Effects of Surface Structure and Morphology of Nanoclays on the

Article pubs.acs.org/IECR

Effects of Surface Structure and Morphology of Nanoclays on the Properties of Jatropha Curcas Oil-Based Waterborne Polyurethane/ Clay Nanocomposites Lingyuan Liao, Xiaoya Li, Yin Wang, Heqing Fu,* and Yongjin Li School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Three kinds of nanoclays with different structure and morphology were modified by γ-aminopropyltriethoxysilane (APTES) and then incorporated into Jatropha oil-based waterborne polyurethane (WPU) matrix via in situ polymerization. The effects of surface structure and morphology of nanoclay on the degree of silylation were characterized by Fourier transform infrared spectroscopy (FTIR) and thermogravimetry analysis (TGA). The results showed that the montmorillonite (MT) with abundant hydroxyl group structure and platelet-like morphology had the highest degree of silylation, while the modified halloysite nanotubes (HT) had the lowest grafting ratio. The effects of different silylated clays on the properties of WPU/clay nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), TGA, dynamic thermomechanical analysis (DMA) and tensile testing machine. SEM images showed that all silylated clays had good compatibility with WPU and were uniformly dispersed into the polymer matrix. WPU/SMT exhibited the best thermal properties owing to its intercalated structure. Dynamic thermomechanical analysis (DMA), atomic force microscope (AFM), and water contact angle results demonstrated that the silylated nanoclays enhanced the degree of microphase separation, surface roughness, and hydrophobicity of WPU/clay nanocomposites.

1. INTRODUCTION Waterborne polyurethane (WPU) has been widely used in various fields, including coatings,1 inks,2 adhesives,3 and intelligent materials4 owing to its outstanding mechanical properties, adhesion properties and zero/low volatile organic compounds (VOCs) emission. Along with the developing of WPU, the demand of WPU is rapidly expanding because the application filed of WPU is expanded increasingly. WPU uses raw materials come from petroleum based polyol which is nonrenewable. The current trend is consumers are inclined to select greener products. Furthermore, manufacturers are constantly looking for alternatives as the price of fossil oil is typically increasing. In recent years, polymers developed from renewable resources, such as Jatropha curcas oil (JOP), soybean oil, and castor oil have attracted much attention due to their economic, environmental, and societal advantages. However, vegetable oil based polyol exhibited disadvantages of inferior physical properties, poor water resistance, and low thermal stability.5−8 The incoporation of nanomaterials is an effective modification method to enhance the properties of WPU.9−12 Amin et al.13 have demonstrated a scalable, organic solvent-free incorporation of nanocellulose into thermoplastic polyurethane and the nanocomposite showed a remarkable improvement (up to 43%) in ultimate tensile strength. Lorenzetti et al.14 have synthesized graphene filled polyurethane foams; it was found © XXXX American Chemical Society

that graphene is able, at very low content (0.3 wt %), to reduce the radiative contribution of the initial thermal conductivity by both decreasing the cell size and increasing the extinction coefficient. However, the preparation process of nanocellulose or carbon material is tedious. Nanoclays, such as montmorillonite (MT), attapulgite (AT), halloysite nanotubes (HT), etc., as one of the low-cost and earth-abundant inorganic nanomaterials, have been used to modify polymers.15−17 However, clays are difficult to introduce into organic matrices for their hydrophilicity. Moreover, the properties of polymer/ clay composites depended on the dispersion of nanoparticles and the interfacial interactions between inorganic and organic phases; the poor dispersion of nanofillers lead to decrease properties.18−20 To solve this problem, organic modifiers are used to improve the compatibility between clay particles and polymer matrix. Two methods, ion exchange with organic cations and covalent bond functionalization, have been reported to modify nanoclays. MT, a swelling clay with Na+ or Ca2+ in its interlayer galleries and abundant hydroxyl groups on its surface, can be modified by these two methods.21,22 However, AT and HT, as one-dimensional nonswelling clay, Received: Revised: Accepted: Published: A

July 1, 2016 September 15, 2016 October 25, 2016 October 25, 2016 DOI: 10.1021/acs.iecr.6b02527 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Surface modification path of three kinds of nanoclays.

Attapulgite (AT) was obtained from Jiangsu province. Isophorone diisocyanate (IPDI) was purchased from Bayer, Germany. Crude jatropha oil was supplied by BATC Development Berhad, Kuala Lumpur, Malaysia. Dimethylolbutanoic acid (DMBA, Tianyu, China), n-methyl pyrrolidone (NMP, Fuchen, China), γ-aminopropyltriethoxysilane (ATPES, Aladdin, China), triethylamine (TEA, Lingfeng, China), and dibutyltin dilaurate (DBTDL, Lingfeng, China) were all purchased in AR grade. Acetone was supplied by Guangzhou Fine Chemical Factory, China. 2.2. Preparation of Organoclays. One gram of nanoclay powder was placed in a 250 mL beaker containing 100 mL 80% aqueous ethanol solution and mixed under ultrasonic oscillation for 30 min to get a homogeneous mixture. Meanwhile, 2 mL APTES and 100 mL 80% aqueous ethanol solution were added into a 500 mL four-necked glass reaction kettle equipped with a mechanical stirrer, thermometer, and condenser and heated at 50 °C with stirring for 1 h. When the fully hydrolyzed APTES solution was prepared, the nanoclay aqueous ethanol solution was poured into the reaction kettle and stirred uniformly under reflux for 12 h at 70 °C. The solid was separated and washed with pure ethanol and deionized water three times each, followed by a drying; the final products were the silylated nanoclays. The mechanism of the surface modification was shown in Figure 1. 2.3. Preparation of Jatropha Oil-Based Polyols. Preparation of jatropha oil-based polyols (JOP) can be decomposed into two steps: first, the epoxidation reaction produces epoxidized jatropha oil and, then, epoxidized jatropha oil ring opening reaction produces polyols.27 The hydroxyl number of Jatropha-based polyol was 184 mg of KOH/g, which was determined by ASTM D4274-99 Test Method C (reflux phthalation) with acidity correction. 2.4. Synthesis of Jatropha Curcas Oil-Based WPU/Clay Hybrid Dispersions. The jatropha curcas oil-based WPU/clay hybrid dispersions were prepared via in situ polymerization. First, soichiometric dehydrated JOP and ultrasonic treated silylated nanoclay n-methyl pyrrolidone solution were added into a dry 500 mL four-necked flask equipped with a thermometer, mechanical stirrer, and nitrogen gas inlet and heated at 80 °C with stirring for 1 h. Then, soichiometric IPDI, DMBA, and the catalyst DBTDL were added into the reactor under nitrogen atmosphere, and the reaction proceeded at 80 °C until the residual NCO content reached the expected value (determined by the standard dibutylamine back-titration method).28 Before discharging the product, some acetone was

ion exchange with organic cation is difficult because of their relatively low cation content. The common modification of AT and HT is through the reaction between organic modifier and the hydroxyl groups of clay.23,24 Preparation of waterborne polyurethane (WPU) nanocomposites using organic modifier modified AT and unmodified AT was reported.11,12 Subramani21 et al. found that the thermal and tensile properties as well as water and xylene resistance of silylated montmorillonite/PUA nanocomposites were superior to the neat SPUA and the commercial clay-based nanocomposites, suggesting the effectiveness of additional nanoclays to enhance WPU performance. However, to the best of our knowledge, research was mainly focused on the effects of different organo-montmorillonite on the properties of polymer nanocomposites;25,26 no work was carried out on the effects of the morphology and surface structure of nanoclays on the properties of Jatropha oil-based WPU/clay nanocomposites. In this study, montmorillonite (MT), attapulgite (AT), and halloysite nanotubes (HT) with platelet-like, rod-like, and tubular structures, respectively, were used to modify Jatropha oil-based WPU. The effects of the morphology and surface structure of nanoclays on the properties of Jatropha oil-based WPU/clay nanocomposites were investigated. As researchers previously reported, once the content of nanomaterials is above 2 wt %, the inorganic particles may aggregate together and result in decreasing properties.11,12,18 While the inorganic filler content is below 2 wt %, the enhancement was not remarkable. In order to obtain high performance WPU/clay nanocomposites, the content of modified nanoclays was chosen as 2 wt %. The nanoclays were functionalized with silane coupling agent γ-aminopropyltriethoxysilane, and then incorporated into a waterborne polyurethane matrix via in situ polymerization. The morphology of nanoclays was characterized by SEM and TEM. FTIR, TGA, and elemental analysis were used to examine the extent of surface modification of nanoclays. The morphology, thermal, and mechanical properties of WPU/clay nanocomposites were characterized by SEM, AFM, TGA, and tensile testing. The hydrophobicity of WPU/clay nanocomposites was charactrized by contact angle measurement.

2. EXPERIMENTAL SECTION 2.1. Materials. Montmorillonite (MT, CEC = 110 m mol/ 100 g) was supplied by Zhejiang Fenghong. Halloysite nanotubles (HTs) was purchased from Hubei province, and B

DOI: 10.1021/acs.iecr.6b02527 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 2. Reaction scheme for the synthesis of jatropha curcas oil base waterborne polyurethane/clay nanocomposites.

were detected by a Bruker 550 infrared spectrophotometer in the wavenumber range from 4000 to 400 cm−1 at 25 °C. XRD analysis was carried out on a Bruker D8 Advance apparatus with a nickel-filtered Cu K radiation (λ = 0.154 nm). TGA was investigated by a Netzsch STGA449C instrument at the heating rate of 10 °C/min under nitrogen atmosphere. Elemental analysis was performed on a Vario ELIIICHNS elemental analyzer. Brunauer−Emmett−Teller (BET) was conducted with nitrogen porosity meter NOVA 2200e, Quatachrome Instrument Inc. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The base pressure in the analysis chamber was about 3 × 10 −10 mbar. Typically the hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for energy referencing. DMA was analyzed by the DMA242C (Netzsch, Germany) under the tension mode from −100 to 70 °C with a fixed frequency of 1 Hz and a heating rate of 2 °C/min. The mechanical properties of films were measured at 25 °C with the Instron tension meter Model 3367. A crosshead rate of 100 mm/min was used to determine the elongation at break and the ultimate tensile strength. For the AFM characterization, the samples were prepared by spincoating the dispersions on a fresh cleared glass substrates with the rate of 1000 rpm, and the films were allowed to dry in a vacuum oven at room temperature for 24 h. The AFM measurement was performed on the instrument (Bruker Multimode 8) with 20 μm × 20 μm scan area. The surface properties of films were measured by the contact angle goniometer (JC2000C1, Powereach, Shanghai Zhongchen). The sessile-drop method was taken at room temperature, and the average value of three replicates was taken.

added to reduce viscosity, then a certain amount of TEA was added to neutralize the carboxylic acid for 30 min at 50 °C. The ionomer was dispersed into quantitative deionized water with vigorous stirring. Finally, the WPU/clay hybrid dispersions with 30% solid content were obtained after removing the acetone by vacuum distillation. The silylated clay content was 2 wt % with respect to the total solid weight. The reaction scheme for this method is shown in Figure 2. The formulation for jatropha curcas oil based WPU/clay hybrid dispersions is listed in Table 1. A series of WPU/clay nanocomposites, WPU/SMT, WPU/ SAT, and WPU/SHT, were prepared. Table 1. Formulation of Jatropha Curcas Oil-Based WPU/ Clay Hybrid Dispersions mole ratio IPDI (NCO)

JOP(−OH)

DMPA(−OH)

TEA

nanoclay (wt %)

3

2.3

0.68

0.68

2

2.5. Preparation of Jatropha Curcas Oil-Based WPU/ Clay Composite Films. The WPU/clay films were prepared by pouring the WPU/clay hybrid emulsion onto a polytetrafluoroethylene (PTFE) mold and then dried at room temperature for a week. Before characterization, the films were placed in a vacuum oven at 50 °C for 24 h to remove the solvent completely. 2.6. Characterization. The morphologies of the nanoclays and nanocomposites were characterized by SEM (Zeiss Merlin Compact). SEM micrographs were carried out after spraying gold onto the fracture surface under vacuum. FTIR spectra C


Article


Figure 3. SEM and TEM image of MT (a, b), AT (c, d), and HT (e, f).

3455 and 1640 cm−1 were observed in all the samples. Compared with the neat clays, all of the modified clays appeared new bands around 2930 and 1560 cm−1 related to the C−H stretching and deformation vibration of NH2 groups, respectively. The new peaks in the spectra of three kind of modified clays indicated that all of the neat clays were successfully modified by APTES. The XRD patterns of clays before and after modification were shown in Figure 5. The characteristic peak of SMT shifted to lower 2θ value, indicating that the grafting silane in the layers of MT occurred and led to the increased d-spacing of SMT. The increase of d-spacing also confirmed that APTES was not only grafted on the external surface of MT but also intercalated in the layers of MT. As nonswelling nanoclays, the characteristic diffraction peak of AT and HT before and after modification remained the same. This was because the silylation modification of AT and HT just occurred on its external surface, and the grafting of APTES did not change the crystalline structure of AT and HT. However, it was worth noting that the crystallinity of clays were decreased after surface modification. Moreover, the increased basal spacing of MT and unchanged structure of AT and HT showed in the XRD patterns were consistent with the results from SEM.

3. RESULTS AND DISCUSSION 3.1. Characterization of Organo-Modified Nanoclays. The morphologies of three nanoclays were characterized by TEM and SEM. As shown in Figure 3a and b, the sodium montmorillonite (MT) showed the distinct regular layered structure, and the layers stacked tightly. Figure 3c and d present the rodlike structure of attapulgite (AT), the diameter of individual fibers is about 20−50 nm, whereas the average length is a few micron. The mean value of the aspect ratios of the fibers (L/d) is in the range of 40−100. Figure 3e and f exhibited the tubular structure of halloysite nanotubes (HT), the morphology of HT is the smooth surface and cylindricalshaped tube, which is with multilayer walls and an open-ended lumen. The external diameter of the nanotubes is about 60−80 nm and the inner diameter is about 20 nm, while the wall thickness is about 25 nm. Figure 4a, b, and c show the FTIR spectra of MT, AT, and HT before and after organic modification, respectively. All of the samples showed strong absorption peaks in 3500−3700 cm−1 associated with the O−H stretching of hydroxyl groups on the surface of clay.29,30 Other characteristic absorption peaks such as perpendicular Si−O stretching at 1094 and 1025 cm−1, and O−H stretching and deformation vibration of water at D


Article


Figure 4. FTIR curves of nanoclays before and after modification MT (a), AT (b), and HT (c).

Figure 5. XRD curves nanoclays before and after modification. MT (a), SMT (b), AT (c), SAT (d), HT (e), and SHT (f).

Figure 6. TGA curves of nanoclays before and after modification.

TGA was employed to analyze the thermal stability and surface modification extent of silylated clays. The TGA curves of three kinds of clays before and after modification were shown in Figure 6. The neat clays mainly presented a two-step weight loss in the temperature range of 30−800 °C. The first weight loss in the temperature range of 30−150 °C was assigned to water desorption formed on the external surfaces or dehydration of the interlayer. The second weight loss occurring in the temperature range of 150−800 °C was attributed to the loss the hygroscopic water, zeolitic water, and the dehydroxylation of hydroxyl groups. For the modified clays, the weight loss in the temperature range of 30−200 °C was assigned to the loss of moisture and absorbed APTES. While in the temperature range of 200−800 °C, the loss of different structure water, grafting APTES and dehydroxylation of hydroxyl groups occurred.23 Owing to the presence of organic modifier, the weight loss of all kinds of modified clays was higher than that of the pristine clays. The weight loss of grafting APTES mainly occurred in the temperature range of 200−600

°C. In this range. In contrast to the weight loss before and after surface modification, the silylated degree of clays increased in the order of SHT < SAT < SMT. The difference in silane grafted amount was attributed to the varied morphology and surface structure of clays. As a layered silicate clay, MT has abundant hydroxyl groups on the surface of clay platelets, providing a large number of reactive sites for covalent functionalization. The lowest hydroxyl group content on the external surface of HT limited the reactive sites for covalent bonding and resulted in the lowest degree of organomodification. The BET surface area of nanoclay was listed in Table 2. As for SMT, the surface area of MMT increased through APTES Table 2. Surface Area of the Clays and the Modified Clays

E

sample

MT

SMT

AH

SAT

HT

SHT

surface area (m2·g−1)

57.4

79.8

98.4

80.1

61.2

49.7


Article


in WPU, N−H groups peaks at 3300, 1530, and 950 cm−1 appeared, which indicated that the NCO group reacted with OH group. The surface morphologies of pure jatropha curcas oil base WPU and jatropha curcas oil base WPU/clay nanocomposite films were investigated by SEM. Figure 8a, b, c, and d showed the SEM microphotographs of pure WPU, WPU/SMT, WPU/ SAT, and WPU/SHT film, respectively. As can be seen in Figure 8, the fracture surface of pure WPU was uneven but relatively smooth. Compared with pure WPU, all of the nanocomposite samples showed a rougher surface, owing to the presence of rigid clays and the associated constraints on the chain mobility of WPU matrix. No exposed clays or clay agglomerates were observed in the fracture sections of three different kinds of WPU/clay nanocomposite films, indicating that the compatibility between inorganic and organic phases was good, and the silylated nanoclays were uniformly dispersed in WPU matrix. It was found that the surface of WPU/SMT showed more pronounced microroughness than those of WPU/SAT and WPU/SHT, which may be due to that the layered structure of SMT had a better effect on restricting the motion of WPU molecular chains and increasing the crosslinking density of the nanocomposites. FTIR was used to characterize the hydrogen-bonding interaction between nanoclays and jatropha curcas oil base WPU matrix. The deconvolution mathematical procedure based on Gaussian adjustments was used to identify the absorption bands related to the stretching vibration of carboxyl groups associated with free ester and urethane bonds (∼1745 cm−1), H-bonded carbonyl groups from ester and urethane bonds (∼1726 cm−1), carbonyl groups of free urea (∼1695 cm−1), and carbonyl groups from H-bonded urea (∼1654 cm−1). The FTIR spectra of reaction studied as a function of time were presented in Figure 9. At the initial stage of reaction, the peak due to free CO stretching is centered at about 1745 cm−1. With the increasing of reaction time, carbonyl groups of free urea at 1695 cm−1 appeared, indicated that −NH2 groups of clay react with NCO groups. The later stage of reaction, the broad shoulder at lower wavenumbers corresponds to hydrogen bonded carbonyl groups in urethane groups. The FTIR spectra of jatropha curcas oil base WPU and jatropha curcas oil base WPU/clay nanocomposites in the wavenumber range from 1800 to 1600 cm−1 were presented in Figure 10. H-Bond formation in PU may be judged by the hydrogen-bonding index (HBI); this parameter is estimated from the ratio of intensities of absorption corresponding to Hbonded carbonyl groups and free carbonyl groups, that is, C O b /C O free . The HBI of carbonyl groups can be calculated.31−33 And the calculated HBI for WPU, WPU/ SMT, WPU/SAT, and WPU/SHT was 0.42, 0.73, 0.62 and 0.54, respectively. The results showed that the HBI of WPU/ clay nanocomposites were higher than that of pure WPU, and the HBI increased in the order of WPU < WPU/SHT < WPU/ SAT < WPU/SMT. The increase of HBI was attributed to the urea bond formed by the reaction of NH2 groups and NCO groups, which could provide more protons to form hydrogen bonds with CO groups. The WPU/SMT nanocomposites with highest HBI can be explained by the following reasons: (1) The maximum amount of grafted APTES made it easier for SMT to react with NCO groups and obtained more urea bonds, resulting in more H-bonded carbonyl groups. (2) The molecular motion of WPU chains was restricted in the layered

modification. According to the results of XRD and BET analysis, after ATPES molecule was introduced to the interlayer structure of MMT, it led to an increase in the interlayer distance and surface area of MMT. This is consistent with the XRD results. BET surface area of Halloysite was 61.2 m2·g−1, the BET surface area of the SHT decreased after modification. It is concluded that the APTES has entered into the lumen of halloysite, which made the surface area decreased compared with pure halloysite. The AT surface area was 98.4 m2·g−1, addition of APTES led to decrease of these parameters to 80.1 m2·g−1, which suggests the reaction of APTES and the attapulgite. And, this is consistent with the FTIR results. XPS was used to analyze concentration of the hydroxyl groups on clays and reactive rate of the −OH. The survey scans of nanoclay and modified clay are shown in Figures S1−S3. Characteristic elements including carbon, oxygen, silicon, and aluminum were detected. Generally, the thickness of clay exceeds the penetration depth of XPS (10 nm). So it only determine element content on the surfaces of nanoclay. As for modified clay, areas relating to C 1s (284.6 eV) appeared to be slightly larger, and peak of N 1s (399.6 eV) was observed. The XPS results suggesting APTES have be grafted to clay. APTES grafted amount of different nanoclays base on the contents of nitrogen was shown in Table S1. The high-resolution XPS spectra can be deconvoluted into multiple peaks. In Figures S1−S3, the spectra of high-resolution O 1s for clay show two resolved peaks at 530.1 and 531.9 eV corresponding to Si−O−Si and O−H, respectively. The the reactive rate of the −OH could be calculated by the following equation: reactive rate = (At(OH,clay) − At(OH,S ‐ clay))/At(OH,clay)

(1)

Where, At(OH,clay) are shown in Figures S2 and S3. The reactive rate of the −OH is displayed in Figures S1−S3. 3.2. Characterization of Jatropha Curcas Oil-Based WPU/Clay Nanocomposite Films. 3.2.1. Morphology and Structure Characterization. The structure of synthesized WPU was confirmed by FT-IR spectroscopy, as shown in Figure 7. It can be seen that the broad absorbance at 3450 cm−1 in the JOP shifted to 3340 cm−1 in the WPU, which indicated that the OH group was converted to a hydrogen bonded NH group. The typical NCO absorption peak at about 2270 cm−1 disappeared

Figure 7. FTIR curves of IPDI, JOP, and WPU. F


Article


Figure 8. SEM images of WPU and WPU/clay nanocomposite with different types of clays: WPU (a), WPU/SMT (b), WPU/SAT (c), and WPU/ SHT (d).

Figure 10. FTIR curves of carbonyl groups of WPU and WPU/clay nanocomposite with different types of clays. (inset) Typical curve fitting result of WPU/SAT nanocomposites. Figure 9. FTIR curves of carbonyl groups of WPU/SMT with different reaction time.

structure of SMT, which facilitated the abundant OH groups remained on the surface of SMT, which formed more hydrogen bonds between clay and WPU molecular chains. Owing to the lowest grafting degree and few OH groups on the external surface of SHT, the WPU/SHT nanocomposites had the lowest hydrogen-bonding index. 3.2.2. Thermal Stability. The thermal stability of jatropha curcas oil base WPU/clay nanocomposite films with different clays was studied by TGA measurement. Figure 11 showed the TGA curves, and the data was summarized in Table 3. The decomposition of WPU and WPU/clay nanocomposites mainly contained a two-step weight loss process. In general, the temperature at 10% weight loss (T10%) was considered as the onset of thermal degradation.30 The beginning of thermal

Figure 11. TGA curves of WPU and WPU/clay nanocomposite with different types of clays. G


Article


3.2.3. Dynamic Thermomechanical Analysis. Figure 12 showed the loss factor (tan δ) curves of WPU and WPU/clay

Table 3. Thermal Properties and Dynamic Mechanical Thermal Analysis of WPU and WPU/Clay Nanocomposites with Different Types of Clays temperature (°C)

DMA data (°C)

sample

T10%

T50%

Tmax

Tgs

Tgh

ΔTg

WPU WPU/SMT WPU/SAT WPU/SHT

209 253 232 221

361 381 376 364

394 413 405 396

−35.2 −24.3 −32.8 −42.1

42.9 60.7 54.2 51.6

78.1 85.0 87.0 93.7

degradation of all samples can be observed from 209−253 °C, and the weight loss of about 10 wt % was 209, 253, 232, and 221 °C for WPU, WPU/SMT, WPU/SAT, and WPU/SHT, respectively. The thermal degradation at the first stage was corresponded to the decomposition of hard segments formed by urethane and urea bonds. These results indicate that modified clay/WPU composites have better thermal stability than that of pure WPU, and it may be explained by the reaction between −NH2 on the surface of clay and −NCO of hard segments. The subsequent stage of degradation was assigned to the dissociation of the soft segments.6 The weight loss of about 50 wt % was 361, 381, 376, and 364 °C for WPU, WPU/SMT, WPU/SAT, and WPU/SHT, respectively. The improvement in thermal stability can be attributed to barrier phenomenon effect of clay, which delays the escape of volatile degradation products and retards the diffusion of heat and mass transfer of products of pyrolysis.34,35 It was obvious that with the incorporation of modified clays, the heat stability of WPU/clay nanocomposites was significantly improved. The enhanced thermal properties was mainly attributed to the following reasons. First, the uniformly dispersed nanoclay particles may serve as heat insulator and mass transport barrier, which slowed the decomposition of nanocomposites, this barrier phenomenon is particularly evident in the WPU/SMT for layered structure of SMT. Second, the modified nanoclays reacted with polyurethane prepolymer and acted as cross-linking sites to increase the cross-linking density of nanocomposites, which made the nanocomposites more heat-stable,7 this cross-linking effect reflected in the WPU/SAT is more obvious. Compared with the WPU/SMT and WPU/SAT, we can conclude that heat insulation effect is better than the cross-linking effect to improve the thermal stability of polyurethane. In addition, it is worth noting that the thermal stability of WPU/SMT nanocomposites at initial degradation stage was improved more significantly, and the heat stability of WPU/ SHT nanocomposites at the second degradation stage was lower than that of WPU/SMT and WPU/SAT nanocomposites. This phenomenon was because the SMT had highest degree of silylation and resulted in more heat-stable urea bonds and easily form H-bonds between hard domains and hydroxyl groups on the surfaces of SMT, which further improved the thermal stability of hard segments. Moreover, the layered structure of SMT dispersed uniformly in polymer matrix could form thermal barrier and mass transport barrier, which would slow down the escape of the volatile products during the process of degradation. Due to the lowest hydrogen bonded index of WPU/SHT nanocomposites resulted from the lowest degree of silylation and few hydroxyl groups on the external surface of SHT, WPU/SHT nanocomposites showed the lowest thermal stability among the WPU/clay nanocomposites.

Figure 12. DMA curves of WPU and WPU/clay nanocomposite with different types of clays.

nanocomposits with different clays. All samples presented two tan δ peaks, and the peak at low temperature corresponded to the glass transition temperature (Tg) of soft segments (Tgs), while the peak at high temperature was related to the Tg of hard segments (Tgh). It was reported that the degree of microphase separation was reflected by the difference of Tgs and Tgh, namely ΔTg (Tgh − Tgs).2 The corresponding data of Tgs, Tgh, and ΔTg was summarized in Table 3. It was clear that the incorporation of nanoclays resulted in the increase in both Tgs and Tgh, except Tgs of WPU/SHT. The modified nanoclays acted as cross-linking agents, which increased the cross-linking density of nanocompistes. The hydrogen bonds and strong interfacial interaction between modified nanoclays and hard domains, caused an increase in Tgh. Compared with SAT and SHT, SMT with the layered structure and highest silylation degree made it easier to form strong interaction with WPU molecular chains, resulting in the highest Tgh. In general, the rigid inorganic filler can confine the motion of soft segments, which increased the Tgs of WPU/clay nanocomposites. However, the Tgs of WPU/SHT was lower than that of WPU, which may be due to the less interfacial interaction between SHT and soft segments, and stronger hydrogen bonding interaction between SHT and hard domains. It was also found that the ΔTg of all samples increased in the order of WPU (78.1 °C) < WPU/SMT (85.0 °C) < WPU/ SAT (87.0 °C) < WPU/SHT (93.7 °C). It indicated that the addition of nanoclays improved the microphase separation of WPU/clay nanocomposites, and the extent of microphase separation increased in accordance with the increase of ΔTg. It was found that the height of tan δ peaks decreased with the incorporation of nanoclays, which was assigned to the improvement of cross-linking density in WPU matrix. The WPU/SMT obtained the lowest tan δ peak value, implying that the layered structure of SMT played a more effective role in increasing the cross-linking density and restring the molecular motion. The mechanical properties of WPU/clay nanocomposites were related to their microphase separation degree, which will be discussed in the following section. 3.2.4. Mechanical Properties. The mechanical properties of WPU and WPU/clay nanocompoistes were examined by tensile testing. It was obvious that the incorporation of nanoclays greatly impact the mechanical properties of WPU/clay H


Article

Industrial & Engineering Chemistry Research nanocomposites, and the reinforcement effect depended on the dispersion status of nanofillers and the interfacial interaction between polymer matrix. Figure 13 showed the stress−strain

Figure 14. Schematic illustration for network formation of WPU/clay nanocomposite: WPU/SMT (a), WPU/SAT and WPU/SHT (b).

over, compared with WPU/SAT, WPU/SHT had higher elongation at break and lower tensile strength. This was because the highest microseparation degree of WPU/SHT detected by DMA, which made the soft segments more flexible. On the other hand, the rod-like structure of SAT and the stronger interfacial interaction between SAT and WPU matrix as well as more formed hydrogen bonds made SAT more effective than SHT to improve the tensile strength of nanocomposites. 3.2.5. Surface Properties. Figure 15 presented the twodimensional (2D) and three-dimensional (3D) AFM images for pure WPU and WPU/clay nanocomposites. The average roughness (Ra), root-mean-square roughness (Rq), and maximum height (Rmax) of all samples were listed in Table 5. It was obvious that the surface morphology of the WPU matrix was changed by the addition of the clays. The surface roughness increased in the order of WPU < WPU/SMT < WPU/SHT < WPU/SAT, indicating that the roughness of WPU matrix increased with the incorporation of nanoclays and depended on the type of clay. Owing to the lowest silylation degree, SHT had the worst compatibility with polymer matrix among the silylated clays, which made SHT easier to migrate to the surface of nanocomposite, leading to a rougher surface. For WPU/SAT nanocomposite, the abundant hydroxyl groups remained on the surface of SAT made the rigid clay rods easier to agglomerate, which greatly improved the roughness of composite film. The surface roughness of WPU/SMT nanocomposite was more smooth than that of other two nanocomposites, which may be attributed to the well compatibility between inorganic and organic phases as well as the formed intercalated structure. 3.2.6. Contact Angle Measurements. It is well-known that the hydrophobicity of a polymer is directly related to the wettability of its surface and can be measured by determining the contact angle of a sessile drop of water on a solid surface.36 In order to examine the effect of nanoclay types on the hydrophobicity of WPU/clay nanocomposites, contact angle measurements were carried out with pure water and the water contact angle of WPU and WPU/clay nanocomposites were shown in Table 5. The incorporation of different nanoclay particles into WPU matrix led to an increase in the water contact angle of the WPU substrate surface, implying that the addition of silylated nanoclays can improve the hydrophobicity of WPU. The improved surface properties was attributed to the introduction of reactive nanoclays, which improved the crosslinking density and hydrogen-bonding index of WPU/clay nanocomposites. Moreover, the improved hydrophobicity was also assigned to the change of surface microstructures of WPU/

Figure 13. Stress−strain curves of WPU and WPU/clay nanocomposite with different types of clays.

Table 4. Mechanical Properties of WPU and WPU/Clay Nanocomposites with Different Types of Clays samples

yield stress (MPa)

tensile strength (MPa)

elongation at break (%)

WPU WPU/SMT WPU/SAT WPU/SHT

2.71 3.43 3.79 6.64

4.38 6.77 7.29 7.63

121 164 143 125

curves, and the corresponding data was summarized in Table 4. From Figure 13, we found that the WPU/SMT has higher tensile strength than WPU/SHT, while the elongation at break is lower. There are two main reasons for this difference. First, SMT has much more active −NH2 groups and larger surface area, which plays a key role in improving cross-link density. Second, the relatively strong polymer−SMT interaction from the hydrogen bonding between the polymer and SMT restricted molecular motion. Both will increase the tensile strength and decrease the elongation at break of the materials. The layered structure and highest silylation degree of SMT made it easier to improve the cross-linking density of nanocomposite and restrict the motion of WPU molecular chains. As Figure 14a shows, in the stretching process, high surface area of MMT contain more amino groups which can produce more cross-linking and combine more polymer chains into a beam, which increases the tensile strength and decreases the ratio of elongation for breaking of the nanocomposites. The WPU/SAT and WPU/SHT nanocomposites exhibited simultaneous enhancement in tensile strength and elongation at break, which could be attributed to their one-dimensional structure and the proper interfacial interplays between inorganic fillers and polymer matrix. As Figure 14b shows, in the stretching process, the one-dimensional silylated nanoclays showed alignment along the tension direction and still bonded to WPU chains, which generated synergistic effects to enlarge the tension of nanocomposites and burdened more stress. MoreI


Article


Figure 15. AFM images of WPU and WPU/clay nanocomposite with different types of clays: WPU (a), WPU/SMT (b), WPU/SAT (c), and WPU/ SHT (d).

thermal stability of nanocomposites. DMA analyses suggested that the interactions between clays and polymer matrix increased the microphase separation degree of WPU/clay nanocomposites, and WPU/SHT obtained the highest microphase separation degree, which could be assigned to that the silylated halloysite nanotubes formed strong interplays with hard segments but weak interactions with soft segments of WPU. Tensile test suggested that the tensile strength and elongation at breakage of WPU/SAT and WPU/SHT were simultaneously enhanced. Moreover, the surface roughness and hydrophobicity were improved by the introduction of nanoclays and the enhancement varied with the types of nanoclays. All obtained results implied that the properties of WPU/clay nanocomposites could be tailored by selecting the clay type.

Table 5. AFM Data and Water Contact Angle of WPU and WPU/Clay Nanocomposites with Different Types of Clays samples WPU WPU/ SMT WPU/ SAT WPU/ SHT

average roughness (Ra nm)

root mean square roughness (Rq nm)

maximum height roughness (Rmax nm)

water contact angle (deg)

6.93 14.6

9.28 18.9

71.7 140.6

61.7 81.5

56.5

69.2

483.7

67.8

27.3

36.8

315.3

72.1

clay nanocomposites by nanoclays via nucleation effect during drying the nanocomposites.30 Among the three different WPU/ clay nanocomposites, the water contact angle of WPU/SMT was the highest, while the WPU/SAT was the lowest. The enhancement of SAT and SHT was inferior to that of SMT, which can be explained by compatibility between different nanoclays and WPU matrix. The lower compatibility was resulted from the lower silylation degree, which made SHT easier to migrate to the surface of the film; on the other hand, AFM characterization suggested that the strong interactions force by the abundant hydroxyls groups on the SAT surface made SAT nanorods agglomerate easily, which greatly improved the surface roughness of WPU/SAT film, resulting in the decrease of water contact angle.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02527. Element contents and APTES grafted amount of different nanoclays, XPS wide scan, and O 1s core level of ATP SMT, SAT, and SHT (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 020 87114919. Fax: +86 020 87112047. E-mail address: [email protected] (H.F.).

4. CONCLUSIONS Three silylated nanoclay respectively with morphology of platelet-like, rod-like, and tubular were incorporated into jatropha curcas oil base WPU matrix via in situ polymerization and the content of silylated clay was 2 wt %. FTIR results indicated that the interaction between clay particles and polymer matrix was strongest in WPU/SMT nanocomposite. The formed intercalated structure in WPU/SMT improved the

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We appreciate the financial support from the Science Foundation of State Key Laboratory of Structural Chemistry, China, under Grant of No. 20160027. J


Article


■

polyurethane filled with low loading level of starch nanocrystals. Polymer 2008, 49, 1860−1870. (21) Subramani, S.; Choi, S.; Lee, J.; Kim, J. H. Aqueous dispersion of novel silylated (polyurethane-acrylic hybrid/clay) nanocomposite. Polymer 2007, 48, 4691−4703. (22) Huskić, M.; Ž igon, M.; Ivanković, M. Comparison of the properties of clay polymer nanocomposites prepared by montmorillonite modified by silane and by quaternary ammonium salts. Appl. Clay Sci. 2013, 85, 109−115. (23) Yuan, P.; Southon, P. D.; Liu, Z.; Green, M. E. R.; et al. Functionalization of Halloysite Clay Nanotubes by Grafting with γAminopropyltriethoxysilane. J. Phys. Chem. C 2008, 112, 15742− 15751. (24) Zhang, Y.; Zhao, J.; Chu, H.; Zhou, X.; Wei, Y. Effect of modified attapulgite addition on the performance of a PVDF ultrafiltration membrane. Desalination 2014, 344, 71−78. (25) Neppalli, R.; Causin, V.; Marega, C.; Modesti, M.; et al. The effect of different clays on the structure, morphology and degradation behavior of poly(lactic acid). Appl. Clay Sci. 2014, 87, 278−284. (26) Al-Malaika, S.; Sheena, H.; Fischer, D.; Masarati, E. Influence of processing and clay type on nanostructure and stability of polypropylene−clay nanocomposites. Polym. Degrad. Stab. 2013, 98, 2400−2410. (27) Gogoi, P.; Boruah, M.; Sharma, S.; Dolui, S. K. Blends of Epoxidized Alkyd Resins Based on Jatropha Oil and the Epoxidized Oil Cured with Aqueous Citric Acid Solution: A Green Technology Approach. ACS Sustainable Chem. Eng. 2015, 3, 261−268. (28) Gao, F.; Ku, B.; Huang, S. Synthesis and characterization of bistriethoxysilane endcapped polyurethane/urea. J. Appl. Polym. Sci. 2011, 122, 798−803. (29) Bertuoli, P. T.; Piazza, D.; Scienza, L. C.; Zattera, A. J. Preparation and characterization of montmorillonite modified with 3aminopropyltriethoxysilane. Appl. Clay Sci. 2014, 87, 46−51. (30) Tan, D.; Yuan, P.; Annabi-Bergaya, F.; Yu, H.; et al. Natural halloysite nanotubes as mesoporous carriers for the loading of ibuprofen. Microporous Mesoporous Mater. 2013, 179, 89−98. (31) Pergal, M. V.; Džunuzović, J. V.; Poręba, R.; Ostojić, S.; et al. Microstructure and properties of poly(urethane-siloxane)s based on hyperbranched polyester of the fourth pseudo generation. Prog. Org. Coat. 2013, 76, 743−756. (32) Princi, E.; Vicini, S.; Castro, K.; Capitani, D.; On.; et al. The Micro-Phase Separation in Waterborne Polyurethanes. Macromol. Chem. Phys. 2009, 210, 879−889. (33) de Oliveira Patricio, P. S.; Pereira, I. M.; Da Silva, N. C. F.; Ayres, E.; et al. Tailoring the morphology and properties of waterborne polyurethanes by the procedure of cellulose nanocrystal incorporation. Eur. Polym. J. 2013, 49, 3761−3769. (34) Salahuddin, N.; Abo-El-Enein, S. A.; Selim, A.; Salah-el-dien, O. Synthesis and characterization of polyurethane-urea clay nanocomposites using montmorillonite modified by oxyethylene-oxypropylene copolymer. Polym. Adv. Technol. 2010, 21, 533−542. (35) Liu, N.; Zhao, Y.; Kang, M.; Wang, J.; et al. The effects of the molecular weight and structure of polycarbonatediols on the properties of waterborne polyurethanes. Prog. Org. Coat. 2015, 82, 46−56. (36) Liu, M.; Jia, Z.; Liu, F.; Jia, D.; Guo, B. Tailoring the wettability of polypropylene surfaces with halloysite nanotubes. J. Colloid Interface Sci. 2010, 350, 186−193.

REFERENCES

(1) Zhao, J.; Zhou, T.; Zhang, J.; Chen, H.; et al. Synthesis of a Waterborne Polyurethane-Fluorinated Emulsion and Its Hydrophobic Properties of Coating Films. Ind. Eng. Chem. Res. 2014, 53, 19257− 19264. (2) Lei, L.; Zhong, L.; Lin, X.; Li, Y.; Xia, Z. Synthesis and characterization of waterborne polyurethane dispersions with different chain extenders for potential application in waterborne ink. Chem. Eng. J. 2014, 253, 518−525. (3) Fu, H.; Yan, C.; Zhou, W.; Huang, H. Nano-SiO2/fluorinated waterborne polyurethane nanocomposite adhesive for laminated films. J. Ind. Eng. Chem. 2014, 20, 1623−1632. (4) 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. (5) Saalah, S.; Abdullah, L. C.; Aung, M. M.; Salleh, M. Z.; et al. Waterborne polyurethane dispersions synthesized from jatropha oil. Ind. Crops Prod. 2015, 64, 194−200. (6) Hazmi, A. S. A.; Aung, M. M.; Abdullah, L. C.; Salleh, M. Z.; Mahmood, M. H. Producing Jatropha oil-based polyol via epoxidation and ring opening. Ind. Crops Prod. 2013, 50, 563−567. (7) Aung, M. M.; Yaakob, Z.; Kamarudin, S.; Abdullah, L. C. Synthesis and characterization of Jatropha (Jatropha curcas L.) oilbased polyurethane wood adhesive. Ind. Crops Prod. 2014, 60, 177− 185. (8) Gogoi, P.; Boruah, R.; Dolui, S. K. Jatropha curcas oil based alkyd/epoxy/graphene oxide (GO) bionanocomposites: Effect of GO on curing, mechanical and thermal properties. Prog. Org. Coat. 2015, 84, 128−135. (9) Zhang, L.; Zhang, H.; Guo, J. Synthesis and Properties of UVCurable Polyester-Based Waterborne Polyurethane/Functionalized Silica Composites and Morphology of Their Nanostructured Films. Ind. Eng. Chem. Res. 2012, 51, 8434−8441. (10) Hsiao, S.; Ma, C. M.; Tien, H.; Liao, W.; et al. Effect of Covalent Modification of Graphene Nanosheets on the Electrical Property and Electromagnetic Interference Shielding Performance of a Water-Borne Polyurethane Composite. ACS Appl. Mater. Interfaces 2015, 7, 2817− 2826. (11) Peng, L.; Zhou, L.; Li, Y.; Pan, F.; Zhang, S. Synthesis and properties of waterborne polyurethane/attapulgite nanocomposites. Compos. Sci. Technol. 2011, 71, 1280−1285. (12) Pan, H.; Chen, D. Preparation and characterization of waterborne polyurethane/attapulgite nanocomposites. Eur. Polym. J. 2007, 43, 3766−3772. (13) Amin, K. N. M.; Amiralian, N.; Annamalai, P. K.; Edwards, G.; et al. Scalable processing of thermoplastic polyurethane nanocomposites toughened with nanocellulose. Chem. Eng. J. 2016, 302, 406−416. (14) Lorenzetti, A.; Roso, M.; Bruschetta, A.; Boaretti, C.; Modesti, M.; et al. Polyurethane-graphene nanocomposite foams with enhanced thermal insulating properties. Polym. Adv. Technol. 2016, 27, 303−307. (15) Niu, X.; Huo, L.; Cai, C.; Guo, J.; Zhou, H. Rod-Like Attapulgite Modified by Bifunctional Acrylic Resin As Reinforcement for Epoxy Composites. Ind. Eng. Chem. Res. 2014, 53, 16359−16365. (16) Liu, M.; Jia, Z.; Jia, D.; Zhou, C. Recent advance in research on halloysite nanotubes-polymer nanocomposite. Prog. Polym. Sci. 2014, 39, 1498−1525. (17) Alateyah, A. I.; Dhakal, H. N.; Zhang, Z. Y. Processing, Properties, and Applications of Polymer Nanocomposites Based on Layer Silicates: A Review. Adv. Polym. Technol. 2013, 32, 21368− 21405. (18) Zhang, S.; Li, Y.; Peng, L.; Li, Q. Synthesis and characterization of novel waterborne polyurethane nanocomposites with magnetic and electrical properties. Composites, Part A 2013, 55, 94−101. (19) Zhao, C.; Zhang, W. Preparation of waterborne polyurethane nanocomposites: Polymerization from functionalized hydroxyapatite. Eur. Polym. J. 2008, 44, 1988−1995. (20) Chen, G.; Wei, M.; Chen, J.; Huang, J.; et al. Simultaneous reinforcing and toughening: New nanocomposites of waterborne K


Effects of Surface Structure and Morphology of Nanoclays on the

Recommend Documents