Design of lamellar structured POSS/BPZ polybenzoxazine nanocomposites as a novel class of ultra low-k dielectric materials

Abstract

A novel class of lamellar structured polyhedral oligomeric silsesquioxane/bisphenol Z (POSS/BPZ) polybenzoxazine (PBz) nanocomposites was successfully designed by a facile one-step copolymerization technique. The chemical structures of the monomer and resulting polymer were characterized by Fourier transform infrared (FTIR) spectroscopy, ¹H, ¹³C, DEPT-135, ²⁹Si NMR (nuclear magnetic resonance) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The desired cross-linked lamellae structural arrangement of POSS/BPZ polybenzoxazine (PBz) nanocomposites was confirmed by transmission electron microscopy (TEM). The BPZ-PBz and POSS-PBz layers were self-assembled by intermolecular hydrogen bonding in such a way as to form the lamellar structure during ring opening polymerization. An advantage of this lamellar structure is that 30% POSS/BPZ polybenzoxazine composite exhibits an ultra low-k value of 1.7 at 1 MHz as well as high thermal stability.

---

Introduction

The design of smart, miniaturized microelectronic devices such as integrated circuits, memory devices, etc., has received enormous research interest directed towards to the negligible time delay which is necessary for ultra-fast electronic devices.¹ In general, the square of the dielectric constant is inversely proportional to the propagation velocity of the signal. Consequently, high speed electronic device circuits require low-k material to realize faster signal transmission without crosstalk.^2,3 To achieve this, it is important to develop the low-k dielectric materials that are required for efficient integrated circuits. It is well established that low-k silica and other related materials can prevent signal crossover with low power consumption.⁴ In this respect, many efforts have been made to reduce the dielectric constant (<1.8) using different kinds of material.^5–7 In particular, nanolevel porous inorganic materials, a variety of polymeric materials, and their combinations are being investigated as low-k dielectrics. At the same time, low-k composite materials should be compatible and have strong adhesion with improved thermal and mechanical stability. Based on these aspects, composite materials of interconnected structures with air gaps and nanopores have been studied extensively. Networked polymers such as polyimide, polyhedral oligomeric silsesquioxane (POSS), PEEK, and polybenzoxazine (PBz) have been widely reported as ultra low-k dielectric materials.^8–13 Among these, POSS is one of the well known nanoporous inorganic materials which contains eight organic groups with a cubic structural arrangement and has been demonstrated to be an excellent building block for high performance applications.⁴

In recent years, benzoxazine-based organic–inorganic hybrid network structures have received great research interest due to their unique structural, thermal and mechanical properties. The heterocyclic benzoxazine ring can be synthesized by Mannich condensation from phenolic derivatives, formaldehyde and primary amine. As well as the advantages of a thermally induced ring opening, addition polymerization of benzoxazine (Bz) does not require any catalyst and there are also no by-products. In addition, polybenzoxazine (PBz) possesses excellent thermal and mechanical properties, low moisture absorption, high carbon residue, low shrinkage and excellent electrical properties.¹⁴ More importantly, polybenzoxazine exhibits both inter- and intramolecular hydrogen bonding in which the intermolecular hydrogen bonding could be beneficial for self-assembly of polybenzoxazine composites.¹⁵ Hence, it is interesting to introduce the benzoxazine group into the porous polyhedral oligomeric silsesquioxane (POSS) compound for the preparation of self-assembled polybenzoxazine nanocomposites with the benefit of reducing the value of the dielectric constant. Pristine polybenzoxazine has a dielectric constant of ∼3.5 and it can be used as an ideal dielectric material for microelectronics applications.¹⁶ Meanwhile, hybridization of benzoxazine with ordered mesoporous materials such as POSS, SBA-15, or SiO₂ reduces the value of the dielectric constant as low as ∼2.^17–20 In addition to the above, their easy compatibility with organic functional materials such as oxazole, fluorinated compounds, polyimide, etc., have increased their scope in high performance applications.^{12,14,16,21–23} Moreover, the characteristic polarization and favorable structural arrangement of functionalized polybenzoxazine greatly reduces the dielectric constant as a low value is needed for practical applications.^12,24 Liu et al. reported methylmethacrylate (MMA) and POSS hybrid composites with an ordered lamellar structure and with an ultra low-k value of 1.47.²⁵ Further, they have found that the lamellar arrangement plays a vital role in reducing the value of the dielectric constant.^24–26 Similarly, Leu et al. reported that POSS/polyimide nanocomposites with an optimum value of 29% POSS exhibit a dielectric constant of 2.3 when compared to that of neat polyimide.¹⁰ To the best of our knowledge, no reports have been published up to the present time with regard to a lamellar structure based on POSS/BPZ-PBz nanocomposites. Specifically, 2-allyl phenol benzoxazine functionalized POSS with the desired network structure has not been studied yet for low-k dielectric applications. Hence, in the present work, an attempt has been made to develop a novel class of POSS/BPZ-PBz nanocomposites with an ordered lamellar structure by the copolymerization of polyhedral oligomeric silsesquioxane benzoxazine (POSS-Bz) and bisphenol Z benzoxazine (BPZ-Bz) with a view to reducing the value of the dielectric constant.

Experimental

Materials

Analytical grades of cyclohexanone, phenol, concentrated hydrochloric acid, acetic acid, aniline, paraformaldehyde, chloroform and toluene, were purchased from SRL, India. High purity tetraethylorthosilicate (TEOS), 2-allyl phenol, tetramethyl ammonium hydroxide (40% in methanol), chlorodimethylsilane, and 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 1,1-bis(4-hydroxyphenyl)-cyclohexane (bisphenol Z) (Scheme 1)

Typical synthesis of bisphenol Z (BPZ) was as follows: 10.37 ml of cyclohexanone (0.22 mol) was charged into a mixture of concentrated hydrochloric acid and acetic acid (2 : 1 v/v), subsequently phenol (19.32 ml, 0.1 M) was added dropwise to the reaction mixture and stirred for 48 h at 55 °C. The required amount of distilled water was added to the reaction mixture and the resulting pink-colored product was isolated by filtration and washed several times with hot water to remove the excess phenol before drying at 70–80 °C. Finally, the end-product was recrystallized using ethanol to obtain an off-white crystalline powder (yield: 78%).

Synthesis of bisphenol Z benzoxazine (BPZ-Bz) (Scheme 1)

To a solution of aniline (6.8 ml, 0.075 mol) in toluene, formaldehyde (5 g, 0.167 mol) was added and stirred for 30 min at 0 °C. Then, 10 g of bisphenol Z (0.037 mol) was added to the reaction mixture and stirred overnight at 80 °C. After completion of the reaction (monitored by TLC), the reaction mixture was extracted with ethyl acetate and washed with 2 N NaOH, water, brine and the organic layer concentrated to yield 95% brownish semi-solid.

Synthesis of 2-allyl phenol benzoxazine (AP-Bz) (Scheme 2)

To a solution of aniline (5 ml, 0.055 mol) in chloroform, formaldehyde (3.3 g, 0.11 mol) was added and stirred for 30 min at 0 °C. Subsequently, 7.12 ml of 2-allyl phenol (0.055 mol) was added to the reaction mixture and stirred overnight at 70 °C. After completion of the reaction, the reaction mixture was extracted with ethyl acetate and washed with 2 N NaOH, water, brine and the organic layer concentrated to yield 97% pale yellow liquid.

Synthesis of OHC-POSS (Scheme 3)

The octahydridocubic polyhedral oligomeric silsesquioxane (OHC-POSS) was synthesized by the procedure as reported.^27,28 Octaanion solution [Me₄N⁺]₈[SiO_2.5⁻]₈ was prepared by mixing tetramethyl ammonium hydroxide (8.04 ml), methanol (3.91 ml), and deionized water (2.93 ml) followed by dropwise addition of tetra-ethoxysilane (4.28 ml) under nitrogen atmosphere. OHC-POSS [HMe₂SiOSiO_1.5]₈ was produced by the reaction of octaanion solution (18.5 ml) with dimethylchlorosilane (10.6 ml) to yield 38% of white crystalline powder.

Synthesis of POSS-Bz (Scheme 4)

Finally, benzoxazine functionalized polyhedral oligomeric silsesquioxane (POSS-Bz) was synthesized by the addition of AP-Bz to OHC-POSS using Pt(dvs) catalyst. To a solution of OHC-POSS (10 g, 0.009 mol) in chloroform, Pt(dvs) catalyst (8 drops) was added under nitrogen atmosphere and stirred for 30 min at 30 °C. Subsequently a solution of AP-Bz (21 g, 0.08 mol) in chloroform was added to the reaction mixture and stirred overnight at 110 °C. After the completion of reaction, the temperature was reduced to 30 °C, activated charcoal was added to the reaction mixture and the mixture filtered through celite. The filtrate was then concentrated with a rotary evaporator to yield 91% of product as a pale yellow semi-solid.

Preparation of neat polybenzoxazine matrix

In a glass mold, solutions of POSS-Bz and BPZ-Bz in tetrahydrofuran (THF) were separately heated at 100 °C overnight to evaporate the solvent, and then each was cured stepwise at 120, 140, 160, 180, 200, 220, 240 or 260 °C for 1 h to obtain a dark brown film.

Preparation of POSS/PBz nanocomposites

Various weight percentages of POSS-Bz (10, 20, 30, 40 and 50 wt%) were added to 2 g of BPZ-Bz dissolved in 10 ml THF. The resulting solution was stirred for 30 min at 30 °C. The solutions were poured into respective glass molds and were heated at 100 °C for 3 h and then each was cured stepwise at 120, 140, 160, 180, 200, 220, 240 or 260 °C for 1 h.

Characterization

¹H and ¹³C NMR spectra were recorded on a Brucker-300 NMR spectrometer. ²⁹Si NMR spectra were obtained with a Brucker-500 NMR spectrometer. Fourier-transform infrared (FTIR) spectra of KBr disks were obtained using a Bruker Tensor 27 FT-IR spectrophotometer. The X-ray diffraction analyses of the samples were carried out using a Rigaku, miniflux II-C X-ray diffractometer (30 kV, 20 mA) with a copper target (1.54 Å) at a scan rate of 4° min⁻¹. Thermogravimetric and differential scanning calorimetric analyses of the 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 with a FEI QUANTA 200F high resolution scanning electron microscope (HRSEM). Samples for high resolution transmission electron microscopy (HRTEM) analysis were first prepared by putting POSS-Bz/BPZ-Bz nanocomposite films into epoxy capsules and curing at 70 °C for 24 h in a vacuum oven. After that, the cured epoxy samples were microtomed with a Leica Ultracut Uct into about 100 nm thick slices and placed over 200 mesh copper nets. HRTEM images were captured using a TECNAI G₂ S-Twin transmission electron microscope, with an acceleration voltage of 250 kV.

Results and discussion

Scheme 1a represents the synthesis route for bisphenol Z (BPZ) and the corresponding FT-IR spectrum is shown in Fig. 1a. The peaks at 2931 cm⁻¹, 2853 cm⁻¹ and 819 cm⁻¹ are assigned to a para-substituted benzene ring, and symmetric and asymmetric stretching C–H bonds, respectively.²⁸ The ¹H NMR spectrum of BPZ (Fig. 2a) shows peaks at 8.25 ppm, 7.05–6.72 ppm and 2.17–1.46 ppm, associated with hydroxyl, aromatic and aliphatic cyclohexyl ring protons, respectively, which confirms the successful formation of BPZ.

Scheme 1 Synthesis of BPZ (a) and BPZ-Bz (b).

Fig. 1 FTIR spectra of (a) BPZ, BPZ-Bz, AP-Bz, OHC-POSS and POSS-Bz and (b) POSS/BPZ-PBz polybenzoxazine nanocomposites.

Fig. 2 ¹H NMR spectra of BPZ (a) and BPZ-Bz (b).

The Mannich condensation of BPZ and aniline to form BPZ-Bz monomer is represented in Scheme 1b. From the FT-IR spectrum shown in Fig. 1a, the bands related to a tri-substituted benzene ring (N–C–O), and (C–O–C) can be seen at 1508 cm⁻¹, 948 cm⁻¹ and 1232 cm⁻¹, respectively.^28,29 The ¹H NMR peaks at 5.31 ppm and 4.58 ppm in Fig. 2b correspond to (O–CH₂–N) and (Ar–CH₂–N) resonance of the benzoxazine ring.

AP-Bz was synthesized by condensation of 2-allyl phenol with aniline as represented in Scheme 2. The FT-IR bands (Fig. 1a) at 941 cm⁻¹ (N–O–C) and 1221 cm⁻¹ (Ar–C–N) confirm the formation of benzoxazine and this was further supported by the ¹H NMR peaks at 5.37 ppm (O–CH₂–N) and 4.62 ppm (Ar–CH₂–N) in the spectrum shown in Fig. 3.

Scheme 2 Synthesis of AP-Bz.

Fig. 3 ¹H NMR spectrum of AP-Bz.

OHC-POSS preparation is shown in Scheme 3 and its chemical structure was confirmed by FT-IR and ¹H NMR spectra as shown in Fig. 1a and 4b, respectively. Strong FT-IR bands are positioned at 1097 cm⁻¹, 2145 cm⁻¹ and 902 cm⁻¹ associated with Si–O–Si, and Si–H stretching and bending vibrations of OHC-POSS.²⁰ The ¹H NMR peaks appeared in the range 4.74–4.72 ppm (Si–H), and 0.26–0.25 ppm [Si(CH₃)₂] as a multiplet, indicating the successful formation of OHC-POSS.

Scheme 3 Synthesis of OHC-POSS.

Fig. 4 XRD pattern (a) and ¹H NMR spectrum (b) of OHC-POSS.

Scheme 4 shows the addition reaction between AP-Bz and OHC-POSS. The appearance of new FT-IR bands at 1166 cm⁻¹ (Si–CH₂–CH₂) and the absence of Si–H at 2145 cm⁻¹ in Fig. 1a confirms the addition reaction to form POSS-Bz. The ²⁹Si NMR spectrum as shown in Fig. 5a ascertains the presence of Si–O–Si (−102.95 ppm) and Si-(CH₃)₂ (18.78 ppm) in POSS-Bz.³⁰ The corresponding ¹H NMR shifts in the regions 1.59–1.57 ppm and 0.65–0.61 ppm are related to Si–CH₂–CH₂ and Si–CH₂ in POSS-Bz cages and are shown in Fig. 5b. The two possible methods of addition are shown in Scheme 4. The precise addition reaction has been confirmed from the ¹³C and DEPT-135 NMR spectra and is illustrated in Fig. 6a and b. The ¹³C NMR spectrum of POSS-Bz shows all the carbon related (C, CH, CH₂, CH₃) peaks but the DEPT-135 NMR only shows the CH and CH₃ peaks that appeared opposite the CH₂ peak, apart from the C peak. In this case, the ¹³C NMR peaks at 152.2, 148.5, 130.5 and 120.29 ppm, are attributed to respective aromatic C peaks which are not observed in DEPT-135 NMR. Moreover, the DEPT-135 NMR shows five CH₂ peaks in the aliphatic up-field region and six CH peaks in the aromatic down-field region. The absence of the CH₃ peak in the aliphatic region indicates the formation of POSS-Bz (b) [Scheme 4].

Scheme 4 Synthesis of POSS-Bz.

Fig. 5 ²⁹Si (a) and ¹H NMR (b) spectra of POSS-Bz.

Fig. 6 ¹³C (a) and DEPT-135 NMR (b) spectra of POSS-Bz.

Fig. 7 Schematic representation of BPZ/POSS polybenzoxazine nanocomposites with cross linked lamellae and a photograph of the film.

The formation of polybenzoxazine nanocomposites was confirmed by FT-IR spectra. Fig. 1b shows the FT-IR spectra of polybenzoxazine nanocomposites. The characteristic absorption bands at 945 cm⁻¹ (out of plane bending vibration of C–H) and 1463 cm⁻¹ (tri-substituted benzene ring) gradually disappeared. Meanwhile, a new absorption band appearing at 1412 cm⁻¹ and attributed to a tetra-substituted benzene ring, indicates the ring-opening polymerization of benzoxazine.²⁸ This is further confirmed by the DSC analysis and described in detail in the thermal analysis section.

The stepwise structural modifications of POSS-Bz copolymer have been investigated by low angle X-ray diffraction. Fig. 8 shows the XRD patterns of respective POSS-Bz monomers and their conjugated polymers. From the pattern, it can be seen that OHC-POSS exhibits (Fig. 4a) highly crystalline features. The peak positions and corresponding d-space values are found at 8.01°, 10.72°, 11.86°, 18.61°, 24.1° and 10.5 Å, 8.0 Å, 7.2 Å, 4.6 Å, 3.6 Å, respectively. These values can be indexed to a rhombohedral crystal structure and are also in good agreement with earlier reports.¹⁰ In contrast, the benzoxazine monomer shows a predominant amorphous phase (Fig. 8). Also, there is no evidence of any crystalline phase in the POSS-Bz monomer. This result further confirms the functionalization of AP-Bz with OHC-POSS which disrupts the crystallinity of OHC-POSS to a significant extent. However, copolymerization of POSS-Bz with BPZ-Bz shows a different diffraction pattern, as can be seen in Fig. 8. With the concentration of POSS-Bz, the crystallinity enhances, in particular, 30% POSS-Bz hybridized polymer composite shows the highest crystallinity and this may lower its dielectric constant to an extremely low value as a result of its beneficial structural arrangement. This observation is consistent with earlier reports.^10,12,31 When the POSS-Bz concentration increases above 30%, the crystallinity of the composites decreases towards amorphous due to the aggregation of POSS-PBz.

Fig. 8 XRD pattern of pristine and POSS-Bz copolymerized nanocomposites.

In order to understand the nature of the surface and incorporation of POSS-Bz in the polymer network, the samples were analyzed by scanning electron microscope (SEM). Fig. 9 shows SEM micrographs of pure BPZ-PBz (Fig. 9a), 30% POSS/BPZ-PBz (Fig. 9b), Si mapping of 30% POSS/BPZ-PBz (Fig. 9c) and 100% POSS-PBz (Fig. 9d). The SEM micrograph of pristine BPZ-PBz film (Fig. 9a) shows the dense morphology with a large number of voids. This mainly arises from ring opening polymerization of cyclohexyl ring functionalized BPZ-Bz that enhances the external porosity of the pure BPZ-Bz as well as the composite polymers. After the copolymerization process, the SEM images show the uniformly distributed crystallite aggregates of POSS-Bz, in addition to the voids created by the BPZ-Bz. Moreover, the visible open pores with dark background in 30% POSS-Bz composite film (Fig. 9b) further increase the free volume space which also reduces the dielectric constant.^32,33 Conversely, the 100% POSS-PBz film (Fig. 9d) shows that a large amount of crystallite aggregates are discernible when compared to the 30% POSS-Bz composite. Fig. 9c describes the results of Si-mapping of the 30% POSS-Bz composite and demonstrates the uniform distribution of POSS-Bz over the surface.

Fig. 9 SEM images of pure BPZ-PBz (a), 30% POSS/BPZ-PBz (b), Si mapping of 30% POSS/BPZ-PBz (c) and 100% POSS-PBz (d) nanocomposites.

The internal microstructure of the 30% POSS-Bz composite film has been observed with HRTEM analysis (Fig. 10). The TEM images clearly indicate the ordered lamellae structure with multilayers. From the higher magnification images shown in Fig. 10b and c, a well separated unidirectional multilayer arrangement with a large number of crosslinks can be seen. It is possible that the observed dark layers are associated with POSS-PBz and the less intense layers correspond to BPZ-PBz. In this order, the layers have been arranged and form the desired lamellae structure as represented in Fig. 7.^10,12 During the ring opening copolymerization process, there is more likelihood for the formation of hydrogen bonding which may self-assemble the BPZ-PBz and POSS-PBz layers together to form the ordered lamellae.

Fig. 10 HRTEM micrographs for 30% POSS-Bz/BPZ-Bz polymer nanocomposites (a 1 μm & b, c 200 nm).

Thermal stability is one of the important factors for interlayer dielectric materials. Herein, the thermal stability of BPZ-Bz and POSS-Bz polymer nanocomposites has been studied by thermogravimetric analysis. Typical TGA curves for pristine and composite films are shown in Fig. 11. As expected, the BPZ-Bz copolymerized with POSS-Bz exhibits higher thermal stability than the pristine BPz-PBz. Thermal stability is directly associated with the presence of a POSS network which strengthens the BPZ-Bz chains during copolymerization. When the POSS concentration is increased, a gradual increment in thermal stability is observed in the resulting composites. Although the maximum thermal stability was obtained for POSS-PBz, this may be attributed to the formation of a completely stable POSS network.¹⁴ In detail, the initial weight loss in the range 240–300 °C is probably the result of removal of solvent residues. The major weight loss above 300 °C is associated with degradation of the polymer network which is consistent with an earlier investigation.³⁴ A typical DSC profile for the monomer shows (Fig. 12a and b) a broad exothermic peak above 175 °C as an indication of the BPZ-PBz curing temperature. With 30% POSS, only a single exothermic peak maximum at 224 °C was observed which indicates co-reaction between the oxazine rings of POSS-Bz and BPZ-Bz.^14,35

Fig. 11 TGA curve of BPZ-PBz and POSS-PBz nanocomposites.

Fig. 12 DSC profile of BPZ-Bz and 30% POSS/BPZ-Bz nanocomposite before (a) and after (b) polymerization.

In microelectronics applications, a material with an extremely low dielectric constant is most desirable, so the development of such novel materials is warranted. Extensive efforts are being made by a number of researchers in order to reduce the dielectric constant to the near-equivalent of air (1).^25,36 Generally, polymer systems which offer low-k values sacrifice other structural, chemical and thermal properties. Therefore, careful study is required before these materials can be used to their full potential in commercial appliances. It is well established that the dielectric constant is a tunable factor and the introduction of mesopores in the polymer matrix can result in lowering of the value of the dielectric constant at an extreme level down to ∼1.7. Similarly, it is also possible to reduce the dielectric constant by altering the chemical and physical structure of the polymer matrix using functionalization processes.^14,16,21 The former method uses mesoporous materials, such as POSS, SBA-15, etc., which create pores in the polymer matrix thus lowering the dielectric constant to a significant extent.^10,20 The latter method requires a tedious synthesis process to alter the structure.

According to earlier reports on benzoxazine-based nanocomposites, they exhibited a dielectric constant of about ∼1.8 and this needed to be improved to meet their full potential in applications.³⁷ In this study, we designed a novel POSS/BPZ-PBz composite film with a layered microstructure and this has achieved an ultra low dielectric constant value of 1.7 ± 0.01 at the optimum concentration of 30% POSS/BPZ-PBz nanocomposites (Table 1). To the best of our knowledge, this is the lowest value reported in the case of POSS/BPZ-PBz hybrid nanocomposites. The novel type of 2-allyl phenol benzoxazine functionalized POSS plays a crucial role in imparting the desired lamellar structure which, in turn, contributes to give an ultra low-k value for the hybrid composites. Interestingly, the neat polymer showed a relatively low k-value of 3.49 which is also comparatively lower than the values reported earlier.^16,17,38 This may be due to the presence of larger voids that enhance the external porosity which in turn reduces the k-value considerably. In the present composite systems, with an increase in the concentration of POSS-Bz up to 30 wt%, the corresponding k-value decreases, but it increases with further increases in POSS-Bz concentration. Neat POSS-PBz exhibits a higher value of dielectric constant (2.9) owing to large aggregates of POSS crystallites and completely packed voids.

Table 1 Weight loss, char yield (Y_c) and dielectric constant (k) of neat BPZ-PBz and POSS/BPZ-PBz nanocomposites

Sample	T5 (°C)	T10 (°C)	Yc (%)	Dielectric constant (k)
BPZ-PBz	186.2	301.2	11.2	3.49 ± 0.01
10%POSS-Bz/BPZ-Bz	184.0	242.8	32.8	2.03 ± 0.01
20%POSS-Bz/BPZ-Bz	204.6	294.0	34.7	1.86 ± 0.01
30%POSS-Bz/BPZ-Bz	229.3	322.5	36.1	1.70 ± 0.01
POSS-PBz	345.7	423.7	63.8	2.89 ± 0.01

---

The observed ultra low-k (Fig. 13) for the POSS/BPZ-PBz composites can be explained by the following structural features; first, the self-assembled layered structure with unidirectional orientation is responsible for reducing the polarization by increasing the inter- and intra-layer distances.^12,24–26 In particular, the layer thickness of the dark region (POSS-PBz) is higher compared to that of the BPZ-PBz layer and this increases the internal porosity by almost two orders of magnitude due to the high volume fraction of POSS in the film. Therefore, the reduction in dielectric constant is strongly influenced by the proportion of POSS in the composites. A 30% POSS-Bz concentration imparts the desired lamellae structure with enhanced crystallinity, which in turn results in the ultra low-k value at this concentration.

Fig. 13 Frequency dependence of dielectric constants of BPZ-PBz and POSS-PBz nanocomposites.

In addition, the cyclohexyl functionalized benzoxazine monomer (BPZ-Bz) provides external voids to enhance the free volume of the composite, and this also plays a significant role in reducing the value of the dielectric constant. By further increasing the POSS concentration up to 100%, the resulting larger aggregates and lower crystalline features enhance the value of the dielectric constant due to a reduction in voids and dense packing. Dielectric loss is one of the key factors in understanding the utilization of dielectric material in microelectronics. Typical frequency versus dielectric loss curves for POSS/BPZ polybenzoxazine nanocomposites are shown in Fig. 14. The observed dielectric loss is ultimately very low (0.0019) at 1 MHz for lamellar structured 30% POSS-Bz composite, which also has an ultra low-k value. From this investigation, it is suggested that the design and introduction of lamellar structure into POSS/BPZ-PBz hybrid nanocomposites could be effectively used as an interesting candidate for microelectronic devices.

Fig. 14 Frequency dependence of dielectric loss of BPZ-PBz and POSS-PBz nanocomposites.

Conclusion

With a view to reducing the dielectric constant, we successfully designed a novel hybrid POSS/BPZ polybenzoxazine nanocomposite system. As expected, the composite with 30% POSS exhibited an ultra low-k value of 1.7 ± 0.01 at 1 MHz. The low-k value is mainly attributed to the highly ordered lamellar network with distinct crosslinks. Meanwhile, the thermal stability and dielectric constant have been tuned by varying the POSS concentration without loss of transparency. This kind of hybrid composite can expect to find potential application in advanced microelectronic devices.

Acknowledgements

The authors thank DST Nanomission (SR/NM/NS-18/2010), New Delhi, Govt. of India., for financial support. The authors thank Dr K. Gunasekaran and Mr M. Kesavan, Dept. of Crystallography and Biophysics, University of Madras, for providing the NMR facility.

References

1. Y. Huang and J. Economy, Macromolecules, 2006, 39, 1850–1853.
2. Y. Watanabe, Y. Shibasaki, S. Ando and M. Ueda, Polym. J., 2006, 38, 79–84.
3. G. Maier, Prog. Polym. Sci., 2001, 26, 3–65.
4. C. Zhang, S. Guang, X. Zhu, H. Xu, X. Liu and M. Jiang, J. Phys. Chem. C, 2010, 114, 22455–22461.
5. K. Su, D. R. Bujalski, K. Eguchi, G. V. Gordon, D. Li Ou, P. Chevalier, S. Hu and R. P. Boisvert, Chem. Mater., 2005, 17, 2520–2529.
6. G. D. Fu, Y. Zhang, E. T. Kang and K. G. Neoh, Adv. Mater., 2004, 16, 839–842.
7. S. Nunomura, K. Kog, M. Shiratani, Y. Watanabe, Y. Morisadal, N. Matsukil and S. Ikedal, Jpn. J. Appl. Phys., 2005, 44, 1509–1511.
8. Z. Geng, J. Ba, S. Zhang, J. Luan, X. Jiang, P. Huo and G. Wang, J. Mater. Chem., 2012, 22, 23534–23540.
9. S. Nagendiran, M. Alagar and I. Hamerton, Acta Mater., 2010, 58, 3345–3356.
10. C.-M. Leu, Y.-T. Chang and K.-H. Wei, Macromolecules, 2003, 36, 9122–9127.
11. M. Abdul Wahab, K. Yi Mya and C. He, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 5887–5896.
12. C.-M. Leu, G. Mahesh Reddy, K.-H. Wei and C.-F. Shu, Chem. Mater., 2003, 15, 2261–2265.
13. Y. Chen, L. Chen, H. Nie and E. T. Kang, J. Appl. Polym. Sci., 2006, 99, 2226–2232.
14. K. Zhang, Q. Zhuang, X. Liu, G. Yang, R. Cai and Z. Han, Macromolecules, 2013, 46, 2696–2704.
15. C.-H. Lu, S.-W. Kuo, W.-T. Chang and F.-C. Chang, Macromol. Rapid Commun., 2009, 30, 2121–2127.
16. Yi-C. Su and F.-C. Chang, Polymer, 2003, 44, 7989–7996.
17. M. R. Vengatesan, S. Devaraju, K. Dinakaran and M. Alagar, J. Mater. Chem., 2012, 22, 7559–7566.
18. A. Chandramohan, K. Dinkaran, A. Ashok Kumar and M. Alagar, High Perform. Polym., 2012, 24, 405–417.
19. S. Devaraju, M. R. Vengatesan, M. Selvi, A. Ashok Kumar, I. Hamerton, J. S. God and M. Alagar, RSC Adv., 2013, 3, 12915–12921.
20. B.-h. Yang, H.-y. Xu, Z.-z. Yangc and X.-y. Liu, J. Mater. Chem., 2009, 19, 9038–9044.
21. K. Zhang, Q. Zhuang, X. Liu, R. Cai, G. Yang and Z. Han, RSC Adv., 2013, 3, 5261–5270.
22. J. Lin and X. Wang, Polymer, 2007, 48, 318–329.
23. K. R. Carter, R. A. DiPietro, M. I. Sanchez and S. A. Swanson, Chem. Mater., 2001, 13, 213–221.
24. H.-C. Liu, W.-C. Sub and Y.-L. Liu, J. Mater. Chem., 2011, 21, 7182–7187.
25. Y.-L. Liu and M.-H. Fangchiang, J. Mater. Chem., 2009, 19, 3643–3647.
26. M.-C. Tseng and Y.-L. Liu, Polymer, 2010, 51, 5567–5575.
27. J. Choi, J. Harcup, A. F. Yee, Q. Zhu and R. M. Laine, J. Am. Chem. Soc., 2001, 123, 11420–11430.
28. M. R. Vengatesan, S. Devaraju, K. Dinakaran and M. Alagar, Polym. Compos., 2011, 32, 1701–1711.
29. T. Agag, C. R. Arza, F. H. J. Maurer and H. Ishida, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 4335–4342.
30. J. Rathore, Q. Dai, B. Davis, M. Sherwood, R. D. Miller, Q. Linb and A. Nelson, J. Mater. Chem., 2011, 21, 14254–14258.
31. S.-W. Kuoa and F.-C. Chang, Prog. Polym. Sci., 2011, 36, 1649–1696.
32. Y. Chena and En-T. Kang, Mater. Lett., 2004, 58, 3716–3719.
33. Y.-J. Leea, J.-M. Huangb, S.-W. Kuoa, J.-S. Lua and F.-C. Chang, Polymer, 2005, 46, 173–181.
34. Y.-L. Liu and Yi-W. Chen, Macromol. Chem. Phys., 2007, 208, 224–232.
35. W.-Y. Chen, K. Shan Ho, T.-H. Hsieh, F.-C. Chang and Y.-Z. Wang, Macromol. Rapid Commun., 2006, 27, 452–457.
36. H. Chen, L. Xie, H. Lu and Y. Yang, J. Mater. Chem., 2007, 17, 1258–1261.
37. G. D. Fu, Z. Yuan, E. T. Kang, K. G. Neoh, D. M. Lai and A. C. H. Huan, Adv. Funct. Mater., 2005, 15, 315–322.
38. C. H. Lin, S. J. Huang, P. J. Wang, H. T. Lin and S. A. Dai, Macromolecules, 2012, 45, 7461–7466.

---

Footnote

† Electronic supplementary information (ESI) available: FTIR and NMR spectra are provided for further information. See DOI: 10.1039/c4ra01905a

---