Application of Metal-Organic Framework as Reactive Filler in Bisphenol-A-Based High-Temperature Thermosets

Abstract

Excellent thermoset monomers, bisphenol-A-based biscyanate ester (BADCy) and bispropargyl ether (BPEBPA), are synthesized and thermally cured to high-temperature thermosetting polymers. The nanoporous aluminum fumarate (Al_FA_A), an interesting Metal-Organic Framework (MOF), is synthesized in an eco-friendly manner and used as a reactive nanoparticle filler. The interaction of fumarate π bonds (trans -CH=CH-) in MOF with the reactive end functional groups (-O-C ≡ N) in cyanate ester (CE) and (-CH2-C ≡ CH) in bispropargyl (BP) ethers is focused in these hybrid nanocomposites. The % decrease in enthalpy of curing in the organic and the inorganic blends (~60% for CE and ~ 10% for BP) indicates the interaction exciting between the MOF and the organic component. The addition of the aluminum fumarate MOF increases the glass transition temperature of the polymers. The amount of heat released for every increase in 1°C during the temperature window of curing (ΔHc/TE-TS) of the neat BADCy resin is approximately 2.4 times higher than the blend (BADCy+Al_FA_A). But BPEBPA shows only a 1% higher temperature curing window compared to its blend with MOF. The metal hotspots present in the hybrid nanocomposites may be the reason for the decrease in the thermal stability, and the % char residue is noted at 700°C. The TG-FTIR studies are done to predict the gaseous products (CO2) evolved during thermal degradation.

1. Introduction

The cyanate ester (CE) and the acetylene terminated (AT) resins are specialty materials and find applications in several fields. Of which, the bisphenol-A-based dicyanate (BADCy) and the bispropargyl (BPEBPA) resins are advanced thermoset materials due to the formation of triazine [1] (polycyanurate) from BADCy [2] and products such as polyene, phenolic allenes by Claisen rearrangement, and chromene from aromatic bispropargyl ether (BPEBPA). The polycyanurate materials have high strength and toughness, high glass transition temperature, low water absorption, low dielectric constant, and good adhesion to a variety of substrates [3], and they combine the advantages of epoxies, fire resistance of phenolics, and high-temperature performance of polyimides. Several authors studied thermal degradation [4], curing kinetics, and blends of bismaleimides with bispropargyl ethers [5, 6, 7, 8]. The authors already have experience in synthesizing the bispropargyl ether with several swivel groups. The most important study is copper-salt-assisted polymerization of bispropargyl of bisphenol-A [9] and the blend of propargyl ether with bismaleimide [10].

Polymer blending with other thermosets (or) with inorganic salts/materials [10, 11] has been studied by different research groups. This created a large scope for investing in the materials through resin modification and reinforcements. The authors work toward the influence of chain extension in BMIM composites and studied the effects of chain extensions on thermal and mechanical properties [12]. In their investigation, Siva Kaylasa Sundari et al. [13] studied the influence of nanoporous aluminum fumarate in cyanate ester matrix. It was reported that the double bond of aluminum MOF was involved in triazine formation. The synthesis, characterization, and thermal degradation kinetics by the model free approach of aluminum fumarate were already presented [14]. This study compares the bulk polymerization and polymer thermal properties having different functional group systems (bisphenol-A-based cyanate ester and bispropargyl ether) with aluminum fumarate MOF as particulate reinforcements. The structures of bisphenol-A-based monomers are presented in Figure 1.

Figure 1.

Structures of bisphenol-A-based monomers.

The exponential increase in the design and development of Metal-Organic Frameworks (MOFs) is due to their tailorable properties and unprecedented functionality. The MOFs have a wide range of advantage and extend applications in several smart fields [15, 16] and also in biomedical applications [17, 18]. MOFs are formed by strong covalent bonds between inorganic metal salts and organic linkers. The author already studied the detailed aspects of synthesis and procedure optimization of aluminum fumarate and its isothermal model free [14] and model fitting kinetics. The formation of MOF-polymer composites can be obtained by the in-situ polymerization reaction, MOF construction using polymeric ligands, post-synthetic grafting of polymers onto the reactive ligands, the post-synthetic introduction of pre-formed polymers and MOFs self-assemble around polymers [19]. Markedly, this invention relates to the solvent-free process for preparing the MOF-polymer composite by the post-synthetic grafting of pre-formed polymers on reactive MOFs. The polymerized material contains allenes, phenolic allenes by claisen rearrangement, chromenes from aromatic propargyl ether, and also the linear polyene. The involvement of fumarate π-bonds in MOFs with reactive double bonds of polymerized bispropargyl provides a complex bulk and random hybrid polymer brushes, which makes the material applicable for functionalized applications.

2. Materials and methods

2.1 Materials

The compounds bisphenol A dicyanate (BADCy), bispropargyl ether (BPEBPA) and the nanoporous aluminum fumarate MOF (Al_FA_A) were synthesized. Bisphenol-A, sodium hydroxide, tetra butyl ammonium bromide, and isopropanol were obtained from Merck India Ltd., Mumbai, India, and were used without any further purification. Propargyl chloride obtained from Grauer & Weil (India) Limited, Vapi, India, was distilled (Boiling point: 57°C) before use. The synthesis procedure and the characterizations of bisphenol-A dicyanate [13] and the nanoporous aluminum fumarate MOF [14] were reported in our previous work.

2.1.1 Synthesis of bispropargyl ether of bisphenol-A (BPEBPA)

A mixture of 0.2 mol of bisphenol-A, 200 mL of 20% aqueous sodium hydroxide (NaOH), and 0.01mol of tetrabutyl ammonium bromide was kept at room temperature. About 0.6 mol of propargyl chloride was added over 10 min to the above mixture with constant stirring, and the resultant reaction mixture was stirred for 16 h. The resulting white crystals were filtered and washed twice with 200 mL of water and then with 50 mL of isopropanol. This results in a yield of 95% of BPEBPA. The reaction for the synthesis is presented in Figure 2.

Figure 2.

Synthesis of bispropargyl ether of bisphenol-A.

2.1.2 Preparation of hybrid nanocomposites

The dried BADCy/BPEBPA and the Al_FA_A (99:1, w/w) were taken in an agate mortar, and the mixture was ground repeatedly to have effective mixing. The mixture was then dried and preserved for polymerization. The pure BADCy/BPEBPA and their blends with nanoporous aluminum MOF were taken in separate borosilicate glass test tubes and flushed with dry oxygen-free-nitrogen gas and polymerized at 240°C for 6 h. After the polymerization, the samples were allowed to cool to room temperature, removed from the test tubes, ground to a coarse powder, packed, and stored for further analysis. The reaction mechanism for the involvement of fumarate in triazine formation is presented in our previous work [13]. The possible products during the polymerization of bispropargyl ether are presented in Figure 3.

Figure 3.

The products formed during the polymerization of bisphenol-A-based bispropargyl ether (BPEBPA).

2.2 Methods

The Fourier Transform Infrared Spectrum (FTIR) of the material was recorded on a Fourier transform infrared-8400S spectrophotometer, Shimadzu, Japan, using the potassium bromide disc technique. The differential scanning calorimetric (DSC) curves were recorded using TA Instruments DSC Q20 using a non-hermetic aluminum pan. The samples were heated from ambient to 350°C at 10°C min⁻¹ in a nitrogen atmosphere (50 mL min⁻¹). The thermal degradation behavior of the material was examined using TGA Model Q50 supplied by TA Instruments, Waters India Pvt., Ltd., Bengaluru – 560,086, India. The measurements were carried out using approximately 3–4 mg of the sample in a high-purity nitrogen atmosphere, and the flow of nitrogen to the balance area was 40 mL min⁻¹, and the sample was swept with a nitrogen flow of 60 mL min⁻¹. The obtained TG and DTG curves were analyzed using the universal analysis 2000 software provided by TA instruments. The TG-FTIR study of polymerized samples was carried out in a TA Instruments TGA Q5000 V3.10 Build 258 at a heating rate of 10°C min⁻¹ from ambient to 800°C. Nearly, 3–4 mg samples was used for the analysis in the nitrogen atmosphere (balance flow: 10 mL min⁻¹ and sample flow: 25 mL min⁻¹), and the FTIR spectra of the volatiles formed during the thermal degradation were recorded every 30 s.

3. Results and discussion

3.1 Aluminum fumarate

The Fourier Transform Infrared Spectroscopy was done for Al_FA_A to find the existence of intermolecular hydrogen bonding between the layers of aluminum fumarate. The Al_FA_A shows an onset degradation temperature at 410°C, attains a maximum at 484°C, and ends at 561°C. The material Al_FA_A is stable up to 400°C, and the amount of residue noted at 700°C is 33%. The crystallinity of Al_FA_A is 89% (Scherrer equation), and the average mean particle diameter of 50 nm. The Brunauer-Emmett-Teller (BET) surface area of Al_FA_A was found to be 937 m²g⁻¹; monolayer volume (V_m) of 215.21 cm³g⁻¹; total pore volume (V_pore) of 0.38 cm³g⁻¹; and a mean pore diameter of 1.63 nm. The thermal degradation behavior of the aluminum fumarate by model free kinetics [14] and the curing behavior of BADCy and BADCy+Al_FA_A were already presented [13].

3.2 FTIR studies

The FTIR spectra for synthesized and polymerized materials are given in Figure 4, and the bands were reported in Table 1. The FTIR bands for the pre and post-polymerization of BADCy and BADCy+Al_FA_A at different temperatures were discussed in detail in our previous work [13]. The compound BPEBPA shows a characteristic band at 2120 cm⁻¹ for C ≡ C stretching, 3259 cm⁻¹ for ≡C-H stretching, and 700cm⁻¹ for ≡C-H deformation. The blending of fumarate MOFs in BPEBPA showed similar spectra to monomers. The polymerization that happened via the claisen polymerization was confirmed by the presence of the ketone group in IR spectra. The propargyl/bispropargyl involved in allene rearrangement then follows the claisen rearrangement to form keto-allene, which undergoes enolization for phenolic allene. The keto-allene further reacts to form the chromene structure [8], which shows a band at 1630 cm⁻¹ and a cyclic ether group at 1150 cm⁻¹.

Figure 4.

FTIR spectra for monomers and polymers.

S.No.	Frequency (cm^−1)	Bands (BADCy)	Bands (BPEBPA)
1.	3259	—	≡C-H stretching
2.	3100	Phenolic -OH stretching	—
3.	2970	C-H stretching in CH3	C-H stretching in CH3
4.	2120	—	C ≡ C stretching
5.	2240 to 2260	C ≡ N stretching	—
6.	1510	C=N-C stretching of triazine ring	—
7.	1400	O-C=N stretching of triazine ring	—
8.	1100 to 1200	C-O-C stretching	C-O-C stretching
9.	820	C-H out of plane deformation	C-H out of plane deformation

Table 1.

The frequency and corresponding bands of cyanate ester and bispropargyl system.

3.3 DSC studies

The DSC thermograms for the monomers and the blends are given in Figure 5. The details regarding the melting and curing behavior values for materials are given in Table 2.

Sample Code	Tm; (°C)	∆Hf; (J g^−1)	TS; (°C)	Tmax; (°C)	TE; (°C)	TE-TS; (°C)	∆HC; (J g^−1)	∆HC/ TE-TS; (J g^−1 °C^−1)
BADCy	82	31	196	252	334	138	211	1.53
BADCy+Al_FA_A	80	16	192	243	329	137	86	0.63
BPEBPA	82	45	205	274	332	127	207	1.63
BPEBPA+Al_FA_A	81	43	211	273	335	124	184	1.48

Table 2.

Parameters from DSC curves.

Figure 5.

DSC thermograms of monomers and blends.

The BADCy shows the onset curing temperature (T_S) at 196°C, the temperature at which the curing rate is maximum (T_max) at 252°C, and the endset temperature (T_E) at 334°C. The addition of nanoporous material decreases the T_S to 192°C, T_max to 243°C and the T_E to 329°C. The BPEBPA shows the T_S at 205°C, the T_max at 274°C, and the T_E at 332°C. But the addition of Al_FA_A increases the T_S to 211°C for BPEBPA. The enthalpy of curing (ΔHc) for BADCy is 211 J g⁻¹, whereas incorporation of Al_FA_A in BADCy drastically decreases the value to 86 J g⁻¹, which amounts to a decrease of 59%. The addition of aluminum fumarate in bisphenol-A-based thermoset decreases the enthalpy of fusion (50%) for BADCy and shows a negligible change in the BPEBPA matrix. The addition of aluminum fumarate increases the glass transition temperature (T_g) of polymer.

The enthalpy of combustion decreases by 60% for BADCy and by 10% for BPEBPA systems. The reduction in the ΔH_f and ΔH_c values for the blend indicates the interaction existing between the organic and inorganic components in the blend. The T_E-T_S values of both the pure resins and the blends show an almost similar rate of curing. The amount of heat released for every increase in 1°C during the temperature window of curing (ΔH_c/T_E-T_S) of the neat BADCy resin is approximately 2.4 times higher than the blend (BADCy+Al_FA_A). But BPEBPA shows only a 1% higher-temperature curing window compared to its blend with MOF. From the DSC curves, a profound effect was observed during the physical blending of aluminum fumarate in bisphenol-A-based thermosets.

3.4 TG studies

The thermal analyses of the polymerized samples are presented in Figure 6. The polymers are less stable than the nanoporous aluminum MOFs. The details regarding the onset and endset degradation temperatures, T_E-T_S value, mass loss (%) and the residue (%) at 700°C are given in Table 3. Compared to the BADCy, the BPEBPA matrix shows broad degradation. The influence of nanoporous material does not affect the degradation pattern in BADCy, but it shows a drastic difference in the BPEBPA polymer. The effect of copper salts (CuCl: cuprous chloride; CuCl₂.2H₂O: cupric chloride dihydrate; CuSO₄.5H₂O: copper sulfate pentahydrate; CuCO₃.Cu(OH)₂: basic copper carbonate; (CH₃COO)₂Cu.H₂O: cupric acetate monohydrate; CuO: cupric oxide) in the BPEBPA matrix reported that the addition of copper salts decreases the onset degradation temperature [9]. Similarly, the addition of Al_FA_A decreases the thermal stability in both cyanate (BADCy) and the bispropargyl (BPEBPA) systems. The limiting oxygen index (LOI) for the nanoporous MOF and the polymers was calculated using Van Krevelen method [20] eq. (1).

Figure 6.

TG and DTG curves of polymers.

Sample Code	First Degradation Stage					Second Degradation Stage				Residue; (%) at; 700°C	LOI
	Ts	Tmax	TE	TE-TS	Mass Loss (%)	Ts	Tmax	TE	Mass Loss (%)
Al_FA_A	410	484	548	138	65	—	—	—	—	33	0.31
P(BADCy)	376	443	623	247	45	—	—	—	—	53	0.39
P(BADCy+Al_FA_A)	327	439	658	331	55	—	—	—	—	42	0.34
P(BPEBPA)	220	476	672	452	42	672	751	797	4	52	0.38
P(BPEBPA+Al_FA_A)	206	435*	662	456	55	—	—	—	—	40	0.34

Table 3.

Degradation parameters of Al_FA_A and polymers.

*

Overlapped degradation.

$$LOI=\frac{17.5+\left(0.4\ast CharResidue\right)}{100}$$E1

3.5 TG-FTIR studies

The 3D TG-FTIR spectrum of aluminum fumarate is presented in Figure 7. The major compounds released from the Al_FA_A were water (H₂O), carbon dioxide (CO₂), carbon monoxide (CO), and acetylene. A detailed discussion regarding the bond dissociation energies of the aluminum fumarate is provided in the previous work [14]. The 3D TG-FTIR spectra for polymer blends are presented in Figure 8.

Figure 7.

3D: TG-FTIR spectra of aluminum fumarate.

Figure 8.

3D: TG-FTIR spectra of polymer blends.

Compared to P(BADCy) and P(BADCy+Al_FA_A), the polymers P(BPEBPA) and P(BPEBPA+Al_FA_A) release more carbon dioxide (CO₂) at 2350 cm⁻¹ at all temperatures. The intensity of CO₂ evolution in P(BADCy) and P(BPEBPA) decreases due to the addition of nanoporous MOF, it indicates that the CO₂ can be captured in the tunnels of the MOF. Vinayagamoorthy et al. [6] and Dhanalakshmi et al. [9] studied the off-line pyrolysis and the evolved gas analysis technique of the bispropargyl ethers, and they concluded that the obtained gaseous degradation products should be able to liberate by cleavage at an activated benzylic (or) propargylic site in BPEBPA system. The thermal, mechanical, and electrical effects of Al MOFs in the di/trifunctional epoxies of epoxy resin composites were studied in our previous work [21, 22].

4. Conclusion

The nanoporous aluminum fumarate (Al_FA_A), bisphenol-A-based cyanate ester (BADCy), and bispropargyl ether (BPEBPA) were synthesized. The high-temperature thermoset polymers were obtained from the monomers and blended with Al_FA_A. The addition of aluminum fumarate MOF decreases the onset curing temperature (T_S) for BADCy and increases T_S for the BPEBPA system without affecting the melting point. The influence of π-fumarate bonds of Al_FA_A in the polymer matrix was observed from the drastic changes in the enthalpy of fusion and enthalpy of combustion values. The onset thermal degradation temperature of polymers was decreased by the influence of MOF. The influence of nanoporous material does not affect the degradation pattern in BADCy, but it shows a substantial difference in the BPEBPA matrix. The major products released from the polymers are water (H₂O), carbon dioxide (CO₂), carbon monoxide (CO), and acetylene.

Acknowledgments

The authors express their sincere thanks to the Management and Principal of Kamaraj College of Engineering and Technology (Autonomous), S.P.G.C. Nagar, K. Vellakulam-625701, Madurai, India, for providing all facilities to do the work.

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.

Funding

No funding was received for this study.

Author statement

Supervision, Writing – Review and Editing: Dr. C. T. Vijayakumar.

Methodology, Writing – Original draft preparation: S. Siva Kaylasa Sundari.

Validation: Dr. M. Arunjunai Raj and.

Validation: Dr. S. Shamim Rishwana.