Enhanced stability and mechanical strength of sodium alginate composite films

Accepted Manuscript Title: Enhanced stability and mechanical strength of sodium alginate composite films Author: Sijun Liu Yong Li Lin Li PII: DOI: Reference:

S0144-8617(16)31425-4 http://dx.doi.org/doi:10.1016/j.carbpol.2016.12.048 CARP 11855

To appear in: Received date: Revised date: Accepted date:

22-1-2016 28-10-2016 19-12-2016

Please cite this article as: Liu, Sijun., Li, Yong., & Li, Lin., Enhanced stability and mechanical strength of sodium alginate composite films.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.12.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced stability and mechanical strength of sodium alginate composite films Sijun Liu, Yong Li, Lin Li* School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

*

Corresponding author. E-mail addresses: [email protected] (L. Li). 1

Highlights 

Three types of nanofillers: graphene oxide (GO), ammonia functionalized graphene oxide (AGO), and triethoxylpropylaminosilane functionalized silica, were respectively added to an aqueous solution of sodium alginate (SA) to prepare SA composite films.



The thermal and wet stability and mechanical strength of SA composite films strongly depended on the interaction between SA and filler.



Both GO and AGO were able to improve stability of SA films at high temperatures and in a wet environment as well as mechanical strength.



AGO is the best filler that could significantly enhance the interaction between AGO and SA, which led to a great increase in stability and mechanical strength of SA composite films, rather GO and silica.

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Abstract This work aims to study how three kinds of nanofillers: graphene oxide (GO), ammonia functionalized graphene oxide (AGO), and triethoxylpropylaminosilane functionalized silica, can affect stability and mechanical strength of sodium alginate (SA) composite films. The filler/sodium alginate (SA) solutions were first studied by rheology to reveal effects of various fillers on zero shear viscosity η0. SA composite films were then prepared by a solution mixing-evaporation method. The structure, morphology and properties of SA composite films were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), contact angle and mechanical testing. Compared to GO and silica, the presence of AGO significantly improved the interaction between AGO and SA, which led to the increase in stability and mechanical strength of the resulting SA composite films. The tensile strength and elongation at break of AGO/SA composite film at 3 wt% AGO loading were increased by 114.9 % and 194.4 %, respectively, in contrast to pure SA film. Furthermore, the stability of AGO/SA composite films at high temperatures and in a wet environment were better than that of silica/SA and GO/SA composite films.

Keywords: Sodium alginate, silica, graphene oxide, composite film, stability

1. Introduction Sodium alginate (SA) is a natural biopolymer extracted from various species of brown seaweed (Gacesa, 1988). It is known as a linear copolymer containing (1 → 4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues respectively, covalently linked together in different sequences (Tallis, 1950). The monomers can appear as homopolymeric blocks of consecutive G units (GG), consecutive M units (MM), and alternating M and G units (GM). SA extracted from different sources differs in M and G contents as well as the length of each block. The G blocks are stiffer and more extended in chain configuration than the M blocks due to a higher degree of hindered rotation around the glycosidic linkages (Matsumoto, Kawai & Masuda, 1992; Sinquin, Hubert & Dellacherie, 1993). In aqueous solution, SA is usually used as a thickening agent to increase the viscosity of a medium. In the presence of divalent cations (e.g., Ca2+, Cu2+, Ba2+, Sr2+, Mg2+), the carboxyl groups of 3

SA give it the ability to undergo a sol-gel transition (Braccini & Pérez, 2001; Lu, Liu & Tong, 2006; Stokke, Draget, Smidsrd, Yuguchi, Urakawa & Kajiwara, 2000). Besides SA hydrogels formed by ionic cross-linking, SA can also form acid gels at pH below the pKa value of the uronic acid residues (Abd El-Ghaffar, Hashem, El-Awady & Rabie, 2012; Draget, Stokke, Yuguchi, Urakawa & Kajiwara, 2003; El-Sherbiny, Abdel-Mogib, Dawidar, Elsayed & Smyth, 2011; Gong, Li, Zhu, Zhang, Du & Jiang, 2011). SA has been found widespread applications in the biomedical field such as scaffolds for tissue engineering, delivery vehicles for drugs and model extracellular matrices for biological studies (Augst, Kong & Mooney, 2006; Gazori, Khoshayand, Azizi, Yazdizade, Nomani & Haririan, 2009; Kong, Smith & Mooney, 2003; Kuo & Ma, 2001; Li, Chen, Sun, Park & Cha, 2011; Rinaudo, 2008). However, compared with conventional synthetic polymers, SA exhibits many disadvantages, such as strong hydrophilic character, low thermal stability and poor mechanical strength. A number of researches have been carried out to improve the properties of SA by compounding SA with other polymers. For example, Li et al. prepared a biodegradable scaffold from SA through mixing with chitosan, which significantly improved mechanical and biological properties of SA (Zhensheng, Ramay, Hauch, Demin & Miqin, 2005). By mixing cellulose into a SA solution, Chang et al. fabricated a material with large pores where the stiff cellulose acted as a support of pore wall and SA as an expander of pore size (Chunyu, Bo & Lina, 2009). Although the blending of SA with other polymers is able to improve the mechanical strength of SA, the resulting blends usually show poor stabilities in a high temperature and wet environment, especially when blended with a hydrophilic polymer. In recent years, the use of inorganic fillers to modify the mechanical strength and stability of SA at high temperature and in wet environment has attracted much interest. For example, by mixing carboxyl multi-walled carbon nanotubes into a SA/polyethylene glycol solution to prepare SA membranes, Jie et al. obtained a high tensile strength up to 1.83 MPa for the SA composites (Jie et al., 2015). He et al. prepared GO/SA composite fibers by a wet spinning method and found the maximum tensile strength and Young’s modulus increased from 0.32 and 1.6 to 0.62 and 4.3 GPa, respectively (He et al., 2012). Mariana et al. found that the presence of 6 wt% graphene oxide (GO) not only increased the tensile strength and Young’s modulus of SA films, but also improved the thermal stability of SA/GO composite films (Ionita, Pandele & Iovu, 2013). The reason of GO can be used to increase the thermal stability and mechanical strength of SA is that GO sheets possess numerous functional groups (e.g. hydroxyl groups and carboxyl groups) in the basal planes and the 4

edges, which could allow GO sheets to form interfacial interaction (hydrogen bonds) with hanging hydroxyl groups of SA. In addition, SA is a polyanion and very sensitive to the presence of positive charges in aqueous solution due to the electrostatic interaction. Many studies have demonstrated that the electrostatic interaction is able to promote the dispersion of inorganic fillers in SA and produce a strong interfacial interaction between SA and fillers in comparison with the hydrogen bonding. For instance, by using tetraethylenepentamine to modify GO, Nie et al. observed that the nitrogen containing functional groups of GO can be protonated to become multivalent nanoparticles, and then readily form strong electrostatic interactions with negatively charged COO− groups of SA, which contributes to the improvement in thermal stability and mechanical strength of SA composites (Nie, Liu, Wang, Shuai, Cui & Liu, 2014). On the other hand, thermal stability and mechanical strength of a composite may also depend on the shape of filler particles. For example, Alishahi et al. studied the effect of shape of the filler particles on the mechanical properties of epoxy composites, and found that diamond and fibrous (carbon nanotube and nanofiber) particles provide better tensile properties while platelet (graphene oxide) leads to a considerable increase in the fracture toughness of the composites (Alishahi, Shadlou, Doagou-R & Ayatollahi, 2013). Salgueiro et al. investigated the effect of spherical and rod-shaped Au nanoparticles on κ-carrageenan hydrogel, and suggested that the anisotropy of the rod-shaped Au affects the aggregation of κ-carrageenan helices leading to the formation of less homogeneous microstructure, which results in the low mechanical strength in contrast to the spherical Au nanoparticles (Salgueiro, Daniel-Da-Silva, Fateixa & Trindade, 2013). However, to the best of our knowledge, there have been no reports on the effects of inorganic fillers with various shapes and functional groups on the stability and mechanical strength of SA films. In the present work, we systematically investigated the effects of silica, GO and ammonia functionalised graphene oxide (AGO) on structure, morphology, stability at high temperatures and in a wet environment, mechanical strength of the resulting SA composite films through Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermal gravimetric analysis (TGA), contact angle and tensile test. The interactions between SA and various fillers were illustrated. The fundamental interaction-property relationships in SA composite films were discussed.

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2. Experimental 2.1. Materials SA was purchased from Sigma-Aldrich (Singapore). According to the supplier, the molecular weight of SA ranged from 100,000 to 150,000 g/mol and G block content was 50 – 60 %. The aqueous solutions of silica (triethoxylpropylaminosilane functionalized, particle size < 30 nm) with 1.158 g/mL and ammonia functionalized graphene oxide (AGO) with 1 mg/mL were obtained from Sigma–Aldrich (Singapore), and the percentage of N element in silica and AGO were 0.5 - 4 % and 3 6 %, respectively. Graphene oxide (GO) was the product of XF NANO (Nanjing, China), the thickness of single sheet ranged from 0.8 to 1.2 nm and the content of O element was 30 – 35 %.

2.2. Preparation of SA composite film The aqueous solutions with various silica, GO or AGO contents were prepared by ultrasonic treatment using deionized water from a Millipore water purifier. A 2 wt% SA aqueous solution was prepared and added into the aqueous solutions of silica, GO and AGO, respectively. The resulting mixture was constantly stirred at room temperature for about 12 hours using a magnetic stirrer. The loading levels of silica, GO and AGO were fixed at 0.5, 1, 2 and 3 wt% based on SA, which were labelled with filler-n/SA where n is the mass fraction of filler. SA composite films with different filler contents were prepared by casting onto a glass plate at ambient temperature. After 72 hours, the films were peeled off from the glass plate and dried under a vacuum oven 60 oC for about 24 hours prior to testing.

2.3. Rheological characterization of filler/SA solutions The filler/SA solutions were analyzed through a rotational rheometer (DHR, TA Instruments, USA) with a cone-plate geometry of 60 mm in diameter and a cone angle of 2 o. For the rheological measurement, the sample, which was the same as that from which the film was cast, was transferred directly from a glass bottle to the bottom plate of rheometer using a pipette. Strain sweeps in the range of 0.1 – 100 % at frequencies of 0.1 – 2 Hz were carried out to determine the linear viscoelastic range of the solutions. Frequency sweeps in the angular frequency range of 0.1 – 100 rad/s were performed at a constant strain of 2.0 % and a fixed temperature (T = 20 oC).

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2.4. Characterization of SA composite films Fourier transform infrared (FTIR) spectra of silica, GO, AGO, pure SA film and filler/SA composite films were recorded on a Shimadzu IR Prestige 21 FTIR spectrometer using the ATR mode in the wavelength range of 4000 – 500 cm−1. X-ray diffraction (XRD) was performed on a powder XRD system (PW1830, Philips) with Cu Ka radiation and wavelength of 0.154 nm. The tube current and voltage were 30 mA and 40 kV, respectively, and the data from the 2θ angular regions between 5 and 50 o were collected. Surface morphologies of pure SA film and filler/SA composite films were observed using field emission scanning electron microscopy (FESEM, JSM-7600F, JEOL, Japan) with an accelerating voltage of 2 kV. The samples were sputtered with gold on their surfaces in vacuum before testing. Thermal stabilities of pure SA film and filler/SA composite films were examined using thermogravimetry thermal analysis (TGA 2950 with a TA 500 controller from TA Instruments) under a constant nitrogen flow rate 40 mL/min. The samples were heated from 40 to 500 °C at a scanning rate of 20 °C/min. Contact angles of deionized water on pure SA film and filler/SA composite films were measured using a contact angle meter (OCA 15Pro, Optical Contact Angle Measurement System) at room temperature. In addition, in order to investigate the stabilities of pure SA film and filler/SA composite films in a wet environment, the films were immersed in deionized water at room temperature, and then the morphologies of films in deionized water were observed after 20 min. Mechanical strength of pure SA film and filler/SA composite films were measured using an Instron 5569 uniaxial test machine with an extension speed of 10 mm/min at room temperature. The size of the test specimens was 6 cm in length and 1 cm in width. A minimum of five specimens were tested for each sample and the average values are reported.

3. Results and discussion 3.1. Rheological properties of filler/SA solutions SA and all the fillers used in this study can be mixed in water to form homogeneous solutions, and the obtained solutions were stable at room temperature. The rheological frequency sweep was carried out in the range of 0.1 – 100 rad/s for the filler/SA solutions containing various filler contents. G" is always larger than G' over the whole frequency range, showing a typical rheological 7

feature of a polymer solution. The complex viscosity η* was plotted as a function of angular frequency in Figure 1a, where a frequency independent behavior of η* is observed at low frequencies for all the solutions. If η* at ω → 0 is defined as zero shear viscosity η0, it is apparent that η0 of 2 wt% silica is larger than that of 1 wt% silica for the silica/SA solutions. Figure 1b demonstrates the effect of various filler contents on η0. It is apparent that η0 increases with increasing filler’s content for all the filler/SA systems, and a linear relationship of η0 with filler content was obtained, which is consistent with the Einstein equation,  = m(1 + 2.5filler), where m is the viscosity of the medium and filler is the volume fraction of filler (Einstein, 1911). The slope increases in the order of GO/SA, silica/SA, and AGO/SA. However, according to the Einstein equation, the slope for the AGO/SA system should be equal to that of GO/SA system since the volume fractions of GO and AGO in the solution are almost the same at the same GO and AGO content. In addition, the volume fraction of GO is larger than that of silica at the same loading level because the density of silica (~2.65 g/cm3) is greater than that of GO (1.8 - 1.9 g/cm3), which would lead to an enhanced gradient for the viscosity curve of the GO/SA system as compared to that for the silica/SA system. However, from Figure 1b, we observe that the slope of the viscosity curve for the silica/SA system is larger than that for the GO/SA system. Both results suggest that the enhancement of the complex viscosity in the filler/SA solution not only depended on the filler content, but also related to the interaction between filler and SA. Considering the experimental results of Nie et al. (Nie, Liu, Wang, Shuai, Cui & Liu, 2014), we can conclude here that the ammonia functionalized fillers, AGO and silica, have a strong interaction with SA in contrast to GO because the amine groups can be protonated to become the positively charged fillers, which is able to form the electrostatic interaction with negatively charged COO– groups on SA chains to result in the large increase in viscosity. Moreover, compared to silica, a large number of hydroxyl groups at the edges of AGO may induce the formation of hydrogen bonding with the hydroxyl groups in SA chains, which leads to much strong interaction between SA and AGO. This is why the slope of AGO/SA solution is larger than that of silica/SA solution even if both AGO and silica contain the amine groups.

8

10-1

* (Pas)

(a)

10-2

SA GO-1/SA Silica-1/SA Silica-2/SA AGO-1/SA 10-3

0.1

1

10

100

Frequency (rad/s) 0.10

(b) GO/SA Silica/SA AGO/SA

0.08

o(Pas)

0.06

0.04

0.02

0.00 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Filler (wt%) Figure 1. (a) complex viscosity η* as a function of angular frequency ω for pure SA solution and filler/SA solutions and (b) dependence of zero shear viscosity η0 on filler content.

3.2. FTIR spectra We know that the stability and mechanical strength of SA composite film depend not only on the interaction between SA and filler as well as filler content but also on SA’s microstructure. Therefore, it is necessary to clarify the interaction between SA and filler in SA composites as well as the effect of various fillers on SA’s microstructure. Figure 2 shows the FTIR spectra of silica, GO, AGO, pure SA film and filler-1/SA composite films. For the pure SA film, the characteristic peaks at 1027, 1405 and 1598 cm−1 were assigned to the stretching vibration of C−O−C, and to the symmetric and asymmetric stretching vibrations of COO−, respectively. The broad peak in the range of 2840 − 3660

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cm−1 may be attributed to the absorbed water. This observation is consistent with Sharma’s results (Sharma, Sanpui, Chattopadhyay & Ghosh, 2012). For the silica, the peaks at 794 and 1096 cm−1 were due to the symmetric and asymmetric stretching vibrations of Si–O–Si, respectively. No peak was found at 960 cm−1, suggesting that there was no the stretching vibration of Si–OH on the surface of silica, which is similar to the study by Wu et al. (Jianbing, Junbao, Lixia, Guozhang & Baojun, 2013). For the GO, the characteristic peaks appeared at 1721, 1623, 1400 and 1040 cm−1, which were assigned to the stretching vibration of C=O, the stretching vibration of C=C corresponding to the remaining sp2 character, the stretching vibrations of C−OH and C−O, respectively. Meanwhile, an absorbed water peak was observed from 2750 to 3480 cm−1. For the AGO, the peaks at 2917, 2842, 1571, 1444 and 1180 cm−1 should be attributed to the stretching vibration of N−H, the bending vibration of C−H, the stretching vibration of C=O, the stretching vibration of C−N, the stretching vibration of C−OH. After adding 1 wt% silica into SA, the FTIR spectrum of the silica-1/SA composite film shows a combination of characteristics similar to that of the pure SA and silica, and the characteristic peaks corresponding to SA shifted to low wave-numbers due to the electrostatic interaction between SA and silica. After mixing 1 wt% GO into SA, the FTIR spectrum of GO-1/SA composite film is approximately similar to that of the pure SA film except that the characteristic peak corresponding to the stretching vibration of O–H was broadened due to the intermolecular hydrogen bonding. The similar FTIR spectra were also observed for the GO/chitosan composites (Han, Yan, Chen & Li, 2011). However, for the AGO-1/SA composite film, the peaks at 1598 and 1405 cm−1 shifted to lower wave-numbers of 1588 and 1438 cm−1, which should be attributed to the merging of the characteristic peaks of SA and AGO (e.g. the 1588 cm−1 came from the merge of 1598 cm−1 of SA and 1571 cm−1 of AGO, and the 1438 cm−1 came from the merging of 1405 cm−1 of SA and 1444 cm−1 of AGO). The peak at 1027 cm−1 for SA shifted to a lower wave-number of 1020 cm−1 due to the intermolecular hydrogen bonding. Furthermore, no peaks at 2917, 2842 and 715 cm−1 corresponding to AGO were observed for the AGO-1/SA composite film. All of these results indicate that the presence of amine groups in AGO led to a strong interaction and great dispersion, which may contribute good adhesion at the interface of AGO/SA composites and further result in an enhanced mechanical strength.

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Figure 2. FTIR spectra of SA, silica, GO, AGO, and filler-1/SA composite films.

3.3. XRD The XRD patterns of SA composite films with various filler contents are shown in Figure 3. For pure SA film, two characteristic diffraction peaks at 2θ = 13.4 and 16.1o are observed, indicating that SA shows a bit of crystallinity. For the silica, the broad diffraction peak from 15.8 to 25.3o is the feature of amorphous silica. After adding silica, the diffraction peak of silica was not observed in the silica/SA composite films as shown in Figure 3a, which indicates that silica was dispersed very well in the SA matrix. However, the diffraction peaks from SA almost keeps constant, and the intensity of diffraction peaks decreases slightly with increasing silica content, indicating that the presence of silica shows a weak influence on the microstructure of SA. Figure 3b displays the XRD patterns of GO and GO/SA composite films. It can be found that GO shows a strong peak at 2θ = 11.5o, which suggests that GO possesses some crystallinity. Based on Bragg’s equation, the d spacing calculated is about 0.79 nm, which is within the range of value that has been reported in the literature (Bissessur, Liu, White & Scully, 2006). After loading GO with the lowest content (0.5 wt%) studied in this paper, the diffraction peaks derived from SA decrease obviously. When GO content was increased to 2 wt%, the diffraction peaks from SA disappear, indicating that the addition of GO destroyed the crystalline structure of SA even if there is only the hydrogen bonding between SA and GO. This result might not be expected because silica shows a strong interaction with SA in contrast to GO. That is to say, the effect of silica on the crystalline structure of SA should be greater than that 11

of GO. Our explanation is that in the SA matrix the effect of a filler on SA’s microstructure depends not only on the interaction between SA and filler but also on the shape of the filler. For 2-dimensional GO, the hydroxyl groups on the edges of GO sheets may interact with the hydroxyl groups on various SA chains, which led to the formation of a network. GO takes a role of crosslinking. The similar phenomenon has also been reported in the carboxymethylcellulose/SA composites filled with GO (Yadav, Rhee & Park, 2014). On the other hand, a small characteristic diffraction peak of GO appears when GO content was increased to 2 wt%. Combining with the observation by FESEM, it may be inferred that a higher loading of GO resulted in aggregation of GO due to the weak interaction between GO and SA. Figure 3c shows the XRD patterns of AGO and AGO/SA composite films. For the AGO, a weak and broad diffraction peak at about 18.2o, which represents amorphous AGO with an average intermolecular distance 0.63 nm, was found. For the AGO/SA composite films, the diffraction peaks corresponding to AGO and SA have not been observed within the experimental range of AGO contents, which indicates that AGO was uniformly dispersed in the SA matrix and built a strong interaction with SA. Meanwhile, there is not any characteristic diffraction peak of AGO even at the highest AGO content, suggesting that the compatibility of AGO with SA was strong.

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Figure 3. XRD patterns of (a) silica/SA composite films, (b) GO/SA composite films and (c) AGO/SA composite films.

3.4. Field emission scanning electron microscopic observation As shown in Figure 4, FESEM studies provide direct information regarding the dispersion of fillers. The surface of pure SA film is smooth (its image is not shown here) and is similar to the surface of AGO-0.5/SA composite film. The silica/SA composite films show gray spots on the surface and the density of gray spots increases with increasing silica loading. There are no obvious aggregates of silica particles observed on the surface even if the content of silica was increased to 3 wt%, which indicates that silica particles were dispersed well in the SA matrix due to the strong interaction between silica and SA. From the result of XRD, we know that the diffraction peak almost

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kept constant by adding silica, suggesting the uniformly dispersed silica has a weak influence on the microstructure of SA. Similar to the surface morphology of pure SA film, the GO/SA composite films with low GO contents display a smooth surface. However, a rough morphology with some salient banded structures was observed at high GO contents. From the XRD result in Figure 3b, it is known that there is a small diffraction peak of GO when the GO loading was increased to 2.0 wt%. Therefore, the salient banded structures should be attributed to the aggregation of GO. The similar structure has also been observed by Han et al. in the GO/chitosan composites.(Han, Yan, Chen & Li, 2011) However, for the AGO/SA composite films with various AGO contents, the surface displays a generally smooth morphology, indicating that the AGO was uniformly dispersed in the SA matrix due to the strong interactions (electrostatic and hydrogen bonding) between AGO and SA.

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Figure 4. FESEM images of surface of silica/SA, GO/SA and AGO/SA composite films with various filler contents. The scale bars are 10 micrometer.

3.5. Thermal stability of SA composite films The thermal stability of pure SA film and SA composite films was studied by thermogravimetric analysis as shown in Figure 5a. The first weight loss before 100 oC should be attributed to the loss of absorbed water. With further increasing temperature, the quick weight loss in the temperature range of 200 – 270 oC should be associated with the decomposition of SA. If the onset temperature of SA decomposition was defined as Tonset, it was found from Figure 5b that SA composite films showed a higher Tonset than pure SA film, and Tonset increased with increasing filler content. These results suggest that the mobility of SA chains was suppressed by interaction between SA and fillers, which improves thermal stability of SA composite films. On the other hand, Tonset for the AGO/SA composite films was higher than those for the GO/SA and silica/SA composite films at the same filler content, suggesting that AGO has a stronger interfacial interaction with SA than GO and silica. Therefore, the AGO/SA composite films have better thermal stability than the GO/SA and silica/SA composite films. 110 100

SA Silica-1/SA GO-1/SA AGO-1/SA AGO-2/SA

Weight (%)

90 80

(a)

70 60 50 40 30 100

200

300

400

o

Temperature ( C)

15

500

214

210

Tonset (oC)

(b)

Silica/SA GO/SA AGO/SA

212

208 206 204 202 200 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Filler (wt%) Figure 5. (a) TGA curves of pure SA film and filler/SA composite films and (b) dependence of the onset decomposition temperature of filler/SA composite films on filler loading.

3.6. Hydrophilicity and wet stability of SA composite films SA is a natural polymer and possesses a high hydrophilicity, which limits its application in many engineering areas. How to improve the hydrophobicity of SA has become a research topic. Figure 6a shows the effects of various fillers on the contact angle of SA composite films. It is obvious that the presence of various fillers enhances the hydrophobicity of SA composite films. At the same filler content, the increase in the hydrophobicity for the AGO/SA composite films is higher than that of silica/SA and GO/SA composite films. On the other hand, we also carried out the immersion experiments to evaluate the water solubility of SA composite films as shown in Figure 6b. The pure SA film was completely dissolved while the silica/SA composite film with 1 wt% silica disintegrated into many small pieces after the film was immersed into deionized water for 20 minutes at room temperature. However, for the GO/SA and AGO/SA composite films at 1 wt% filler content, no disintegration was observed and the composite films still remained in a whole shape, indicating that the addition of GO and AGO greatly enhanced the stability of SA in a wet environment.

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75

Contact angle (o)

70

(a)

65 60 55 50

Silica/SA GO/SA AGO/SA

45 40 35 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Filler (wt%)

Figure 6. (a) Effect of various fillers on contact angle and (b) photographs of pure SA film and filler-1/SA composite films immersed in deionized water for 20 minutes.

3.7. Mechanical strength of SA composite films Figure 7a shows the stress-strain curves for pure SA film and filler-1/SA composite films under uniaxial elongation with a speed of 10 mm/min. Compared with the pure SA film and silica/SA composite film, the GO/SA and AGO/SA composite films exhibit the high tensile stress and elongation at break. The tensile strength of SA/silica composite films increases slightly with increasing loading of silica as shown in Figure 7b. This suggests that the addition of silica could improve the mechanical strength of SA. However, the addition of silica leads to the decrease in the elongation at break as shown in Figure 7c. For the GO/SA and AGO/SA composite films, GO and AGO had an obvious reinforcing effect on SA matrix. The maximum of tensile strength reaches to 7.26

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MPa for GO/SA composite film with 1 wt% GO loading and 10.63 MPa for AGO/SA composite film with 3 wt% AGO loading (improved by 114.9 % and 194.4%, respectively), indicating a considerable reinforcing effect from GO and AGO. It is noted that increasing GO or AGO content also improved the elongation at break, which is different from silica/SA composite films. This is to say, the improvement of the mechanical strength of the GO/SA or AGO/SA composite films is mainly attributed to the specific interactions such as hydrogen bonding between SA and GO as well as both hydrogen bonding and electrostatic interaction between SA and AGO, rather than crystallization of SA. This result sounds unreasonable because the decrease in polymer crystallinity usually leads to the decline in the stiffness and strength of the polymer. From the XRD patterns shown in Figure 3, the addition of GO or AGO affects greatly the crystalline structure of SA. Therefore the crystallinity plays a less important role in the mechanical strength of GO/SA or AGO/SA composites, due to the limited crystallinity or almost an amorphous state of SA in its composites. The same observation was reported in the literature for a chitosan film filled with 0.5 wt% GO that slightly affected crystallinity but significantly increased the Young’s modulus and elongation at break (Yang, Tu, Li, Shang & Tao, 2010). On the other hand, the tensile strength and elongation at break of GO/SA composite films decrease when GO loading was increased to 2 wt%. From the result of XRD in Figure 3b and the surface morphology in Figure 4, it can be inferred here that the decrease in the tensile strength and elongation at break should be attributed to the agglomeration of GO sheets, which weaken the interfacial interaction between GO and SA. The improvement of tensile strength with further increasing AGO content is attributed to the good dispersion of AGO within the SA matrix and the strong interfacial interaction between AGO and SA, which also greatly improves the elongation at break of SA composite films. It is considered that the amine groups of AGO have a lot of nitrogen active sites and some of them can be protonated. Thus, protonated AGO can make strong electrostatic interaction with SA besides the hydrogen bonding. Therefore, the AGO/SA composite films have better tensile stress and elongation at break (10.63 MPa and 15.2 %) than those of GO/SA composite films (7.26 MPa and 10.4 % respectively).

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Figure 7. (a) Stress-strain curves, (b) tensile strength, and (c) elongation at break of the SA composite films at uniaxial elongation with a speed of 10 mm/min at room temperature.

On the basis of experimental results, we propose a schematic diagram for the interaction between SA and AGO as shown in Figure 8. On the surface of AGO there are a large number of hydroxyl, carboxyl and amine groups. The hydroxyl groups between SA and AGO interact to form the hydrogen bonding to facilitate the dispersion of AGO in the SA matrix. On the other hand, the amine groups on the surface and edges of AGO sheets can be protonated to become the positively charged NH3+ groups, which then readily form the electrostatic interaction with the negatively charged COO− groups on SA chains. This strong ionic interaction can improve the compatibility between filler and matrix. Therefore, AGO can form a much stronger interfacial interaction with SA than other fillers, which then contributes to the improvement in the stability and mechanical properties of SA. As for the GO/SA system, the hydrogen bonding interaction between GO and SA is not strong, so that the agglomeration of GO may take place at the higher contents of GO as observed in Figure 3b and Figure 4.

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Figure 8. Schematic diagram for the interaction between SA and AGO.

Conclusions In this study, we prepared a series of silica/SA, GO/SA and AGO/SA composite films by a solution mixing-evaporation method. The results indicated that ammonia functionalized silica and AGO have an improved compatibility with SA, which led to a good dispersion of silica and AGO in the SA matrix. Compared to the AGO/SA and silica/SA composites, pristine GO tends to aggregate in the SA matrix when GO content is high. The thermal and wet stability and mechanical strength of resulting composite films strongly depended on the interaction between SA and filler as well as the effect of filler on microstructure of SA. The addition of silica showed a weak influence on mechanical strength of SA films. The presence of GO and AGO greatly improved the stability at high temperatures and in a wet environment as well as the mechanical properties of SA films. Compared to GO, the improvement in thermal and wet stability as well as mechanical properties was significant for the SA composite films filled with AGO due to the strong electrostatic interaction between SA and AGO. Therefore, the AGO/SA composite films with water-resistance and high mechanical strength will find promising applications as biomaterials or packing materials.

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