As viscoelastic materials, hydrogels show energy storage and energy dissipation characteristics. The corresponding storage modulus (G′) and loss modulus (G″) can be measured to reveal the structural difference of hydrogels ( 33 ). To confirm the cogelation of PVA and chitosan, we prepared a PVA/PPy hydrogel as the blank sample (i.e., control sample without chitosan). The higher G′ values compared with G″ values confirm the cross-linked polymeric skeleton of these hydrogels ( Fig. 2D ). The higher G′ value of the h-LAH compared with PVA/PPy indicates stronger mechanical strength due to the introduction of chitosan, while the higher G″ value reveals the steric hindrance of chitosan polymer chains ( 33 ). In addition, the comparison of G′ and G″ values between PVA/chitosan hydrogel and the h-LAH confirms that PPy is interpenetrated in the polymer network composed of PVA and chitosan (fig. S1). These above results demonstrate that the PPy is interpenetrated in the PVA/chitosan hybrid polymer network.

The in situ cogelation method is used to construct the polymer network that consists of PVA and chitosan. The PPy was used as an additive endowing the h-LAH with a light-absorbing functionality (see section S1 for details). The as-prepared h-LAHs were black and flexible ( Fig. 2A ). The configuration of h-LAHs, such as the size and shape, depends on the mold used for gelation, indicating desirable scalability. Scanning electron microscopy (SEM) imaging reveals the cross-section morphology of the freeze-dried h-LAH ( Fig. 2B ). Pores with a diameter of several microns are uniformly distributed in the h-LAH, which is a typical structural feature, indicating homogeneous gelation throughout the resultant hydrogel. To analyze the chemical composition, we show the Fourier transform infrared (FTIR) spectra of pure PVA, PPy, chitosan, and the h-LAH in Fig. 2C . In the spectrum of PVA (black curve), the peak at 1087 cm −1 represents the C─O stretching, which is a characteristic peak of PVA ( 31 ). The spectrum of PPy (red curve) shows the absorption signal at 1451 cm −1 , corresponding to the C═C stretching in the pyrrole rings ( 32 ). The blue curve represents the spectrum of chitosan in which the characteristic peaks are located at 1626 and 1374 cm −1 , corresponding to the amide peak of chitosan ( 31 ). All the characteristic peaks of PVA, PPy, and chitosan can be found in the spectrum of the h-LAH (purple curve), confirming the existence of PPy in the PVA and chitosan hybrid polymer network.

The h-LAH is made by infiltrating PPy absorbers into a matrix consisting of PVA and chitosan. As PVA can play the role of surfactant, the PPy chains are dispersed uniformly in the cross-linked PVA and chitosan polymer network. Upon exposure to the solar irradiation, h-LAH can generate water vapor using solar energy. The floating h-LAH consists of the hydratable polymer network based on cross-linked PVA and chitosan, which is interpenetrated by the light-absorbing PPy. The containing water has three different water types—bound water, IW, and FW. Wherein, the IW can be effectively evaporated with significantly reduced energy demand.

Tunable water state in the h-LAH

According to the difference of intermolecular hydrogen bonding, including water/polymer bonding, weakened water/water bonding, and normal water/water bonding (Fig. 3A), the water in the hydrated polymer network has been classified into three types: FW (Fig. 3A, light blue color), IW (Fig. 3A, yellow color), and bound water (Fig. 3A, dark blue color), respectively. We analyze the Raman spectra in the region of O─H stretching to show the hydrogen bonding distinction of water molecules in the h-LAH (see details in section S2.3.1), revealing the water state in the h-LAH (Fig. 3B). The peaks at 3233 and 3401 cm−1 correspond to FW (Fig. 3B) with four hydrogen bonds (two protons and two lone electron pairs are involved in hydrogen bonding with adjacent water molecules), while the peaks at 3514 and 3630 cm−1 are associated with weakly hydrogen-bonded IW (Fig. 3B) (19). Hence, the stronger IW peaks compared with FW peaks indicate a higher proportion of IW in the h-LAH.

Fig. 3 Water state in the h-LAH. (A) Schematic of the water in the hydratable polymer network of the h-LAH, showing water/polymer bonding, weakened water/water bonding, and normal water/water bonding. (B) Raman spectra showing the fitting peaks representing IW and FW in the h-LAH. (C) Differential scanning calorimetry (DSC) curves of the h-LAH with different water fraction (i.e., swollen level, 100% refers to the fully swollen state).

Given that the water in each state shows characteristic phase change behaviors (34), such as freezing and melting, the involved energy transfer could be monitored to further confirm that the h-LAHs induced the differentiation of water state. To quantitatively describe the water content (i.e., hydration level) of h-LAHs, we first define a water fraction (W H 2 o ) as W H 2 O = W / W s where the W and W s are the weight of water in the h-LAH and the saturated water content of a fully swollen h-LAH, respectively. We use the differential scanning calorimetry (DSC) to reveal the phase change of water (Fig. 3C). The h-LAH with a low W H 2 o of 1% (i.e., almost dried sample) presents a straight line without signals of endothermic process (black curve). It has been demonstrated that bound water, which strongly interacts with hydrophilic polymer chains, is nonfreezable water, while the IW and FW are freezable (34). Therefore, the water molecules are captured by the polymer network of the h-LAH, forming bound water, when the W H 2 o is 1%. In contrast, there are two peaks located at 0° and ~5°C corresponding to the melting of IW and FW, respectively, that could be observed in the fully hydrated h-LAH (i.e., W H 2 o is 100%; green curve). In addition, the measured melting point of FW is shifted to a higher temperature with the increase in W H 2 o (red, blue, and green curves), which could be attributed to the postponed heating of samples induced by endothermic melting. In contrast, the steep signal of IW (purple curve) is independent of the water content of the h-LAH, indicating that the generation of IW relies on the hydratable polymer network. It should be mentioned that the melting points of IW and FW are close because of the almost similar melting enthalpy of ices with different crystal structures (35).

To obtain a tailored water state that is capable of providing high-proportioned IW, we constructed polymer networks consisting of PVA and chitosan with a different proportion. Wherein, the h-LAH samples with various PVA/chitosan weight ratios from 1:0 (i.e., no chitosan additive), 1:0.05, 1:0.1, and 1:0.175 to 1:0.25 are noted as h-LAH1 to h-LAH5, respectively. Because of the similar carbon/oxygen ratio (i.e., molar ratio of hydrophobic and hydrophilic groups) of PVA and chitosan (Fig. 4A), the obtained polymer networks in h-LAHs show a similar ability to capture water molecules to form bound water (see details in fig. S2, A and B). The amount of IW highly depends on the hydrability of polymer networks, which presents the ability of polymer networks to swell water and can be indicated by the saturated water content of h-LAHs. The saturated water content (Q s ) of h-LAHs is represented by Q s = W / W d where W and W d are the weights of the water in the fully swollen sample and the corresponding dried aerogel sample, respectively. The Q s of h-LAHs rises with the proportion of chitosan (Fig. 4B), indicating an increased hydrability. This phenomenon could be attributed to the presence of highly hydratable –NH 2 groups on chitosan chains (36).

Fig. 4 Tunable water state and water vaporization enthalpy of the h-LAHs. (A) Schematic illustration of PVA and chitosan structure. (B) The saturated water content of h-LAH samples, where the h-LAHs with PVA/chitosan weight ratios of 1:0 (i.e., no chitosan additive), 1:0.05, 1:0.1, 1:0.175, and 1:0.25 are noted as h-LAH1 to h-LAH5, respectively. (C) The ratio of IW to FW in h-LAHs. (D) The equivalent water vaporization enthalpy of bulk water and water in h-LAH1 to h-LAH5.