The preparation strategy and working principles of IG were illustrated in Figure 1. The yellow-colored matrix (Figure 1a) was composed by gelation of PVDF (Figure 1b) and PEG (Figure 1c). While, the blue-colored SHM (Figure 1d), which has shown fluffy and disordered motifs (Figure 1e), was prepared via in-situ exfoliation (Figure 1f) of hetero-layered 2D MOFs [Co(NCS)2(pyz)2] and [Zn(NCS)2(pyz)2] (Figure 1g, Figure S1 and S2). It should be note that, SHM can be well dispersed in ethanol solution to give a colloidal suspension with a significant Tyndall effect (Figure 1h), suggesting the atomically thin 2D nature (Figure S3). In addition, the key aspects of SHM for humidity capture lies on the larger 2D surfaces area, as well as the high polarity of pyrazine linker and NCS ligand. Hence, the more exposure of the 2D surfaces, the higher working capacity of SHM. At this point, the yellow-colored blank gel can act as versatile bracket to dispersed evenly the SHM into atomically scale (Figure S4). As a consequence, the resulted IG (Figure 1i) was highly sensitive to atmospheric humidity, which can capture humidity effectively with continuously color-change (Figure 1j) and in-situ liquefaction (Figure 1k), and this process can be recyclable, providing a pretty good model for atmospheric humidity manipulation at the device level.
The successfully application of IG was highly dependent on the flexibility of the polymer framework, the good compatibility with SHM and appropriate flex channel for water delivery are required. To verify these aspects, the flexibility of IG with weight ratio of SHM:PVDF = 3% was investigated. A sky-blue colored cubic of IG with side-length of 6 mm was prepared (Figure 2a), upon exposure to ambient air, continuous water adsorption was visible to the naked eye, with the color of the cubic has changed into grey at 10 min (Figure S5) and finally into light-pink (Figure 2b). However, no increase in side-length can be observed,
suggesting a good adaptability of IG to water. More importantly, Raman measurements have demonstrated that upon water capture, the spectrum of SHM have been disappear with only the spectrum of water presented (Figure S6), suggesting that the water molecules were adsorbed on the 2D surfaces of SHM. While upon heating under 40 ºC for 1h with continuous water release, the light-pink colored cubic has shrinking into a blue-colored cubic with side-length of about 4 mm, demonstrating the good flexibility of the polymer framework. The SEM images of IG under the above-mentioned three different state were then investigated. For the as-prepared state, channels with different sizes (average diameter of about 5 µm) were fill the whole gel (Figure 2d, e), which are expected to promote greatly the efficiency of water adsorption on IG. While upon water adsorption, the sizes of channels were expanded about 8 times, with average diameter to be about 40 µm (Figure 2f, g), providing a wide and unobstructed passage for shuttle of water molecules. It should be note, the expand of channels did not induced the increase the whole volume of gel, suggesting the intelligent nature of this gel with superior adaptability to water.
At this point, water release must lead to shrinking of the channels on IG, as expected, the diameter of the channel has decreased to be about 1µm (Figure 2h, i). Note again, this kind of 40 times shrinking in channels has only induced about 3.4 times decrease of the gel volume, further demonstrating the intelligence of the gel with superior adaptability to water.
For practical application, resume of the shrinking gel is need. On considering the cross-linking structure, immersion in benign solvent is expected to swell the shrinking gel. We first choose aqueous solution, for the sky-blue-colored samples, the immersion in water have given birth to a white cubic (Figure 3a), however, no increase in volume can be observed even immersed for 12h (Figure 3b), suggesting that water is the bad solvent for IG, then the in-situ liquefaction of the adsorbed water in IG can be expected. While for the samples after heated under 40 ºC for 40 min (with side-length of 4 mm), the immersion in DMF solution has shown continuously fast increase in side-length of the cubic (Figure 3c), from the begin 4mm to 5.5 mm at 5 min, then 6.5 mm at 10 min, and finally to 7 mm at 50 min. Interestingly, for the swelling cubic with side-length of 7 mm, the blue-colored SHM was distributed in the inner space within 5 mm, almost similar with the as-prepared state. That is, the swelling process did not affect the distribution position of SHM, suggesting the well stability of IG during the process of water adsorption and release. In addition, this kind of IG has shown a much fast swelling process than the shrinking (Figure 3d), the former has reached a 3.5 ratio within 50 min, while the latter got a 0.7 ratio at 100 min, providing a 10 times disparity in efficiency. One thing should be stressed is that, heating under 40 ºC for more than 2h (with side-length of 3.5 mm) will destroy the elasticity of IG, as no swelling of the shrinking gel can be observed even been immersed in DMF for more than 50 min (Figure 3e), accompanied by a 3 time decrease in water adsorption capacity (Figure 3f).
The above-mentioned results have demonstrated the intelligent, elastic and stable aspects of the polymer framework, then the optimal doping of SHM in IG needs to be explored, for the purpose of fully expose the 2D surface as well as high polarity pyrazine linker and NCS ligands, to get the optimum performance of IG. The doping of SHM in IG was evaluated by weight ratio of SHM:PVDF, which have been set as 3, 10, 20, 30, 40, 50, and 70%. As have shown in Figure 4 and Figure S7, for all of the ratios, fast water adsorption, continuously dramatic color change, as well as continuously in-situ liquefaction, all have been observed within 3h upon exposure the gel cubic to ambient atmosphere. However, the difference still remains (Figure 4a), that is: for ratios less than 10% (3%) and larger than 40% (40, 50, 70%), the color change were all not obviously, the former can be attribute to the too small amounts of SHM, while the latter was because of too much amounts of SHM that were accumulated over dense, which adverse to the fully exposure the 2D surfaces as well as high polarity pyrazine linker and NCS ligands.
Actually, for the cubic gels with SHM: PVDF weight ratio of 10, 20 and 30%, the fast water adsorption, the sensitive and huge contrast of colors induced by adsorption capacities, the efficient in-situ liquefaction with no overflow of SHM (Figure 4a), all have demonstrated that this kind of intelligent gel has shown promising applications in atmospheric humidity detection, spontaneous capture and in-situ liquefaction. While for these three valid ratios, the time-dependent water uptake profile has suggested that the cubic gel with SHM: PVDF weight ratio of 30% has shown the largest water adsorption capacity, reaching a value of 0.77 g/g (IG) at 1500 min (Figure 4b), which was followed successively by ratio of 50, 20, 10 and 40%. One thing should be stressed is that, the water adsorption of IG was only owe to SHM. Hence, the water adsorption capacity based on the amount of SHM deserves to be investigated. Figure 4c has shown the SHM-based water adsorption capacity, where, the ratio of 30% have displayed a value as high as 26856 g/g(SHM), which was substantially larger than the value of 21485, 18979, 14925, and 11976 g/g(SHM), for ratio of 20, 10, 40, and 50%, respectively. Compared with the water adsorption capacity of 6.58 g/g (SHM) for the powdered state SHM, the present results have provide the following three beneficial aspects: (1) The integration into IG has provide a practical model for application of SHM; (2) The doping of SHM into polymer framework, with any ratios, have improved significantly the performance of SHM (3) The optimal doping ratio is SHM:PVDF = 30%, which has get a 4081 times increase in working capacity than the individual SHM.
The subtle integration of super hygroscopic material (SHM), hydrophilic polymer (PEG-6000), and hydrophobic polymer (PVDF), to construct an intelligent gel (IG) for practical atmospheric humidity detection, spontaneous capture and in-situ liquefaction, under ambient conditions, have been demonstrated. Within such a design strategy, the division of labor was clear-cut for each part: (1) SHM was responsible for fast water adsorption and in-situ liquefaction; (2) PEG-6000 was act as a modifier to assist PVDF to form porous polymer framework for water storage; (3) PVDF was the main framework of IG, which form crisscross channels for water molecules adsorption and release. This synergistic effect involves three parts has improved 4081 times the working capacity of SHM, which are expected to open new avenue for next-generational water management systems.