Physicochemical Properties of Amino–Silane-Terminated Vegetable

Research Article pubs.acs.org/journal/ascecg

Physicochemical Properties of Amino−Silane-Terminated Vegetable Oil-Based Waterborne Polyurethane Nanocomposites T. Gurunathan*,†,‡ and Jin Suk Chung† †

School of Chemical Engineering, University of Ulsan, Namgu, Daehakro 93, Ulsan 680-749, Republic of Korea Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India

‡

ABSTRACT: A series of biobased polyurethane−siloxane cross-linked coatings were prepared using castor oil, partially biobased Tolonate, and 3-aminopropyl trimethoxysilane. The 29 Si NMR spectra was used to determine the formation of cross-linking structure of the polyurethane−siloxane system. FT-IR studies verified that a hydrogen bonding interaction existed at the interface of the castor oil-based polyurethane− siloxane moieties, thereby shifting the characteristic peak position of N−H and CO groups to higher field values. The curing process of the dispersions was followed by means of gel content measurements. The thermal properties of neat waterborne polyurethane and its nanocomposite films were tested by thermogravimetric analysis and differential scanning calorimetric. TGA tests proved that adding silica nanoparticles increased the thermal stability of the nanocomposites. DSC measurements showed that the addition of silica nanoparticles increased the glass transition temperature. Silica nanoparticles in the polyurethane matrix had a significant influence on mechanical properties: The charging level increased from 0% to 5%, and the Young’s modulus and tensile strength of the polyurethane nanocomposites increased from 26.3 to 109.3 and 13.4 to 25.9 MPa, respectively. Atomic force microscopy was also used to represent the surface topography of the polyurethane−siloxane films. The surface wettability and rheological properties of the films were also evaluated. The overall performance of coatings revealed that biorenewable-based polyurethane−siloxane can be successfully used as coatings in various applications. KEYWORDS: Polyurethane−siloxane nanocomposites, Castor oil, Partially biobased Tolonate, Physicochemical properties

■

excellent elasticity of its films, and good processability. Unfortunately, dried WPU films have some shortcomings, including reduced film formation, inferior mechanical properties, low chemical resistance, low pH stability, and inadequate outdoor durability of PUs.8 For the purpose of improving the properties of WPU, several methods have been researched.4−7 Over the years, there have been various strategies proposed for increasing the properties of organic−inorganic hybrid nanocomposites. These nanocomposite materials manifest good adhesion between the polymer matrix fillers because of the nanosize and surface-to-volume ratio of the nanoscale building blocks.9 Nanosized fillers include silica,10 montmorillonite (MMT),11 clay nanoparticles,12 and carbon nanotubes (CNT).13 Among them, silica nanoparticles are expected to offer an attractive potential in polymers reinforcement. The extensively used method to add silica nanoparticles into the polymeric matrix is a sol−gel process.14 The main advantage of using a sol−gel method is the uniform nanostructure achievable at low temperature used to prepare silica nanoparticles. However, there are some disadvantages of the sol−gel process

INTRODUCTION Biorenewable polymers are as ubiquitous as they are essential for a profusion of consumer and high technology applications. Vegetable oil-based polymeric materials have a number of distinguished characteristics, such as ready availability from horticultural resources, biodegradability, low cost, nontoxicity and high thermal stability.1 Among them, castor oil is an important triglyceride of fatty acids in which ricinoleic acid is the major ingredient (about 90% of castor oil). The general structure of ricinoleic acid is HOOC(CH 2 ) 7 CH CHCH2CHOH(CH2)5CH3, with a secondary hydroxyl group that can allow the use of castor oil as a polyol to synthesize cost-effective and biorenewable polyurethane without additional modification.2,3 Recently, traditional applications of solvent-based polyurethanes (PUs) have been confronted with some limitations due to environmental regulations, and therefore, vegetable oil-based cationic and anionic waterborne polyurethane dispersions (PUDs) are proper substitutes for these polymers.4−6 Traditional PUs are insoluble in water; therefore, hydrophilic groups (including ionic or nonionic agents) must be added to their backbones in order to disperse these polymers in water.7 Furthermore, waterborne polyurethanes (WPU) have advantages, such as versatile structure−property relationships, © XXXX American Chemical Society

Received: April 14, 2016 Revised: June 17, 2016

A

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of Castor Oil-Based COPU-Si Waterborne PU Nanocomposites

properties over conventional PUs and the biostability of silicon rubber.17 Recently, Meera et al.18 reported castor oil-based polyurethane−siloxane cross-linked film structures for hydrophobic surface coatings with improved mechanical, thermal, and optical properties. This kind of PU-silica material has positive applications in the medicinal field, as it has good compatibility with living material.19 There are numerous polymer methods used for the development of polymer−silica nanocomposites. Of which, PU has received much attention due to the special microphase structure formed between the hard and soft segments.20 The hard domains expand their rigidity through physical crosslinking (hydrogen bonding between hard segments) and give

such as large volume shrinkage and cracking during drying. In order to overcome this insufficiency, cross-linking agents were used. The 3-aminopropyl trimethoxysilane (APTMS) is one of the cross-linkers used to generate cross-linked PU, in which a methoxy group undergoes a cross-linking reaction to produce stable siloxane network structures with a silylated PU moiety.15 APTMS has unique properties such as low surface energy, high flexibility, transparency, hydrophobicity, and excellent thermal stability.16 Furthermore, the inclusion of silica nanoparticles into the polymer matrix enhances the performance of the polymer and allows some new properties to be added to the polymer matrix. It has also been reported that the cross-linked polyurethane−siloxane structures can adjust the mechanical B

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering filler-like reinforcement to the soft segment. The strong intermolecular interaction between urethane units is preferred because it can limit the degree of phase separation or improve the compatibility of the polymer−filler interaction.21 The main advantage of having PU is the performance of tailor-made properties like elasticity with reasonably large mechanical strength, controllable hardness, and damping ability by changing the molecular chain structure.16 Polyurethanes describe a class of polymers that has widespread use in adhesives, coatings, paints elastomers, foams, and synthetic skin.16 The complex microstructure of PU and its connection to macroscopic properties has attracted researchers for decades17 and continues to do so.22 In view of the above merits, we prepared waterborne nanocomposites by reacting different contents of APTMS with isocyanate-covered castor oil-based PU prepolymers. The purpose of this study is to develop biorenewable-based polyurethane−siloxane nanocomposites with good thermal stability and enhanced mechanical properties. The castor oilbased PU is chemically linked with the silica nanoparticles, and the cross-link density of the nanocomposites is enhanced substantially. The obtained polyurethane−siloxane nanocomposites were characterized for getting their structural, surface, thermal, and mechanical properties using different physicochemical methods in order to utilize their physicochemical features. The polyurethane−siloxane nanocomposites may be good for the practical application of high-performance organic− inorganic hybrid coatings.

■

MEK was eliminated to yield a COPU-Si nanocomposite dispersion with 30 wt % solid content. The final wt % values of the silicon amounts in the dispersion were 1, 3, and 5 wt %. The COPU-Si nanocomposite dispersions are designated as COPU-Si1%, COPU-Si3%, and COPU-Si5%, respectively. For comparison, the sample without APTMS was also made, and the residual NCO reacted with water to form amines, which can then react with the residual NCO to form a polyurea. This measurement sample is designated as COPU-Si0. Films were made by casting the dispersion onto a Teflon plate at 50 °C for 24 h to get transparent films. The resulting films were heated overnight in an oven at 30 °C under a pressure of 2−3 mmHg. Characterization Techniques. 29Si Solid Nuclear Magnetic Resonance (Si NMR). 29Si NMR was used to define the final structure of the synthesized polymers after drying the films at room temperature for at least 1 week. The tests were reported in a Fourier transform Bruker 300 MHz spectrometer (model Avance 300 DPX). The rotor (7 mm) spin rate was 4 kHz, with a delay time of 2 s, accumulating 2000 transients. Fourier Transform Infrared (FT-IR) Spectroscopy. The FT-IR of the samples was performed using a ThermoScientific Nicolet 6700 spectrometer. Each FT-IR spectrum consisted of 68 scans recorded over the wavenumber range from 400 to 4000 cm−1 with a resolution of 4 cm−1 at room temperature. The region of 1600−1800 and 3000− 3700 cm−1 were chosen for the separate of CO and N−H zone, respectively. Tensile Test. The tensile properties were held in accordance with the ASTM D638 using specimens of dimensions 80 mm × 10 mm (length × width) at a crosshead rate of 5 mm/min and a standard length of 50 mm in the Universal testing machine, (UTM, Instron 3386, U.K.). The analysis was carried out at standard laboratory requirements of 23 ± 5 °C and 55% relative humidity. The toughness of the polymer, which is the fracture energy per unit volume of the sample, was taken from the area under the corresponding tensile stress−strain curves. An average value of at least five replicates of every sample was used, and the average value of tensile properties are summarized. Differential Scanning Calorimetry (DSC). The DSC experiments were carried out on a thermal analyzer (TA Instruments DSC Q200, USA). The sample of ∼3 mg was heated at a rate of 10 °C/min from room temperature to 300 °C to remove the thermal history, equilibrated at −100 °C, and then heated to 300 °C at a rate of 10 °C/min. The Tg of the samples was discovered from the midpoint in the heat capacity change in the second DSC scan. Thermogravimetric Analysis (TGA). The thermogravimetric analyzer (TA Instruments TGA Q500, U.S.A.) was used to cover the weight loss of the films under N2 atmosphere. The samples (∼7 mg) were scanned from room temperature to 800 °C at a heating rate of 10 °C/min with N2 purge flow of 20 mL/min. Contact Angle. Contact angles of the dispersion were measured with a G10 (KRUSS) system (Hamburg, Germany) applying the sessile drop technique, and surface energy was determined using the equation of state. The hydrophilic and hydrophobic characteristics of the films were investigated by contact angle measurements. The tests were conducted at room temperature, and the results summarized are the average values of at least five runs. Water Swell and Gel Content. Swelling was estimated by preserving the films in water at room temperature. The rate of swelling for a particular film was determined by measuring its weight progress as a function of time. The water swell was calculated by the following equation:

EXPERIMENTAL SECTION

Materials and Reagents. Castor oil (CO) was purchased from the local supplier and was dried at 100 °C under vacuum for 2 h before use. The characteristic properties of hydroxyl and acid values of 156− 165 and 1.27−3 mg KOH/g were estimated according to ASTM standards D1639-89 and D4274-94, respectively. The commercial grade of isocyanate Tolonate X-FLO 100 (partially biobased aliphatic isocyanate) was kindly supplied by M/s. Vencorex Chemical, France. 3-Aminopropyltrimethoxysilane (APTMS), 2,2-bis(hydroxymethyl) propionic acid (bis-MPA), dibutylamine (DBA), dibutyltin dilaurate (DBTDL), and methyl ethyl ketone (MEK) were obtained from Aldrich (Milwaukee, WI, USA). Triethylamine (TEA) and acetone were procured from SD. Fine chemicals (Mumbai, India) and reagents were utilized as received without additional purification. Double distilled water was used throughout the study. Synthesis of Castor Oil-Based Polyurethane−Silica (COPUSi) Nanocomposites. The overall reaction processes to synthesize the COPU-Si nanocomposites are shown in Scheme 1. The preparation of COPU-Si nanocomposites consists of a two-step reaction sequences. In the first step, castor oil, isocyanate, bis-MPA, and 1 drop of DBTDL as a catalyst were charged to a four-necked flask equipped with a thermometer, condenser, mechanical stirrer, and nitrogen inlet. The mole ratio of the NCO groups of the IPDI, the OH groups of the CO, and the OH groups of the bis-MPA was 2.0:1.0:0.9. The reaction was carried out at 80 °C for 1 h, and then 60 mL MEK was added to modify the viscosity and inhibit gelation. During this time, isocyanate (−NCO) content was observed at constant time intervals by a standard DBA back-titration method. The reaction was kept for another 2 h at 80 °C. After cooling the reaction mixture to room temperature, the PU prepolymers were neutralized by TEA (3 equiv. per bis-MPA) and stirred for 30 min. After 3 h of reaction, a different wt % of APTMS was added and allowed to react with the extra amount of NCO groups at room temperature for 1 h. Formation of the COPU-Si nanocomposite was accomplished by gradually adding water to the neutralized MEK solution of the PU prepolymers at ambient temperature with an agitation speed of 550 rpm. The above COPU-Si dispersion was transferred to a rotary evaporator, and the

Swell (%) =

W1 − W2 × 100 W2

(1)

where W2 is the weight of dried film and W1 is weight after water absorption. The degree of chain formation of the nanocomposites films was cut into 2 cm × 2 cm pieces to determine their weight (W2). The dried film was dipped in toluene for 48 h and dehydrated for 72 h at 30 °C C

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering to provide a weight of W1. The gel content was assessed according to the following formula:

Gel content (%) =

W2 × 100 W1

data about the insertion of the alkoxysilane into the polyurethane matrix. Furthermore, evidence of hydrogen bonding and phase separation behavior was also gathered by following the change in frequency and intensity of the peaks in the mid-IR spectral region. The presence of a broad absorption band at 3347 cm−1 is associated with the stretching vibration of the −N−H bond in the urethane segments (−NH−CO−O). The synthesis of polyurethane was established by the disappearance of the characteristic −NCO peak at 2270 cm−1, which is clearly showing that all the NCO groups were reacted with the amino group of APTMS. The peaks appearing in the range of 2847−2935 cm−1 correspond to the −CH stretching vibrations (asymmetric and symmetric vibrations) of the alkyl chain present in castor oil. As demonstrated in Figure 2, the absorption band 1736 cm−1 can be assigned to the −C

(2)

where W2 is the weight the initially dried film and W1 is the weight of dried film after immersing in the solvents. Rheological Properties. A modular advanced rheometer (MARS III, Thermo Fisher Scientific, Germany) was used for rheological measurements under oscillator mode with 25 mm diameter parallel plates. Dynamic storage modulus (G′) and complex viscosity (η*) were estimated from 0.1 to 100 rad/s a fixed strain of 0.04. The experiment was analyzed at 190 °C with a gap width of 1 mm. Atomic Force Microscopy (AFM). The surface topology of the samples was studied using an atomic force microscope (M/s Park Scientific Instrument, XE-100, U.S.A.). The apparatus was performed in drawing mode using an etched silicon tip (normal radius 8 nm) with a cantilever length of 225 μm and a resonance frequency of about 75 kHz. AFM measurements were conducted at room temperature with a scan rate of 1 Hz and a scan angle of 0°.

■

RESULTS AND DISCUSSION Solid-State 29Si NMR Study. During the functionalization and polymerization processes, the terminal alkoxysilane bearing PU prepolymers can support hydrolysis of the silicon methoxy groups in water and silanol polycondensation reactions, in order to make sure that the PU chains are chemically linked to the silica nanoparticles to improve the cross-link density of the COPU-Si nanocomposites. Figure 1 shows the 29Si NMR spectrum of a sample containing COPU-Si5% nanocomposites. In 29Si solid-state

Figure 2. FT-IR spectra of (a) COPU-Si0, (b) COPU-Si1%, (c) COPU-Si3%, and (d) COPU-Si5%.

O stretching frequency (contribution from amide I: −CO stretching vibrations). Special peaks are at 1500−1600 cm−1 (amide II: δN−H + νC−N + νC−C; sensitive to chain formation and intermolecular hydrogen bonding),25 1372−1381 cm−1 (νC−N), 1252−1256 cm−1 (amide III: νC―N, N−H bending and C−Cα), 767−771 cm−1 (amide IV; N−H out-of-plane), 720 (CH2 rocking) and 695 cm−1 (amide V).26 The introduction of the coordination peak at 1206 and 1020 cm−1 represents stretching of Si−O in Si−O−Si groups of a tetrahedral sheet,27 while a broad band at 1041 cm−1 is attributed to Si−O stretching, which demonstrates the structure of a siloxane linkage. A partial condensation might have occurred by the Si−O−Si asymmetric stretching mode at 1100 cm−1, as well as Si−O−Si symmetric stretching vibration at 1120 cm−1 and the symmetric Si−O−Si stretching mode observed at 868 cm−1. The −N−H urethane region of COPU-Si0 appears at 3347 cm−1, whereas, in the case of COPU-Si nanocomposites films, they show the −N−H peak at 3362 cm−1 with an increase in intensity. The increase in the wavenumber of the −N−H region of COPU-Si 0 to 5% is mainly due to the behavior of silica nanoparticles, which actively interacted with the polyurethane moiety and also the hydrogen bonding present in the nanocomposites, thereby shifting the peak position of −N−H groups (Figure 3). Furthermore, the peak areas of hydrogen bonded and nonhydrogen bonded −CO groups imply a higher contribution of the hydrogen-bonded structure for COPU-Si nanocomposites films. It is shown in Figure 4 that

Figure 1. 29Si solid-state NMR spectrum of COPU-Si5%.

NMR spectra, silicon atoms are un-, mono-, di-, tri-, and tetrasubstituted siloxane linkages [(SiO)2Si(OH)2], [Si(OSi)3OH] and [Si(OSi)4] and the bonds are designated as Qn (un-), Q1 (mono-), Q2 (di-), Q3(tri-), and Q4 (tetra-), respectively. In Figure 1, chemical shifts Q2, Q3, and Q4 were identified at −91, −96.18, and −107.31 ppm, respectively, which reveals the appearance of di-, tri- and tetra-substituted siloxane bonds.23 The strong Q4 peak makes sure that alkoxysilane groups in contact with water undergo full condensation of the silanols. The surface of the Q3 peak denotes the silicon with one unreacted hydroxy group, and the Q2 peak is attached to a silicon bearing two hydroxyl groups. The strong Q4 peak intimates that the degree of silanol condensation is high, suggesting silica clusters are developed.24 Characterization of Nanocomposites by FT-IR. FT-IR spectroscopy is a valuable analytical method to get qualitative D

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. Stress−strain curves of (a) COPU-Si0, (b) COPU-Si1%, (c) COPU-Si3%, and (d) COPU-Si5%.

109.3 from 26.3 MPa, with silica content increases from 1 to 5 wt %. At the same time, the tensile strength is enhanced to 25.9 from 13.4 MPa. Both of these intensifications can be ascribed to the interfacial interaction between the silica filler and the polymer matrix, which carries the load from the polymer matrix to inorganic silica. On the other hand, the elongation at break dropped significantly from 382.1% to 174.9% with the development in the cross-link density, as expected. An enhanced cross-link density enhances the tensile strength but drops the elongation at break.28 This interfacial bonding provides the load transfer capability from the flexible polymer to the stable continuous phase and decreases the slippage through straining. In addition, this cooperation may be able to modify the static microphase morphology of the COPU-Si0 in such a process that results in heightened mechanical properties.29 The improvement of tensile properties mainly depends on the adaptability and physical cross-linking of silica filler in the polyurethane matrix. Overall, the uncertainty in the tensile properties of PU nanocomposite films may be due to the behavior of some agglomerates of silica nanoparticles, which may reconstruct the mechanical properties. DSC Analysis of COPU-Si0 and Its Nanocomposites. Figure 6 displays the DSC thermograms of the COPU-Si0 and COPU-Si nanocomposites. The control COPU-Si0 shows glass transition temperature (Tg) corresponding to the soft segment (SS) at −33.3 °C and the melting temperature (Tm) of the hard segment (HS) at 279.1 °C (COPU-Si0). Compared to the Tg

Figure 3. FT-IR spectra for the expanded N−H region (3500−3300 cm−1): (a) COPU-Si0, (b) COPU-Si1%, (c) COPU-Si3%, and (d) COPU-Si5%.

Figure 4. FT-IR spectra for the expanded CO region (1800−1600 cm−1): (a) COPU-Si0, (b) COPU-Si1%, (c) COPU-Si3%, and (d) COPU-Si5%.

raising the silica nanoparticles content shifts the peak position of the −CO groups. The increase in the intensity and peak positions of the −N−H and − CO groups clearly indicate the creation of a comprehensive and regular network structure of PU. Effects of APTMS on Mechanical Properties of COPUSi Nanocomposites. Table 2 summarizes the tensile properties of the freestanding films of COPU-Si0 and COPUSi nanocomposites film samples with silica content ranging from 1 to 5 wt %, and their stress−strain curves are shown in Figure 5. The neat COPU-Si0 shows a tensile strength of 13.4 MPa and elongation at break of 382.1%. In the case of COPUSi nanocomposite film samples, Young’s modulus is raised to

Figure 6. DSC scans of COPU-Si0 and COPU-Si nanocomposites films. E

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

CO2.34 The third stage, the degradation above 400 °C, is mainly due to the castor oil chain scission and the curve of nanocomposites attributed to the decomposition of organic molecular chains from functionalized silica and remnant polyurethane. Furthermore, the thermo-oxidative degradation of the polyurethane films takes place above 500 °C. Overall, the incorporation of APTMS enhances the thermal stability of nanocomposites; it can be demonstrated that the existence of inorganic oxide could form nanocomposites with high thermal stability. With more cross-linking, the formed Si−O−Si linkages and hydrogen bonding brings the polymer backbones closer and thus reduces the molecular mobility and increases the thermal stability. In addition, the incorporation of a siloxane unit in the COPU system also reduces the amount of more combustible organic components and produces siliceous residue barrier layers that inhibit heat and mass transfer, which also contributed to the increased thermal stability.35 Contact Angle and Gel Content. Contact angle is a useful index to determine the hydrophilicity and hydrophobicity of hybrid films. For this, drops of water were distributed in different areas of the film surface using a microsyringe, and the average was considered as the real value of the contact angle. The larger contact angle value stands for excellent hydrophobicity. From the effects of contact angle analysis, as shown in Table 1, the contact angle of the neat COPU-Si0 film is

values of COPU-Si0, silica nanoparticles incorporating COPUSi nanocomposites have higher Tg with an increasing silane ratio. The addition of APTMS has a notable effect on the Tg value of COPU-Si0, which can be observed from the incremental change in Tg values. This phenomenon can be demonstrated by the large dispersion APTMS in the COPU-Si0 matrix, which reduces the polymer chain movement at higher cross-link densities and the expanded free volume of the nanocomposites.30,31 The Tg values of the COPU-Si nanocomposites are −33.7, −32.6, and −29.7 °C for 1, 3, and 5 wt % of APTMS, respectively. However, the incorporation of APTMS does not alter much of the melting temperature (Tm) values. The melting enthalpy (ΔH) value of the COPUSi0 sample at 6.64 J g−1, whereas the ΔH value of the silicaincorporated COPU-Si nanocomposite are 6.8, 11.4, 12.7, and 11.4 J g1− for 1, 3, and 5 wt % of APTMS, respectively. This reveals that the ΔH extended upon increasing the fraction of silica up to 3 wt %, which may be due to the appearance of strong interfacial interaction between the silica nanoparticles and the hard segment in the polyurethane chain.32 However, the melting enthalpy value starts to decrease over 3 wt % of APTMS. This interfacial interplay contains the molecular motion of the hard segment in polyurethane as mentioned earlier. TGA Analysis of COPU-Si0 and Its Nanocomposites. TGA is used to estimate the thermal stability of the COPU-Si nanocomposite films as a function of silica content. The thermal degradation study of COPU-Si0 and COPU-Si nanocomposites with different APTMS loading were done in an N2 environment at a heating rate of 10 °C/min (Figure 7).

Table 1. Contact Angle Measurements of COPU-Si0 and Its Nanocomposites sample designation

contact angle (θ, deg)

surface energy (mN m−1)

water swell (%)

gel content (wt %)

COPU-Si0 COPU-Si1% COPU-Si3% COPU-Si5%

69.8 75.5 78.7 83.1

38.9 35.3 31.6 30.1

7.2 4.9 4.4 3.8

78 81 86 91

69.8°. The introduction of APTMS to COPU-Si0 increases the contact angle from 69.8° (COPU-Si0) to 83.1° (COPU-Si5%), which indicates a water-repellant surface. This is due to the evidence that with improving APTMS concentration more Si− O−Si network structures are made in the COPU-Si nanocomposite films. The increase in cross-linking of the Si−O−Si network structure in the COPU-Si0 matrix moves toward the surface because of its low surface energy. Hence, the surface polarity is decreased, and the contact angle is increased. Usually, siloxanes stratify the surface during the film formation and cross-linking, which reduces the surface energy.36 The surface energy of the COPU-Si nanocomposite is reported in Table 1. The results show that the surface free energy decreases with increasing contact angle. The introduction of an Si−O−Si unit changes the surface roughness, which influences the contact angle and surface energy values. The appearance of a small amount of siloxane provides a thermodynamic driving force for movement to the material, thereby covering and rendering the surface hydrophobic.37 The contact angle values proclaimed were the average of five measurements taken at five different locations of the film surface, which shows the homogeneity of the surface. Table 1 displays the gel content of the COPU-Si nanocomposite films measured after 20 days of preserving at room temperature to define the extent of solvent resistivity. The APTMS cross-linked COPU-Si nanocomposite films exhibited higher gel contents, suggesting their superior solvent resistance.

Figure 7. TGA thermograms of COPU-S i0 and COPU-Si nanocomposites films.

Polyurethanes exhibit comparatively low thermal stability generally because of the behavior of labile urethane bonds, which decompose below 250 °C, depending upon the isocyanate and polyols employed for the synthesis.33 The TGA curve of COPU-Si0 has two weight loss stages, and COPU-Si nanocomposites exhibit three different weight loss stages. The initial weight loss in the range of 25−250 °C is connected to the evaporation of residual water and loss of oligomer being in COPU-Si0 and its nanocomposites. The second weight loss in the temperature range of 250−300 °C is due to the separation of the urethane bonds to create isocyanates, alcohols, primary and secondary amines, and F

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering The addition of APTMS content in the nanocomposites films would influence cross-linking due to self-condensation; this will induce greater gel content of the film to increase its solvent resistivity. The Si−O−Si unit can perform as a physical crosslinker and enhance the cross-link density of films, which was confirmed by the high gel contents of the films. Furthermore, the nanocomposite films exhibit different swelling ratios in water (Table 2). For instance, COPU-Si nanocomposite films Table 2. Tensile Properties of COPU-Si0 and Its Nanocomposites sample designation

Young’s modulus (MPa)

COPU-Si0 COPU-Si1% COPU-Si3% COPU-Si5%

26.3 ± 6.6 48.1 ± 4.4 59.6 ± 7.8 109.3 ± 9.2

tensile strength (MPa) 13.4 17.6 21.1 25.9

± ± ± ±

0.2 0.7 1.1 1.4

elongation at break (%) 382.1 311.5 246.7 174.9

± ± ± ±

28.1 25.4 18.6 15.2

Figure 9. Rheological behaviors of COPU-Si0 and its nanocomposites: Complex viscosity (η*) as a function of angular frequency.

exhibit lower swelling than that of the neat COPU-Si0. The more Si−O−Si network structures developed due to the condensation might have transferred onto the surface to limit water molecules from making the bulk of the polymer films. However, the Si−O−Si unit, being hydrophobic, would reduce the water swelling nature of the films.38 Rheological Behavior of COPU-Si0 and Its Nanocomposites. The determination of dynamic melt rheological characteristics of polymeric materials below the molten state is important to gain a significant understanding of the nature of the processability and the structure−property relationships for these materials. The angular frequency dependence of both complex viscosity (η*) and storage modulus (G′) of COPU-Si nanocomposites is shown in Figures 8 and 9. As expected, the

all the COPU-Si shows non-Newtonian behavior, i.e., a decrease in viscosity with developing shear rate, probably due to the interruption of the internal structure of the hybrid film. However, a larger silica content of the hybrid solution would appear in higher viscosity, showing a higher degree of interaction. AFM Study of COPU-Si0 and Its Nanocomposites. Atomic force microscopy (AFM) is a high-resolution imaging method for analyzing surfaces and has been employed to provide the morphology (qualitative parameter) and the roughness (quantitative parameter) of the biopolymers at the nanometer scale that is usually inaccessible by any additional experimental technique.39 The results of the particles emerging and the chemical reaction were recorded by AFM. The twoand three-dimensional surface microtopography of the COPUSi0 and COPU-Si nanocomposites films obtained from AFM is shown in Figure 10. The surface roughness of the film structure changes with growing silane ratio, which indicates that a lot of

Figure 8. Rheological behaviors of COPU-Si0 and its nanocomposites: Storage modulus (G′) as a function of angular frequency.

η* and G′ increased substantially with the increase in silica content of the matrix, and the values were higher in the case of COPU-Si5% nanocomposites. This is due to the interplay between colloidal silica and polyurethane or urea (CO groups) developing a network internal structure, which is effective at longer silica content. Another probability would be the creation of a siloxane network at the larger concentration of silica. Thus,

Figure 10. AFM images of (a) COPU-Si0 and its (b) COPU-Si5% nanocomposites. G

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering silane particles reside on the surfaces of hybrid films. When particles of the dispersion overlap with each other, only the edge parts of the silane particles can merge together, and then the unmerged parts of the silane particles exposed on the surface of the film, which indicates lots of particle traces in the body of the film. The COPU-Si5% film displays a rough surface because the number of reactive groups on silane is reduced, leading to an inferior extent of the siloxane cross-linking with the organic phase and implement good compatibility.40,41

(4) Lu, Y.; Larock, R. C. Soybean-oil-based waterborne polyurethane dispersions: effects of polyol functionality and hard segment content on properties. Biomacromolecules 2008, 9, 3332−3340. (5) Fu, C.; Zheng, Z.; Yang, Z.; Chen, Y.; Shen, L. A fully bio-based waterborne polyurethane dispersion from vegetable oils: From synthesis of precursors by thiol-ene reaction to study of final material. Prog. Org. Coat. 2014, 77, 53−60. (6) Gurunathan, T.; Mohanty, S.; Nayak, S. K. Effect of reactive organoclay on physicochemical properties of vegetable oil-based waterborne polyurethane nanocomposites. RSC Adv. 2015, 5, 11524−11533. (7) Rosthauser, J. W.; Nachtkamp, K. Waterborne polyurethanes. J. Ind. Text. 1986, 16, 39−79. (8) Chattopadhyay, D. K.; Sreedhar, B.; Raju, K. V. Effect of chain extender on phase mixing and coating properties of polyurethane ureas. Ind. Eng. Chem. Res. 2005, 44, 1772−1779. (9) Ciprari, D.; Jacob, K.; Tannenbaum, R. Characterization of polymer nanocomposite interphase and its impact on mechanical properties. Macromolecules 2006, 39, 6565−6573. (10) Xia, Y.; Larock, R. C. Preparation and properties of aqueous castor oil-based polyurethane−silica nanocomposite dispersions through a sol−gel process. Macromol. Rapid Commun. 2011, 32, 1331−1337. (11) Wang, M. C.; Lin, J. J.; Tseng, H. J.; Hsu, S. H. Characterization, antimicrobial activities, and biocompatibility of organically modified clays and their nanocomposites with polyurethane. ACS Appl. Mater. Interfaces 2012, 4, 338−350. (12) Kim, B. K.; Seo, J. W.; Jeong, H. M. Morphology and properties of waterborne polyurethane/clay nanocomposites. Eur. Polym. J. 2003, 39, 85−91. (13) Gogoi, S.; Karak, N. Biobased biodegradable waterborne hyperbranched polyurethane as an ecofriendly sustainable material. ACS Sustainable Chem. Eng. 2014, 2, 2730−2738. (14) Hazarika, D.; Karak, N. Waterborne sustainable tough hyperbranched aliphatic polyester thermosets. ACS Sustainable Chem. Eng. 2015, 3, 2458−2468. (15) Subramani, S.; Lee, J. M.; Cheong, I. W.; Kim, J. H. Synthesis and characterization of water-borne crosslinked silylated polyurethane dispersions. J. Appl. Polym. Sci. 2005, 98, 620−631. (16) Zou, H.; Wu, S.; Shen, J. Polymer/silica nanocomposites: preparation, characterization, properties, and applications. Chem. Rev. 2008, 108, 3893−3957. (17) Sarkar, B.; Alexandridis, P. Block copolymer−nanoparticle composites: Structure, functional properties, and processing. Prog. Polym. Sci. 2015, 40, 33−62. (18) Seeni Meera, K. M.; Murali Sankar, R.; Jaisankar, S. N.; Mandal, A. B. Physicochemical Studies on Polyurethane/Siloxane Cross-Linked Films for Hydrophobic Surfaces by the Sol−Gel Process. J. Phys. Chem. B 2013, 117, 2682−2694. (19) Ciriminna, R.; Fidalgo, A.; Pandarus, V.; Béland, F.; Ilharco, L. M.; Pagliaro, M. The sol−gel route to advanced silica-based materials and recent applications. Chem. Rev. 2013, 113, 6592−6620. (20) Lee, H. T.; Lin, L. H. Waterborne polyurethane/clay nanocomposites: novel effects of the clay and its interlayer ions on the morphology and physical and electrical properties. Macromolecules 2006, 39, 6133−6141. (21) Gorrasi, G.; Tortora, M.; Vittoria, V. Synthesis and physical properties of layered silicates/polyurethane nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 2454−2467. (22) Toki, S.; Sics, I.; Burger, C.; Fang, D.; Liu, L.; Hsiao, B. S.; Tsou, A. H.; Datta, S. Structure evolution during cyclic deformation of an elastic propylene-based ethylene-propylene copolymer. Macromolecules 2006, 39, 3588−3597. (23) Siuzdak, D. A.; Start, P. R.; Mauritz, K. A. Surlyn®/silicate hybrid materials. I. Polymer in situ sol−gel process and structure characterization. J. Appl. Polym. Sci. 2000, 77 (13), 2832−2844. (24) Sardon, H.; Irusta, L.; Fernández-Berridi, M. J.; Lansalot, M.; Bourgeat-Lami, E. Synthesis of room temperature self-curable

■

CONCLUSIONS We have demonstrated the environmentally friendly approach over the conventional petrochemical routes to synthesize a COPU-Si nanocomposite using castor oil as a biobased material. The 29Si solid state NMR spectra indicate the respective chemical shift, which confirmed the successful formation of a siloxane network structure. The FT-IR study of N−H and CO suggests that higher hydrogen bonding interaction was detected from the N−H stretching and carbonyl regions in the COPU-Si nanocomposites by the introduction of APTMS. APTMS in the polymer matrix plays an important role in improving the thermal stabilities due to the formation of more thermally stable Si−O−Si linkages in the matrix. The Si−O−Si network acts as a thermal insulator and mass transfer barrier for volatile compounds generated during degradation. The contact angle measurement result suggests that the incorporation of silica nanoparticles increases the contact angle from 69.8° (COPU-Si0) to 83.1° (COPU-Si5%), which results in surface tension increment. The tensile strength and Young’s modulus of the COPU-Si nanocomposites were increased from 13.4 to 25.9 and 26.3 to 109.3 MPa, respectively. These materials display significant mechanical properties, especially above Tg. The viscoelastic properties of biorenewable COPU-Si nanocomposite films increased dramatically with the interaction between the silica and the polymer matrix. AFM images show that the incorporation of silica nanoparticles gives a good surface roughness, which is mainly from the Si−O−Si network. The results of this study obtained from various physicochemical techniques will have a high impact on broadening the various uses and applications of high biorenewable-based polymer nanocomposite materials.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the University of Ulsan (UOU) for their financial support of this work. We thank the Council of Scientific and Industrial Research (CSIR) for the analysis.

■

REFERENCES

(1) Xia, Y.; Larock, R. C. Vegetable oil-based polymeric materials: synthesis, properties, and applications. Green Chem. 2010, 12, 1893− 1909. (2) Gurunathan, T.; Mohanty, S.; Nayak, S. K. Isocyanate terminated castor oil-based polyurethane prepolymer: Synthesis and characterization. Prog. Org. Coat. 2015, 80, 39−48. (3) Ogunniyi, D. S. Castor oil: A vital industrial raw material. Bioresour. Technol. 2006, 97, 1086−1091. H

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering waterborne hybrid polyurethanes functionalized with (3-aminopropyl) triethoxysilane (APTES). Polymer 2010, 51, 5051−5057. (25) Lee, J. Y.; Painter, P. C.; Coleman, M. M. (1988). Hydrogen bonding in polymer blends. 4. Blends involving polymers containing methacrylic acid and vinylpyridine groups. Macromolecules 1988, 21, 954−960. (26) Mishra, A. K.; Chattopadhyay, D. K.; Sreedhar, B.; Raju, K. V. S. N. FT-IR and XPS studies of polyurethane-urea-imide coatings. Prog. Org. Coat. 2006, 55, 231−243. (27) Lee, S. K.; Yoon, S. H.; Chung, I.; Hartwig, A.; Kim, B. K. Waterborne polyurethane nanocomposites having shape memory effects. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 634−641. (28) Choi, S.; Lee, K. M.; Han, C. D. Effects of triblock copolymer architecture and the degree of functionalization on the organoclay dispersion and rheology of nanocomposites. Macromolecules 2004, 37, 7649−7662. (29) Finnigan, B.; Jack, K.; Campbell, K.; Halley, P.; Truss, R.; Casey, P.; Martin, D.; Cookson, D.; King, S. Segmented polyurethane nanocomposites: Impact of controlled particle size nanofillers on the morphological response to uniaxial deformation. Macromolecules 2005, 38, 7386−7396. (30) Ma, J. Z.; Hu, J.; Zhang, Z. J. Polyacrylate/silica nanocomposite materials prepared by sol−gel process. Eur. Polym. J. 2007, 43, 4169− 4177. (31) Xu, L.; Fu, Y.; Du, M. Investigation on structures and properties of shape memory polyurethane/silica nanocomposites. Chin. J. Chem. 2011, 29, 703−710. (32) Lee, S. I.; Hahn, Y. B.; Nahm, K. S.; Lee, Y. S. Synthesis of polyether-based polyurethane-silica nanocomposites with high elongation property. Polym. Adv. Technol. 2005, 16, 328−331. (33) Xia, Y.; Larock, R. C. Preparation and Properties of Aqueous Castor Oil-based Polyurethane−Silica Nanocomposite Dispersions through a Sol−Gel Process. Macromol. Rapid Commun. 2011, 32, 1331−1337. (34) Petrović, Z. S.; Yang, L.; Zlatanić, A.; Zhang, W.; Javni, I. Network structure and properties of polyurethanes from soybean oil. J. Appl. Polym. Sci. 2007, 105, 2717−2727. (35) Chattopadhyay, D. K.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34, 1068− 1133. (36) Bai, C.; Zhang, X.; Dai, J.; Wang, J. Synthesis of UV crosslinkable waterborne siloxane−polyurethane dispersion PDMSPEDA-PU and the properties of the films. J. Coating. Technol. Res. 2008, 5, 251−257. (37) Stiubianu, G.; Nicolescu, A.; Nistor, A.; Cazacu, M.; Varganici, C.; Simionescu, B. C. Chemical modification of cellulose acetate by allylation and crosslinking with siloxane derivatives. Polym. Int. 2012, 61, 1115−1126. (38) Choi, H. Y.; Bae, C. Y.; Kim, B. K. Nanoclay reinforced UV curable waterborne polyurethane hybrids. Prog. Org. Coat. 2010, 68, 356−362. (39) Elofsson, C.; Dejmek, P.; Paulsson, M.; Burling, H. Atomic force microscopy studies on whey proteins. Int. Dairy J. 1997, 7, 813−819. (40) Jena, K. K.; Mishra, A. K.; Raju, K. V. S. N. Thermal and thermomechanical properties of hyperbranched polyurethane-urea/ cenosphere hybrids. J. Appl. Polym. Sci. 2008, 110, 4022−4033. (41) Huang, H. H.; Orler, B.; Wilkes, G. L. Structure-property behavior of new hybrid materials incorporating oligomeric species into sol-gel glasses. 3. Effect of acid content, tetraethoxysilane content, and molecular weight of poly (dimethylsiloxane). Macromolecules 1987, 20, 1322−1330.

I

DOI: 10.1021/acssuschemeng.6b00768 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX