Control of Crystalline-Amorphous Structures of Polyhedral Oligomeric Silsesquioxanes Containing Two Types of Ammonium Side-Chain Groups and Their Properties as Protic Ionic Liquids

Polyhedral oligomeric silsesquioxanes (POSSs), Am-POSS(x,y), prepared by hydrolytic condensation, contains two types of ammonium side-chain groups, where the numbering of x and y represents the type of ammonium ions in the POSS structure, corresponding to primary (1), secondary (2), tertiary (3), and quaternary (4) ammonium ions. Mixtures of the two starting materials selected from organotrialkoxysilanes containing primary, secondary, and tertiary amines and a quaternary ammonium salt [(RO)3Si(CH2)3R′, R = CH3 or CH2CH3, R′ = NH2, NHCH3, N(CH3)2, and N(CH3)3Cl] were dissolved in dimethyl sulfoxide (DMSO). The hydrolytic condensation was performed in the presence of bis(trifluoromethansulfonyl)imide (HNTf2) and water. All Am-POSS(x,y) structures consisted of a cage-type octamer (T8-POSS), as confirmed by 29Si NMR spectrometry and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses indicated that Am-POSS(1,3), Am-POSS(1,4), and Am-POSS(2,4) had amorphous structures. These POSSs have two or three differences in the number of methyl groups between the two types of ammonium side-chains. Conversely, Am-POSS(1,2), Am-POSS(2,3), and Am-POSS(3,4) had crystalline structures. The difference in the number of methyl groups between the two types of ammonium side-chains in these POSSs is only one. Therefore, the crystalline-amorphous structure of Am-POSS(x,y) is controlled by the side-chain group combinations. Furthermore, Am-POSS(1,3), Am-POSS(1,4), and Am-POSS(2,4) are protic ionic liquids with relatively low flow temperatures.


Introduction
Polyhedral oligomeric silsesquioxanes (POSSs), which are generally prepared by the hydrolytic condensation of trifunctional silane compounds, such as organotrialkoxysilanes and organotrichlorosilanes, have attracted much attention in recent years as compounds having both inorganic and organic characteristics. In addition to the remarkable thermal and chemical stability derived from siloxane-bond frameworks, good dispersibility and solubility are provided due to regulated cage structures and suitable side-chain groups [1][2][3][4]. POSSs are, therefore, in great demand as dispersible inorganic compounds for the development of organic-inorganic hybrid materials [5][6][7][8][9][10].
Well-known POSSs are typically crystalline, cage-type octamers, because of their isotropic cubic structure. The amorphization of POSSs is one of the recent interesting topics in basic research and material applications. For example, amorphous POSS dimers [11] and polymers [12][13][14] are prepared by linking a plurality of POSSs, and are applied as transparent thermostable materials. There are some examples of large-sized POSSs (e.g., cage-type decamers) [15,16] and incomplete POSSs [17], which are also amorphous.
As a simple method to amorphize POSSs, we have prepared POSS with randomly arranged two types of ammonium side-chain groups (3-aminopropyl and 3-(2-aminoethylamino)propyl groups protonated with trifluoromethanesulfonic acid (HOTf)) [18]. In addition, it was also found that quaternary ammonium and imidazolium salt-containing POSS is amorphous, having no melting point (T m ) [19]. This POSS had a glass transition point (T g ) of −8 • C, and the temperature at which it tilted and flowed was 30 • C, showing the property of room temperature ionic liquid (IL).
As mentioned above, amorphous POSSs with randomly arranged two types of side-chain groups exhibited characteristics that are relevant for various material applications. However, no detailed investigation has been performed on how side-chain combinations affect the amorphous or crystalline morphology.
In this study, the effect of combinations of the two side-chains on the crystalline-amorphous structure of the POSSs containing two types of ammonium side-chain groups (Am-POSS(x,y)) was investigated, where the numbering of x and y represents the type of ammonium ions in the POSS structure, corresponding to primary (1), secondary (2), tertiary (3), and quaternary (4) ammonium ions. In addition, the properties of Am-POSS(x,y) as ionic liquids (ILs) were evaluated.

Preparation and Characterization of POSSs Containing One Type of Ammonium Side-Chain Group (Am-POSS(x))
For comparison, prior to the preparation and characterization of Am-POSS(x,y), POSSs containing one type of ammonium side-chain group (Am-POSS(x), where the numbering of x represented the type of ammonium ion in the POSS structure, corresponding to primary (1), secondary (2), tertiary (3), and quaternary (4) ammonium ions) were prepared and characterized. In our previous studies on the preparation of ammonium-functionalized POSSs using superacid catalysts, the solvent water was yielded a mixture of cage-type octamer (main product) and decamer (minor product) [20], while applying the solvent dimethyl sulfoxide (DMSO) selectively provided only a cage-type octamer in higher yield at short reaction times [21]. Therefore, DMSO was used as a solvent in this study.

Am-POSS(x) Water DMSO DMF Methanol Acetone Acetonitrile Chloroform Toluene n-Hexane
Am-POSS(1) The 1 H NMR spectra of Am-POSS(x) in DMSO-d6 showed the signals attributable to the side-chains of the corresponding POSSs ( Figure 1). The signals for the methoxy or ethoxy group of the starting materials were not observed (Figure 1), pointing out that the starting materials were not present in the products. The energy-dispersive X-ray (EDX) pattern of Am-POSS(4) contained no peaks originating from Cl (ca. 2.6 and 2.8 keV). The Si:S atom ratios were estimated to be 1.00:1.95 for Am-POSS(1), 1.00:2.02 for Am-POSS(2), 1.00:1.96 for Am-POSS(3), and 1.00:2.00 for Am-POSS(4), respectively, indicating that the molar ratios of ammonium cations to NTf2 anions in all products were ca. 1:1 ( Figure S1).

Am-POSS(x) Water DMSO DMF Methanol Acetone Acetonitrile Chloroform Toluene n-Hexane
Am-POSS(1) The 29 Si NMR spectra of Am-POSS(x) in DMSO-d6 at 40 °C showed only the sharp signals due to the T 3 structure (CSi(OSi)n(OH)3-n, n = 3) in the range between δ −66.6 and −67.3 ppm. These peaks are attributable to cage-type octamer (T8-POSS) ( Figure 2). To support these 29 Si NMR results, the matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) was performed on these POSSs. The MALDI-TOF MS of Am-POSS(1), Am-POSS(2), Am-POSS(3), and Am-POSS(4) showed several peaks corresponding to the masses of T8-POSS ( Figure S2-S5). These results indicated that the products consisted of T8-POSS only, no other POSSs, such as cage-type decamer (T10-POSS) and cage-type dodecamer (T12-POSS), were present in the products. The 29 Si NMR spectra of Am-POSS(x) in DMSO-d 6 at 40 • C showed only the sharp signals due to the T 3 structure (CSi(OSi) n (OH) 3-n , n = 3) in the range between δ −66.6 and −67.3 ppm. These peaks are attributable to cage-type octamer (T 8 -POSS) ( Figure 2). To support these 29 Si NMR results, the matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) was performed on these POSSs. The MALDI-TOF MS of Am-POSS(1), Am-POSS(2), Am-POSS(3), and Am-POSS(4) showed several peaks corresponding to the masses of T 8 -POSS ( Figures S2-S5). These results indicated that the products consisted of T 8 -POSS only, no other POSSs, such as cage-type decamer (T 10 -POSS) and cage-type dodecamer (T 12 -POSS), were present in the products.

Evaluation of Crystalline-Amorphous Structures of Am-POSS(x)
The X-ray diffraction (XRD) patterns of all Am-POSS(x) showed sharp diffraction peaks, indicating their crystalline structures ( Figure 3). In addition, in the differential scanning calorimetry  (Figure 4d), respectively. Multiple endothermic peaks were observed for POSSs, except Am-POSS(4), potentially caused by the different crystalline forms of these POSSs. A more detailed study on the crystalline forms of these POSSs containing one type of ammonium side-chain group is currently in progress.

Evaluation of Crystalline-Amorphous Structures of Am-POSS(x)
The X-ray diffraction (XRD) patterns of all Am-POSS(x) showed sharp diffraction peaks, indicating their crystalline structures ( Figure 3). In addition, in the differential scanning calorimetry (DSC) curves  (Figure 4d), respectively. Multiple endothermic peaks were observed for POSSs, except Am-POSS(4), potentially caused by the different crystalline forms of these POSSs. A more detailed study on the crystalline forms of these POSSs containing one type of ammonium side-chain group is currently in progress.       Solubilities of Am-POSS(x,y) are summarized in Table 2. All Am-POSS(x,y) were soluble in highly polar organic solvents but insoluble in water and low polarity organic solvents.

Preparation and Characterization of POSSs Containing Two Types of Ammonium Side-Chain Groups (Am-POSS(x,y))
Am-POSS(x,y)s were prepared by the hydrolytic condensation of a mixture of two starting materials selected from APTMS, MAPTMS, DMAPTMS, and TMTESPAC. DMSO was used as a solvent together with HNTf2 and water (the feed molar ratio of amine: HNTf2:H2O was 1.0:1.5:5.0) (Scheme 2). Solubilities of Am-POSS(x,y) are summarized in Table 2. All Am-POSS(x,y) were soluble in highly polar organic solvents but insoluble in water and low polarity organic solvents.  Table 2. The solubility of Am-POSS(x,y).

Am-POSS(x,y) Water DMSO DMF Methanol Acetone Acetonitrile Chloroform Toluene n-Hexane
Soluble at room temperature, −: Insoluble at room temperature; DMSO: Dimethyl sulfoxide, DMF: The 1 H NMR spectra of Am-POSS(x,y) in DMSO-d6 showed the signals attributable to the side-chains of both the two selected organotrialkoxysilane components ( Figure 5). The signals for the methoxy or ethoxy group of the two starting materials were not observed ( Figure 5). The compositional ratio of the two starting material components in Am-POSS(x,y) was estimated to be ca. 1:1, considering the integrated ratio of the signals in the 1 H NMR spectrum, i.e., (a + a′):e′  Table 2. The solubility of Am-POSS(x,y).  4), respectively, indicating that the molar ratios of ammonium cations to NTf 2 anions in all products were ca. 1:1 ( Figure S6).

Am-POSS(1,2)
Am-POSS(1,4), 1.00:2.06 for Am-POSS(2,3), 1.00:2.00 for Am-POSS(2,4), and 1.00:1.98 for Am-POSS (3,4), respectively, indicating that the molar ratios of ammonium cations to NTf2 anions in all products were ca. 1:1 ( Figure S6). The 29 Si NMR spectra of Am-POSS(x,y) in DMSO-d6 at 40 °C showed only the sharp signals in the range between δ −66.6 and −67.3 ppm, corresponding to the T 3 structure ( Figure 6). These peaks correspond to cage-type octamer (T8-POSS). To support these 29 Si NMR results, MALDI-TOF MS was performed on these POSSs. The MALDI-TOF MS of all Am-POSS(x,y) showed several peaks ascribed to the masses of T8-POSS ( Figure S7-S12). In most POSSs, the number of combinations of the two side-chain groups ranged from 1:7 to 7:1. This indicates that the products consisted of T8-POSS only, no other POSSs, such as T10-and T12-POSSs were present. The 29 Si NMR spectra of Am-POSS(x,y) in DMSO-d 6 at 40 • C showed only the sharp signals in the range between δ −66.6 and −67.3 ppm, corresponding to the T 3 structure ( Figure 6). These peaks correspond to cage-type octamer (T 8 -POSS). To support these 29 Si NMR results, MALDI-TOF MS was performed on these POSSs. The MALDI-TOF MS of all Am-POSS(x,y) showed several peaks ascribed to the masses of T 8 -POSS (Figures S7-S12). In most POSSs, the number of combinations of the two side-chain groups ranged from 1:7 to 7:1. This indicates that the products consisted of T 8 -POSS only, no other POSSs, such as T 10 -and T 12 -POSSs were present.

Evaluation of Crystalline-Amorphous Structures of Am-POSS(x,y)
In  Table 3), respectively, and endothermic peaks due to Tms were not observed, suggesting amorphous structures. Conversely, Am-POSS(1,2), Am-POSS(2,3), and Am-POSS (3,4) had Tms at 154 °C (Figure 7a, Run 1 in Table 3 Table 3), respectively, indicating crystalline structures. In the XRD patterns of these products, immediately recorded after preparation, sharp diffraction peaks were observed only for Am-POSS(1,2) (Figure 8a). Repeating the XRD after more than two weeks resulted in sharp diffraction peaks for Am-POSS

Evaluation of Crystalline-Amorphous Structures of Am-POSS(x,y)
In the DSC curves of Am-POSS(1,3), Am-POSS (1,4), and Am-POSS(2,4), baseline shifts due to T g s were observed at −6 • C (Figure 7b, Run 2 in Table 3 Table 3), respectively, indicating crystalline structures. In the XRD patterns of these products, immediately recorded after preparation, sharp diffraction peaks were observed only for Am-POSS(1,2) (Figure 8a). Repeating the XRD after more than two weeks resulted in sharp diffraction peaks for Am-POSS(2,3) and Am-POSS(3,4) (Figure 9d       Based on the DSC and XRD results, we concluded that crystalline and amorphous POSSs were selectively prepared, depending on the combination of side-chain groups. Am-POSS(1,3), Am-POSS (1,4), and Am-POSS(2,4) have two or three differences in the number of methyl groups between two types of ammonium side-chains. Because of the low molecular symmetry of the POSSs containing two different randomly distributed side-chain groups, crystallization was suppressed, leading to amorphous structures. Conversely, the molecular symmetry of Am-POSS(1,2), Am-POSS(2,3), and Am-POSS (3,4) were maintained because of the difference in the numbers of methyl groups between the two types of ammonium side-chains was only one. Therefore, it seems difficult to suppress the crystallization of these POSSs.

Evaluation of Am-POSS(x,y) as ILs
ILs are generally defined as salts that melt below 100 °C (there is also a definition of less than 150 °C). The fundamental thermodynamic properties of ILs have been extensively studied [22][23][24] for applications, such as green solvents [25][26][27][28] and electrolyte materials [29][30][31]. ILs present unique properties, such as high thermal stability, low vapor pressure, and high ionic conductivity. ILs are classified as aprotic ILs (AILs) or protic ILs (PILs), depending on the absence or presence of active protons. PILs are prepared by a simple neutralization (proton transfer) reaction between a Brønsted acid and a Brønsted base [32,33]. Upon heating, PILs formation can be easily reversed to obtain the original acid and base, showing inferior thermal stability to that of AILs. Therefore, the thermal stability of PILs needs to be improved.
A POSS with IL properties was first developed by Chujo, Tanaka, and co-workers [34]. This POSS contained anionic carboxylate side-chains and imidazolium cations as counterions, and its melting point (Tm) was 23 °C . Another IL containing a POSS structure was reported by Feng, Zhang, Based on the DSC and XRD results, we concluded that crystalline and amorphous POSSs were selectively prepared, depending on the combination of side-chain groups. Am-POSS(1,3), Am-POSS(1,4), and Am-POSS(2,4) have two or three differences in the number of methyl groups between two types of ammonium side-chains. Because of the low molecular symmetry of the POSSs containing two different randomly distributed side-chain groups, crystallization was suppressed, leading to amorphous structures. Conversely, the molecular symmetry of Am-POSS(1,2), Am-POSS(2,3), and Am-POSS(3,4) were maintained because of the difference in the numbers of methyl groups between the two types of ammonium side-chains was only one. Therefore, it seems difficult to suppress the crystallization of these POSSs.

Evaluation of Am-POSS(x,y) as ILs
ILs are generally defined as salts that melt below 100 • C (there is also a definition of less than 150 • C). The fundamental thermodynamic properties of ILs have been extensively studied [22][23][24] for applications, such as green solvents [25][26][27][28] and electrolyte materials [29][30][31]. ILs present unique properties, such as high thermal stability, low vapor pressure, and high ionic conductivity. ILs are classified as aprotic ILs (AILs) or protic ILs (PILs), depending on the absence or presence of active protons. PILs are prepared by a simple neutralization (proton transfer) reaction between a Brønsted acid and a Brønsted base [32,33]. Upon heating, PILs formation can be easily reversed to obtain the original acid and base, showing inferior thermal stability to that of AILs. Therefore, the thermal stability of PILs needs to be improved.
A POSS with IL properties was first developed by Chujo, Tanaka, and co-workers [34]. This POSS contained anionic carboxylate side-chains and imidazolium cations as counterions, and its melting point (T m ) was 23 • C. Another IL containing a POSS structure was reported by Feng, Zhang, and co-workers. It contained cationic imidazolium side-chains and dodecyl sulfate anions as counterions and had a T m of 18 • C [35]. We have also reported the preparation of ILs containing randomly structured silsesquioxanes [36,37] and POSS with two types of side-chains [19]. However, all these ILs containing silsesquioxane frameworks were not PILs but AILs.
The flow temperatures of POSSs were confirmed as follows: Samples in glass vessels were maintained in a horizontal position at 200 • C for 15 min, and then cooled to room temperature (ca. 25 • C). Next, the vessels were kept horizontally at various temperatures (with 5 • C intervals) for 10 min and then tilted for 15 min. Following this procedure, it was confirmed that Am-POSS(1,2), Am-POSS (1,3), Am-POSS(1,4), Am-POSS(2,3), Am-POSS(2,4), and Am-POSS(3,4) flowed at 140 • C (Figure 10a, Run 1 in Table 3), 45 • C (Figure 10b, Run 2 in Table 3), 60 • C (Figure 10c, Run 3 in Table 3 Finally, the thermal stabilities of Am-POSS(x,y) were investigated by thermogravimetric analyses (TGA) ( Figure S13). The pyrolysis temperatures of 5% (T d5 ) and 10% (T d10 ) weight losses of Am-POSS(x,y) are summarized in Table 3. It was found that the T d5 values of all Am-POSS(x,y) exceeded 370 • C. These values indicate excellent thermal stabilities and results from the suppression of the molecular tumbling by the silsesquioxane framework [34]. and co-workers. It contained cationic imidazolium side-chains and dodecyl sulfate anions as counterions and had a Tm of 18 °C [35]. We have also reported the preparation of ILs containing randomly structured silsesquioxanes [36,37] and POSS with two types of side-chains [19]. However, all these ILs containing silsesquioxane frameworks were not PILs but AILs. Although our group reported that cyclic oligosiloxanes containing 3-aminopropyl or 3-(2-aminoethylamino)propyl side-chain groups protonated with HNTf2 were of PILs nature [38], POSS containing the same side-chain groups did not show IL characteristics, because of the high crystallinity. The amorphous Am-POSS (1,3), Am-POSS (1,4), and Am-POSS (2,4) prepared in this study have active protons and showed PILs characteristics.

Measurements
The 1 H and 29 Si NMR spectra were recorded using an ECX-400 spectrometer (JEOL RESONANCE Inc., Tokyo, Japan). The absence of Cl and the elemental ratios of Si:S in the products were confirmed by EDX analyses using an XL30 scanning electron microscope (FEI Company, Hillsboro, OR, USA). The MALDI-TOF MS analyses of the products were performed using a Voyager Biospectrometry Workstation Ver. 5.1 (SHIMADZU Corporation, Kyoto, Japan) with 2,5-dihydroxybenzoic acid (DHB) as the matrix under positive ion mode. The DSC analyses were performed on a DSC-60 Plus (SHIMADZU Corporation, Kyoto, Japan). The samples placed in an aluminum capsule were cooled to −140 • C at a rate of 20 • C min −1 under a nitrogen flow (300 mL min −1 ) and then heated from −140 • C to 240 • C at the same rate. The T g and T m values were determined as the onset of the baseline shift and as the tops of the endothermic peaks, respectively, in the curves of the third scan (from −140 • C to 240 • C at a rate of 20 • C min −1 ) to eliminate the heat history in the samples. The XRD patterns were recorded at a scanning speed of 2θ = 9.0 • min −1 using an X'Pert Pro diffractometer (PANalytical, Almelo, Netherlands) with Ni-filtered Cu Kα radiation (0.15418 nm). The TGA was performed on a TGA-50 (SHIMADZU Corporation, Kyoto, Japan) at a heating rate of 10 • C min −1 up to 1000 • C under nitrogen flow (100 mL min −1 ). The pyrolysis temperatures were determined from the 5% (T d5 ) and 10% (T d10 ) weight losses.