Dielectric and thermal behaviors of POSS reinforced polyurethane based polybenzoxazine nanocomposites

Abstract

POSS (polyhedral oligomeric silsesquioxane) reinforced polyurethane (PU) based polybenzoxazine (PBz) nanocomposites were prepared as an interlayer low k dielectric material for microelectronics applications based on the concept of polarization and porosity of the composite. The hydroxyl terminated benzoxazine (OH–Bz) materials containing a less polar long aliphatic chain and hydroxyl terminated nanoporous POSS (OH–POSS) material were synthesized and copolymerized with hexamethylenediisocyante (HMDI) to obtain POSS–Bz–PU nanocomposites. Dielectric analysis of different concentrations of POSS reinforced PU–PBz nanocomposites indicates that the incorporation of POSS into the polymer matrix significantly reduced the value of the dielectric constant. Despite this, the reduction of dielectric constant by reinforcement is limited up to 30% POSS–PU–PBz (k = 1.94) and beyond this concentration the reverse trend was observed which might be due to the increasing density of the resulting composites by agglomeration of POSS nanoparticles. The SEM images of 40% POSS–PU–PBz composites evidently support the agglomerate formation of POSS particles. Besides, the thermal stability of the resulting POSS reinforced PU–PBz nanocomposites also increased to an appreciable extent.

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Introduction

With the rapid development of advanced technologies, the miniaturization of integrated circuitry devices require low k dielectric materials as an insulator to separate the conducting metal wires, due to their desirable properties such as reducing crosstalk, lowering power consumption and reducing resistance–capacitance time delay.^1,2 However, the dielectric constant of a material mainly depends on the polarization and density of that material; lower polarization and lower density of a material significantly reduces the value of dielectric constant.³ Accordingly, to acquire low k materials, hybrid organic–inorganic nanocomposites have been prepared by the introduction of a less polar organic backbone and porous inorganic silica based materials.^4–8 In recent years, the dielectric properties of several polymers have been studied. Despite this, the insufficient properties of polymers has meant that they need to be modified with inorganic materials to improve their thermo-mechanical and electrical properties.^9,10 Polyurethanes constitute one of the most versatile classes of polymeric materials with significant electrically insulating properties. However, polyurethanes have several shortcomings such as low resistance to moisture, low resistance to polar solvents, and inferior thermal stability.^11,12 To overcome these drawbacks, benzoxazine moieties can be introduced into the polyurethane system which can be further polymerized to form polyurethane–polybenzoxazine networks as high performance materials.^13–16 Polybenzoxazine is one of the excellent phenolic resins because of its excellent physical and chemical properties such as good thermal and mechanical properties, low moisture uptake, being flame retardant and admirable electrical properties.² To obtain polybenzoxazine, the benzoxazine monomer was synthesized and polymerized through ring opening and addition polymerization by a simple thermal treatment without using any catalyst. Benzoxazine monomer has been synthesized by Mannich condensation of phenol, amine and formaldehyde.¹⁷ In addition, the choice of precursors with less polarity are important in the reduction of value of dielectric constant of a material.³ Hence, the long chain aliphatic bridged benzoxazines and diisocyanates can be used widely to prepare low dielectric polyurethane based polybenzoxazine materials. In order to improve the thermal, mechanical and low k dielectric properties to an extent, the inorganic porous POSS (polyhedral oligomeric silsesquioxane) material has been incorporated into the polymer matrix to obtain high performance polymer nanocomposites.^2,7

POSS is a nanometer-sized cube like molecule, has porous inorganic core surrounded by eight organic tether groups, which have the general formula (RSiO_1.5)_n.¹⁸ The functionalized POSS has been demonstrated to be excellent platforms and building blocks for technological applications and architecture of novel organic–inorganic hybrids. The polymers can be attached covalently to functionalized POSS derivatives to form high performance hybrid materials.^2,4,19 The physical properties of POSS reinforced polymer nanocomposites are strongly influenced by the miscibility of the host polymer and the POSS derivative. The porous POSS cage enables them to significantly reduce the dielectric constant of the hybrid materials with improved thermal and mechanical properties.²⁰ The nanosized POSS particles have advantages over clays or conventional fillers because the POSS have monodispersion, a well-defined structure, high thermal stability, no trace metals, and can accommodate a sizable number of interfacial interactions between composite particles and polymer segments.^19,20 Consequently, the combination of polyurethane, polybenzoxazine and POSS based hybrids has great interest to the advancement of low k materials.

This study proposes, the preparation of POSS reinforced polyurethane based polybenzoxazine (POSS–PU–PBz) hybrids with a view to obtain low k dielectric material. Hexamethylenediisocyante was reacted with different ratios of hydroxyl terminated POSS and hydroxyl terminated benzoxazine to form polyurethane, simultaneously, which was further polymerized through ring opening and addition polymerization by thermal treatment to obtain POSS–PU–PBz hybrid composites.

Experimental

Materials

Analytical grades of hexamethylenediisocyante (HMDI), hexamethylene diamine, 4-hydroxy benzyl alcohol, aniline, paraformaldehyde, methanol, toluene, N-methylpyrrolidine (NMP) and de-ionized water were purchased from SRL, India. High purity tetraethylorthosilicate (TEOS), eugenol, dibutyltin dilaurate (DBTD), tetramethyl ammonium hydroxide (40% in methanol), chlorodimethylsilane, platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene [Pt(dvs)] catalyst were purchased from Sigma-Aldrich and were used as received without further purification.

Synthesis of hydroxyl terminated benzoxazine (OH–Bz) (Scheme 1)

To a solution of hexamethylene diamine (0.06 mol) in toluene, the formaldehyde (0.24 mol) was added and stirred for 30 minutes at 0 °C. Followed by 4-hydroxy benzyl alcohol (0.12 mol) was added to the reaction mixture and stirred for overnight at 100 °C. After the completion of reaction (monitored by TLC), the reaction mixture was extracted with ethyl acetate and washed with 0.5 N NaHCO₃ solution, water, brine and concentrated the organic layer. The product was obtained as brownish semi solid with 88% yield.

¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.11–6.74 (6H, Ar), 4.84 (4H, O–CH₂–N), 4.26 (Ar–CH₂–OH), 3.96 (4H, Ar–CH₂–N), 2.75–2.72 (4H, N–CH₂), 1.87–1.84 (4H, N–CH₂–CH₂), 1.72–1.61 (4H, N–CH₂–CH₂–CH₂).

Synthesis of H–POSS

The octahydridocubic polyhedral oligomeric silsesquioxane (H–POSS) was synthesized by the procedure as reported earlier.² Octaanion solution [Me₄N⁺]₈[SiO_2.5⁻]₈ was prepared by mixing of tetra-methyl ammonium hydroxide (2.01 ml), methanol (0.98 ml), and de-ionized water (0.73 ml) followed by dropwise addition of tetraethoxysilane (1.07 ml) under nitrogen atmosphere. Then the octanion solution (4.6 ml) was slowly added to the solution of dimethylchlorosilane (2.65 ml) in hexane (100 ml) at room temperature. The product H–POSS [HMe₂SiOSiO_1.5]₈ was obtained as white crystalline powder (yield 37%).

Synthesis of OH–POSS (Scheme 2)

Hydroxyl terminated polyhedral oligomeric silsesquioxane (OH–POSS) was synthesized by the addition of eugenol with H–POSS using Pt(dvs) catalyst. To a solution of H–POSS (0.004 mol) in toluene, Pt(dvs) catalyst (5 drops) was added under nitrogen atmosphere and stirred for 30 minutes at 30 °C. Followed by eugenol (0.032 mol) was added to the reaction mixture and stirred for overnight at 110 °C. After the completion of reaction, the vessel temperature was reduced to 30 °C and activated charcoal was added to the reaction mixture and filtered through celite, then the filtrate was concentrated by reduced vacuum using rotary evaporator to obtain the product (yield 93%).

¹H NMR (400 MHz, CDCl₃) δ (ppm): 6.83–6.65 (3H, Ar), 3.84 (3H, –OCH₃), 2.56–2.53 (2H, Ar–CH₂), 1.65–1.62 (2H, Ar–CH₂–CH₂), 0.65–0.62 (2H, Si–CH₂), 0.13 (6H, CH₃–Si–CH₃).

Synthesis of POSS–Bz–PU and POSS–PU–PBz (Scheme 3)

The procedure for preparation of polyurethane and polybenzoxazine is as follows; to a stirred solution of OH–Bz (0.0048 mol) and HMDI (0.0048 mol) in N-methylpyrrolidine (NMP), different weight percentages of OH–POSS were added separately and followed by the addition of a drop of dibutyltin dilaurate as a catalyst. Immediately the reaction mixture was poured into a respective glass mold, which are placed in an oven and the temperature was raised to 80 °C and allowed to stand for 3 h at the same temperature to form POSS and benzoxazine based polyurethane (POSS–Bz–PU) as well as the evaporation of solvent. Subsequently the polyurethane containing benzoxazine was cured stepwise at 120, 140, 160, 180, 200, 220 and 240 °C for 1 h each to obtained POSS and polyurethane based polybenzoxazine (POSS–PU–PBz).

Characterization

¹H NMR spectra were recorded on a Bruker-300 NMR spectrometer. Fourier-transform infrared (FTIR) spectra of KBr disks were obtained using a Bruker Tensor 27 FT-IR spectrophotometer. Thermogravimetric analysis of polybenzoxazine films were carried out with an Exstar 6300 at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The surface overview of the composites was identified from VEGA3 TESCAN scanning electron microscope (SEM). Sample required for the high resolution transmission electron microscopy (HRTEM) analysis was prepared by dispersions in ethanol and sonicated for 15 minutes. After that, the dispersed solution was dropped over the mesh of 200 copper nets. HRTEM images were captured using TECNAI G₂ S-Twin transmission electron microscope, with an acceleration voltage of 250 kV. Dielectric constant was determined by Broad band Dielectric Spectrometer (BDS), NOVOCONTROL Technologies GmbH & Co. (model Concept 80) at 30 °C.

Results and discussion

Characterization of monomers and polymer nanocomposites

Hydroxyl terminated benzoxazine (OH–Bz) and POSS surrounded by hydroxyl group (OH–POSS) were synthesized and confirmed using FTIR and ¹H NMR spectra. Scheme 1 shows the molecular structure and synthesis route of OH–Bz. Fig. 1 shows the FTIR spectrum of OH–Bz, the bands appeared in the ranges between 2948 cm⁻¹ and 2828 cm⁻¹ represents the aliphatic group of OH–Bz. The absorption bands related to N–C–O and C–O–C of benzoxazine ring are appeared at 942 cm⁻¹ and 1222 cm⁻¹, respectively. The band appeared at 1478 cm⁻¹ is attributed to the tri-substituted benzene ring.^7,9 The formation of benzoxazine ring (OH–Bz) was further supported by the ¹H NMR spectrum (Fig. 3), the peaks related to aromatic group are appeared in the ranges between 6.74 and 7.11 ppm. The peaks appeared at 4.84 and 3.96 ppm are correspond to (O–CH₂–N) and (Ar–CH₂–N) resonance of the benzoxazine ring, respectively.

Scheme 1 Synthesis of OH–Bz.

Fig. 1 FTIR spectra of OH–Bz and OH–POSS.

Fig. 2 FTIR spectra of neat PU–PBz and POSS–PU–PBz.

Fig. 3 ¹H NMR spectrum of OH–Bz.

Scheme 2 shows the synthesis method of OH–POSS. Fig. 1 shows the FTIR spectrum of OH–POSS, the absorption bands related to Si–O–Si stretching frequencies are appeared in the ranges from 1165 cm⁻¹ to 1081 cm⁻¹. The broad absorption band is appeared between 3527 cm⁻¹ and 3426 cm⁻¹ represents the hydroxyl group of POSS. The ¹H NMR spectral (Fig. 4) peaks evidently support that the formation of OH–POSS through the addition of eugenol with H–POSS. The appearance of peaks at 3.84 ppm and 0.13 ppm represents the methyl protons of methoxy group and dimethylsilane respectively. The peaks appeared in the range between 6.83 and 6.65 ppm represent aromatic protons and similarly the aliphatic protons are appeared in the ranges from 2.56 to 2.53, 1.65 to 1.62 and 0.65 to 0.62 ppm indicates the successful formation of OH–POSS. Further the synthesized OH–Bz and OH–POSS were copolymerized with hexamethylenediisocyanate (HMDA) to form polyhedral oligomeric silsesquioxane (POSS) and benzoxazine (Bz) based polyurethane (POSS–Bz–PU) as an intermediate and simultaneously which were polymerized through thermal process to obtain POSS and polyurethane based polybenzoxazine (POSS–PU–PBz) (Scheme 3). Subsequently, the prepared POSS–PU–PBz nanocomposites were characterized and confirmed using FTIR analysis as shown in Fig. 2. The disappearance of bands at 942 cm⁻¹ (N–C–O), 1222 cm⁻¹ (C–O–C) and 1478 cm⁻¹ (tri-substituted benzene ring) and the appearance of band appeared at 1366 cm⁻¹ (tetra-substituted benzene ring) indicate the occurrence of ring opening and addition polymerization of benzoxazine and the band appeared between 1108 and 1027 cm⁻¹ represents the presence of Si–O–Si bond. In addition, the bands appeared at 3422 cm⁻¹, 1701 cm⁻¹ and 1572 cm⁻¹ represents NH, C=O and C–N vibrational frequencies, respectively which indicate the successful formation of polyurethane based POSS reinforced polybenzoxazine network (POSS–PU–PBz).

Scheme 2 Synthesis of OH–POSS.

Fig. 4 ¹H NMR spectrum of OH–POSS.

Scheme 3 Preparation of POSS–PU–PBz nanocomposites.

Morphological properties

The formation of surface morphology of neat PU–PBz and POSS–PU–PBz were examined using scanning electron microscope (SEM) images. Fig. 5a–c shows the SEM images of neat PU–PBz (a), 30% POSS–PU–PBz (b) and 40% POSS–PU–PBz (c). The neat PU–PBz exhibits smooth surface morphology, whereas that of the 30% POSS reinforced PU–PBz shows bright dots with homogeneous distribution which might be the POSS particles. Besides, the 40% POSS reinforced PU–PBz shows heterogeneous surface with agglomeration of POSS particles which were identified as bright dots in the SEM image (Fig. 5c). Moreover, the uniform distribution of POSS particles might be contributes to the reduction of value of dielectric constant by introducing pores into the resulting POSS–PU–PBz nanocomposites. However, the agglomeration of POSS nanoparticles considerably increases the density of the nanocomposites which could rather increase the value of dielectric constant, due to the number of pores of the composites were decreased. In addition, the SEM images (Fig. 6a and b) of OH–POSS shows the porous cubic structure. Hence, the OH–POSS reinforced PU–PBz nanocomposites could possess porous behavior.

Fig. 5 SEM images of neat PU–PBz (a), 30% POSS–PU–PBz (b) and 40% POSS–PU–PBz.

Fig. 6 SEM images of OH–POSS (a and b).

The formation of internal microstructure of the composites are also contributes to the reduction in the value of dielectric constant of the materials, which have been studied by high resolution transmission electron microscope (HRTEM) images. Fig. 7 shows the HRTEM images of 30% POSS–PU–PBz which indicates the formation of porous and complex structured polymer network. The formation of homogenous distribution of POSS particles ascertained from the higher and lower magnifications of the TEM images of 30% POSS–PU–PBz (Fig. 7a and b). Thus the formation of porous structure might be contributes to the reduction of value of dielectric constant to an appreciable extent.

Fig. 7 HRTEM images of 30% POSS–PU–PBz (a and b).

Thermal properties

The thermal behavior of neat PU–PBz and POSS–PU–PBz nanocomposites were examined by thermogravimetric analysis (TGA) and are presented in Fig. 8. The weight loss of the polymer nanocomposites were measured as the temperature increases. In detail, the degradation temperature at 10% weight loss of polymer composites are listed in Table 1. The occurrence of initial degradation at below 200 °C is attributed to the removal of adsorbed moisture and residual solvent. The degradation temperature at above 300 °C represents the simultaneous degradation of polyurethane and polybenzoxazine polymer network and finally to yield char at 800 °C. The char yield of neat PU–PBz and POSS–PU–PBz nanocomposites are varied with varying weight percentages of POSS reinforcement and are listed in Table 1. In addition, the higher char yield of the polymer nanocomposites could have significant flame retardant properties.^21,22 However, 30% POSS reinforced PU–PBz possesses good thermal stability and higher char yield than that of neat PU–PBz. Thus this type of thermally stable polymer composite materials can be utilized in the microelectronics devices as an insulator.

Fig. 8 TGA curves of neat PU–PBz and POSS–PU–PBz nanocomposites.

Table 1 Weight loss, char yield (Y_c), dielectric constant and dielectric loss of neat PU–PBz and POSS–PU–PBz nanocomposites

Sample	T10 (°C)	Yc (%) at 800 °C	Dielectric constant (ε′) at 1 MHz	Dielectric loss (ε′′) at 1 MHz
Neat PU–PBz	321.0	9.8	2.95 ± 0.01	0.0181 ± 0.001
10% POSS–PU–PBz	311.0	19.1	2.41 ± 0.01	0.0161 ± 0.001
20% POSS–PU–PBz	285.9	26.2	2.11 ± 0.01	0.0144 ± 0.001
30% POSS–PU–PBz	326.6	33.9	1.94 ± 0.01	0.0124 ± 0.001
40% POSS–PU–PBz	350.2	36.8	2.26 ± 0.01	0.0140 ± 0.001

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Dielectric properties

The frequency dependency of dielectric constant of neat PU–PBz and POSS–PU–PBz are presented in Fig. 9 and also the values of dielectric constant at 1 MHz are listed in Table 1. Most of the aromatic based polybenzoxazine materials possess the dielectric constant value of about 3.5 at 1 MHz,^7,9 whereas the long aliphatic chain based neat PU–PBz exhibits the lower value of dielectric constant of 2.95. This might be due to the contribution of less polar nature of long aliphatic chain, hence the introduction of hexamethylene groups in both isocyanate unit and benzoxazine unit significantly reduces the polarization throughout the matrix and the value of dielectric constant as well. The earlier reports also submitted that the value of dielectric constant decreases with increasing the aliphatic chain length.^4,23 Despite, the neat PU–PBz shows higher value of dielectric constant than the requirement (k < 2.1) of international technology roadmap for semiconductors (ITRS).²⁴ Consequently, the development of dielectric behavior and thermal stability of PU–PBz is warranted for utilizing in the microelectronic devices. Hence, in order to improve the thermal stability as well as to reduce the value of dielectric constant of PU–PBz, the inorganic material of porous natured POSS was introduced into the PU–PBz matrix through covalent linkages. Due to the introduction of porous material (POSS) significantly contributes to reduce the density as well as in the reduction of dielectric constant value of POSS–PU–PBz by reducing the polarization throughout the system. Because of the pores were filled by air/vacuum which has dielectric constant value of about 1. Consequently, different weight percentages (10, 20, 30 and 40 wt%) of OH–POSS and OH–Bz were copolymerized with HMDI through the formation of urethane units and simultaneously the formation of POSS–PU–Bz was polymerized to obtain POSS–PU–PBz composites by thermal treatment. The values of dielectric constant of POSS–PU–PBz composites were decreased while increasing the weight percentages of OH–POSS (Table 1). Despite, the dielectric constant value was reduced up to 30% POSS–PU–PBz (k = 1.94) and beyond this weight percentage (40% POSS–PU–PBz, k = 2.26) the reverse trend was observed which might be due to the agglomeration of POSS nanoparticles as shown in SEM images of POSS–PU–PBz (Fig. 5). However, the 30% POSS–PU–PBz shows the lower value of dielectric constant of 1.94 than that of neat PU–PBz. Thus the synergistic properties of less polar nature of organic material and porous inorganic material mainly contribute to the reduction in the value of dielectric constant by reducing the polarization of the resulting nanocomposites. In addition, the dielectric loss of a material causes more power consumption. Fig. 10 shows the dielectric loss of neat PU–PBz matrix and POSS–PU–PBz nanocomposites with respect to frequency. The incorporation of POSS into the PU–PBz composites considerably reduces the value of dielectric loss and the values are listed in Table 1. 30% POSS–PU–PBz shows the lowest value of dielectric loss of 0.0124 which could contributes to the reduction of power consumption of a dielectric material.

Fig. 9 Dielectric constant of neat PU–PBz and POSS–PU–PBz nanocomposites.

Fig. 10 Dielectric loss of neat PU–PBz and POSS–PU–PBz nanocomposites.

Conclusion

With a view to develop low k dielectric materials, POSS (polyhedral oligomeric silsesquioxane) reinforced polyurethane (PU) based polybenzoxazine (PBz) nanocomposites have been prepared and their dielectric and thermal properties were examined. The neat polyurethane based polybenzoxazine material exhibits the value of dielectric constant of 2.95. The reinforcement of POSS into PU–PBz significantly reduces the dielectric constant values up to 30% POSS–PU–PBz (1.94) and beyond this weight percentage of POSS the reverse trend was observed which might be due to the formation of agglomeration of POSS nanoparticles. The thermal stability and char yield of the POSS reinforced polybenzoxazine nanocomposites were increased as increasing the concentration of POSS. This work provides a vital piece of information for producing low-k materials with high thermal stability and these materials can be utilized for the miniaturization of electronic devices as an effective insulating material.

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