Acoustical study on interaction of thymol-guanidine-formaldehyde copolymer resin with polar protic and aprotic solvents

Journal of Molecular Liquids 222 (2016) 225–232

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Acoustical study on interaction of thymol-guanidine-formaldehyde copolymer resin with polar protic and aprotic solvents S.D. Kukade a,⁎, S.K. Singh b, R.R. Naik a, S.V. Bawankar a a b

Department of Chemistry, Jankidevi Bajaj College of Science, Wardha 442001, India Department of Chemistry, Visvesvaraya National Institute of Technology, Nagpur 440010, India

a r t i c l e

i n f o

Article history: Received 8 January 2016 Received in revised form 14 July 2016 Accepted 15 July 2016 Available online 17 July 2016 Keywords: Solute-solvent interaction Thymol Guanidine Copolymer resin

a b s t r a c t The various proportions of Thymol-guanidine hydrochloride-formaldehyde copolymer resins have been synthesized by the condensation polymerization of reacting monomers i.e. thymol, guanidine hydrochloride and formaldehyde using microwave irradiation technique. The synthesized copolymer resins have been characterized by spectral methods viz. UV–visible, IR, 1H NMR and 13C NMR spectroscopy. The ultrasonic velocity, density and viscosity of these copolymer resins in different solvents at 303 K have been measured. Based on the data obtained, various acoustical parameters like adiabatic compressibility, intermolecular free length, acoustic impedance, relaxation time and Gibb's free energy have been calculated. The result has been discussed on the basis of various types of solute-solvent interactions present between various ratios of copolymer resin with different solvent system. The increasing trends of ultrasonic velocity in different solvents is in the order: DMSO N DMF N 1,4dioxane N n-butyl alcohol N ethyl alcohol, which indicates the strong intermolecular interactions of synthesized copolymer resin with DMSO and 1,4-dioxane due to strong hydrogen bonding. © 2016 Published by Elsevier B.V.

1. Introduction In the 21st century, we see the numbers of product which are used in our daily life are made from polymers, hence we can say that polymer are the backbone of our present society [1–5]. Phenol-formaldehyde type polymer has many applications but now they are used in the preparation of coating materials, adhesive, electrical devices and ion exchange resins [6–13] while some polymeric resins used in the paint industry as a binder [14]. The solvents (termed as paint thinner) which are used in the paint industry are used to reduce viscosity of paint to provide better protection and decoration to the surface [15]. Initially polymer have been prepared using conventional heating which required more time but now a day's microwave irradiation technology has solve these problem and various researchers reported the application of microwave in the synthesis of terpolymeric resin [16–18]. In the recent years, nature of molecular interactions and types of forces present in the various solute-solvent and solvent-solvent systems have been studied using ultrasound. An ultrasonic study is non-destructive and very effective technique for the investigation of various thermodynamic parameters. Today this technique is extensively used in the medicinal, agricultural, industrial, polymer and solution chemistry [19–25].

⁎ Corresponding author. E-mail address: [email protected] (S.D. Kukade).

http://dx.doi.org/10.1016/j.molliq.2016.07.059 0167-7322/© 2016 Published by Elsevier B.V.

The ultrasonic velocity of a liquid is fundamentally related to the binding forces in between the atoms or molecules which are successfully employed in understanding the nature of molecular interactions present between solute and solvent [26–30]. The variations in the ultrasonic velocity and related acoustical and thermodynamical parameters with respect to varying temperature and concentration have been much interested for study to find out structural changes associated with interacting components present in the solution. Various researchers have been studied and reported the interaction present in different solute-solvent system [31–45]. By looking into the literature, it is seen that no solute-solvent interaction studies of thymol-guanidine-formaldehyde copolymer resin in different solvents has been reported so far. Therefore, it was planned to study such type of copolymer resin using acoustical methods. In the present paper, the copolymers resin has been synthesized in different molar ratios using microwave irradiation. The ultrasonic velocity, density and viscosity of various molar proportions of TGF copolymer resin in different solvents at 303.15 K have been measured. Based on the data obtained, various acoustical parameters like adiabatic compressibility, intermolecular free length, acoustic impedance, relaxation time, relative association and Gibb's free energy have been calculated. The variations of these parameters with different molar ratios of copolymer resin in various solvents have found to be useful in understanding the physical and chemical interactions among the atoms of the copolymer and organic solvents.

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2. Experimental section

Table 1 Reaction details of different ratios of TGF copolymer resins.

All the chemicals used as starting materials in the synthesis of the copolymer were of AR or chemically pure grade. 2.1. Microwave synthesis of TGF copolymeric resins The TGF copolymer resin was prepared by condensation polymerization of thymol (0.01 mol) and guanidine hydrochloride (0.01 mol) with formaldehyde (0.02 mol) in the molar ratio of 1:1:2 and in presence of 2 M HCl at 80±2 °C using microwave irradiation technique. It has been purified and characterized by various spectral methods [18]. Similarly different copolymer resins by varying the molar ratios of reactant monomers were also synthesized viz. TGF (2:1:3), TGF (3:1:4) and TGF (4:1:5). The reaction route is given in Scheme 1. The synthesized copolymer resins along with m.p., product yield and other reaction details are incorporated in Table 1. 2.2. Spectral analysis The electronic absorption spectra of the various TGF copolymer resins were recorded in ethanol on Shimadzu UV–visible spectrometer in the region of 190–400 nm and the infrared spectra were recorded on Shimadzu FTIR spectrometer in the region of 4000–400 cm−1. While the 1H NMR and 13C NMR spectra were recorded on Bruker Avance-II NMR spectrometer at 400 MHz using CDCl3 as a solvent and TMS as a reference.

Copolymer resin abbreviation Monomers Thymol (mole) Guanidine hydrochloride (mole) Formaldehyde (mole) 2 M HCl (ml) Time required for completion of reaction after microwave irradiation (minute) Molar ratio of reactants Yield (%) Melting point (K)

TGF-1 TGF-2 TGF-3 TGF-4 0.01 0.01 0.02 100 15

0.02 0.01 0.03 100 12

0.03 0.01 0.04 100 11

0.04 0.01 0.05 100 10

1:1:2 51.12 385

2:1:3 12.07 391

3:1:4 53.04 346

4:1:5 74.78 393

2.3. Acoustical study of TGF copolymer resin The solutions of different molar ratios of TGF copolymer resin were prepared by using various pure solvents viz. ethyl alcohol, n-butyl alcohol, dimethyl formamide (DMF), dimethyl sulphoxide (DMSO) and 1,4dioxane. The various solutions are prepared whose concentration is kept constant i.e. 0.1% but molar ratio of copolymer resin and solvents are varied. The ultrasonic velocity (U) of these various solutions has been measured using Digital Ultrasonic Echo Pulse Velocity Meter, Model VCT-70 (Vi Microsystem Pvt. Ltd., Chennai-96) at frequency 2 MHz with an accuracy of 0.1%. The viscosities (η) of these solutions were determined using Ostwald's viscometer by calibrating with doubly

Scheme 1. Synthetic route of TGF copolymer resins A) TGF-1 B) TGF-2 C) TGF-3 D) TGF-4.

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distilled water with an accuracy of ±0.001 Pa·s. while the density (ρ) of these solutions were measured accurately using 25 mL specific gravity bottle in an electronic balance, precisely and accurately. The basic parameter U, η and ρ were measured for different molar ratios of TGF copolymer resin in different solutions at 303 K. The various acoustical parameters viz. adiabatic compressibility, intermolecular free length, acoustic impedance, relaxation time, relative association and Gibb's free energy have been calculated from U, η and ρ values using standard formulae. 1. Adiabatic compressibility: β ¼ 1

 U2 ρ

2. Acoustic impedance (Z): Z =U × ρ 3. Intermolecular free length: Lf = KT √ β where KT is temperature dependent constant known as Jacobson's constantKT = (93.875 + 0.375 T) × 10‐8, where T = absolute temperature in Kelvin  4. Relaxation time: τ ¼ 4ηβ 3 5. Gibb's free energy (from  ΔG ¼ RT ln½kTτ h 

Eyring–Polanyi

Equation):

where, k = Boltzmann's constant (1.3806 × 10−23 J K−1),R = gas constant (8.314 J K−1 mol−1)h = Planck's constant (6.6250 × 10−34 J·s). 3. Results and discussion The synthesized TGF copolymer resins are yellow in color and soluble in ethyl alcohol, n-butyl alcohol, dimethyl formamide, dimethyl sulphoxide and 1,4-dioxane. 3.1. Electronic spectra The electronic absorption spectra of all the TGF copolymer resins were recorded in 95% ethanol (Fig. 1). These copolymer resins give rise to two characteristic bands in the region 200–300 nm. The band in the region 240–244 nm may be π-π* transitions in aromatic C_C as well as in the C_N group while the characteristic band between 279 and 288 nm indicates the n-π* transitions in the C_N group. This shift in the absorption band towards longer wavelength may be due to the presence of auxochrome group (C_NH group) present in copolymer resins [12,13,18,23,49,50].

Fig. 1. Electronic absorption spectra of TGF copolymer resins.

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3.2. Functional group analysis The IR spectra of all the TGF copolymer resins provide useful information about the linkages and functional groups present in the copolymer resins (Fig. 2) and the data is summarized in Table 2. The band at 3353 cm−1 may be due to the N\\H stretching (symm. & asymm.) while the broad band in the region 3300–3200 cm−1 may be due to the intermolecular hydrogen bonded phenolic hydroxyl groups (O\\H stretching). The sharp band at 1236 cm−1 and 1075 cm−1 may be due to C\\O stretching. The band present between the region 3000–2800 cm−1 may be due to aromatic C\\H stretching and band appearing in the region 1668 cm−1 may be due to C_C stretch which are the characteristics feature of aromatic region. The band appearing at 1452 cm− 1 is attributed due to C\\H def. in \\CH2\\ while the band at 1303 cm−1 may be due to C\\H def. in\\C(CH3)2\\. The band at 1668 cm−1 may be due to the\\C_N stretching (imines) and 890– 690 cm−1 is of ortho and para substituted benzene ring.

3.3. NMR spectra (1H NMR & 13C NMR) The 1H NMR spectra of all the TGF copolymer resins are shown in Fig. 3A and the data are tabulated in Table 3. It is seen that, the spectral features of all TGF copolymer resins are qualitatively similar to each other. The signal appear at δ 7.24 ppm which is due to intermolecular hydrogen bonding probably tells the presence of phenolic (–OH) proton while protons on methylenic linkage of Ar–CH2–N moiety absorbs at δ 3.11 ppm. The unsymmetrical pattern at δ 6.79–7.04 ppm indicates the presence of aromatic ring protons (Ar–H). The signal appearing in the region δ 3.74–4.18 ppm is of protons present on \\NH\\ bridging and at δ 6.57 ppm may be due to C_NH proton (imine). The occurrence of signal at δ 1.22 ppm may be due to the protons present on methyl group which is directly attached to phenolic ring of copolymer resin. The signal appeared in the region δ 1.08–1.14 and δ 2.04–2.38 ppm may be due to the protons present on methyl group and –CH– group present on isopropyl moiety of TGF copolymer resin. While the 13C NMR spectra of all TGF copolymer resins are shown in Fig. 3B–D. The spectrum shows that there are signals in the region δ 117–135.2 ppm indicates the presence aromatic carbons (sp2 hybridized) present in the thymol ring of TGF copolymer resins. The peak appearing at δ 26.8 is of methylenic carbon of NH\\CH2 bridge and at

Fig. 2. IR spectra of TGF copolymer resins.

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Table 2 FTIR spectral data of TGF copolymer resins. Assignment

Phenolic\ \OH stretch \ \NH stretch (imide) \ \NH bend (imide) Aromatic C_C stretch Phenolic C\ \O stretch Methylic bridge (\ \CH2) modes rock Methylic bridge (\ \CH2) modes bend Methylic bridge (\ \CH2) modes wagging C_N (imines) Ortho and para substituted benzene ring

Observed band frequency (cm−1) TGF-1

TGF-2

TGF-3

TGF-4

3300–3200 3353 680 1452 1236/1075 800–710 1452 1303 1729 850–690

3300–3200 3353 680 1452 1236/1075 800–710 1452 1303 1729 850–690

3300–3200 3353 680 1452 1236/1075 800–710 1452 1303 1729 850–690

3300–3200 3353 680 1452 1236/1075 800–710 1452 1303 1729 850–690

δ 150.6 ppm (highly deshielded) is due to the carbon of C_N moiety present in the resin. The signal at δ 15.8 ppm may be observed due to the methyl carbon attached directly to the aromatic ring. The chemical shift at δ 19.1 ppm is of methyl carbon and at δ 22.8 ppm is of carbon of –CH– group present on isopropyl moiety of TGF copolymer resin. There is very sharp peak appearing in the region 76–78 ppm is of CDCl3 which was used as reference in this work [23,49,50].

Expected band frequency (cm−1)

References

3750–3200 3500–3300 800–600 1650–1450 1310–1050 800–710 1485–1300 1300–1200 1790–1740 850–690

[12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,13,18,23] and [46–50] [12,18,23] and [46–50] [12,13,18,23] and [46–50]

3.4. Acoustical study of TGF copolymer resins The measured values of ultrasonic velocity, viscosity, density and related acoustical parameters like adiabatic compressibility, intermolecular free length, acoustic impedance, relaxation time and Gibb's free energy of different molar ratios of TGF copolymer resin with different solvents are incorporated in Figs. 4, 6–8.

Fig. 3. (A) 1H NMR & (B–D) 13C NMR spectra of TGF copolymer resins.

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Table 3 1 H NMR spectral data of TGF copolymer resins. Nature of proton assigned in the NMR spectra

Aromatic (Ar-H) Phenolic (Ar-OH) C_NH proton Ar-CH2-N moiety \ \NH bridging Ar-CH3 \ \CH(CH3)2 \ \CH(CH3)2

Observed chemical shift (δ) in copolymer resin (ppm) TGF-1

TGF-2

TGF-3

TGF-4

6.79–7.04 7.24 6.57 3.11 3.74–4.18 1.22 2.04–2.38 1.08–1.14

6.79–7.04 7.24 6.57 3.11 3.74–4.18 1.22 2.04–2.38 1.08–1.14

6.79–7.04 7.24 6.57 3.11 3.74–4.18 1.22 2.04–2.38 1.08–1.14

6.79–7.04 7.24 6.57 3.11 3.74–4.18 1.22 2.04–2.38 1.08–1.14

The variations of acoustical parameters of various ratios of TGF copolymer resin are studied in different solvents. The results have been discussed in terms of various interactions observed and nature of bonding present in different solutions of TGF copolymer resin. The increase in the value of ultrasonic velocity in different solvents follows the orderDMSO N DMF N 1,4-dioxane N n-butyl alcohol N ethyl alcohol, thus this increase in the value of ultrasonic velocity and decrease in the value of intermolecular free length may be due to strong dipole-dipole interactions in DMSO than in ethyl alcohol [23]. But by increasing the molar ratio of thymol and formaldehyde in TGF copolymer resin from TGF-1 to TGF-4, in different solvents, ultrasonic velocity decreases significantly in ethyl alcohol (Fig. 4A). The ultrasonic velocity values is found to be nearly constant in DMSO, 1,4-dioxane and n-butyl alcohol. The ultrasonic velocity is higher in medium with rigid or well-arranged molecular structure due to strong solvent-solvent interactions. The decrease in the value of ultrasonic velocity in ethyl alcohol is due to strong solutesolvent hydrogen bonding interactions and higher solvation strength resulting in slow transfer of sound energy from one solvent molecule to another. While increase in the value of ultrasonic velocity supports the weak solute-solvent interactions, weak or absent solute-solvent hydrogen bonding and lower solvation strength in DMF as compared to other solvents which is shown by Fig. 4A [19,23,25,31,41,42]. From the Fig. 4B, it is observed that the values of the densities in different solvents increases continuously as the molar ratio of TGF copolymer resin increases. The increase in the value of density may be due to association of solute particles in the solution which supports stronger attraction of unlike (solute-solvent) molecules (Fig. 5) [19,25,41,42]. It is seen that, pure DMSO shows higher value of density as compared to other solvents. Viscosity is the drag force between layers of liquid that resist its free flow. Higher the solvent-solvent attractive interaction, higher is the viscosity of liquid. The value of the viscosities increases in TGF copolymer resin solutions as compared to pure solvent in case of ethyl alcohol and DMF. This increase in viscosity is attributed to

Expected chemical shift (δ) (ppm)

References

6–9 4–12 6.57 2.5–4.8 4–8 0.9–2 2–4 0.9–2

[12,13,18,23,46–50] [12,13,18,23,46–50] [12,18,23,46–50] [12,13,18,23,46–50] [12,13,18,23,46–50] [18,46–50] [18,46–50] [18,46–50]

solute-solvent hydrogen bonding interactions. However, with increase in the TGF copolymer ratio, there are increased phenolic OH available for hydrogen bonding. Therefore, the viscosity increases minutely (apparently constant in Fig. 6A) from TGF-1 to TGF-4 in ethyl alcohol and DMF, except for 1,4-dioxane. This may due to weaker dipole moment of 1,4-dioxane and less polar C\\O\\C bonds for participation in hydrogen bonding interactions. We know from Fig. 6A that DMSO and n-butyl alcohol are high viscous solvent as compared to ethyl alcohol, DMF and 1,4-dioxane. This is due to stronger solvent-solvent interactions in DMSO and n-butyl alcohol. The viscosity value decreases in case solutions of DMSO and n-butyl alcohol compared to pure DMSO and nbutyl alcohol solvent. This indicates interference of solute (polymer) molecules in the solvent-solvent interactions. Further increase in the

Fig. 5. Solvation sphere of TGF copolymer resin with 1,4-dioxane.

Fig. 4. Variation of (A) ultrasonic velocity (B) density with different molar ratios of TGF copolymer resin in different solvents.

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Fig. 6. Variation of (A) viscosity (B) adiabatic compressibility with different molar ratios of TGF copolymer resin in different solvents.

viscosity from TGF-1 to TGF-4 solutions is again increased phenolic OH available for hydrogen bonding (Fig. 6A). The increase in the value of viscosity may be due to the strong Van der Waals forces of attraction and strong hydrogen bonding present between copolymer resin molecules and molecules of pure n-butyl alcohol and DMSO [19,23,31,44,45]. Adiabatic compressibility is the measure of relative change of volume of a liquid with change in pressure or under stress. The higher the compressibility, the lower will be the intermolecular attractive forces like dipole-dipole, hydrogen bonding, etc. Also, compressibility is proportional to the square of the intermolecular free length. Hence, stronger the interactions between liquid molecules, the lower will be the adiabatic compressibility and shorter will be the intermolecular free length [51]. As the molar ratio of thymol and formaldehyde in TGF copolymer resin increases from TGF-1 to TGF-4, then values of the adiabatic compressibility and intermolecular free length decreases in n-butyl alcohol, DMSO and 1,4-dioxane continuously while its value showed non-linear trend in ethyl alcohol and DMF [Figs. 6B and 7B]. The decrease in the value of intermolecular free length and adiabatic compressibility supports strong solute-solvent interaction which shows that there may be strong intermolecular hydrogen bonding between copolymer resin and n-Butyl alcohol, DMSO and 1,4-dioxane molecules while nonlinear trends (ethyl alcohol and DMF) in the value of intermolecular free length and adiabatic compressibility may be due to weak interactions between copolymer resin molecules and solvent molecules [19,24,31,41–45]. From the Fig. 7A, it is seen that the values of the acoustic impedance for different copolymer resin solutions increase continuously in ethyl alcohol, n-butyl alcohol, DMSO, DMF and 1,4-dioxane. The increase in the value of acoustical impedance may be due to strong interactions between the solute-solvent molecules. But there are nonlinear trends observed in DMF, which may be due to more interactions between solute-

solute and solvent-solvent molecules as compared to solute-solvent molecules [19,23,25,41,42]. The relaxation time and Gibb's free energy is an important parameter to determine the nature of interaction and spontaneity of reaction between solute-solvent molecules. With increase in the molar ratio of TGF copolymer resin, the values of the relaxation time as well as Gibb's free energy decrease in ethyl alcohol and 1,4-dioxane while its value increases in n-butyl alcohol, DMSO and DMF. The increase in the value of relaxation time and Gibb's free energy may be due to weak solute-solvent interactions while decrease in the value of relaxation time and Gibb's free energy supports stronger interactions between solute-solvent molecules due to intermolecular hydrogen bonding (Fig. 8) [23, 41,42,45]. Gibb's free energy showed a steep decrease compared to pure solvent in case of DMSO and later, with increase in molar ratio of TGF copolymer resin, the Gibb's free energy also increased. Combining these observations, we concluded that the TGF copolymer resin showed strongest solute-solvent interactions with DMSO at lowest molar ratio (TGF-1). Further, solution of TGF-1 in DMSO was evaluated for enthalpy and entropy thermodynamic parameters. For this Gibb's free energy of this solution at various temperature was plotted and linear fitted in accordance to basic relation ΔG = ΔH − TΔS [52,53]. The fitted plot is shown in Fig. 9. The entropy value of 5.32 × 10− 24 J molecule−1 (3.204 KJ/mol) and enthalpy value of 0.47 × 10− 20 J molecule−1 (2.830 KJ/mol). Positive value of entropy indicated increase in entropy favoring dissolution of polymer in solvent via strong solute-solvent interactions [54]. 4. Conclusion In the present study, different molar ratios of copolymer resin TGF was synthesized from thymol and guanidine hydrochloride with

Fig. 7. Variation of (A) acoustic impedance (B) intermolecular free length with different molar ratios of TGF copolymer resin in different solvents.

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Fig. 8. Variation of (A) relaxation time (B) Gibb's free energy with different molar ratios of TGF copolymer resin in different solvents.

formaldehyde by using microwave irradiation technique. The synthesized copolymer resins have been characterized by spectral methods viz. UV–visible, IR, 1H NMR and 13C NMR spectroscopy. The spectral data suggests the formation of expected TGF copolymer resins. The ultrasonic velocity, viscosity and density of the TGF copolymer resin solutions with different solvents viz. ethyl alcohol, n-butyl alcohol, DMSO, DMF and 1,4-dioxane has been calculated and based on data obtained, other acoustical parameter such as adiabatic compressibility, intermolecular free length, acoustic impedance, relaxation time and Gibb's free energy were determined at 303 K. The study of acoustical parameters provides the wealth information regarding the nature of molecular interaction present in the solution. The increase in ultrasonic velocity and decrease in intermolecular free length with increase in molar ratios of TGF copolymer resin with DMSO as solvent suggests the strong intermolecular interaction between copolymer resin molecules and solvent molecules through\\OH and _NH groups. The Gibb's free energy decrease in DMSO and 1,4-dioxane solutions as compared to pure solvents indicated favorable solute-solvent interactions in solutions which is due to dipole-dipole interactions and intermolecular hydrogen bonding respectively as also indicated by ultrasonic velocities and intermolecular free length. It means that copolymer forms stronger interaction in polar aprotic solvents than in polar protic solvents.

Fig. 9. Linear curve fitted to the plot of Gibb's free energy vs temperature.

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