Green synthesis of new and natural diester based on gallic acid and polyethylene glycol

Apparatus

Fourier Transformed Infrared (FTIR) Spectroscopy; The FTIR spectra were checked in utilizing a Brucker Tensor-27. The scan was done between 4000 and 400 cm-1. All spectra were baseline-corrected with Opus software.

X-ray Diffraction (XRD); The Cristallinity of the products were determined by Brucker D8 Advance diffractometer DAVINCI model, operating in Bragg–Brentano geometry, with Cu Kα radiation (λ = 1.5418 Å).

Nuclear Magnetic Resonance (NMR); Proton and carbon nuclear magnetic resonance (1H NMR) and (13C NMR) spectra were recorded on Bruker 300 MHZ NMR spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) as solvent.

Ultraviolet-Visible (UV–Vis); UV-Vis absorption spectra were recorded using Evolution 60S spectrophotometer (Thermo Fischer Scientific). The spectra were recorded in Dimethylsulfoxide (DMSO) solvent.

Scanning Electron Microscopy (SEM); The analysis of nanocomposites powder image was carried out Quanta FEG 250 instrument (FEI, Hillsboro).

Thermogravimetric analysis (TGA); it was carried out on a Setsys Evolution analyzer (ATG Setaram, Caluire, France) equipped with an aluminium cell, using aluminium pans to encapsulate the samples. Typically, samples were heated at a constant rate of 10 °C/min from room temperature up to 550 °C, under a helium flow of 50 mL/min. The thermal decomposition temperature was taken at the onset of significant (≥5%) weight loss from the heated sample.

Materials

The gallic acid was purchased from BioChem, Polyethylene glycol glycol 2000 (PEG, M = 2000g/mole) was obtained from Fluka, sulfuric acid was purchased from Emsure and dichloromethane was obtained from Sigma Aldrich.

Synthesis of of polyethylene glycol diester

In this work, we performed a green esterification synthesis using the solid-solid reaction. PEG-digallates was prepared from gallic acid (0.02mol) and polyethylene glycol (PEG) (0.01mol) at 110°C using sulfuric acid (0.5ml) as a catalyst for 12 hours. Thereafter, 50mL of dichloromethane was added and filtred to remove non-reagent amounts of gallic acid and polyethylene glycol. The synthetic scheme is shown in Figure 1.

Figure 1. Formation process of Di-gallate.

Vibrational characterization by FTIR spectroscopy

FTIR analysis was used to identify changes or structure inter¬actions caused by the addition of PEG to Gallic acid. The FT-IR spectra of gallic acid, PEG and PEG-digallate complex treated using Origin Pro (v.2018), are given in Figure 2. As proven in PEG-digallate, the peaks around 3000-3500 cm-¹ represented the stretching and bending vibration of -OH, indicating the attendance of hydroxyl groups. The high absorption peaks at 2876, 1707 and 1279 cm-¹, were assigned to methylene (=CH₂), carboxyl (–COOH), and phenol (–OH); groups in pure PEG, respectively; at the same time as the peaks at 1609 and 1104 cm-¹ were attributed to aromatic ester and ether groups.²³ It is worth noting that, the FT-IR spectra of the obtained complex, pure PEG and GA are relatively similar with several other peaks observed around 1700–1000 cm-¹ which are attributed to C–O bond stretching vibration and O–H bond bending vibration into gallic acid. In contrast, changes in the stretching frequency of the current groups indicated the interaction of PEG with the gallic acid functional group.

Figure 2. FT-IR spectra of the prepared composite (PEG-digallate), pure PEG and gallic acid.

NMR analysis

Nuclear magnetic resonance (NMR) spectroscopy was used to determine the molecular structure and chemical composition of the complex PEG-digallate synthesized.

The ¹H-NMR spectra of PEG-digallate and gallic acid have been acquired in DMSO-d6 at a 5 mM concentration. As proven in Figures 3 & 4, the chemical shift of every proton for gallic acid and the obtained product were studied in DMSO-d₆ earlier than and after reaction with PEG. The signals at 6.95, 8.13, and 8.31 ppm correspond to the ortho protons of the benzene ring, the para and meta hydroxyl (O-H) protons, respectively. Notably, the disappearance of the acidic group (COOH) proton signal and the presence of the polyethylene glycol proton CH2-CH2 are indicated at 3.51 ppm. Additional signals appear at 3.4 and 3.6 ppm, corresponding to protons at the ends of the polymer chains (protons c and b; Figure 3). Comparing the ¹H-NMR spectra of gallic acid and PEG-digallate, we note that at 12.26 ppm there is no signal corresponding to the -COOH proton of the acid functional group, but there is a signal corresponding to the -OH proton of the hydroxyl functional group at 8.13 and 8.31 ppm, confirming that gallic acid has reacted with the acid function and that the resinous hydroxyl function is free.

Figure 3. ¹H-NMR spectra of PEG-digallate in DMSO-d₆.

Figure 4. ¹H-NMR spectra of gallic acid in DMSO-d₆.

Furthermore, comparing the ¹H NMR spectra of PEG-digallate and pure polyethylene glycol (From literature²⁴), we take a look at the same strong signal at 3.51 ppm, corresponding to the CH₂-CH₂ of polyethylene glycol.

The CH₂-CH₂ protons at the ends of the polymer chains maintain the same signal at 3.4 and 3.6 ppm.

Based at the evaluation of ¹³C-NMR spectra of the obtained complex PEG-digallate and gallic acid (Figure 5 and 6), which revealed the presence of a carbon signal (O=C-O) at 167.94 ppm corresponds to the ester function. Another peak at 138.4 ppm corresponds to the C-OH carbon in the para position, and a peak at 145.83 ppm corresponds to the two C-OH carbons in the meta position. The signal at 120.87 ppm corresponds to the -C- carbon in the para position, and the two carbons in the ortho position occur at 109.14 ppm on the benzene ring. The strong signal at 70.24 corresponds to the CH₂-CH₂ carbons of polyethylene glycol. The signals at 72.78 ppm and 60.68 ppm correspond to the C-O ether and C-O ester carbons at the end of the chain, respectively.

Figure 5. ¹³C-NMR spectra of PEG-digallate in DMSO-d₆.

Figure 6. ¹³C-NMR spectra of Gallic acid in DMSO-d₆.

Comparing these results with the literature demonstrates the presence of signals for all carbons corresponding to gallic acid.²⁵ The results in Figure 6 are compared with the ¹³C-NMR spectrum of polyethylene glycol esters, showing the same strong signal at 70.24 ppm, corresponding to the CH₂-CH₂ carbons of polyether, and the shift peak of C-O-ester carbons appearing at 60, 68 ppm.²⁶

X-ray diffraction analysis

X-ray diffraction (XRD) analysis was performed to examine the physical state of the obtained PEG-digallate complex. Figure 7 displays the results of the XRD examinations of gallic acid, PEG, and the composite PEG-digallates. Gallic acid was discovered to have numerous strong diffraction peaks of crystallinity between a diffraction angle of 2 θ = 7-50° in the X-ray diffractogram, indicating that it is exists as a crystalline substance.^27,28 Pure PEG displayed two distinct high intensity peaks at approximately 2 θ = 19.20° and 23.20° with a few minor peaks at: 2 θ =26.35, 36.18, 39.84 and 45.29°, respectively. Complex development, where the typical PEG peaks of the composite PEG-DG are present with slightly decreased strength and the elimination of huge diffraction peaks, where it is no longer feasible to detect the characteristic peaks of the composite, both contributed to a partial amorphization. The findings showed that gallic acid is no longer a crystalline substance and that the PEG complex it was successfully formed into still exists in a semi-amorphous condition.

Figure 7. X-ray diffraction patterns of pure gallic acid, pure PEG and the composite PEG-DG.

Scanning electron microscopy

Figure 8 displays SEM images of the complex PEG-Digallate, pure PEG, and gallic acid; they were taken with zooms up to 500, 50, and 10 μm, respectively. Smaller, regular-shaped crystals with a surface that appeared smooth define the pure gallic acid, which also has smaller, more crystalline forms (Figure 8, A). In contrast to the PEG-digallate composite, pure PEG has an extremely smooth and flat surface and very regular compact structure (Figure 8, B). Scanning electron micrographs reveal that the surface morphology of the PEG-digallate (Figure 8, C) composite was amorphous; this may have contributed to the absence of crystal structures (confirming the X-ray diffraction results) and may be related to the uniform dispersion of GA in the PEG matrix.

Figure 8. SEM images of GA (A; 500μm, A1; 50μm, A2; 10μm), PEG (B; 500μm, B1; 50μm, B2; 10μm), and PEG-DG (C; 500μm, C1; 50μm, C2; 10μm).

UV-Vis absorption measurement

The UV-Vis absorption spectra of PEG, GA and PEG-digallate dissolved in DMSO were recorded at room temperature, as shown in Figures 9 & 10.

Figure 9. UV-Vis spectra of gallic acid and PEG-digallate in DMSO as solvent.

Figure 10. UV-Vis spectrum of polyethylene glycol in DMSO.

The absorption spectral changes of PEG-digallate in the presence and absence of PEG are shown in Figure 9. From the obtained spectrum, the absorbance maximum of gallic acid was discovered at 277 nm, which was related to the absorption of aromatic amino acids. The peaks of PEG-digallate increased with the addition of GA, and a red shift at 295 nm was noted. The outcomes suggested that polyethylene glycol and gallic acid had formed a novel combination. A transparent compound in a spectral domain, when taken in an isolated state, can become absorbent if it is put in the presence of a species with which it interacts by a mechanism of the donor-acceptor (D-A) type. According to the UV-Vis absorption spectra of PEG in DMSO, there was no discernible absorption between 200 and 800 nm, demonstrating that pure polyethylene glycol has a relatively low total absorption in the UV region (Figure 10). Important for electrochromic applications is this observation.

Thermal analysis (TGA)

Thermal gravimetric analysis (TGA) was used to investigate the thermal degradation of PEG-digallate. The TG and DTG curves of PEG, GA, and PEG-digallate were shown in Figure 11 from room temperature to 600 C in a N2 environment at a heating rate of 10°C/min.

Figure 11. Thermogravimetric analysis TGA of (A) gallic acid, (B) PEG and (C) PEG-digallate.

The Figure 11 C confirms that the surface medicine was successful. PEG-digallate TG and DTG curves (Figure 11 C) differ from those of PEG and GA (Figure 11 A & B). The PEG TG and DTG curves show that PEG has a single breakdown phase that begins at 350 °C and ends about 405 °C. Thermal breakdown of PEG is anticipated to occur at both the backbone chains -C-O- and -C-C- bonds. Figure 11 (A) illustrates that in GA, which has three decomposition phases, the compound is stable up to 87°C when the first mass loss (8.27%) occurs up to 105°C due to hydration water. Gallic acid is stable beyond this temperature until 200°C, with no mass loss and no endo or exothermal processes. The second phase (36.38%) occurs between 220-264°C, and the DTA curve shows an endothermic peak (250°C). The oxidation of organic matter immediately causes the third mass loss (18.22%) and two related exothermic peaks (at 400 and 428°C, respectively). The final residue of gallic acid breakdown was 1.5% of the total original mass (carbon residue). The TG and DTG curves of PEG-digallate in Figure 11C show that the majority of weight loss occurs below 100 °C, which can be attributed to trapped water. Thermal deterioration of composites, on the other hand, occurs primarily around 210 °C and 420 °C, which can be attributed to the GA layer stably coating on PEG. By counting the weight loss from 200 °C to 700 °C, the TG curve reveals that the amount of GA coated on PEG is around 3 to 4 wt.%. Because to the reduction in surface area for alterations, the aggregated PEG-digallate explains the limited amount of coated GA.