Journal of Molecular Structure 1032 (2013) 29–34

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Journal of Molecular Structure

journal homepage: www.elsevier .com/ locate /molst ruc

Synthesis, structural characterization and properties of a cubicocta-n-propylsilsesquioxane inorganic–organic hybrid material

Hui Liu, Qingzeng Zhu ⇑, Lei Feng, Bingjian Yao, Shengyu FengKey Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China

h i g h l i g h t s

" Cubic octa-n-propylsilsesquioxanes was synthesized with a 68.9% yield." Soluble in common organic solvents." A triclinic system crystal with a P-1 space group." Sublimate above 200 �C and not to decompose until 524 �C under a nitrogen atmosphere.

a r t i c l e i n f o

Article history:Received 25 June 2012Received in revised form 22 July 2012Accepted 24 July 2012Available online 2 August 2012

Keywords:n-PropylsilsesquioxaneCubicX-ray single crystal diffractionSublimateTGA

0022-2860/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.molstruc.2012.07.045

⇑ Corresponding author.E-mail address: qzzhu@sdu.edu.cn (Q. Zhu).

a b s t r a c t

Synthesis, structural characterization and property studies were carried out on cubic octa-n-propyl-silsesquioxanes (n-Pr-POSS) in this paper. n-Pr-POSS was synthesized by an acid-catalyzed hydrolytic con-densation of n-propyltriethoxysilane with a 68.9% yield. Common organic solvents, such as benzene,chloroform, tetrahydrofuran, diethyl ether, dichloromethane, toluene, cyclohexane, hexane and pentanecan be used to dissolve n-Pr-POSS; however, n-Pr-POSS is insoluble or poorly soluble in acetone, dichloro-ethane, chlorobenzene, dimethylformamide, xylene, methanol, alcohol and isopropanol. The cubic struc-ture and crystal morphology of n-Pr-POSS have been investigated by Fourier transform infraredspectroscopy (FTIR), transmission electron microscopy (TEM), optical microscope, 1H, 13C and 29Si nuclearmagnetic resonance (NMR), X-ray single crystal diffraction and X-ray powder diffraction (XRD) methods.Crystalline n-Pr-POSS is a triclinic system crystal with a P-1 space group. Thermogravimetric analysis(TGA) indicates that n-Pr-POSS begins to sublimate above 200 �C and does not decompose until 524 �Cunder a nitrogen atmosphere.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Over the past several decades, polyhedral oligomeric silsesquiox-anes (POSSs), including octameric silsesquioxanes (RSiO1.5)8, havereceived an increasing amount of scientific and technological atten-tion [1–4]. POSS represent a class of three-dimensional organosiliconoligomers that have well-defined architectures with size from 1 to3 nm in diameter, while the silica cores are hard particles with a0.45–0.53 nm diameter and the organic substituents (R) are attachedto Si atoms at the inorganic cage corners. POSS are typical nano-sizedinorganic–organic hybrid materials, having high symmetry and goodchemical and thermal stability. These compounds have uses in elas-tomers, plastics, liquid crystalline polymers, dendrimers, low dielec-tric materials, heterogeneous catalysts, medical materials, high-temperature composites, biology, space-resistant materials andother applications [1,5–10].

ll rights reserved.

Despite the tremendous attention garnered by POSS in recentyears, relatively little information is available concerning the poly-hedral oligomeric n-propylsilsesquioxane, which has n-propylattachments on the Si atoms at the inorganic cage corners. Barryet al. reported that n-propylsilsesquioxanes were isolated after re-peated recrystallization from the distillate paste obtained fromthe alkali-catalyzed thermal rearrangement of n-propyltrichlorosi-lane hydrolyzates [11]. However, the yield for this process wasnot given in the literature. In 1958, Olsson described the processto prepare oligomeric silsesquioxanes, (RSiO1.5)8, where R wasmethyl, ethyl, n-propyl, isopropyl, n-butyl, or phenyl, formed byhydrolysis of the corresponding alkyl or aryltrichlorosilanes understrongly acidic conditions [12]. Larsson investigated several POSS,such as (CH3SiO1.5)8, (C2H5SiO1.5)8, (n-C3H7SiO1.5)8, (i-C3H7SiO1.5)8,(n-C4H9SiO1.5)8, and he found that (n-C3H7SiO1.5)8, (i-C3H7SiO1.5)8

and (n-C4H9SiO1.5)8 had two melting points. Voronkov presumedthat the dual melting points were associated with polymorphism[13,14]. Frey synthesized a series of 8-fold alkyl-substituted octa-silsesquioxane including n-propylsilsesquioxanes by hydrosilation

Fig. 1. Chemical structure for a cubic octa-n-propylsilsesquioxane cage.

Fig. 2. FTIR spectrum of cubic octa-n-propylsilsesquioxanes.

30 H. Liu et al. / Journal of Molecular Structure 1032 (2013) 29–34

of alkenes with octa(hydridosilsesquioxane) [15]. Comparativelystudy on the different longer n-alkyl chains was carried out.

In the aforementioned references, the properties of polyhedral n-propylsilsesquioxanes were stated in a cursory way. In this paper,we present a systematic study of cubic octa-n-propylsilsesquiox-anes (n-Pr-POSS). The crystalline octa-n-propylsilsesquioxanewas synthesized by an acid-catalyzed hydrolytic condensation ofn-propyltriethoxysilane with a high yield. The structure and proper-ties of n-Pr-POSS were investigated using Fourier transform infraredspectroscopy (FTIR), 1H, 13C and 29Si nuclear magnetic resonance(NMR), X-ray powder diffraction (XRD), X-ray single crystal diffrac-tion, transmission electron microscopy (TEM), scanning electronmicroscopy (SEM) and thermogravimetric analysis (TGA) methods.

2. Experimental

2.1. Materials

n-Propyltriethoxysilane (95% purity) was purchased from Jingz-hou Jianghan Fine Chemicals Co., Ltd., and it was purified by distil-lation before use (purified grade, 99.9%). Methanol andhydrochloric acid (wt.% = 37.0%) were of analytical grade.

2.2. Synthesis of cubic octa-n-propylsilsesquioxanes

n-Pr-POSS was prepared by an acid-catalyzed hydrolytic con-densation of n-propyltriethoxysilane. In a typical reaction, n-pro-pyltriethoxysilane (8.8 mL, 0.038 mol) was added to a roundbottom flask to mixture of 200.0 mL methanol and 12.0 mL hydro-chloric acid (37.0 wt.%). After stirring a half hour, the reactant washeld still at room temperature. The crystalline n-propylsilsesquiox-ane was obtained after sixteen days. The resulting n-Pr-POSS wasthen washed with cold methanol and dried under reduced pres-sure. The yield was typically 68.9%.

2.3. Characterizations

FTIR was performed on a Bruker Tensor 27 spectrometer in therange of 4000–400 cm�1. The sample was prepared for analysis bycasting a 2.0 wt.% CH2Cl2 solution onto KBr windows, and then thesample was placed in an oven at 50 �C for 48.0 h under vacuum tocompletely remove the solvent. 1H NMR, 13C NMR and 29Si NMRspectra were recorded on a 400 MHz UltraShield™ magnet (Bru-ker) spectrometer using CDCl3 as the solvent, and the chemicalshifts (d) were given in ppm. TEM was performed on a JEOL JEM-2100 electron microscope that operated at an accelerating voltageof 120 kV. SEM was performed on a JEOL JSM-7600F instrument.Samples on conductive tape were tested after spraying platinum.XRD analyses were performed on a Bruker D8 Advance X-ray dif-fractometer with Cu Ka radiation (k = 1.5406 Å); The 2h angle ran-ged from 10� to 80�, and the step size was 0.08�. X-ray singlecrystal diffraction was detected using a Bruker–Nonius SMARTAPEX II CCD at 25 �C. TGA was performed using a Mettler TGA/AD-TA851e thermal analyzer. The samples (6.0–10.0 mg) were heatedfrom ambient temperature to 800 �C at a heating rate 10 �C/minunder a continuous flow of nitrogen or air atmosphere (50 mL/min).

3. Results and discussion

3.1. Solubility

Once prepared, n-Pr-POSS is a colorless crystalline substance,and its chemical structure is shown in Fig. 1. It is readily solublein common organic solvents, such as benzene, chloroform, tetrahy-

drofuran, diethyl ether, dichloromethane, toluene, cyclohexane,hexane and pentane, and it is insoluble or poorly soluble in ace-tone, dichloroethane, chlorobenzene, dimethylformamide, xylene,methanol, alcohol and isopropanol.

3.2. FTIR analyses

n-Pr-POSS was carefully studied by FTIR. The FTIR spectrum ofthe synthesized n-Pr-POSS showed no absorption for a hydroxylgroup (Fig. 2). The strong band observed at 1114 cm�1 was thecharacteristic asymmetric stretching vibration for the Si–O–Siframework of the cage silsesquioxanes. Because of the highly sym-metric cage structure, the peak shape and wavelength are some-what different from linear and branched polysiloxanes. The peaksat 2958 cm�1, 2930 cm�1 and 2872 cm�1 were attributed to theC–H stretching vibration of propyl groups, and the C–H bendingvibrations were located at 1464–1372 cm�1. The peak at1220 cm�1 was due to the characteristic stretching vibration ofSi–C.

3.3. NMR analyses

n-Pr-POSS was characterized using 1H NMR, 13C NMR and 29SiNMR techniques. In the 1H NMR spectrum (Fig. 3), the proton sig-nals for propyl groups were observed at 0.63 ppm (t, Si–CH2, 2H),0.98 ppm (t, CH3, 3H) and 1.47 ppm (m, CH2, 2H) with an intensityratio of 2:3:2. The single peak at 7.28 ppm was a result of a smallamount of chloroform from the chloroform-d NMR solvent. Theproton signals for Si–OCH2CH3 group and Si–OH group did not ap-pear in the 1H NMR spectrum, which revealed that both the hydro-lysis and condensation reactions had occurred. As illustrated inFig. 4, the 13C NMR spectrum exhibits three sets of carbons(14.40 ppm, 16.33 ppm and 17.33 ppm) with an intensity ratio of1:1:1. These peaks were assigned to the three different chemical

Fig. 3. 1H NMR spectrum of cubic octa-n-propylsilsesquioxanes.

Fig. 4. 13C NMR spectrum of cubic octa-n-propylsilsesquioxanes.

Fig. 5. 29Si NMR spectrum of cubic octa-n-propylsilsesquioxanes.

H. Liu et al. / Journal of Molecular Structure 1032 (2013) 29–34 31

environment carbon atoms of the –CH2CH2CH3 group. The reso-nance of CDCl3 in the 13C NMR spectra is at 77.00 ppm. Fig. 5 dis-played the 29Si NMR spectrum of n-Pr-POSS, and it revealed thatthere was only one characteristic 29Si chemical shift at�66.93 ppm, which was for sp3 carbons attached to silicon. Thesingle peak in the 29Si NMR indicated that there was only onestructural conformation for the silicon atom in n-Pr-POSS. Thechemical shift was assigned to the silicon atom connected to thepropyl group (C3H7SiO1.5). One set of silicon atom also evidencedthat the product was of high purity.

On the basis of the FTIR and NMR results, we could confirm thatthe target compound, n-Pr-POSS, was successfully obtained as acage structure silsesquioxane instead of random structure or lad-der structure silicone resins.

3.4. Microscopy characterizations

The morphology and structure of n-Pr-POSS were investigatedby optical microscope and TEM. Fig. 6 shows the crystal image ofn-Pr-POSS. The prepared crystals were rectangular and had up to

Fig. 6. Optical microscope image of octa-n-propylsilsesquioxanes crystals.

Fig. 8. X-ray diffraction pattern of cubic octa-n-propylsilsesquioxanes.

32 H. Liu et al. / Journal of Molecular Structure 1032 (2013) 29–34

millimeter dimensions. TEM was used to provide images of the n-Pr-POSS that were obtained by the acid-catalyzed hydrolytic con-densation of n-propyltriethoxysilanes with a considerable excessof water at relatively low concentrations. As shown in Fig. 7, theocta-n-propylsilsesquioxanes obtained were triangular crystals.The electron diffraction spots were arranged on a hexagonal array,which suggests that the product has a high degree of crystallo-graphic regularity.

3.5. X-ray diffraction studies

The XRD pattern for n-Pr-POSS was determined experimentallyby X-ray powder method. As indicated in Fig. 8, the XRD patternshowed several sharp peaks that were centered at 2h = 11.57�,13.04�, 17.60�, 18.80�, 20.40�, 21.12� and 26.40�, correspondingto d-spacings of 7.52 Å, 6.89 Å, 5.04 Å, 4.72 Å, 4.35 Å, 4.20 Å and3.37 Å, respectively. The 3.37 Å and 5.04 Å spacings are attributedto the distance between opposite Si4O4 faces of the POSS core andthe n-Pr-POSS cage diagonal distance, respectively [16–19]. It is anano-sized silicon-oxygen cage.

The pure n-Pr-POSS crystals were successfully prepared in asolution system. X-ray single crystal diffraction was used to struc-turally characterize the crystalline n-Pr-POSS, and it providedinformation about the bond distances and angles on the basis ofwhich plots of the octa-n-propylsilsesquioxanes structure can bedrawn. The crystallographic data and structure refinement forthe n-Pr-POSS crystal (0.10 mm � 0.15 mm � 0.15 mm) are given

Fig. 7. TEM of crystalline octa-n-propylsilsesquioxanes. The insert is electrondiffraction pattern.

in Table 1. The crystal composition was determined asC24H56Si8O12. Tables 2 and 3 summarize the relevant bond lengths(Å) and bond angles (�) with estimated standard deviations inparentheses. The ORTEP plot with the atom numbering schemefrom the X-ray single crystal diffraction study of n-Pr-POSS isshown in Fig. 9.

Analysis of the collected data revealed that the as-synthesizedocta-n-propylsilsesquioxane crystallizes in the triclinic system andP � 1 centrosymmetric space group. n-Pr-POSS has a slightly dis-torted cage with the O–Si–O angles in the range 108.62(9)–109.46(19)� (av. = 109.04�), the O–Si–C angles in the range108.9(2)–111.9(2)� (av. = 110.4�), Si–O–Si angles in the range146.1(2)–151.6(2)� (av. = 148.85�) and the Si–O and Si–C bondlengths in the ranges 1.612(4)–1.623(4) Å (av. = 1.6175 Å) and1.818(5)–1.843(5) Å (av. = 1.8305 Å). The distorted structure ofthe cage is because the Si atoms tend to keep a tetrahedral geome-try, which causes the O atoms to point outwards from the cubeedges. The steric hindrance of the n-propyl groups connected to sil-icon atoms may also contribute to the distorted structure. Never-theless, all the data of the core of the crystal are in the normalrange of octasilsesquioxanes: O–Si–O angle (107–112�) (av. =109�); Si–O–Si angle (145–152�) (av. = 148.5�); Si–O bond (1.55–1.65 Å) (av. = 1.60 Å) [20–27].

Table 1Crystal data and structure refinement for octa-n-propylsilsesquioxanes.

Empirical formula C24H56Si8O12

Formula weight 760.88Temperature 298(2) KWavelength 0.71073 ÅCrystal system TriclinicSpace group P � 1

Unit cell dimensionsa, b, c (Å) 9.9796(18), 13.931(3), 14.794(3)a, b, c (�) 90.224(2), 99.334(2), 99.283(2)Volume 2002.11(6) Å3

Z 2Density (calculated) 1.2629 mg/m3

Absorption coefficient 0.32 mm�1

F(000) 808Crystal size 0.10 � 0.15 � 0.15 mm3

Theta range for data collection 2.32–27.60�Index ranges �12 � h � 12, �18 � k � 18, �19 � l � 19

Table 2Selected bond lengths d (Å) with estimated standard deviations in parentheses forcubic octa-n-propylsilsesquioxanes (C24H56Si8O12).

dSi–O (Å) dSi–C (Å)

Si1–O2 1.610(4) Si1–C6 1.829(5)Si1–O3 1.615(4) Si2–C2 1.839(5)Si1–O10 1.619(4) Si4–C5 1.818(5)Si2–O1 1.619(4) Si6–C4 1.843(5)Si2–O2 1.619(4)Si2–O9 1.613(4)Si4–O3 1.623(4)Si4–O4 1.615(4)Si4–O9 1.615(4)Si6–O1 1.617(4)Si6–O4 1.616(4)Si6–O10 1.612(4)

Table 3Selected bond angles a (�) with estimated standard deviations in parentheses forcubic octa-n-propylsilsesquioxanes (C24H56Si8O12).

aSi–O–Si (�) aO–Si–O (�) aO–Si–C (�)

Si1–O2–Si2 151.3(2) O2–Si1–O3 108.74(19) O2–Si1–C6 110.7(2)Si1–O3–Si4 146.1(2) O2–Si1–O10 109.10(19) O3–Si1–C6 108.9(2)Si4–O9–Si2 151.3(2) O3–Si1–O10 109.12(19) O10–Si1–C6 110.2(2)Si4–O4–Si6 151.6(2) O2–Si2–O1 109.46(19) O1–Si2–C2 109.7(2)Si6–O1–Si2 145.9(2) O9–Si2–O1 108.69(19) O2–Si2–C2 109.8(2)Si6–O10–Si1 150.8(2) O9–Si2–O2 108.84(19) O9–Si2–C2 110.4(2)

O4–Si4–O3 108.95(19) O3–Si4–C5 109.9(2)O9–Si4–O3 108.64(19) O4–Si4–C5 110.9(2)O4–Si4–O9 108.80(19) O9–Si4–C5 109.6(2)O10–Si6–O1 108.62(19) O1–Si6–C4 109.5(2)O1–Si6–O4 109.06(19) O4–Si6–C4 109.0(2)O10–Si6–O4 108.83(19) O10–Si6–C4 111.9(2)

Fig. 9. The ORTEP plot of cubic octa-n-propylsilsesquioxanes.

Fig. 10. TGA curves of cubic octa-n-propylsilsesquioxanes under nitrogen atmo-sphere (line and point) and under air (line and circle).

H. Liu et al. / Journal of Molecular Structure 1032 (2013) 29–34 33

Crystallographic data show that the crystalline n-Pr-POSS be-longs to triclinic system crystal. This is consistent with the TEMelectron diffraction pattern.

3.6. TGA studies

The thermal stability and decomposition pathways for n-Pr-POSS were studied by TGA under air and nitrogen atmospheres(Fig. 10). Under an air atmosphere, n-Pr-POSS decomposed in twostages. The first stage was a rapid mass loss process beginning atapproximately 217 �C, while the second stage starting at 312 �Cwas correspondingly much slower. The 28.2% residual mass wassignificantly lower than expected. Theoretically, the char residueexpected for (C3H7SiO1.5)8 with complete conversion to SiO2 is63.2%. The lower residual mass for n-Pr-POSS was due to the sub-limation of n-Pr-POSS in the first stage (217–312 �C). According tothe TGA curve, the amount of sample sublimation was 57.3%. The42.7% residual n-Pr-POSS began to decompose and convert toSiO2 above 312 �C. Based on 63.2% theoretical ceramic yield of n-Pr-POSS, the char residue should be 27.0%, which is very close tothe experimental value 28.2%. This same trend was also observedon the TGA curve under nitrogen atmosphere. The first mass lossregion was at 200–296 �C. In this period, the amount of n-Pr-POSSsublimation was 76.7%. The TGA showed a ceramic yield of 13.5%under nitrogen atmosphere, which is consistent with the calcu-lated value of 14.7%. The weight loss for n-Pr-POSS resulted fromthe sublimation of sample and from the cleavage of chemicalgroups, and the sublimation process was primary in the mass loss.The TGA curve under nitrogen atmosphere revealed that the onsetof decomposition occurred at 524 �C, which indicated a high ther-mal stability for n-Pr-POSS.

4. Conclusions

In summary, this paper presents a systematic study of nano-sized octa-n-propylsilsesquioxanes inorganic–organic hybrid mate-rials. A high yielding synthetic method is explored to prepare cubicocta-n-propylsilsesquioxanes through an acid-catalyzed hydrolyticcondensation of n-propyltriethoxysilane. The cubic structure of n-Pr-POSS has been confirmed by FTIR, 1H NMR, 13C NMR and 29SiNMR, TEM, XRD and X-ray single crystal diffraction studies. Crystal-line n-Pr-POSS is a triclinic system crystal, and n-Pr-POSS can dis-solve in most organic solvents. n-Pr-POSS begins to sublimateabove 200 �C and does not decompose until 524 �C in a nitrogenatmosphere, which makes clear that thermal degradation resistanceof n-Pr-POSS is good. As an intrinsic nanometer scale inorganic–or-

34 H. Liu et al. / Journal of Molecular Structure 1032 (2013) 29–34

ganic hybrid material, n-Pr-POSS have potential application in theimprovement of conventional materials and the creation of novelfunctional materials.

Acknowledgements

The authors are grateful for the financial support of ShandongProvince Natural Science Foundation (ZR2009BM038), ChinaPostdoctoral Science Foundation (20090461212), Science andTechnology Development Planning of Shandong University(2009GG20003016), Independent Innovation Foundation ofShandong University (IIFSDU, 2011JC012), and National BasicResearch Program of China (2009CB930103).

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