Cubic silsesquioxane–polyimide nanocomposites with improved thermomechanical and dielectric properties

Acta Materialia 53 (2005) 2395–2404 www.actamat-journals.com

Cubic silsesquioxane–polyimide nanocomposites with improved thermomechanical and dielectric properties Junchao Huang

b

a,*

, Poh Chong Lim a, Lu Shen a, Pramoda Kumari Pallathadka a, Kaiyang Zeng b, Chaobin He a,*

a Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

Received 6 July 2004; received in revised form 1 February 2005; accepted 1 February 2005 Available online 2 April 2005

Abstract Two series of nanoporous polyimide nanocomposites with well-defined tether architecture have been prepared from octa(aminophenyl)silsesquioxane. Transmission electron microscopy, solid-state 29Si nuclear magnetic resonance, model reaction, and density measurement show that the nanostructures of the polyimide nanocomposites are well defined and can be adjusted accordingly. The polyimide nanocomposites exhibit tunable dielectric constant with a lowest value of 2.29. The thermomechanical properties of the polyimide have been improved significantly with the addition of octa(aminophenyl)silsesquioxane; for instance, glass transition temperature increases by
80 °C and storage modulus by 46%, as compared to the neat polyimide. Moreover, thermal stability, coefficient of thermal expansion, hardness, and moisture absorption of the polyimide nanocomposites are also improved significantly. Ó 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc. Keywords: Polymer matrix composites; Nanostructure; Porous material; Dielectric; Silsesquioxane

1. Introduction Organic–inorganic nanocomposites with well-defined architectures have attracted increasing attention because of their potential to provide materials with controlled morphology at nanometer scale [1–4]. These organic– inorganic hybrid nanocomposites exhibit many unique properties associated with both nanometer size and the multifunctionality arising from the organic and inorganic components [5–12]. Recently, it has been demonstrated that octafunctionalized cubic silsesquioxane, regarded as nanobuilding blocks, offers an efficient route in developing novel hybrid nanocomposites [13–29]. The * Corresponding authors: Tel.: +65 68748145; fax: +65 68727528 (C. He); Tel.: +65 68741972 (J. Huang). E-mail addresses: [email protected] (C. He), [email protected] (J. Huang).

cubic silsesquioxane has a well-defined nanometer-sized structure with high surface area, controlled porosity, and various functionalities. It consists of a rigid, hollow silica-like core with 0.53 nm body diagonal and the eight equally reactive functional groups that link covalently to its eight vertices. Due to the stable silica structure and controllable functionalities, polyhedral oligomeric silsesquioxane (POSS) has been widely utilized to fabricate hybrid materials with enhanced thermomechanical properties [13,17,19], good thermal stability [13,17,19], atom oxygen resistance [30], abrasion resistance [30] and low water uptake [15]. The combination of high thermal stability and porosity makes this hybrid nanocomposite a good candidate for low k materials. According to the National Technology Roadmap for Semiconductors [31,32], as the feature size in the large scale integration of microelectronic circuits shrinks to 0.15 lm or less, the

1359-6454/$30.00 Ó 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc. doi:10.1016/j.actamat.2005.02.001

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resistance–capacitance (RC) delay and the crosstalk noise between metal interconnects would offset the previous gain in chip performance derived from the device size miniaturization. In order to avoid the crosstalk and the RC delay, a significantly lower dielectric constant (k = 2.0–2.5) material is required. Polyimide has been widely used in the microelectronic industry due to its good thermal mechanical properties, however, the relative high dielectric constant of the polyimide, normally in a range of 3.1–3.5, limits its application as a dielectric material for the next generation of integrated circuits. As a result, modified polyimides with low dielectric constant are needed for miniaturized device fabrication. To this end, various approaches have been used in the attempt to alter the physical or chemical structure of polyimides. The synthesis of fluorinated polyimides has been investigated. Although the dielectric constant of fluorinated polyimide is low (2.5–2.9), this advantage for microelectronic application is offset by their poor mechanical properties [33,34]. Another approach is to introduce air voids (k = 1) in the polyimide substrate by the so-called sacrificial porogen (pore generator) approach, that is, the selective removal of the organic macromolecular phase from phase-separated mixtures of polyimide/polymer blends. However, the complete removal of the residual organic component and the control of size and shape of the pores remain a considerable challenge [35–37]. Organic–inorganic hybrid composites from silsesquioxane have been synthesized to modify the mechanical and dielectric properties of the polyimide matrices [19,21]. However, the dielectric and mechanical properties of their hybrid composites could not be improved simultaneously [21]. Furthermore, the mechanisms of processes [19,21], direct characterization of POSS structure in solid nanocomposites [19,21], morphologies [19], dielectric constant [19], and water uptake [19,21] have not been investigated, and the pot-life of the poly(amic acid) (PAA) mixtures in previous studies is so short that the PAA could not be applied using a typical spin-coating technique due to immediate gelation after mixing the amine solutions with the anhydride solutions [19]. In our previous studies, we presented a series of conventional polyimide/ POSS nanocomposites with improved thermomechanical properties [13]. In this paper, we reported two systems of polyimide nanocomposites by introducing the nanoporous functional POSS into the fluorinated polyimide matrices. Both dielectric properties and thermomechanical properties of the fluorinated polyimides were significantly improved. In our systems, porosities were introduced by POSS molecules in order to improve dielectric properties of the polyimide, the networks based on the covalent bonds between POSS and polyimide improved thermomechanical properties of the polyimide.

2. Experimental 2.1. Materials 4,4 0 -Diaminodiphenylmethane (DADPM), phthalic anhydride and 4,4 0 -(hexafluoroisopropylidene)-diphthalic anhydride (6F-DA) were purchased from Aldrich. Aniline was obtained from Sino Chemical Co. Ltd. Anhydrous grade N-methyl-2-pyrrolidinone (NMP) was purified by distillation under a nitrogen atmosphere and dried over molecular sieves. Octa(aminophenyl)silsesquioxane (OAPS) was synthesized following the previous literature [13,20]. Other chemicals were used as purchased unless mentioned. 2.2. Characterizations The nanocomposite structure was imaged using a JEOL 3010 high-resolution transmission electron microscope (TEM). Samples for the TEM experiments were prepared by burying the nanocomposite films in the epoxy capsules and curing at room temperature for 24 h. The cured epoxy samples were microtomed using a diamond knife on a Leica Ultracut, and the thin slice (90 nm) was placed on mesh 200 carbon coated copper grid for TEM observation. TEM was operated at an acceleration voltage of 300 kV. 1 H nuclear magnetic resonance (NMR), 13C NMR spectra were collected using a Bruker 400 spectrometer, using chloroform-d as the solvent and tetramethylsilane as the internal standard. Solid-state 29Si NMR spectra were also recorded on the Bruker 400 spectrometer with the frequency of 79.5 MHz. Fourier transform infrared (FTIR) spectra were recorded on a Perkin–Elmer SPECTRUM-2000 FTIR spectrophotometer. Dynamic mechanical analysis (DMA) measurements were conducted using a TA Instruments DMA 2980; samples were 4 mm wide and 30 mm long, and a heating rate of 3 °C/min and a frequency of 1 Hz were used. Dielectric analyses were recorded on a TA instrument DEA 2970 dielectric analyzer. Double-sided Au electrodes were deposited on the film surface by the mask-sputtering method. The following protocol was used for every sample: heating from room temperature to 150 °C at 5 °C/ min, holding at 150 °C for 30 min, cooling to 40 °C at 5 °C/min, and then reheating from 40 to 250 °C. Data were collected during the second heating step. Thermomechanical analyses (TMA) were performed using a TA Instruments TMA 2940 at a heating rate of 3 °C/min. The thermal stabilities were characterized using a TGA 2050 thermogravimetric analyzer of TA Instruments at a heating rate of 20 °C/min. All the thermal analysis experiments were conducted in nitrogen atmosphere. The mechanical properties of the neat polyimide and the polyimide nanocomposites were tested on an Instron universal tester at a displacement rate of 1 mm/min.

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Nanoindentation measurement was done using an UMIS-2000 Nano-indenter (Australian Scientific Instruments) with a Berkovich indenter (three-faced pyramid diamond). The load and depth of penetration were independently measured by two linear variable differential transformer sensors. The indentation procedure performed on all samples was as follows: load to 5 mN as the maximum load with a constant strain rate (0.05 s1 in present test), hold for 60 s at the maximum load, and then unloading in the same rate to 2% of the maximum load. At least 10 indents were made on each specimen. Averaged hardness and modulus values are presented. The densities of the nanocomposites were measured and calculated by the following equation, Ds = DwWs/ (Ws  Ww), where Ds is the density of sample, Dw the density of pure water at 25 °C (0.99707 g/cm3), Ws the weight of the sample, and Ww is the weight of the sample in pure water. The samples for water uptake measurement were dried at 100 °C for 24 h under vacuum, and then put in a sealed container at 25 °C and 100% relative humidity until equilibrium state (one week). Three samples were tested and the average values were taken.

tively. A typical approach for the nanocomposites of formulation B was as follows. The fixed amount of DADPM and 6F-DA were dissolved separately in the 25 ml NMP. The two monomer solutions were mixed in a three-neck flask that had been purged with nitrogen gas to remove moisture. The mixture was stirred under N2 at room temperature for 6 h, and a viscous poly(amic acid) (PAA) solution was obtained. To this PAA solution, a pre-determined amount of OAPS/NMP solution was added. The mixture was stirred at room temperature for an additional 2 h to give a transparent solution. The solution was then cast on a smooth glass substrate and thermally treated at 80 °C for 8 h, 200 ° C for 2 h, and 300 °C for 2 h. The films were stripped from the glass substrates with the aid of deionized water and dried at 100 °C in a vacuum oven; the thickness of the films was about 50 lm. Additionally, the polyimide/OAPS nanocomposites of the formulation A were prepared in a similar way, except that the OAPS/NMP solution and PAA/NMP solution were mixed and cast immediately on the glass substrate to avoid gelation.

2.3. Nanocomposites preparation

In a three-neck flask aniline (0.93 g, 10 mmol), phthalic anhydride (1.48 g, 10 mmol), and NMP (9 ml) were charged under nitrogen purge and the solution was stirred for 1 h at 25 °C. The intermediate, N-phenylphthalamic acid (NPPAA), was formed in the solution. OAPS (0.145 g, 0.125 mmol) was dissolved in

The polyimide nanocomposites were prepared by a conventional two-step method. The compositions of two series of the nanocomposites are listed in Table 1 (formulation A) and Table 2 (formulation B), respec-

2.4. Model reaction

Table 1 Formulation A of OAPS modified polyimide nanocomposites Sample

OAPS wt%b

OAPS vol%c

DADPM (mmol)

6F-DA (mmol)

OAPS (mmol)

Neat PI PI(9:10:0.25)a PI(8:10:0.5) PI(6:10:1) PI(4:10:1.5)

0 4.4 8.7 17.0 24.8

0 5.7 11.1 21.2 30.2

10 (1.98 g) 9 (1.78 g) 8 (1.59 g) 6 (1.19 g) 4 (0.79 g)

10 10 10 10 10

0 (0 g) 0.25 (0.29 g) 0.5 (0.58 g) 1 (1.15 g) 1.5 (1.73 g)

(4.44 g) (4.44 g) (4.44 g) (4.44 g) (4.44 g)

a

The mole ratio of diamine:dianhydride:OAPS is 9:10:0.25. Theoretical value. c The volume percentage of OAPS in the nanocomposite is theoretically calculated from the densities of the pure OAPS (1.09 g/cm3) and the neat polyimide (1.43 g/cm3). b

Table 2 Formulation B of OAPS modified polyimide nanocomposites Sample

OAPS wt%b

OAPS vol%c

DADPM (mmol)

6F-DA (mmol)

OAPS (mmol)

Neat PI PI(10:10:0.25)a PI(10:10:0.5) PI(10:10:1) PI(10:10:1.5)

0 4.3 8.2 15.2 21.2

0 5.6 10.5 19.0 26.1

10 10 10 10 10

10 10 10 10 10

0 (0 g) 0.25 (0.29 g) 0.5 (0.58 g) 1 (1.15 g) 1.5 (1.73 g)

a

(1.98 g) (1.98 g) (1.98 g) (1.98 g) (1.98 g)

(4.44 g) (4.44 g) (4.44 g) (4.44 g) (4.44 g)

The mole ratio of diamine:dianhydride:OAPS is 10:10:0.25. Theoretical value. c The volume percentage of OAPS in the nanocomposite is theoretically calculated from the densities of the pure OAPS (1.09 g/cm3) and the neat polyimide (1.43 g/cm3). b

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the 3 ml of NMP in glass vial and was added into the flask under nitrogen. The mixture was stirred for additional 1 h, and then the mixture solution was poured into a Petri dish and heated at 80 °C for 8 h, 200 °C for 2 h, and 300 °C for 2 h. After thermal imidization, the white needle-shape powders and the brown pellets adhering to the surface of Petri dish were obtained. The light needle-shape powders were collected by mild nitrogen stream. The brown pellets were dissolved in dichloromethane and precipitated into hexane. According to NMR spectra, the needle-shape powders were N-phenylphthalimide (NPPI), and the brown pellets were octa(phthalimidephenyl)silsesquioxane (OPIPS). NPPI 1H NMR(CDCl3): 7.87(m, Ph), 7.71(m, Ph), 7.44(t, Ph), 7.38(s, Ph), 7.33(t, Ph); 13C NMR(CDCl3, ppm): 167.5, 134.6, 132.0, 131.9, 129.3, 128.3, 126.8, 123.9; m.p.: 210 °C. OPIPS 1H NMR(CDCl3, ppm): 6.7–8.1(b); 13C NMR(CDCl3, ppm): 167.7, 134.5, 132.4, 131.6, 128.9, 123.6; 29Si solid-state NMR(ppm): 66.2 (b); GPC(THF): Mn 1805, Mw 1974, Mw/Mn 1.09.

3. Results and discussion We first prepared the PI-OAPS nanocomposites at the stoichiometric ratio (Table 1), that is, the mole ratio of anhydride/amine groups was kept at 1:1. As shown in Scheme 1 (formulation A), the processibility of this system was poor due to instant gelation after the mixing of the amine and the anhydride solutions. The resulting films also fractured extensively because of the volume shrinkage during the imidization process. We tried offstoichiometry formulation as shown in Table 2. In these systems the mole ratio of diamine and dianhydride equals 1:1, and additional OAPS was added. It was a surprise to observe that the resulting nanocomposites showed a higher glass transition temperature, improved thermomechanical properties as compared to the neat polyimide, indicating that the cross-linking network was formed based on OAPS and polyimide molecules. A possible interpretation was proposed as exhibited in Scheme 1 (formulation B): the amine groups in OAPS could react with the pendant carboxylic groups of poly(amic acid) at high temperature, which was followed by the formation of the imide linkages between OAPS and polyimide chains. 3.1. Model reaction In order to verify the above interpretation, a model reaction as shown in Scheme 2 was designed. The characterization results confirmed the formation of octa(phthalimidephenyl)silsesquioxane (OPIPS) in the model reaction. The products of the model reaction were

the mixture of the brown pellets adhering to the surface of Petri dish and the white needle-shape powders. The white powders could be easily separated from the brown pellets by nitrogen stream. From analysis of 1H NMR spectra it was concluded that the white powders were NPPI, and the brown pellets were OPIPS. 1H NMR, 13 C NMR and solid-state 29Si NMR spectra of OPIPS also agree well with the previous results [13,20]. The formation of OPIPS in the model reaction confirms our hypothesis that the amine groups in OAPS could bond to the polyimide molecules by the imide linkages. 3.2. FTIR and solid-state

29

Si NMR

The neat polyimide and the polyimide nanocomposites were normally prepared by reaction of diamine and carboxylic dianhydride. Fig. 1 shows the FTIR spectra of the neat polyimide and the nanocomposites (PI(6:10:1), PI(10:10:1)). A complete imidized structure is confirmed by the absorption bands at 1781 cm1 (asymmetric m(C@O) in imide groups), 1729 cm1 (symmetric m(C@O) in imide groups) [38,39], and the absorption band of POSS (asymmetric m(Si–O–Si)) appears at 1103 cm1. The retention of the cubic cage structure of OAPS was directly confirmed by solid-state 29 Si NMR spectra as exhibited in Fig. 2. The peak of each 29Si NMR spectrum corresponds to the silicon atoms in the cubic cage structure. To our knowledge, the direct characterization of the silsesquioxane structures in solid-state nanocomposites was reported for the first time. 3.3. Morphology of polyimide nanocomposites As shown in Fig. 3, the micrographs of the polyimide nanocomposites were characterized by TEM. Fig. 3(a) shows the TEM image of cross sections of the polyimide nanocomposite (formulation B) at the 8.2 wt% OAPS loading, a limited amount of dark spots were observed, where OAPS molecules aggregate in a loose manner. Fig. 3(b) shows that as the loading of OAPS increases to 21.2 wt% the aggregation regions in the nanocomposites grow larger as compared to the former TEM image. TEM images of another series polyimide nanocomposites (formulation A) also give similar results, TEM images (Fig. 3(c)) of the nanocomposite of 8.7 wt% OAPS do not display any dark spots, indicating that the OAPS molecules are homogeneously dispersed in the nanocomposites. However, at the high loading of OAPS (24.8 wt%), the OAPS molecules are observed to self-assemble into dark rods with diameters of about 60–70 nm as seen in Fig. 3(d); this is possibly caused by the chain-to-chain arrangement of the OAPS molecules and the polyimide chains, and a similar observation was also reported by Wei et al. in their studies [22].

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Formulation A F3 C

O

CF3

N H

O O

O

CH2

CF3

F3 C

O

O

O CH2

N H

n

HO

OH

CF3

O

N H

OH

O

O

F3 C

O

N H

PAA

O O

OH

O

OAPS O O SiO Si Si Si O O O O O Si Si OSi O SiO O

O

F3C

CF3

N H

NH

CH2

CF3

O

N H

O CH2

N H

O

O

F3 C

CF3

OH

O HN O

OH

O

OH

Gelation

Imidization O O SiO Si Si Si O O O O O Si Si OSi O SiO O

O Si OSi O O Si Si O O O Si Si OSi O SiO O

O

N H

n

HO

OH

O

HO O

F3C

O

O

O O SiO Si Si Si O O O O O Si Si OSi O SiO O O O SiO Si Si Si O O O O O Si Si OSi O SiO O

O O SiO Si Si Si O O O O O Si Si OSi O SiO O

Formulation B F 3C

O

CF3

N H

O

O

O

F3 C

O

O

O Si SiO O

N H

O

O

Si Si O O O Si OSi O

O

CH2

NH2

n

HO

OH

O SiO Si O

CF3

N H

CH2

O

PAA

OH

Rreaction between two groups occurs in the imidization process NH2 8

Imidization O O SiO Si Si Si O O O O O Si Si O OSi O Si O

O

O SiO Si Si Si O O O O O Si Si OSi O SiO O

O O SiO Si Si Si O O O O O Si Si O OSi O Si O

O O SiO Si Si Si O O O O O Si Si O OSi O Si O

Scheme 1. Reaction of OAPS with PAA macromolecules to form the polyimide nanocomposites.

The incorporation of nanoporous OAPS molecules leads to nanoporous polyimide nanocomposites with reduced density. As shown in Table 3, the densities of the nanocomposites decrease with the increasing loading of OAPS. In addition to the nanoporosity in the core of the OAPS molecules, OAPS also introduced external porosity between OAPS molecules and polyimide chains, and external porosity is also introduced in the aggregated domains of OAPS molecules in the nanocomposites. The porosity of POSS was also confirmed by the low

densities of the pure OAPS, 1.09 g/cm3. It is noted that the density of OAPS is significantly lower than that of the neat polyimide (1.43 g/cm3). 3.4. Dielectric analysis (DEA) Dielectric constants of OAPS modified polyimide nanocomposites were measured with a dielectric analyzer (DEA). A typical dielectric spectrum of the neat fluorinated polyimide at various frequencies is

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J. Huang et al. / Acta Materialia 53 (2005) 2395–2404 O

+ H2N

O

O

O O SiO Si O

O N OH

O

+

O Si

OH

Si Si O O O Si OSi O

Si

NH2

O

NPPAA

80:1 mole ratio

8

OAPS

Imidization at high temperature O N

+

O SiO Si O

O

Si Si O O O O Si Si O O OSi Si O

O

O N

O

NPPI

8

OPIPS

Scheme 2. Model reaction between N-phenylphthalamic (NPPAA) and octa(aminophenyl)silsesquioxane (OAPS).

acid

1103

Absorbance

Si-O-Si

PI(10:10:1) 1729

Neat PI

3.5. Dynamic mechanical analysis (DMA)

C=O 1781 Si-O-Si

PI(6:10:1)

4000

shown in Fig. S1 for reference (supporting information). As shown in Fig. 4, with the addition of OAPS molecules the polyimide nanocomposites exhibit a lower dielectric constant than that of the neat polyimide; the reduction of dielectric constant is dependent on the amount of OAPS in the nanocomposites. Compared with the dielectric constant of the neat polyimide (k = 2.96 at 50 °C), the dielectric constant decreases to 2.29 for the nanocomposites of 24.8 wt% OAPS (formulation A) and 2.40 for the nanocomposites of 21.2 wt% OAPS (formulation B). A few possible interpretations for dielectric constant reduction are put forward. It is most likely due to the nanoporosity in the core of the OAPS molecules and the external porosity introduced by the large sweep volume of OAPS molecules. Another possible contribution to the dielectric constant reduction is the limited ability of the polarizable units in the nanocomposites to orient fast enough to keep up with the oscillations of an alternating electric field. Owing to the high crosslinking density and the short tether length between the vertices of OAPS molecules, the alignment of dipoles, such as the imide groups, in response to the electric field requires more time than those in the linear polyimide. In addition, it was reported that the dielectric constant (
2.1) of POSS cubic silica structure is lower than that of the neat polyimide, resulting in the reduction in the dielectric constant of OAPS modified nanocomposites [40].

3000

2000

1000

cm

-1

Fig. 1. FTIR spectra of the neat polyimide and the polyimide nanocomposites.

Fig. 2. Solid-state 29Si NMR spectra of octa(phthalimidephenyl)silsesquioxane (OPIPS) and the polyimide nanocomposites: (a) OPIPS; (b) PI(4:10:1.5); (c) PI(10:10:1.5).

The results of dynamic mechanic analyses (DMA) of the polyimide nanocomposites of various OAPS loading are shown in Fig. 5. The polyimide nanocomposites of formulation A exhibit increased Tg and higher storage moduli as compared to the neat polyimide. As can be seen from Fig. 5 (formulation A), the tan d peak of each nanocomposite obviously shifts to higher temperatures, the peaks become significantly weaker and broader, and the storage moduli of each nanocomposite increases obviously. Compared with the neat polyimide, the polyimide nanocomposite of 24.8 wt% OAPS shows Tg of 379.8 °C (
70 °C increase), the glass state modulus of 4.26 GPa at 50 °C (30% increase), and the rubbery state modulus of 0.54 GPa at 379 °C (
11 times the modulus of the neat polyimide at its Tg). The increase in glass transition temperatures of the nanocomposites could be for two reasons. First, incorporation of OAPS increases the cross-linking density of the resulting nanocomposites, and OAPS is a rigid body, increasing the rigidity of the composite system. Second, the incorporation of OAPS leads to the porosities (free volume) in the nanocomposites; this effect has, however, been counteracted by an increase in the cross-linking density.

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Fig. 3. TEM images of the PI-OAPS nanocomposites: (a) PI(10:10:0.5); (b) PI(10:10:1.5); (c) right part of image, PI(8:10:0.5); (d) PI(4:10:1.5).

Table 3 Mechanical and thermal properties of the polyimide nanocomposites at various OAPS loading Sample

Tg (°C)b

Td (°C)c

CTE (ppm/K)

Density (g/cm3)

Modulus (GPa)

Break elongation (%)

Max. stress (MPa)

Water uptake (wt%)

Neat PI PI(9:10:0.25)a PI(8:10:0.5) PI(6:10:1) PI(4:10:1.5) PI(10:10:0.25) PI(10:10:0.5) PI(10:10:1) PI(10:10:1.5)

308.1 320.6 332.1 363.2 379.8 322.8 341.0 365.8 385.7

513.5 519.7 508.1 507.9 495.6 522.2 523.8 528.6 540.2

55.2 50.1 49.1 47.0 43.5 51.4 50.5 49.9 45.6

1.43 1.42 1.40 1.36 1.31 1.39 1.37 1.32 1.28

2.01 ± 0.04 – – – – 2.37 ± 0.09 2.73 ± 0.03 2.82 ± 0.06 2.96 ± 0.04

7 ± 1.2 – – – – 9 ± 1.2 5±1 3±1 2±1

95.6 ± 3.4 – – – – 110.0 ± 8.1 105.1 ± 5.3 80.3 ± 4.9 44.3 ± 2.1

2.73 2.57 2.24 1.69 1.26 2.39 1.97 1.38 1.14

a b c

The mole ratio of diamine:dianhydride:OAPS is 9:10:0.25. The glass transition temperatures were measured by DMA. The on-set decomposition temperatures were measured by TGA.

For the nanocomposites of formulation B, similar DMA results (Fig. 5) were observed. Compared with the neat polyimide, the nanocomposites exhibit higher Tg, higher moduli, and broader tan d peaks. The DMA results evidenced that the cross-linking density of the nanocomposites increases with the increase of OAPS loading. In these systems an equal amount of diamine and dianhydride were added; this means that no more dianhydride groups will react with the amine groups of OAPS, and the cross-linking density should not increase any more. An interpretation of the cross-linking density increase is that the pendant carboxylic groups of poly

(amic acid) will be bound to the amine groups of OAPS at high temperature, and the imide linkages are formed during the imidization process; this hypothesis has already been verified by the results of model reaction. 3.6. Thermomechanical analysis (TMA) The coefficients of thermal expansion (CTE) of the nanocomposites are shown in Table 3. The CTEs of the polyimide nanocomposites decrease with the increasing OAPS loading. The CTEs of the neat PI and the nanocomposites of 24.8 wt% (formulation A) and

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J. Huang et al. / Acta Materialia 53 (2005) 2395–2404 3.0

Formulation A 2.7

'ε

Neat PI PI(9:10:0.25) PI(8:10:0.5) PI(6:10:1) PI(4:10:1.5)

0.3Mhz

2.4

50

100

150

200

250

Temperature (oC)

3.7. Thermogravimetric analysis (TGA)

3.0

The thermal stabilities of OAPS modified polyimide nanocomposites were investigated by TGA (Fig. 6). The nanocomposites were prepared in two formulations (Tables 1 and 2). For the nanocomposites of the formulation A, the onset decomposition temperature (Td) of nanocomposite of 4.4 wt% OAPS increases from 513.5 °C (neat polyimide) to 519.7 °C; further addition of OAPS leads to nanocomposites of lower Td. This most likely arises from the early decomposition of the

Formulation B

'ε

21.5 wt% (formulation B) are 55.2, 43.5 and 45.5 ppm/ K, respectively; it is observed that at
20 wt% OAPS loading the CTEs of nanocomposites decrease by about 20%. The reduction in CTE of OAPS modified nanocomposites could be attributed to the low CTE of rigid POSS. OAPS consists of one cubic silica structure with eight aminophenyl on its vertices; the addition of OAPS results in the nanocomposites with high rigidity, and the CTE of nanocomposites will consequently decrease. Also, the cross-linking density of the nanocomposites increases significantly by incorporation of OAPS, which also reduces the CTE of the nanocomposites.

2.7 Neat PI PI(10:10:0.25) PI(10:10:0.5) PI(10:10:1) PI(10:10:1.5)

0.3Mhz

2.4 50

100

150

200

250

o Temperature ( C)

Fig. 4. DEA curves of the neat polyimide and the polyimide nanocomposites at a frequency of 0.3 MHz.

Tan δ

Storage modulus (MPa)

0.8

Neat PI PI(9:10:0.25) PI(8:10:0.5) PI(6:10:1) PI(4:10:1.5)

Weight (%)

1.2

Formulation A 4000

2000

100

Formulation A 80

Neat PI PI(9:10:0.25) PI(8:10:0.5) PI(6:10:1) PI(4:10:1.5)

60

0.4

200

400

600

800

o

0 100

200

300

Temperature ( C)

0.0 400

Temperature (oC)

100 1.2

0.8

2000

Neat PI PI(10:10:0.25) PI(10:10:0.5) PI(10:10:1) PI(10:10:1.5)

80

Formulation B Neat PI PI(10:10:0.25) PI(10:10:0.5) PI(10:10:1) PI(10:10:1.5)

0.4

60

0 100

Weight (%)

4000

Tan δ

Storage modulus (MPa)

Formulation B

200

300

0.0 400

o

Temperature ( C) Fig. 5. DMA curves of the neat polyimide and the polyimide nanocomposites at various OAPS loading.

200

400

600

800

o Temperature ( C)

Fig. 6. TGA curves of the neat polyimide and the polyimide nanocomposites at various OAPS loading.

J. Huang et al. / Acta Materialia 53 (2005) 2395–2404

residual polyimide of low molecular weight in these systems, particularly in the systems of high OAPS loading. The polyimide of low molecular weight is caused by the unequal amount of diamine and dianhydride added in these systems. However, all systems of formulation B show better thermal stability than the neat polyimide: at the OAPS loading of 21.2 wt% the Td of the nanocomposite is 540.2 °C, increasing by
30 °C, because the effect of the polyimide molecular weight is negligible, and the OAPS molecules possess good thermal stability and are bonded to the polyimide chains. 3.8. Nanoindentation Fig. 7 shows the nanoindentation hardness and outof-plane compressive moduli of the neat polyimide and the polyimide nanocomposites. For the systems of both formulations, the modest increase in hardness and outof-plane moduli is observed as the loading of OAPS increases until about 15 wt%; further addition of OAPS leads to the slight decrease in the hardness and out-ofplane compressive modulus. It is noted that the slight decrease in hardness and out-of-plane compressive modulus at high OAPS loading in the nanocomposites is different from the results reported by Choi et al. [19]. A possible reason is proposed here. At low OAPS loading, hardnesses and out-of-plane moduli of the systems increase as a result of the increase in the cross-link density. However at high OAPS loading, hardnesses and out-ofplane moduli are more sensitive to the porosities in the

Modulus (GPa)

3

Modulus Hardness

PI(4:10:1.5)

PI(6:10:1)

PI(8:10:0.5)

Neat PI

4

PI(9:10:0.25)

0.5

Formulation A

0.4

Hardness (GPa)

0.6

5

2403

nanocomposites in view of the in-plane oriented polyimide molecules [12]. 3.9. Mechanical properties The polyimide nanocomposite films of formulation A fracture extensively as a result of the tensile stresses during the thermal imidization. It is impossible to cut the resulting films into 5 mm wide and 60 mm long strips using a razor blade. In contrast, it was easy to obtain the samples of the above size for the nanocomposite films of formulation B. Typical stress–strain curves and the mechanical properties of the polyimide nanocomposites of formulation B are shown in Fig. 8 and Table 3, respectively. As the loading of OAPS increases to 8.2 wt%, the modulus and tensile strength of the nanocomposites increase by 15.0% and 17.9%, respectively, as compared to the neat polyimide. Further increase in the loading of OAPS leads to the reduction in the elongation at break and tensile strength; however, the modulus continues to increase as the loading of OAPS increases, the nanocomposite of 21.2 wt% OAPS shows modulus of 2.96 GPa that increase by 47.3% (2.96 vs. 2.01). 3.10. Water uptake The water absorption of the polyimides has a remarkable effect on their dielectric properties: the absorbed water in the polyimides increases the dielectric constant of the dielectric materials (polyimide) and promotes the corrosion of metal conductor in microelectronic devices. It is consequently necessary to develop a modified polyimide of low water uptake. As shown in Table 3, the neat polyimide has a water uptake value of 2.73%; with the increasing loading of OAPS the water uptakes of the polyimide nanocomposites decrease to 1.26% (formulation A) and 1.14% (formulation B), respectively, and the maximum reduction in water uptake is 53.8% for

0.3

0.2

Modulus Hardness

Formulation B

PI(10:10:1.5)

P I ( 10 : 1 0 : 1 )

PI(10:10:0.5)

PI(10:10:0.25)

3

Neat PI

Modulus (GPa)

0.5 4

0.4

Ha rdnes s (G P a)

0.6

5

Tensile Stress (MPa)

Formulation B

100

Neat PI PI(10:10:0.25) PI(10:10:0.5) PI(10:10:1) PI(10:10:1.5)

50

0.3

0 0.2

Fig. 7. Hardness and out-of-plane compressive modulus of the neat polyimide and the polyimide nanocomposites at various OAPS loading as measured by nanoindentation.

0

3 6 Tensile Strain (%)

9

Fig. 8. Typical stress–strain curves of the neat polyimide and the polyimide nanocomposites at various OAPS loading.

2404

J. Huang et al. / Acta Materialia 53 (2005) 2395–2404

the nanocomposites of formulation A and 58.2% for the nanocomposites of formulation B. The significant reduction in water uptake of the polyimide nanocomposites is most likely due to the higher hydrophobicity induced by the addition of POSS; similar results were also obtained in a few polyimide–silsesquioxane hybrid materials [15,19].

4. Conclusion Nanoporous polyimide nanocomposites with a low dielectric constant and excellent thermal mechanical properties were prepared by copolymerization between octa(aminophenyl)silsesquioxane and polyimide. Two series of the polyimide nanocomposites were investigated. A loose aggregation of OAPS was observed in polyimide/OAPS nanocomposites at high OAPS concentration by TEM and density measurement. As the loading of OAPS increased, glass transition temperatures, and in-plane moduli of the nanocomposites increased significantly while the dielectric constant decreased obviously. Additionally, hardness, out-ofplane compressive modulus, water uptake, thermal stability and mechanical properties of the nanocomposites were improved with the incorporation of OAPS into the polyimide matrices.

Acknowledgment Financial support was provided by the DSO national lab of Singapore and the Institute of Materials Research and Engineering.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.actamat.2005.02.001.

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