The Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffractometry (XRD) and particle size analysis (PSA) of the synthesized copper fumarate MOF were done and discussed below. The FTIR spectra showed the band at around 3000 cm− 1 for intermolecular hydrogen bonding and the strong band for a double bond was found at 1600 cm− 1. The diffractogram of Cu_FA_A shows its crystalline nature. The complete decomposition of the framework, carbonization and copper oxide formation occurs only when the sample was annealed over 300 °C. In agreement with the XRD, the TG-FTIR spectra show that the carbonization temperature was > 300 °C. The detailed discussion regarding the characterization of aluminum fumarate MOFs was discussed in our previous work [12]. The particle size analysis data for Al_FA_A and Cu_FA_A are presented in Fig. 1. The particle size ranges from 10 to 50 µm and approximately around 250 µm for Cu_FA_A. This shows that the as-synthesized Cu fumarate MOF is gap grade and it showed the absence of particle size from 50 to 200 µm.
The FTIR spectra of the monomer, blends and polymers were discussed below. The compound SPE shows a characteristic band at 2120 cm− 1 for C≡C stretching, 3259 cm− 1 for ≡C-H stretching and 700 cm− 1 for ≡C-H deformation. The sulfone group in SPE was noted by the band at 1150 and 1340 cm− 1. The blending of fumarate MOFs in SPE showed similar spectra to monomers. The polymerization that happened via the Claisen rearrangement 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 [13, 14] which shows a band at 1630 cm− 1 and a cyclic ether group at 1150 cm− 1. The thermophysical properties of the monomers, blends and polymers were characterized by DSC and TG studies. The elimination of water molecules from the framework was noticed by an endotherm at 100 °C and 200 °C for aluminum fumarate and copper fumarate MOFs respectively. The DSC curves for the monomer (SPE) and its blends with Al and Cu fumarate MOFs are presented in Fig. 2. The monomer SPE shows a melting point of 187 °C. The addition of Al_FA_A and Cu_FA_A MOFs decreases the enthalpy of fusion and enthalpy of curing by 60% without any change in the melting point. The values for melting point (Tm), enthalpy of fusion (ΔHf), curing onset (TS), the temperature at which curing rate is maximum (Tmax) and endset curing temperature (TE) are given in Table 1. In the value of enthalpy, the amount of heat released for every increase in 1 ⁰C in the curing window (ΔHc/TE-TS) of Cu_FA_A blended SPE, is found to be 3.2 Jg− 1 °C− 1 and is 2.5 times lower than pure SPE indicating lesser thermal stress during curing. Low-temperature processability can be achieved in SPE by the addition of Cu fumarate MOF.
Table 1
Parameters derived from the DSC curves of monomer - SPE and its blend with aluminum and copper fumarates (SPE + Al_FA_A and SPE + Cu_FA_A)
| Sample Code | Tm (°C) | ΔHf (Jg− 1) | TS (°C) | Tmax (°C) | TE (°C) | TE-TS (°C) | ΔHc (Jg− 1) | ΔHc/TE-TS (Jg− 1 °C− 1) |
| SPE | 187 | 141 | 214 | 300 | 346 | 127 | 1079 | 8.5 |
| SPE + Al_FA_A | 187 | 56 | 215 | 289 | 345 | 130 | 447 | 3.4 |
| SPE + Cu_FA_A | 187 | 53 | 195 | 252 | 341 | 146 | 465 | 3.2 |
The thermogravimetric and differential thermogravimetric curves of polymers (SPE; SPE + Al_FA_A and SPE + Cu_FA_A) are presented in Fig. 3. The thermal degradation of the P(SPE) is nearly overlapping bimodal degradation having degradation rate maximum being at temperatures 431°C and 554°C. The MOF bearing SPE based hybrid composites behave completely differently in their thermal degradation behavior. The presence of MOF reduces the thermal degradation temperature. The total difference in the DTG curves is excellent proof of the interaction of the MOFs with the SPE monomer during polymerization and leading to a different polymer network. The MOF-containing polymers show much faster thermal degradation, the rate being maximum at 400°C for P(SPE + Al_FA_A) and at 300°C for P(SPE + Cu_FA_A). So, with sufficient loss in thermal stability, the addition of MOF to SPE provides low-temperature curing. The P(SPE) has higher thermal stability because of the rigid chromene formation during polymerization. The addition of fumarate MOFs to SPE decreases the thermal stability of the polymer SPE - P(SPE). The decrease in thermal stability is compensated by the wide degradation window of polymerized SPE with fumarate MOFs.
Guang Yang et al [15] studied the synthesis, characterization and heat resistant properties of linear propargyl ether-terminated polymers with the Si-H group. The authors stated that the propargyl terminated resins with high glass transition temperature and high char residue had good heat resistance and could be applied at high-temperature conditions. Nechausov et al [16] studied the dual curing of propargyl/phthalonitrile monomers for composites by vacuum infusion process. The TG-FTIR for polymers is presented in Fig. 4. The detailed discussions and the possible products evolved during the thermal degradation of aluminum fumarate [12], aluminum isophthalate and aluminum terephthalate MOFs [17] were reported in our previous work.
The presence of free phenolic groups in the thermally cured SPE and its polymers with Al and Cu fumarate MOFs is confirmed by the band at 3500–3550 cm− 1. The release of CO2 and CO from the degrading polymers was confirmed by the appearance of the band at 2300–2500 cm− 1. The addition of aluminum fumarate MOF favors the degradation of the polymer earlier than the P(SPE). The intensity of the CO2 and CO band increases as the temperature increases in P(SPE + Cu_FA_A). But for the P(SPE + Al_FA_A) matrix, degradation occurs faster as evidenced by thermal degradation behavior. The products formed during the thermal degradation were represented in the graphical abstract. The possible products were benzene, phenol, benzyl alcohol and chromene and products like acetylene and substituted acetylene also evolved during thermal degradation.
