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Preparation of nanoporous poly(methyl silsesquioxane) films using core-shell silsesquioxane as porogen
Materials Chemistry and Physics 114 (2009) 736–741
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation of nanoporous poly(methyl silsesquioxane) films using core-shell silsesquioxane as porogen Hung-Wen Su a,c , Wen-Chang Chen a,b,∗ a
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4 Roosevelt Rd., Taipei 106, Taiwan c AGI Corporation, Taipei 114, Taiwan b
a r t i c l e
i n f o
Article history: Received 11 April 2008 Received in revised form 30 September 2008 Accepted 12 October 2008 Keywords: Microporous materials Thin films Poly(methyl silsesquioxane) Porogen
a b s t r a c t Nanoporous poly(methyl silsesquioxane) (PMSSQ) thin films were successfully prepared using a new thermally sacrificing porogen, dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS). The core-shell silsesquioxane was synthesized from Acrylo POSS with 3dimethylaminopropylamine (DMAPA) and 2-dimethylamino ethyl acrylate (DMAEA) using Michael addition reaction. Miscible hybrid materials were obtained through strong hydrogen-bonding interaction between the Si–OH end group in MSSQ and the tertiary amino groups in DMA-POSS. Nanopores in PMSSQ matrix were generated by thermal decomposing DMA-POSS at 425 ◦ C. The AFM and FESEM studies suggested that nanopores were homogeneously distributed in the prepared thin films. As the porosity increased up to 16.4%, the refractive index and the dielectric constant of the nanoporous films were decreased from 1.379 to 1.307 and 2.8–2.2, respectively. The tertiary amino substitutes could promote the silanol condensation and improve the mechanical properties. Therefore, the hardness retained around 1 GPa as the porosity increased. The low-dielectric constant, good thermal and mechanical property suggested the potential applications of these films as low-dielectric constant materials. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanoporous thin films possess unique physicochemical properties [1] and are very attractive for various applications, especially for low-dielectric constant materials [2–5]. Poly(methyl silsesquioxanes) (PMSSQ) are promising candidates for low dielectrics as their inherent low-dielectric constant (k = 2.8), low-water uptake, and high-thermal stability. For ultralow-k applications, incorporating nanopore of air (k = 1) into polymer matrix is a general approach to reduce the dielectric constant [2]. One potential approach to generate nanoporous films is to use sacrificial porogen approach [3]: the selective removal of thermally liable pore generator (porogen) from phaseseparated organic/inorganic matrices. The successful porogens include dendrimers [6,7], star-shaped polymers [8–12], nitrogen containing copolymers [13–15], amphiphilic block copolymers [16–19], and others [20–24]. The pore morphology generated by this approach strongly depends on the composition of the
∗ Corresponding author at: Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4 Roosevelt Rd., Taipei 106, Taiwan. Tel.: +886 2 23628398; fax: +886 2 23623040. E-mail address: [email protected] (W.-C. Chen). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.10.035
hybrid, the interaction between matrix and porogen, and the molecular architecture of the porogen. To prevent macroscopic phase separation, the porogen must be initially miscible in the matrix by chemical or physical bond. For example, the nitrogen atom in poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) (P(MMA-co-DMAEMA)) [14] or poly(styrene-block2-vinylpyridine) (PS-b-P2VP) [14] might have a hydrogen bond with the Si–OH of PMSSQ to promote the miscibility of each other. We demonstrated that the molecular architecture of porogen played an important role in the morphological formation of the MSSQ-based mesoporous materials [25]. Octafunctional silsesquioxanes with a size of 1.2–1.4 nm in diameter have been shown as a potential porogen for generating nanoporous materials [26–29], such as octa(2,4dinitrophenyl)-silsesquioxane (ODNPSQ) [28] and poly(ethylene oxide)-polyhedral oligomeric silsesquioxane (PEO-POSS) [29]. However, specific chemical or physical bonding in the above silsesquioxane with PMSSQ has not been fully explored yet. In this study, dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS) with a core-shell structure (one silsesquioxane core covered by dimethylamino groups as shell) was designed to as the new porogen for imprinting nanopores in poly(methyl sisesquioxane) films. DMA-POSS was synthesized from Acrylo POSS with 3-dimethylaminopropylamine (DMAPA) and 2-
H.-W. Su, W.-C. Chen / Materials Chemistry and Physics 114 (2009) 736–741
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Scheme 1. Synthesis of the porogen DMA-POSS and preparation of the nanoporous PMSSQ films: (a) Michael addition, (b) blending DMA-POSS and PMSSQ precursor, and (c) thermal curing and pyrolysis.
dimethylamino ethyl acrylate (DMAEA) using Michael addition reaction [30,31], as shown in Scheme 1. The PMSSQ precursor was synthesized according to our previous report [32]. Miscible hybrid materials would be obtained through the proposed intermolecular hydrogen bond between the tertiary amino group of DMA-POSS and the Si–OH group of PMSSQ. Then, it was followed by multi-step curing to obtain porous PMSSQ film. The tertiary amino substitutes could promote the silanol condensation and improve the mechanical properties. The morphology was characterized by AFM and FESEM. The properties of the obtained porous films were analyzed, including refractive index, dielectric constant, and hardness. The experimental results suggested the formation of nanopores in the PMSSQ films and thus low-dielectric constant films with good film hardness were obtained. 2. Experimental 2.1. Materials Acrylo POSS@ cage mixture (MA0736, (C6 H9 O2 )n (SiO1.5 )n , n = 8, 10, 12) with Mw = 1322 was purchased from Hybrid Plastics Co. (Hattiesburg, USA). 3-dimethylaminopropylamine (DMAPA, 99%) and 2-dimethylaminoethyl ethyl acrylate (DMAEA, 98%) were purchased from Acros (Gell, Belgium) and Alfa Aesar (Ward Hill, USA), respectively. Tetrahydrofuran (THF, 99.9%) and n-butyl alcohol (BuOH, 99.9%) were obtained from TEDIA (Fairfield, USA). Hydrochloric acid (HCl, 37.5%) and methyl trimethoxysilane (MTMS, 98%) were obtained from Scharlau Chemie (Barcelona, Spain) and Aldrich (St. Louis, USA), respectively. 2.2. Synthesis of dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS) 7.32 g (0.044 mole of acrylic group) of MA0736 dissolved in 14 ml of THF was added into a three-necked bottle equipped with stirrer and nitrogen purging. Under cooling (0–5 ◦ C), 4.50 g (0.044 mol) of DMAPA dissolved in 6 ml of
THF was added during 1 h. After the cooling bath was removed, the mixture was allowed to stand overnight at room temperature. Then, 6.30 g (0.044 mol) of DMAEA dissolved in 6 ml of THF was added and the mixture stood overnight at room temperature. Subsequently, THF and un-reacted monomers were evaporated using vacuum rotary at 100 ◦ C. The products obtained were slightly yellow, viscous oils. The reaction yield of the above Michael addition reaction was about 85.4% based on the estimation of tertiary amine composition. 1 H NMR (ppm) (d-CDCl3 ): ı = 0.6 (Si–CH2 ), ı = 1.6 and 1.7 (C–CH2 –C), ı = 2.2 and 2.3 (N–CH3 ), ı = 2.4–2.8 (CH2 ), ı = 4 and 4.1 (ester CH2 ). FTIR (cm−1 ): 2950 (CH2 ), 2883 and 2760 (N–CH3 ), 1730 (C O).
2.3. Synthesis of poly(methyl silsesquioxane) precursor (PMSSQ) The synthesis and characterization of PMSSQ precursor was according to our previous reports [32], except that only THF was used as solvent. PMSSQ precursor was synthesized from MTMS in THF by sol–gel process using the molar ratio of the water to MTMS equal to 3.0 and pH of 2.0. The weight average molecular weight of the prepared PMSSQ determined by GPC was 3880. The obtained PMSSQ precursor consisted of both symmetric (cage-like, 1120 cm−1 ) and non-symmetric (networklike, 1030 cm−1 ) structures were identified by FTIR. The OH content estimated from FTIR analysis was 6.72% from the comparison with a reference PMSSQ sample from Gelest Co. with a reported OH content of 5%.
2.4. Preparation of the nanoporous PMSSQ films using DMA-POSS as templates The PMSSQ precursor in THF solution and the porogen of DMA-POSS were mixed with different ratio. The number (X) of the obtained nanoporous PMSSQ films (PMSSQ-X) was the weight percent of DMA-POSS loading, as listed in Table 1. Another solvent, BuOH, was used to adjust the solid content of the solution to be 20 wt%. Then, the solution was loaded into a syringe and filtered through a 0.2 m PTFE filter. The filtered solution was dropped directly onto a clean Si(1 0 0) wafer and the coated substrate was spun at 2000 rpm for 20 s. The thermal curing of the PMSSQ and the pyrolysis of the porogen were carried out in a quartz furnace at 2 ◦ C min−1 to 425 ◦ C and held for 2 h under nitrogen purge. The thickness of the nanoporous PMSSQ film was 500–1000 nm depending on the spin-coating process.
Table 1 Properties of nanoporous PMSSQ thin films.
PMSSQ PMSSQ-10 PMSSQ-20 PMSSQ-30 PMSSQ-40
Porogen (wt%)
Porosity (%)
Thickness (nm)
Rq (nm)
Refractive index
Dielectric constant
Hardness (GPa)
0 10 20 30 40
0 4.3 8.7 12.6 16.4
890 810 650 625 530
0.194 0.224 0.242 0.301 0.449
1.379 1.358 1.340 1.324 1.307
2.8 2.7 2.4 2.3 2.2
1.19 1.00 1.08 1.07 1.06
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Fig. 1.
1
H NMR spectra of (a) Acrylo POSS and (b) DMA-POSS in CDCl3 .
2.5. Characterization 1
H NMR spectra were recorded by Bruker Avance DRX 500 MHz spectrometer in CDCl3 . FTIR spectra of the prepared films on the doubly polished silicon wafers were obtained with a PerkinElmer PARAGON 1000. TGA thermal analysis was con-
Fig. 3. FTIR absorption spectra of as-spun PMSSQ/(DMA-POSS) hybrid thin films before pyrolysis in the wavenumber ranging from 875 to 1000 cm−1 : (a) PMSSQ, (b) PMSSQ-10, (c) PMSSQ-20, (d) PMSSQ-30, and (e) PMSSQ-40.
ducted on a PerkinElmer pyris 1 TGA under continuous nitrogen flow, at a heating rate 10 ◦ C min−1 . An atomic force microscope (AFM, Nanoscope Inc., Model DI 5000) was operating in the tapping mode with the silicon cantilever/tips (Nano-Sensors, NCH-type) to probe the surface morphology of the prepared thin films. The silicon tips with diameter of 10–20 nm, force constant of 42 N m−1 , and resonance frequency of 250–270 kHz were used. Field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F) were used to examine the surface morphology and the inner structure of the coated films. The MTS Nano Indenter was used to measure the hardness. Dielectric constant was measured by mercury (Hg) probe current–voltage (I–V) method at 1 MHz. Refractive index and the film thickness were measured using a Metricon Prism Coupler (Model 2100). The porosity of the prepared nanoporous
Fig. 2. FTIR absorption spectra of (a) DMA-POSS, (b) PMSSQ, (c) PMSSQ-40 before thermal pyrolysis, and (d) PMSSQ-40 after thermal pyrolysis.
Fig. 4. TGA curves of (a) PMSSQ, (b) PMSSQ-20, and (c) DMA-POSS before thermal pyrolysis.
H.-W. Su, W.-C. Chen / Materials Chemistry and Physics 114 (2009) 736–741 PMSSQ films was determined by the Maxwell–Garnett mode as Eq. (1) [18]: n2PMSSQ − n2air
n −1 = (1 − p) 2 n2 + 2 nPMSSQ + 2n2air 2
(1)
where n, nPMSSQ , and nair are the refractive index of the prepared nanoporous PMSSQ film, the parent PMSSQ film and the air, respectively. p is the porosity of the nanoporous PMSSQ film relative to the parent PMSSQ film
3. Results and discussion Fig. 1 shows the 1 H NMR spectrum of the original Acrylo POSS and the synthesized DMA-POSS in CDCl3 . The resonances of the
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acrylic groups in Acrylo POSS (ı = 5.8, 6.1, and 6.4 ppm) are completely disappeared after Michael addition. The proton resonances of the dimethylamino groups in DMAPA and DMAEA are clearly observed at 2.2 and 2.3 ppm, respectively. It suggests that DMAPA and DMAEA have been successfully incorporated into POSS using the Michael addition [30,31]. The resonance of the Si–CH2 – appears at 0.6 ppm confirm the successful linkage of inorganic core (POSS) and organic shell (DMA). The vinyl group of acrylate, primary and secondary amine peaks at around 1640, 1650 and 1560 cm−1 , respectively, disappear in the FTIR spectrum of Fig. 2(a). Meanwhile, the dimethylamino bands, resulted from the two proton sponges of the nitrogen, are observed at 2883 and 2760 cm−1 .
Fig. 5. AFM images of the nanoporous PMSSQ films: (a) PMSSQ, (b) PMSSQ-10, (c) PMSSQ-20 (1 m × 1 m), (d) PMSSQ-20 (500 nm × 500 nm), (e) PMSSQ-30, and (f) PMSSQ-40.
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The NMR and FTIR results suggest the successful synthesis of DMA-POSS. Fig. 2(b) and (c) show the FTIR spectra of PMSSQ and PMSSQ40 films before thermal curing, respectively. The primary bands of interest for PMSSQ include the OH stretching mode in the 3100–3600 cm−1 regions and the Si–OH stretching vibration at 905 cm−1 . After the porogen DMA-POSS is loading, the carbonyl (–C O) stretching and the dimethylamino bands (N–CH3 ) are observed at 1730 and 2700–2900 cm−1 , respectively. Besides, the peak intensities of silanol bands at 3100–3600 are significantly decreased and the other silanol band at 905 cm−1 of PMSSQ is shifted to 940 cm−1 of PMSSQ-40. Silanol condensation catalyzed by amine is well known and leads to improved mechanical properties of the cured PMSSQ [15,33,34]. The mechanism might be that the proton released by the amine is transferred to the PMSSQ, producing a protonated leaving group that can be displaced by water in an SN2 reaction. Therefore, the above results suggest that the tertiary amino groups in DMA-POSS could serve as a catalyst for the silanol condensation reaction. Fig. 2(d) shows the FTIR spectrum of PMSSQ-40 film after pyrolysis, in which no organic peaks are observed. Also, the PMSSQ peaks are observed at 2950 cm−1 (SiCH3 ), 1275 cm−1 (SiCH3 ), 1120 cm−1 (Si–O–Si, sym), and 1030 cm−1 (Si–O–Si, non-sym), respectively. Hydrogen bonding interaction between the organic and inorganic moieties is very important to avoid the macrophase separation and leads to homogeneously nanoscopic pores. Fig. 3 shows the absorption bands assigned to the Si–O stretching on the Si–OH bands of the PMSSQ/DMA-POSS hybrid films. As observed in Fig. 3, the position of the Si–OH band gradually shifts from 905 to 940 cm−1 as increasing the loading of DMA-POSS. It indicates that the formation of the hydrogen bond between the silanol groups of PMSSQ and the tertiary amine groups or the carbonyl groups of DMA-POSS. Such hydrogen bond decreases the bond strength between the H and Si–O of the Si–OH bond, thus results in the shifting of the Si–O stretching band to a higher wavenumber [18]. The thermal decomposition of DMA-POSS alone and blend with PMSSQ have been monitored by thermal gravimetric analysis, as shown in Fig. 4. The thermal decomposition of DMA-POSS occurs in three distinct steps, 200–300, 300–450, and 450–600 ◦ C, respectively. The weight loss of 200–300 and 300–450 ◦ C might be attributed from the loss of dimethylamino and acrylate segments, respectively. These results are similar to the blend of PMSSQ/P(MMA-co-DMAEMA) nanocomposites [15]. As shown in Fig. 4(c), the organic fraction of DMA is almost decomposed in the temperature range of 400–500 ◦ C. The thermal decomposition of PMSSQ with DMA-POSS (PMSSQ-20) at different stage is similar to DMA-POSS alone. The char yield at 800 ◦ C is about 79 wt%, which is close to the theoretical value (20 wt%) of organic fraction in the hybrid materials. It suggests that DMA-POSS could be an ideal porogen for preparing nanoporous materials. Understanding the surface morphology of the porous thin films could help us elucidate the phase separation in the hybrid. Fig. 5 shows the AFM micrographs of the nanoporous PMSSQ thin films imprinted by DMA-POSS loading ranging from 0 to 40 wt%. For the case of PMSSQ and PMSSQ-10, there is no clear domain size shown on the surface and the surface roughness (Rq) is very small (0.194 nm for PMSSQ and 0.224 nm for PMSSQ-10). Since the porogen size of DMA-POSS is around 1.2–1.4 nm in diameter, the relatively smooth surface may suggest that the pore size is too small to be observed in the hybrid film. With increasing the porogen loading, for PMSSQ-20, uniform spherical pores are shown on the surface and the size estimated from the AFM height profile image is around 5–10 nm. The larger pore size than that single DMA-POSS suggests its self-aggregation at a high-porogen loading. However, such pore size is still smaller than 10 nm and the film surface is
Fig. 6. FESEM image of the nanoporous PMSSQ thin films: (a) PMSSQ-10, (b) PMSSQ20, (c) PMSSQ-30, and (d) PMSSQ-40.
relative smooth (Rq = 0.242 nm). In our previous study using PS-bP2VP as porogen at the same loading ratio, uniform spherical pores with the size are around 13–15 nm [18]. It suggests the DMA-POSS could be a superior porogen for generating nanoporous materials than PS-b-P2VP. When the DMA-POSS porogen loads up to 30 and 40%, the isolated pore is gradually aggregated together and the surface roughness is increased (Rq = 0.301 nm for PMSSQ-30 and Rq = 0.449 nm for PMSSQ-40), as shown in Fig. 5. Fig. 6 shows the cross-sectional FESEM images of the nanoporous thin films. For PMSSQ-10 and PMSSQ-20, very uniform PMSSQ images without significant aggregation are observed in Fig. 6(a) and (b). The images of PMSSQ-30 and PMSSQ-40 exhibit slightly rough surface but no significant porous domain, as shown in Fig. 6(c) and (d). It suggests the porosity in the PMSSQ film is relatively small. The refractive index, the dielectric constant and the porosity estimated by Eq. (1) are listed in Table 1. As the DMA-POSS loading increases from 0 to 40 wt%, the refractive index decreases from 1.379 to 1.307 and the estimated porosity increases from 0 to 16.4%, as shown in Fig. 7. The corresponding dielectric constant decreases from 2.8 to 2.2, respectively. The incorporated voids in the PMSSQ matrix are contributed to the decreased refractive index and dielectric constant. The hardness of the nanoporous PMSSQ films is quantified using nanoindentation measurements [17], shown in Table 1. The hardness of the pure PMSSQ film is about 1.19 GPa
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Taiwan University (Excellent Research Projects), and Ministry of Economic Affairs of Taiwan (96-EC-17-A-08-S1-015) are highly appreciated. References
Fig. 7. Variations on the refractive index and the porosity of the nanoporous film with the porogen loading ratio.
and retains the high hardness of 1 GPa even at the porosity of 16.4%. The retention of these small-cage (POSS) structures dispersed homogeneously in PMSSQ matrix is the primary reason for improved hardness. Meanwhile, the tertiary amine group probably catalyzes the silanol condensation during thermal curing and improves the mechanical properties. It explains the high hardness for the prepared nanoporous materials. These results demonstrate that nanoporous PMSSQ thin film has been successfully created using DMA-POSS porogen. 4. Conclusions Nanoporous poly(methyl silsesquioxane) PMSSQ thin films were successfully fabricated through thermally sacrificing porogen of dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS). Nanopores were homogeneously dispersed in PMSSQ matrix through the hydrogen-bonding interaction and composition optimization. The low-dielectric constant (k = 2.2) of porous PMSSQ was obtained by incorporating 16.4% porosity and the hardness retained at about 1 GPa. The present study suggests that the low-dielectric constant porous materials could be successfully prepared using the new core-shell silsesquioxane. Acknowledgements The financial supports from Taiwan Semiconductor Co., National Science Council of Taiwan (NSC 97-2221-E002-024-MY3), National
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation of nanoporous poly(methyl silsesquioxane) films using core-shell silsesquioxane as porogen Hung-Wen Su a,c , Wen-Chang Chen a,b,∗ a
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4 Roosevelt Rd., Taipei 106, Taiwan c AGI Corporation, Taipei 114, Taiwan b
a r t i c l e
i n f o
Article history: Received 11 April 2008 Received in revised form 30 September 2008 Accepted 12 October 2008 Keywords: Microporous materials Thin films Poly(methyl silsesquioxane) Porogen
a b s t r a c t Nanoporous poly(methyl silsesquioxane) (PMSSQ) thin films were successfully prepared using a new thermally sacrificing porogen, dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS). The core-shell silsesquioxane was synthesized from Acrylo POSS with 3dimethylaminopropylamine (DMAPA) and 2-dimethylamino ethyl acrylate (DMAEA) using Michael addition reaction. Miscible hybrid materials were obtained through strong hydrogen-bonding interaction between the Si–OH end group in MSSQ and the tertiary amino groups in DMA-POSS. Nanopores in PMSSQ matrix were generated by thermal decomposing DMA-POSS at 425 ◦ C. The AFM and FESEM studies suggested that nanopores were homogeneously distributed in the prepared thin films. As the porosity increased up to 16.4%, the refractive index and the dielectric constant of the nanoporous films were decreased from 1.379 to 1.307 and 2.8–2.2, respectively. The tertiary amino substitutes could promote the silanol condensation and improve the mechanical properties. Therefore, the hardness retained around 1 GPa as the porosity increased. The low-dielectric constant, good thermal and mechanical property suggested the potential applications of these films as low-dielectric constant materials. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanoporous thin films possess unique physicochemical properties [1] and are very attractive for various applications, especially for low-dielectric constant materials [2–5]. Poly(methyl silsesquioxanes) (PMSSQ) are promising candidates for low dielectrics as their inherent low-dielectric constant (k = 2.8), low-water uptake, and high-thermal stability. For ultralow-k applications, incorporating nanopore of air (k = 1) into polymer matrix is a general approach to reduce the dielectric constant [2]. One potential approach to generate nanoporous films is to use sacrificial porogen approach [3]: the selective removal of thermally liable pore generator (porogen) from phaseseparated organic/inorganic matrices. The successful porogens include dendrimers [6,7], star-shaped polymers [8–12], nitrogen containing copolymers [13–15], amphiphilic block copolymers [16–19], and others [20–24]. The pore morphology generated by this approach strongly depends on the composition of the
∗ Corresponding author at: Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4 Roosevelt Rd., Taipei 106, Taiwan. Tel.: +886 2 23628398; fax: +886 2 23623040. E-mail address: [email protected] (W.-C. Chen). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.10.035
hybrid, the interaction between matrix and porogen, and the molecular architecture of the porogen. To prevent macroscopic phase separation, the porogen must be initially miscible in the matrix by chemical or physical bond. For example, the nitrogen atom in poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) (P(MMA-co-DMAEMA)) [14] or poly(styrene-block2-vinylpyridine) (PS-b-P2VP) [14] might have a hydrogen bond with the Si–OH of PMSSQ to promote the miscibility of each other. We demonstrated that the molecular architecture of porogen played an important role in the morphological formation of the MSSQ-based mesoporous materials [25]. Octafunctional silsesquioxanes with a size of 1.2–1.4 nm in diameter have been shown as a potential porogen for generating nanoporous materials [26–29], such as octa(2,4dinitrophenyl)-silsesquioxane (ODNPSQ) [28] and poly(ethylene oxide)-polyhedral oligomeric silsesquioxane (PEO-POSS) [29]. However, specific chemical or physical bonding in the above silsesquioxane with PMSSQ has not been fully explored yet. In this study, dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS) with a core-shell structure (one silsesquioxane core covered by dimethylamino groups as shell) was designed to as the new porogen for imprinting nanopores in poly(methyl sisesquioxane) films. DMA-POSS was synthesized from Acrylo POSS with 3-dimethylaminopropylamine (DMAPA) and 2-
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Scheme 1. Synthesis of the porogen DMA-POSS and preparation of the nanoporous PMSSQ films: (a) Michael addition, (b) blending DMA-POSS and PMSSQ precursor, and (c) thermal curing and pyrolysis.
dimethylamino ethyl acrylate (DMAEA) using Michael addition reaction [30,31], as shown in Scheme 1. The PMSSQ precursor was synthesized according to our previous report [32]. Miscible hybrid materials would be obtained through the proposed intermolecular hydrogen bond between the tertiary amino group of DMA-POSS and the Si–OH group of PMSSQ. Then, it was followed by multi-step curing to obtain porous PMSSQ film. The tertiary amino substitutes could promote the silanol condensation and improve the mechanical properties. The morphology was characterized by AFM and FESEM. The properties of the obtained porous films were analyzed, including refractive index, dielectric constant, and hardness. The experimental results suggested the formation of nanopores in the PMSSQ films and thus low-dielectric constant films with good film hardness were obtained. 2. Experimental 2.1. Materials Acrylo POSS@ cage mixture (MA0736, (C6 H9 O2 )n (SiO1.5 )n , n = 8, 10, 12) with Mw = 1322 was purchased from Hybrid Plastics Co. (Hattiesburg, USA). 3-dimethylaminopropylamine (DMAPA, 99%) and 2-dimethylaminoethyl ethyl acrylate (DMAEA, 98%) were purchased from Acros (Gell, Belgium) and Alfa Aesar (Ward Hill, USA), respectively. Tetrahydrofuran (THF, 99.9%) and n-butyl alcohol (BuOH, 99.9%) were obtained from TEDIA (Fairfield, USA). Hydrochloric acid (HCl, 37.5%) and methyl trimethoxysilane (MTMS, 98%) were obtained from Scharlau Chemie (Barcelona, Spain) and Aldrich (St. Louis, USA), respectively. 2.2. Synthesis of dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS) 7.32 g (0.044 mole of acrylic group) of MA0736 dissolved in 14 ml of THF was added into a three-necked bottle equipped with stirrer and nitrogen purging. Under cooling (0–5 ◦ C), 4.50 g (0.044 mol) of DMAPA dissolved in 6 ml of
THF was added during 1 h. After the cooling bath was removed, the mixture was allowed to stand overnight at room temperature. Then, 6.30 g (0.044 mol) of DMAEA dissolved in 6 ml of THF was added and the mixture stood overnight at room temperature. Subsequently, THF and un-reacted monomers were evaporated using vacuum rotary at 100 ◦ C. The products obtained were slightly yellow, viscous oils. The reaction yield of the above Michael addition reaction was about 85.4% based on the estimation of tertiary amine composition. 1 H NMR (ppm) (d-CDCl3 ): ı = 0.6 (Si–CH2 ), ı = 1.6 and 1.7 (C–CH2 –C), ı = 2.2 and 2.3 (N–CH3 ), ı = 2.4–2.8 (CH2 ), ı = 4 and 4.1 (ester CH2 ). FTIR (cm−1 ): 2950 (CH2 ), 2883 and 2760 (N–CH3 ), 1730 (C O).
2.3. Synthesis of poly(methyl silsesquioxane) precursor (PMSSQ) The synthesis and characterization of PMSSQ precursor was according to our previous reports [32], except that only THF was used as solvent. PMSSQ precursor was synthesized from MTMS in THF by sol–gel process using the molar ratio of the water to MTMS equal to 3.0 and pH of 2.0. The weight average molecular weight of the prepared PMSSQ determined by GPC was 3880. The obtained PMSSQ precursor consisted of both symmetric (cage-like, 1120 cm−1 ) and non-symmetric (networklike, 1030 cm−1 ) structures were identified by FTIR. The OH content estimated from FTIR analysis was 6.72% from the comparison with a reference PMSSQ sample from Gelest Co. with a reported OH content of 5%.
2.4. Preparation of the nanoporous PMSSQ films using DMA-POSS as templates The PMSSQ precursor in THF solution and the porogen of DMA-POSS were mixed with different ratio. The number (X) of the obtained nanoporous PMSSQ films (PMSSQ-X) was the weight percent of DMA-POSS loading, as listed in Table 1. Another solvent, BuOH, was used to adjust the solid content of the solution to be 20 wt%. Then, the solution was loaded into a syringe and filtered through a 0.2 m PTFE filter. The filtered solution was dropped directly onto a clean Si(1 0 0) wafer and the coated substrate was spun at 2000 rpm for 20 s. The thermal curing of the PMSSQ and the pyrolysis of the porogen were carried out in a quartz furnace at 2 ◦ C min−1 to 425 ◦ C and held for 2 h under nitrogen purge. The thickness of the nanoporous PMSSQ film was 500–1000 nm depending on the spin-coating process.
Table 1 Properties of nanoporous PMSSQ thin films.
PMSSQ PMSSQ-10 PMSSQ-20 PMSSQ-30 PMSSQ-40
Porogen (wt%)
Porosity (%)
Thickness (nm)
Rq (nm)
Refractive index
Dielectric constant
Hardness (GPa)
0 10 20 30 40
0 4.3 8.7 12.6 16.4
890 810 650 625 530
0.194 0.224 0.242 0.301 0.449
1.379 1.358 1.340 1.324 1.307
2.8 2.7 2.4 2.3 2.2
1.19 1.00 1.08 1.07 1.06
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Fig. 1.
1
H NMR spectra of (a) Acrylo POSS and (b) DMA-POSS in CDCl3 .
2.5. Characterization 1
H NMR spectra were recorded by Bruker Avance DRX 500 MHz spectrometer in CDCl3 . FTIR spectra of the prepared films on the doubly polished silicon wafers were obtained with a PerkinElmer PARAGON 1000. TGA thermal analysis was con-
Fig. 3. FTIR absorption spectra of as-spun PMSSQ/(DMA-POSS) hybrid thin films before pyrolysis in the wavenumber ranging from 875 to 1000 cm−1 : (a) PMSSQ, (b) PMSSQ-10, (c) PMSSQ-20, (d) PMSSQ-30, and (e) PMSSQ-40.
ducted on a PerkinElmer pyris 1 TGA under continuous nitrogen flow, at a heating rate 10 ◦ C min−1 . An atomic force microscope (AFM, Nanoscope Inc., Model DI 5000) was operating in the tapping mode with the silicon cantilever/tips (Nano-Sensors, NCH-type) to probe the surface morphology of the prepared thin films. The silicon tips with diameter of 10–20 nm, force constant of 42 N m−1 , and resonance frequency of 250–270 kHz were used. Field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F) were used to examine the surface morphology and the inner structure of the coated films. The MTS Nano Indenter was used to measure the hardness. Dielectric constant was measured by mercury (Hg) probe current–voltage (I–V) method at 1 MHz. Refractive index and the film thickness were measured using a Metricon Prism Coupler (Model 2100). The porosity of the prepared nanoporous
Fig. 2. FTIR absorption spectra of (a) DMA-POSS, (b) PMSSQ, (c) PMSSQ-40 before thermal pyrolysis, and (d) PMSSQ-40 after thermal pyrolysis.
Fig. 4. TGA curves of (a) PMSSQ, (b) PMSSQ-20, and (c) DMA-POSS before thermal pyrolysis.
H.-W. Su, W.-C. Chen / Materials Chemistry and Physics 114 (2009) 736–741 PMSSQ films was determined by the Maxwell–Garnett mode as Eq. (1) [18]: n2PMSSQ − n2air
n −1 = (1 − p) 2 n2 + 2 nPMSSQ + 2n2air 2
(1)
where n, nPMSSQ , and nair are the refractive index of the prepared nanoporous PMSSQ film, the parent PMSSQ film and the air, respectively. p is the porosity of the nanoporous PMSSQ film relative to the parent PMSSQ film
3. Results and discussion Fig. 1 shows the 1 H NMR spectrum of the original Acrylo POSS and the synthesized DMA-POSS in CDCl3 . The resonances of the
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acrylic groups in Acrylo POSS (ı = 5.8, 6.1, and 6.4 ppm) are completely disappeared after Michael addition. The proton resonances of the dimethylamino groups in DMAPA and DMAEA are clearly observed at 2.2 and 2.3 ppm, respectively. It suggests that DMAPA and DMAEA have been successfully incorporated into POSS using the Michael addition [30,31]. The resonance of the Si–CH2 – appears at 0.6 ppm confirm the successful linkage of inorganic core (POSS) and organic shell (DMA). The vinyl group of acrylate, primary and secondary amine peaks at around 1640, 1650 and 1560 cm−1 , respectively, disappear in the FTIR spectrum of Fig. 2(a). Meanwhile, the dimethylamino bands, resulted from the two proton sponges of the nitrogen, are observed at 2883 and 2760 cm−1 .
Fig. 5. AFM images of the nanoporous PMSSQ films: (a) PMSSQ, (b) PMSSQ-10, (c) PMSSQ-20 (1 m × 1 m), (d) PMSSQ-20 (500 nm × 500 nm), (e) PMSSQ-30, and (f) PMSSQ-40.
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The NMR and FTIR results suggest the successful synthesis of DMA-POSS. Fig. 2(b) and (c) show the FTIR spectra of PMSSQ and PMSSQ40 films before thermal curing, respectively. The primary bands of interest for PMSSQ include the OH stretching mode in the 3100–3600 cm−1 regions and the Si–OH stretching vibration at 905 cm−1 . After the porogen DMA-POSS is loading, the carbonyl (–C O) stretching and the dimethylamino bands (N–CH3 ) are observed at 1730 and 2700–2900 cm−1 , respectively. Besides, the peak intensities of silanol bands at 3100–3600 are significantly decreased and the other silanol band at 905 cm−1 of PMSSQ is shifted to 940 cm−1 of PMSSQ-40. Silanol condensation catalyzed by amine is well known and leads to improved mechanical properties of the cured PMSSQ [15,33,34]. The mechanism might be that the proton released by the amine is transferred to the PMSSQ, producing a protonated leaving group that can be displaced by water in an SN2 reaction. Therefore, the above results suggest that the tertiary amino groups in DMA-POSS could serve as a catalyst for the silanol condensation reaction. Fig. 2(d) shows the FTIR spectrum of PMSSQ-40 film after pyrolysis, in which no organic peaks are observed. Also, the PMSSQ peaks are observed at 2950 cm−1 (SiCH3 ), 1275 cm−1 (SiCH3 ), 1120 cm−1 (Si–O–Si, sym), and 1030 cm−1 (Si–O–Si, non-sym), respectively. Hydrogen bonding interaction between the organic and inorganic moieties is very important to avoid the macrophase separation and leads to homogeneously nanoscopic pores. Fig. 3 shows the absorption bands assigned to the Si–O stretching on the Si–OH bands of the PMSSQ/DMA-POSS hybrid films. As observed in Fig. 3, the position of the Si–OH band gradually shifts from 905 to 940 cm−1 as increasing the loading of DMA-POSS. It indicates that the formation of the hydrogen bond between the silanol groups of PMSSQ and the tertiary amine groups or the carbonyl groups of DMA-POSS. Such hydrogen bond decreases the bond strength between the H and Si–O of the Si–OH bond, thus results in the shifting of the Si–O stretching band to a higher wavenumber [18]. The thermal decomposition of DMA-POSS alone and blend with PMSSQ have been monitored by thermal gravimetric analysis, as shown in Fig. 4. The thermal decomposition of DMA-POSS occurs in three distinct steps, 200–300, 300–450, and 450–600 ◦ C, respectively. The weight loss of 200–300 and 300–450 ◦ C might be attributed from the loss of dimethylamino and acrylate segments, respectively. These results are similar to the blend of PMSSQ/P(MMA-co-DMAEMA) nanocomposites [15]. As shown in Fig. 4(c), the organic fraction of DMA is almost decomposed in the temperature range of 400–500 ◦ C. The thermal decomposition of PMSSQ with DMA-POSS (PMSSQ-20) at different stage is similar to DMA-POSS alone. The char yield at 800 ◦ C is about 79 wt%, which is close to the theoretical value (20 wt%) of organic fraction in the hybrid materials. It suggests that DMA-POSS could be an ideal porogen for preparing nanoporous materials. Understanding the surface morphology of the porous thin films could help us elucidate the phase separation in the hybrid. Fig. 5 shows the AFM micrographs of the nanoporous PMSSQ thin films imprinted by DMA-POSS loading ranging from 0 to 40 wt%. For the case of PMSSQ and PMSSQ-10, there is no clear domain size shown on the surface and the surface roughness (Rq) is very small (0.194 nm for PMSSQ and 0.224 nm for PMSSQ-10). Since the porogen size of DMA-POSS is around 1.2–1.4 nm in diameter, the relatively smooth surface may suggest that the pore size is too small to be observed in the hybrid film. With increasing the porogen loading, for PMSSQ-20, uniform spherical pores are shown on the surface and the size estimated from the AFM height profile image is around 5–10 nm. The larger pore size than that single DMA-POSS suggests its self-aggregation at a high-porogen loading. However, such pore size is still smaller than 10 nm and the film surface is
Fig. 6. FESEM image of the nanoporous PMSSQ thin films: (a) PMSSQ-10, (b) PMSSQ20, (c) PMSSQ-30, and (d) PMSSQ-40.
relative smooth (Rq = 0.242 nm). In our previous study using PS-bP2VP as porogen at the same loading ratio, uniform spherical pores with the size are around 13–15 nm [18]. It suggests the DMA-POSS could be a superior porogen for generating nanoporous materials than PS-b-P2VP. When the DMA-POSS porogen loads up to 30 and 40%, the isolated pore is gradually aggregated together and the surface roughness is increased (Rq = 0.301 nm for PMSSQ-30 and Rq = 0.449 nm for PMSSQ-40), as shown in Fig. 5. Fig. 6 shows the cross-sectional FESEM images of the nanoporous thin films. For PMSSQ-10 and PMSSQ-20, very uniform PMSSQ images without significant aggregation are observed in Fig. 6(a) and (b). The images of PMSSQ-30 and PMSSQ-40 exhibit slightly rough surface but no significant porous domain, as shown in Fig. 6(c) and (d). It suggests the porosity in the PMSSQ film is relatively small. The refractive index, the dielectric constant and the porosity estimated by Eq. (1) are listed in Table 1. As the DMA-POSS loading increases from 0 to 40 wt%, the refractive index decreases from 1.379 to 1.307 and the estimated porosity increases from 0 to 16.4%, as shown in Fig. 7. The corresponding dielectric constant decreases from 2.8 to 2.2, respectively. The incorporated voids in the PMSSQ matrix are contributed to the decreased refractive index and dielectric constant. The hardness of the nanoporous PMSSQ films is quantified using nanoindentation measurements [17], shown in Table 1. The hardness of the pure PMSSQ film is about 1.19 GPa
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Taiwan University (Excellent Research Projects), and Ministry of Economic Affairs of Taiwan (96-EC-17-A-08-S1-015) are highly appreciated. References
Fig. 7. Variations on the refractive index and the porosity of the nanoporous film with the porogen loading ratio.
and retains the high hardness of 1 GPa even at the porosity of 16.4%. The retention of these small-cage (POSS) structures dispersed homogeneously in PMSSQ matrix is the primary reason for improved hardness. Meanwhile, the tertiary amine group probably catalyzes the silanol condensation during thermal curing and improves the mechanical properties. It explains the high hardness for the prepared nanoporous materials. These results demonstrate that nanoporous PMSSQ thin film has been successfully created using DMA-POSS porogen. 4. Conclusions Nanoporous poly(methyl silsesquioxane) PMSSQ thin films were successfully fabricated through thermally sacrificing porogen of dimethylamino-functionalized polyhedral oligomeric silsesquioxane (DMA-POSS). Nanopores were homogeneously dispersed in PMSSQ matrix through the hydrogen-bonding interaction and composition optimization. The low-dielectric constant (k = 2.2) of porous PMSSQ was obtained by incorporating 16.4% porosity and the hardness retained at about 1 GPa. The present study suggests that the low-dielectric constant porous materials could be successfully prepared using the new core-shell silsesquioxane. Acknowledgements The financial supports from Taiwan Semiconductor Co., National Science Council of Taiwan (NSC 97-2221-E002-024-MY3), National
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