Thermoresponsive Polyrotaxane Aerogels: Converting Molecular

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Thermo-responsive polyrotaxane aerogels: converting molecular necklaces into tough porous monoliths Jin Wang, Ran Du, and Xuetong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18741 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Thermo-responsive polyrotaxane aerogels: converting molecular necklaces into tough porous monoliths Jin Wang,† Ran Du, ‡ and Xuetong Zhang†* †

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou

215123, P. R. China; ‡ Physical Chemistry, TU Dresden, Bergstrasse 66b, Dresden 01062, Germany. *corresponding author: [email protected]

KEYWORDS: molecular necklace, aerogel, polyrotaxane, supramolecular system, cyclodextrin

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ABSTRACT: Polyrotaxanes (PRs) are supramolecular systems which combine with several cyclic molecules threaded on a linear molecule, and have been widely applied in molecular abacus, stimuli-responsive systems, self-healing materials, etc. The fabrication of macroscopic porous PR monoliths, which is expected to impart more functions, has not been realized yet. PR aerogels featuring with high specific surface area (232 m2/g), high strength (74.7 MPa) and temperature-responsiveness have been synthesized in this work by introducing poly(Nisopropylacrylamide) to the molecular necklaces followed by chemical cross-linking with a rigid crosslinker and supercritical fluid drying, which might open a new door towards smart macroscopic porous materials.


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Supramolecular systems are noncovalent-bonded aggregates built upon relatively weak interactions such as hydrogen bonding,1 metal coordination,2 hydrophobic interaction,3 π-π stacking,4 van der Waals force,5 and mechanical interlocking.6 Supramolecular systems, such as protein folding, nucleic acid assembly, and phospholipid membrane,7 have extensively existed in nature for millions of years since the origin of life on earth, It was not until the 1980s that Lehn et al. systematically investigated on the host-guest complexes and proposed the concept of supramolecular chemistry.8 A variety of functional supramolecular systems have been created in recent years.9 Molecular necklaces (MNs), also referred to as polyrotaxanes (PRs) or pseudopolyrotaxanes (PPRs), are supramolecular systems which combine with several cyclic molecules threading on a linear molecule (as illustrated in Scheme 1a) with (for PR) or without (for PPR) end-capping groups on the linear chain,10 and have attracted extensive attention since its advent in 1990. Because there is no strong interaction between the rings and the chain, the cyclic molecules can freely rotate and slide on the linear molecule, which result in diverse interesting properties for MNs to be applied in molecular shuttles,11 molecular abacus,12 stimuliresponsive systems,13 shape-memory or self-healing materials,14,15 etc. The application of the MNs usually relys on the presence of solvents, and transforming the wet supramolecular system into solvent-free, mechanically strong, and free-standing monolith with preserved porous structure remains challenge. The non-covalent supramolecular structures (micelles, tubes, vesicae, etc.) or networks (hydrogels) are always fragile, and they may be easily collapsed during ambient drying process due to intensively capillary force.16 The synthesis of a solvent-free and structure-preserved MN monolith with tailored function may not only provide a certain insight into controlling material’s microstructure, but also offer a brand-new material, e.g., aerogel.17-19 Aerogels are highly porous materials with large specific surface area, high


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porosity, and low thermal conductivity, etc., and have so far exhibited great potentials in the fields of thermal insulation, catalysis, energy storage, etc. A variety of aerogels, such as silica aerogel,18 polymeric aerogel,20,21 and organic-inorganic hybrid aerogel,18 has been reported elsewhere. Function-led design of aerogels with specific building blocks, such as 0D nanoparticle, 1D nanowire, and 2D nanosheet, has attracted extensive attention.22 As a result, a cyclodextrin (CD)-based PR is used as a new building block to construct a toughly porous organic aerogel with stimulus responsiveness by introducing poly(N-isopropylacrylamide) (PNIPAAm) into the PRs. The purpose for the use of the PNIPAAm is to impart thermoresponsiveness of the final product as well as to improve the stability of the MNs. 3D network (organogel) was created by chemically cross-linking the above MNs with a rigid crosslinker at a high cross-linking degree, in order to restrain the structure deformation during the following process. Supercritical liquid (SCL) drying technique was finally used to relieve the capillary force incurred during the drying process.16 By using this protocol, polyrotaxane aerogels (PRAs) with high strength (Young’s module up to 74.7 MPa), large specific surface area (up to 232 m2/g) and unique temperature-responsive hydrophilic-hydrophobic transition behavior have been fabricated. Scheme 1a illustrated three possible strategies to synthesize MN networks (organogels or hydrogels) for the resulting porous monoliths. In strategy 1, a MN network was directly formed via the physical interaction between the cyclic molecules.19 Serious phase separation occurred in this system, and the dried monolith was weak and could be completely dissolved in various solvents (e.g. DMF, MDSO, water).19 In strategy 2, a cross-linking reaction happened among the cyclic molecules in the PPR, and the resulting crosslinked product could be dissociated during the solvent-exchange or washing step due to the absence of end-capping groups, so a monolith


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may even not be formed.23 Cross-linking the cyclic molecules in a PR with end-capping groups, a mechanically stable polymer network could be prepared as shown in strategy 3. A slide-ring gel was synthesized with a low cross-linking density, in which the pulley effect was prominent and the cyclic molecules could slide along a linear molecule in the presence of solvents. Nevertheless, the slide-ring gel was also shrunk during drying, even by SCL drying.24,25 However, increasing the cross-linking density in a PR, as illustrated in Scheme 1b, the pulley effect might be hindered due to the losing of figure-of-eight structure (see details in Figure S1 of the Supporting Information),24 which resulted in a mechanically strong network that could be transformed into a solvent-free porous monolith.17


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Scheme 1. (a) Various potential approaches for gel syntheses from molecular necklaces. Molecular necklace A (MN-A) is also referred to as PPR, and molecular necklace B (MN-B) is a PR. Strategy 1 is physically cross-linked gel, while strategy 2 and 3 are chemically cross-linked gels starting from the cyclic molecules. In strategy 3, increasing the cross-linking density in a high coverage ratio may result in mechanically strong network that could be transformed into solvent-free porous monolith. Besides, the end-capping groups could be well preserved, as well as their functions and properties. (b) Schematic illustration of the structure and merits of the PRA


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precursor (CD, PNIPAAm, and TTI), with a high feeding amount of TTI. Several CD molecules may be crosslinked together. Strategy 3 was performed by using a specific-designed MN as the building block. As shown in Scheme 1b and Figure S2, MN-B composed of β-cyclodextrin (β-CD) (host), PEO-PPO-PEO triblock copolymer Pluronic F127 (guest), and PNIPAAm (end-capping group) was used as the building block.26,27 The hydroxyl groups in CDs could be easily crosslinked by various crosslinkers.17 It should be pointed out that the cross-linking density and the flexibility of the crosslinkers might determine whether the resulting gel could be transformed into an ideal solvent-free monolith, because a low cross-linking density or a soft crosslinker may result in a swollen gel, which would shrunk to form a nonporous power or bulk during drying process19 due to a lot of voids and soft network in the wet gel. Thereafter to obtain a structure-preserved solvent-free monolith, triphenylmethane-4,4’,4’’-triisocyanate (TTI) was selected as the crosslinker.17 Then various cross-linking densities were performed in this work. The samples were named as PR-TTIn, where PR indicated the building block, TTI indicated the crosslinker, n stood for the feeding amount of the crosslinker (representative of cross-linking density). The results were presented in Table S1 in Supporting Information. As expected, highly porous but tough PRAs were synthesized, the BET specific areas (SAs) of the PRAs were around 200 m2/g, and the Young’s modulus of the PRAs were in the range of 6.3 to 74.7 MPa. The results indicated that a solvent-free MN monolith could be designed through the molecular structure and the synthetic technique. For comparison, hexamethylene diisocyanate (HDI) was also used as a crosslinker, and only powders or broken plates (see photo images in Figure S3) with high density, low SA and low pore volume were obtained. In these cases, increasing the cross-linking density or varying the solvent-exchange media (ethanol and acetone) could not help to produce a


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toughly porous monolith. It may be explained by the fact that HDI is too soft (because of its long and non-conjugated chain) to support a stable network, especially when the gel is subjected to drying.24,25 In contrast, a tough and plastic-like porous monolith was produced when TTI was used as the crosslinker. The chemical structure of the PR precursor was confirmed by 1H NMR as shown in Figure 1a, where the signal from CD, Pluronic F127, and PNIPAAm could be easily recognized. For the PRA, its chemical structure was confirmed by FTIR spectra as shown in Figure 1b. The vibration absorption peaks corresponding to the urethane bond located in the range from 1470 to 1700 cm1

were identified, while the O-H stretching peak around 3330 cm-1 and the intense C-O stretching

peak at 1028 cm-1 confirmed the existence and intactness of β-CDs. Besides, PRs are normally powdery materials due to its channel-type crystalline structure with characteristic diffraction peak locating at 120 (Figure S4). However, the PRAs are amorphous (Figure S4), which indicates that the channel type crystalline structure has not been formed after the cross-linking of CDs.26,27


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Figure 1. (a) 1H NMR spectra of PR and PNIPAAm; (b) FTIR spectra of PR, CD, and the HDI/TTI crosslinked PR solids; (c) Compressive stress-strain curves of the PR aerogels; (d) Illustration of the chemical structures among nanoparticles in the silica aerogel and the PR aerogel. Figure 2 shows the SEM images of the PRAs with different cross-linking densities. It can be seen that all the PRAs were formed by interconnected spherical nanoparticles, similar to that of the silica aerogel reported elsewhere.18 Nano-sized pores were formed among these nano-sized particles, possibly due to the phase separation occurred during the sol-gel transition as described in Figure S5 (Supporting Information). Besides, no macropores were observed in the low magnification SEM images as shown in Figure S6. The results indicated that highly porous MN


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monoliths could be successfully produced by cross-linking of the PRs with rigid TTI together with SCL drying. The importance of the SCL drying was demonstrated by a parallel test. When dried under ambient condition, the PR-TTI organogel was collapsed and cracked to form a brittle and nonporous xerogel due to capillary force16,19 (N2 adsorption measurement gives BET SA ca. 0, possibly due to the absence of micropores and mesopores, and porous structure also could not be observed from the SEM image, see Figure S7). Note that, when TTI was replaced by the soft HDI, product derived from the PR-HDI gel showed few pores even by combining a high crosslinking density with SCL drying, as confirmed by the SEM image (Figure S8) and the BET SA test (Figure S9). These results further support the hypothesis that the soft cross-linker, i.e. HDI, cannot support a stable solvent-free network.


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Figure 2. SEM images of (a) PR-TTI150, (b) PR-TTI200, (c) PR-TTI250, and (d) PR-TTI300, each inset is the corresponding photo image of the PRA. The PRAs showed typical type IV isotherms with hysteresis loops (Figure S10), and a BET SA up to 232 m2/g for PR-TTI200, as well as a pore volume up to 1.05 cm3/g for PR-TTI250, was obtained. The results indicated that PRAs were successfully synthesized by cross-linking PRs with the rigid TTI crosslinker. Besides, the PRAs possessed high thermal stability (>320 oC) with two decomposition stages being observed, which corresponded to the decomposition of the PR and crosslinker, respectively (Figure S11). Mechanical properties are critical to evaluate materials. The compressive strain-stress curves shown in Figure 1c indicated that the PRAs possessed high Young’s modulus. For instance, the PR-TTI150 with a relatively low cross-linking density featured a modulus of 6.3 MPa, while that of PR-TTI200, PR-TTI250, and PR-TTI300 were 23.1, 39.8, and 74.7 MPa, respectively. The results indicated that increasing the cross-linking density could significantly enhance the modulus of the PRAs and thus improve the mechanical strength of materials. Moreover, the tough PRAs remained intact when compressed up to 70 % (Figure S12), suggesting a stable network. Taken PR-TTI250 for example (Figure S13), it showed a moderate elastic range up to ca. 10 % strain, followed by plastic deformation until 50 % strain, and then densification and plastic hardening process. Overall, the sample did not crack during the hardening process, as had been observed from the SEM image (Figure S14). The fragility of the nanoparticle-built aerogels (e.g. silica aerogel) mainly resulted from the weak connection at the neck regions among nanoparticles,18 and the high toughness of the PRAs might result from the high cross-linking density among the nanoparticles and the flexibility of the PR structure (Figure 1d and Figure S5).


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These results further indicated that MN might be used to mechanically enforce the traditional fragile inorganic aerogels.18,25 Beside mechanical properties of the supramolecular systems, preserving their stimuliresponsive behavior in the solvent-free state is another challenge.28 In virtue of the introduction of the functional end-capping PNIPAAm molecules, the PRAs showed a temperature-responsive behavior as demonstrated by the contact angle (CA) measurements (Figure 3). The CA of the PRA was ca. 85o at 20 oC, and it increased up to ca. 98o when the surface temperature of PRA increased up to 50 oC. The transition angle across 90o is attractive, because dewetting/wetting may happen in this region according to the Young’s equation:29 γSV =γSL +γLVcosθ where γSV, γSL, and γLV represent solid/gas, solid/liquid, and liquid/gas interface tension, respectively. When θ > 90o, the surface is considered to be hydrophobic, when θ < 90o, the surface is considered to be hydrophilic. As a demonstration, the PRA could float on the water at temperatures higher than lower critical solution temperature (LCST), and sink in the water at room temperature (Figure S15). The reason could be attributed to the introduction of PNIPAAm in the PR precursor, which showed a LCST at ca. 36 oC (see the reversible dissolving-precipitation transition in Figure S16).27 Because the PRs were only crosslinked by the CDs, the structure of PNIPAAm could be well preserved (Figure 1b) and may be distributed homogenously in the gel network (as confirmed by SEM-EDS data shown in Figure S17). As a consequence, thermo-responsive PRAs were produced.


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Figure 3. (a) Contact angle measurements of PNIPAAm, PR, and PRA (PR-TTI250) at different temperature, the samples for PNIPAAm and PR were prepared by dropping the polymer solutions on a clean glass plate and dried slowly at room temperature then in vacuum; (b) Proposed mechanism of the thermo-responsive behavior of the PRA. The CA of PNIAAm changed from ca. 61o (20 oC) to ca. 71o (50 oC) (Figure 3a) due to both increased intramolecular hydrogen bonding and decreased intermolecular hydrogen bonding,


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similar to the result reported elsewhere.30 While the CA of the PR at room temperature was only ca. 39o, mainly due to the existence of both hydrophilic CDs and PEO blocks in the PR. The CA of PR increased to ca. 72o when heated up to 50 oC, similar to that of PNIPAAm (Figure 3a). So why the CA of PRAs switched around 90o ? The phenomenon might be explained by the fact that, after incorporating of the TTI into the MNs, the hydrophobicity of the product would be increased accordingly because TTI was hydrophobic (the molecular structure of the TTI shown in Figure S2). Thus, it may be foreseen that the CA of the PRAs might be regulated by varying the amount of crosslinker, PNIPAAm, and CDs, etc. To the best of our knowledge, this is the first report on the porous surapmolecular monolith featuring both high toughness and thermal responsiveness in the solvent-free state. In summary, to synthesize functional and monolithic MN materials and to expand the potential applications in supramolecular systems, a series of designs has been proposed. PNIPAAm is employed as an end-capping reagent to endow stability and thermo-sensitivity of the MNs, and a large amount of rigid TTI is introduced as a chemical crosslinker to create high-strength PRbased network restraining structural deformation during the drying process. Supercritical fluid drying technique is adopted to relieve the capillary force in desolvation process. As a result, the first PRA featuring high specific SA (up to 232 m2/g), high strength (Young’s modulus of up to 74.7 MPa) has been successfully fabricated. Moreover, the PRA displays intriguing temperatureinduced wettability modulation properties (water contact angle switch around 90°), enabling controlled hydrophilic-hydrophobic transition useful in diverse fields. The results shown here not only provide an unambiguous tactic in designing functional PRA and even the supramolecular aerogels, but may also shed light on creating a variety of smart materials promising in smart devices, functional engineering materials, and intelligent aerogels, etc.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of the materials, synthesis, characterizations, table of composition and properties of the PRAs, SEM images, SEM-EDS spectra, XRD, photo images of PRA and PR, the N2 adsorption isotherms of PRAs, compressive stress-strain curves, and TGA traces (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jin Wang: 0000-0003-1573-574X Xuetong Zhang: 0000-0002-1268-9250 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51572285, 21373024, 21404117, 51773225), the National Key Research and Development Program of China (2016YFA0203301) and the Natural Science Foundation of Jiangsu Province (BK20151234, BK20170428).

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Thermoresponsive Polyrotaxane Aerogels: Converting Molecular

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