Waterborne polyurethanes based on macrocyclic thiacalix[4]arenes as novel emulsifiers: synthesis, characterization and anti-corrosion properties

Abstract

Thiacalix[4]arenes (TC4As) are macrocyclic compounds with a cavity shape, host–guest properties, and modifiable structure for desired application. In this study, two 1,3-alternate derivatives of TC4As were employed as new macrocyclic emulsifiers bearing sulfonate groups in mixture with dimethylolpropionic acid (DMPA) to synthesize waterborne polyurethanes (WPUs). The effect of type and content of TC4As on the particle size, storage stability and viscosity of the dispersions was studied. Moreover, ATR-FTIR, ¹HNMR, XRD, SEM, TGA and DMTA analysis were used to investigate the structure, morphology and thermo-mechanical properties of polyurethane films derived from synthesized WPU dispersions. The morphological inspection of samples exhibited that the microphase separation degree strongly depends on the type and content of TC4As. Furthermore, water contact angle measurements confirmed that the WPUs containing TC4As have higher hydrophobicity in comparison to WPU samples containing only DMPA as emulsifier. This improvement in hydrophobicity of TC4As-based WPUs can be explained in terms of the more hydrophobic structure of the TC4As compared to DMPA. The anti-corrosion performance of the WPUs as environmental friendly coatings for mild steel in 3.5% NaCl solution was also evaluated using PDS and EIS techniques. The results revealed that the sample including macrocyclic SuTC4A as 5 mol% of total emulsifiers has the best corrosion protection ability which was also affirmed by SEM analysis.

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1. Introduction

Polyurethanes (PUs), a most versatile class of polymers, are widely used in different applications from foams and elastomers to adhesives and coatings. Polyurethanes show quite interesting properties because of their many possibilities in the selection of monomers from a great variety of diisocyanates, polyols, and chain extenders.^1–3

Traditional polyurethane products usually contain considerable toxic volatile organic compounds (VOCs) which are harmful to the environment and human health. Recently, they have been gradually replaced by waterborne polyurethanes (WPUs) as real alternatives in the lessening of volatile organic compounds emissions.^4,5 WPU dispersions with low viscosity and convenient film formation ability are binary colloidal systems in which the PU particles are dispersed in an aqueous phase due to the presence of ionic groups in their structure, which act as internal emulsifiers. WPUs have been used in various fields including adhesives and have applications as coatings for textiles, paper, wood, concrete, leather, metal and some polymers.^6–8

Polyurethanes are composed of soft segment and hard segment sequences. The soft segments are derived from polyols, while hard segments derived from diisocyanates and chain extenders. Microphase separation may take place in polyurethanes which can control the morphology, crystallinity and thermo-mechanical properties. The degree of microphase separation depends on the chemical structure of the diisocyanates, polyols, and the chain extenders employed in backbone of polyurethanes.^9,10

The hydrophilic group is of primary importance in the synthesis of WPUs. The dimethylolpropionic acid (DMPA) with two reactive hydroxyl groups bearing carboxyl group is one of the most commonly internal emulsifiers to prepare WPUs which cause the dispersibility of the polyurethane chains in aqueous medium.¹¹ To prepare WPU dispersions, some compounds bearing sulfonate groups, as well as DMPA, can also been used as internal emulsifier. In some few studies, sulfonated diols or diamines such as N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid sodium salt (BES sodium salt),¹² sodium-2,4-diaminobenzenesulfonate (SDBS),¹³ polyether sodium sulfonate (PESS),¹¹ and 2-[(2-aminoethyl)amino]ethyl sulfonic acid sodium salt (A95),¹⁴ have been reported as internal emulsifier to synthesize WPU dispersions.

Thiacalix[4]arenes (TC4As) are macrocyclic compounds with high thermal stability, which their cavity shape and host–guest property make them attractive candidates for many applications such as supramolecular chemistry, molecular sensing and separation.^15–18 Furthermore, calix[4]arenes family compounds are introduced in the literature as new corrosion inhibitors due to their aromatic planar cycles, which are able to interact with and metallic surfaces through p-electrons.^19,20 Recently, it has been disclosed that incorporation of some ionic functionality into the hard segment units of polyurethane chains leads to enhanced corrosion protection ability of PUs.^21,22 Thus, it is estimated that the incorporation of the TC4As derivatives bearing ionic groups as novel emulsifiers into polyurethanes can makes them potential candidate for anti-corrosive applications with specific properties. Up to now, only two our previous studies have been undertaken to covalently incorporate thiacalix[4]arenes into polyurethanes. In these studies, we described the preparation of p-tert-butyl-thiacalix[4]arene imbedded flexible polyurethane foam as an efficient cationic dye adsorbent and synthesis of thiacalix[4]arenes-based polyurethane elastomers.^10,18

The purpose of this study is to prepare and characterize a series of waterborne polyurethanes based on two types of thiacalix[4]arenes macrocycles bearing sulfonate groups as novel anionic internal emulsifiers in mixture with DMPA. The WPUs were synthesized by a pre-polymer process from isophorone diisocyanate, poly(tetramethylene oxide) (PTMO) and thiacalix[4]arenes macrocycles (TC4As) including p-tert-butyl-dipropyl sulfonate thiacalix[4]arene (SuTC4A) and dibutyl tetrasodium thiacalix[4]arene tetrasulfonate (BuTSTC4AS) in mixture with dimethylolpropionic acid (DMPA). The introduction of TC4As into polyurethane backbone can be carried out through the reactive hydroxyl groups of TC4As reacting with the functional group of diisocyanates. The effects of type and content of TC4As on the viscosity, particle size, and storage stability of the dispersions, and also on the structural, morphological, and thermo-mechanical properties of the obtained films were investigated. Additionally, we utilized these environmentally friendly waterborne polyurethanes as anti-corrosion coatings for mild steel in 3.5% NaCl solution. On the other hand, to the best of our knowledge, the use of thiacalix[4]arene derivatives as novel internal emulsifiers in preparation of waterborne polyurethanes has not been studied.

2. Experimental

2.1. Materials

poly(tetramethylene oxide) (PTMO) (M_w = 2000 g mol⁻¹) as polyether polyol, was supplied by Solvay Chemicals Co. Isophorone diisocyanate (IPDI) and dimethylolpropionic acid (DMPA) were used without further purification and purchased from Sigma Aldrich. Dibutyltin dilaurate (DBTDL, Merck), N-methyl pyrrolidinone (NMP, Merck), triethylamine (TEA, Merck), and 1,4-butanediol (BDO, Aldrich), were all purchased in analytical reagent grade. Deionized water was used as the dispersing phase. PTMO and BDO were dried at 100 °C under vacuum for 2 h to remove residual water that may otherwise interfere with the isocyanate reactions. Other chemicals used in the synthesis of TC4As were as follows: sulfur, p-tert-butyl phenol, sodium hydroxide, tetraethylene glycol dimethyl ether (tetraglyme), 1,3-propane sultone, butyl iodide, potassium carbonate and sulfuric acid.

2.2. Synthesis of thiacalix[4]arene derivatives (TC4As)

2.2.1. Synthesis of p-tert-butyl thiacalix[4]arene (p-BTC4A). p-BTC4A was prepared based on already reported method.¹⁵ A mixture of p-tert-butyl phenol (8 g), elemental sulfur S8 (3.5 g), and NaOH (1.20 g) in tetraethylene glycol dimethyl ether (2.5 mL) was stirred under nitrogen. The stirred mixture was heated gradually to 230 °C over a period of 4 h and kept at this temperature for further 3 h under nitrogen atmosphere. The obtained dark red mixture was cooled to room temperature and diluted with 35 mL of toluene and 50 mL of 4 M aqueous sulfuric acid solution, followed by addition of 50 mL of diethyl ether with stirring to give a suspension. The precipitate was afforded by filtration, recrystallized from ethanol and dried under vacuum at 100 °C for 2 h.

Yield: 60%. FT-IR (KBr, cm⁻¹): 3247 (O–H), 2958 (C–H), 1559 (C=C), 1457 (C–H), 1254 and 1196 (C–O) and 690 (C–S). ¹H NMR (400 MHz, CDCl₃, δ, ppm): 9.61 (s, 4H, OH), 7.56 (s, 8H, ArH), 1.24 (s, 36H, t-Bu).

2.2.2. Synthesis of tetrasodium thiacalix[4]arene tetrasulfonate (TSTC4AS). TSTC4AS was synthesized based on the literature method.²³ At first, 1.5 g of p-BTC4A was mixed with 80 mL of sulfuric acid (98%), and the suspension was heated at 80 °C for 4 h under stirring. The solution was then cooled and poured into 500 mL of ice-cold water, and the resulting purple solid was filtered. Then, 100 g of sodium chloride was added to the filtrate to afford sodium salt. The obtained salt was dried and dissolved in 20 mL of water, and then ethanol was added to form a milky precipitate. The white precipitate was filtered and dried in vacuum oven at 90 °C for 4 h.

Yield: 65%. FT-IR (KBr, cm⁻¹): 3449 (O–H), 1138 and 1044 (S=O). ¹H NMR (400 MHz, D₂O, δ, ppm): the H of –OH disappeared on exchange with D₂O, 7.91 (s, 8H, ArH).

2.2.3. Synthesis of p-tert-butyl-dipropyl sulfonate thiacalix[4]arene (SuTC4A). In a three-necked, round-bottomed flask equipped with a condenser, p-BTC4A (0.005 mol) and K₂CO₃ (0.005 mol) were added to acetonitrile (100 mL) and the mixture was heated at reflux for 1 h under nitrogen atmosphere. 1,3-Propane sultone (0.01 mol) was subsequently added to the flask and the reaction was allowed to proceed at reflux for 24 h. After filtration, the crude product was obtained by rotary vacuum evaporation of the filtrate. Recrystallization of the product with ethanol gave a pure product. The stepwise procedure for the synthesis of SuTC4A from p-BTC4A is illustrated in Fig. 1a.

Fig. 1 The schematic illustration of synthesis methods of TC4As.

Yield: 85%. FT-IR (KBr, cm⁻¹): 3338 (O–H), 2961 (C–H), 1629 and 1565 (C=C), 1457, 1395 and 1365 (C–H), 1270 and 1242 (C–O) and 738 (C–S), 1190 and 1046 (S=O). ¹H NMR (400 MHz, DMSO-d₆, δ, ppm): 9.58 (s, 2H, OH), 7.73 (s, 4H, ArH), 7.61 (s, 4H, ArH), 3.42 (t, 2H, O–CH₂–), 2.45 (t, 2H, –CH₂–), 1.72 (m, 3H, –CH₃), 1.18 (s, 18H, t-Bu), 1.15 (s, 18H, t-Bu).

2.2.4. Synthesis of dibutyl tetrasodium thiacalix[4]arene tetrasulfonate (BuTSTC4AS). In a three-necked round-bottomed flask equipped with a condenser, TSTC4AS (0.005 mol), and K₂CO₃ (0.005 mol) were added to acetonitrile (100 mL) and the mixture was heated at reflux for 1 h under nitrogen atmosphere. Butyl bromide (0.01 mol) was subsequently added to the flask and the reaction was allowed to proceed at reflux for 24 h. After filtration, the crude product was obtained by rotary vacuum evaporation of the filtrate. The crude product was purified using recrystallization with ethanol. The stepwise procedure for the synthesis of BuTSTC4AS from TSTC4AS is illustrated in Fig. 1b.

Yield: 76%. FT-IR (KBr, cm⁻¹): 3448 (O–H), 1110 and 1047 (S=O). ¹H NMR (400 MHz, DMSO-d₆, δ, ppm): 9.56 (s, 2H, OH), 7.71 (s, 4H, ArH), 7.57 (s, 4H, ArH), 3.45 (t, 2H, O–CH₂–), 2.07 (m, 2H, –CH₂–), 1.33 (m, 2H, –CH₂–), 1.17 (t, 3H, –CH₃).

2.3. Synthesis of WPU dispersions

The WPU dispersions were synthesized by a prepolymer mixing method. The preparation process of WPU dispersions was divided into three steps: (1) formation of isocyanate-terminated prepolymer, (2) incorporation of emulsifiers into PU chains and then neutralization, and (3) chain extension with BDO for higher molecular weight and subsequently dispersion in aqueous medium. The preparation scheme of WPU dispersions are shown in Fig. 2. The prepolymer was prepared in a 250 mL three-necked round bottom flask equipped with a mechanical stirrer, nitrogen inlet, thermometer, and condenser. For this purpose, PTMO and IPDI were charged into the dried flask, and then DBTDL (0.2% based on the total solid content) were added as catalyst. The resulted mixture was heated with stirring at a speed of 250 rpm under dry nitrogen atmosphere at 80 °C for 2 h to form the NCO-terminated prepolymer. Subsequently, the prepolymer mixture reacted with suitable amounts of emulsifiers (dissolved in NMP) which contained DMPA alone or in mixture with TC4As at 80 °C for 30 min according to the formulations given in Table 1. Afterward, the BDO was added to accomplish the chain extension at 80 °C until the NCO peak (2270 cm⁻¹) in the IR spectra of WPUs had disappeared. Then, the reaction mixture was cooled down to 60 °C, TEA (DMPA equiv.) was added to neutralize the acidic groups for 30 min. Finally, the obtained mixture was cooled down to 35 °C and WPU dispersions were obtained with addition of certain amounts of deionized water to get 30% solid content under vigorous stirring (900 rpm) for 20 min.

Fig. 2 Synthesis procedure of WPUs.

Table 1 Sample code designation and formulation of WPUs

Sample	Emulsifier type	Molar ratio
		(PTMO : IPDI : BDO : DMPA : TC4As)
WPU	DMPA	1 : 3:1 : 1 : 0
TC-WPU1	DMPA + SuTC4A	1 : 3:1 : 0.95 : 0.05
TC-WPU2	DMPA + SuTC4A	1 : 3:1 : 0.9 : 0.1
TS-WPU1	DMPA + BuTSTC4AS	1 : 3:1 : 0.95 : 0.05
TS-WPU2	DMPA + BuTSTC4AS	1 : 3:1 : 0.9 : 0.1

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2.4. Preparation of the polyurethane films

Some properties of the polyurethanes were measured in solid films that were prepared by placing appropriate amount of WPU dispersion in a Teflon mold and allowing slow evaporation of the water at room temperature for 72 h. Afterwards, the mold was put in an oven at 80 °C for 24 h to allow the complete removal of solvent in the polyurethane. Then, the films with Teflon were stored in a desiccator at room temperature for further analysis.

2.5. Measurements

2.5.1. Characterization of the WPU dispersions. The mean particle size and the particle size distribution of the WPU dispersions were measured by dynamic light scattering (DLS) using a laser diffraction (Malvern MasterSizer, UK) at 25 °C. The samples were homogenized after dilution in deionized water to 0.1%. The test was followed by experiment duration of 60 s and refractive index of 1.59.

The Brookfield viscosity of the dispersions was measured at 25 °C using a Brookfield viscometer (DV-II + Pro, UL Adapter spindle).

The pH value of WPU dispersions was determined at 25 °C using a pH-meter (Aqualytic AL15). The pH was calculated as the average of three experimental determinations.

The storage stability of WPU dispersions was examined after six months at 25 °C using a ROTINA-380R high-speed centrifuge at centrifugal rate of 9000 rpm for 10 minutes. The observation of any residual sediment would imply the storage stability of WPU dispersions.

2.5.2. Characterization of the WPU films. Fourier transform infrared (FTIR) spectroscopy of WPU films was performed on a spectrometer (EQUINOX55, Bruker Co, Germany) using the attenuated total reflectance (ATR) technique. All spectra were collected with 32 scans at resolution of 2 cm⁻¹ over a wavenumber range of 500–4000 cm⁻¹.

¹H NMR spectra of samples were recorded on a 400 MHz Bruker Avance DRX NMR spectrometer. Deuterated chloroform (CDCl₃), dimethylsulfoxide (DMSO-d₆) and D₂O were used as solvents. Tetramethylsilane (TMS) served as an internal standard.

Thermogravimetric analyses (TGA) and derivative thermogravimetry (DTG) experiments were performed on Perkin Elmer Pyris 1 (Perkin Elmer, USA) to measure the weight loss of the WPU films under N₂ atmosphere. The samples were heated from room temperature to 600 °C with a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere.

Wide angle X-ray diffraction (WAXD) measurement was carried out to analyze the crystallinity of the polyurethane films with a D-5000 X-ray diffractometer (SIEMENS, Germany) at room temperature using CuKα radiation (λ = 1.540598 Å). The XRD spectra were recorded with a scan rate of 1° min⁻¹ between 2θ = 5° and 2θ = 30°.

The dynamic mechanical thermal analysis (DMTA) were performed using a Dynamic Mechanical Thermal Analyzer (Polymer Laboratories, MK II, UK) using tensile mode at a frequency of 1 Hz with a heating rate of 5 °C min⁻¹ from −80 to 40. The dimensions of the WPU films were 20 mm × 10 mm × 0.5 mm.

The surface morphology of the polyurethane films was evaluated by scanning electron microscopy (SEM) using a scanning electron microscope (Tescan Vega-II, USA). Samples were fractured in liquid nitrogen and covered with a gold layer to obtain better conductivity.

Water droplet contact angle of the film samples was measured at room temperature via sessile drop method by a Kruss G10 instrument (KRUSS, Germany). The deionized water was used as medium to measure the contact angle. All the data presented were average of three measurements.

2.5.3. Anti-corrosion performance tests. To investigate the effect of novel TC4As emulsifiers on the anti-corrosion performance of WPUs coating on the mild steel substrate, electrochemical impedance spectroscopy (EIS) and potentiodynamic scan (PDS) tests were implemented. In order to measure the electrochemical corrosion performance of samples coated on mild steels, WPU dispersions were first coated onto the mild steels by dip-coating technique. Then, a 1.0 × 1.0 cm² area at the center of each specimen was exposed to the electrolytes and the backs and the edges of the specimen were sealed by beeswax. The prepared electrodes were kept in aqueous 3.5 wt% sodium chloride solution as corrosive environment for one week prior evaluation. EIS and PDS measurements were performed with a Autolab PGSTAT 30 (Metrohm) and conventional standard three-electrode electrochemical test system. The three electrodes are an Ag/AgCl (KCl sat.) reference electrode, a platinum counter electrode, and the exposed sample (1 cm² and with a coating thickness of 50 ± 5 μm) as a working electrode, immersed in a 3.5% NaCl solution (w/w). All EIS measurements were carried out at the frequency range of 10 mHz to 100 kHz at the open circuit potential (OCP). EIS spectrum was also obtained for bare steel specimen. Polarization curves of the WPUs-coated mild steels and bare steel were recorded under potentiodynamic conditions in the potential range of ±1 V with respect to OCP at a scan rate of 5 mV s⁻¹. Corrosion surface morphology of WPUs-coated mild steels was also examined by scanning electron microscopy (SEM) using a scanning electron microscope (Tescan Vega-II, USA) before and after one week immersion in 3.5% NaCl solution.

3. Results and discussion

TC4As-based waterborne polyurethanes were successfully prepared using novel TC4As emulsifiers as a part of internal emulsifier as well as DMPA (Table 1). An efficient, one-step, nucleophilic ring opening reaction between p-BTC4A and 1,3-propan sultone and also nucleophilic substitution reaction between TSTC4AS and butyl bromide produced SuTC4A and BuTSTC4AS, respectively in high yields (Fig. 1). WPUs were synthesized by a prepolymer process from isophorone diisocyanate, poly(tetramethylene oxide) (PTMO) and thiacalix[4]arenes macrocycles (TC4As) including p-tert-butyl-dipropyl sulfonate thiacalix[4]arene (SuTC4A) and dibutyl tetrasodium thiacalix[4]arene tetrasulfonate (BuTSTC4AS) in mixture with DMPA (Fig. 2). The effects of the type and content of TC4As emulsifiers on the viscosity, particles size, and stability of the dispersions, and also on the structural, morphological, and thermo-mechanical properties of the obtained films were investigated. Furthermore, anti-corrosion protection ability of WPU coatings on mild steel in 3.5% NaCl solution was evaluated.

3.1. Characteristics of the WPU dispersions

The effects of type and content of TC4As on the particle size, viscosity, pH, and storage stability of WPU dispersions were investigated and summarized in Table 2. DLS was used to determine the particle size and the particle size distribution of the resulting WPU dispersions. Fig. 3 shows the particle size distribution (expressed in number percent) of the WPU dispersions. It is found that the average particle size of WPUs is variable depending on the type and content of TC4As emulsifiers (Table 2). According to the particle size distribution, all samples except TC-WPU2 show roughly narrow particle size distribution indicating that the most particles are small. However, there are particles with larger particle size, but in a small fraction.

Table 2 Some properties of the waterborne polyurethane dispersions

Samples	Brookfield viscosity (mPa s)	Average particle size (d, nm)	pH	Storage stability^a
a The storage stability after 6 months.
WPU	14.2	109	7.01	Stable
TC-WPU1	16.7	115	6.96	Stable
TC-WPU2	18.8	191	6.75	Stable
TS-WPU1	17.6	142	7.19	Stable
TS-WPU2	13.3	87	7.47	Stable

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Fig. 3 Particle sizes and distributions of WPU dispersions.

It is assumed that, incorporation of TC4As into polyurethane backbones results a decrease in flexibility and hydrophilicity of polyurethanes which causes the PU chains not easy to be cut and deform by water to form small emulsion particles.²⁴ For TC-WPU1 sample, addition of 5 mol% of SuTC4A as a portion of emulsifier as well as DMPA induce higher hydrophobicity to PU chains and thereby larger particles size compared to WPU sample (containing only DMPA as emulsifier) was resulted. In case of TC-WPU2 sample, more content of SuTC4A emulsifier (10 mol% of total emulsifiers) resulted larger particle size compared to TC-WPU1. Based on the emulsion polymerization theory, with increasing the hydrophobic segments, the size of latex particles tend to raise to keep the emulsion stable.²⁵ In addition, the wide particle size distribution of TC-WPU2 sample could be due to its excessive viscosity which makes the dispersibility of the polymer chains more difficult during the dispersion step and consequently generates irregular-sized particles. TS-WPU1 sample showed larger particle size compared to TC-WPU1 which can be interpreted by higher microphase separation degree and its mostly crystalline structure which traps the hydrophilic groups and diminishes the water penetration into polyurethane chains. Comparing to TS-WPU1, the particle size of the TS-WPU2 dispersion exceptionally decreased from 142 to 87 nm when the BuTSTC4AS content increased from 5 to 10 mol%. Upon increasing the BuTSTC4AS content to 10 mol%, PU chains become more dissociated because of reduction in interchain interactions by repulsive force between negatively charged sulfonate groups. As a result, the enhanced chain mobility compared to TS-WPU1 sample leads to lower viscosity and subsequently migration of sulfonate and carboxylate groups to the particle's surface is facilitated and produced smaller particles by dispersion in water.⁷

The viscosity of the WPU dispersions was assessed by Brookfield viscosity measurement. As shown in Table 2, the viscosity of dispersions is associated with the particle size. The storage stability of dispersions was also evaluated with centrifugation after six month from synthesis. The observation of any residual sediment after six months and also during centrifugation for 10 minutes confirms the desired storage stability of WPU dispersions.

3.2. Characterization of the polyurethane films

3.2.1. Structural characterization. Fig. 4 shows the ATR-FTIR spectra of prepared WPU films. WPU sample, containing only DMPA as emulsifier, was characterized with stretching vibrations of urethane N–H bond at 3320 cm⁻¹, asymmetric and symmetric stretching vibrations of urethane carbonyl at 1713 and 1701 cm⁻¹, respectively. It should be noted that the characteristic peaks at 1652 and 1620 cm⁻¹ are related to vibrations of the DMPA carbonyl groups.¹² Absorptions at 1530 are due to urethane N–H out-of-plane bending vibration at 1537 cm⁻¹. The C–H asymmetric and symmetric stretching vibrations appeared at 2940 and 2859 cm⁻¹, respectively. The absorption bands at 1460 and 1305 cm⁻¹ were assigned to CH₂ bending and CH₂ wagging vibrations, respectively. C–N stretching vibration was appeared at 1368 cm⁻¹. The C–O–C asymmetric and symmetric stretching vibrations corresponds to the ether oxygen of the soft segments were observed at 1239 and 1105 cm⁻¹, respectively.²⁶ The asymmetric stretching vibration of the NCO groups at 2270 cm⁻¹ was used to determine the completeness of the reaction. The disappearance of this characteristic peak indicates that the NCO groups completely reacted with OH groups.²⁴ As can be shown in Fig. 4, the TC4As-based WPUs (TC-WPU1, TC-WPU2, TS-WPU1 and TS-WPU2 samples) show similar bands compared to WPU sample without significant shift in peaks position, and furthermore, all spectra confirm the formation of well-defined polyurethane structures. For TC4As-based WPUs, it was observed that the characteristic urethane carbonyl and urethane N–H out-of-plane bending vibration peaks are broader and more intense than WPU sample. This observation affirmed an increase in diversity of hydrogen bonding interactions with introduction of TC4As into PU chains. Moreover, the band at 1040 cm⁻¹ relating to symmetric stretching vibrations of S=O bonds was appeared for TC4As-based WPUs. All these evidences confirm successful covalently incorporation of TC4As as novel macrocyclic emulsifiers into the polyurethane chains.²²

Fig. 4 ATR-FTIR spectra of different WPU films.

¹H NMR spectra of WPU, TC-WPU1 and TS-WPU1 samples are shown in Fig. 5. Terminal methyl groups of DMPA, TEA, TC4As and isophorone moieties appeared at 0.87–1.20 ppm. The central methylene groups of PTMO, isophorone ring and butanediol moieties were observed at 1.85–2.35 ppm. Methylene groups of DMPA, PTMO and BDO attached to urethane oxygen atom appeared at 3.8–4.3 ppm. Methylene and methine groups of IPDI, attached to urethane nitrogen atom were observed around 2.6–2.8 and ppm. Methylene groups of PTMO moieties, attached to ether oxygen atom, appeared near 3.6 ppm. Weak peaks appearing at 6.5–7.2 ppm were attributed to urethane NH groups.²⁷ Considering the NMR spectra of TC-WPU1 and TS-WPU1 samples, appearance of the peaks around 7.5–7.8 ppm arises from the aromatic protons which endorse incorporation of the TC4As emulsifiers into polyurethane chains.²² The appearance of new peaks around 1.2–1.5 ppm, can presumably ascribed to the central methylene groups of propyl sulfonate and butyl substitutions on lower rim of SuTC4A and BuTSTC4AS moieties in polyurethane chains. In conclusion, the ¹H NMR spectra were in accordance with the structure of synthesized WPUs.

Fig. 5 ¹H NMR spectra of WPU films.

3.2.2. XRD study. Wide angle X-ray diffraction (WAXD) measurement was used to investigate the crystallinity of the WPU films. Fig. 6 shows the wide angle X-ray diffraction profiles for all WPUs. As can be observed from the XRD patterns, all the diffractograms are similar with a large broad diffraction peak, indicating that they have a low degree of crystallinity. In polyurethanes, the crystallinity is commonly achieved by the ordered structure which can be obtained by the microphase separation of soft segments and hard segments.²⁸ According to the X-ray diffractions, crystallinity and microphase separation degree depends on the type and amount of TC4As emulsifiers used in the polyurethane backbone. The all WPU films showed two sharp peaks with different intensity at 19.9 and 24.2° as well as large broad peaks, which implies the formation of a semicrystalline structure because of low microphase separation degree. According to literature, XRD pattern of PTMG polyol (2000 g mol⁻¹) shows two sharp crystalline peaks at 2θ of 19.6 and 24°.²⁹ Since crystalline peak positions for all WPUs corresponded well to crystalline peaks of PTMG, it is likely to consider that observed sharp peaks are related to crystallization of soft segment domains. Thus, it is possible to say that all samples have some crystalline soft segments dispersed in amorphous medium.⁸ The intensity of diffraction peaks of TC-WPU1 is lower than that of WPU because of the lower microphase separation degree arising from introduction of 5 mol% of SuTC4A emulsifier.²⁵ Moreover, for TC-WPU2, the intensity of crystalline sharp peaks more decreased with the increase of SuTC4A content to 10 mol% compared to TC-WPU1 sample. It is proposed that the disruption of hydrogen bonding interactions in hard segments may be caused by bulky structure of SuTC4A which inhibit formation of more packed hard segments and hence more decrease in microphase separation degree can occur with increase of SuTC4A.^25,30 In addition, SuTC4A moieties due to its hydrophobic upper rim can raise the hydrophobicity in hard segments and thereby hard segments can be highly mixed with hydrophobic PTMO and improve the phase mixing. On the other hand, the diffraction crystalline peaks for TS-WPU1 sample were considerably more intense compared to WPU sample. This observation suggests that the imbedding 5 mol% of BuTSTC4AS into polyurethane chains favor the formation of more ordered polyurethane structure owing to increase in microphase separation degree. This enhancement in microphase separation degree can be originating from the low miscibility of hard segments containing hydrophilic upper rim with hydrophobic PTMO soft segments. In contrast, TS-WPU2 sample exhibited only a broad amorphous diffraction peak without any sharp peak, implying that increase in BuTSTC4AS content to 10 mol% disfavor the formation of an ordered structure due to highly repulsive forces between negatively charged sulfonate groups in hard segments. It should be noted that the disruption in interchain interactions and regularity of hard segments can obstructs the crystallization of soft segments in polyurethanes.¹³ Therefore, it's concluded that there's almost no notable microphase separation between soft and hard segments in TS-WPU2 sample.

Fig. 6 Wide angle X-ray diffraction of WPU films.

3.2.3. SEM morphology. Properties of polyurethanes depend strongly on their morphologies and miscibility of soft and hard segments. Soft segments are formed from polyols and hard segments derived from chain extenders and diisocyanates.⁹ In PU morphology inspection, for miscible soft and hard segments only one phase is observed, whereas in case of immiscible soft and hard segments, two different phases coexist.^9,31 Fig. 7 illustrates the SEM morphology of the WPU films. As it can be seen from SEM images, microphase separation morphology depends on type and content of emulsifiers used in WPUs. The SEM photomicrographs showed that the hard domains (bright regions) dispersed in the polyurethane matrices. The SEM images correlate well with XRD results in morphology properties. In the case of WPU, TC-WPU1 and TC-WPU2 samples, SEM image elucidated smaller size and randomly distribution of hard segments within the continuous soft segments compared to TS-WPU1 sample resulting from their low microphase separation degree.²⁹ As it is clearly evident from SEM image of TS-WPU1 sample, hard domains are more extensive than the hard domains in other samples. This observation can be assigned to well-defined microphase-separated morphology which was confirmed earlier with XRD results. In addition, the size of the hard domains in the TS-WPU2 sample noticeably decreased and no hard domains is observable due to high degree of microphase mixing which causes homogeneously distribution of hard segment within the polyurethane matrix.²⁸

Fig. 7 SEM images of different WPU films.

3.2.4. Thermogravimetric study. TGA and DTG analysis were used to evaluate the thermal stability and decomposition behavior of the WPU films under nitrogen atmosphere. The TGA and DTG curves and obtained results are presented in Fig. 8 and Table 3. Many investigations confirm that polyurethanes are unstable at high temperature and depending on their constituents, degradation can take place during heating.³² It is known that the hard segments are more prone to thermal decomposition than the soft segments in polyurethanes. In consequence, the thermal stability of PUs is mainly dependent on the thermal stability of hard segments.³⁰ The thermal decomposition of all samples was evaluated at onset degradation temperature (T_onset) as a criterion for thermal stability, and also at 10 and 50% weight loss temperatures (T_10% and T_50%). The thermal decomposition of WPU sample starts at 234 °C (T_onset), which can be related to the depolymerization of urethane linkages. Moreover, for WPU sample, the T_10% and T_50% temperatures were found 287 and 404 °C, respectively. Comparing with the WPU sample, containing only DMPA as emulsifier, it was observed that T_onset and T_10% shifted toward higher temperature for TC4As-based WPUs samples including TC4As emulsifiers as well as DMPA. This improvement in thermal stability could be attributed to high thermal stability of thiacalix[4]arenes due to its four aromatic rings which delays the PU degradation under a nitrogen atmosphere.¹⁸ It is also obvious from TGA curves (Fig. 8a) and Table 3 that T_onset of TC-WPU2 and TS-WPU2 samples is higher than that of TC-WPU1 and TS-WPU1 samples and are more thermally stable. This occurs as a result of higher content of TC4As in TC-WPU2 and TS-WPU2 samples which delay the degradation more than TC-WPU1 and TS-WPU1 samples. Moreover, in the case of TS-WPU2 sample, higher thermal stability compared to TC-WPU2 sample with equal content of TC4As may be justified by presence of four sulfonate groups attached to upper rim of the BuTSTC4AS compared to SuTC4A bearing two sulfonate groups in lower rim. It is accepted that the sulfonate groups is more stable than carboxylate groups against attacking of organic radicals to form new active species and therefore the TS-WPU2 sample is more thermally stable than TC-WPU2 sample.^12,33 It is worth noting that the decomposition of the soft segments takes place between 350–450 °C and the weight losses amounts relating to soft segments of TC4As-based WPUs decreased by incorporating the TC4As into hard segments compared to WPU sample.

Fig. 8 TGA (a) and DTG (b) curves of WPU films.

Table 3 Thermal properties of WPU films

Sample code	Tonset^a (°C)	T10%^b (°C)	T50%^c (°C)	Tmax,1^d (°C)	Tmax,2 (°C)	Tmax,3 (°C)
a Onset degradation temperature (temperature at which polymer degradation starts).b Temperature at 10% weight loss.c Temperature at 50% weight loss.d Temperatures with the maximum thermal degradation rate.
WPU	234	287	404	271	320	418
TC-WPU1	237	287	401	277	327	416
TC-WPU2	242	294	399	277	334	415
TS-WPU1	237	292	402	279	329	418
TS-WPU2	242	298	401	280	338	418

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The thermal degradation of polyurethane samples usually takes place in two main steps. The first degradation step occurs roughly from 200 to 350 °C and is ascribed to decomposition of the hard segments and the second main degradation step occurs roughly from 350 to 500 °C and is related to soft segment degradation.³¹ Petrovic et al. found that degradation of PUs in the initial step is mainly depending on the type and amount of hard segments and degradation products may include isocyanates, alcohols, amines, olefins and carbon dioxide.³⁴ The DTG curves in Fig. 8b clearly manifested that all samples have a similar thermal degradation behavior and the degradation of WPUs occurs mainly in two main steps which first step was further divided into two stages. This observation is associated with the presence of different chain extenders (BDO and diol emulsifiers) within the hard segments and therefore the first degradation process corresponds to hard segments containing BDO and the second one to hard segments containing emulsifiers.³⁵ An inspection of the DTG results in Table 3 indicates that incorporation of TC4As as a portion of emulsifiers into WPUs cause an increase in T_max,1 and T_max,2 (temperatures at which rate of hard segments mass loss is maximum) values corresponding to hard segments degradation steps in comparison with WPU sample, respectively. The increase in T_max,1 and T_max,2 for TC4As-based WPUs can also be attributed to higher thermal stability of TC4As compared to DMPA moieties.

3.2.5. Thermo-mechanical study. The viscoelastic properties of the WPUs were studied by means of dynamic mechanical thermal analysis (DMTA) which can distinguish different kinds of thermal transitions.³⁶ Fig. 9 shows the variation of logarithm of storage modulus (log E′) and tan δ as a function of temperature for the WPUs with different emulsifier type and content. According to Fig. 9a, in the glassy and rubbery regions, lowest module depression and highest onset flow temperature of TS-WPU1 sample among WPU samples can presumably ascribed to its high microphase separation degree which was confirmed earlier with XRD and SEM analysis. Moreover, TS-WPU2 sample exhibited the lowest initial modulus due to its highly phase mixing.³⁷ It can be seen from tan δ curves (Fig. 9b) that all the samples exhibit two thermal transitions which attributed to the thermal glass transitions of soft segments (T_g,SS) in lower temperature and hard segments in higher temperature (T_g,HS).³⁸ Table 4 reports the T_g,SS and T_g,HS values and also the log E′ at temperature of 25 °C for WPU films. The T_g values of ionic polyurethanes really depend on the chemical structure of diols, diisocyanates, ionic compounds, and the amount of ionic compounds. Moreover, the T_g decreases or increases with the introduction of ionic compounds and influenced by the various interactions occurring in the PU chains.³¹ It should be noted that T_g,HS and T_g,SS of TS-WPU1 sample occurred at higher temperature in comparison with WPU sample. This observation is due to highly phase separated degree of TS-WPU1 which causes restriction in chain movements of the soft and hard segments and hence higher temperature is needed for increasing the chain mobility.^24,35 In addition, the tan δ curves also showed that both T_g,HS and T_g,SS of TS-WPU2 samples shifted to lower temperature compared to WPU sample. This is because of its highly phase mixed morphology which promote the mobility of the polyurethane chains.³⁵ Compared to WPU sample, T_g,HS and T_g,SS of TC-WPU1 moves toward higher temperatures with introduction of 5 mol% of SuTC4A emulsifier. This increase can be explained by decreasing in chain mobility, which arises from rigid and bulky structure of SuTC4A. But, increase of SuTC4A up to 10 mol% for TC-WPU2 sample cause a decrease in T_g,SS compared to TC-WPU1 due to its lower phase separation degree compared to TC-WPU1. Moreover, among WPUs, the tan δ curves of the samples with more degree of microphase separation (WPU and TS-WPU1) exhibited more clear T_g,HS and T_g,SS than others.

Fig. 9 Storage modulus (a) and tan δ (b) curves of the WPU films.

Table 4 Thermal glass transition temperatures and storage modulus of WPUs

Samples	Tg,SS^a (°C)	Tg,HS^b (°C)	log E′ (at 25 °C)
a Glass transition temperature of soft segment.b Glass transition temperature of hard segment.
WPU	−59	−31	6.71
TC-WPU1	−57	−24	7.70
TC-WPU2	−61	−27	7.30
TS-WPU1	−47	−19	7.87
TS-WPU2	−61	−33	6.02

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3.2.6. Surface wettability. The degree of wettability of WPU films was investigated by measuring the contact angles formed between liquid drops and the surface of WPU films. The results showed that the contact angles increased with embedding TC4As emulsifiers into polyurethane chains, which manifests higher hydrophobicity of TC4As compared to DMPA. The water contact angle of the TC4As-based WPU films was in the range of 64–71°, which was higher than that of WPU sample with contact angle of 62°. Upon increasing the SuTC4A content from 5% for TC-WPU1 to 10 mol% for TC-WPU2, the water contact angles increased from 71 to 76°, respectively. In addition, upon increasing the BuTSTC4AS content from 5% for TS-WPU1 to 10 mol% for TS-WPU2, the water contact angles increased from 64 to 67°, respectively. The results showed that incorporation of SuTC4A caused more increase in contact angle in comparison with BuTSTC4AS. This can be explained by the fact that the presence of more sulfonate salt substitution in upper rim of BuTSTC4AS imparted more hydrophilic characteristics to BuTSTC4AS compared to SuTC4A.

3.3. Anti-corrosion properties

In this work, the anti-corrosion properties of the WPUs-coated mild steels were investigated in a 3.5% NaCl solution by potentiodynamic scan (PDS) method and electrochemical impedance spectroscopy (EIS) measurements. For comparison, the same measurements were conducted on bare steel. By PDS method, corrosion protection ability of WPUs-coated on mild steels can be investigated from the values of corrosion potential (E_corr) and corrosion current (I_corr). Fig. 10 shows the potentiodynamic polarization curves (Tafel plots) of bare and coated mild steels and the values of the corrosion parameters (E_corr and I_corr) obtained from polarization curves are summarized in Table 5.

Fig. 10 Potentiodynamic polarization curves for bare steel and WPUs-coated steel in 3.5% NaCl solution.

Table 5 The corrosion parameters of bare and WPUs-coated mild steels

Samples	Icorr (A cm^−2)	E (V)
Bare steel	2.1 × 10^−4	−0.81
WPU	2.2 × 10^−5	−0.39
TC-WPU1	5.2 × 10^−6	−0.20
TC-WPU2	9.7 × 10^−5	−0.74
TS-WPU1	8.7 × 10^−5	−0.67
TS-WPU2	5.1 × 10^−5	−0.32

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According to Tafel plots and Table 5 data, the corrosion current density (I_corr) values for the WPUs-coated mild steels are much lower than bare steel with I_corr of 2.1 × 10⁻⁴ A cm⁻². Also the corrosion potential of the WPUs-coated mild steels shifted toward more positive potentials in comparison to bare steel with E_corr of −0.81 V. This behavior is a good indicator for improved corrosion protection ability of WPUs-coated mild steels compared to bare steel. This phenomenon can be attributed to the barrier effect and good adherence of WPU films on the surface of mild steels.²⁷ Besides, the corrosion properties of WPUs-coated steels depend on the type and content TC4As used as emulsifiers in polyurethane chains. Among the TC4As-based WPUs, TC-WPU1 and TS-WPU2 showed the best corrosion protection ability. It is proposed that the corrosion properties of WPUs strongly are influenced by particle size of WPU dispersions. It is notable that, WPUs with smaller particle size (TC-WPU1, TS-WPU2 and WPU) showed better corrosion properties. This observation can be assigned to effective barrier behavior arising from more ability of small particles to form highly packed polyurethane films on surface of mild steels which impede the penetration of corrosive ions at the coating/metal interface and subsequently enhance the corrosion protection ability of PU coatings.³⁹ Moreover, better performance of TC-WPU1 (around −0.20 V in the E_corr and 5.2 × 10⁻⁶ A in the I_corr) in comparison with TS-WPU2 (around −0.32 V in the E_corr and 5.1 × 10⁻⁵ A in the I_corr) can be attributed to more hydrophobic structure and also higher microphase separation degree of TC-WPU1 compared to TS-WPU2 which lead to an effective barrier behavior and hence improve the capability of TC-WPU1 to passivate mild steel electrode. In addition, TC-WPU2 and TS-WPU1 samples showed lower corrosion protection ability compared to WPU sample due to larger particles size. Presence of the pores originated from large particle size stimulates penetration of corrosive ions into the coatings and finally leads to corrosion process start more quickly beneath the polymer film.⁴⁰ According the results, the TC-WPU1 sample provides excellent protection efficiency against mild steel corrosion. Therefore it can act as an effective corrosion protection layer on mild steel and may be considered as a suitable candidate for future industrial evaluation.

EIS is one of the most powerful techniques used for the investigation corrosion protection performance of coatings. EIS measurements were carried out in order to gain more insight into the anti-corrosion behavior of WPU coatings. The Nyquist impedance and bode plots of bare steel and coated steels are shown in Fig. 11a and b, respectively. In Nyquist plot, the diameter of the semi-circle was considered as the charge transfer resistance value (R_ct). Evaluation of R_ct is the best way for the estimation of corrosion protection of coatings. The high value of R_ct indicates that the coating has good corrosion protection ability.⁴¹ WPUs-coated steels show higher R_ct value in comparison to bare steel. The coating obtained from WPU dispersions has high ability to produce passive metal oxide layer on the metal surface and coatings interface due to engagement in redox reactions leading to the formation of metal oxide passive layer. Therefore, increased R_ct value is related to formation of passive layer beneath the coating.²⁷ Among WPUs, TC-WPU1 shows the highest value of R_ct. This finding is in good agreements with PDS results and the reasons for this excellent protection ability was mentioned earlier in PDS analysis.

Fig. 11 The Nyquist (a) and Bode (b) plots of bare steel and WPUs-coated steel in 3.5% NaCl solution.

The Bode plot in Fig. 11b depicts the absolute values of impedance (ohm cm²) of the bare steel and WPUs-coated mild steels versus frequency (Hz). The Bode plots showed high impedance values at low and high frequencies for all samples. An increase in impedance values of WPUs coated mild steels can be attributed to the barrier effect of coatings which blocked the diffusion pathway of corrosive ions.^38,42 From Fig. 11b, it can be clearly seen that the TC-WPU1 coating with higher impedance value in a lower as well as in a higher frequency region, provides best corrosion protection to the mild steel substrate, which was also supported by PDS and Nyquist plots.

The surface morphology of mild steels coated with WPU, TC-WPU1 and TS-WPU1 samples was observed by scanning electron microscopy (SEM) before and after one week immersion in 3.5% NaCl solution (Fig. 12). The degree of surface defects in SEM images indicates the corrosion protection efficiency of samples. As can be seen from Fig. 12, after one week immersion, the degree of surface defects in TC-WPU1 image is lower than others indicating considerable reduction of corrosion rate. This observation in surface morphology can be ascribed to significant corrosion protection efficiency of TC-WPU1 which was emphasized in previous sections. In case of WPU and TS-WPU1 samples, the remarkably surface degradation along with the formation of cracks and particles would imply that these coating systems provided less corrosion protection efficiency than TC-WPU1 sample.⁴²

Fig. 12 SEM images of WPU coatings before and after one week immersion in 3.5% NaCl solution.

4. Conclusion

Waterborne polyurethanes have been successfully prepared by importing two 1,3-alternate derivatives of thiacalix[4]arenes as new macrocyclic emulsifiers bearing sulfonate groups into the polyurethane chains. The emulsion stability measurements indicated that the storage stability of WPU dispersions is more than 6 months. ATR-FTIR and ¹H NMR spectroscopy showed that the TC4As was successfully incorporated into PU chains. The TC4As-based WPUs offered higher thermal stabilities relative to WPU containing only DMPA. The improvement in thermal stability was attributed to the incorporation of high thermally stable TC4As moieties into polyurethane chains. XRD and SEM analysis revealed that the microphase separation degree and crystallization significantly depend on type and content of TC4As emulsifiers. According to DMTA results, in the glassy and rubbery regions, lowest module depression and highest onset flow temperature of TS-WPU1 sample can presumably ascribed to its high microphase separation degree. TS-WPU2 sample also exhibited the lowest initial modulus due to its mostly amorphous structure. It should be noted that T_g,HS and T_g,SS of TS-WPU1 sample occurred at higher temperature in comparison with WPU sample due to increase in the crystallinity and high microphase separated morphology. Furthermore, water contact angle measurements confirmed that the WPUs containing TC4As have higher hydrophobicity in comparison to WPU sample containing only DMPA as emulsifier. The improvement of the hydrophobicity of the films can be ascribed to more hydrophobic structure of TC4A derivatives compared to DMPA. Anti-corrosion performance of the WPU dispersions as environmental friendly coatings for mild steel in 3.5% NaCl solution was also evaluated using PDS and EIS techniques. The results revealed that TC-WPU1 sample including 5 mol% of SuTC4A has the best corrosion protection ability among WPUs and thereby has a promising potential for industrial application as corrosion protective coating.

Acknowledgements

The authors gratefully acknowledge the support of the University of Mazanadaran.

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