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octenylsuccinated starch incorporated with different concentrations of soybean oil
Journal Pre-proofs Preparation and evaluation of hydrophobic biodegradable films made from corn/octenylsuccinated starch incorporated with different concentrations of soybean oil Wei Gao, Wen Wu, Pengfei Liu, Hanxue Hou, Xiangyang Li, Bo Cui PII: DOI: Reference:
S0141-8130(19)35999-9 https://doi.org/10.1016/j.ijbiomac.2019.09.108 BIOMAC 13353
To appear in: Received Date: Revised Date: Accepted Date:
30 July 2019 9 September 2019 14 September 2019
Please cite this article as: W. Gao, W. Wu, P. Liu, H. Hou, X. Li, B. Cui, Preparation and evaluation of hydrophobic biodegradable films made from corn/octenylsuccinated starch incorporated with different concentrations of soybean oil, (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.09.108
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Preparation and evaluation of hydrophobic biodegradable films made from corn/octenylsuccinated starch incorporated with different concentrations of soybean oil Wei Gaoa, b, c,1, Wen Wud,1, Pengfei Liua, b, Hanxue Houc, Xiangyang Lic, Bo Cui a, b a
State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of
Technology, Shandong Academy of Sciences, Ji’nan, 250353, China b
School of Food Science and Engineering, Qilu University of Technology, Shandong Academy
of Sciences, Jinan, Shandong 250353, China c
Department of Food Science and Engineering, Shandong Agricultural University, Tai’an,
271018, China d
Department of Blood Transfusion, PLA 960 Hospital, Jinan 250031, China
1
Wei Gao and Wen Wu contributed equally to this work and are co-first authors
Abstract Corn/octenylsuccinated starch (C/OS) composite films incorporated with soybean oil (SO) at 0, 0.5%, 1.0%, 1.5% and 2.0% (w/w) were prepared to investigate their physicochemical properties. Fourier transform infrared analysis indicated that low concentrations of SO could facilitate molecular interaction and the formation of hydrogen bonds between starch molecules. All the films exhibited similar diffractograms and lower relative crystallinity values. Scanning electron microscopy and atomic force microscopy showed that the irregular and coarse surface structures of the films were obtained more frequently with increasing SO concentration. A higher contact angle of 76.14° and lower water vapor permeability of 2.46×10-12 g.cm/cm2. s.
Corresponding author. Qilu University of Technology (Shandong Academy of Sciences), Daxue Road, Changqing District, Ji’nan City, Shandong Province 250353, China. E-mail address: [email protected] (B. Cui)
Pa were obtained with increasing SO content, with the exception of the 2.0% SO sample. The highest tensile strength value of 6.54 MPa was obtained by the C/OS-1.0% SO composite film, while the optimum elongation at break of 71.84% was exhibited by the C/OS-1.5% SO composite film. Keywords: Corn starch; Octenylsuccinated starch; Soybean oil. 1. Introduction Along with the development of biodegradable materials, in the past few years more attention has been paid to natural and renewable polymers such as starch, cellulose, and gelatine [1-5]. Starch-based materials, comparatively speaking, were considered the most promising alternative to conventional plastics because of their wide sourcing, low price and thermoplastic behaviour [6-11]. However, starch-based films exhibit poor water barrier properties due to the strong hydrophilicity of starch, which greatly limits their application in food packaging [12-14]. Several attempts have been made to reduce the hygroscopicity of starch-based films in order to expand their applications. Based on our previous research, five different hydrophilic organic modified clays were used as reinforcing agents to prepare starch-based films, and a lower water vapor permeability (WVP) of 3.69×10-12 g.cm/cm2. s. Pa was obtained [7]. Colivet and Carvalho [15] found that the mechanical and water barrier properties were both significantly improved for the films prepared from cross-linked and acetylated cassava starch. According to Sun et al. [16], the films exhibited lower WVP of 3.78×10-12 g.cm/cm2. s. Pa due to the presence of low content (12%) polyhydroxyalkanoate. It was reported that the addition of waxy maize starch nanocrystals decreased the WVP of the pea starch films from 3.11 ×10-8 g.cm/cm2. s. Pa to 1.18 ×10-8 g.cm/cm2. s. Pa [17]. However, the improvement of the hydrophobic property of
the composite film was not entirely satisfactory due to the poor processing performance, compatibility and security of additives. Octenylsuccinated starch (OS starch), which exhibits a higher hydrophobicity than native starch, can be produced by esterification with octenyl succinic anhydride (OSA) [18, 19]. OS starch, as an emulsion in oil-in-water (O/W) system, could be theoretically used to prepare starch-lipid composite films. The lipid may be uniformly distributed or migrate to the surface of starch film to form a lipid-rich surface layer [20]. Thermoplastic starch foams with OS starch were obtained by the compression method, and the changes in the properties caused by the addition of hydrophobic starch were also explained [21]. Mixed octenylsuccinated/native starches at the proportion of 1:1 were successfully used to prepare sweet potato starch films, which were stretchable and exhibited better barrier properties [22]. Zhang et al. [23] prepared agar/maltodextrin composite films incorporated with hydrophobic agents, which significantly improved the physicochemical properties of films by increasing their mechanical and water vapor barrier properties. However, the eff ects of different concentrations of soybean oil (SO) on the properties of corn/octenylsuccinated starch (C/OS) composite films have hardly been investigated. The process of molecular self - assembly of film - forming components is shown in Fig.1. In our study, different concentrations of SO were incorporated into C/OS composite films. Due to the presence of OS starch, which is an emulsifier with amphiphilic properties [18], the molecular evolution of SO and starch in C/OS composite films was studied in this paper. The novelty of the research is precisely to obtain a highly hydrophobic edible film emulsified by OS starch with different concentrations of SO and to explain the changes in the properties of
C/OS composite films. 2. Experimental 2.1 Materials Corn starch was provided by Zhucheng Xingmao Corn Developing Co., Ltd. (Weifang, China). OS starch (Product No. S25500) was purchased from Yuanye Biotechnology Co., Ltd. (Shanghai, China). Glycerol was obtained from Tianjin Fuyu Chemical Co., Ltd. (Tianjin, China). SO was provided by Yihai Grain and Oil Industry Co., Ltd. (Yantai, China). 2.2 Film formation C/OS composite films were prepared by solvent casting method as reported in previous works [24, 25]. The corn starch and OS starch were blended (6 g) in a 1:1 weight ratio and mixed with 100 mL of distilled water. Glycerol (2 g) and SO were added to the above solution. The film formulations and the SO to blends ratios in the film-forming solutions are shown in Table 1. The mixture was heated in a water bath for 60 min with heating temperature and stirring speed of 90 ℃ and 400 rpm, respectively. 30g of the solution was then poured onto a round smooth plastic plates with radius 10cm. The plastic plates were placed in the oven at 50 ℃ for at least 5 h, gradually lost water and then formed a film. 2.3 Fourier transform infrared (FT-IR) analysis FT-IR spectra of C/OS composite films were recorded with a Nexus 670 spectrometer (Thermo Fisher Scientific Co., Ltd., USA) in the wavelength range of 400-4000 cm-1. The scanning resolution and number of cumulative scanning were 4 cm-1 and 32, respectively. 2.4 X-ray diffraction (XRD) Crystalline structure analysis of C/OS composite films was carried out with an X-ray
diffractometer (Bruker-AXS, Germany). The diffraction angle (2θ) ranged from 4° to 40° with a step of 0.02° and step time of 1 sec. The test voltage and current were set at 40 kV and 40 mA, respectively [26]. 2.5 Scanning electron microscopy (SEM) Microstructural analysis of C/OS composite films was performed with transmission electron microscope (ZEISS-Supra 55, Jena, Germany) at a voltage of 5 kV. The C/OS composite films were fixed on a conductive carbon tape and coated with Au/Pd. 2.6 Atomic force microscope (AFM) The surface roughness of C/OS composite films was observed by AFM (Bruker Multimode8, Madison, USA). The scan size of the sample was set at 10μm×10μm with the scanning speed of 0.977 Hz. NanoScope Analysis software was used to analyse AFM topographic image. 2.7 Contact angle (CA) Contact angle goniometer (JC2000C1, Zhongchen Digital Technology Co., Ltd, Shanghai, China) was used to measure the CA which is illustrated with a small liquid droplet resting on a flat horizontal film surface. The external image of the droplet was obtained by microlens and camera, and then the contact angle values in the image were calculated by digital image processing and some algorithms. 2.8 Mechanical properties Tensile strength (TS, MPa) and elongation at break (E, %) of the C/OS composite films were measured with a TA-XT2i texture analyzer (Stable Micro System Company, UK). All of the tested films were accurately cut into strips (15 mm × 100 mm). The initial distance between the grips was 50 mm with a test speed of 1 mm/s. Each sample was measured by six replicates
in order to obtain reliable results [25, 27]. 2.9 Water vapor permeability (WVP) Perme W3/030 Water Vapor Transmittance Tester (Labthink Instruments Co., Ltd., Jinan, China) was used to determine the WVP of C/OS composite films. Films were cut into round shapes (80 mm in diameter) using a special sampler and tested in the condition of 38.0 ℃ and 90% RH. The preheating time and weighing interval were located at 4 h and 120 min, respectively. Three separate tests were necessary for each sample.
2.10 Statistical analysis By running the SPSS 20, the test results of film properties were analyzed by performing analysis of variance (ANOVA) and Duncan’s multiple range test (p < 0.05) to evaluate mean values and differences among mean values. 3. Results and Discussion 3.1 Molecular interactions Fig. 2 shows the FTIR spectra curves of C/OS composite films with different concentrations of SO. Not surprisingly, there were some position changes, although all the FTIR spectra curves presented similar characteristics. The typical peak approximately 3290 cm-1 was attributed to the stretching vibration of O-H, which was associated with the formation of nonplasticizing hydrogen bonds in the film matrix [7, 11]. The characteristic peak at 3289 cm-1 was shifted to 3288, 3286, 3287 and 3290 cm-1, respectively, with increasing SO content from 0 to 2.0%. The results indicated that stronger hydrogen bonds between starch molecules were formed by the incorporation of certain concentrations of SO, thus facilitating formation of the network, except for 2.0% SO. The newly
derived peak around at 1745 cm-1, especially for the films with 1.0% concentration of SO, were related to the ester group band of fatty acids and triglycerides in oil [28, 29]. Interestingly, when the concentration of SO exceeded 1.0%, the intensity of characteristic peaks did not increase but decreased. This phenomenon could be attributed to the oxidation and decomposition of oil that migrated to surface of the film under higher addition during film forming process. However, an increase in peak intensity (C=O) of starch-chitosan-OEO film was completely attributed to an increase in the concentration of the oregano essential oil (OEO) [30]. The peak approximately 1645 cm-1 was a symbol of the tightly bound water, while the peak shift from 920 to 1180 cm-1 was related to C-O bonding stretch associated with plasticization [31]. As shown in Fig. 2, the peaks at 1645 cm-1 in the C/OS composite films shifted towards higher wavenumbers with the increasing concentration of SO, indicating certain concentrations of SO decreased WVP except for 2.0%. Due to the emulsification of OS starch, a certain concentration of SO was evenly distributed in the starch matrix, squeezed the space of starch molecules and mandated a tortuous pathway for water molecules to traverse the film matrix, thus decreasing the WVP. Meanwhile, as the concentrations of SO were located at 0.5 and 1.0%, the characteristic peaks shifted from 1016 cm-1 to 1015 cm-1 and 1014 cm-1, respectively. These results suggested that low concentrations of SO could facilitate molecular interaction and compatibility in the C/OS composite films due to the good emulsification performance of OS starch. 3.2 Crystalline structure The XRD diffractograms of C/OS composite films with different concentrations of SO are shown in Fig. 3. As described in the literature, corn starch showed A-type crystal structure with
peak positions appearing at 15º, 17º, 18º and 22º of 2θ [32-34]. However, the characteristic peaks disappeared for the C/OS composite films with different concentrations of SO due to the presence of OS starch and SO, indicating all the films had a much lower crystallinity and showed a more amorphous structure. The phenomenon was attributed to the steric hindrance of OSA groups that compacted starch molecules by disrupting strong intermolecular hydrogen bonds [35, 36]. In the presence of OS starch with a high degree of substitution, the polymorph structure of films was changed from A-type to V-type, thus decreasing the crystallinity. Meanwhile, the characteristic peaks of SO with low concentrations (≤1.0%) were not detected due to its good dispersion in the C/OS composite films. Due to its liquid form, there were also no characteristic peaks for SO in high concentration. Similar results were obtained by Sartori and Menegalli [10]. They prepared the unripe banana starch films incorporated with solid and liquid fatty acids which led to a disordered structure. Tang and Copeland [37] have also presented a maximum oil addition that inhibited the complex formation and exhibited an amorphous region. 3.3 Morphology and Roughness The microstructure of films is affected by the distribution of SO in the starch matrix. The micrographs of the samples incorporated with SO are shown in Fig. 4a. The control film exhibited a smooth and uniform surface without obvious incompletely broken starch granules. As expected, the irregular and coarse surface structures of the films were obtained by increasing the SO concentration. As shown in Fig. 4a, the amount of micro-starch granules or oil droplets increased, thus exhibiting a non-uniformity and heterogeneous structure on the surface of film. The presence of obvious salient points through the SEM images was attributed to the location
of
oil
droplets
or
incompletely
broken
starch
granules.
Based on
previous research by other scholars, the addition of lemon essential oil into corn and wheat starch films with different concentrations ranging from 0.5% to 2.0%, inhibited particle breakage and caused the formation of a discontinuous phase [38]. According to Acevedo-Fani et al. [39], the microstructure of alginate-based films containing essential oils was rougher than the control film. These results were attributed to the migration of oil droplets and further aggregation, flocculation during drying process of the film forming formulations [23, 38, 40]. The topographic images and roughness of films by AFM are shown in Fig. 4b. The scale of the surface roughness for all the films is in the nanometer range. The control film surface showed no residual starch granules with a lower mean roughness (Ra) of 12.2 nm and maximum height of apexes (69.9 nm). As seen in Fig. 4b, the maximum height of the apexes and Ra increased as the SO content increased, which indicated that SO did not facilitate a more homogeneous and denser film [41]. 3.4 Surface hydrophobicity Water contact angle (CA) is measured to determine the wettability of the liquid to the material surface. The low CA values are related to the better hydrophilicity of surface [42]. As shown in Fig. 5, it could be seen that the CA values were 39.89, 45.44, 58.64, 76.14 and 37.00 for SO additions of 0, 0.5%, 1.0%, 1.5%, and 2.0%, respectively. These results indicated that the different concentrations of SO significantly increased the contact angle for the films, thus improving the hydrophobic character of surface, except for 2.0% concentration of SO. In the presence of a certain concentration of SO, a stronger interaction between starch molecules can inhibit the migration of water to the surface of the film. According to Slavutsky and Bertuzzi
[43], the addition of essential lemon oil to the film exhibited an increase in contact angle and enhanced the hydrophobic character of the film surface. It was also found that the CA values significantly increased from 49.46° to 88.57° with increasing concentration of cellulose nanofibers from 0 to 20% [42]. However, when the SO concentration increased to 2.0%, it interrupted the hydrogen bonds between starch molecules, inhibited the formation of continuous phase and led to the aggregation of oil droplets in the film rather than migration to the surface, thus obtaining a lower CA value. Generally, the above results showed that the surface hydrophobicity of films was not only depended on the type and concentration of the hydrophobic agent, but also affected by the existence form in the polymer matrix. 3.5 Mechanical properties TS and E of the film are primary parameters for evaluating its performance when it is used for food packaging. It was shown in Table 2 that the mechanical properties of starch films, including TS and E, varied significantly with the concentration of soybean oil (P < 0.05). Compared to the control film of 4.38 MPa, the highest TS value of 6.54 MPa was observed, when 1.0% SO was incorporated into the film matrix. However, the values of TS decreased ranging from 6.54 MPa to 3.58 MPa, with the increase of SO content from 1.0% to 2.0%. Meanwhile, the changes in E of the films exhibited an opposite trend, with the exception of film containing 2.0% SO, which showed the lowest E value of 17.42%. The results indicated that the presence of low concentration SO (<1%) in the film matrix could facilitate the formation of a strong network structure. Due to the good emulsification performance of OS starch, low concentration SO was well distributed in the starch film matrix, which could facilitate the contact and reaction of starch molecules. A cross-linker effect was
produced by the strong interaction between the starch molecules, which decreased the molecular mobility of the starch and led to a higher value of TS and lower value of E. Meanwhile, film-forming molecules could also form entanglement and physical crossover under a certain concentration of hydrophobic agent, and thus obtaining a higher TS value. When a higher concentration of SO (>1.0%) was added to the film, the formed film exhibited a heterogeneous and non-uniform structure. The stronger polymer-polymer interactions were partly replaced by weaker polymer-oil interactions in the film network, which could usually weaken the network structure and led to a lower TS value. Moreover, the higher concentration of SO, presenting in the film in the form of oil droplets, could easily be deformed and result in an increasing E value of the film. Interestingly, the film containing 2.0% SO showed the poorest mechanical properties among all of the films, which could be attributed to phase separation between the film-forming polymer and SO. Peng et al. [44] found that the TS and E decreased by 50.0% and 62.5% for the chitosan film containing 2% of lemon essential oil, respectively, compared with the control film. However, as compared to the control film, the TS increased about 74%, 73%, and 55% for agar/maltodextrin films with 10% of beeswax, shortening, and liquid paraffin, respectively. Meanwhile, the E also showed an increasing trend [23]. The different test results should be attributed to the difference in film-forming materials, crystallinity, as well as the type and concentration of lipid component [38]. 3.6 Water vapor barrier property The WVP is an important factor to evaluate the barrier property of the film. As observed in Table 2, the water vapor barrier properties of the films were enhanced significantly (P < 0.05) by the presence of different concentrations of SO, with the exception of the 2.0% SO sample.
The WVP of control film without SO was 2.93×10-12 g.cm/cm2. s. Pa. The WVP of the composite films gradually decreased from 2.72×10-12 g.cm/cm2. s. Pa to 2.46×10-12 g.cm/cm2. s. Pa, when the SO concentration increased from 0.5% to 1.5%. From these results, it can also be seen that the WVP of film containing 2.0% SO exhibited the highest WVP value of 3.03×1012
g.cm/cm2. s. Pa. The ability of starch film to bind water is weakened by the incorporation of
SO due to its hydrophobic property. It is the dispersion of SO in starch matrix that results in discontinuities in the hydrophilic phase, increases the effective path length for diffusion and decreases the film’s WVP. It is interesting that a higher concentration of SO (2.0%) could interrupt the hydrogen bonds between the starch molecules, destroy the network structure and expand the molecular interstice of the films, thus leading to the highest WVP value. Two kinds of starch films were prepared in our previous studies, for which the WVP values were 3.69×1012
g.cm/cm2. s. Pa and 4.78×10-12 g.cm/cm2. s. Pa, respectively [7, 45]. By contrast, a
hydrophobic biodegradable film exhibiting a lower WVP value of 2.46×10-12 g.cm/cm2. s. Pa was obtained due to the incorporation of different concentrations of SO in this study. According to the statistical results of some scholars mentioned in introduction, the addition of SO could enhance the water resistance of starch film more effectively than other treatments methods [16, 17]. For instance, essential oil was also regarded as one of the hydrophobic agents that can improve the water vapor barrier properties of polymer-based films due to its hydrophobic nature. The effects of essential oil on the moisture barrier properties of the films were investigated by many researchers. The values of WVP ranged from 0.75×10-11 g.cm/cm2. s. Pa to 1.01×10-11 g.cm/cm2. s. Pa for the chitosan-tea tree essential oil composite films compared to the control film, for which the WVP value was 1.24×10-11 g.cm/cm2. s. Pa [46]. Vargas et al. [47]
investigated the water vapor barrier properties of chitosan films with or without essential oils, and found that the WVP value of the film with higher concentration of essential oil (0.76×1011
g.cm/cm2. s. Pa) was lowered than the control film (1.65×10-11g.cm/cm2. s. Pa). Meanwhile,
the incorporation of oregano oil or mustard essential oil into film-forming matrix had no remarkable effect on the WVP of the films [48, 49]. In general, water vapor barrier properties of films are determined by the type and content of lipids, as well as their physical state, extent of distribution, crystallinity, and the interactions with the other film components [50]. In comparison, SO seems to be more suitable as a hydrophobic agent to improve the water vapor barrier properties of starch films due to its low price and wide sources. 4. Conclusion The properties of C/OS composite films were observed to be significantly affected by the incorporation of different concentrations of SO. The presence of SO gradually improved the water barrier properties and surface hydrophobicity, with the exception of 2.0% SO. In the presence of OS starch, low concentrations of SO facilitated molecular interaction and the formation of hydrogen bonds between starch molecules, thus increasing the TS value. Different concentrations of SO had a negative effect on the E of the films except for 1.5% SO. The surface of film was nonuniform and coarse by the incorporation of SO. The incorporation of SO into C/OS composite films had no significant effect on the diffractograms and relative crystallinity of the films. A higher concentration (2.0%) of SO into the C/OS composite film significantly weakened its comprehensive properties. Acknowledgements The authors appreciate the financial support of the Key Research and Development Program
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nanoemulsions:
Physicochemical
characterization
and
antimicrobial properties, Food Hydrocoll. 47 (2015) 168-177. [40] L. Sánchez-González, M. Cháfer, M. Hernandez, A. Chiralt, C. González-Martínez, Antimicrobial activity of polysaccharide films containing essential oils, Food Control 22 (2011) 1302-1310.
[41] L.M. Dai, J. Zhang, F. Cheng, Effects of starches from different botanical sources and modification methods on physicochemical properties of starch-based edible films, Int. J. Biol. Macromol. 132 (2019) 897-905. [42] M. Li, X. Tian, R.F. Jin, D. Li, Preparation and characterization of nanocomposite films containing starch and cellulose nanofibers, Ind. Crops Prod. 123 (2018) 654-660. [43] A.M. Slavutsky, M.A. Bertuzzi, Improvement of water barrier properties of starch films by lipid nanolamination, Food Packaging Shelf. 7 (2016) 41-46. [44] Y. Peng, L. Yin, Y.F. Li, Combined effects of lemon essential oil and surfactants on physical and structural properties of chitosan films, Int. J. Food Sci. Tech. 48, (2012) 44-50. [45] W. Gao, P.F. Liu, X.Y. Li, L.Z. Qiu, H.X. Hou, B. Cui, The co-plasticization effects of glycerol and small molecular sugars on starch-based nanocomposite films prepared by extrusion blowing, Int. J. Biol. Macromol. 133, (2019) 1175-1181. [46] L. Sánchez-González, C. González-Martínez, A. Chiralt, M. Cháfer, Physical and antimicrobial properties of chitosan-tea tree essential oil composite films, J. Food Eng. 98 (2010) 443-452. [47] M. Vargas, A. Albors, A. Chiralt, C. Gonza´lez-Martı´nez, Characterization of chitosan– oleic acid composite films, Food Hydrocoll. 23 (2009) 536-547. [48] G. Chen, B. Liu, Cellulose sulfate-based film with slow-release antimicrobial properties prepared by incorporation of mustard essential oil and β-cyclodextrin, Food Hydrocoll. 55 (2016) 100-107. [49] M.A. Rojas-Grau, R.J. Avena-Bustillos, C. Olsen, M. Friedman, P.R. Henika, O. MartinBelloso, Z.L. Pan, T.H. McHugh, Effects of plant essential oils and oil compounds on
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Table 1 Formulation of C/OS composite films with different concentrations of SO. Specimen
Corn starch (g)
OS starch (g)
Soybean oil (g)
Glycerol (g)
Proportion of soybean oil in solid blends (%)
C/OS
3
3
0
2
0
C/OS-0.5% SO
3
3
0.03
2
0.5
C/OS-1.0% SO
3
3
0.06
2
1.0
C/OS-1.5% SO
3
3
0.09
2
1.5
C/OS-2.0% SO
3
3
0.12
2
2.0
Table 2 Mechanical properties and WVP of C/OS composite films with different concentrations of SO. Specimen
Proportion of SO in solid blends (%)
TS (MPa)
E (%)
WVP (10-12g.cm/cm2. s. Pa.)
0
4.38±0.07c
45.67±4.11b
2.93±0.10ab
C/OS-0.5% SO
0.5
5.21±0.20b
36.54±6.36b
2.72±0.13bc
C/OS-1.0% SO
1.0
6.54±0.25a
26.31±4.35c
2.67±0.14cd
C/OS-1.5% SO
1.5
3.70±0.11d
71.84±6.31a
2.46±0.04d
C/OS-2.0% SO
2.0
3.58±0.19d
17.42±5.59c
3.03±0.15a
C/OS
a–d
Different lowercase letters in the same column indicated significant differences (p < 0.05).
Figure captions Fig. 1. Process of molecular self - assembly of film - forming components. Fig. 2. FT-IR spectra of C/OS composite films with different concentrations of SO. Fig. 3. XRD diffractograms of C/OS composite films with different concentrations of SO. Fig. 4a. SEM micrographs of C/OS composite films with different concentrations of SO. Fig. 4b. AFM images of C/OS composite films with different concentrations of SO. Fig. 5. Water contact angles of C/OS composite films with different concentrations of SO.
Fig. 1. Process of molecular self - assembly of film - forming components.
Fig. 2. FT-IR spectra of C/OS composite films with different concentrations of SO.
Fig. 3. XRD diffractograms of C/OS composite films with different concentrations of SO.
C/OS
C/OS-0.5%SO
C/OS-1.0%SO
C/OS-1.5%SO
C/OS-2.0%SO Fig. 4a. SEM micrographs of C/OS composite films with different concentrations of SO.
C/OS
C/OS-1.0%SO
C/OS-0.5%SO
C/OS-1.5%SO
C/OS-2.0%SO Fig. 4b. AFM images of C/OS composite films with different concentrations of SO.
C/OS
C/OS-0.5%SO
C/OS-1.0%SO
C/OS-1.5%SO
C/OS-2.0%SO Fig. 5. Water contact angles of C/OS composite films with different concentrations of SO.
Highlights
Octenylsuccinated starch could facilitate the distribution of soybean oil in film forming matrix.
The barrier and mechanical properties of corn/octenylsuccinated starch composite films were significantly improved due to the incorporation of certain concentration soybean oil.
High concentration of soybean oil had a negative effect on physicochemical properties of the corn/octenylsuccinated starch composite films.
Graphical abstract
S0141-8130(19)35999-9 https://doi.org/10.1016/j.ijbiomac.2019.09.108 BIOMAC 13353
To appear in: Received Date: Revised Date: Accepted Date:
30 July 2019 9 September 2019 14 September 2019
Please cite this article as: W. Gao, W. Wu, P. Liu, H. Hou, X. Li, B. Cui, Preparation and evaluation of hydrophobic biodegradable films made from corn/octenylsuccinated starch incorporated with different concentrations of soybean oil, (2019), doi: https://doi.org/10.1016/j.ijbiomac.2019.09.108
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Preparation and evaluation of hydrophobic biodegradable films made from corn/octenylsuccinated starch incorporated with different concentrations of soybean oil Wei Gaoa, b, c,1, Wen Wud,1, Pengfei Liua, b, Hanxue Houc, Xiangyang Lic, Bo Cui a, b a
State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of
Technology, Shandong Academy of Sciences, Ji’nan, 250353, China b
School of Food Science and Engineering, Qilu University of Technology, Shandong Academy
of Sciences, Jinan, Shandong 250353, China c
Department of Food Science and Engineering, Shandong Agricultural University, Tai’an,
271018, China d
Department of Blood Transfusion, PLA 960 Hospital, Jinan 250031, China
1
Wei Gao and Wen Wu contributed equally to this work and are co-first authors
Abstract Corn/octenylsuccinated starch (C/OS) composite films incorporated with soybean oil (SO) at 0, 0.5%, 1.0%, 1.5% and 2.0% (w/w) were prepared to investigate their physicochemical properties. Fourier transform infrared analysis indicated that low concentrations of SO could facilitate molecular interaction and the formation of hydrogen bonds between starch molecules. All the films exhibited similar diffractograms and lower relative crystallinity values. Scanning electron microscopy and atomic force microscopy showed that the irregular and coarse surface structures of the films were obtained more frequently with increasing SO concentration. A higher contact angle of 76.14° and lower water vapor permeability of 2.46×10-12 g.cm/cm2. s.
Corresponding author. Qilu University of Technology (Shandong Academy of Sciences), Daxue Road, Changqing District, Ji’nan City, Shandong Province 250353, China. E-mail address: [email protected] (B. Cui)
Pa were obtained with increasing SO content, with the exception of the 2.0% SO sample. The highest tensile strength value of 6.54 MPa was obtained by the C/OS-1.0% SO composite film, while the optimum elongation at break of 71.84% was exhibited by the C/OS-1.5% SO composite film. Keywords: Corn starch; Octenylsuccinated starch; Soybean oil. 1. Introduction Along with the development of biodegradable materials, in the past few years more attention has been paid to natural and renewable polymers such as starch, cellulose, and gelatine [1-5]. Starch-based materials, comparatively speaking, were considered the most promising alternative to conventional plastics because of their wide sourcing, low price and thermoplastic behaviour [6-11]. However, starch-based films exhibit poor water barrier properties due to the strong hydrophilicity of starch, which greatly limits their application in food packaging [12-14]. Several attempts have been made to reduce the hygroscopicity of starch-based films in order to expand their applications. Based on our previous research, five different hydrophilic organic modified clays were used as reinforcing agents to prepare starch-based films, and a lower water vapor permeability (WVP) of 3.69×10-12 g.cm/cm2. s. Pa was obtained [7]. Colivet and Carvalho [15] found that the mechanical and water barrier properties were both significantly improved for the films prepared from cross-linked and acetylated cassava starch. According to Sun et al. [16], the films exhibited lower WVP of 3.78×10-12 g.cm/cm2. s. Pa due to the presence of low content (12%) polyhydroxyalkanoate. It was reported that the addition of waxy maize starch nanocrystals decreased the WVP of the pea starch films from 3.11 ×10-8 g.cm/cm2. s. Pa to 1.18 ×10-8 g.cm/cm2. s. Pa [17]. However, the improvement of the hydrophobic property of
the composite film was not entirely satisfactory due to the poor processing performance, compatibility and security of additives. Octenylsuccinated starch (OS starch), which exhibits a higher hydrophobicity than native starch, can be produced by esterification with octenyl succinic anhydride (OSA) [18, 19]. OS starch, as an emulsion in oil-in-water (O/W) system, could be theoretically used to prepare starch-lipid composite films. The lipid may be uniformly distributed or migrate to the surface of starch film to form a lipid-rich surface layer [20]. Thermoplastic starch foams with OS starch were obtained by the compression method, and the changes in the properties caused by the addition of hydrophobic starch were also explained [21]. Mixed octenylsuccinated/native starches at the proportion of 1:1 were successfully used to prepare sweet potato starch films, which were stretchable and exhibited better barrier properties [22]. Zhang et al. [23] prepared agar/maltodextrin composite films incorporated with hydrophobic agents, which significantly improved the physicochemical properties of films by increasing their mechanical and water vapor barrier properties. However, the eff ects of different concentrations of soybean oil (SO) on the properties of corn/octenylsuccinated starch (C/OS) composite films have hardly been investigated. The process of molecular self - assembly of film - forming components is shown in Fig.1. In our study, different concentrations of SO were incorporated into C/OS composite films. Due to the presence of OS starch, which is an emulsifier with amphiphilic properties [18], the molecular evolution of SO and starch in C/OS composite films was studied in this paper. The novelty of the research is precisely to obtain a highly hydrophobic edible film emulsified by OS starch with different concentrations of SO and to explain the changes in the properties of
C/OS composite films. 2. Experimental 2.1 Materials Corn starch was provided by Zhucheng Xingmao Corn Developing Co., Ltd. (Weifang, China). OS starch (Product No. S25500) was purchased from Yuanye Biotechnology Co., Ltd. (Shanghai, China). Glycerol was obtained from Tianjin Fuyu Chemical Co., Ltd. (Tianjin, China). SO was provided by Yihai Grain and Oil Industry Co., Ltd. (Yantai, China). 2.2 Film formation C/OS composite films were prepared by solvent casting method as reported in previous works [24, 25]. The corn starch and OS starch were blended (6 g) in a 1:1 weight ratio and mixed with 100 mL of distilled water. Glycerol (2 g) and SO were added to the above solution. The film formulations and the SO to blends ratios in the film-forming solutions are shown in Table 1. The mixture was heated in a water bath for 60 min with heating temperature and stirring speed of 90 ℃ and 400 rpm, respectively. 30g of the solution was then poured onto a round smooth plastic plates with radius 10cm. The plastic plates were placed in the oven at 50 ℃ for at least 5 h, gradually lost water and then formed a film. 2.3 Fourier transform infrared (FT-IR) analysis FT-IR spectra of C/OS composite films were recorded with a Nexus 670 spectrometer (Thermo Fisher Scientific Co., Ltd., USA) in the wavelength range of 400-4000 cm-1. The scanning resolution and number of cumulative scanning were 4 cm-1 and 32, respectively. 2.4 X-ray diffraction (XRD) Crystalline structure analysis of C/OS composite films was carried out with an X-ray
diffractometer (Bruker-AXS, Germany). The diffraction angle (2θ) ranged from 4° to 40° with a step of 0.02° and step time of 1 sec. The test voltage and current were set at 40 kV and 40 mA, respectively [26]. 2.5 Scanning electron microscopy (SEM) Microstructural analysis of C/OS composite films was performed with transmission electron microscope (ZEISS-Supra 55, Jena, Germany) at a voltage of 5 kV. The C/OS composite films were fixed on a conductive carbon tape and coated with Au/Pd. 2.6 Atomic force microscope (AFM) The surface roughness of C/OS composite films was observed by AFM (Bruker Multimode8, Madison, USA). The scan size of the sample was set at 10μm×10μm with the scanning speed of 0.977 Hz. NanoScope Analysis software was used to analyse AFM topographic image. 2.7 Contact angle (CA) Contact angle goniometer (JC2000C1, Zhongchen Digital Technology Co., Ltd, Shanghai, China) was used to measure the CA which is illustrated with a small liquid droplet resting on a flat horizontal film surface. The external image of the droplet was obtained by microlens and camera, and then the contact angle values in the image were calculated by digital image processing and some algorithms. 2.8 Mechanical properties Tensile strength (TS, MPa) and elongation at break (E, %) of the C/OS composite films were measured with a TA-XT2i texture analyzer (Stable Micro System Company, UK). All of the tested films were accurately cut into strips (15 mm × 100 mm). The initial distance between the grips was 50 mm with a test speed of 1 mm/s. Each sample was measured by six replicates
in order to obtain reliable results [25, 27]. 2.9 Water vapor permeability (WVP) Perme W3/030 Water Vapor Transmittance Tester (Labthink Instruments Co., Ltd., Jinan, China) was used to determine the WVP of C/OS composite films. Films were cut into round shapes (80 mm in diameter) using a special sampler and tested in the condition of 38.0 ℃ and 90% RH. The preheating time and weighing interval were located at 4 h and 120 min, respectively. Three separate tests were necessary for each sample.
2.10 Statistical analysis By running the SPSS 20, the test results of film properties were analyzed by performing analysis of variance (ANOVA) and Duncan’s multiple range test (p < 0.05) to evaluate mean values and differences among mean values. 3. Results and Discussion 3.1 Molecular interactions Fig. 2 shows the FTIR spectra curves of C/OS composite films with different concentrations of SO. Not surprisingly, there were some position changes, although all the FTIR spectra curves presented similar characteristics. The typical peak approximately 3290 cm-1 was attributed to the stretching vibration of O-H, which was associated with the formation of nonplasticizing hydrogen bonds in the film matrix [7, 11]. The characteristic peak at 3289 cm-1 was shifted to 3288, 3286, 3287 and 3290 cm-1, respectively, with increasing SO content from 0 to 2.0%. The results indicated that stronger hydrogen bonds between starch molecules were formed by the incorporation of certain concentrations of SO, thus facilitating formation of the network, except for 2.0% SO. The newly
derived peak around at 1745 cm-1, especially for the films with 1.0% concentration of SO, were related to the ester group band of fatty acids and triglycerides in oil [28, 29]. Interestingly, when the concentration of SO exceeded 1.0%, the intensity of characteristic peaks did not increase but decreased. This phenomenon could be attributed to the oxidation and decomposition of oil that migrated to surface of the film under higher addition during film forming process. However, an increase in peak intensity (C=O) of starch-chitosan-OEO film was completely attributed to an increase in the concentration of the oregano essential oil (OEO) [30]. The peak approximately 1645 cm-1 was a symbol of the tightly bound water, while the peak shift from 920 to 1180 cm-1 was related to C-O bonding stretch associated with plasticization [31]. As shown in Fig. 2, the peaks at 1645 cm-1 in the C/OS composite films shifted towards higher wavenumbers with the increasing concentration of SO, indicating certain concentrations of SO decreased WVP except for 2.0%. Due to the emulsification of OS starch, a certain concentration of SO was evenly distributed in the starch matrix, squeezed the space of starch molecules and mandated a tortuous pathway for water molecules to traverse the film matrix, thus decreasing the WVP. Meanwhile, as the concentrations of SO were located at 0.5 and 1.0%, the characteristic peaks shifted from 1016 cm-1 to 1015 cm-1 and 1014 cm-1, respectively. These results suggested that low concentrations of SO could facilitate molecular interaction and compatibility in the C/OS composite films due to the good emulsification performance of OS starch. 3.2 Crystalline structure The XRD diffractograms of C/OS composite films with different concentrations of SO are shown in Fig. 3. As described in the literature, corn starch showed A-type crystal structure with
peak positions appearing at 15º, 17º, 18º and 22º of 2θ [32-34]. However, the characteristic peaks disappeared for the C/OS composite films with different concentrations of SO due to the presence of OS starch and SO, indicating all the films had a much lower crystallinity and showed a more amorphous structure. The phenomenon was attributed to the steric hindrance of OSA groups that compacted starch molecules by disrupting strong intermolecular hydrogen bonds [35, 36]. In the presence of OS starch with a high degree of substitution, the polymorph structure of films was changed from A-type to V-type, thus decreasing the crystallinity. Meanwhile, the characteristic peaks of SO with low concentrations (≤1.0%) were not detected due to its good dispersion in the C/OS composite films. Due to its liquid form, there were also no characteristic peaks for SO in high concentration. Similar results were obtained by Sartori and Menegalli [10]. They prepared the unripe banana starch films incorporated with solid and liquid fatty acids which led to a disordered structure. Tang and Copeland [37] have also presented a maximum oil addition that inhibited the complex formation and exhibited an amorphous region. 3.3 Morphology and Roughness The microstructure of films is affected by the distribution of SO in the starch matrix. The micrographs of the samples incorporated with SO are shown in Fig. 4a. The control film exhibited a smooth and uniform surface without obvious incompletely broken starch granules. As expected, the irregular and coarse surface structures of the films were obtained by increasing the SO concentration. As shown in Fig. 4a, the amount of micro-starch granules or oil droplets increased, thus exhibiting a non-uniformity and heterogeneous structure on the surface of film. The presence of obvious salient points through the SEM images was attributed to the location
of
oil
droplets
or
incompletely
broken
starch
granules.
Based on
previous research by other scholars, the addition of lemon essential oil into corn and wheat starch films with different concentrations ranging from 0.5% to 2.0%, inhibited particle breakage and caused the formation of a discontinuous phase [38]. According to Acevedo-Fani et al. [39], the microstructure of alginate-based films containing essential oils was rougher than the control film. These results were attributed to the migration of oil droplets and further aggregation, flocculation during drying process of the film forming formulations [23, 38, 40]. The topographic images and roughness of films by AFM are shown in Fig. 4b. The scale of the surface roughness for all the films is in the nanometer range. The control film surface showed no residual starch granules with a lower mean roughness (Ra) of 12.2 nm and maximum height of apexes (69.9 nm). As seen in Fig. 4b, the maximum height of the apexes and Ra increased as the SO content increased, which indicated that SO did not facilitate a more homogeneous and denser film [41]. 3.4 Surface hydrophobicity Water contact angle (CA) is measured to determine the wettability of the liquid to the material surface. The low CA values are related to the better hydrophilicity of surface [42]. As shown in Fig. 5, it could be seen that the CA values were 39.89, 45.44, 58.64, 76.14 and 37.00 for SO additions of 0, 0.5%, 1.0%, 1.5%, and 2.0%, respectively. These results indicated that the different concentrations of SO significantly increased the contact angle for the films, thus improving the hydrophobic character of surface, except for 2.0% concentration of SO. In the presence of a certain concentration of SO, a stronger interaction between starch molecules can inhibit the migration of water to the surface of the film. According to Slavutsky and Bertuzzi
[43], the addition of essential lemon oil to the film exhibited an increase in contact angle and enhanced the hydrophobic character of the film surface. It was also found that the CA values significantly increased from 49.46° to 88.57° with increasing concentration of cellulose nanofibers from 0 to 20% [42]. However, when the SO concentration increased to 2.0%, it interrupted the hydrogen bonds between starch molecules, inhibited the formation of continuous phase and led to the aggregation of oil droplets in the film rather than migration to the surface, thus obtaining a lower CA value. Generally, the above results showed that the surface hydrophobicity of films was not only depended on the type and concentration of the hydrophobic agent, but also affected by the existence form in the polymer matrix. 3.5 Mechanical properties TS and E of the film are primary parameters for evaluating its performance when it is used for food packaging. It was shown in Table 2 that the mechanical properties of starch films, including TS and E, varied significantly with the concentration of soybean oil (P < 0.05). Compared to the control film of 4.38 MPa, the highest TS value of 6.54 MPa was observed, when 1.0% SO was incorporated into the film matrix. However, the values of TS decreased ranging from 6.54 MPa to 3.58 MPa, with the increase of SO content from 1.0% to 2.0%. Meanwhile, the changes in E of the films exhibited an opposite trend, with the exception of film containing 2.0% SO, which showed the lowest E value of 17.42%. The results indicated that the presence of low concentration SO (<1%) in the film matrix could facilitate the formation of a strong network structure. Due to the good emulsification performance of OS starch, low concentration SO was well distributed in the starch film matrix, which could facilitate the contact and reaction of starch molecules. A cross-linker effect was
produced by the strong interaction between the starch molecules, which decreased the molecular mobility of the starch and led to a higher value of TS and lower value of E. Meanwhile, film-forming molecules could also form entanglement and physical crossover under a certain concentration of hydrophobic agent, and thus obtaining a higher TS value. When a higher concentration of SO (>1.0%) was added to the film, the formed film exhibited a heterogeneous and non-uniform structure. The stronger polymer-polymer interactions were partly replaced by weaker polymer-oil interactions in the film network, which could usually weaken the network structure and led to a lower TS value. Moreover, the higher concentration of SO, presenting in the film in the form of oil droplets, could easily be deformed and result in an increasing E value of the film. Interestingly, the film containing 2.0% SO showed the poorest mechanical properties among all of the films, which could be attributed to phase separation between the film-forming polymer and SO. Peng et al. [44] found that the TS and E decreased by 50.0% and 62.5% for the chitosan film containing 2% of lemon essential oil, respectively, compared with the control film. However, as compared to the control film, the TS increased about 74%, 73%, and 55% for agar/maltodextrin films with 10% of beeswax, shortening, and liquid paraffin, respectively. Meanwhile, the E also showed an increasing trend [23]. The different test results should be attributed to the difference in film-forming materials, crystallinity, as well as the type and concentration of lipid component [38]. 3.6 Water vapor barrier property The WVP is an important factor to evaluate the barrier property of the film. As observed in Table 2, the water vapor barrier properties of the films were enhanced significantly (P < 0.05) by the presence of different concentrations of SO, with the exception of the 2.0% SO sample.
The WVP of control film without SO was 2.93×10-12 g.cm/cm2. s. Pa. The WVP of the composite films gradually decreased from 2.72×10-12 g.cm/cm2. s. Pa to 2.46×10-12 g.cm/cm2. s. Pa, when the SO concentration increased from 0.5% to 1.5%. From these results, it can also be seen that the WVP of film containing 2.0% SO exhibited the highest WVP value of 3.03×1012
g.cm/cm2. s. Pa. The ability of starch film to bind water is weakened by the incorporation of
SO due to its hydrophobic property. It is the dispersion of SO in starch matrix that results in discontinuities in the hydrophilic phase, increases the effective path length for diffusion and decreases the film’s WVP. It is interesting that a higher concentration of SO (2.0%) could interrupt the hydrogen bonds between the starch molecules, destroy the network structure and expand the molecular interstice of the films, thus leading to the highest WVP value. Two kinds of starch films were prepared in our previous studies, for which the WVP values were 3.69×1012
g.cm/cm2. s. Pa and 4.78×10-12 g.cm/cm2. s. Pa, respectively [7, 45]. By contrast, a
hydrophobic biodegradable film exhibiting a lower WVP value of 2.46×10-12 g.cm/cm2. s. Pa was obtained due to the incorporation of different concentrations of SO in this study. According to the statistical results of some scholars mentioned in introduction, the addition of SO could enhance the water resistance of starch film more effectively than other treatments methods [16, 17]. For instance, essential oil was also regarded as one of the hydrophobic agents that can improve the water vapor barrier properties of polymer-based films due to its hydrophobic nature. The effects of essential oil on the moisture barrier properties of the films were investigated by many researchers. The values of WVP ranged from 0.75×10-11 g.cm/cm2. s. Pa to 1.01×10-11 g.cm/cm2. s. Pa for the chitosan-tea tree essential oil composite films compared to the control film, for which the WVP value was 1.24×10-11 g.cm/cm2. s. Pa [46]. Vargas et al. [47]
investigated the water vapor barrier properties of chitosan films with or without essential oils, and found that the WVP value of the film with higher concentration of essential oil (0.76×1011
g.cm/cm2. s. Pa) was lowered than the control film (1.65×10-11g.cm/cm2. s. Pa). Meanwhile,
the incorporation of oregano oil or mustard essential oil into film-forming matrix had no remarkable effect on the WVP of the films [48, 49]. In general, water vapor barrier properties of films are determined by the type and content of lipids, as well as their physical state, extent of distribution, crystallinity, and the interactions with the other film components [50]. In comparison, SO seems to be more suitable as a hydrophobic agent to improve the water vapor barrier properties of starch films due to its low price and wide sources. 4. Conclusion The properties of C/OS composite films were observed to be significantly affected by the incorporation of different concentrations of SO. The presence of SO gradually improved the water barrier properties and surface hydrophobicity, with the exception of 2.0% SO. In the presence of OS starch, low concentrations of SO facilitated molecular interaction and the formation of hydrogen bonds between starch molecules, thus increasing the TS value. Different concentrations of SO had a negative effect on the E of the films except for 1.5% SO. The surface of film was nonuniform and coarse by the incorporation of SO. The incorporation of SO into C/OS composite films had no significant effect on the diffractograms and relative crystallinity of the films. A higher concentration (2.0%) of SO into the C/OS composite film significantly weakened its comprehensive properties. Acknowledgements The authors appreciate the financial support of the Key Research and Development Program
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Table 1 Formulation of C/OS composite films with different concentrations of SO. Specimen
Corn starch (g)
OS starch (g)
Soybean oil (g)
Glycerol (g)
Proportion of soybean oil in solid blends (%)
C/OS
3
3
0
2
0
C/OS-0.5% SO
3
3
0.03
2
0.5
C/OS-1.0% SO
3
3
0.06
2
1.0
C/OS-1.5% SO
3
3
0.09
2
1.5
C/OS-2.0% SO
3
3
0.12
2
2.0
Table 2 Mechanical properties and WVP of C/OS composite films with different concentrations of SO. Specimen
Proportion of SO in solid blends (%)
TS (MPa)
E (%)
WVP (10-12g.cm/cm2. s. Pa.)
0
4.38±0.07c
45.67±4.11b
2.93±0.10ab
C/OS-0.5% SO
0.5
5.21±0.20b
36.54±6.36b
2.72±0.13bc
C/OS-1.0% SO
1.0
6.54±0.25a
26.31±4.35c
2.67±0.14cd
C/OS-1.5% SO
1.5
3.70±0.11d
71.84±6.31a
2.46±0.04d
C/OS-2.0% SO
2.0
3.58±0.19d
17.42±5.59c
3.03±0.15a
C/OS
a–d
Different lowercase letters in the same column indicated significant differences (p < 0.05).
Figure captions Fig. 1. Process of molecular self - assembly of film - forming components. Fig. 2. FT-IR spectra of C/OS composite films with different concentrations of SO. Fig. 3. XRD diffractograms of C/OS composite films with different concentrations of SO. Fig. 4a. SEM micrographs of C/OS composite films with different concentrations of SO. Fig. 4b. AFM images of C/OS composite films with different concentrations of SO. Fig. 5. Water contact angles of C/OS composite films with different concentrations of SO.
Fig. 1. Process of molecular self - assembly of film - forming components.
Fig. 2. FT-IR spectra of C/OS composite films with different concentrations of SO.
Fig. 3. XRD diffractograms of C/OS composite films with different concentrations of SO.
C/OS
C/OS-0.5%SO
C/OS-1.0%SO
C/OS-1.5%SO
C/OS-2.0%SO Fig. 4a. SEM micrographs of C/OS composite films with different concentrations of SO.
C/OS
C/OS-1.0%SO
C/OS-0.5%SO
C/OS-1.5%SO
C/OS-2.0%SO Fig. 4b. AFM images of C/OS composite films with different concentrations of SO.
C/OS
C/OS-0.5%SO
C/OS-1.0%SO
C/OS-1.5%SO
C/OS-2.0%SO Fig. 5. Water contact angles of C/OS composite films with different concentrations of SO.
Highlights
Octenylsuccinated starch could facilitate the distribution of soybean oil in film forming matrix.
The barrier and mechanical properties of corn/octenylsuccinated starch composite films were significantly improved due to the incorporation of certain concentration soybean oil.
High concentration of soybean oil had a negative effect on physicochemical properties of the corn/octenylsuccinated starch composite films.
Graphical abstract