Abstract
To overcome the agglomeration tendency of nanosilver composite hydrogels and to improve their anti-mold properties, a method of preparing N-isopropylacrylamide/acrylic acid/nanosilver composite hydrogel was developed using the free radical polymerization method. The composite hydrogel was characterized via infrared spectroscopy, dynamic light scattering, transmission electron microscopy, and X-ray photoelectron spectroscopy in order to explore the effects of the acrylic acid content on particle size and dispersion properties of the composite hydrogels. The elemental composition, microstructure, and anti-mold properties of the bamboo strips treated with the composite hydrogel were also determined. The results showed that the composite hydrogel prepared using the novel method described in this study had good dispersity. Composite hydrogels with the smallest particle size and optimized dispersion were produced when AAc concentration was 0.64 µL/mL. The composite hydrogel effectively filled and covered the bamboo cells after treatment. Moreover, it displayed good anti-mold properties as well as retaining the color of the bamboo.
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Preparation of Uniformly Dispersed N-isopropylacryl-amide/Acrylic acid/Nanosilver Composite Hydrogel and its Anti-Mold Properties
Rui Peng,a Huilong Yu,b Chungui Du,a,* Jingjing Zhang,a Ailian Hu,a Qi Li,a Yating Hua,a Hongzhi Liu,a and Shimin Chu a
To overcome the agglomeration tendency of nanosilver composite hydrogels and to improve their anti-mold properties, a method of preparing N-isopropylacrylamide/acrylic acid/nanosilver composite hydrogel was developed using the free radical polymerization method. The composite hydrogel was characterized via infrared spectroscopy, dynamic light scattering, transmission electron microscopy, and X-ray photoelectron spectroscopy in order to explore the effects of the acrylic acid content on particle size and dispersion properties of the composite hydrogels. The elemental composition, microstructure, and anti-mold properties of the bamboo strips treated with the composite hydrogel were also determined. The results showed that the composite hydrogel prepared using the novel method described in this study had good dispersity. Composite hydrogels with the smallest particle size and optimized dispersion were produced when AAc concentration was 0.64 µL/mL. The composite hydrogel effectively filled and covered the bamboo cells after treatment. Moreover, it displayed good anti-mold properties as well as retaining the color of the bamboo.
Keywords: N-isopropylacrylamide (NIPAm); Acrylic acid (AAc); Nanosilver (AgNPs); Composite hydrogel; Dispersity; Bamboo strips; Anti-mold properties
Contact information: a: School of Engineering, Zhejiang A & F University, Hangzhou, Zhejiang 311300 China; b: National Furniture Product Quality Supervision and Inspection Center, Ganzhou, Jiangxi 341400 China; *Corresponding author: chunguidu@163.com
INTRODUCTION
As a new renewable resource with a short growth cycle and a high economic value, bamboo has been widely used in interior decoration, architecture, automobile manufacturing, and other fields (Chung and Yu 2002; Sharma et al. 2015; Yuan et al. 2017; Liu et al. 2019; Wu et al. 2019; Liu et al. 2020). However, bamboo is vulnerable to mold invasion, and the resulting loss accounts for approximately 10% of the total bamboo output (Zhang et al. 2020). Therefore, it is of great importance to study the mold prevention of bamboo. The author’s team used N-isopropyl acrylamide (NIPAm) as a monomer to synthesize a thermo-sensitive silver-borne nano-hydrogel, which was applied in order to mildew-proof bamboo material, which achieved good mildew-proofing effects (Yu et al. 2017). However, the authors found that the addition of nanosilver (AgNPs) in the composite hydrogel would cause a small amount of precipitation, agglomeration, or oxidation over time, which resulted in reduced antibacterial properties. In addition, the oxidized AgNPs would turn the bamboo black during the mold-proofing process, thus affecting the beauty of the bamboo.
Dispersion protectants are often used to overcome the shortcomings of AgNPs, e.g., precipitation, agglomeration, etc. (Eghbalifam et al. 2015; Ge et al. 2017; Gao et al. 2018; Qi et al. 2018; Zhou et al. 2019). The dispersion protectants and the AgNPs particles undergo chemical or physical adsorption, and a molecular protection layer will form on the particle surfaces, preventing the aggregation of AgNPs particles. Polyvinyl pyrrolidone (PVP) is a non-ionic surfactant with good solubility, high polymer surface activity, complexability, as well as other desirable properties (Begum et al. 2016; Gilcrease et al. 2020; Hu et al. 2020). It is commonly used as a dispersive protective agent for AgNPs (He et al. 2017a,b). However, the addition of a single PVP does not provide major dispersion effects for AgNPs, so the concentration of AgNPs must be increased, which will lead to the tedious removal process of the subsequent surfactants. A large number of studies have found that combinations of dispersing protectants used together with the polymer monomers can produce better dispersing stability effects than a single dispersing agent. Acrylic acid (AAc) is a hydrophilic monomer containing carboxyl groups, which have strong complexation to Ag+ (Hu et al. 2014). Therefore, with NIPAm and AAc as the hydrogel matrix and PVP as the protective agent, the authors prepared a novel composite hydrogel via the free radical polymerization method (Wei et al. 2016, 2018; Huang et al. 2019), and evaluated the addition amount of AAc in terms of the composite hydrogel particle size, dispersion performance, and bamboo anti-mold properties. This preparation method has good anti-mold properties due to the addition of NIPAm/AgNPs and should provide a theoretical reference compound for antibacterial hydrogel production.
EXPERIMENTAL
Materials
The N-Isopropyl acrylamide (NIPAm AR) was purchased from TCI Chemicals Co., Ltd. (Tokyo, Japan). The acrylic acid (AAc), N,N’-methylenebisacrylamide (MBA), tetramethylenediamine (TMEDA), potassium persulfate (KPS), silver nitrate (AgNO3), sodium borohydride (NaBH4), and polyvinylpyrrolidone (PVP) were all supplied by Aladdin Reagent Co., Ltd. (Pico Rivera, CA). The deionized water was prepared in the laboratory.
The bamboo strips were purchased from the Zhejiang Yongyu Bamboo Industry Co., and the purchased Moso bamboo was processed into strips with the dimensions of 50 mm × 20 mm × 5 mm (length × width × thickness) so that no node was formed and a 10% moisture content was maintained.
Methods
Preparation of the composite hydrogels
A 250 mL 3-neck round bottom flask containing 100 mL of deionized water was purged with high purity N2 at room temperature while stirring until the flask was completely free of oxygen. Different concentrations of AAc were prepared in the flasks (AAc concentrations were 0, 0.16, 0.32, 0.48, 0.64, and 0.80 µL/mL, respectively). This was followed by the addition of 0.682 g of NIPAm monomer, 0.1 g of PVP protectant, 1700 µL of MBA crosslinker (0.015 g/mL, prepared in the lab), and 93 µL of TEMED catalyst. The solution was properly mixed and kept in the dark. Under a N2 atmosphere, 600 µL of AgNO3 solution (1.0 M, prepared in the lab) was slowly added and stirred for 1 h, followed by the addition of 1000 µL of KPS initiator (0.030 g/mL). The mixture was allowed to polymerize for 6 h under a N2 atmosphere, resulting in a clear and transparent nano-hydrogel solution. Afterwards, 1200 µL of NaBH4 solution (1.0 M) was added, and the solution was kept in the dark for 12 h. Finally, the reaction mixture was purified with a dialysis bag (MWCO: 8000 to 14000) to remove any unreacted monomers, and the different types of composite hydrogels (P0, P16, P32, P48, P64, and P80) were obtained and stored at room temperature for use. Figure 1 shows the preparation of the composite hydrogels.
Fig. 1. Schematic diagram of the preparation of the composite hydrogels
Infrared spectroscopic (FT-IR) analysis
The lyophilized composite hydrogel samples were ground into a powder and mixed with potassium bromide at a mass ratio of 1 to 100 and pressed into thin films. Its molecular structure was analyzed via Fourier-transform infrared spectroscopy (FT-IR) (IRPrestige-21 spectrometer) with a resolution of 4 cm–1 and a scanning range from 4000 cm–1 to 400 cm–1. Each sample was scanned in triplicate, and the spectra were required to match at least twice.
Dynamic light scattering (DLS) analysis
A few drops of the composite hydrogel solution were added into a quartz cell and diluted. Bubbles in the solution were removed by shaking, and the particle size of the composite hydrogel was analyzed via a DLS instrument (ZetaPALS-31484). The measurement conditions were as follows: a laser emission of 658 nm, a laser energy of 4.0 mW, a background scattering of 173°, and temperature of 25 °C. Each sample was scanned five times, and the results were averaged.
Transmission electron microscopy (TEM) analysis
A drop of the composite hydrogel solution was diluted three times and mixed with a drop of 2.0 wt.% uranyl acetate solution. It was then placed on a copper mesh covered with carbon film, and the excess liquid was absorbed with clean filter paper after approximately 4 min. After the mesh naturally dried, the composite hydrogel was scanned via a TEM (JEOL JEM-1200) instrument working at an accelerating voltage of 80 kV to observe the microstructure of the composite hydrogel.
Impregnation of the bamboo strips with the composite hydrogel
The bamboo strips were treated with boiling water for 2 h, followed by 1.0 wt.% NaOH solution for 2 h, deionized water for 2 h, 1.0 wt% HCl for 2 h, and finally dried to a moisture content of approximately 10.0%. The treated strips were then immersed in the composite hydrogel solution for 1.5 h at room temperature and a pressure of 0.5 MPa before removing for use.
X-ray photoelectron spectrometric (XPS) analysis of the treated bamboo strips
After treatment with the composite hydrogels, the bamboo strips were sliced, and the surface elemental composition of the slices was analyzed via XPS (ESCALAB 250Xi). The analysis conditions were as follows: the radiation type was Al Kα, the power was 350 W, and the pressure was 10–9 Torr.
Scanning electron microscopic (SEM) analysis of the treated bamboo
The treated bamboo slices were further analyzed via SEM (Hitachi TM 1000) to observe the cross-sectional microscopic structures. After fixing conductive glue on the sample station, the sample was coated with gold for 60 s with an acceleration voltage of 5.0 kV.
Anti-mold properties of treated bamboo strips
The anti-mold properties of the bamboo strips treated with the composite hydrogels were tested according to the GB/T standard 18261 (2013). The strain suspensions of Penicillium citrinum (PC), Trichoderma viride (TV), Aspergillus niger (AN), and mixed molds (PC, TV, and AN in a 1 to 1 to 1 ratio, abbreviated as PTA) were spread evenly on a Petri dish containing the plate medium and put in sterilized U-shaped solid glass rods. Afterwards, the dish containing the molds was cultured in an incubator at a temperature of 28 °C ± 2 °C and a relative humidity of 85% ± 5%. After successfully culturing the mold, the bamboo was placed on the U-shaped glass rods, and the edges of the Petri dish were sealed with a sealing film. Finally, the bamboo strips were placed in a Petri dish in the incubator for anti-mold testing.
Table 1. Classification Standard of Surface Infection Levels of Samples
During the test, the bamboo strips in the incubator were observed and recorded for mold infection every alternate day. After 28 d, the bamboo samples were photographed, the area of the bamboo strips infected by the fungus were observed and analyzed to determine the infection levels of the bamboo strips (Table 1) (Zhang et al. 2020), each group was repeated three times and the results were averaged. Finally, in order to analyze the anti-mold performance of the NIPAm/AAc/Ag NPs composite hydrogels, the authors compared the anti-mold results of the composite hydrogels with the results of the silver-loaded thermo-sensitive nanogels (Yu et al. 2017), and paid attention to the color change of bamboo material after the anti-mold testing procedure.
RESULTS AND DISCUSSION
Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis of the Composite Hydrogels
Figure 2 shows the FT-IR spectra of the different types of composite hydrogels prepared by adding different amounts of AAc. The characteristic stretch vibrational absorption peak at 1620 cm–1 for the C=C conjugation as well as the out-of-plane deformation vibrational peak at 990 cm–1 for the terminal alkene hydrogen atoms in the NIPAm spectra disappeared in the spectra of the different types of composite hydrogels. This indicated the successful polymerization of NIPAm, forming PNIPAm hydrogels.
Fig. 2. FT-IR spectra of the composite hydrogels
In spectra of hydrogel samples P0 to P80, there was a wide absorption band at 3441 cm–1, which was the stretching vibration peak of -OH and the result of water adsorption via PVP molecules. The peak at 1650 cm–1 was a stretching vibration peak of the C=O of the amide group, while the peak at 1548 cm–1 was a variable angle vibration peak caused by the stretching vibration of the N-H and C-N bonds. The carboxyl group in the AAc formed hydrogen bonds with Ag+ (Meng et al. 2016; Hong et al. 2017); however, it weakened the hydrogen bonding force between the amide groups in PNIPAm and Ag+ (Devaki et al. 2014; Ma et al. 2019). Thus, the absorption bands of these functional groups in the composite hydrogels were only slightly altered when AAc was added. The change was minor, and the original molecular structure was mostly maintained (Guo et al. 2014; Wang et al. 2016).
Dynamic Light Scatting (DLS) Analysis of the Composite Hydrogels
A dynamic light scattering particle size (DLS) instrument was used to measure the particle size distribution of the different types of composite hydrogels and the effects of the addition of AAc on particle size. The results are shown in Fig. 3.
Figure 3a demonstrated that the particle size of the different types of composite hydrogels ranged from 25 nm to 343 nm and showed a regular Gaussian distribution pattern. Figure 3b indicates that the particle sizes of the composite hydrogels were greatly influenced by the concentration of AAc. When the concentration of AAc ranged from 0 to 64 µL/mL, the particle size rapidly decreased as the concentration of AAc increased. In contrast, when the concentration of AAc ranged from 0.64 µL/mL to 0.80 µL/mL, the particle size of the composite hydrogel rapidly increased as the concentration of AAc increased. When no AAc was added, the composite hydrogel displayed the largest particle size (147.0 nm). When the concentration of AAc was 0.64 µL/mL, the composite hydrogel had the smallest particle size (59.4 nm). Thus, 0.64 µL/mL of AAc was the turning point in the particle size of the synthetic composite hydrogel. This was also the optimal concentration of AAc for preparation of the composite hydrogel.
Fig. 3. The particle size distribution of the composite hydrogels and the effect of AAc on particle size: (a) Size distribution curves; and (b) Effects of the amount of AAc on particle size
The rationale was that when the concentration of AAc was in the range 0 to 0.64 µL/mL, the carboxyl group on AAc formed a strong ionic attraction with the amide group on the PNIPAm chains. The strength of this ionic attraction was stronger than the original hydrogen bond formed between the amide groups and the water molecules. This weakened the hydrophilic effect of the molecular chain of the PNIPAm hydrogel. Meanwhile, the carboxyl groups weakened the binding ability between the amide group and Ag+, which led to an overall reduced particle size of the composite hydrogel. When the concentration of AAc ranged from 0.64 µL/mL to 0.80 µL/mL, the extra AAc attached to the molecular chains of PNIPAm due to the formation of ionic attraction between the AAc and the PNIPAm. The continued addition of AAc further increased the number of carboxyl groups in the hydrogel system, along with the formation of additional hydrogen bonds with water, therefore increasing the hydrogel particle size. Moreover, the adsorption of the Ag+ ions by -COOH groups in AAc was another important factor contributing to the increase in the particle size (He et al. 2017a,b).
Transmission Electron Microscopy (TEM) Analysis of the Composite Hydrogels
All types of composite hydrogels displayed a uniformly dispersed state. However, it was unclear whether the AgNPs in the composite hydrogels were also uniformly dispersed. For this reason, the microscopic morphology of the different types of composite hydrogels was tested via transmission electron microscopy (TEM), and the results are shown in Fig. 4.
Due to the scattering effect of the Ag nuclei on the electron rays, Ag appeared black on the TEM image. In the TEM images of the P0 to P80 samples (scale bar = 500 nm), the composite hydrogel showed an obvious reticular structure, and the dispersion of the AgNPs was observed without any agglomeration (Wu et al. 2010). In the magnified TEM images of the P0 to P80 samples (scale bar = 50 nm), the spherical morphology of the AgNPs was revealed with variable dispersion depending on the composite hydrogel type.