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.