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New Water Vapor Barrier Film Based on Lamellar AliphaticMonoamine-Bridged Polysilsesquioxane Cong Zhang,†,⊥ Ce Zhang,†,⊥ Ruimin Ding,*,† Xinmin Cui,†,⊥ Jing Wang,†,⊥ Qinghua Zhang,*,§ and Yao Xu*,‡ †

Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an, 710119, China § Chengdu Fine Optical Engineering Research Center, Chengdu, Sichuan 610041, China ⊥ University of Chinese Academy of Sciences, Beijing, 100049, China ‡

S Supporting Information *

ABSTRACT: Siloxane-based hybrid lamellar materials with ordered nanostructure units paralleling to the substrate have been widely used for water vapor barrier. However, it is very difficult to control the orientation of the lamellar units at molecular level. In this Research Article, a new lamellar bridged polysilsesquioxane (BPSQ) film, whose voids between lamellae were filled by pendant alkyl chains in the organic bridge, was prepared via the stoichiometric reaction between 3-glycidoxypropyltrimethoxysilane and aliphatic monoamine at 60 °C without catalyst. Experimental evidence obtained from FT-IR, MS, NMR, and GIXRD techniques suggested that the as-prepared BPSQ films were constructed by lamellar units with disordered orientation. Nonetheless, they possessed satisfactory water vapor barrier performance for potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) optical crystals, and the water vapor transmission rate through BPSQ film with thickness of 25 μm was as low as 20.3 g·m−2·d−1. Those results proved that filling the voids between molecular lamellae with alkyl chains greatly weakened the effect of lamellar unit orientation on the vapor barrier property of BPSQ film. KEYWORDS: water vapor barrier, lamellar polymer, bridged polysilsesquioxane, protective film

1. INTRODUCTION Water vapor barrier is a key issue in many fields, such as biomedical implants,1 organic photovoltaic devices encapsulation,2,3 metal corrosion protection,4 food packaging,5,6 and optical crystal protection,7,8 etc. Siloxane-based hybrid materials have been widely reported to be used for water vapor barrier for its high optical transmittance, good thermal stability, and flexibility.9−12 From the structure, it can be divided into two kinds. The traditional one blocks diffusion of water vapor via its disordered and compact structure,8 while the other kind barriers water vapor via its special lamellar structure.7,9 In this paper, we focused on the kind of siloxane-based hybrid materials with lamellar structure. However, the water vapor barrier property of siloxane-based hybrid lamellar materials mainly depended on the orientation of its lamellar units. When the lamellar units were parallel to the substrate surface, they could keep water vapor molecules away from the substrate. © XXXX American Chemical Society

While the lamellar units were perpendicular to the substrate surface, the voids between lamellae were exposed out, through which the water vapor molecule could diffuse and finally arrive at the substrate (shown in Scheme 1a, redraw according to ref 7). As it is a very difficult process to control the orientation of the lamellar units at molecular level. Here, we put forward an interesting idea as illustrated by Scheme 1b. If the voids between molecular lamellae were filled with flexible alkyl chains, would the barrier property of material have little to do with the orientation of the ordered nanostructure units? Based on this idea, the filled flexible alkyl chains between lamellae will increase the diffusion pathway to barrier the diffused water molecules away from substrate, especially when the lamellar Received: January 22, 2016 Accepted: May 25, 2016

A

DOI: 10.1021/acsami.6b00878 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Water Vapor Barrier Mechanism of (a) the Traditional Lamellar Moisture Barrier Materials and (b) the New Lamellar Moisture Barrier Material Proposed in This Paper

Scheme 2. Reaction Routine from GPTMS and BA (HA, OA) to BPSQ

between lamellae effectively while the generated expansion effect could be neglected. Four aliphatic monoamine-bridged polysilsesquioxane with different alkyl chain lengths were chosen for this research. The amine-bridged polysilsesquioxane was chosen to check our idea because it can be easily prepared with high yield according to the epoxy-opening reaction under mild condition.20−22 In addition, we explored and found out the optimum reaction condition between organic amine and 3glycidoxypropyltrimethoxysilane (GPTMS) in our previous study,8 which laid a foundation for the synthesis of bridged silsesquioxane (BSQ) precursors in this paper. Considering the safety and environment issues in the process of preparation, nbutylamine (BA), n-hexylamine (HA), and n-octylamine (OA) were chosen as the bridging molecules, and, GPTMS offered the organic bridge and inorganic silica matrix.

units are perpendicular to the substrate. The water vapor barrier film prepared based on this idea might be much more convenient and controllable than the traditional lamellar material with fixed molecular layer orientation. Bridged polysilsesquioxane is a kind of typical siloxane-based hybrid material with a wide range of structures13−19 that was relatively easy to be controlled. Therefore, lamellar bridged polysilsesquioxane (BPSQ) should be an appropriate research object to verify our idea. The first aim of this work was to prepare lamellar BPSQ with voids were filled by alkyl chains but its lamellar units did not show a fixed orientation. Then we carried out detailed structural study on the obtained lamellar BPSQ and verified whether the obtained BPSQ had satisfactory water vapor barrier performance or not. However, when the alkyl chains were introduced into the two adjacent lamellae, they could not only fill up the void between the two lamellae, but also extend the lamellae spacing. Considering the filling effect and the expansion effect of the introduced alkyl chains, there should be a balanced state, in which the alkyl chains could fill the voids

2. EXPERIMENTAL SECTION 2.1. Chemicals. 3-Glycidoxypropyltrimethoxysilane (GPTMS, 99%), n-butylamine (BA, 99+%), hexylamine (HA, 99%), and noctylamine (OA, 99+%) were purchased from Acros Company. B


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ACS Applied Materials & Interfaces Anhydrous ethanol (EtOH) and ammonia−water (27%, wt %) were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) crystal with size of 23 × 23 × 8 mm3 were purchased from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. All these were used as received. Water was deionized and twice distilled. 2.2. Material Synthesis and Preparation. 2.2.1. Synthesis of the BSQ Precursors. BSQ precursors were synthesized via the stoichiometric reaction between GPTMS and aliphatic monoamine. GPTMS (2.36 g, 0.01 mol) was dissolved in 50 mL of anhydrous EtOH and stirred. BA (0.37 g, 0.005 mol), HA (0.51 g, 0.005 mol) or OA (0.65 g, 0.005 mol) was dissolved in 50 mL of anhydrous EtOH and the solution was stirred to dissolve thoroughly. Then, the solution of GPTMS was added into the monoamine solution, and the mixed solution was stirred at 60 °C for 72 h. The concentration of the final BSQ precursor solution was 0.1 mol/L. The product obtained by the reaction of GPTMS and BA was noted as BG-BSQ. Similarly, the other three products obtained by HA and OA were noted as HG-BSQ and OG-BSQ. For a clear description, the synthesis process was illustrated by Scheme 2. 2.2.2. Synthesis of BPSQs. The BPSQs were synthesized through hydrolysis and condensation of the corresponding BSQ precursors, and were denoted as BG-BPSQ, HG-BPSQ, and OG-BPSQ. In this process, ammonia was added to speed up hydrolysis and condensation of BSQ precursors. Meanwhile, it provided necessary water for hydrolysis of trimethoxysilyl groups in BSQ precursors. In a typical synthesis, 0.81 mL of ammonia aqueous solution (14.3 mol/L) was added to the above solution of BSQ precursors, and the total volume of final solution was about 103 mL. The resultant sol was stirred at 60 °C and aged for 7 days to prepare BPSQ films. 2.2.3. Preparation of BPSQ films. KDP (DKDP) crystals were cleaned with toluene before use. Then, the KDP (DKDP) crystal was immersed into BPSQ sol for 1 min in clear air at 25 °C and RH 30− 40%, and withdrawn from the sol at a speed of 200 mm·min−1. The viscosity of BPSQ sols used for preparing BPSQ films was 1.45 mPa·s for BG-BPSQ sol, 1.36 mPa·s for HG-BPSQ sol, and 1.44 mPa·s for OG-BPSQ sol, which were measured at 25 °C with a shearing rate of 400 s−1. The as-prepared film was dried for 24 h at room temperature and noted as F-BG, F-HG, F-OG, and F-IG respectively. Its thickness was around 120 nm. The BPSQ films coated on silicon wafer substrates were used for structure analysis and films on KDP and DKDP crystals were used for water vapor barrier tests. 2.3. Characterizations. 2.3.1. BSQ Precursors. The chemical structure of BSQ precursors was characterized by Fourier-transform infrared (FT-IR), elemental analysis, mass spectra (MS), and nuclear magnetic resonance (NMR). FT-IR spectra were recorded in the range of 400−4000 cm−1 on a Nicolet IS50 spectrometer using a DTGS detector with the transmittance mode. Element contents were measured on a Vario EL CUBE elemental analyzer. High resolution MS was measured by a Bruker microTOF QIII quadrupole time-offlight mass spectrometer. 1H liquid NMR, 13C liquid NMR, and 29Si liquid NMR spectra were recorded on a Bruker AVANCE IIITM 400 MHz (solution) spectrometer. The solvent was CDCl3, and the relaxation agent was acetyl acetone chromium. The resonance frequency of 1H NMR, 13C NMR, and 29Si NMR was 400.1, 100.6, and 79.5 MHz, respectively. 2.3.2. BPSQs. The condensation degree of BPSQ was characterized by 29Si MAS NMR recorded on a Bruker AVANCE IIITM 600 MHz spectrometer. The sample was dried in oven to obtain the gel. The solvent was CDCl3 and the resonance frequency was 79.5 MHz. The thermostability of BPSQ was characterized by thermogravimetric analysis (TGA, SDTA851, Mettler Toledo Star) in N2 atmosphere. The temperature was from 25 to 1000 °C, with heating rate of 10 °C min−1 and gas flow 500 mL min−1 on sample of about 10 mg. The viscosity of BPSQ sols was measured by a RheolabQC Rheometer (Anton Paar) with DG42/SS/QC-LTD double-slit rotator. 2.3.3. BPSQ Films. The optical transmittance of the BPSQ film was recorded on a UV−visible−NIR spectrometer (UV-4100, Hitachi), in the wavelength range of 240−1200 nm, and the precision of the

transmittance value was 0.0001%. The refractive index and thickness of films were measured on a spectroscopic ellipsometry (SC620, Sanco) and fitted with Cauchy’s dispersion model. The incident angle was 60°. In the process of date fitting, the thickness of Si substrate was supposed to be 1.0 mm, and supposing that there was a SiO2 oxide layer with thickness of 2.76 nm on Si substrate surface. The water contact angle of BPSQ films was measured on a contact-angle meter (SL200B). The nanostructure of BPSQ films was investigated by 2D Grazing incidence X-ray diffraction (GIXRD), which was performed at BL14B1 beamline of Shanghai Synchrotron Radiation Facility of China. The incidence X-ray wavelength λ was 0.124 nm and the camera length was 304.18 mm. A two-dimensional detector with 2048 × 2048 pixels was used to collect diffractive data. The incident angle of primary beam to sample surface was chosen at 0.3°, just above the critical angle. 2.3.4. Water Vapor Barrier of BPSQ Films. The water vapor barrier property of films was directly evaluated by comparing their optical transmittance before and after treated in air condition of 60% relative humidity (RH) for 7 months at 25 °C. As a reference experiment, the water vapor transmission rate (WVTR) of film was tested using a purpose-built apparatus according to a modified ASTM (1995) method E96 (Figure S1). The stand-free BPSQ membrane used for WVTR (g·d−1·m−2) test were prepared by solvent evaporation. The BPSQ sol was poured into a Teflon plate, after drying in the 60 °C oven for 24 h, the obtained stand-free BPSQ membrane could be used for WVTR test. The thickness of stand-free BPSQ film used for test was about 20 μm, whose cross-section SEM image can be found in Figure S1. In this test, a special container with a circular hole in the middle was filled up sodium chloride saturated aqueous solution. Next, the container was sealed by a stand-free BPSQ film covered on the hole, and the void between film and the hole was sealed with vacuum sealant. Then this container with BPSQ film was placed into a sealed desiccator in which there was a certain amount of anhydrous calcium chloride. The weight of the anhydrous calcium chloride was measured every 12 h for the periods up to 72 h. Finally, WVTR value of BPSQ films was calculated from the weight change of the anhydrous calcium chloride on unit area and during unit time. The obtained WVTR values were normalized to a film thickness of 25 μm using the following equation for comparison with the reported values,

normalized WVTR = non‐normalized WVTR ×

l 25

(1)

All the WVTR values where l (μm) is an actual film thickness. shown below are normalized. The error range of the water vapor transmission rate measurements was ±1.0 g·m−2·d−1. 23,24

3. RESULTS AND DISCUSSION 3.1. Structure Determination of BSQ Precursor. 3.1.1. FT-IR. If we want to get the expected lamellar BPSQ bearing a pendant hydrophobic chain in organic bridge successfully, the precursor synthesis should be processed in the way as shown by Scheme 2. According to this ideal reaction, each N−H bond of monoamine molecule was involved in the ring-opening reaction of the epoxy group in GPTMS. Figure 1 shows FT-IR spectra of GPTMS, HA and HG-BSQ. In Figure 1a, the NH2 group in HA had two characteristic IR absorption bands at 3366 and 3289 cm−1 (stretching vibration of N−H), and one at around 1604 cm−1 (bending vibration of H−N− H).25 Besides, the weak shoulder peak at 3052 and 910 cm−1 in GPTMS spectrum shown in Figure 1b was corresponding to the stretching vibration of CH2 and the asymmetrical ring stretching vibration in epoxy group.25 These characteristic bands could not be detected in HG-BSQ spectrum (Figure 1c) suggesting that the epoxy rings of GPTMS had been opened and almost all the HA had been consumed up in the reaction. The same conclusion can be obtained from BG-BSQ and OGBSQ shown in Figure S2. C


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In mass spectra of BSQ precursors shown in Figure S3, the impurity peaks with higher m/z can be attributed to the fragment of some byproducts from the possible side-reaction shown by Figure S4. The corresponding fragment structures of these byproducts can be found in Figure S5 for the detail. 3.1.3. 1H Liquid NMR, 13C Liquid NMR, and 29Si Liquid NMR. To further define the chemical structure of BSQ precursors, liquid NMR experiments were carried out. Figure 2 shows 13C liquid NMR spectra of GPTMS, HA and HG-BSQ, which gave a clear indication of the reaction occurrence. Because there were little byproduct residues in BSQ precursors, some additional weak peaks can be found in Figure 2c. As the chemical structure of byproduct was very similar to that of corresponding BSQ precursors, the chemical shifts of byproduct were also close with that of BSQ precursors. The case for BG-BSQ and OG-BSQ shown in Figures S6 and S7 were same as that of HG-BSQ. The 13C NMR spectrum of HA (Figure 2a) was characterized by the signal at 41.9 ppm (C1) of the CH2 carbon attached to the primary amine group. The

Figure 1. FT-IR spectra of (a) HA, (b) GPTMS, and (c) HG-BSQ.

3.1.2. Elemental Analysis and MS. To confirm chemical components of BSQ products, elemental analysis, and MS researches were carried out. Elemental analysis results of three BSQ precursors were collected in Table 1. From Table 1, their experimental molecular formulas were obtained.

Table 2. 13C Chemical Shifts of Carbon Atoms in Different Molecules

Table 1. Elemental Components of Three BSQ Precursors element content (wt %) sample

C

H

O

Si

N

experimental molecular formula

BGBSQ HGBSQ OGBSQ

48.82

9.48

29.24

9.84

2.62

C22H51O5Si2N

50.67

9.53

27.83

9.54

2.43

C24H55O5Si2N

52.12

9.88

26.67

8.98

2.35

C26H59O5Si2N

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15

Figure S3 shows mass spectra of three BSQ precursors. The mass-to-charge ratio (m/z) and its corresponding fragment of the molecular ion peaks were collected in Tables S1, S2, and S3. In the case of HG-BSQ, m/z of the quasi-molecular ion peak was 574.3436, the molecular formula was determined as C24H55O5Si2N combined with its elemental analysis result. According to the structure of reactants and the fragments in mass spectra, the molecular structure of HG-BSQ could be inferred out. Therefore, it could be thought that the practical HG-BSQ synthesis was completed according to the mechanism shown in Scheme 2. The same results can be obtained for BGBSQ and OG-BSQ.

BA

HA

OA

GPTMS

41.9 36.2 20.1 13.9

42.4 34.1 31.9 26.7 22.8 14.1

42.4 34.0 32.0 29.6 29.4 27.0 22.8 14.1

49.5 4.8 22.7 73.1 71.3 50.5 43.2

BGBSQ

HGBSQ

OGBSQ

50.4 5.1 22.7 73.5 68.2 57.8 55.1 29.1 27.1 20.4 14.0

50.5 5.1 22.5 73.6 68.2 57.9 55.6 31.7 27.0 25.0 23.9 23.4 13.9

50.2 4.9 22.4 73.3 68.1 57.9 55.6 31.6 29.1 27.4 26.7 24.9 23.8 23.4 13.9

chemical shifts of 13C NMR signals were listed in Table 2. The 13 C NMR spectrum of GPTMS (Figure 2b) were characterized by the signals at 49.5 ppm (C6) and 43.2 ppm (C7) of the CH ring and CH2 carbons, respectively.19,26 All of these three

Figure 2. 13C liquid NMR spectra of (a) HA, (b) GPTMS, and (c) HG-BSQ. The peaks marked with asterisk (*) were signals of the byproduct. D


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was an important mean to identify different Si atoms according to their chemical shift in different chemical environment. No other obvious T0 signals were found suggested that the amount of byproducts was very small. Considering that so small amount of byproducts had little influence on the scientific conclusion, the byproducts could be neglected. Besides, only T0 signal also indicated that the BSQ precursor had not yet started the hydrolysis and condensation reaction. 3.2. Structure Determination of BPSQ. 3.2.1. 29Si Solid NMR. The 29Si MAS NMR spectra of HG-BPSQ gel aging for different times were shown in Figure 4, from which we can decide when the BPSQ sols could be used to deposit films. According to Figure 4, the suitable aging time was 7 days. At this moment, the polycondensation degree of Si−O−Si reached 98% and did not change. Signals produced by trifunctional silanes were generally symbolized as Tn, in which n represented the Si−O−Si number linked with a silicon atom, and Tn = −Si(OSi)n(OH)3−n (n = 0, 1, 2, 3).19,27 As shown in Figure 4c, four peaks are identified. The leftmost peak at −45.2 ppm was the characteristic peak of T0 species, −Si(OH)3. The next peak present at −52.7 ppm could be assigned to T1 structure, −Si(OSi) (OH)2, the following one at 60.1 ppm was T2 structure, −Si(OSi)2OH, and the rightmost peak at −66.3 ppm was attributed to T3 structure, −Si(OSi)3. The degree of condensation (DC) of the BPSQ can be calculated according to the following formula

signals were completely disappeared and two new signals at 57.9 and 55.6 ppm were present in 13C NMR spectrum of HGBSQ (Figure 2c), clearly indicating that the epoxy ring had been opened by the active hydrogen of HA. Figure 3 shows 1H liquid NMR spectra of three BSQ precursors. 1H liquid NMR spectra of reactants GPTMS, BA,

Figure 3. 1H liquid NMR spectra of (a) BG-BSQ, (b) HG-BSQ, and (c) HG-BSQ.

HA, and OA can be found in Figure S8. The structure details of BSQ precursors were in good agreement with the elemental analysis and MS results. The chemical shifts of the 1H NMR signals associated with different molecules were listed in Table 3. From Figure 3, a weak signal at around 4.1 ppm marked with

DC (%) =

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16

HA

OA

GPTMS

1.38 2.63 1.30 1.14 0.86

1.16 2.41 1.02 1.02 0.95 0.95 0.61

1.42 2.66 1.26 1.26 1.26 1.26 1.26 1.26 0.86

3.51 0.66 1.69 3.45 3.71 3.13 2.78

BGBSQ

HGBSQ

OGBSQ

3.57 0.66 1.68 3.43 3.44 3.49 3.86 2.59 2.30 1.49 1.27 0.88

3.57 0.65 1.68 3.42 3.42 3.48 3.85 2.59 2.30 1.47 1.28 1.28 1.28 0.89

3.57 0.66 1.68 3.43 3.44 3.49 3.86 2.58 2.30 1.47 1.40 1.39 1.35 1.32 1.30 0.93

(2)

where A(Tn) was the integral area of the Tn species derived from deconvolution calculation of the 29Si MAS NMR spectra.19,27 From Figure 4, DC values of 4%, 32%, 71%, 88%, and 98% were obtained for HG-BPSQ aging for 1, 3, 4, 5, and 7 days. In the same way, DC of 97% and 95% can be obtained for BG-BPSQ and OG-BPSQ respectively. The high condensation degree of three BPSQs made it possible to form the ideal BPSQ structure as shown in Scheme 2. 3.2.2. GIXRD. The practical structure of the obtained BPSQ can be further confirmed according to GIXRD analysis as follows. Figure 5 displays 2D GIXRD patterns of BPSQ films dip-coated on silicon wafer substrate. The ordinate axis is identified as the out-of-plane direction, whereas the abscissa axis is identified as the in-plane direction. The diffraction information on the out-of-plane direction reflects the nanostructure of sample with orientation parallel to the substrate, and the information on the in-plane direction reflect the nanostructure of sample with orientation perpendicular to the substrate. As GIXRD has a typical penetration depth which ranges between 10 and 100 nm, these results reflect statistic structure of sample films. Each diffraction ring present in 2D GIXRD image corresponds to a specific period ordered structure unit in film, and the continuous and uninterrupted diffraction ring illustrates that the nanostructure in film almost has no obvious difference between the out-of-plane direction and the in-plane direction. In this case, there are three continuous diffraction rings in each GIXRD pattern, suggesting that there are three periodic ordered units in the BPSQ film structure. Figure 6 shows one-dimensional GIXRD spectra derived from the 2D GIXRD patterns. In which the diffraction peak corresponds to the diffraction ring in Figure 5 at the same position. The broad peak indicated that the ordered nanostructure of BPSQ was far from perfect. The spacing

Table 3. 1H Chemical Shifts of Hydrogen Atoms in Different Molecules BA

2 1 A(T) A(T2) + A(T3) 1 + 3 3

asterisk (*) appeared in each spectrum of BSQ precursors, which might be ascribed to the signals of hydroxyl in methanol generated by side reaction shown in Figure S4. On the basis of the above analysis, the chemical structure of the BSQ precursors could be determined. Figure S9 shows 29Si liquid NMR spectra of three BSQ precursors. Each sample clearly shows only one signal with chemical shift at around 41.50 ppm, which was known as the 29 Si liquid NMR T0 signal of −Si(OR)3 groups.19,27 As NMR E


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Figure 4. 29Si MAS NMR spectra of HG-BPSQ gels aged for (a) 1, (b) 3, (c) 4, (d) 5, (e) 7 days.

Figure 5. 2D GIXRD images of (a) F-BG, (b) F-HG, and (c) F-OG. The yellow dotted lines show the position of the diffraction rings.

of organic bridge separating inorganic region, and d3 was the characteristic periodic arrangement distance of the inorganic Si−O−Si structure. Combining the above discussions, we simulated a 3D structure of BPSQ based on the theoretical speculation and calculation, which was shown in Figure 7a. As shown in Figure 6 and Table 4, increasing length of the alkyl chains in organic bridge (from 4−8 carbon atoms) resulted in the increase of d1 value, whereas d2 and d3 almost have no change. As mentioned above, d1 was the spacing between two adjacent lamellae. The value of d1 spacing was related with two kinds of interactions in BPSQ structure. One was the interaction between adjacent molecular layers,28,29 and the other was between alkyl chains filled in the adjacent molecular layers.30,31 The interaction strength between two adjacent molecular layers was associated with chemical structure of molecular layers, while the interaction strength between alkyl chains was related to its length. Therefore, when these two kinds of interaction reached a balance state, the layer spacing between adjacent molecular layers must be different, and the voids between alkyl chains were also different. Interestingly, HG-BPSQ was the only one whose experimental d1 was almost equal to the theoretical value that was just the distance of a tail to tail association of the two pendant hexane groups. This indicated that when the alkyl chain length was 6 carbon atoms, the structure of the obtained BPSQ might be denser than the other two BPSQs. The experimental d1 of BGBPSQ and OG-BPSQ were 1.52 and 2.61 nm, longer than their theoretical value 1.13 and 2.26 nm respectively, indicating that the lamellar spacing of BG-BPSQ and OG-BPSQ were not filled up by alkyl chains and there were some voids left between alkyl chains. This phenomenon might be interpreted by the stronger repulsion of the alkyl chain than its filling effect as well. Figure 7b showed possible structures of the three BPSQs. The

Figure 6. One-dimensional GIXRD spectra of BPSQ films.

distance (d) related to the periodic ordered units could be calculated according to the following equation: d = λ/2*sin(Θ), where λ was the wavelength of the incident X-ray and Θ was the angle between the incident X-ray and sample surface. The corresponding experimental and theoretically calculated values of three BPSQ films were shown in Table 4. The theoretical d values were estimated from the interatomic distances and bond angles using the software ChemiBio 3D Ultra. d1 was the spacing between the two adjacent lamellae; d2 was the distance Table 4. Distances Corresponding to Diffraction Peaks in GIXRD Spectra of BPSQ Films experimental value (nm)

theoretical value (nm)

sample

d1

d2

d3

d1

d2

d3

F-BG F-HG F-OG

1.52 1.70 2.61

0.93 0.90 0.91

0.43 0.42 0.43

1.13 1.69 2.26

1.90 1.90 1.90

0.43 0.43 0.43 F


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Figure 7. (a) Theoretical simulated 3D-structure of BPSQ, (b) the possible structures of three BPSQ, and (c) the overall structure of BPSQ film.

refractive index of BPSQ films at 630 nm was 1.41 ± 0.03 for FBG, 1.45 ± 0.03 for F-HG, and 1.43 ± 0.03 for F-OG. Considering the refractive index as a probe of film density, it was obvious that HG-BPSQ was denser than the others, which agreed with the above results. In addition, as the distance d2 meant the length of the organic bridge separating two Si atoms in BPSQ, the three BPSQs should have the same d2 value. In fact, the experimental d2 values of BG-BPSQ, HG-BPSQ and OG-BPSQ were very similar but were shorter than the theoretical calculated value 1.90 nm, indicating that the organic bridging chain was not fully expanded but presented a bending state in practice. The experimental d3 of three BPSQs were about 0.43 nm, very close to the theoretical calculated value. All of these confirmed the inferred structure of BPSQ shown in Figure 7a and 7b. The ordered regions in BPSQ film could be detected in both out-of-plane direction and in-plane direction, suggesting that the ordered structure did not have a fixed orientation. Taken together, the overall structure of BPSQ film was shown in Figure 7c. 3.3. Morphology of BPSQ Films. The surface morphologies of BPSQ films are shown in the AFM height images in Figure 8a−c. There are no obvious particle boundaries observed on film surface, indicating that the film surface was smooth. Besides, the roughness of film was so low that the light scattering generated from the film surface could be neglected. Therefore, the as-prepared BPSQ films had potential applications in the field of optical films. Figures 8d and S10 show the SEM images of BPSQ films. Some ordered layer-like regions can be clearly observed in SEM images suggesting that the film had lamellar nanostructures. AFM surface analysis and SEM oblique-section analysis all indicated that there were no pores visible in BPSQ films. Furthermore, the specific surface area of three BPSQs was too low to be detected by N2 adsorption−desorption technology, suggesting that there was no open pores in our BPSQ films. Therefore, they should have good water vapor barrier performance. 3.4. Thermal Stability of BPSQ. Figure 9 shows the thermal decomposition behavior of BPSQ under N2 flow. Three stages are observed from the TG curves. The first stage of the mass loss were attributed to the volatilization of the solvent resided in BPSQ gel system. With temperature rising in the second stage, the mass of BPSQ decreased sharply because

Figure 8. AFM height images (5 μm × 5 μm) of films (a) F-BG, (b) F-HG, (c) F-OG, and (d) SEM image of the oblique-section of F-HG.

Figure 9. TGA curves of the BPSQ films under nitrogen flow. Td10 were noted as the temperature at which 10% weight loss.

the high temperature made the organic chain (C−C bonds and C−H bonds) decomposed. After the organic structure were decomposed thoroughly, there were mainly inorganic SiO2 framework. Therefore, the mass did not change and the TG curve tend to be a horizontal line in the third stage. To study G


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Figure 10. Average transmittance in the wavelength range of 300−1200 nm of (a) bare KDP and KDP coated with different BPSQ films (b) bare DKDP and DKDP coated with different BPSQ films before and after treated under 60% RH at 25 °C for 7 months.

alkyl chain might be the optimum one to satisfy these needs. From the AT variation of different treated DKDP shown in Figure 10b, the same conclusion could be obtained. To further verify the effect of filling voids between molecular lamellae with alkyl chains, we tried to prepare a lamellar material without pendant alkyl groups for comparison, but there was no appropriate amine could be reacted with GPTMS to synthesis a BSQ precursor without pendant alkyl groups. Methylamine seemed useful to act as a bridge to obtain a similar lamellar material with only a methyl group. Unfortunately, methylamine is a dangerous flammable gas under normal temperature and pressure, so that it is very difficult to control the reaction between methylamine and GPTMS. Considering the experiment safety, we chose isopropyl amine (IA) as the bridge molecule to prepare a similar material with shorter alkyl chain for comparison. The comparison result shown in Figure 10 suggested that the water vapor barrier performance of IG-BPSQ film was far weaker than the three BPSQ films with C4, C6, and C8 pendant chains, illustrating that our idea filling the voids between molecular lamellae with alkyl chains was benefit for water vapor barrier property of BPSQ films. The WVTR value of the as-prepared BPSQ film was measured according to a modified ASTM (1995) method E96. Table 5 shows normalized water vapor transmission rate

the thermal stability of three BPSQ, Td10 of BG-BPSQ, HGBPSQ, and OG-BPSQ under N2 flow were counted as 371 °C, 357 and 348 °C respectively, which were comparable to that of polyhedral oligomer silsesquioxane32 (Td10 = 368 °C) and siloxane-based lamellar hybrid material9 (Td10 = 400 °C). This showed that the as-prepared BPSQ had a satisfactory thermal stability. 3.5. Water Vapor Barrier Performance of BPSQ Films. To verify the water vapor barrier performance of the asprepared BPSQ films, two experiments were designed. The first experiment was to study the moisture barrier performance of the BPSQ film in a defined moist environment on KDP and DKDP crystals, which were easy deliquesced by absorbing moisture in the air, resulting in the decline of its optical transmittance. Here, we defined the average transmittance as AT, whereas AT = ∫ λλ11 Tdλ/(λ2 − λ1). λ was the wavelength of ultraviolet spectrum. The AT variation of bare crystal and crystal coated with different BPSQ films before and after treated under relative humidity of 60% at 25 °C for 7 months were shown in Figure 10, and the original transmittance spectra of different treated crystals are shown in Figures S11 and S12. From Figure 10a, AT of bare KDP declined much more than KDP coated with BPSQ films, suggesting that these films were good protection for KDP crystals from the moisture. As there were so many hydroxyl groups in the BPSQ structure, and the water contact angle of BPSQ films were about 70° (Figure S13), the good moisture barrier property of the films should be attributed to its special structure shown in Scheme 1b. As mentioned in GIXRD result, the lamellar units in BPSQ film did not have the fixed orientation parallel to the substrate. Therefore, the good moisture barrier property of the BPSQ films powerfully confirmed that our idea was practicable. When the voids between molecular lamellae were filled up with flexible organic chains, the barrier property of BPSQ films would have little to do with the orientation of the lamellar units. Besides that, the AT decline value of F-BG was only 1.74% slightly lower than other films, indicating that the F-HG had better moisture barrier performance and it was attributed to its denser structure for there still were some voids existed in BGBPSQ and OG-BPSQ molecular lamellae shown in Figure 7b. This also proved the inference that the alkyl chain could not only fill the void between the adjacent two lamellae, but also extend the spacing of the adjacent two lamellae because of its structural space effect. However, there was a balanced state, which the alkyl chains could fill the voids between lamellae effectively while the generated expansion effect could be neglected. According to the above result, 6 carbon atoms of

Table 5. Normalized Water Vapor Transmission Rate of Different Polymer Films at 25°C no. 1 2 3 4 5 6 7 8 9 10

WVTR (g·m−2·d−1)

film 33

polyethylene polypropylene33,34 polyurethane35 poly-L-lactide23 siloxane-based hybrid lamellae9 ladderlike BPSQ7 poly(e-caprolactone)24 F-BG F-HG F-OG

New Water Vapor Barrier Film Based on Lamellar ... - ACS Publications

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