2. Experimental section
2.1 Reagents and Materials. Analytically pure Mg(NO3)2·6H2O, Al(NO3)3·9H2O were purchased from Beijing Chemical Plants. Urea and ammonia were obtained from Xilong Scientific Co., Ltd. TA was purchased from Aladdin Biotechnology Co., Ltd. PEO was obtained from Maclean Biochemical Technology Co., Ltd. Natural rubber (NR) substrate was purchased from ASONE (Shanghai) Trading Co., Ltd. All the reagents were used without further purification. Deionized (DI) and decarbonated water was used in all the experiments.
2.2 Preparation of colloidal MgAl-LDH nanosheets suspension.MgAl(CO3)-LDH was prepared by urea-assisted hydrothermal method.14 Specifically, 25.64 g of Mg(NO3)2·6H2O, 18.70 g of Al(NO3)3·9H2O, and 30.03 g of urea were dissolved in 500 ml of water and placed in a stainless steel autoclave lined with polytetrafluoroethylene. After hydrothermal treatment at 110 °C for 24 h, it was then washed several times with a centrifuge using deionized water and ethanol. After cooling, the precipitate of MgAl(CO3)-LDH nanoplatelets was washed several times with deionized water and ethanol and dried at room temperature.
Then the interlayered CO32− were exchanged by NO3 ions through the salt-acid treatment of MgAl(CO3)-LDH, to facilitate delamination.35 The sample of 1 g MgAl(CO3)-LDH nanoplatelets was added into 1 L aqueous salt-acid solution (311 μL of HNO3, 98 wt% and 127 g of NaNO3) and stirred for 24 h in a nitrogen atmosphere. The MgAl(NO3)-LDH nanoplatelets were collected by centrifugation and washed with water and ethanol and dried in a vacuum oven. Afterward, samples of 0.1 g MgAl(NO3)-LDH nanoplatelets were placed into 100 mL formamide for 48 h with stirring to prepare single-layer nanosheets suspension.36
2.3 Preparation of TA@LDH suspension. TA was put into formamide and stirred at room temperature until complete dissolution. Under the protection of N2, LDH nanosheet suspension was mixed with formamide solution of TA and stirred at room temperature for 24 hours. TA@LDH suspension can be obtained through the above operations. To study the effect of TA: LDH ratio on the gas resistance and tensile properties of the films, different TA@LDH suspensions with various TA: LDH ratios were prepared. To ensure that the total concentration of TA@LDH suspension is 0.1 wt% and pH is 8.0, four groups of experiments were carried out according to TA: LDH = 1:1, 2:1, 3:1 and 4:1. Normally, the expression of TA@LDH represents the sample with TA: LDH = 3:1, unless specified.
2.4 Fabrication of (TA@LDH/PEO)nmultilayer films. An aqueous solution of PEO (0.1 wt%) was stirred at 60 °C for 2 h to achieve homogeneity. The pH of each PEO solution and rinsing DI water was altered to 8.0 using ammonia. Multilayer films of (TA@LDH/PEO)n were fabricated by applying the LBL assembly procedure. Quartz glass, silicon wafer and NR film (thickness: ~1 mm) were chosen as substrates for various characterizations and tests. Quartz glass substrate was used for UV-vis absorption spectra characterization, and silicon wafer was applied for SEM and AFM study. All the other measurements were based on the samples on NR substrate. Before assembly, every NR film was rinsed with DI water, soaked in n-propanol at 40 °C for 10 min, then treated with n-propanol and deionized water by ultrasound at 40 °C, and dried naturally. Quartz glass and silicon wafer were firstly washed in 30% H2O2/NH3·H2O (v/v=3:7) and concentrated H2SO4 for 30 min each to make the substrate surface hydrophilic and negatively charged. The overall LBL process consists of a cyclic repetition of the following steps: (a) dipping the pre-treated substrate into a colloidal TA@LDH suspension for 10 min, followed by rinsing with DI water thoroughly; (b) immersing into an aqueous solution of PEO for 10 min and washed with DI water. Multilayer films of (TA@LDH/PEO)n were fabricated by alternate deposition of TA@LDH suspension and PEO solution for n cycles. The resulting films were finally rinsed with water and dried at ambient temperature. As for comparison samples, the (TA/LDH)n , (TA/PEO)n and (LDH/PEO)n films were prepared by a similar assembly procedure in a sequence of LDH nanosheets suspension, TA, PEO aqueous solution. All films were prepared using an automatic dip-coating machine (Figure S1, Supporting Information). A stretching instrument (Figure S2, Supporting Information) was used to apply different strains to the films with and without nanocoatings for 1 min.