Development of a polybenzoxazine/TSBA-15 composite from the renewable resource cardanol for low-k applications

Abstract

The current work describes the synthesis of a novel cardanol-based benzoxazine monomer (CBz) from the renewable resource cardanol using caprolactamdiamine (CPLDA) by a solvent-free method. Thiol-functionalized mesoporous silica (TSBA-15) was incorporated into the cardanol-based benzoxazine matrix (PCBz) and the structure of thiol-functionalized mesoporous silica/cardanol based polybenzoxazine(TSBA-15/PCBz) composite was confirmed by FT-IR and NMR analysis. The surface morphology of TSBA-15/PCBz was determined using SEM and TEM. From the TEM results, it was observed that the dispersion of thiol-functionalized mesoporous silica into PCBz forms a fibrous material containing free volume. The thermal properties of this fibrous materials were studied by using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The thermal properties were observed to depend on the weight percentage of the TSBA-15 material. Dielectric data obtained from impedance analysis shows that the TSBA-15/PCBz composites have lower dielectric constant values, and dielectric loss than that of neat PCBz material. The results indicate that TSBA-15/PCBz composites obtained in a renewable manner from waste cardanol can find applications in microelectronics as electrical resistors.

---

Introduction

Inorganic materials that are traditionally used as dielectrics are on the one hand brittle and use considerable amounts of energy for processing, but on the other hand possess several superior qualities such as excellent thermal, dielectric and magnetic properties.¹ In contrast, polymers are flexible with high resistivity and good processibility, while showing poor thermal and dielectric properties. Combining these two materials in the form of polymer hybrid composites could lead to the development of a new type of material with improved dielectric and electrical insulation properties. Both materials with low dielectric constants (k) and those with high values of k are essential in the electronics industries. A low dielectric constant is required for insulators. The applications of these passivation materials are wide ranging and have included isolation of signal-carrying conductors from each other, fast signal propagation, interlayer dielectric to reduce resistance–capacitance (RC) time delays, crosstalk and power dissipation in high-density and high-speed integration.² In advanced integrated circuit (IC) chips, polymers such as polyimides, polybenzoxazine, polyarylethers, fluoropolymers, and polysilsesquioxanes, have been employed as potentially low-k materials.^3,4 Among the polymeric dielectrics, polybenzoxazines constitute a relatively new class of novel thermoset polymeric materials that are formed by thermally activated ring opening polymerization without any catalyst and without generating any byproducts.^5,6

Polybenzoxazines exhibit unique properties such as nearly zero shrinkage upon curing, good thermal stability, good chemical resistance, low water absorption and a wide processing window.^7–11 Though the dielectric constants of polybenzoxazines are low, they need to be reduced further to find better utility in microelectronics applications. Different approaches have been investigated to reduce the dielectric constant of polybenzoxazine, such as the incorporation of a fluorinated substituent into the polybenzoxazine skeleton,¹² the use of nano-porous poly(caprolactone-cobenzoxazine)¹³ and the introduction of an air gap into the interconnected structures by blending them with nanoporous materials.^14–16 The dielectric constant can be reduced by embedding voids of air, which has a dielectric constant of one, into the polymer matrix.^17,18 The mesoporous silica nanofillers have received much attention due to their ordered structures, high surface area and ease with which their nanopores can be functionalized. The nanopores are sufficiently porous to accommodate macromolecules, which lead to intimate interactions between the polymer and the inorganic phase.¹⁹

Benzoxazines are generally synthesized from petrochemicals, which constitute a depleting resource. Several research groups have reported the synthesis of benzoxazine monomers from renewable sources.^20–22 The current report describes, for the first time to the best of our knowledge, the preparation of a novel dielectric materials from the renewable agrochemical cardanol using caprolactamdiamine in a solvent-free method. A novel attempt was made to improve the properties of polybenzoxazine using cardanol as a renewable resource. Further, the effects of weight percentage of thiol-functionalized mesoporous silica SBA-15 on thermal and dielectric properties of polybenzoxazine were investigated. In order to obtain a well-dispersed composite with enhanced interfacial adhesion, the surfaces of the nanoparticles were modified with MPTMS. Thus, the prepared TSBA-15/polybenzoxazine nanocomposite is expected to display flexibility along with low dielectric characteristics.

Experimental

Materials

Cardanol was procured from Satya Cashew Chemicals Pvt. Ltd. Chennai, (India), paraformaldehyde, 6-aminocaproic acid, 4,4′-methylene dianiline and N-methylpyrrolidone (NMP) were obtained from Sigma-Aldrich.

Synthesis of a caprolactam diamine (CPLDA)

Caprolactam-based diamine (CPLDA) was synthesized by using caprolactam, 4,4′-diaminodiphenylmethane, NMP and alkali hypophosphate as a catalyst. A volume of 100 mL of NMP, 10 g (0.0883 mol) of caprolactam, 7.8 g (0.0442 mol) of 4,4′-diaminodiphenylmethane and 5 mg of alkali hypophosphate catalyst were added into a 250 mL two-necked round-bottomed flask equipped with a condenser. The above reaction mixture was allowed to react at 160 °C for 24 h with efficient agitation to facilitate completion of reaction. The resulting product was filtered as a solid material with a yield of 85% (Scheme 1).

Scheme 1 Synthesis of N,N-(4,4-methylene bis(4,1-phenylene))bis(6-aminohexamine) (CPLDA).

Synthesis of cardanol benzoxazine monomer (CBz)

The cardanol-based benzoxazine monomer was synthesized by using a single-step condensation reaction of aromatic caprolactamdiamine (CPLDA), cardanol and paraformaldehyde by following a solvent-free method.²³ A mixture of cardanol (6.59 g, 0.022 mol), paraformaldehyde (1.39 g, 0.046 mol), and CPLDA (5 g, 0.011 mol) was added into a 500 mL three-neck round-bottom flask equipped with a magnetic stirrer and refluxing condenser. The reaction mixture was refluxed at 85 °C for 1 h and at 130 °C for another 5 h. Then the reaction mixture was allowed to attain room temperature. Chloroform (100 mL) was added to the above reaction mixture and the organic layer was washed with aqueous sodium hydroxide (2 N) followed by distilled water. The organic layers were combined, dried over sodium sulphate and filtered to give a red oil. The solvent was removed under reduced pressure and the residue was dried at 353 K under vacuum to give benzoxazine (CBz) as a brown viscous solid (Scheme 2). ¹H-NMR (300 MHz, CDCl₃): 0.86–0.93 (t, –CH₃, 6H (a)), 1.2–1.3 (m, –CH₂, 20H (b)), 1.4–1.86 (m, –CH₂CH=, 8H (c)), 1.96–2.25 (–CH₂–CO (d)), 2.6 (t, –CH₂–N– (e)), 2.8–3.0 (t, CH₂Ar (f)), 6H, 3.2 (s, Ar–CH₂–Ar (g)), 3.8 (s, ArCH₂N– (h)), 4.6 (s, O–CH₂–N– (j)), 5.20–5.50 (m, –CH=CH– (k)), 6.63–6.80 (Ar–H, 6H (l)), 6.9–7.40 (Ar–H, m (m, n)), 6.90 (–N–H (p)). FTIR (KBr disc): 3008, 2950, 2848, 1620, 1247, 1172, 1100 and 960 cm⁻¹. MALDI mass: m/z = 1075.6.

Scheme 2 Synthesis of cardanol benzoxazine (CBz).

Synthesis of SBA-15

About 4 g of Pluronic polymer was dissolved in 150 mL of 1.6 M HCl (pH < 2). After heating the above reaction mixture to 40 °C, 0.3 g of TMB (swelling agent) and 9.2 mL of TEOS (inorganic precursor) were added under vigorous stirring conditions. The reaction mixture was then transferred into a tightly sealed Teflon bottle. The bottle was aged at 80 °C for 24 h. The mixture was then cooled, after which it was filtered by vacuum filtration to produce a white powder sample. This powder was allowed to dry in an air under vacuum for 24 h. The dried sample was calcinated in air at 550 °C for 6 h with a heating rate of 1 °C min⁻¹ to yield the final mesoporous product, which is denoted as SBA-15.

Preparation of thiol-functionalized SBA-15 (TSBA-15)

Thiol-functionalized SBA-15 (TSBA-15) was synthesized according to the reported procedure.²⁴ Mesoporous silica (8 g) was dispersed in ethanol under ultrasonic agitation for 30 min, and then 8 g of MPTMS was added to the above reaction mixture, and refluxed for 24 h under vigorous stirring. Finally the reaction mixture was filtered, and the obtained product was washed with ethanol followed by hexane, in order to remove the unreacted MPTMS, and then dried in the vacuum oven at 50 °C for 24 h (Scheme 3).

Scheme 3 Synthesis of thiol-functionalized SBA-15 (TSBA-15).

Preparation of polybenzoxazine (PCBz)

The CBz monomer was dissolved in CHCl₃, and the solution was poured onto a silane-coated glass plate and then cured at 120 °C for 1 h, 160 °C for 1 h, 180 °C for 1 h, 200 °C for 2 h, and 220° C for 2 h to produce a transparent film for comparative studies.

Preparation of cardanol-based polybenzoxazine (PCBz)/TSBA-15 hybrid composites

Cardanol-based polybenzoxazine/TSBA-15 hybrid films were synthesized according to Scheme 4. About 2 g of benzoxazine monomer was dissolved in 5 mL CHCl₃. Various weight percentages of TSBA-15 (1%, 3% and 5%) dissolved in CHCl₃ were added gradually to the benzoxazine monomer solution and sonicated for 1 h. The solution was poured onto a silane-coated glass plate and then cured at 120 °C for 1 h, 160 °C for 1 h, 180 °C for 1 h, 200 °C for 2 h, and 220° C for 2 h in order to produce semi-transparent films (Scheme 4).

Scheme 4 Preparation of TSBA-15/PCBz hybrid composites.

Characterization techniques

FT-IR spectra were recorded on a KBr disc with a Perkin Elmer 6X FT-IR spectrometer. ¹H NMR spectra were recorded on a Bruker 300 spectrometer using CDCl₃ as the solvent. A Netzsch DSC-200 differential scanning calorimeter was used for the calorimetric analysis at a heating rate of 10 °C min⁻¹ under a continuous flow of nitrogen. Thermo-gravimetric analysis was performed in a DSC-2920 coupled with a TA-2000 control system at a scanning rate of 10 °C min⁻¹ under a nitrogen atmosphere. The dielectric constant (DC) of PCBz/TSBA-15 systems were determined with the help of an impedance analyzer (Solartron impedance/gain phase analyzer 1260) using a Pt electrode at 40 °C in a frequency range of 20 Hz to 1 MHz. Powder X-ray diffraction patterns (XRD) were recorded using a Rigaku MiniFlex diffractometer with Cu-KR radiation. A JEOL JSM-6360 field emission scanning electron microscope was used for studying the morphologies of the samples. The phase morphology of PBCz/TSBA-15 nanocomposites was characterized by using a HR-TEM (JEM-3010, JEOL, Tokyo, Japan), operating at 80 kV with a measured point-to-point resolution of 0.23 nm.

Result and discussion

FTIR studies

Fig. 1 shows the FTIR spectrum of caprolactamdiamine (CPLDA). The characteristic absorption peak appeared at 1510 cm⁻¹, which indicates the existence of aromatic C=C bonds, and the presence of NH₂ was confirmed by the peak observed at 3400 cm⁻¹. The couple of absorption peaks observed at 2925 cm⁻¹ and 2852 cm⁻¹ indicates C–H aliphatic stretching vibration, and the peak at 1660 cm⁻¹ indicates the presence of C=O stretching of an amide-linked amine group present in the CPLDA.

Fig. 1 FTIR spectrum of caprolactamdiamine (CPLDA).

The molecular structure of cardanol benzoxazine (CBz) monomer was confirmed by analysis of the FTIR spectrum in Fig. 2.

Fig. 2 FTIR spectrum of cardanol benzoxazine (CBz).

The characteristic absorptions peaks that appeared at 1247 cm⁻¹ and 1172 cm⁻¹ were assigned, respectively, to asymmetric stretching of Ar–O–C and symmetric stretching of C–O–C in cardanol benzoxazine (Fig. 2). The peaks that appeared at 2950 cm⁻¹ and 2848 cm⁻¹ arose from C–H aliphatic stretching vibrations. The peaks observed at 1620 cm⁻¹ and 1508 cm⁻¹ confirm the presence of the 1,2,3-trisubstituted benzene ring in cardanol benzoxazine. Furthermore, N–C–O stretching and Ar–O–C symmetric stretching vibrations appeared at 960 cm⁻¹ and 1100 cm⁻¹, respectively.

In the IR spectrum of TSBA-15, the appearance of an absorption peak at 2565 cm⁻¹ confirms the presence of an –SH group in functionalized mesoporous silica [Fig. 3]. The strong bands at 2925–2854 cm⁻¹ and 1437 cm⁻¹ were attributed to C–H stretching and bending vibrations, respectively. In addition, the typical Si–O–Si bands that were obtained at around 1100 cm⁻¹ and 807 cm⁻¹ confirm the formation of a condensed silica network.

Fig. 3 Shows the FTIR spectrum of thiol-functionalized SBA-15 material.

The curing reaction of CBz and its composite with TSBA-15 were further confirmed by FTIR analysis. Fig. 4 shows the FTIR spectra of PCBz and its composite with TSBA-15 material.

Fig. 4 FTIR spectra of PCBz and TSBA-15/PCBz composites.

Polybenzoxazine (PCBz) as well as TSBA-15-reinforced PCBz composites yielded a new absorption peak at 1451 cm⁻¹ that is due to the presence of a tetrasubstituted benzene ring (Fig. 4), which confirms the ring-opening polymerization of CBz (5–8). However, such a peak was not found in the case of the CBz monomer (Fig. 2). The absorption peaks appeared at 2925 cm⁻¹ and 2850 cm⁻¹, confirming the presence of C–H aliphatic groups. The absorption peak observed at 3008 cm⁻¹ represents the aromatic C–H stretching. The absorption peak that appeared at 1614 cm⁻¹ corresponds to an aliphatic C=C stretching frequency. In addition, the absorption peak observed at 1090 cm⁻¹ confirms the formation of an –Si–O–Si– linkage due to the occurrence of hydrolysis of TEOS followed by condensation reactions of the resulting Si–OH. The broad absorption peak that appeared at 3384 cm⁻¹ corresponds to vibration of O–H bond of the phenolic group present in PCBz. For TSBA-15 reinforced PCBz composites with 1, 3 and 5 wt% values of TSBA-15, this peak was observed to broaden even further and shifted to 3377 cm⁻¹, 3369 cm⁻¹ and 3362 cm⁻¹ respectively. The shift and broadening of the peak may be explained (Fig. 4) by the presence of hydrogen-bonding interactions between TSBA-15 and cardanol. The presence of the thiol group in mesoporous silica was confirmed by the absorption peak at 2565 cm⁻¹ in the IR spectrum of TSBA-15 (Fig. 3). However in the IR spectra of TSBA-15-incorporated PCBz composites, the peak at 2565 cm⁻¹ disappeared, due to chemical interactions between S–H groups and PCBz (Fig. 4).^25,26

NMR spectrum of caprolactamdiamine (CPLDA)

The structure of caprolactamdiamine (CPLDA) was confirmed by ¹H NMR. Fig. 5 shows the NMR spectrum of CPLDA.

Fig. 5 ¹H NMR spectrum of caprolactamdiamine (CPLDA).

The peak that appeared at 3.13–3.30 ppm (c) was assigned to methylene (–CH₂–NH₂–) protons. The peaks at 6.59 ppm (f) and 6.94 ppm (h) were assigned to aromatic protons. Furthermore, the peaks that appeared at 1.62 ppm (a), 2.42 ppm (b) and 3.8 ppm (e) correspond to aliphatic protons, and the peak at 3.59 ppm (d) was assigned to –NH₂. Thus, the ¹H NMR study confirms the formation of caprolactamdiamine.

NMR and MALDI mass spectra of cardanol benzoxazine (CBz)

The molecular structure of the monomer (CBz) was confirmed by ¹H NMR, ¹³C NMR and MALDI mass techniques (see Fig. 6–8).

Fig. 6 ¹H NMR spectrum of cardanol benzoxazine (CBz).

Fig. 7 ¹³C NMR spectrum of cardanol benzoxazine (CBz).

Fig. 8 MALDI mass spectrum of cardanol benzoxazine (CBz).

The characteristic protons of the oxazine ring that appeared at 4.6 ppm were assigned to O–CH₂–N– and the peak at 3.8 ppm was due to Ar–CH₂–N–. The peaks observed from 6.63 to 7.40 ppm were assigned to methylene protons attached to aromatic hydrogens. These observations provide evidence that the hydroxyl functional group of cardanol was involved in the formation of the oxazine ring structure and hence ascertains the successful formation of the CBz monomer. The peak that appeared at 5.51 ppm was ascribed to –CH=CH–, which was originally present in the long alkyl side chain of the cardanol monomer. The above results suggest the formation of the oxazine ring structure and hence the successful formation of the cardanol-based benzoxazine monomer.

Furthermore, formation of the cardanol-based benzoxazine monomer was confirmed by analysis of a ¹³C NMR spectrum (Fig. 7). The peak that appeared at 80 ppm was assigned to the –O–CH₂–N carbon and that at 52 ppm shows the presence of the ph-CH₂–N carbon, which confirms the formation of the oxazine ring. The peak that appeared at 130 ppm shows the presence of aliphatic –C=C– in the side chain and indicates the presence of the cardanol moiety in the newly synthesized benzoxazine monomer. The ¹³C NMR peak observed at 178 ppm indicates the presence of carbonyl carbon atoms and the peaks from 115 ppm to 145 ppm confirm the presence of aromatic carbon atoms. The peaks observed from 14 ppm to 42 ppm confirm the presence of aliphatic carbon atoms. Thus, the ¹³C NMR spectrum also confirms the formation of the cardanol-based benzoxazine monomer.

MALDI mass analysis is an important technique used to determine the molecular weights of proteins, peptides, monomers and polymers, etc. The formation of the cardanol-based benzoxazine monomer as well as its molecular weight were further confirmed by using MALDI mass spectroscopy (Fig. 8). The spectral analysis shows the molecular weight of the benzoxazine monomer to be 1076, which is in good agreement with the theoretically calculated molecular weight value. Taken together, the above results (¹H NMR, ¹³C NMR and MALDI mass) confirm the successful formation of the cardanol-based benzoxazine monomer.

Thermal properties

DSC. The curing characteristics of the monomer was studied by DSC. The melting temperature, onset curing temperature, exothermic enthalpy and the heat of the curing reaction were obtained from the DSC curve, and their determined values are listed in Table 1.

Table 1 Characteristic parameters of CBz

Parameters	Temperature (°C)
Melting point	85.9
Onset cure temperature	220.8
Peak temperature	245.6
End temperature	271.4
Change in enthalpy (J g^−1)	−150.3

---

The sharp endothermic peak centered at 85.9 °C (Fig. 9) may be attributed to the melting point of CBz. The exothermic behavior observed at the high-temperature region was associated with the ring-opening polymerization of the oxazine rings.

Fig. 9 DSC of monomer cardanol benzoxazine.

The onset polymerization temperature of CBz has found to be 220.8 °C. The difference between the melting point and onset polymerization temperatures for CBz is 134.9 °C, which indicates its good processing behavior. After initial polymerization, CBz may impart steric hindrance on continued polymerization due to the long alkyl substituent, hence shifting the exothermic peak to a high temperature of 245.6 °C. The reaction enthalpy of the polymerization was found to be 150.3 J g⁻¹. The curing temperature of the p-cresol-based benzoxazine was found to be 272 °C (ref. 27) and that of benzoxazine from cardanol–furfural resin was 275 °C.²⁸ These results illustrated that the curing temperature of the synthesized CBz was much lower than that of p-cresol-based benzoxazine, which could be attributed to the presence of aliphatic bridging units of caprolactamdiamine into the cardanol-based benzoxazine.

The glass transition temperature for neat PCBz and TSBA-15 (1, 3 and 5 wt%)-reinforced nanocomposites are shown in Fig. 10 and Table 2.

Fig. 10 DSC thermograms of PCBz and TSBA-15/PCBz composites.

Table 2 Results from TGA, DSC and dielectric analysis

Sample	Tg (°C)	T10 (°C)	T20 (°C)	T30 (°C)	Char yield% (800 °C)	Dielectric constant at 1 MHz
Neat PCBz	90.1	348	378	405	12.24	3.52
1% TSBA-15/PCBz	105.6	356	385	413	14.05	2.99
3% TSBA-15/PCBz	109.4	361	394	422	17.26	2.32
5% TSBA-15/PCBz	113.9	368	409	427	18.98	1.947

---

From Table 2, it is clear that the T_g values of the TSBA-15/PCBz composites increased with increasing amounts of TSBA-15. The neat PCBz was observed to have a T_g value of 90.1 °C and incorporation of 5% TSBA-15 shifted the T_g value to 113.9 °C. The increase in the value of T_g for the TSBA-15/PCBz composite is due to the restricted motion of bulky polybenzoxazine molecular chains, the presence of the long alkyl side chains of the cardanol, and to the incorporation of TSBA-15, which imparts rigidity.

TGA. The thermal stability of PCBz and TSBA-15/PCBz hybrids were evaluated by TGA and their results are listed in Table 2. Fig. 11 shows the thermal stability of hybrid films and the temperatures at which 10%, 20% and 30% of the mass was lost. All TSBA-15/PCBz composites showed similar decomposition curves indicative of good thermal stability, which is due to the presence of long alkyl side chain of the cardanol units in the polymer network. The char yield at 800 °C for neat PCBz, 1% TSBA-15/PCBz, 3% TSBA-15/PCBz and 5% TSBA-15/PCBz were 12.24%, 14.05%, 17.26% and 18.98%, respectively.

Fig. 11 TGA thermogram of PCBz and various weight% values of TSBA-15 in PCBz.

The increasing char residue of hybrids resulted from the interaction of more inorganic TSBA-15 particles in the polybenzoxazine matrix. Furthermore, the TSBA-15 nanofiller has high thermal stability and was observed to start decomposing only at a higher temperature. Thus TSBA-15 was not observed to experience weight loss between 30 and 800 °C. When the temperature was increased to 800 °C, only the component of polybenzoxazine was decomposed. Further the incorporation of TSBA-15 increases the crosslinking density of the resulting nanocomposites and also restricts the chain mobility, which leads to an increase in the T_g values.

Dielectric properties

The dielectric constants of the polymeric materials depend on the contribution of dipole, electronic and atomic polarization. For heterogeneous materials such as composites, there is also the possibility for interfacial polarization, which arises due to the differences in conductivities of the two phases. Fig. 12 shows the frequency dependence of the dielectric constant of PCBz, and various weight% values of TSBA-15 in PCBz, which are shown in Table 2. From Table 2, it is clear that the dielectric constant decreases with increasing content of thiol-modified SBA-15. Fig. 13 shows the dielectric constants at 1 MHz for PCBz and various weight% values of TSBA-15 in PCBz. A similar type of observation (decreasing values for the dielectric constant with increasing TSBA-15 content) was also noticed for measurements taken at 1 MHz (Fig. 13). According to previous reports, low-k values (less than 2.5) can be achieved by introducing porosity since air has a dielectric constant close to unity.^29,30 Free volume is another important factor in determining the dielectric constant. The introduction of alkyl groups, flexible bridging units and bulky groups, which limit chain packing density, has been utilized to enhance free volume.³¹ The presence of free volume in the form of pores results in a decrease in the dielectric constant as these pores become occupied with air. Using careful chemical design, porosity can also be introduced into the solid composites by forming ordered arrangements of voids between the particulates and the pores. Furthermore, the dielectric behavior of polymeric materials is generally known to depend on the cross-linking density, structure, and orientation relaxation of dipoles in the applied electric field. In addition, the process of dipole polarization is accompanied by the movement of polymer chain segments. Hence, in the present case, increasing the content of TSBA-15, which can increase the number of pores and voids at the micro scale level as observed in the SEM and TEM images, was found to affect the dielectric property of polymer composites significantly. Being a mesoporous material, the incorporation of TSBA-15 into the cardanol-based benzoxazine matrix creates free space that may be occupied by air, which has a dielectric constant of 1. The value of the dielectric constant was decreased when the weight percentage of TSBA-15 was increased, due to the formation of air voids and pores. Fig. 14 shows dielectric loss of the neat PCBz and various weight% values of TSBA-15-incorporated PCBz. The presence of a strongly bonded interface between TSBA-15 and PCBz may suppress the dielectric loss. The presence of effective interface interactions between TSBA-15 and PCBz may suppress the dielectric loss. The decrease in the values of the dielectric constant and dielectric loss are also due to the formation of a highly crosslinked network structure and inorganic –Si–O–Si– linkages due to its insignificant polarizability. The porosity or free volume also reduces polarizability, which in turn reduces the dielectric constant of the polymer composite.^32,33 A strong interaction between TSBA-15 and the PCBz matrix was indicated by SEM, and the well-ordered arrangement of the pores and voids in the composite are visualized from the TEM image. The mesoporous nature of TSBA-15 was also found to be retained in the composites. Thus the presence of pores, voids and the retained mesoporous nature of TSBA-15 in the composite material decreased the value of the dielectric constant. The strong interfacial adhesion between filler and matrix leads to low dielectric loss. The low dielectric constant as well as low dielectric loss of the materials suggest that these materials can be used as effective interlayer insulators in integrated circuitry devices for microelectronics applications.

Fig. 12 Dielectric constants of PCBz and various weight% values of TSBA-15 in PCBz at varying frequencies.

Fig. 13 Dielectric constants of PCBz and various weight% values of TSBA-15 in PCBz at 1 MHz.

Fig. 14 Dielectric loss of PCBz and various weight% values of TSBA-15 in PCBz.

XRD characteristics of PCBz/TSBA-15

The XRD patterns of TSBA-15 and TSBA-15 incorporated in PCBz are presented in Fig. 15. TSBA-15 yielded peaks at 0.88 and 1.40 (2θ) for the silica planes (100) and (110), respectively. A decrease in the peak intensity was observed for the addition of different weight percentages of TSBA-15 to PCBz. Even though such a decrease in peak intensity was observed, there was only a slight change in the peak position, which may be attributed to the highly restricted mobility of polybenzoxazine as well as to the scattering contrast between the silica walls and pore channels.

Fig. 15 XRD patterns of TSBA-15 and PCBz/TSBA-15 composites.

Morphological studies of PCBz and PCBz/TSBA-15

Dark brown hybrid films were obtained by incorporating TSBA-15 into PCBz and by curing this composite in a step-wise manner. The morphologies of PCBz and TSBA-15/PCBz hybrid films were investigated using SEM (Fig. 16). For PCBz, the surface of the polymeric film was very smooth due to the single neat composition. When TSBA-15 was incorporated into PCBz, the films became less smooth due to the formation of rope-like domains of mesoporous silica³⁴ with cylindrical pores.

Fig. 16 SEM micrograph of (a) neat PCBz, (b) 1% TSBA-15/PCBz, (c) 3% TSBA-15/PCBz, and (d) 5% TSBA-15/PCBz nanocomposites.

The TEM image of Fig. 17 shows the presence of TSBA-15 in the polybenzoxazine matrix as a chain-like fibrous material. This chain-like appearance is due to the introduction of a thiol group in SBA-15 using MPTMS and the resulting strong interfacial interactions between the TSBA-15 and PCBz matrix. The long chain-like structure with ordered cylindrical pores confirms a well-defined structure for the TSBA-15/PCBz composite (Fig. 17). The created free volume lowers the dielectric constant significantly. The TEM image revealed a nanometer level dispersion of TSBA-15 in the PCBz matrix, which could influence the thermal and electrical properties of the resulting TSBA-15/PCBz nanocomposites.

Fig. 17 TEM micrograph of 5% TSBA-15/PCBz nanocomposite.

Conclusion

In the present work, a new approach was used to apply a waste material to a valuable technology by synthesizing a novel benzoxazine monomer from the renewable bio waste cardanol with a low curing temperature. TSBA-15 was used as an inorganic nanofiller, and a hybrid composite of TSBA-15 and PCBz was synthesized; this composite was shown to possess a low dielectric constant and high thermal stability. The TEM image clearly revealed the presence of a long fibrous material that imparted free volume. Significant improvements in the glass transition temperature, dielectric constant, thermal stability and char yield were observed Thus, these nanocomposites can be used as an advanced composite materials in engineering applications involving microelectronics.

Acknowledgements

The authors thank DST/Nanomission, New Delhi, India for financial support to carry out this work and establishment of Nanotech Research Lab through grant no. SR/NM/NS-05/2011(G). The authors also thank Dr M. Mandhakini, Department of Nanoscience and Technology, ACTech, Chennai for timely help and moral support, and Mr Satya Priya, Satya Cashew Chemicals Pvt. Ltd. Chennai for providing cardanol for research purposes.

References

1. S. F. Wang, T. R. Wang, K. C. Cheng and Y. P. Hsiao, Ceram. Int., 2009, 35, 265.
2. R. Tummala and E. J. Rymaszewski, Microelectronics Packaging Handbook, 1989, p. 1.
3. S. Numata, K. Fujisaki, D. Makino and N. Kinjo, Proceedings of the 2nd Technical Conference on Polyimides, Society of Plastic Engineers, Ellenville Inc., New York, 1985, p. 164.
4. M. Ree, Macromol. Res., 2006, 14, 1.
5. H. Schreiber, German Patent, 2, 1973, vol. 255, p. 504.
6. H. Schreiber, German Patent, 2, 1973, vol. 323, p. 936.
7. X. Ning and H. Ishida, J. Polym. Sci., Part A: Polym. Chem., 1994, 32, 1121.
8. X. Ning and H. S. Ishida, J. Polym. Sci., Part B: Polym. Phys., 1994, 32, 921.
9. H. Ishida and D. J. Allen, J. Polym. Sci., Part B: Polym. Phys., 1996, 34, 1019.
10. N. N. Ghosh, B. Kiskan and Y. Yagci, Prog. Polym. Sci., 2007, 32, 1344.
11. S. Wirasate, S. Dhumrongvaraporn, D. J. Allen and H. Ishida, J. Appl. Polym. Sci., 1998, 70, 1299.
12. Y. C. Su and F. C. Chang, Polymer, 2003, 44, 7989.
13. Y. C. Su, W. C. Chen, K. L. Ou and F. C. Chang, Polymer, 2005, 46, 3758.
14. M. C. Tseng and Y. Liu, Polymer, 2010, 51, 5567.
15. Y. W. Chen and E. T. Kang, Mater. Lett., 2004, 58, 3716.
16. C. M. Leu, Y. T. Chang and K. H. Wei, Chem. Mater., 2003, 15, 3721.
17. Q. R. Hung, W. Volksen, E. Huang, M. Toney, C. W. Frank and R. D. Miller, Chem. Mater., 2002, 14, 3676.
18. Q. R. Hung, H. C. Kim, E. Huang, D. Mecerreyes, J. L. Hedrick and W. Volksen, Macromolecules, 2003, 36, 7661.
19. J. E. Mark, Acc. Chem. Res., 2006, 12, 881.
20. E. Calo, A. Maffezzoli, G. Mele, F. Martina, S. E. Mazzetto, A. Tarzia and C. Stifani, Green Chem., 2007, 9, 754.
21. B. Lochab, I. K. Varma and J. Bijwe, Journ. Calorim. Anal. Therm., 2010, 102, 769.
22. B. S. Rao and A. Palanisamy, React. Funct. Polym., 2011, 74, 148.
23. J. Bijwe and P. V. Gurunath, PCT Int. Appl WO., 2008149380 A2 20081211, 2008.
24. J. Park, J. Park and H. Kim, Int. Proc. Chem., Biol. Environ. Eng., 2011, 24, 78.
25. K. Wilson, A. F. Lee, D. J. Macquarrie and J. H. Clark, Appl. Catal., A, 2002, 228, 127.
26. V. Ganesan and A. Walcarius, Langmuir, 2004, 20, 3632.
27. M. A. Espinosa, V. Cádiz and M. Galia, J. Appl. Polym. Sci., 2003, 90, 470.
28. S. F. Li, S. Yan, J. Yu and B. Yu, J. Appl. Polym. Sci., 2011, 122, 2843.
29. B. D. Hatton, K. Landskron, W. J. Hunks, M. R. Bennett, D. Shukaris, D. Perovic and G. A. Ozin, Mater. Today, 2006, 9, 22.
30. S.-W. Kuo and F.-C. Chang, Prog. Polym. Sci., 2011, 36, 1649–1696.
31. G. Wypych, Handbook of Fillers, Chemical Technology Publishing, 2nd edn, 1999, p. 2.
32. Y. S. Negi, Y. Suzuki and I. Kawamura, et al., J. Polym. Sci., Part A: Polym. Chem., 1992, 30, 2281.
33. T. M. Long and T. M. Swager, J. Am. Chem. Soc., 2003, 125, 1411–1439.
34. S. Ravi and M. Selvaraj, Dalton Trans., 2014, 5299.

---