Binary Crystallized Supramolecular Aerogels Derived from Host

Oct 29, 2015 - solution are collapsed onto the crystalline domain and result in interconnected pores among the cross-linking domains. To demonstrate t...
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Binary Crystallized Supramolecular Aerogels Derived from Host Guest Inclusion Complexes Jin Wang† and Xuetong Zhang*,†,‡ †

Suzhou Institute of Nano-tech & Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China and ‡School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China

ABSTRACT Aerogels with low density and high porosity show

outstanding properties such as large surface area and low thermal and acoustic conductivity. However, great challenges remain to convert hydrophilic polymer based hydrogels to corresponding aerogels. Here, we report a structurally new type of aerogels, supramolecular aerogels (SMAs), derived from supramolecular hydrogels formed by self-assembling of poly(ethylene glycol) and R-/γ-cyclodextrin. The SMAs posses a characteristic binary crystallized nanosheet structure due to their supramolecular cross-linking nature, and their specific surface areas and nanosheet structures are tunable. Furthermore, we demonstrated application of the aerogels as solid solid phase change materials with tunable latent heat, reversible melting-crystallization cycle while keeping the microstructure of the SMAs unchanged. KEYWORDS: aerogel . supramolecular hydrogel . host guest inclusion complexes . phase change materials

A

erogels are highly porous solid materials having extremely low density, low thermal conductivity, large open pores and high specific surface areas. Due to these outstanding structural properties, they have found wide applications as thermal and acoustic insulators, catalyst supports, drug carriers, cosmic dust collectors, etc.1 4 There were first proposed and synthesized by Kistler in the 1930s, who defined them as gels in which the liquid had been replaced by air and with very moderate shrinkage of the solid network.5 Up to now, various single-component aerogels and composite aerogels have been synthesized, such as silica aerogels,6 9 metal oxide aerogels,10 13 element aerogels,14,15 resorcinol/formaldehyde (RF) aerogels,16 carbon aerogels,17,18 cellulose aerogels, 19 21 carbon nanotube (CNT) aerogels, 22,23 and graphene (rGO/GO) aerogels.24 28 It has been expected that almost any gels could be converted into their corresponding aerogels.4,5 However, as one of the most widely investigated and applied gels, the hydrophilic polymer based hydrogels such as poly(ethylene glycol) (PEG), polyacrylamide (PAM), poly(vinyl alcohol) (PVA), and hyaluronic WANG AND ZHANG

acid derived hydrogels29,30 are not yet converted into single-component aerogels, possibly due to their weak and soft skeleton networks as compared to those of silica gels (which are strong and rigid) (Scheme 1A).1 4 Moreover, the as prepared aerogels are normally composed of noncrystalline matter,4 and great challenges remain to synthesize intrinsic aerogels built up with crystallized networks.31,32 Attempts to resolve the aforementioned issues had been proposed by various strategies. For instance, composite aerogels involving PEG and PVA, etc. have been prepared by using rigid template, such as clay, GO, SiO2, and CNT,33 35 and crystallized aerogels have been fabricated by thermal treatment of metal oxide based aerogels.11,12 Nevertheless, much efforts should be made to fulfill the synthesis of single-component aerogels derived from hydrophilic polymers while simultaneously forming crystallized networks. A strategy may be practicable by introducing crystalline domain as a rigid crosslinking point as proposed in Scheme 1A. Similar to the silica wet gels, the solid crosslinking phase might be strong enough to maintain its skeleton network during and after VOL. XXX

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* Address correspondence to [email protected]. Received for review August 23, 2015 and accepted October 29, 2015. Published online 10.1021/acsnano.5b05281 C XXXX American Chemical Society

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RESULTS AND DISCUSSION

Scheme 1. (A) Schematic diagram of the network structures of silica gels, polymer-based hydrogels, and proposed hydrogels with a solid phase (cross-linking point) and polymer solution phase. (B) Schematic description of the synthesis of supramolecular aerogels derived from host guest inclusion complexes based on the self-assembly of poly(ethylene glycol) and R-cyclodextrin.

supercritical liquid drying (SCLD), while the polymers in solution are collapsed onto the crystalline domain and result in interconnected pores among the cross-linking domains. To demonstrate this proposal, herein we report a structurally new type of single-component aerogel supramolecular aerogel (SMA), derived from PEG and R-cyclodextrins (R-CD) based host guest inclusion complexes, that is the skeleton of the SMAs built up with a supramolecular polypseudorotaxane (PPR) structure, rather than a mixture or composite of PEG and R-CDs. As shown in Scheme 1B, supramolecular hydrogels (SMHGs) were formed via the self-assembling of PEG and R-CD in a wide range of concentration and molar ratios, where the formation of channel-type crystallized PPR acted as a physical cross-linking phase, while the uncovered PEG was dissolved in the aqueous phase.36 39 As expected, SMAs were successfully prepared via SCLD of the SMHGs with little volume shrinkage. Interestingly, the networks of the SMAs are built up with interconnected graphene-like two-dimensional (2D) nanosheets, which is rare among organic aerogels and silica-based aerogels (normally formed with colloidal particles or nanofibers).2,3 Moreover, the SMA can support a weight which is 1000 times over its own weight, which is unexpected and interesting because PPRs are normally powdery materials, possibly due to their supramolecular interaction and highly crystallized structure. Moreover, the SMA is stable under high temperatures (>100 °C) for days (no visible change in WANG AND ZHANG

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appearances and microstructures), which endows the SMA with the possible application as solid solid phase change materials (PCM). Furthermore, the SMA can be facilely dissolved in water due to its supramolecular assembly and dissociation nature between PEG and R-CD.

PPR is a kind of supramolecular material formed by the inclusion of a linear polymer chain with several macro cyclic molecules.40 42 In the case of PPR formed by the self-assembly of PEG and R-CD, PPR precipitates are produced when the number-average weight (Mn) of PEG is lower than 3500. However, with higher Mn values, SMHGs can be formed in that the threaded PEG chain by R-CDs forms a PPR cross-linking domain due to hydrogen bonding, while the unthreaded PEG chain is dissolved in the solution (Scheme 1B).36 39,43 Therefore, PEGs with Mn value of 10 000 (PEG10K) and 20 000 (PEG20K) were used as guests in the present work. For the successful conversion of the SMHGs to SMAs, two key issues need receive significant consideration: (1) preserving the network structure during solventexchange and SCLD, and (2) washing out the unthreaded PEG and free R-CDs. Though solvent-exchange (which is a requisite for the supercritical (SC) CO2 liquid drying) with DMF, THF, or ethanol may dissolve or disrupt the SMHGs, we luckily found that the SMHGs were stable in acetone (see Figure S1 in the Supporting Information), and the corresponding acetone-gel can be dried under SC CO2. On the other hand, the free R-CDs and unthreaded PEG could be washed out by extraction with SC CO2, while the PPR can be reserved in it.44,45 As shown in Table 1, the densities of the SMAs are lower than 100 mg/cm3, and high porosities from 93.8 to 96.0% have been achieved. It should be pointed out that the densities of the SMAs are smaller than the theoretic values, and it might be attributed to the weight loss of free CD and PEG during solvent extraction of SC CO2, though a little volume shrinkage was happened after drying. Interestingly, the density of the cryogel (127.8 mg/cm3, entry 1) is higher than the theoretic value (97.8 mg/cm3), this is because the freeze-drying result in large volume shrinkage and the free CDs and PEG are trapped in the cryogel. Effects on the morphology and physical property of the SMAs were first investigated by varying feed molar ratio of R-CD to PEG. Consequently, various SMHGs with different molar ratio of CD to PEG were prepared, and the corresponding aerogels were named as SMAn-1, where n indicates the volume ratio of CD solution to PEG solution (see Table 1 and Methods for details). Figure 1A F shows the SEM images of the SMAs (entries 1 6 in Table 1). Interestingly, all the SMAs (entries 2 6) are built up with graphene-like 2D nanosheets with thickness smaller than 50 nm VOL. XXX

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average pore a

entry

sample name

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

SMA2-1 cryogel SMA1-1 SMA2-1 SMA4-1 SMA15-1 SMA20-1 SMA4-1-07W SMA4-1-05W SMA4-1-03W SMA4-1-05D SMA15-1-05D SMA15-1-03D SMA2-1-PEG10K SMA4-1-PEG10K SMA10-1-PEG10K SMA15-1-PEG10K

volume ratio CD/PEG/solvent

b

c

2

c

densityd (mg/cm3)

surface areas (m /g)

diameter (nm)

measured (theoretic)

porosity (%)

0.047 11.37 31.75 33.99 41.45 27.69 45.03 66.19 53.81 69.52 54.03 61.72 22.35 33.48 62.26 41.47

11.0 10.5 9.8 10.7 12.2 12.1 12.4 11.9 11.8 11.0 11.4 10.9 9.7 14.2 11.6

127.8 (97.8) 95.3 (98.3) 91.5 (97.8) 84.0 (97.4) 93.1 (96.9) 89.4 (96.8) 86.3 (90.1) 75.1 (81.1) 62.3 (69.5) 79.5 (81.1) 86.4 (91.1) 81.2 (86.1) 90.0 (97.8) 86.7 (97.4) 82.5 (97.0) 88.1 (96.9)

91.7 93.8 94.1 94.6 94.0 94.2 94.4 95.1 96.0 94.9 94.4 94.8 94.2 94.4 94.7 94.3

2:1: -e 1:1: 2:1: 4:1: 15:1: 20:1: 4: 1:0.4 (water) 4:1: 1 (water) 4:1: 2 (water) 4:1: 1 (DMSO) 15:1: 1 (DMSO) 15:1: 2 (DMSO) 2:1: 4:1: 10:1: 15:1: -

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TABLE 1. Recipe, Identification, And Structure Parameters of the SMAs

a SMA2-1 cryogel is dried by freeze-drying. b The Mn of PEG in entries 1 12 is 20K, while that of entries 13 16 is 10K. The concentration of R-CD is 0.0967 g/mL, while that of PEG is 0.100 g/mL. c Determined from the N2 adsorption desorption measurement. d The measured density of the SMAs is determined by dividing their weights by its volume, and the theory density is calculated from the total weights of CD and PEG and measuring the wet-gel volume (no shrinkage). e CD and PEG aqueous solution were used in these samples without adding extra solvents.

Figure 1. SEM images of (A) SMA2-1 Freeze Dry, (B) SMA1-1, (C) SMA2-1, (D) SMA4-1, (E) SMA15-1, and (F) SMA20-1; (G) N2 adsorption desorption isotherms of the indicated SMAs.

(take SMA4-1 for instance, the thickness of the nanosheet is ca. 36 nm as revealed by SEM, TEM, and AFM shown in Figure S2), while the cryogel (Figure 1A, entry 1) exhibits surface relief resulted from ice. Besides, the sizes of the nanosheets decrease with the increase of n. Moreover, the BET surface area of SMA2-1 WANG AND ZHANG

is 31.75 m2/g, which is 676 times higher than that of the cryogel (entry 1, 0.047 m2/g) prepared from the same SMHGs2-1. These results suggest that the network of the SMHGs could be well preserved by SCLD drying. On the contrary, freeze-drying results in macroporous bulky cryogels, while ambient pressure drying (APD) results in bulky film without any pores or bricks from the SMHGs or acetone-exchanged gels (Figure S3), which were possibly due to the collapse of the nanosheet networks. As can be seen in Figure 1G, all the SMAs show typical type IV isotherms and H3 hysteresis loops associated with layered pores according to the IUPAC classification indicating the coexistence of mesoand macro-pores. The specific surface areas (SAs) of the SMAs calculated by the Brunauer Emmett Teller (BET) method are summarized in Table 1. By adjusting the feed molar ratio of CD to PEG, SAs are varied from 11.37 (for SMA1-1) to 41.15 m2/g (for SMA15-1). These results are accorded with theoretically calculated values (Figure S4). The increment of SAs was mainly resulted from the decrease of the nanosheet size rather than the decrease of the thickness, as can be seen from the SEM images shown in Figure 1. Moreover, the volume of macropores was remarkably reduced with the increase of feed molar ratio of CD to PEG, which may be resulted from the increase of the concentration of the inclusion complexes. The powder XRD spectra of the SMAs shown in Figure 2A are all characterized by a diffraction peak at 2θ = 19.8°, which are corresponding to the channeltype crystalline structure of PPR formed by R-CD and PEG.46,47 The result indicates that the nanosheets are crystallized with a channel-type structure, which is rare VOL. XXX

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Figure 2. (A) Power WXRD diffraction curves of the SMAs, PEG, and PPR; (B) DSC curves of the SMAs (heating run); (C) TGA traces of the SMAs.

in organic aerogels. Furthermore, as for SMA2-1 and SMA4-1, another two characterized peaks at 2θ = 19.2° and 23.3° corresponding to PEG crystal are clearly identified, while they are absent in the spectra of SMA15-1 and SMA20-1. The result is also demonstrated by the DSC analysis as presented in Figure 2B (the cooling run is shown in Figure S5), where the melting peak corresponding to PEG around 63 °C (Table S2) can be seen for SMA1-1, SMA2-1, and SMA4-1, while no melting peak can be observed for SMA15-1 and SMA20-1. These results confirmed that the nanosheet networks of the SMAs are crystallized. More interestingly, for the SMAs with low molar ratio of CD to PEG (1:1, 2:1, and 4:1), binary crystallized aerogels are obtained. To the best of our knowledge, this is the first time observing a synthetic aerogel formed by 2D nanosheets with binary crystalline structures. The supramolecular PPR structures in the SMAs were further confirmed by TGA as shown in Figure 2C (the DTG curves and their peak temperatures are presented in Figure S6 and Table S3). First, two stage degradation temperatures are obtained, which are due to the degradation of R-CD and PEG, respectively. Second, the degradation temperatures of R-CD in the SMAs are higher than those of pure R-CD, indicating the formation of PPRs.40,41 Moreover, the weight ratio of CD/PEG increases with the increment of feed volume ratio (see Table S1), which indicated that the coverage of PEG chain is controllable. The formation mechanism of the 2D nanosheet network, as well as the binary crystalline structure and its molar ratio reliance, was proposed in Figure 3. In the early stage, R-CDs were threaded onto the PEG chain due to the hydrophobic interaction between the internal cavity of R-CD and PEG chain, and the hydrogen bonding between neighboring CDs. Then, the self-assembly of PPR was driven by hydrogen bonding forming a channel-type crystalline nanoplate,48 50 which led to the formation of SMHGs. The crystal WANG AND ZHANG

Figure 3. Proposed mechanism and microstructures of the supramolecular hydrogels and SMAs.

structure of the SMHG was demonstrated by in situ XRD analysis presented in Figure S7 (see Supporting Information). The diffraction peak appeared at 2θ = 19.8°, confirming the formation of PPR crystalline domain, while no PEG signal can be observed, which indicated that the unthreaded PEG was dissolved in the aqueous phase. After solvent exchange with acetone, the diffraction peak at 2θ = 19.8° was enhanced, and still no PEG signal appeared. Thus, the model of the wet gel was proposed in Figure 3. After SCLD, the unthreaded PEG were collapsed onto the PPR nanoplate. Therefore, a sandwich-like nanosheet structure was developed;a channel-type PPR crystalline domain covered by two PEG crystalline domains. However, in the case of SMA15-1 and SMA20-1, no PEG crystals were formed; this can be explained by the fact that more than 70% of PEG is covered by CDs (Table S1), which prevents the crystallization of PEG.40 From the proposed model, it would be interesting to estimate that the thickness of the nanosheet might be controlled by varying the molar ratio of CD to PEG. For example, reducing the molar ratio of CD to PEG may decrease the number of CD molecules threaded on each PEG chain (chain coverage), thus the thickness of the PPR crystalline domain might be smaller. However, the fact is that decreasing the n value indeed results in a lower coverage (see Table S1), but at the same time, the uncovered PEG chain is increased, which may result in a thicker PEG domain. Therefore, the thickness of the nanosheets in all SMAs was similar (from 30 40 nm as obtained from SEM results). It is should be pointed out that in the case of high n values, such as SMA15-1, whose coverage is 70.5%, the theoretical length of PPR must be 160  0.79 = 126.4 nm. However, CDs were not successively arranged on the PEG chain but formed separate PPR domains due to the VOL. XXX

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long PEG chain, because when the number of CD reaches 12, it tends to form PPR aggregates and induce phase separation.37,46 Thus, thicker PPR plate may not be formed in the present cases. Alternatively, if nonsolvents of PPR were added before hydrogel formation, the growth of the nanosheet might be hindered and result in a smaller nanosheet. Therefore, by the addition of either acetone or ethanol into the mixture of R-CD and PEG, gels were not formed. Interestingly, by the addition of different amount of either water or DMSO, gels can be formed (see Table 1, entries 7 12). Figure 4A,B shows the N2 adsorption desorption isotherms of the SMA4-1 and SMA15-1 series prepared by adding extra water or DMSO, and the SA values of all the SMA are remarkably increased (even multiplied); e.g., SA values of 66.19 (entry 8) and 69.52 m2/g (entry 10) have been achieved by adding fixed volume of water and DMSO to the mixture of CD and PEG, respectively. As can be seen from the SEM images of these SMAs presented in Figure 4D,E, as well as in Figure S8 and S9 in Supporting Information, the sizes of the nanosheet are considerably decreased when compared to those of SMAs without adding of extra solvents, which accounts for the increment of SAs. The effects of extra water and DMSO on the nanosheets' size may be explained by two aspects: (1) they act as good solvent for both R-CD and PEG; thus, the addition of water and DMSO decreases CD concentration, which may hinder WANG AND ZHANG

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Figure 4. N2 adsorption desorption isotherms of the indicated SMAs: (A) SMA4-1 series by adding extra water and DMSO, (B) SMA15-1 series by adding DMSO, and (C) SMAs based on PEG10000; SEM images of (D) SMA4-1-005D, (E) SMA15-1-003D, and (F) SMA10-1-PEG10K.

gel formation due to the dissociation of R-CD and PEG;37 39 and (2) for DMSO, it may hinder the large aggregation of PPRs, and as a result, smaller size nanosheets were formed.51 The Mn of PEG significantly affects the SA values of the SMAs but forming similar 2D nanosheet structure. As shown in Figure 4C, all the SMAs, by using PEG10K as guests, exhibited IV isotherms and H3 hysteresis loops associated with layered pores, similar to those of PEG20K derived SMAs. The SEM images of the SMA illustrated in Figure 4F and Figure S10 further confirm that the SMAs are built up by nanosheet networks. However, the molar ratio of CD to PEG seems to be greatly determined by the SAs, for instance, the SA of SMA2-1-PEG10K is 22.35 m2/g, while that of SMA10-1PEG10K is considerably increased to 62.26 m2/g. More interestingly, when γ-CDs were used as hosts instead of R-CDs, regular square nanosheet were observed for all the γ-CD based SMAs (see SEM image in Figure S11). The XRD results (Figure S12, diffraction peak at 2θ = 7.5°) demonstrated that the SMAs also preserved channel-type crystalline PPR structures. The N2 adsorption desorption isotherm of one γ-CD based SMA is presented in Figure S13, and the SA is 8.86 m2/g, which is much lower than that of R-CD based SMAs. A proposed model of the γ-CD based SMAs was illustrated in Figure S14, which may explain the morphology differences between the R-CD and γ-CD based SMAs. Because the PEG is threaded in a bended fashion in γ-CDs,52,53 the PPR plate is easily to be aggregated and results in larger and thicker nanosheets (higher than 50 nm); consequently, the BET SA is low as compared to that of R-CD based SMAs. Since the SMAs with low molar ratio of R-CD to PEG have double crystallized structure, and the PEG crystalline domain can be reversibly melt and crystallized while the PPR crystalline domain is stable up to 120 °C (see Figure 2B; actually, the melting temperature of PPR is higher than its degradation temperature, so the melting of PPR has not yet been reported40,41), it would be interesting to determine if these novel SMAs could be used as solid solid PCM without leaking. In fact, as shown in Figure S15, no obvious change has been observed for the nanosheet structures of SMA4-1 before and after thermal annealing at 120 °C for 2 h. Moreover, the N2 adsorption desorption of the SMAs, take SMA1-1, SMA4-1, SMA15-1, and SMA20-1 for example, is almost overlapped after degassing at various temperatures (100, 150, and 200 °C) for 4 h (see Figure S16 in Supporting Information), except for SMA1-1, in which the BET SA is slightly decreased when degassed at 200 °C, possibly due to its low CD coverage and the fact that the stability of the high content of unthreaded PEG layer is lower than that of PPR. These results indicate that the microstructure of the SMAs is very stable, even though the PEG crystalline domains are melted. The thermal stability of the microstructure

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of the SMAs could be explained when we consider the proposed model presented in Figure 3, where the skeleton network of the aerogels (nanosheets) is built up by PPR channel-type crystalline plate and covered by PEG crystalline or amorphous layer depending on molar ratio of CD to PEG; as a result, the melting and crystallization of PEG may result in surface changes but not the nanosheet morphology. Table S2 collected the latent heat values of the SMAs. As can be seen, SMAs showing variable latent heat depending on the molar ratio of CD to PEG (or the PEG weight content), and a value of 71.8 J/g have been obtained for SMA1-1, in which the PEG content is 52.6% as determined by TGA. Interestingly, as for SMA4 1, whose PEG content is only 24%, it also has a latent heat of 5.4 J/g. However, the values are lower than the theoretic values (see Table S2 in Supporting Information), which may be due to the residence of CDs on the PEG chain so that the crystallization of PEG was affected.54 This would also be explained why these values are lower than those of solid solid PCM based on PEG composites or chemical branched, grafted, and blocked copolymers with the comparable mass content.55,56 The changes of SMA4-1 and PEG plate under thermal annealing at 100 °C were monitored using infrared digital camera; for comparison, PS plate and silica aerogel plate with similar thickness were also presented. The digital and infrared photo images of these plates taken at different times are shown in Figure 5A. As can be seen, the pure PEG plate (thermal conductivity of the solid state at 25 °C is 0.3078 W/mK) starts to melt after 10 min and completed molting in 20 min. In contrast, no visible change can be observed for the SMA4-1. Moreover, the temperature of the upside surface of SMA4 1 is lower than 60 °C even while on a heater of 100 °C for 0.5 h, and no longer increases possibly due to its low thermal conductivity (0.0523 W/mK), which is only a bit higher than that of silica aerogels (0.0431 W/mK). Thus, the SMA, which is a combination of PCM with aerogel, would offer interesting applications for heat insulation and preservation as depicted in Figure 5C. For the traditional thermal insulation materials, such as silica aerogels and polyurethane foams, little of the heat can be passed through during the daytime, but it may also reversely pass through during night (if the indoor temperature is higher than that of outside during night) (Figure 5B); no heat energy was preserved because the traditional thermal insulation cannot preserve heat by itself. In contrast, when the SMAs were used as thermal insulation materials, not only was most of the heat insulated, but also heat was reserved and released from the insulation materials due to the presence of PEG, which can reserve heat at high temperature and release it at low temperature. As a result, heat in room may not be lost

Figure 5. (A) Infrared and digital photo images of PEG plate, PS plate, silica aerogel, and SMA4-1 heated under 100 °C for 30 min and cooled for 10 min as indicated. The thickness of the PEG plate, silica aerogel, and SMA4-1 are around 8 mm. The sample in the fourth photo seems a bit yellow; this mainly resulted from the condition of shooting. However, no color changes were observed by our bare eyes. (B) Illustration of conventional thermal insulation materials for heat insulation. (C) Illustration of SMA, a combination of PCM and aerogel, for heat insulation and preservation.

but replenished by the insulation materials containing PCM (Figure 5C). Thus, this proposed application of the SMAs may open a new avenue for the combination of thermal insulation materials with PCM, in which the rapid heat storage rate is not demanded. CONCLUSIONS In summary, a structurally new type of singlecomponent aerogels, supramolecular aerogels, has been synthesized by SCLD, derived from of supramolecular hydrogels formed by the self-assembly of R/γ-CD and PEG. On the other hand, by carefully introducing a rigid and solid crystalline cross-linking phase (PPR), the soft and water-soluble PEG based hydrogels are successfully converted into aerogels. The network of the SMAs is composed of graphene-like 2D nanosheets, and it preserves a channel-type crystalline PPR domain and a PEG crystalline or amorphous domain (tunable by varying the molar ratio of CD to PEG). The thickness of the nanosheets is smaller than 50 nm, and the size of nanosheets can be controlled by introducing water or DMSO. Interestingly, a high BET surface area of 69.5 m2/g has been obtained. Moreover, the molecular weight of PEG also plays an important role in surface area; e.g., a value of 62.3 m2/g for the PEG-10K is observed, while that of PEG-20K is 41.2 m2/g. In addition, when γ-CDs are used as hosts instead of R-CDs, much more regular square nanosheets were formed. For the first time, we demonstrate the application of the SMAs as PCM, and a high latent heat of 71.8 J/g is recorded when the PEG weight content is 53%. Thus, the present work opened a new strategy to VOL. XXX

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phase, while simultaneously forming binary crystalline networks with a 2D nanosheet morphology.

METHODS

samples for mechanical (stress strain) measurement by cutting SMA with a very thin blade; however, the samples were easily broken and no reliable data could be obtained. Further work is carried out to improve its mechanical properties by introducing chemically bonded cross-linking domains. Conflict of Interest: The authors declare no competing financial interest.

Materials. PEG with Mn of 10 and 20 K, R/γ-CDs (g98% HPLC) were purchased from Aladdin Industrial Corporation and used as received. Distilled water was used, and all the other reagents are of analytical purity and used as received unless otherwise noted. Aqueous solutions of PEG-20K (0.1 g/mL), PEG-10K (0.1 g/mL), R-CDs (0.0967 g/mL) and γ-CDs (0.2 g/mL) were prepared and used in this work. Synthesis of SMAs. Take the preparation of SMA2-1 as example, 2 mL of R-CDs aqueous solution and 1 mL of PEG solution (n = 2) were mixed and the mixture was ultrasonicated for 5 min at room temperature. Then, the mixture was kept at 5 °C for 24 h for gelation (SMHG2-1). The white and opaque hydrogel was solvent-exchanged with acetone for 4 days by changing fresh acetone every 12 h. Finally, supramolecular aerogel was obtained by SC CO2 extraction at 42 °C and 10 MP for 10 h. For the preparation of SMA with extra water or DMSO, a determined amount of the corresponding solvent was added before ultrasonication. The SMAs are named as SMAn-1, SMAn-1-xW/D, SMAn-1-PEG10K. The SMAn-1 series are based on R-CDs and PEG 20K, and n indicates the volume ratio of R-CDs solution to PEG20K solution. SMAn-1-xW/D is the same as SMAn-1 series, and x indicates the concentration of PEG after adding water (W) or DMSO (D). SMAn-1-PEG10K is based on R-CDs and PEG 10K, and n indicates the volume ratio of R-CDs solution to PEG10K solution. For the γ-CDs based hydrogels, PEG 20K with a concentration of 0.3 g/mL was used. The composition and chemical structures of the SMAs were investigated by FTIR (see Figure S17) and EDX (see Figures S18 and S19 in Supporting Information). Characterizations. XRD was performed on a Bruker D8 Advance spectrometer. The radiation source used was Ni-filtered Cu KR radiation with a wavelength of 0.154 nm. Samples were mounted in a sample holder and scanned from 4.5° to 60° at a speed of 5° min 1. The pore size distributions and average pore diameters of the aerogels were analyzed by the BJH nitrogen adsorption and desorption method (ASAP 2020, Micromeritics). The surface areas of the aerogels were determined by the Brunauer Emmett Teller (BET) method, based on the amount of N2 adsorbed at pressures 0.05< P/P0 < 0.3, and the degassing process were under 100 °C for 4 h. The FTIR spectra were measured on Thermo Scientific Nicolet iN10 spectrometer using a transmission mode. Field-emission scanning electron microscopy (Quanta 400 FEG) was performed to determine the microstructure of the aerogels, and energy dispersive X-ray spectroscopy (EDX) was used to investigate the atoms and their percentage in the SMAs. The samples were coated with Au nanopowder under current of 20 mA for 2 min. TEM measurement was carried on a Tecnai G2 F20 S-TWIN. Samples were prepared by carefully putting the gels in contact with copper grid and drying under air for 1 week. AFM was carried out on a Veeco Dimension 3100, and samples were prepared by dispersing SMAs in acetone by vigorous stirring and then dropped on mica plate. Thermal gravimetric analysis (TGA) and DTG were carried out using a TG 209F1 Libra (NETZSCH) analyzer with a heating rate of 10 °C/min in a nitrogen atmosphere. DSC analysis was performed on a DSC 200F3 NETZSCH with a heating and cooling rate of 10 °C/min. Infrared photos were taken with a MinIR (M1100150) camera. Densities of the aerogels were calculated by weighting the samples and measuring the volumes. Porosity of the aerogels was calculated according to the following equation: porosity = (1 Fb/Fs)  100%, where Fb is the bulk density of the SMA and Fs is the skeleton density of the ICs (1.55 g/cm3, bulky plate without pores by ambient pressure drying). Thermal conductivity of the aerogel was measured using transient hot wire method (XIATECH TC3000, China), and the data was collected three times with a 5 min interval between each measurement. We attempted to acquire

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convert hydrophilic polymer based hydrogels into corresponding aerogels by introducing rigid cross-linking

Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (51572285, 21373024, 21404117), the Natural Science Foundation of Jiangsu Province (BK20151234), the 100 Talents Program of the Chinese Academy of Sciences and the Innovation Program of the Beijing Institute of Technology. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05281. Molar and weight ratio of CD to PEG, latent heat values of the SMAs, DTG peak temperatures, additional figures (including SEM, TEM, and AFM images, XRD patterns, EDX spectra), proposed model of γ-CD based SMAs, FTIR spectra, N2 adsorption desorption isotherms (PDF)

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