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montmorillonite nanocomposite prepared by aqueous solution intercalating
Applied Clay Science 171 (2019) 14–19
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Research Paper
Influence of pH on the structure and properties of soy protein/ montmorillonite nanocomposite prepared by aqueous solution intercalating
T
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Gaiping Guoa, Huafeng Tianb, , Qiangxian Wuc a
College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China School of Material and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, China c Green Polymer Laboratory, College of Chemistry, Central China Normal University, Wuhan 430079, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Soy protein Montmorillonite Nanocomposite pH
The soy protein isolate (SPI)/montmorillonite (Mt) nanocomposites were successfully prepared in aqueous media with different pH values. And then, the SPI/Mt composite plastics with glycerol as plasticizer were obtained via a compression molding process. The electrostatic surface potential has been calculated to investigate the surface charge distribution of the soy protein subunits. The results revealed that the structure of the SPI/Mt nanocomposites was strongly depended on the pH environment during the preparation process. The intercalated nanocomposites were obtained at pH = 1, 4.6 and 12.5, whereas exfoliated ones were prepared at pH = 3 and 10, indicating a significant influence of the solution pH values. The heterogeneous distribution of the surface positive charge caused the positive-charge-rich domains for soy protein bearing net negative charge to anchor into the interlayer space of Mt The mechanical strength of the SPI/Mt plastics was significantly improved as a result of the strong interaction and the fine dispersion of the Mt layers in the SPI matrix.
1. Introduction For the serious white pollution aroused by wide application of nonbiodegradable petroleum based polymers, such as polyethylene, polypropylene, polystyrene, materials based on natural occurring resources such as starch, cellulose, chitin/chitosan, protein, etc., are attracting more and more intentions by both scientific and industry experts (Dani et al., 2011; Jagadeesh et al., 2011; Ma et al., 2016; Muthulakshmi et al., 2017; Tian et al., 2018a). Soy protein has been regarded as a readily renewable biopolymer and potential source for biodegradable plastics and adhesives because of the low cost and eco-friendly characters (Kim and Netravali, 2010; Kumar et al., 2016; Tian et al., 2018b). The common soy protein raw material is soy protein isolate (SPI) with > 90% protein consisting of 18 kinds of amino acids. Usually, bioplastics obtained from SPI possess good biodegradability but poor flexibility (Guo et al., 2015a; Wu et al., 2017; Tian et al., 2018c), so plasticizer is usually incorporated to overcome the brittleness of the soy protein based plastics, which unavoidably leads to significant decrease of its tensile strength (Reddy et al., 2009; Guo et al., 2015b; Renoux et al., 2018). Usually these bioplastics possess fine mechanical performance at dry state, however lowered strength at a high RH atmosphere. Therefore, methods such as crosslinking etc., would lead to an improved water resistance and higher mechanical strength at a humidity
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state. In addition, it is expected that the suitable reinforcing fillers should be involved in the SPI/plasticizer systems to obtain the flexible materials with high strength (Rahman et al., 2014; Gonzalez and Igarzabal, 2015; Guo et al., 2015c; Garrido et al., 2016; Rahman and Netravali, 2017; Xiang et al., 2017; Matas Ortiz et al., 2018; Yu et al., 2018). The factors influencing the reinforcing effect involve dispersion effect, modulus ratio of the matrix/filler, as well as the interactions between them, etc. In recent years, polymer/layer silicates nanocomposites have attracted great interest for their outstanding physical properties (Li et al., 2015; Fan et al., 2018). When compared with virgin polymer or conventional micro- and macro-composites, the polymer composites with the nanometric dispersion of silicate layers show remarkable improvements in material properties, such as mechanical properties and vapor permeability. The crystal structure of a layer silicate such as montmorillonite (Mt) consists of two-dimensional layers by the combination of two tetrahedral silica with Mg or Al to form an octahedral metal oxide. Isomorphic substitution within the layers generates negative charges that are normally counterbalanced by cation (such as Na+, K+, Ca2+) in the interlayer space. Usually, there are three dominant ways to achieve exfoliated nanocomposites: solution intercalation, melt intercalation and intercalative polymerization (Ray and Okamoto, 2003).
Corresponding author. E-mail address: [email protected] (H. Tian).
https://doi.org/10.1016/j.clay.2019.01.020 Received 20 November 2018; Received in revised form 16 January 2019; Accepted 27 January 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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(Nicolet, USA) in the range from 4000 to 400 cm−1 using a KBr-pellet method. The electrostatic surface potential distribution was calculated and visualized by a Swiss PdbViewer 3.7 (SP5) program using the Coulomb law (Guex and Peitsch, 1997). The pH value of the aqueous media in calculation was fixed at 12.5. The contouring values of the potential were set as −1.8 kT/e for negative potential and 1.8 kT/e for positive potential, and the negative and positive potential were colored in blue and red, respectively. In this case, only charged residues (Arg, Lsy, Glu, Asp) were taken into account for their contribution to the surface potential, and the charges were located at the corresponding (non-H) atom positions. The original files of the amino acid sequences for the αc´, β and A1aB1b homotrimers were downloaded from RCSB Protein Data Bank (PDB). X-ray diffraction (XRD) was carried out on a D8 Advance diffractometer (Bruker, USA) equipped with a Cu Kα radiation source (λ = 0.154 nm). The Diffraction data were collected from 2θ = 1–10° in a fixed time mode with a step interval of 0.02°. The structures of the nanocomposites were visualized by a transmittance electron microscope (TEM) [JEM-2010 FEF (UHR), JEOL, Japan] at an accelerating voltage of 200 kV. Either the sheet or the powder samples were embedded in epoxy resin, and the ultrathin sections were obtained on an LKB-8800 ultratome. The ultrathin films of the samples were directly placed on a Formbar-backed carbon-coated copper grid. The glass transition behaviors of the nanocomposites sheets were analyzed on a differential scanning calorimetry (DSC) analyzer (DSC204, Netzsch Co., Germany) equipped with a liquid nitrogen cooling system, and was calibrated with an indium standard (Tf = 156.6 °C). The sample in capsule was quenched to −120 °C, and then heated to 250 °C under nitrogen atmosphere with a heating rate of 20 °C min−1. The tensile strength (σb), elongation at break (εb), and Young's modulus (E) of the sheets were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., Shenzhen, China) with a tensile rate of 5 mm/min according to ISO527-3:1995(E). Five parallel measurements were carried out for each sample.
Some high-performance natural protein/clay materials with intercalated or exfoliated structures have been achieved reported in literatures (Dang et al., 2010; Kumar et al., 2010; Echeverria et al., 2014; Ribeiro et al., 2018). The high affinity of the protein provides the possibility to prepare the nanocomposites with improved properties though a convenient solution intercalation process (Chen and Zhang, 2006). The high affinity of the clays to the protein molecules bearing neutral, positive and even negative charges could be proved in aqueous media (Yu et al., 2013), thus, pH may directly affect the interaction and distribution of nanoclays in protein matrix (Zhang et al., 2006; CortesTrivino and Martinez, 2018; Felix et al., 2018). For the importance of the solution pH on the interaction between the protein matrix and clay nanolayers, in the present study, soy protein/montmorillonite nanocomposites were prepared in aqueous media with different pH values and the interactions between SPI and Mt were investigated. The SPI/Mt plastics with glycerol as plasticizer were prepared though a compression molding process, and their structure and properties were also characterized in detail. It was found that the structures of the SPI/Mt nanocomposites and plastics were strongly depended on the pH environment in the preparation process. 2. Experimental part 2.1. Materials Commercial soy protein isolate (moisture ca. 6.5 wt%) was purchased from DuPont-Yunmeng Protein Technology Co. Ltd. (Yunmeng, China). Na+-montmorillonite (Na+-Mt, trade name: NANNOLIN DK0) was supplied by Fenghong Clay Chemical Corporation in China. The cation exchange capacity of Mt was 110 meq/100 g with a particle dimension of 25 μm in dry state. Other chemicals were obtain from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. 2.2. Preparation of Mt/SPI nanocomposites and plastic sheets
3. Results and discussion 10 g SPI was dissolved in 160 mL distilled water at ambient temperature under stirring to obtain an emulsion. At the same time, 0.8 g Mt was dispersed in 40 mL distilled water and stirred for 30 min. The pH value of SPI emulsion was adjusted by adding 1 mol/L HCl or NaOH to 1.0, 3.0, 4.6, 10.0 and 12.5 before the Mt suspension was added into it, and then the resultant mixture was stirred violently at 60 °C for 2 h. 200 mL acetone were added and the resultant mixture was centrifuged with a speed of 9000 rpm for 15 min. The precipitate after centrifugation was suspended in 100 mL of acetone and then filtered. The resulting sample was vacuum-dried for 24 h to obtain a yellow nanocomposite powder. According to the pH values in preparation process, the nanocomposite powders were coded as PPH1, PPH3, PPH4.6, PPH10 and PPH12.5 corresponding to the solution pH of 1.0, 3.0, 4.6, 10.0 and 12.5, respectively. The PPH-series powders with an addition of 30 wt% glycerol as plasticizer were mixed in a kitchen beater (HR1704, PHILIPS Ltd., Zhuhai, China) for 15 min. Subsequently, the mixture of the SPI/Mt nanocomposite powders and glycerol were compression-molded at 140 °C and 20 MPa. The resultant plastic sheets were coded as FPH1, FPH3, FPH4.6, FPH10 and FPH12.5, respectively. The pure SPI sheets without Mt were also prepared through the same way and were coded as BFPH1, BFPH3, BFPH4.6, BFPH10 and BFPH12.5 according to the pH values. Neat soy protein sheets without Mt and pH changing in preparation was coded as BFSPI. The nanocomposite powders and sheets were conditioned in a desiccator with P2O5 as desiccant for one week at room temperature before characterization.
3.1. Influence of pH on structure of nanocomposites Fig. 1 shows the XRD patterns for the Na+-Mt and the nanocomposite powders (PPH1, PPH3, PPH4.6, PPH10 and PPH12.5). The d001-spacing of the SPI/Mt nanocomposites prepared through solution intercalation can be calculated on the basis of Bragg's law [d001 = λ / (2sinθ)] according to the peak position. The basal spacing of pristine Na+–Mt was 1.4 nm from a diffraction peak at 2θ = 6.4o, whereas the Mt-free SPI powder shows no diffraction peak in the 2θ range from 1 to 10o. This indicates the soy protein has no ordered structure in the dimension range. The diffraction peak of the Mt is absent in the X-ray diffraction spectrum when the pH values are 3.0 and 10.0 for PPH3 and PPH10. It suggests that the Mt layers are highly disordered and exfoliated in the nanocomposite powder. From the XRD patterns, an apparent interlayer spacing of about 5.4, 3.8 and 6.3 nm has been calculated for the PPH1, PPH4.6 and PPH12.5 powders, suggesting the formation of intercalated structures of Mt The results show that the interactions between Mt and SPI are strongly influenced by the environment's pH values. The layered Mt is delaminated in the aqueous medium by the soy protein macromolecules under proper pH values, whereas the intercalated structure of Mt in soy protein matrices is dominant when the pH is too high, too low pH or at the isoelectric point of SPI. TEM can be applied to verify the extent of delamination and exfoliation achieved directly. Fig. 2 presents the TEM images of the PPH3, PPH4.6 and PPH12.5. In view of the image of Fig. 2(a), the Mt layers are well dispersed in the soy protein matrix. Although the Mt layers still retain the orientation in some degree, the tactoid structure of Mt is highly delaminated into some thin lamellas by soy protein with a
2.3. Characterization FTIR spectroscopy of was recorded on a Nicolet 5700 spectrometer 15
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prepared in the neutral aqueous media (Chen and Zhang, 2006). In this case, the heterogeneous distribution of the surface positive charges provided the possibility for negatively charged soy protein to intercalate and exfoliate the negatively charged Mt layers. The positivecharge-rich domains on the SPI molecules provide the active positivecharged sites for the replacement of Na+ and for the anchoring of the soy protein molecules into the negatively charged Mt interlayer space. Thus in the viewpoint of electrostatic interaction, the low pH value and more positive charges on the surface of SPI molecules are beneficial to the intercalation to Mt layers. Interestingly, from the results of XRD and TEM, the intercalated structure of SPI/Mt nanocomposite is maintained at the lowest pH in the series, whereas highly exfoliated structure has been obtained when pH is 3.0. This could be attributed to the competition between positively charged surface of SPI and too many H3O+ ions at over low pH value, leading to the decrease of the negative sites on the Mt layer for SPI molecules. When the pH value is changed to the isoelectric point of SPI (pH~4.6), the solubility of SPI in water decreased sharply comparing to other pH values (Renkema et al., 2002). Under this circumstance, the solvated SPI molecules reduced, which is equal to the increase of the concentration of Mt The dependence of the nanocomposite structure (exfoliated or intercalated) is highly associated on the Mt content. Therefore, explanation could be drawn that this is the reason why the intercalated structure, not the exfoliated structure formed at the isoelectric point of SPI. Furthermore, through the simulation of electrostatic surface potential distribution of three typical soy protein homotrimers aligned by A1aB1b, αc´ and β subunits (Fig. 3), it is obvious that even at high pH value (pH = 12.5), the positively charged area still exist on the surface of SPI molecules. This explains the formation of exfoliated structure when pH is 10.0. However, because the NaOH solution was used to adjust the pH of the system, there were too many Na+ ions existed in the solution. Na+ in the Mt layers is difficult to diffuse. Subsequently, only the intercalated structure was formed. In conclusion, in the pH range of 1.0–12.5, the existence of positive-charge-rich domains, more or less, is the key point of SPI molecules to intercalate or exfoliate the Mt layers. However, the final result about whether the SPI molecules enter the interlayer of Mt and exfoliate the layers is determined by the complicated factors, such as electrostatic interaction between SPI and Mt, competition between other cations and positive-charge-rich domains of SPI, and the diffusion behavior of Na+ in the layers of Na+-Mt Therefore, the intercalated structure formed when pH is too high, too low pH or at isoelectric point, and exfoliated structure obtained at proper pH values. It has been known that the amide and molecules containing C]O are able to form hydrogen bond with the polar groups on the Mt layered surfaces. Structurally, the surface of the Mt interlayer space contains Si–O–Si and –OH groups, and they could act as hydrogen bonding sites for the guest molecules. The effects of hydrogen-bond should be taken into account for the intercalation of protein molecules into the negatively charged Mt Because of the low symmetry of the dioctahedral Mt
Fig. 1. XRD patterns of Mt, SPI and PPH-series composites.
dimension of about 1–2 nm in thickness. When pH is 4.6, the layered structure of the Mt is generally maintained, and the entrance of protein molecules into the Mt layers makes the d-spacing expand from 1.4 nm to ca. 3–4 nm. When the pH is 12.5, the d-spacing of intercalated Mt layers further expand to about 6 nm. These results meet a good agreement with the results from the XRD experiments. On the basis of the evidences from XRD and TEM, it is clearly revealed that the SPI/Mt nanocomposites with an exfoliated or intercalated structure can be obtained via solution intercalation process in the aqueous media under different pH values. The results mentioned above obviously indicate that a high affinity between soy protein and Mt exists, and it is related to the environment's pH values. 3.2. Interaction between soy protein and Mt Basically, proteins have a net positive charge when the pH is lower than the isoelectric point and a net negative charge when the pH is higher than the isoelectric point. As a consequence, the pH could highly influence the properties of protein materials, including soy protein (Felix et al., 2018), gelatin (Hernandez-Izquierdo and Krochta, 2008), etc., such as film-forming, gelling, emulsifying, and foaming properties. In previous studies, exfoliated SPI/Mt nanocomposite has been
Fig. 2. TEM images of PPH3 (a), PPH4.6 (b) and PPH12.5 (c). 16
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Fig. 3. Distribution of electrostatic surface potential of three typical homotrimers of soy protein at pH = 12.5, (a) β homotrimer of β-conglycinin, (b) α´ homotrimer of β-conglycinin and (c) A1aB1b homotrimer of glycinin. The negative potential (< −1.8 kT/e) is colored in red and positive potential (> 1.8 kT/e) is colored in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. FTIR spectra at 1800–1400 cm−1 (a) and 1200–900 cm−1 (b) for Mt, SPI and PPH-series.
Fig. 6. DSC thermograms of FPH-series SPI/Mt plastics. Table 1 Experimental results from tensile testing of FPH and BFPH-series plastic sheets.
FPH1 FPH3 FPH4.6 FPH10 FPH12.5
σb (MPa)
εb (%)
7.0 8.9 8.8 7.9 4.0
10.0 22.7 30.1 36.9 170
BFPH1 BFPH3 BFPH4.6 BFPH10 BFPH12.5
σb (MPa)
εb (%)
4.0 7.2 7.8 4.6 1.3
19.3 60.5 68.3 115.5 111.0
layer, the SieO absorbance of Na+-Mt is split into four individual peaks at 1120 (peak I), 1087 (peak II), 1040 (peak III) and 1014 (peak IV) cm−1. The peaks I, III and IV are assigned to the in-plane SieO stretching bands. The peak II at 1087 cm−1 ascribed to the out-of-plane vibration is related to the orientation of the Mt layers. Especially, the disordering of the clay lamellas should cause the increase of the
Fig. 5. XRD patterns of FPH-series sheets.
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intensity of this band, and this has been well proved by the polarized attenuated total reflectance (ATR)-FTIR experiments (Ras et al., 2003). Fig. 4 shows the FTIR spectra in two typical ranges (1800–1600 cm−1 and 1300–950 cm−1) for Mt and the nanocomposite powders. The pristine Na+-Mt presents an unobvious absorbance at 1087 cm−1, whereas the out-of-plane SieO vibration absorbance become sharp and separated for the SPI/Mt nanocomposites. This distinct change means the orientation structure of Mt has been strongly perturbed by the presence of the soy protein. In addition, compared with the amide bands of the Mt-free SPI, the band of the amide II (δN–H) shifts to higher wavenumbers, whereas no obvious change occurs for the band of amide I (νC=O). In view of the obvious interference to the protein amide II absorbance and the SieO stretching peak II, it is believed that the intermolecular hydrogen bond between SPI and Mt exists in the nanocomposites. The hydrogen bonding sites may be the oxygen atoms on the silicate layer surface and the hydrogen atoms in the peptide bonds.
4. Conclusion
3.3. Influence of pH on structure and properties of SPI/Mt plastics
Acknowledgments
With 30 wt% glycerol as plasticizer, the SPI/Mt plastic sheets could be obtained through a compression-molding process. The XRD patterns of the nanocomposite plastics are presented in Fig. 5. Similar to the nanocomposites powders, there is no diffraction peak in FPH3 and FPH10. This indicates that the high-degree disordering structure of SPI/ Mt nanocomposite is maintained in the compression-molding process. And the difference is, compare to the XRD patterns of PPH1, PPH3 and PPH12.5, the d-spacing of FPH1, FPH3 and FPH12.5 have been enlarged to 6.9, 7.0 and 6.8 nm respectively, according to the lower angle shifting of the diffraction peak. This phenomenon suggests that a melting intercalation process has been involved when the nanocomposites were compression-molded leading to the enhancement of the d-spacing. The DSC thermograms of FPH3, FPH4.6, FPH10 and BFSPI are shown in Fig. 6. The Mt-free sheet (BFSPI) exhibits two glass transitions (Tg1 and Tg2) at −52.1 °C and 98.3 °C, respectively, ascribed to the glass transition temperature of the glycerol-rich domains and protein-rich domains of glycerol plasticized soy protein plastics(Chen and Zhang, 2005). The Tg2 is related to the protein chains and segments with a low compatibility to glycerol. However, in the exfoliated-structured FPH3 and FPH10 sheets, the glass transition of the glycerol-rich domains is split into two individual ones. The new glass transition (Tg3) appears at −27.0 and-18.3 °C, respectively. The occurrence of Tg3 indicates that the fine dispersion of Mt layers effectively restricts the segmental motion of the soy protein molecules on the interface of the clay layers (Lu and Nutt, 2003). There is no Tg3 formed in the intercalated-structured FPH4.6 sheet, but the Tg1 shifts to −33.1 °C. This phenomenon suggests that the segmental motion of the soy protein molecules is still restricted. This can be explained that compared to the exfoliated structure, the aspect ratio of Mt layers sharply diminished, and the restricted segments reduced. Under such circumstance, the Tg of restricted segments could not be separated individually, consequently, resulting in the increase of the whole Tg1 of the glycerol-rich domains. It is concluded that the exfoliated and intercalated Mt layers mainly exist in the glycerolrich domains of the SPI/Mt nanocomposite sheets. Table 1 presents the tensile testing results of the SPI/Mt plastic sheets. To eliminate the influence of different pH environments, the mechanical properties of Mt-free BFPH series sheets were prepared and tested at the same time. It is obvious that the tensile strength (σb) of the sheets is improved at the same pH value, which evidences the reinforcement effect of the exfoliated or intercalated Mt layers. Meanwhile, the elongation at break (εb) of FPH1~FPH10 sheets decrease. It is worth noted that the hydrogen bonds of protein molecules are destroyed mostly under strong basic environment and the protein molecule chains become stretched. So the FPH12.5 and BFPH12.5 sheets show the characteristic of elastomer. As a result, both of σb and εb of FPH12.5 are higher than those of BFPH12.5 sheets.
The authors would like to express their appreciation for the financial support from the National Natural Science Foundation of China under grant No. 51573066.
Soy protein isolate/montmorillonite nanocomposites were successfully prepared in aqueous media with the different pH values, and the SPI/Mt plastics with glycerol as plasticizer were obtained via a compression molding process. The results proved that the structures of the SPI/Mt nanocomposites and plastics were strongly depended on the pH environment in the preparation process. When pH = 1.0, 4.6 and 12.5, the resultant SPI/Mt composites were intercalated structure; when pH = 3.0 and 10.0, the exfoliated SPI/Mt nanocomposites were obtained. In the pH range from 1.0 to 12.5, the existence of positivecharge-rich domains is the key point of SPI molecules to intercalate or exfoliate the Mt layers. However, the exfoliated composites were obtained only at certain pH values. Moreover, the introduction of Mt to the SPI bulk distinctly improved the tensile strength of the nanocomposite sheets.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Influence of pH on the structure and properties of soy protein/ montmorillonite nanocomposite prepared by aqueous solution intercalating
T
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Gaiping Guoa, Huafeng Tianb, , Qiangxian Wuc a
College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China School of Material and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, China c Green Polymer Laboratory, College of Chemistry, Central China Normal University, Wuhan 430079, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Soy protein Montmorillonite Nanocomposite pH
The soy protein isolate (SPI)/montmorillonite (Mt) nanocomposites were successfully prepared in aqueous media with different pH values. And then, the SPI/Mt composite plastics with glycerol as plasticizer were obtained via a compression molding process. The electrostatic surface potential has been calculated to investigate the surface charge distribution of the soy protein subunits. The results revealed that the structure of the SPI/Mt nanocomposites was strongly depended on the pH environment during the preparation process. The intercalated nanocomposites were obtained at pH = 1, 4.6 and 12.5, whereas exfoliated ones were prepared at pH = 3 and 10, indicating a significant influence of the solution pH values. The heterogeneous distribution of the surface positive charge caused the positive-charge-rich domains for soy protein bearing net negative charge to anchor into the interlayer space of Mt The mechanical strength of the SPI/Mt plastics was significantly improved as a result of the strong interaction and the fine dispersion of the Mt layers in the SPI matrix.
1. Introduction For the serious white pollution aroused by wide application of nonbiodegradable petroleum based polymers, such as polyethylene, polypropylene, polystyrene, materials based on natural occurring resources such as starch, cellulose, chitin/chitosan, protein, etc., are attracting more and more intentions by both scientific and industry experts (Dani et al., 2011; Jagadeesh et al., 2011; Ma et al., 2016; Muthulakshmi et al., 2017; Tian et al., 2018a). Soy protein has been regarded as a readily renewable biopolymer and potential source for biodegradable plastics and adhesives because of the low cost and eco-friendly characters (Kim and Netravali, 2010; Kumar et al., 2016; Tian et al., 2018b). The common soy protein raw material is soy protein isolate (SPI) with > 90% protein consisting of 18 kinds of amino acids. Usually, bioplastics obtained from SPI possess good biodegradability but poor flexibility (Guo et al., 2015a; Wu et al., 2017; Tian et al., 2018c), so plasticizer is usually incorporated to overcome the brittleness of the soy protein based plastics, which unavoidably leads to significant decrease of its tensile strength (Reddy et al., 2009; Guo et al., 2015b; Renoux et al., 2018). Usually these bioplastics possess fine mechanical performance at dry state, however lowered strength at a high RH atmosphere. Therefore, methods such as crosslinking etc., would lead to an improved water resistance and higher mechanical strength at a humidity
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state. In addition, it is expected that the suitable reinforcing fillers should be involved in the SPI/plasticizer systems to obtain the flexible materials with high strength (Rahman et al., 2014; Gonzalez and Igarzabal, 2015; Guo et al., 2015c; Garrido et al., 2016; Rahman and Netravali, 2017; Xiang et al., 2017; Matas Ortiz et al., 2018; Yu et al., 2018). The factors influencing the reinforcing effect involve dispersion effect, modulus ratio of the matrix/filler, as well as the interactions between them, etc. In recent years, polymer/layer silicates nanocomposites have attracted great interest for their outstanding physical properties (Li et al., 2015; Fan et al., 2018). When compared with virgin polymer or conventional micro- and macro-composites, the polymer composites with the nanometric dispersion of silicate layers show remarkable improvements in material properties, such as mechanical properties and vapor permeability. The crystal structure of a layer silicate such as montmorillonite (Mt) consists of two-dimensional layers by the combination of two tetrahedral silica with Mg or Al to form an octahedral metal oxide. Isomorphic substitution within the layers generates negative charges that are normally counterbalanced by cation (such as Na+, K+, Ca2+) in the interlayer space. Usually, there are three dominant ways to achieve exfoliated nanocomposites: solution intercalation, melt intercalation and intercalative polymerization (Ray and Okamoto, 2003).
Corresponding author. E-mail address: [email protected] (H. Tian).
https://doi.org/10.1016/j.clay.2019.01.020 Received 20 November 2018; Received in revised form 16 January 2019; Accepted 27 January 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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(Nicolet, USA) in the range from 4000 to 400 cm−1 using a KBr-pellet method. The electrostatic surface potential distribution was calculated and visualized by a Swiss PdbViewer 3.7 (SP5) program using the Coulomb law (Guex and Peitsch, 1997). The pH value of the aqueous media in calculation was fixed at 12.5. The contouring values of the potential were set as −1.8 kT/e for negative potential and 1.8 kT/e for positive potential, and the negative and positive potential were colored in blue and red, respectively. In this case, only charged residues (Arg, Lsy, Glu, Asp) were taken into account for their contribution to the surface potential, and the charges were located at the corresponding (non-H) atom positions. The original files of the amino acid sequences for the αc´, β and A1aB1b homotrimers were downloaded from RCSB Protein Data Bank (PDB). X-ray diffraction (XRD) was carried out on a D8 Advance diffractometer (Bruker, USA) equipped with a Cu Kα radiation source (λ = 0.154 nm). The Diffraction data were collected from 2θ = 1–10° in a fixed time mode with a step interval of 0.02°. The structures of the nanocomposites were visualized by a transmittance electron microscope (TEM) [JEM-2010 FEF (UHR), JEOL, Japan] at an accelerating voltage of 200 kV. Either the sheet or the powder samples were embedded in epoxy resin, and the ultrathin sections were obtained on an LKB-8800 ultratome. The ultrathin films of the samples were directly placed on a Formbar-backed carbon-coated copper grid. The glass transition behaviors of the nanocomposites sheets were analyzed on a differential scanning calorimetry (DSC) analyzer (DSC204, Netzsch Co., Germany) equipped with a liquid nitrogen cooling system, and was calibrated with an indium standard (Tf = 156.6 °C). The sample in capsule was quenched to −120 °C, and then heated to 250 °C under nitrogen atmosphere with a heating rate of 20 °C min−1. The tensile strength (σb), elongation at break (εb), and Young's modulus (E) of the sheets were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., Shenzhen, China) with a tensile rate of 5 mm/min according to ISO527-3:1995(E). Five parallel measurements were carried out for each sample.
Some high-performance natural protein/clay materials with intercalated or exfoliated structures have been achieved reported in literatures (Dang et al., 2010; Kumar et al., 2010; Echeverria et al., 2014; Ribeiro et al., 2018). The high affinity of the protein provides the possibility to prepare the nanocomposites with improved properties though a convenient solution intercalation process (Chen and Zhang, 2006). The high affinity of the clays to the protein molecules bearing neutral, positive and even negative charges could be proved in aqueous media (Yu et al., 2013), thus, pH may directly affect the interaction and distribution of nanoclays in protein matrix (Zhang et al., 2006; CortesTrivino and Martinez, 2018; Felix et al., 2018). For the importance of the solution pH on the interaction between the protein matrix and clay nanolayers, in the present study, soy protein/montmorillonite nanocomposites were prepared in aqueous media with different pH values and the interactions between SPI and Mt were investigated. The SPI/Mt plastics with glycerol as plasticizer were prepared though a compression molding process, and their structure and properties were also characterized in detail. It was found that the structures of the SPI/Mt nanocomposites and plastics were strongly depended on the pH environment in the preparation process. 2. Experimental part 2.1. Materials Commercial soy protein isolate (moisture ca. 6.5 wt%) was purchased from DuPont-Yunmeng Protein Technology Co. Ltd. (Yunmeng, China). Na+-montmorillonite (Na+-Mt, trade name: NANNOLIN DK0) was supplied by Fenghong Clay Chemical Corporation in China. The cation exchange capacity of Mt was 110 meq/100 g with a particle dimension of 25 μm in dry state. Other chemicals were obtain from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. 2.2. Preparation of Mt/SPI nanocomposites and plastic sheets
3. Results and discussion 10 g SPI was dissolved in 160 mL distilled water at ambient temperature under stirring to obtain an emulsion. At the same time, 0.8 g Mt was dispersed in 40 mL distilled water and stirred for 30 min. The pH value of SPI emulsion was adjusted by adding 1 mol/L HCl or NaOH to 1.0, 3.0, 4.6, 10.0 and 12.5 before the Mt suspension was added into it, and then the resultant mixture was stirred violently at 60 °C for 2 h. 200 mL acetone were added and the resultant mixture was centrifuged with a speed of 9000 rpm for 15 min. The precipitate after centrifugation was suspended in 100 mL of acetone and then filtered. The resulting sample was vacuum-dried for 24 h to obtain a yellow nanocomposite powder. According to the pH values in preparation process, the nanocomposite powders were coded as PPH1, PPH3, PPH4.6, PPH10 and PPH12.5 corresponding to the solution pH of 1.0, 3.0, 4.6, 10.0 and 12.5, respectively. The PPH-series powders with an addition of 30 wt% glycerol as plasticizer were mixed in a kitchen beater (HR1704, PHILIPS Ltd., Zhuhai, China) for 15 min. Subsequently, the mixture of the SPI/Mt nanocomposite powders and glycerol were compression-molded at 140 °C and 20 MPa. The resultant plastic sheets were coded as FPH1, FPH3, FPH4.6, FPH10 and FPH12.5, respectively. The pure SPI sheets without Mt were also prepared through the same way and were coded as BFPH1, BFPH3, BFPH4.6, BFPH10 and BFPH12.5 according to the pH values. Neat soy protein sheets without Mt and pH changing in preparation was coded as BFSPI. The nanocomposite powders and sheets were conditioned in a desiccator with P2O5 as desiccant for one week at room temperature before characterization.
3.1. Influence of pH on structure of nanocomposites Fig. 1 shows the XRD patterns for the Na+-Mt and the nanocomposite powders (PPH1, PPH3, PPH4.6, PPH10 and PPH12.5). The d001-spacing of the SPI/Mt nanocomposites prepared through solution intercalation can be calculated on the basis of Bragg's law [d001 = λ / (2sinθ)] according to the peak position. The basal spacing of pristine Na+–Mt was 1.4 nm from a diffraction peak at 2θ = 6.4o, whereas the Mt-free SPI powder shows no diffraction peak in the 2θ range from 1 to 10o. This indicates the soy protein has no ordered structure in the dimension range. The diffraction peak of the Mt is absent in the X-ray diffraction spectrum when the pH values are 3.0 and 10.0 for PPH3 and PPH10. It suggests that the Mt layers are highly disordered and exfoliated in the nanocomposite powder. From the XRD patterns, an apparent interlayer spacing of about 5.4, 3.8 and 6.3 nm has been calculated for the PPH1, PPH4.6 and PPH12.5 powders, suggesting the formation of intercalated structures of Mt The results show that the interactions between Mt and SPI are strongly influenced by the environment's pH values. The layered Mt is delaminated in the aqueous medium by the soy protein macromolecules under proper pH values, whereas the intercalated structure of Mt in soy protein matrices is dominant when the pH is too high, too low pH or at the isoelectric point of SPI. TEM can be applied to verify the extent of delamination and exfoliation achieved directly. Fig. 2 presents the TEM images of the PPH3, PPH4.6 and PPH12.5. In view of the image of Fig. 2(a), the Mt layers are well dispersed in the soy protein matrix. Although the Mt layers still retain the orientation in some degree, the tactoid structure of Mt is highly delaminated into some thin lamellas by soy protein with a
2.3. Characterization FTIR spectroscopy of was recorded on a Nicolet 5700 spectrometer 15
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prepared in the neutral aqueous media (Chen and Zhang, 2006). In this case, the heterogeneous distribution of the surface positive charges provided the possibility for negatively charged soy protein to intercalate and exfoliate the negatively charged Mt layers. The positivecharge-rich domains on the SPI molecules provide the active positivecharged sites for the replacement of Na+ and for the anchoring of the soy protein molecules into the negatively charged Mt interlayer space. Thus in the viewpoint of electrostatic interaction, the low pH value and more positive charges on the surface of SPI molecules are beneficial to the intercalation to Mt layers. Interestingly, from the results of XRD and TEM, the intercalated structure of SPI/Mt nanocomposite is maintained at the lowest pH in the series, whereas highly exfoliated structure has been obtained when pH is 3.0. This could be attributed to the competition between positively charged surface of SPI and too many H3O+ ions at over low pH value, leading to the decrease of the negative sites on the Mt layer for SPI molecules. When the pH value is changed to the isoelectric point of SPI (pH~4.6), the solubility of SPI in water decreased sharply comparing to other pH values (Renkema et al., 2002). Under this circumstance, the solvated SPI molecules reduced, which is equal to the increase of the concentration of Mt The dependence of the nanocomposite structure (exfoliated or intercalated) is highly associated on the Mt content. Therefore, explanation could be drawn that this is the reason why the intercalated structure, not the exfoliated structure formed at the isoelectric point of SPI. Furthermore, through the simulation of electrostatic surface potential distribution of three typical soy protein homotrimers aligned by A1aB1b, αc´ and β subunits (Fig. 3), it is obvious that even at high pH value (pH = 12.5), the positively charged area still exist on the surface of SPI molecules. This explains the formation of exfoliated structure when pH is 10.0. However, because the NaOH solution was used to adjust the pH of the system, there were too many Na+ ions existed in the solution. Na+ in the Mt layers is difficult to diffuse. Subsequently, only the intercalated structure was formed. In conclusion, in the pH range of 1.0–12.5, the existence of positive-charge-rich domains, more or less, is the key point of SPI molecules to intercalate or exfoliate the Mt layers. However, the final result about whether the SPI molecules enter the interlayer of Mt and exfoliate the layers is determined by the complicated factors, such as electrostatic interaction between SPI and Mt, competition between other cations and positive-charge-rich domains of SPI, and the diffusion behavior of Na+ in the layers of Na+-Mt Therefore, the intercalated structure formed when pH is too high, too low pH or at isoelectric point, and exfoliated structure obtained at proper pH values. It has been known that the amide and molecules containing C]O are able to form hydrogen bond with the polar groups on the Mt layered surfaces. Structurally, the surface of the Mt interlayer space contains Si–O–Si and –OH groups, and they could act as hydrogen bonding sites for the guest molecules. The effects of hydrogen-bond should be taken into account for the intercalation of protein molecules into the negatively charged Mt Because of the low symmetry of the dioctahedral Mt
Fig. 1. XRD patterns of Mt, SPI and PPH-series composites.
dimension of about 1–2 nm in thickness. When pH is 4.6, the layered structure of the Mt is generally maintained, and the entrance of protein molecules into the Mt layers makes the d-spacing expand from 1.4 nm to ca. 3–4 nm. When the pH is 12.5, the d-spacing of intercalated Mt layers further expand to about 6 nm. These results meet a good agreement with the results from the XRD experiments. On the basis of the evidences from XRD and TEM, it is clearly revealed that the SPI/Mt nanocomposites with an exfoliated or intercalated structure can be obtained via solution intercalation process in the aqueous media under different pH values. The results mentioned above obviously indicate that a high affinity between soy protein and Mt exists, and it is related to the environment's pH values. 3.2. Interaction between soy protein and Mt Basically, proteins have a net positive charge when the pH is lower than the isoelectric point and a net negative charge when the pH is higher than the isoelectric point. As a consequence, the pH could highly influence the properties of protein materials, including soy protein (Felix et al., 2018), gelatin (Hernandez-Izquierdo and Krochta, 2008), etc., such as film-forming, gelling, emulsifying, and foaming properties. In previous studies, exfoliated SPI/Mt nanocomposite has been
Fig. 2. TEM images of PPH3 (a), PPH4.6 (b) and PPH12.5 (c). 16
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Fig. 3. Distribution of electrostatic surface potential of three typical homotrimers of soy protein at pH = 12.5, (a) β homotrimer of β-conglycinin, (b) α´ homotrimer of β-conglycinin and (c) A1aB1b homotrimer of glycinin. The negative potential (< −1.8 kT/e) is colored in red and positive potential (> 1.8 kT/e) is colored in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. FTIR spectra at 1800–1400 cm−1 (a) and 1200–900 cm−1 (b) for Mt, SPI and PPH-series.
Fig. 6. DSC thermograms of FPH-series SPI/Mt plastics. Table 1 Experimental results from tensile testing of FPH and BFPH-series plastic sheets.
FPH1 FPH3 FPH4.6 FPH10 FPH12.5
σb (MPa)
εb (%)
7.0 8.9 8.8 7.9 4.0
10.0 22.7 30.1 36.9 170
BFPH1 BFPH3 BFPH4.6 BFPH10 BFPH12.5
σb (MPa)
εb (%)
4.0 7.2 7.8 4.6 1.3
19.3 60.5 68.3 115.5 111.0
layer, the SieO absorbance of Na+-Mt is split into four individual peaks at 1120 (peak I), 1087 (peak II), 1040 (peak III) and 1014 (peak IV) cm−1. The peaks I, III and IV are assigned to the in-plane SieO stretching bands. The peak II at 1087 cm−1 ascribed to the out-of-plane vibration is related to the orientation of the Mt layers. Especially, the disordering of the clay lamellas should cause the increase of the
Fig. 5. XRD patterns of FPH-series sheets.
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intensity of this band, and this has been well proved by the polarized attenuated total reflectance (ATR)-FTIR experiments (Ras et al., 2003). Fig. 4 shows the FTIR spectra in two typical ranges (1800–1600 cm−1 and 1300–950 cm−1) for Mt and the nanocomposite powders. The pristine Na+-Mt presents an unobvious absorbance at 1087 cm−1, whereas the out-of-plane SieO vibration absorbance become sharp and separated for the SPI/Mt nanocomposites. This distinct change means the orientation structure of Mt has been strongly perturbed by the presence of the soy protein. In addition, compared with the amide bands of the Mt-free SPI, the band of the amide II (δN–H) shifts to higher wavenumbers, whereas no obvious change occurs for the band of amide I (νC=O). In view of the obvious interference to the protein amide II absorbance and the SieO stretching peak II, it is believed that the intermolecular hydrogen bond between SPI and Mt exists in the nanocomposites. The hydrogen bonding sites may be the oxygen atoms on the silicate layer surface and the hydrogen atoms in the peptide bonds.
4. Conclusion
3.3. Influence of pH on structure and properties of SPI/Mt plastics
Acknowledgments
With 30 wt% glycerol as plasticizer, the SPI/Mt plastic sheets could be obtained through a compression-molding process. The XRD patterns of the nanocomposite plastics are presented in Fig. 5. Similar to the nanocomposites powders, there is no diffraction peak in FPH3 and FPH10. This indicates that the high-degree disordering structure of SPI/ Mt nanocomposite is maintained in the compression-molding process. And the difference is, compare to the XRD patterns of PPH1, PPH3 and PPH12.5, the d-spacing of FPH1, FPH3 and FPH12.5 have been enlarged to 6.9, 7.0 and 6.8 nm respectively, according to the lower angle shifting of the diffraction peak. This phenomenon suggests that a melting intercalation process has been involved when the nanocomposites were compression-molded leading to the enhancement of the d-spacing. The DSC thermograms of FPH3, FPH4.6, FPH10 and BFSPI are shown in Fig. 6. The Mt-free sheet (BFSPI) exhibits two glass transitions (Tg1 and Tg2) at −52.1 °C and 98.3 °C, respectively, ascribed to the glass transition temperature of the glycerol-rich domains and protein-rich domains of glycerol plasticized soy protein plastics(Chen and Zhang, 2005). The Tg2 is related to the protein chains and segments with a low compatibility to glycerol. However, in the exfoliated-structured FPH3 and FPH10 sheets, the glass transition of the glycerol-rich domains is split into two individual ones. The new glass transition (Tg3) appears at −27.0 and-18.3 °C, respectively. The occurrence of Tg3 indicates that the fine dispersion of Mt layers effectively restricts the segmental motion of the soy protein molecules on the interface of the clay layers (Lu and Nutt, 2003). There is no Tg3 formed in the intercalated-structured FPH4.6 sheet, but the Tg1 shifts to −33.1 °C. This phenomenon suggests that the segmental motion of the soy protein molecules is still restricted. This can be explained that compared to the exfoliated structure, the aspect ratio of Mt layers sharply diminished, and the restricted segments reduced. Under such circumstance, the Tg of restricted segments could not be separated individually, consequently, resulting in the increase of the whole Tg1 of the glycerol-rich domains. It is concluded that the exfoliated and intercalated Mt layers mainly exist in the glycerolrich domains of the SPI/Mt nanocomposite sheets. Table 1 presents the tensile testing results of the SPI/Mt plastic sheets. To eliminate the influence of different pH environments, the mechanical properties of Mt-free BFPH series sheets were prepared and tested at the same time. It is obvious that the tensile strength (σb) of the sheets is improved at the same pH value, which evidences the reinforcement effect of the exfoliated or intercalated Mt layers. Meanwhile, the elongation at break (εb) of FPH1~FPH10 sheets decrease. It is worth noted that the hydrogen bonds of protein molecules are destroyed mostly under strong basic environment and the protein molecule chains become stretched. So the FPH12.5 and BFPH12.5 sheets show the characteristic of elastomer. As a result, both of σb and εb of FPH12.5 are higher than those of BFPH12.5 sheets.
The authors would like to express their appreciation for the financial support from the National Natural Science Foundation of China under grant No. 51573066.
Soy protein isolate/montmorillonite nanocomposites were successfully prepared in aqueous media with the different pH values, and the SPI/Mt plastics with glycerol as plasticizer were obtained via a compression molding process. The results proved that the structures of the SPI/Mt nanocomposites and plastics were strongly depended on the pH environment in the preparation process. When pH = 1.0, 4.6 and 12.5, the resultant SPI/Mt composites were intercalated structure; when pH = 3.0 and 10.0, the exfoliated SPI/Mt nanocomposites were obtained. In the pH range from 1.0 to 12.5, the existence of positivecharge-rich domains is the key point of SPI molecules to intercalate or exfoliate the Mt layers. However, the exfoliated composites were obtained only at certain pH values. Moreover, the introduction of Mt to the SPI bulk distinctly improved the tensile strength of the nanocomposite sheets.
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