. The spectra were computed using the 29Si δii parameters in Table S1.

whereas for sample 1, the line shape is almost unchanged. This result suggests that polymer 1, with the shortest linker, is more rigid than the former one. Very similar results are obtained through a heating procedure for structure 2; when temperatures exceeded 50 °C, a reduction of the Ω parameter, with respect to room temperature measurements, is noted (this is not true for model 1). 4. Study of Host−Guest Interactions via Relaxation Times Measurements. Comparative analysis of the relaxation times is an invaluable NMR tool for studying the nature of host−guest interactions. For instance, such an approach has been reported in a few papers of Kowalewski and co-workers, who analyzed the dynamics of inclusion complexes of chloroform, dichloromethane, and Cryptophane-E.54−56 The longitudinal relaxation times and the heteronuclear steady-state nuclear Overhauser enhancement (NOE) were employed for inspection of the molecular motions of aromatic frame, linkers, and guest molecules.

presence of solvent reduces the chemical shift anisotropy (CSA) of linkers to almost isotropic values. Figure 6 shows 29Si CP MAS spectra for samples 1 and 2 and their complexes with toluene. These spectra were recorded at a spinning rate of 800 Hz to see a sufficient number of spinning sidebands, which were further used for calculations of CSTs. The obtained values of 29Si CSTs are collected in Table S1. It is rather surprising that 29Si CST parameters are also reduced for POSS. Span parameters Ω expressed by the equation Ω = δ11 − δ33 are significantly smaller for solvated polymers. This effect is better seen when the static 29Si line shapes for pure host and host− guest species are compared. Figure 7 shows the calculated static spectra computed with the values given in Table S1. From inspection of 29Si CST parameters, it is apparent that for pure host polymers the tensors are axially symmetric, with almost equal δ11 and δ22 elements (black lines in Figure 7). When guest molecules were included in the polymer pores, a change of the line shape of POSS knots is observed for sample 2, F


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Figure 8. 29Si spin−lattice decays curves of 1 (A,B) and 2 (C−E) for native samples (blue) and their host−guest complexes with toluene (violet), chloroform (red), and ethyl acetate (green) at 25 °C and positions of POSS cage (A, C), −CH2−SiMe2−O− (B, D), and −O−SiMe2−O− (E).

In our project, we focused our attention on 13C and 29Si longitudinal relaxation times. The 13C and 29Si T1 were measured by employing the standard Torchia sequence.57 Figure 8 shows the decay of appropriate 29Si signals as a function of Torchia delay. The values of relaxation times are collected in Table S2. As one can see, the solvent molecules have a significant influence on the changes in T1, decreasing the relaxation times for POSS knots as well as for the hybrid linker in 1; however, this is not the case for the −CH2−SiMe2−O and −O−SiMe2−O- residues of 2. The explanation of these experimental facts is not trivial. The change in relaxation times can be due to a mobility increase of the components of host molecules when solvents integrate with the polymer lattice or dipole−dipole interactions between host and guest molecules or due to both effects (cross-relaxation). The most challenging question to answer is regarding the dynamics of POSS molecules, and to answer this question more advanced NMR methodology is required. The variation of the T1 relaxation time for 13C is much lower (Table S3) than that for 29Si and changes in the range from 3.3 s for native to 0.4 s for host−guest samples; these general trends are also true for 29 Si. 5. Study of Dynamics of POSS Employing CenterBand Only Detection of Exchange (CODEX). To provide a more thorough understanding of the molecular motion of host polymers and host−guest complexes, in this section, we report on results obtained using the CODEX sequence. This technique specializes in the analysis of slow molecular processes.58 It can be performed in two variants, i.e., the socalled τm and N·TR dependencies, yielding information about the correlation time and the topology of reorientation, respectively.59 Any reorientational process can be indicated by a signal decay in either one of the experimental variants. The pulse sequence of the 1D CODEX experiment is shown in Figure 9. The analysis of CODEX data began with the measurements at room temperature. To gain the information on correlation times/jump rates, the CODEX τm experiment was performed. The τm was varied from 0.1 up to 1000 ms. The N·TR value was optimized and set to 0.8 ms. Under these conditions, no significant decay for linker nuclei (13C as well as 29Si) was noted (see Supporting Information). In contrast, a different behavior

Figure 9. Pulse sequences of the 29Si CODEX experiment.

was observed for POSS cage 29Si resonances (−70 ppm). For sample 1 a very subtle decay was seen, while for 2 a small distinction in decays was apparent. These preliminary results prompted testing the CODEX profiles in a broad range of temperatures. The τm curve for the POSS cage signals is shown in Figure 10. For data fitting the following equation was employed β

τm S = a + (1 − a) ·e−( τc ) S0

(1)

where “a” is related to the available site number of dynamics, N, a = 1/N; τc is the correlation time of the dynamic process; and β is the distribution parameter (0 < β < 1). An increase in temperature leads to a systematic shift of the decay curves toward shorter values of τm (i.e., a shorter τc with increasing temperature). The opposite situation is noted for 1, where the sample was heated to 100 °C and did not reveal a significant decay of the S/S0 value. The results for 1 can be addressed by a correlation time of movement much longer than 1000 ms. Even at 100 °C, the molecular motion is too slow for quantitative analysis by CODEX. In contrast, sample 2 is found to exhibit dynamic processes at a low temperature (10 °C). It reached a plateau of S/S0 = 0.25 from a τm of 100 ms at 40 °C. Because molecular mobility is determined by intramolecular interaction potentials acting on the local structure, this result could be related to differences in the sample morphologies. Table 1 presents the numerical results of the best fitting for experimental 2D CODEX τm data points to eq 1. The value “a” is related to the number of exchanging sites in the jump process, M, as a = 1/M. For both samples, the plateau value is reached (model 2) close to 0.25, which suggests a four-site jump. At lower temperatures, the parameter a is higher than G


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Figure 11. Arrhenius plot of τc for 29Si (POSS cage) of the 2 sample. Asterisk indicates omitted point due to low accuracy of the determined value.

Arrhenius equation was fitted to the experimental points with R2 = 0.99. Adding the solvent (toluene was chosen as reference) causes a lack of CODEX decay for models 1t and 2t (Figure 12). Due

Figure 10. CODEX τm dependence for (S/S0) ratios for the 29Si POSS cage signal. Results for (A) structure 1 at 50 °C (□), 80 °C (○), and 100 °C (Δ) simply connected by a dotted line, (B) structure 2 at 10 °C (□), 20 °C (○), 25 °C (+), 30 °C (◊), and 40 °C (Δ). The dashed lines show best fittings to the experimental data (B and C), using S/S0 = a + (1 − a)*exp(−(τm/τc)β). νROT = 5 kHz.

Table 1. Numerical Results for Fitting 29Si CODEX τm Experimental Points of the POSS Cage Signal (ca. −70 ppm) to Equation 1 for Sample 2a parameters a τc β a τc β a τc β a τc β a τc β

temp.

structure 2

10 °C

0.74 224 0.68 0.42 132 0.61 0.24 73 0.52 0.21 29 0.63 0.21 7 0.64

[ms] 20 °C [ms] 25 °C [ms] 30 °C [ms] 40 °C [ms]

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Figure 12. CODEX τm dependencies of (S/S0) ratios for the 29Si POSS cage signals. The results for model 1t (Δ) and 2t (□) at 25 °C are simply connected by dotted lines. νROT = 5 kHz.

0.02 58 0.07 0.02 14 0.03 0.03 12 0.02 0.02 3 0.04 0.02 1 0.05

to the high mobility of the solvent, measurements of host− guest interactions were carried out at 25 °C (to avoid evaporation of the solvent during experimentation). At temperatures ≤25 °C, we did not observe any decay of signal that could be fitted to eq 1. In all cases, the introduction of solvent increased the values of S/S0 up to 0.90 or more, even for a τm of 1000 ms. This observation, when compared with CSA analysis (section 3), confirms the fast anisotropic motion leading to averaged chemical shift tensors in host−guest models. It can be concluded that the slow molecular dynamics observed for native samples speed up, in the case of host−guest models, leading to a fast molecular motion limit that is outside the CODEX time window (τc ≪ 1 ms). From the CODEX results presented supra, it is apparent that slow molecular motion for cross-linked polymers in the knot position (POSS cage) is possible, and the dynamics processes strongly depend on the length of the hybrid organic−inorganic chains. The next objective is to understand the topology of POSS reorientation. In the case of the polymers under discussion, there are few options. We can consider small amplitude wobbling around the diagonal of the cube, smallangle rotation around a symmetry axis, or both processes together. The technique which is dedicated for analysis of topology of dynamics processes is a modification of CODEX known as N· TR CODEX. Figure 13 shows the 29Si N·TR CODEX curves for samples 1 and 2 at different temperatures with a mixing time

a

For all other discussed models of decay, the S/S0 values were too low to perform the fitting procedure (dynamics were out of the CODEX time window).

0.25, probably due to the effect of spectral spin diffusion, which interferes with the molecular dynamics at τm values larger than approximately 100 ms. The temperature dependency of τc was used to determine the activation energy of the dynamic process using the Arrhenius equation. Figure 11 shows the temperature dependence of τc for the POSS cage signal of sample 2. Dotted lines indicate the fittings to the Arrhenius equation, which allows the determination of activation energy associated with dynamic process. The Ea values are found to be 114.2 ± 5.0, and the H


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°C, with the S/S0 decay at 70 °C yielding a smaller value; such a plot can be confusing without additional comments. This unexpected behavior can be explained by assuming that the dynamic process at this temperature is faster than the CODEX time scale, causing the reduction of S/S0 decay in the CODEX experiment. The argument confirming this assumption is the reduction of CSA parameters at 70 °C for model 2, as shown in section 3 (Figure 6). Since the characteristics of N·TR and τm dependencies are in the quantitative form equal to each other, only the N·TR analysis was performed to show speed up of the dynamic process over the CODEX window time. Thus, this proved that our analysis between 10 and 70 °C considers full information about the slow dynamic process. Figure 14 shows the computed decays of the N·TR CODEX curves employing a four-site jump model. Such a model fits

Figure 13. CODEX N·TR dependencies of (S/S0) intensity ratios for the 29Si signals of the POSS cage with a fixed τmix of 40 ms for structures 1 (A) and 2 (B) at 10 °C (green), 50 °C (blue), and 70 °C (red). Experimental points are shown with error bars and simply connected by dotted lines. νROT = 5 kHz.

equal to τm = 40 ms, which is short enough to avoid the influence of spin diffusion.56 Only model 2 exhibits a significant decay in the POSS cage signal, whereas almost no change in the S/S0 value is observed for structure 1. These observations can be easily related to the length of the R linkers, and because of the short linker between the POSS cages in model 1, we observed the largest polymer chain rigidity and an absence of dynamic processes. These results can be explained by the mobile (in the slow molecular motion regime) POSS cage for structure 2, whereas model 1 is completely rigid in the time frame of the CODEX experiment. To quantify the N·TR decay, knowledge of the change in the reorientation of the CSA tensor during the mixing time is required. Because we are not able to determine the orientation of the CSA tensor, we cannot precisely define the jump angle. On the other hand, we can qualitatively interpret the observed decays in terms of different topologies by comparing our results with previous data reported in the literature.60 We observed the opposite CODEX behavior for the linker signals: In this case, a very small decay was observed (Figure S3). We also observed very similar results (no decay) for 13C N·TR CODEX dependencies of linker signals, which is fully consistent with the above observations for 29Si CODEX spectra. Analysis of the N·TR results in a range of temperatures from 10 to 70 °C did not reveal any dynamic process for 1 (i.e., S/S0 ∼ 1.0). The most interesting results are those obtained for model 2, where increasing temperature produces larger amplitudes (up to upper limit of the CODEX time window), which evidence that the mobility of the POSS cage is a strongly temperature-dependent process. At 10 °C, we observed a very small decay of S/S0 for model 2, meaning that dynamic processes at this temperature only slightly cross the lower limit of the CODEX time window. The maximum decay of S/S0 (i.e., the maximum amplitude of dynamic process) is observed at 50

Figure 14. CODEX N·TR simulation for the four-site jump model with a fixed τmix of 40 ms and assuming helical jump motions with different jump angles (5, 10, 20, 30, 40, and larger). An example of experimental points is shown for structure 2 at 50 °C.

very well with the experimental data discussed supra. The S/S0 experimental points for 2 at 50 °C correlate perfectly with the curve representing a 20° jump angle. It should be stressed that the experimental decay cannot be fitted to two- and three-site jump models (see SI, Figure S4). For these models, the plateau of decay is different from the experimental one. In summary, by employing the CODEX approach we show that a complex foursite jump motion for the POSS unit of sample 2, including small-angle rotation of the POSS cages and wobbling around the cube diagonal, is justified. 6. Variable-Temperature PALS Measurements for Samples 1 and 2. Host−Guest Interactions by Means of PALS. The PALS measurements were performed for samples 1 and 2 as a function of increasing temperature and, in the case of sample 2, also as a function of time to determine the dynamics of toluene removal. The results of the PALS measurements vs temperature for both samples are shown in Figure 15. Collecting the data for the PALS spectra analysis by the MELT program is time-consuming; thus, for the measurements acquired as a function of temperature a standard LT 9.2 program28 was used. For this program, the number of exponentials is fixed prior to data collection. With an increase in the number of components, the statistical uncertainty is augmented; for sample 1, which needed to assume five components, the results increased the scatter of data. The presence of two types of free volume in sample 2 was assumed, as found in the MELT analysis (Figure 15b): two oI


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Figure 15. O-Ps lifetimes and intensities in 1 (a) and 2 (b) as a function of increasing temperature. A detailed description is presented in the text.

Figure 16. Number of counts in a narrow range of the spectrum (left) and the respective o-Ps parameters (right) as a function of time for sample 2.

the POSS cage size at temperatures immediately above −2 °C are smaller (D = 4.6 Å) than below this temperature (5.8 Å), the trend of increasing size with temperature continues. The longer-lived component varies with temperature: the diameter of FV is 7.40 and 8.58 Å at −20 °C and +60 °C, respectively. To check the reproducibility of these results, additional measurements were performed (full points in Figure 15b). The new data fitted well with the initial results, confirming the repeatability of these measurements. This indicates that the structure of the material possesses high stability toward temperature cycling. In sample 1, such drastic changes of PALS parameters are not observed. The shortest o-Ps lifetime (triangles in Figure 15a), corresponding to the POSS cage, is visible as a small stepwise increase at approximately 13−15 °C (dashed line in Figure 15a), but the number of this FV-type remains constant (see: intensity Io‑Ps). This means that below 15 °C the cage size is much smaller (D = 3.87 ± 0.30 Å) than at room temperature (D = 5.32 Å). Two other o-Ps components also change at approximately 15 °C. The intensity of the middle one (ϒ2) decreases, but the size of the FV, as described by this component, does not change in the studied temperature range, there is a linear increase in size from 7.13 to 8.38 Å. The changes in intensity of the intermediate component are correlated with changes, i.e., shortening of the longest o-Ps

Ps components were reported in the literature18,48,49 for similar materials. At −2 °C, an unexpected phenomenon appeared in sample 2all parameters describing the o-Ps components changed stepwise: the intensity of the shorter component decreased by half, while that of the longer one increased to almost 40%. The lifetime of the shorter component decreased from 2 to 1.5 ns. Such behavior is indicative of a structural change, which suggests the possibility of transitions from smaller to larger FVs. If so, the decay constant of o-Ps in the smaller FV, λ3 = 1/τ3, is now the sum of the decay constant by a pick-off process λ03, which is described by the Tao-Eldrup model, and of the transition rate K. Thus, the raw results of the MELT measurements for this component are distorted, and the correct lifetime for the FV size determination is observed below −2 °C, i.e., τo‑Ps ≈ 2 ns, yielding an FV diameter of 5.74 Å. Additionally, by maintaining an increasing trend as a function of temperature, then at 25 °C, the FV diameter should be approximately 5.9 Å. The nature of the transition occurring at −2 °C and the mechanism of Ps transfer require further study. A small change in the lifetime of the longer-lived component is observed at −2 °C as well. The intensity of the shorter-lived o-Ps component (Δ) decreases from 16% to 8%, while the intensity of the longerlived component (○) increases to approximately 40%. Though J


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temperature range. We assumed that this sample is more crosslinked, which can lead to disorder and the distribution of pore sizes observed by PALS. It is worth emphasizing that the polymers under investigation can be used as molecular containers for small organic compounds. The host−guest interactions of these inclusion complexes were studied by solid-state NMR spectroscopy. It was concluded that the presence of organic solvents has a significant influence on the molecular motion of the POSS unit for polymer 2, while sample 1 remains rigid. The formation of host−guest complexes is a reversible process, and guest molecules can be replaced by other species. Thus, the basic criterion requested for molecular carriers is fulfilled. Finally, we wish to comment on the PALS data for sample 2. Although this technique is not dedicated to the analysis of dynamic processes in the solid state, our measurements clearly proved the sensitivity of the method to molecular motion.

lifetime (dots, Figure 15a). The two longer-lived components changes suggest that just below 15 °C there is a reorganization of the structure associated with the linkers. As was shown earlier, adding toluene to sample 2 produces much narrower lines in the 29Si CP/MAS spectra; therefore, additional information about the localization of guests in the polymer network and the depths of pore penetration is needed. The PALS results obtained at 25 °C suggest an answer. Sample 2, soaked with toluene (10 min), shows an important difference in the longer-lived PALS componenta shortening of the lifetime from 3.5 ns (native sample 2) to 3.15 ns (sample 2t) and an increase in intensity of approximately 3%. The second, shorter component remains unchanged. This value (3.15 ns) is typical for organic liquids and suggests that sample soaked with toluene does not have any “free” volume and only volumes “filled” by toluene are presented. Sample 2t was left in the measuring chamber for 20 h, during which time it showed no change in its PALS spectrum. Following this, toluene was evacuated from the sample and retained at a low level of vacuum (the pressure in the chamber was reduced to several tens of Pa). The solvent removal occurred quickly, shorter than the standard measurement time per spectrum, and led to the restoration of the original pretoluene PAL spectra parameters (Figure 16, empty points). The relatively short toluene removal time required the use of another type of spectral analysis (see Experimental Procedure). The measurement time per spectrum was limited to 10 min and enabled the determination of the time required to remove the toluene from the material (Figure 16). The estimated time constant of the toluene evacuation process during pumping is 22 min. The PALS spectrum obtained at the end of the toluene evacuation cycle was compared with a spectrum of the original (not soaked) sample. The results show that the spectra are identical, and the toluene was completely removed. Because the presence of toluene modified (shortened) the lifetime of the oPs in larger volumes without any changes in the shorter o-Ps lifetime, we concluded that toluene filled the intercube pores (spacer pores) only.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08868. Schematic representation of the synthesis procedure (Scheme S1), TEM images (Figure S1), and FT-IR spectra (Figure S2) as well as 1H and 13C spectra (Figures S5−S7) for the investigated objects. Tables with NMR chemical shift tensors and spin−lattice relaxation results for all models (Tables S1−S4), a CODEX plot of the NTR dependency for linker signals, and simulated decays for two- and three-sided jump models (Figures S3−S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Polish National Center of Sciences (NCN) for financial support, Grant No. 2013/09/D/ ST2/03712. The computational resources were partially provided by the Polish Infrastructure for Supporting Computational Science in the European Research Space (PL-GRID) and ACK CYFRONET AGH grant no. MNiSW/IBM_BC_HS21/ CBMMPAN/029/2011. TP acknowledges support of the Foundation of Polish Science. We thank Dr. Artur Rózȧ ński for technical support (TEM images).

CONCLUSIONS In this work, we report the synthesis and structural studies of two new hybrid organic−inorganic porous materials with flexible siloxane spacers of different lengths bonding POSS cages. Both polymers were investigated using solid-state NMR spectroscopy and positron annihilation lifetime spectroscopy (PALS); calculations were employed to assist in the analyses. The PALS measurements clearly proved that the simple scenario, i.e., a longer spacer leads to a polymer with a larger pore size, is wrong. Using a shorter linker (sample 1), we obtained a sample with significant disorder and scatter in pore size. The other sample contains a longer spacer linking the POSS cages (sample 2) and is very well organized and homogeneous. Both samples show a dependence of pore size on temperature. For sample 2, above −2 °C, positronium migration from a smaller to a larger volume occurs. Unique information about the slow molecular motion of the POSS cages in sample 2 was derived using NMR spectroscopy. This dynamic process, investigated by solid-state NMR (CODEX experiment), was found to be temperature dependent, and its scale and amplitude increase with increasing temperature. For sample 1, we did not observe the molecular motion of the POSS unit within the CODEX time scale and over a broad

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