Ethanol–NaOH solidification method to intensify chitosan/poly(vinyl alcohol)/attapulgite composite film

Abstract

Chitosan/poly(vinyl alcohol) (CS/PVA) films have recently attracted considerable attention. However, CS/PVA film has a significant flaw. It may swell in an aqueous solution and then lose its mechanical strength. Many efforts have been engaged to strengthen the CS/PVA film by adding various reinforcing materials, but the effect of different post-processing methods on the crystallization and mechanical behaviors of the film was rarely investigated. In this study, we have introduced natural nanoscale attapulgite (APT) into CS/PVA film to enhance its mechanical properties. Based on this, the resultant CS/PVA/APT nanocomposite film was further treated with a solution of NaOH and ethanol to regulate the crystallization degree of the polymer chains. The results reveal that the APT nanorods that are uniformly dispersed in the CS/PVA matrix can form a stronger hydrogen-bonding network with the polymer chains to improve the thermal stability and tensile strength. After treatment with NaOH and ethanol, the crystallization degree of the nanocomposite film was greatly enhanced, which caused an evident improvement in its mechanical properties. This method provides a simple and green approach to enhance the mechanical properties of polymer-based films.

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1. Introduction

Biodegradable polymer-based nanocomposites have recently attracted considerable attention owing to their excellent properties and eco-friendly advantages.^1,2 Among them, the chitosan/poly(vinyl alcohol) (CS/PVA) composite, as a candidate for substituting petroleum-based materials, has been intensively investigated in both academic and industrial areas.^3,4 Chitosan (CS) is the second most abundant natural cationic polysaccharide with renewable, biodegradable, non-toxic and biocompatible properties.^5,6 It has been frequently used to fabricate various composite materials and thus brings the intrinsic advantages of CS into the materials. However, CS is highly hydrophilic and may swell in moist conditions or in an aqueous solution, which sharply decreases the mechanical performance of the CS-based materials. Thus, how to overcome these drawbacks by a simple but effective method is extremely significant to all the applications, but it is also a challenge.

In general, CS has better biodegradable properties and excellent film-forming ability.^7,8 Moreover, CS is a crystalline polysaccharide with strong intramolecular and intermolecular hydrogen bonding interactions among its chains. This makes the neat CS film too brittle to be applied in many areas, e.g., food packing. Thus, poly(vinyl alcohol) (PVA) was introduced into CS-based materials in order to improve its flexibility and mechanical performance. PVA is a biodegradable and nontoxic linear polyhydroxy polymer that can serve as a reinforcing agent.^9–12 In addition, PVA has excellent film-forming properties, tensile strength and flexibility.^13–15 The composite of CS and PVA can overcome the brittle drawback of CS and improve the flexible properties of the film. However, neat organic films still have evident disadvantages such as poor mechanical performance in moist conditions.

Nanocomposites are a promising technology to overcome the drawbacks of organic polymeric materials because it may bring the advantages of nanomaterials into polymeric materials. Attapulgite (APT) is a type of natural hydrated Mg–Al silicate clay mineral with nanorod-like crystal morphology, plentiful nanopores and excellent thermal stability. It was widely applied in many areas, e.g., colloidal agents,¹⁶ catalysis,^17–19 adsorption,^20,21 as a slow-release carrier of drugs,²² and hybrid pigments.^23,24 In addition, APT has one-dimensional nanostructure, high aspect ratio, and reactive Si–OH groups,^25,26 and therefore, it has good dispersion and compatibility with a polymer matrix and has been frequently used as ideal reinforce filler to fabricate nanocomposites.^27,28 APT may affect the crystallization behavior of polymers and their mechanical properties.

The moderate post-treatment of a film is very important to control its properties. Cheng et al. found that the solidification of CS/PVA film with NaOH solution may enhance its mechanical performance.²⁹ However, the intensification of the nanocomposite film by employing a simple post-processing method has been rarely investigated. In this work, we prepared a series of CS/PVA/APT composite films by introducing APT and NaOH–ethanol solidification methods. The main influence factors, including the content of APT, NaOH treatment, and ethanol–NaOH treatment, were optimized, and the mechanical properties of the drying film and wet-immersed film were evaluated. The composite film was characterized by scanning electron microscopy (SEM), thermogravimetric (TG) analysis, X-ray diffraction (XRD), and ultraviolet-visible (UV-vis) transmittance spectra, and the ethanol dehydration was developed as an important process to form a compact and ordered structure. Compared with natural evaporation, the NaOH-solidified film under ethanol dehydration treatment shows better mechanical properties. This provides a new approach to improve further the performance of the polymer film.

2. Experimental

2.1 Materials

APT mineral was produced from Mingguang Mine of Anhui province of China and was provided by Jiuchuan Nanomaterial Technology Co. (Jiangsu, China). CS (degree of deacetylation 90%; molecular weight, 600 kDa) was from Yuhuan Ocean Biology Company (Zhejiang, China). PVA (polymerization degree, 1799; hydrolysis degree, 99%) was purchased from Lanzhou Chemical Reagent Co. Ltd. (Gansu, China).

2.2 Preparation of nanocomposite films

PVA was dissolved in distilled water at 95 °C to form a 10 wt% PVA solution, and CS was dissolved in an aqueous solution of acetic acid (1 wt%) to form a 2 wt% CS solution. Then, 100 mL of the PVA solution (10 wt%) was added into 200 mL of the CS solution (2 wt%) to obtain a PVA/CS mixture with the mass ratio of 5/2. APT was dispersed in distilled water under mechanical agitation at 500 rpm for 1 h at room temperature to obtain suspensions with different solid content. The obtained APT dispersion was added slowly to the CS/PVA mixture solutions with the APT loading of 1%, 2%, 3%, 4%, 5% and 6% (w/w) (refers to the mass of the polymer). Then, glycerol (GE) was added to the mixture at a dosage of 20% of the PVA mass, and the mixture was stirred at 500 rpm for 4 h to obtain a homogeneous solution. After that, the mixture was poured into a glass dish and evaporated under atmospheric conditions for 72 h and the composite films were obtained.

These films were immersed into NaOH solutions at different concentrations (0.5%, 1%, 2%, 3% and 4%) for 30 min and washed with distilled water until the pH is neutral. Then, the films were immersed in an ethanol solution for 30 min and the ethanol–NaOH-films were obtained.

2.3 Tensile tests

The tests adopted the GB/T 1040.3-2006 national standard. A new SANS universal material testing system (CMT4304) equipped with a 200 N load cell at 20 °C was used. The experimental results were evaluated as an average of ten measurements. NaOH-film was dried by evaporation under atmospheric conditions, and the ethanol–NaOH-film was dried by ethanol dehydration. The films immersed for 24 h were tested after removing the water from their surface using cotton adsorbent.

2.4 Characterization

XRD analysis was carried out using a D8 Advance (Bruker) X-ray diffractometer with Cu K-alpha radiation (λ = 0.15418 nm), running at 40 kV and 30 mA, and scanning from 3° to 40° at a rate of 3° min⁻¹.

Thermogravimetric (TG) analyses were performed on a Diamond TG-DTA 6300 thermoanalyzer under an oxygen atmosphere from 30 to 800 °C at a heating rate of 10 °C min⁻¹. The samples were pre-dried at 40 °C for 24 h in a vacuum oven before measurements.

The UV-vis transmittance spectra of the composite films were recorded with a UV-vis Spectrophotometer (Specord 200, Analytic Jena, Germany). The analysis was conducted in the spectral range of 250–700 nm at room temperature and the plotted data were recorded every 1 nm.

SEM observation of the obtained products was performed using a field emission scanning electron microscope (JSM-6701F). High resolution transmission electron microscopy (HRTEM) using a JEM 2100F microscope operated at 200 kW was used to observe the morphology of the CS/PVA/APT composite.

3. Results and discussion

3.1 SEM analysis

Fig. 1 shows the SEM images of the CS/PVA, CS/PVA/APT, NaOH–CS/PVA/APT and ethanol–NaOH–CS/PVA/APT films. It is clear that the surface of the CS/PVA film is smooth (Fig. 1a), indicating that there is no evident phase separation between CS and PVA. Fig. 1b shows that slight APT can be observed on the surface of the CS/PVA/APT film. It was found from Fig. 1c that the film shows a smooth surface after treating with NaOH solution, which means that the treatment of film by NaOH solution leads to the formation of a compact surface. As shown in Fig. 1d, the ethanol–NaOH-film has a smoother surface than the NaOH-film, and the ethanol dehydration process is favourable to form a compact structure. In summary, Fig. 1b–d shows a homogeneous smooth surface, which indicates that the addition of small dosage of APT does not significantly affect the surface morphology of the films. The dispersion of APT in the matrix can be well observed in the TEM image (Fig. 2). The individual APT rods can be observed in the polymeric matrix without aggregation and are almost embedded in the matrix to form a nanocomposite structure.

Fig. 1 SEM images of (a) CS/PVA, (b) CS/PVA/APT, (c) NaOH (2%)–CS/PVA/APT and (d) ethanol–NaOH (2%)–CS/PVA/APT (APT 3% w/w).

Fig. 2 TEM images of ethanol–NaOH (2%)–CS/PVA/APT (APT 3% w/w).

3.2 XRD analysis

Fig. 3 shows the XRD patterns of the CS/PVA, CS/PVA/APT, NaOH–CS/PVA/APT and ethanol–NaOH–CS/PVA/APT films. The diffraction peaks at about 2θ = 19.6° are attributed to the diffraction of the CS/PVA matrix,^30,31 and the diffraction peaks at about 2θ = 8.3° are attributed to the (110) diffraction of APT.³² The diffraction peak at about 2θ = 19.6° becomes broader and the peak area decreases from 4998.13 (for CS/PVA) to 3661.67 (for CS/PVA/APT-film) after adding APT, which indicates a decrease of crystallinity and confirms that the addition of APT reduces the crystallinity of the CS/PVA-film.

Fig. 3 XRD patterns of (a) CS/PVA, (b) CS/PVA/APT, (c) NaOH (2%)–CS/PVA/APT and (d) ethanol–NaOH (2%)–CS/PVA/APT (APT 3% w/w).

After treating with NaOH, the diffraction peaks becomes sharp, and the peak area increases from 3661.67 (for CS/PVA/APT-film) to 5253.41 (for NaOH-film) and 4953.61 (for ethanol–NaOH-film), which indicates an increase of crystallinity and the NaOH-film has relatively higher crystallinity than the ethanol–NaOH-film. This is because the NaOH may neutralize the acetic acid and then decrease the solubility of CS, which cause increase in the crystallinity of the NaOH-film.^33–35 Moreover, the films treated by evaporation under atmospheric conditions and the ethanol dehydration process have different crystallinity, which means that the mechanical properties of the CS/PVA/APT composite films could be controlled by altering the dehydration process.

3.3 Thermal analysis

Thermogravimetric (TG) analysis was used to study the effect of the solidification methods on the thermal stability of the CS/PVA/APT film. Fig. 4 shows the TGA and DTG curves of the composite films. As shown in Fig. 4a, the CS/PVA-film and CS/PVA/APT-film have a relatively steady weight loss in the temperature range of 30–500 °C. The NaOH-film and ethanol–NaOH-film have a relatively higher thermal decomposition temperature in comparison with the CS/PVA-film and CS/PVA/APT-film, which can be attributed to the increased degree of the density of the polymer network. Therefore, the NaOH-film and ethanol–NaOH-film have a slow weight loss rate in the temperature range of 30–250 °C, and it starts to decompose at about 250 °C, as evidenced by the rapid weight loss.

Fig. 4 The (a) TGA curves and (b) DTG curves of the composite films.

In the DTG curves (Fig. 4b), all samples are decomposed at about 275 °C.³⁶ For the NaOH-film and ethanol–NaOH-film, it is also evident that the new sharper and stronger decomposition temperature were observed at about T_max = 346 and 400 °C, which does not clearly appear in the curves of CS/PVA and CS/PVA/APT films. Thus, the increase in the decomposition temperature is attributed to the increased cross-linking density and intensified interaction among the polymer chains as well as the heat barrier effect of PAL.^37,38 This indicates that the ethanol–NaOH solidification method has an active effect to intensify the composite film.

3.4 Effect of APT content

The effects of APT dosage on the mechanical properties of CS/PVA/APT composite films (APT content varied from 0 to 6% w/w) are presented in Fig. 5. It is clear that the elongation at break decreased upon increasing the dosage of APT from 0 to 6% (w/w), whereas the tensile strength clearly increased upon increasing the content of APT, reaching the maximum value (45.39 MPa) at an APT content of 3% (w/w) and then decreased. At this point, the elongation at break is 190.58%. By considering all the data, the composite film with 3% (w/w) of APT content has the best mechanical performance.

Fig. 5 Effect of APT content on the mechanical properties.

The contribution of clay on the mechanical properties of the film can be ascribed to the higher mechanical strength of clay itself and the stronger interfacial interactions.³⁹ The negative charges on the APT surface can interact with CS at acidic pH conditions through electrostatic attraction. In addition, plenty of intermolecular hydrogen bonds might be formed among the APT, PVA and CS, which could build a three-dimensional network. Moreover, the large aspect ratio of the APT nanorods is also beneficial to stress transfer.^28,40,41 The tensile strength and elongation at break began to decrease as the APT content increased over 3 wt%, which is possibly due to the aggregation of APT nanorods with higher surface energy in the polymer matrix.⁴² Therefore, 3% (w/w) of APT content was selected to prepare the composite film for the other tests.

3.5 Effect of solidification methods on the mechanical performance

Different post-processing methods may affect the interaction of the polymer chains and cause change in their properties. As shown in Fig. 6, the films treated with NaOH and ethanol–NaOH solidification methods show superior mechanical properties. In Fig. 6a, the tensile strength of the treated films was evidently enhanced when compared with the untreated film (Fig. 5), but the elongation at break decreased. With increase in the concentration of NaOH solution, the tensile strength of the film increases, but the excess addition of NaOH causes the film to become crisp. Therefore, the tensile strength and elongation at break decreased when the concentration of NaOH is higher than 3%.

Fig. 6 Effect of (a) NaOH treatment and (b) ethanol–NaOH treatment on the mechanical properties. The content of APT is 3 wt%.

Compared with NaOH-films, the ethanol–NaOH treated film has better mechanical performance. It can be seen from Fig. 6b that the films present good mechanical properties. The ethanol–NaOH (2%) composite film achieved a tensile strength of 53.22 MPa and elongation at break of 118.71%. However, using ethanol–NaOH (0.5%), the film has high tensile strength and low elongation at break because of the film tends to swell at low concentrations of NaOH solution, whereas dehydration with ethanol may remove too much water, which contributes to form a compact but not ordered structure.

3.6 Mechanical performances of the films after immersing them for 24 h

Water is commonly found in real environments. Therefore, it is an important tensile test when film is moistened with water. Fig. 8 indicates that the ethanol–NaOH-film has excellent elongation at break after immersion, but the CS/PVA/APT-film was excessively swollen in an aqueous solution, which causes the CS/PVA/APT-film to lose its mechanical properties.

Fig. 7 The mechanical properties of the ethanol–NaOH–CS/PVA/APT and CS/PVA/APTA films after immersing them for 24 h.

Fig. 8 Mechanical properties of the (a) NaOH-films and (b) ethanol–NaOH-films tested after immersing them in an aqueous solution for 24 h.

It was found from Fig. 8 that the films have extremely higher elongation at break, but the tensile strength is below 10 MPa. Because the NaOH solution has solidification effect on the film, the solidification is not complete at a low concentration, which leads to the poor mechanical performances. However, the higher concentration of NaOH causes a fierce chemical reaction, which makes the CS aggregate and weakens its interaction with the PVA chains, and therefore, the films become crisp. As a result, when the concentration of NaOH solution is 1%, the film has the best mechanical performance (Fig. 8a), the tensile strength is 7.62 MPa and the elongation at break is 239.25%.

Other than Fig. 8a, the ethanol–NaOH-films have the best mechanical performance at a NaOH concentration of 2% (Fig. 8b), the tensile strength is 9.35 MPa and the elongation at break is 266.96%. It is because ethanol can take water from the swelling composite film, and the NaOH treatment and ethanol dehydration process fabricates a stronger and dense polymer network than the CS/PVA/APT-film (Fig. 3), which caused the films to present different mechanical performance after immersion for 24 h (Fig. 7).

3.7 Analysis of mechanical properties

Mechanical performance is also an important aspect to respond the modification effect of the composite film. Fig. 9a shows the stress–strain curves of the CS/PVA/APT-film, NaOH-film, and ethanol–NaOH-film. It is clear that the performance of these films exhibits different change tendency: NaOH-film was rapidly broken after yielding; ethanol–NaOH-film has an evident yield point, and it will elongate for some time after yielding; CS/PVA/APT-film does not have an evident yield point, and it keeps elongating until it was broken.

Fig. 9 Mechanical properties of (a) stress–strain curves and (b) ethanol–NaOH-films.

The appearance of a high yield point means that the material is not easily deformed under outside pressure. Fig. 9b shows that the ethanol–NaOH-films have a relatively higher yield point, and they can be used in a wide variety of applications.

3.8 Relevant mechanisms

Because CS cannot be dissolved in an aqueous solution, a solution of acetic acid was used to dissolve CS.⁴³ It may cause the CS/PVA/APT composite film to swell in an aqueous solution. The traditional solidifying method of CS/PVA film is to use a NaOH solution with the aim of obtaining a better mechanical performance, and the NaOH treatment method can prevent the film from excessive swelling in an aqueous solution.^33–35 This is because NaOH may neutralize acetic acid during treatment, and the solubility of CS is decreased and the hydrogen bonding network is formed again.^44,45 However, NaOH-film simultaneously improves the tensile strength to reduce the elongation at break after drying.

Evaporation under atmospheric conditions and washing with ethanol can remove the water from the composite film. However, the mechanisms of the two dehydration processes are different and may have different influences on the structure and mechanical performance. As shown in the Fig. 10, after treating with ethanol, the surface of the ethanol–NaOH-film has many ordered dots, and the surface of ethanol–NaOH-film is more ordered and smoother than the surface of NaOH-film. In addition, the NaOH-film has a higher crystallinity (Fig. 10a) than the ethanol–NaOH-film (Fig. 10b). The higher crystallinity endows NaOH-film with higher tensile strength and lower elongation at break. It means that the ethanol dehydration process has an evident effect on the structure of film other than evaporation under atmospheric conditions. Furthermore, it can be seen from Fig. 11 that the original film has the best transmittance and the ethanol–NaOH-film has a higher transmittance than the NaOH-film. This further confirms that there is no phase separation among PVA, CS and APT, and the structure of the films could be adjusted by employing different treatment methods.

Fig. 10 The surface of (a) NaOH-film and (b) ethanol–NaOH-film.

Fig. 11 Effect of different treatments on the transmittance rate.

Compared with the conventional evaporation drying method, ethanol treatment can cause the film to form a compact and ordered structure. Therefore, the ethanol–NaOH-film has better mechanical performance than NaOH-film either under dried or immersed conditions.

4. Conclusions

In summary, the CS/PVA/APT composite films were prepared using a simple mixing and washing method. The excessive swelling problem of the film in an aqueous solution was resolved: (i) the composite film was immersed in NaOH solution; (ii) the acetic acid in the composite film reacted with the NaOH solution; (iii) a compact and ordered structure was formed by ethanol dehydration. The composite film treated with NaOH and ethanol solutions exhibits good mechanical performance. The tensile strength is 9.35 MPa and the elongation at break is 266.96% when the ethanol–NaOH-film was immersed in an aqueous solution for 24 h. In addition, the dried ethanol–NaOH-film achieved a tensile strength of 53.22 MPa and the elongation at break of 118.71%. Moreover, XRD shows that the NaOH-film has a higher crystallinity than the ethanol–NaOH-film, and the crystallinity affects the mechanical properties. The TG analysis shows that the ethanol–NaOH solidification method has a signification effect to enhance the decomposition temperature. The light transmittance shows that different treatment methods affect the transmittance. The fabrication of a CS/PVA/APT composite by simple mixing and washing is easy to be scaled up, which exhibits great advantages as a new composite film for applications in a wide field.

Acknowledgements

The authors thank the joint support from the National Natural Science Foundation of China (no. 51403221) and the open fund of Key Laboratory for Palygorskite Science and Applied Technology of Jiangsu Province (no. HPK201201).

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