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DOI: 10.1039/C6RA14032G (Paper) RSC Adv., 2016, 6, 71425-71430

Investigation on polyvinyl-alcohol-based rapidly gelling hydrogels for containment of hazardous chemicals†

Le Yuana, Lijun Renb, Xingtao Tianb, Zhiping Huangb, YanHua Xiaob, Sichen Weia and Zhihua Wang*a
aState Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China. E-mail: zhwang@mail.buct.edu.cn
bInstitute of Chemical Defense, Beijing 102205, P. R. China

Received 30th May 2016 , Accepted 20th July 2016

First published on 20th July 2016

Abstract

The specific problem that is of concern here is to stop or at least significantly retard the diffusion of hazardous chemicals which are accidentally released into public places. The solution proposed is to employ rapidly gelling hydrogels that display an effective barrier property to cover the hazardous chemicals. Rapidly gelling hydrogels were prepared by blending a solution of polyvinyl alcohol (PVA) and borax. Sodium alginate (SA) was incorporated so as to improve the stretchability of the PVA–borax crosslinked system. The PVA–SA–borax hydrogels with PVA contents of 15 wt%, SA contents of 0.6 wt%, and borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]25 can be formed within 1 minute, which exhibited self-healing capability and favorable thermal adaptability. The barrier properties of the PVA–SA–borax hydrogels to sodium cyanide, dichlorvos and phorate were investigated, and more than 99% of the sodium cyanide, dichlorvos and phorate can be confined for 24 h by using 10 mm thick hydrogels.

1. Introduction

It is extremely necessary to immediately block hazards when hazardous chemicals are accidentally released into public places. Covering the hazardous chemicals with a rapidly forming material seems like a good idea. The covering materials may be prepared via an in situ reaction by spraying the reactants from some device. Rapidly gelling hydrogels, a class of hydrogel materials that can solidify from liquid states at application sites, will probably be an available covering material.

Different strategies have been utilized to obtain rapidly gelling hydrogels, due to their potential use in biomedical applications, pharmaceutical applications and daily-care applications.1–6 For example, a way to accelerate silk fibroin (SF) gelation using an anionic surfactant, sodium dodecyl sulfate (SDS), as the gelling agent was reported.7 A class of hydrogels derived from oxidized alginates and gelatins were studied by Biji Balakrishnan and coworkers.8 Most hydrogels exhibit biocompatibility, non-toxicity, biodegradability and good stretchability. However, little attention has been focused on the rapidly gelling hydrogels as the covering materials to block hazardous chemicals. Additionally, the complicated pre-production and the high cost have limited the use of hydrogels as the covering materials of hazardous chemicals.9,10 These disadvantages can be overcome through simple preparation processes, which is an important area worth pursuing.

PVA is an inexpensive and soluble polymer, generally employed in the preparation of hydrogels.11,12 The thiolated PVA reacting with different crosslinkers such as sodium trimetaphosphate, boric acid and glyoxal have been reported with the gelation time from 15 to 60 min.13 The acid catalysed cross-linking of PVA with varying concentrations of glutaraldehyde was analyzed and the cross-linked PVAs were utilized as membrane separators in single chambered microbial fuel cells.14 Due to the hydroxyl groups of each repeating molecular unit, PVA hydrogels15–17 can form physically crosslinked hydrogels by freeze–thaw processes. As an important property of PVA, borax crosslinks PVA via “di–diol” complexation.18 Many researches have been carried out on PVA–borax system. In order to understand the viscoelastic properties of the PVA–borax system, efforts have been devoted to the rheology study of PVA–borate complex aqueous system over the past several decades,19,20 and the effects of polymer concentration, molecular weight and temperature on the dynamic viscoelasticity of PVA in aqueous borax solutions have been investigated. However, to the best of our knowledge, we are unaware of any reports on rapidly gelling hydrogels based on polyvinyl alcohol and borax. It is possible that the stretchability of this kind of hydrogels is poor, and the hydrogels will easily rupture.

Semi-interpenetrating polymer network (semi-IPN) is composed of a crosslinked polymeric network and a linear polymer,21–26 and has been widely used to improve the stretchability of hydrogels. Some natural polysaccharide and derivatives thereof are often chosen to be entrapped into the hydrogel network, e.g. chitosan, sodium alginate and silk fibroin. Therefore, introducing another polymer into the PVA–borax hydrogels is one of the promising options to obtain the covering material with excellent stretchability.

The rapidly gelling hydrogel were prepared via the reaction of PVA with borax within two minutes at room temperature in our work. Furthermore, introducing SA to the PVA–borax hydrogels significantly enhanced their stretchability.

In the present studies, the compositions of the PVA–SA–borax hydrogels were optimized according to the characterizing of the gelation time and the stretchability. The thermal adaptability, stretchability, self-healing capability and barrier property to hazardous chemicals of the PVA–SA–borax hydrogels were characterized. The rapidly gelling hydrogels as a covering material for the containment of hazardous chemicals were ideal.

2. Experimental

2.1 Materials

Polyvinyl alcohol 05-88 (MW ∼ 22[thin space (1/6-em)]000 g mol−1) was supplied by Kaidu Reagent Corporation (Shanghai, China). Borax was purchased from Fuchen Chemistry Reagent Factory (Tianjin, China). Sodium alginate was purchased from Xilong Chemical Corporation (Guangdong, China). Sodium cyanide was purchased from Tianjin Chemistry Reagent Factory (Tianjin, China). Dichlorvos and phorate were supplied by Chenguang Chemistry Factory (Beijing, China). All the chemicals were used as received.

2.2 In vitro hydrogel formation and gelation time

2.2.1 Preparation of PVA–borax hydrogels. 1 mL of PVA solution (62.0–366.0 mg mL−1) was mixed with 1 mL of borax solution (0.6–73.2 mg mL−1) in glass vials (length: 8 cm and inner diameter: 2.5 cm) and stirred at 120 rpm min−1. Hydrogels with different PVA contents (3–15 wt%) and borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA mass ratios (1[thin space (1/6-em)]:[thin space (1/6-em)]100–1[thin space (1/6-em)]:[thin space (1/6-em)]5) can be obtained by varying the concentrations of the two solutions, as summarized in Table S1 in the ESI.† The time period required for the bar to stop stirring was defined as the gelation time of the mixed solution.7,8,10 The values reported are the average of 3 times of determination.
2.2.2 Preparation of PVA–SA–borax hydrogels. 1 mL of aqueous solution containing PVA (355 mg mL−1) and SA (15 mg mL−1) was mixed with 1 mL of borax solution (14 mg mL−1) in glass vials (length: 8 cm and inner diameter: 2.5 cm) and stirred at 120 rpm min−1 till the hydrogels were formed. In the resulting hydrogels, the PVA contents were 15 wt%, the SA contents were 0.6 wt%, and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios were 1[thin space (1/6-em)]:[thin space (1/6-em)]25. The values reported are the average of 3 times of determination.

2.3 In situ hydrogel formation and characterization

By employing a double syringe applicator assembly (FibriJet SA-6110, America), wherein one syringe is filled with the PVA solution or the mixed solution of PVA and SA and the other with an equal volume of borax solution, hydrogels for tensile test, scanning electron microscopy and barrier property test were formed in situ.
2.3.1 Tensile test. Hydrogels were prepared in a plastic box measuring 17 cm × 9.5 cm × 1 cm by using a double syringe applicator assembly. Then tensile stress–strain properties of the hydrogels samples were analyzed by using a commercial test machine (Labthink-FPT-F1, China). Prior to the test, the gel was cut into strips of 50 mm × 15 mm × 1 mm. The experimental parameters were as follows: fixed speed of 300 mm min−1 and stretch time of 70 s.
2.3.2 Scanning electron microscopy (SEM). Samples were frozen at −50 °C for 12 h and freeze-dried for 48 h by using a LABCONCO freeze drying system to prevent the collapsing of the porous structure. A scanning electron microscope (JSM-6701F SEM, Japan) was used to characterize the morphology of the hydrogels after the samples were coated by a layer of gold film.
2.3.3 Barrier property test. Fig. 1 presents a schematic diagram of the device used to collect the hazardous chemicals through the hydrogels. It consists of a sample chamber (inner diameter of 56 mm and height of 45 mm), two absorption bottles (5 mL) in a cryotrap, and an air sampler (flow rate of 0.2 L min−1).
image file: c6ra14032g-f1.tif
Fig. 1 Schematic diagram of the device used to collect the toxicants through the hydrogels.

The hazardous chemicals were uniformly distributed at the bottom of the sample chamber. Subsequently, in situ formed gels of different thicknesses were prepared over the hazardous chemicals by using a double syringe applicator assembly, in which one syringe was filled with the mixed solution of PVA (355 mg mL−1) and SA (15 mg mL−1) and the other with an equal volume of borax solution (14 mg mL−1). After a fixed duration, the surfaces of the hydrogels were washed with a solvent, wherein the said solvent is exactly the same substance as the absorbing liquid used for absorbing the hazardous chemicals. Consequently, the penetrating hazardous chemicals that will be quantitatively measured later were the combination of those from the surfaces of the hydrogels and those from the contents of the absorption bottles. The values reported are the average of 3 times of determination.

3. Results and discussion

3.1 The optimal composition of the PVA–borax hydrogels

In order to effectively cover the hazardous chemicals, the hydrogels should be gelatinized as soon as the sprayed reactants mixed; in addition, the hydrogels should exhibit good stretchability to avoid rupturing. Therefore, the composition of the PVA–borax hydrogels was optimized according to the characterizing of the gelation time and the stretchability.
3.1.1 The influences of the PVA contents and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios on the gelation time of the PVA–borax hydrogels. The fact that the PVA solutions present high viscosity and the borax shows poor solubility in aqueous media limits the alternation of the ranges of the PVA contents and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA radios. For that reason, the PVA–borax hydrogels with the PVA contents of 3–15 wt% and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]100–1[thin space (1/6-em)]:[thin space (1/6-em)]5 were investigated. The gelation times of the hydrogels of the varied PVA contents and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios were shown in Fig. 2, and the data are summarised in Table S2 in the ESI.†
image file: c6ra14032g-f2.tif
Fig. 2 The gelation times with different PVA contents and borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios.

When the PVA contents of the hydrogels were below 6 wt%, there was completely no gelation, even after 12 h, demonstrating that a minimum PVA content is essential for rapid gelation. When the PVA contents were higher than or equal to 6 wt%, hydrogels were formed within 2 minutes, with the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios ranging from 1[thin space (1/6-em)]:[thin space (1/6-em)]40 to 1[thin space (1/6-em)]:[thin space (1/6-em)]5. A further decreasing of the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios required a higher PVA content to form hydrogels. The PVA contents had little effect on the gelation time with constant borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios. The increasing of the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios with constant PVA contents resulted in longer gelation times.

In our experiments, it was found that the forming of the hydrogels includes two processes: the rapid crosslinking reaction of PVA and borax, followed by the slower water absorbing swelling of the crosslinked products. When the PVA contents were below 6 wt%, the quantities of the crosslinked products were not enough to absorb the water, so the hydrogels were not able to be formed.

When the PVA contents were greater than or equal to 6 wt%, increasing the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios led to a more compact network. As a result, the process of the water absorbing swelling became slower.

3.1.2 The influences of the PVA contents and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios on the stretchability of the PVA–borax hydrogels. Stress–strain analysis was performed to study the influences of the PVA contents and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios on the stretchability. The stress–strain curves of the hydrogels are presented in Fig. 3, and the data are summarised in Table S3 in the ESI.†
image file: c6ra14032g-f3.tif
Fig. 3 The stress–strain curves of the PVA–borax hydrogels, displaying the influences of: (a) borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios (PVA content for all of the spectra: 6%), (b) PVA contents (borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA for all of the spectra: 1[thin space (1/6-em)]:[thin space (1/6-em)]25).

As displayed in Fig. 3a, with the PVA contents remaining unchanged, the increasing of the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios increased the maximum stresses of the hydrogels. At the PVA contents of 6 wt%, the maximum stress increased from 0.67 kPa to 80.50 kPa, with the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios increasing from 1[thin space (1/6-em)]:[thin space (1/6-em)]40 to 1[thin space (1/6-em)]:[thin space (1/6-em)]5. However, the hydrogels with the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]5 fractured when the tensile strain reached 2.12 mm mm−1. The hydrogels with lower borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios were capable of stretching to the maximum strain without fracturing. The scanning electron microscope images that highlight the influence of the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios on the hydrogel morphology are presented in Fig. 4. At a higher borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratio such as 1[thin space (1/6-em)]:[thin space (1/6-em)]5 in Fig. 4a, a three-dimensional network morphology is evident following the water removal. These ordered voids can improve the tensile stress, but at the same time the hydrogels with the higher borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratio displayed a reduced free volume and a more confined structure, leading to a reduced macromolecular chain mobility, resulting in fracturing. Decreasing the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios (Fig. 4b–d) yielded less regular networks and more free PVA chains, leading to easier macromolecular chain mobility, resulting in decreased tensile stress and increased tensile strain. Consequently, the hydrogels with the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]25 or 1[thin space (1/6-em)]:[thin space (1/6-em)]15 showed good stretchability.


image file: c6ra14032g-f4.tif
Fig. 4 The SEM micrographs of the influence of the PVA[thin space (1/6-em)]:[thin space (1/6-em)]borax ratios on hydrogel morphology (a) 1[thin space (1/6-em)]:[thin space (1/6-em)]5, (b) 1[thin space (1/6-em)]:[thin space (1/6-em)]15, (c) 1[thin space (1/6-em)]:[thin space (1/6-em)]25, and (d) 1[thin space (1/6-em)]:[thin space (1/6-em)]40 (PVA content for all of the spectra: 6%).

Fig. 3b displays the influence of the PVA contents on the stretchability. At a constant borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]25, increasing the PVA contents in the hydrogels enhanced the maximum stress from 7.33 kPa (6 wt%) to 10.00 kPa (9 wt%), 13.27 kPa (12 wt%) and 36.59 kPa (15 wt%). The hydrogels with higher PVA contents had more chains, interactions, entanglements and hydrogen bonds therewith; therefore, the crosslinking built up a stronger network. Accordingly, the product of the PVA content of 15% showed good stretchability.

3.2 The characterization of the PVA–SA–borax hydrogels as a material for covering hazardous chemicals

Based on the foresaid optimal composition of the PVA–borax hydrogels, SA was chosen to be entrapped into the crosslinked network to improve the stretchability of the PVA–borax crosslinked system. SA form very viscous solutions, even at low concentrations, and are difficult to handle.8 By preparing the PVA–SA–borax hydrogels with the PVA contents of 15 wt%, the SA contents of 0.6 wt% and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]25, we observed the thermal adaptability, stretchability, self-healing behavior and barrier property to hazardous chemicals of the PVA–SA–borax hydrogels.
3.2.1 The thermal adaptability of the PVA–SA–borax hydrogels. Considering the hydrogels application environment, the influence of temperature to gelation was investigated. Table 1 shows the gelation time of the PVA–SA–borax hydrogels. It indicated that the temperature have no significant impact on gelation time.
Table 1 The gelation time of the PVA–SA–borax hydrogels at different temperatures
Temperature (°C) Gelation time (second)
0 36
10 33
20 36
25 39
30 38
40 42

3.2.2 The stretchability of the PVA–SA–borax hydrogels. The hydrogels were prepared by using a double syringe applicator assembly. The stress–strain curves of the hydrogels are illustrated in Fig. 5, and the data are summarised in Table S4 in the ESI.†
image file: c6ra14032g-f5.tif
Fig. 5 Stress–strain curves of the PVA–SA–borax hydrogels (for all of the curves: PVA content 15 wt%, and borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratio 1[thin space (1/6-em)]:[thin space (1/6-em)]25).

The maximum stress of the PVA–borax hydrogels was 36.59 kPa, and the maximum stress of the PVA–SA–borax hydrogels was 69.25 kPa. That enhancing in stretchability can be attributed to the forming of semi-interpenetrating networks. The strong interfacial interactions between the SA and PVA imparted restrictions on segments of polymer chains during deformation, leading to enhanced tensile stress. A proposed structure of the hydrogels, showing both crosslinking and hydrogen bonding is presented in Fig. 6.


image file: c6ra14032g-f6.tif
Fig. 6 The crosslink bonding between PVA (blue), SA (purple), borax (red) and water (green). Hydrogen bonding is represented by black dashed lines.
3.2.3 The self-healing behavior of the PVA–SA–borax hydrogels. The hydrogels were probably ruptured by sharps on the ground in the practical application, resulting in the penetration of the hazardous chemicals. Interestingly, it was noted that these hydrogels exhibited particular recovery in our experiments. As shown in Fig. 7.
image file: c6ra14032g-f7.tif
Fig. 7 Photographs showing the self-healing behavior of the PVA/SA/borax hydrogels: (a) the process of two single hydrogels merging into one single hydrogel, (b) the process of the hydrogel covering a nail.

Upon the two single hydrogels moving towards each other, they came into adhering thereto, resulting in an integration phenomenon (Fig. 7a). They can completely merge in three hours.

To further demonstrate its self-healing capability, another piece of the hydrogel was perforated by using a nail, and then we covered the nail with a piece of the hydrogel (Fig. 7b). As expected, the nail gradually disappeared because of the integrating of the two blocks of the hydrogels, without additional external stimuli. In another word, the hydrogel filled up the broken hole made by the nail perforating. The deformation and adherency of the two block hydrogels were attributed to the high water content. The key to completely merging is to have a good balance between a sufficient amount of free hydroxyl groups of PVA on surfaces required for forming interchain H-bonds and enough chain mobility ensuring chain diffusion across the interface.18,19,27,28 The self-healing capability was an advantage of the covering material.

3.2.4 Barrier property of the PVA–SA–borax hydrogels. The barrier properties of the PVA–SA–borax hydrogels to sodium cyanide, dichlorvos, and phorate were investigated. In all of the resulting hydrogels, the PVA contents were 15 wt%, the SA contents were 0.6 wt%, and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios were 1[thin space (1/6-em)]:[thin space (1/6-em)]25. In the experiment, the penetration rates of the hazardous chemicals were used to represent the barrier property to hazardous chemicals of the hydrogels. The penetration rate is the amount of the hazardous chemicals penetrating through the hydrogels per square meter.
3.2.4.1 Sodium cyanide and dichlorvos. Hazardous chemicals (100 g m−2) was uniformly distributed at the bottom of the sample chamber, and the hazardous chemicals were covered by using the in situ formed PVA–SA–borax hydrogels. Deionized water was chosen as the absorbing liquid for sodium cyanide and dichloromethane for dichlorvos (Fig. 1). The amount of the sodium cyanide transferred through the hydrogels was quantitatively determined by using a cyanide concentration meter (HANA) after a fixed duration. The gas chromatography (Agilent 6890) was used to quantitatively determine the amount of the dichlorvos. The penetration rates of the sodium cyanide and dichlorvos transferred through the hydrogels are presented in Table 2.
Table 2 The barrier property to sodium cyanide and dichlorvos of the PVA–SA–borax hydrogels
Hazardous chemicals Thickness (mm) Penetration rate (mg m−2)
4 h 8 h 12 h 16 h 20 h 24 h
Sodium cyanide 3 1793.5 1943.8 2221.1 2254.5 2262.1 2639.6
5 277.4 304.5 400.3 413.6 489.5 633.7
10 6.4 90.2 120.3 126.3 130.1 157.5
Dichlorvos 3 16.4 106.1 114.5 164.0 441.0 647.5
5 0.0 9.4 85.8 135.7 187.5 360.8
10 0.0 0.0 3.0 5.4 13.6 52.2

The penetration rates of the sodium cyanide increased over time, and it gradually decreased with the increasing of the thickness of the hydrogels. When the thickness of the hydrogels was 3 mm, more than 97.3% of the sodium cyanide can be confined for one day, while 99.8% of the sodium cyanide can be confined with the 10 mm thick hydrogels.

When the thickness of the hydrogels was 3 mm, more than 97.3% of the sodium cyanide can be confined for one day, while no dichlorvos penetrated through the hydrogels within 4 h with the 5 mm, or 8 h with the 10 mm, thick hydrogels. The solubility of dichlorvos in water was about 10 g L−1 at room temperature, which was much lower than that of sodium cyanide. Therefore, the permeating of dichlorvos in hydrogels was slower than that of sodium cyanide. As a result, the hydrogels exhibited effective barrier property to dichlorvos.


3.2.4.2 Phorate. Phorate (100 g m−2) was uniformly distributed at the bottom of the sample chamber, and the phorate was covered by using the in situ formed PVA–SA–borax hydrogels. Dichloromethane was chosen as the absorbing liquid (Fig. 2). The amount of the phorate transferred through the hydrogels was quantitatively determined by using gas chromatography (Agilent 6890).

As shown in Table 3, when the thickness of the hydrogels was 1 mm, more than 99.9% of the phorate can be confined for one day, while no dichlorvos penetrated through the hydrogels within 4 h with the 2 mm, or 24 h with the 3 mm, thick hydrogels. Phorate is insoluble in water. So there was little phorate permeating the hydrogels and the hydrogels presented excellent barrier property to phorate.

Table 3 The barrier property to phorate of the PVA–SA–borax hydrogels
Thickness (mm) Penetration rate (mg m−2)
4 h 8 h 12 h 16 h 20 h 24 h
1 10.0 14.2 14.3 16.2 17.2 19.1
2 0.0 3.1 3.3 4.1 6.9 7.0
3 0.0 0.0 0.0 0.0 0.0 0.0

4. Conclusions

The PVA-based rapidly gelling hydrogels were prepared for the containment of hazardous chemicals. It was found that the PVA–borax hydrogels can be formed within 2 minutes at the PVA contents of 6–15 wt% and the borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]40–1[thin space (1/6-em)]:[thin space (1/6-em)]5. The stretchability of the PVA–borax systems was enhanced by the incorporation of SA. The composition of the PVA–SA–borax hydrogels is 15 wt% PVA, 0.6 wt% SA and 1[thin space (1/6-em)]:[thin space (1/6-em)]25 borax[thin space (1/6-em)]:[thin space (1/6-em)]PVA ratios. The PVA–SA–borax hydrogels displayed good thermal adaptability, self-healing capability, and excellent barrier properties to sodium cyanide, dichlorvos, and phorate. Those results suggested that the rapidly gelling PVA–SA–borax hydrogels could be a promising barrier material to hazardous chemicals. The hydrogels can be prepared in situ via spraying the PVA–SA mixed solution and borax solution from a double-medium spray head device.

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Footnote

† Electronic supplementary information (ESI) available: Experimental variables and stress–strain data. See DOI: 10.1039/c6ra14032g
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