Polyurethane/Silver Nanoparticle Composite Thermal Behavior and Electrical Conductivity Enhancement Based on Chalconic Moiety

INTRODUCTION

The scientific and economic interest in polyurethane polymers is high, and their use in industry is on the rise [1, 2]. Polyurethane products and many nanomaterials, which have been widely used to produce composites or as reinforcers of thermoplastic and thermoset polymer matrices in textile, car, packaging, civil construction industry applications, and so on, are most widely used in the transportation sector[1–5]. Polyurethanes’ low cost high resilience and lifetime make them a good choice for many medical applications [6]. This quality of polymeric materials has made them superior to their more conventional counterparts, such as ceramics and metals. Epoxies, unsaturated polyesters, and phenolics are all examples of thermoplastic and thermoset polymers; polyurethanes are in the same family. In a highly exothermic reaction, an isocyanate group (-N=C=O) is converted into a urethane linkage by reacting with an alcohol (-OH). The polyaddition reaction generates polyurethanes, and the core of an inflated foam can reach temperatures of 150–70 °C. In recent years, several organic successive polymers have been discovered, that can become electrically conductive through doping with either acceptors or strong electron donors. Then the two scientists Heeger and MacDiarmid noticed for the first time that the conductivity of polyacetylene increases significantly when the polymer is treated with an oxidizing or reducing agent [7-10].

As a result of their antibacterial properties, optical qualities, electrical conductivities and potential antioxidant action, polymer-silver composites have seen extensive use [11, 12] Superior electrical properties, high stability, and antibacterial uses make many materials such as liquid crystals, silver, zinc oxide and iron oxide nanoparticles modified with diverse polymers of great interest [13- 19]. Conductive fibers with high electrical conductivity while also offering great flexibility and stretchability require the mixing of conductive fillers in a soft polymer. Additionally, there are typically trade-off features between conductivity and stretchability in conductive composite materials. Numerous articles have been published on the topic of synthesizing polyurethane and polyurethane-zinc composites. However, there is a lack of research into the production of polyurethane-silver composites. 

As a chain extender in polyurethane synthesis, we have designed and synthesized chalconic units in the present study. The incorporation of chalconic units into the structure boosted the resulting PU’s thermal stability. This led us to develop novel, previously unreported polyurethane and polyurethane-silver composites. Silver composites based on polyurethane were newly created by dispersing silver particles in dimethylformamide (DMF) in a simple technique. Silver nanoparticles stabilized by polyurethanes have been shown to significantly enhance conductivity measurements and are hence of considerable interest. Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy (XRD), and thermogravimetric analysis (TGA) have all been used to analyze these produced substances. Composites’ morphological characteristics were analyzed using scanning electron microscopy with energy dispersive spectroscopy. The Ossila four-point probe approach with the T2001A3 electrometer was used to determine the conductivity values of the composites. There is a wide variety of uses that could benefit from these materials’ novel structures, characteristics, and morphological models.

MATERIALS AND METHODS 

Aldrich-Sigma compounds Co. provided all compounds, which were used in their original form. Using a Shimadzu FT-IR 8400s spectrometer, we performed a Fourier transform infrared (FT-IR) spectrum study from 400 to 4000 cm^-1. Using deuterated DMSO as the solvent and TMS as an internal reference, 500 MHz ¹H NMR spectra were obtained on a Bruker DMX-300 spectrometer. The thermogravimetric analysis (TGA) was performed using an SDT-Q600 to determine the degradation properties of the produced polymers. Under a nitrogen environment, temperatures were measured from 25 to 800 °C at a rate of 20 °C/min. Using a Cu K radiation source at 30 kV and 20 mA at ambient temperature, powder X-ray diffraction (XRD) was carried out on a HAOYUAN Dx2700BH Dandong. The radiation source’s wavelength was 1.5406. Conductivity measurements of composites were determined with Ossila four-point probe technique using a T2001A3 electrometer.

Synthesis of Monomer A

In a round-bottomed flask, 20 mL of methanol, 40 mmol of p-hydroxybenzaldehyde, and 45 mmol of anhydrous CaCl₂ were combined with 3.2 mmol (0.4 mL) of BF₃•Et₂O. A temperature of 50 degrees Celsius was reached in the mixing process. After waiting 12 hours, 20 mmol of acetone (about 1.5 mL) was added to 3.5 mL of methanol and added to the reaction mixture. 500 mL of water was added to the reaction mixture. The obtained precipitate was dried in a vacuum oven at 60 degrees Celsius for 12 hours (yield: 95%) [14]. ¹H NMR (500 MHz, DMSO-d6) δ 10.09 (s, 2H), 7.68−7.62 (m, 4H), 7.16−7.06 (m, 4H), 6.88−6.55 (m, 4H,). FTIR (KBr) ν_max = 3290,1630, 1593, 1512 cm⁻¹.

Synthesis of Monomer B

In a round-bottomed flask, we combined 20 mmol (2.44 g) of p-hydroxybenzaldehyde, 20 mmol (2.72 g) of p-hydroxy acetophenone, and 27 mmol (3.0 g) of anhydrous CaCl₂ with 0.4 mmol (0.05 mL) of BF₃•Et₂O and 10 mL of methanol. After 10 hours of reaction at 50 degrees Celsius, the mixture was diluted by adding 500 millilitres of water. The precipitate was gathered and then dried under vacuum at 60 degrees Celsius for 12 hours (yield: 95 %) [20]. ¹ H NMR (500 MHz, DMSO-d6) δ 10.35 (s, 1H), 10.03 (s, 1H), 8.05−8.03 (d, 2H), 7.72−7.59 (m, 4H), 6.89−6.82 (m, 4H,). FTIR (KBr) ν_max = 3344, 1647, 1604,1542 cm⁻¹.

Synthesis of Monomer V 

To a round-bottomed flask containing 10 mmol of vanilla, 10 mmol of p-hydroxy acetophenone, and 27 mmol of anhydrous CaCl₂ were added 0.8 mmol of BF₃•Et₂O (0.1 mL) and 15 mL of methanol. After 8 hours of reaction at 50 degrees Celsius, the mixture was diluted by adding 500 millilitres of water. For 12 hours at 80 degrees Celsius under vacuum, the precipitate was dried (yield 95%) [14]. ¹H NMR (500 MHz, DMSO-d6) δ 10.38 (s, 1H), 9.77 (s, 1H), 8.08−8.05 (d, 2H), 7.84−7.39 (m, 4H),7.26-7.25(d,1H), 6.97−6.81 (m, 2H,), 3.87 (s,3H). FTIR (KBr) ν_max= 3244, 1645, 1603,1591 cm⁻¹.

General synthesis of polyurethanes PAT, PBT, and PVT

6.3 mmol of suitable monomer and 10 mL of dimethylformamide (DMF) were added to a round-bottomed flask. While the solution was mixed with magnetic stirring, 6.3mmol of TDI was added and stirred well for 10 minutes at room temperature. The reaction temperature was increased to 60 ⁰C for 4h. Gel formation was observed, and the precipitate was washed with methanol several times then collected and dried at 40 °C in vacuum for 4 h.

General synthesis of polyurethanes PAM, PBM, and PVM

6.3 mmol of suitable monomer and 10 mL of dimethylformamide (DMF) were added to a round-bottomed flask. While the solution was mixed with magnetic stirring, 6.3mmol of TDI was added and stirred well for 10 minutes at room temperature. The reaction temperature was increased to 60 ⁰C for 4h. Gel formation was observed, and the precipitate was washed with methanol several times then collected and dried at 40 °C in vacuum for 4 h.

General synthesis of polyurethanes PAH, PBH, and PVH

6.3 mmol of suitable monomer and 10 mL of dimethylformamide (DMF) were added to a round-bottomed flask. While the solution was mixed with magnetic stirring, 6.3 mmol of HDI was added and stirred well for 10 minutes at room temperature. The reaction temperature was increased to 60 ⁰C for 4h. Gel formation was observed, and the precipitate was washed with methanol several times then collected and dried at 40 °C in vacuum for 4 h.

Preparation of silver nanoparticles by sol-gel method

(0.001 M, 0.17 g) of silver nitrate AgNO₃ was added to a round-bottomed flask with 100 mL of deionized water, the mixture was heated with stirring at 90 °C, and then the flask was covered with aluminium foil to prevent oxidation of silver nitrate, (100 mL) of sodium citrate was added slowly, prepared by adding (0.01 g 0.26 M) sodium citrate to 100 ml of deionized water. The color solution changed from colorless to Pale yellow, the solution was put in a cold-water bath, and then drops of 10% NaOH were added with Continuous stirring until the pH reached (10-11). Then the precipitate was collected in a centrifuge machine at a speed of (5000 rpm). Then the deionized water was added, and the separation process was repeated twice to remove unreacted dissolved ions, the product was collected and dried at room temperature for two days away from light. The precipitate black powder is placed in plastic bottles with a capacity of (1.5 - 2.2 mL) [21].

Preparation of polyurethane-nano silver particle composite membranes

The polyurethane-nano silver particle composite was prepared by dissolving 0.05 g of polyurethane (PBT, PVT, and PVM) in 1 mL DMSO. To this solution, 0.1 g NaCl dissolved in deionized water was added. After that, 0.1g of silver nanoparticles were added. Then the mixture was sonicated in an ultrasonic bath (30 ⁰C for 5 min). After the mixture becomes homogeneous, the mixture is placed on a glass slide and left until the solvent volatilizes, thus making it ready for electrical measurements.

RESULTS AND DISCUSSION 

The synthetic route of polyurethane’s series and the structures of monomers A, B and V are illustrated in Fig. 1. The solubility of the obtained polymers was tested in the commonly used organic solvents. The results show that polymers PAH, PBH and PVH are soluble poorly in polar solvents and PAT, PBT, and PVT are soluble well in some polar solvents like DMSO, and DMF. The solubility of polymer containing TDI moiety is higher than polymer containing HDI and MDI due to the presence of methoxy group in structure leading to the increase in the polarization of this polymer. Due to the poor solubility of polymers containing HDI and MDI in the solvent, its NMR spectra could not be.

Spectral studies

¹H NMR spectra of PAT and PVT polyurethane prepared from Toluene Diisocyanate and dihydroxy chalcone in DMSO-d6 are demonstrated in Fig. 2. In general, the two signals observed at 1.98 and 2.199 ppm are assigned to Methyl groups on aromatic rings, two aromatic structures expected in absence of side reactions [22-25]. The two singlet signals observed at the most downfield region, δ=9.94 and 8.89 ppm, supported the presence of two N-H amide linking groups in the aromatic rings of TDI. Aromatic protons and Olefinic protons are observed within the chemical shift range of δ=8.43-6.32ppm. Singlet resonate at δ=4.74 ppm to the methoxy protons that are directly connected to an aromatic ring of vanillin in chalconic moiety. Proves that the polymerization reactions have occurred successfully and the amide groups NHCOO have formed in the structure of polymers.

FT-IR spectra of some polyurethane are shown in Fig. 3. The appearance of two new bands at 3200- 3300 and 1710-1720 cm^-1 indicates the presence of NH and carbonyl groups, respectively in linking groups of polyurethane bands NHCOO. The analysis results of the resonance signals of the protons in the polymers and FT IR spectra are summarized:

P AT: Yellow solid; m.p 230-232 ⁰C, yield 72 %, FT-IR (Nujol, ν_max in cm⁻¹ ): 3282 (N-H urethane NHCOO), 3032 (C-H aromatic), 2920-2858 (C-H aliphatic), 1710 (C=O NHCOO), 1647 (C=O ketone), 1595 (C=C aromatic), 1600(C=C Olefine), 1543-1423(C=C aromatic), 1269(C-N). ¹ H NMR (400 MHz, DMSO-d6, TMS, ppm) δ 9.02 (s, 1H, N-Ha), 8.89 (s, 1H, N-Hb), 8.43−6.32 (m, 16H, Ar-H and -HC=CH-), 2.14 (m, 3H, CH₃), 1.98(m, 3H, CH₃). 

P BT. A yellow solid; m.p 200-212 ⁰C, FT-IR (Nujol, ν_max in cm⁻¹): 3387 (N-H urethane NHCOO), 3020 (C-H aromatic), 2920-2858 (C-H aliphatic), 1735 (C=O urethane NHCOO), 1654 (C=O ketone), 1597 (C=C Olefine), 1504-1423(C=C Ar), 1215(C-N). ¹ H NMR (400 MHz, DMSO-d6, TMS, ppm) δ 9.41 (s, 1H, N-Ha), 9.12 (s, 1H, N-Hb), 8.61−6.82 (m, 16H, Ar-H and -HC=CH-), 2.61 (m, 3H, CH₃), 2.21(m, 3H, CH₃). 

P VT. A yellow solid; m.p 230-232⁰C, FT-IR (Nujol, ν_max in cm⁻¹): 3292 (N-H urethane NHCOO), 3070 (C-H aromatic), 2920-2856 (C-H aliphatic), 1705 (C=O urethane NHCOO), 1649 (C=O ketone), 1597 (C=C Olefine), 1539-1413 (C=C aromatic), 1219 (C-N). ¹ H NMR (400 MHz, DMSO-d6, TMS, ppm) δ 9.02 (s, 1H, N-Ha), 8.99 (s, 1H, N-Hb), 8.43−6.32 (m, 16H, Ar-H and -HC=CH-), 4.74 (s, 3H, OCH₃), 2.14 (m, 3H, CH₃), 1.88(m, 3H, CH₃). 

PAM. A yellow solid; m.p, 240 ⁰C decompose, FT-IR (Nujol, ν_max in cm⁻¹): 3367 (N-H urethane NHCOO), 3028 (C-H aromatic), 2900-2839 (C-H aliphatic), 1720 (C=O NHCOO), 1674 (C=O ketone), 1597 (C=C Olefine), 1508-1423(C=C aromatic), 1230(C-N). 

P BM. A yellow solid; m.p 223-225 ⁰C, FT-IR (Nujol, ν_max in cm⁻¹): 3352 (N-H urethane NHCOO), 3026 (C-H aromatic), 2900-2837 (C-H aliphatic), 1720 (C=O urethane NHCOO), 1671 (C=O ketone), 1508 (C-H aromatic), 1597 (C=C Olefine), 1431-149(C=C aromatic), 1230(C-N). 

PVM. A yellow solid; m.p 210⁰C, FT-IR (Nujol, ν_max in cm⁻¹): 3352 (N-H urethane NHCOO), 3024 (C-H aromatic), 2900-2840 (C-H aliphatic), 1750 (C=O urethane NHCOO), 1670 (C=O ketone), 1597 (C=C Olefine), 1573-1408 (C=C aromatic), 1230(C-N). 

P AH. A yellow solid; m.p 210-212 ⁰C, FT-IR (Nujol, ν_max in cm⁻¹): 3414 (N-H urethane NHCOO), 3070 (C-H aromatic), 2924-2854 (C-H aliphatic), 1720 (C=O urethane NHCOO), 1651 (C=O ketone), 1573 (C=C aromatic), 1615 (C=C Olefine), 1543-1423(C=C aromatic), 1215 (C-N). 

PBH. A yellow solid; m.p 213-215⁰C, FT-IR (Nujol, ν_max in cm⁻¹): 3317 (N-H urethane NHCOO), 3026 (C-H aromatic), 2927-2854 (C-H aliphatic), 1705 (C=O urethane NHCOO), 1662 (C=O ketone), 1610 (C=C Olefine), 1573 (C=C aromatic), 1531-1423(C=C aromatic), 1215(C-N). 

PVH. A yellow solid; m.p 240⁰C decompose FT-IR (Nujol, ν_max in cm⁻¹): 3363 (N-H urethane NHCOO), 3066 (C-H aromatic), 2931-2854 (C-H aliphatic), 1739 (C=O urethane NHCOO), 1662 (C=O ketone), 1604 (C=C Olefine), 1573-1408 (C=C aromatic), 1203(C-N). 

Thermal Stability of polyurethane

TGA was used to evaluate the thermal characterization of prepared polyurethanes, Fig. 4 shows the TGA curves in a nitrogen atmosphere with a heating rate of 20 ⁰C min^−1. In general, it was observed that the sequence of degradation takes place in three stages depending on the mass loss in different temperature ranges. In the first stage, these polymers start with slight mass loss owing to the water molecule below T=100 °C and then fragments of the backbone and low molecular weight oligomers [26,27]. TGA curves show that the polyurethane PAH has the highest stability among prepared polymers, the initial step of degradation at 260 – 445 ⁰C a corresponding mass loss of 23%, leaving a total residual mass of about 73%. While TGA curves of PBT, PAT and PVT polymers have the lowest stability and show three stages of decomposition, we see an initial step of degradation at 190 ⁰C, 220 ⁰C, and 220 ⁰C corresponding with an abrupt total mass loss (94.36%, 93.13%, and 90.15%) respectively. The TGA data obtained from the TGA curves about the initial decomposition temperature and the mass loss at different temperatures are listed in Table 1.

The morphology and structure of polyurethane/silver Nanocomposite

XRD patterns were considered one of the most important techniques in determining the phase formation and crystal structure of prepared silver nanoparticles. Fig. 5 shows the X-ray diffraction pattern of prepared Ag nanoparticles given six peaks. As shown, (210), (122), (111), (200), (220), and (311), planes are agreed with JCPDS No. 04-0783. On the other side, the crystal structure of the model is Cubic. The mean crystal size (L) for Ag nanoparticles was calculated by utilizing Scherrer’s equations [28, 29].

L = k λ / β Cos θ                                                      (1)

Where k = 0.9 is the constant crystal lattice, λ, β and θ are indicated to shape constant, the wavelength of the radiation, Bragg diffraction angle, and full width at half maximum intensity (FWHM). The result indicates that it has a nano-size equal to 22.006 nm.

TEM images showed the formation of Ag nanosilver with prolate spheroidal phase structure Fig. 6. confirmed that the particles became irregular in shape and highly agglomerated upon silver. TEM images for Ag nano silver particles showed the presence of Ag+ on the surface of the samples

TEM micrographs of of B-M-Ag nanosilver are shown in Fig. 7. TEM study showed that the B-M-Ag nanosilver particles with 79 nm were formed with spherical geometry. Furthermore, a small number of black spots were observed on the surface of the B-M-Ag nanosilver.

Electrical conductivity study

The electrical conductivities studies of polyurethane and polyurethane/Ag NPs/NaCl composite were carried out in membrane form. The four-probe method was used to carry out the study using an electrometer. The electrical conductivities of PBM, PVM, and PBT were found to be at 0 S/cm. Herein, it was seen that the electrical conductivities increased when the polymer was doped with silver. Table 2 shows the data for electrical conductivities of some prepared polyurethane and polyurethane/ Ag NPs /NaCl composite and it was found that the conductivity increases steadily with doping silver, conductivities of PBT/Nano silver were measured as 188.64 S/cm, the increase in electrical conductivity of polymers imply that the charge-transfer between polymers and dopant silver is formed without interrupting, the highest conductivity was observed in of PBT when use dopants nano silver and NaCl at 199.77 S/cm. The maximum conductivity values for PBM/Ag NPs/NaCl, and PVM/Ag NPs/NaCl, were found to be 198.16 and 180.02 S/cm respectively. According to these values, only small, noteworthy differences were found in electrical conductivity values of polyurethane /nano silver / NaCl composite [30,31].

CONCLUSION 

This work presents a successful polyurethanes design for the generation of polyurethanes /AgNPs composite. Three monomers based on a chalconic units, and nine new polyurethanes have been successfully synthesized by condensation reaction with different types of isocyanates MDI, TDI and HDI. To develop the electrical conductivities of polyurethane, conductive nanoparticles, such as silver nanoparticles were added to the synthesized polyurethane nanocomposites exhibiting excellent conducting performance when a combination of silver nanoparticles, NaCl and prepared polyurethanes. The results of the TGA curves show that the obtained copolymers showed high thermal stability. The silver nanoparticle was investigated by XRD diffraction and TEM micrographs study showed the formation of Ag nano silver with prolate spheroidal phase structure and the B-M-Ag nano silver particles were formed with spherical geometry.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.