A Facile Approach To Prepare Tough and Responsive Ultrathin

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A Facile Approach to Prepare Tough and Responsive Ultrathin Physical Hydrogel Films as Artificial Muscles Ye Tian, Xiaoshuang Wei, Zhi Jian Wang, Pengju Pan, Fangyuan Li, Daishun Ling, Zi Liang Wu, and Qiang Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10652 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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ACS Applied Materials & Interfaces

A Facile Approach to Prepare Tough and Responsive Ultrathin Physical Hydrogel Films as Artificial Muscles Ye Tian,1 Xiaoshuang Wei,1 Zhi Jian Wang,1 Pengju Pan,2 Fangyuan Li,3 Daishun Ling,3 Zi Liang Wu,1,* and Qiang Zheng1 1

Key Laboratory of Macromolecular Synthesis and Functionalization of Ministry of Education, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China; 2

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China;

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Institute of Pharmaceutics, College of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, China. *Corresponding author. E-mail: [email protected]

Abstract We report a facile approach to prepare ultrathin physical hydrogel films based on Marangoni effect, which drives ethanol solution of poly(stearyl acrylate-co-acrylic acid) (P(SA-co-AAc)) to rapidly spread on water surface. The subsequent solvent exchange leads to sol-gel transition, where the long alkyl chains of SA units segregate to form physical crosslinking junctions. The resultant disk-shaped single-network (SN) gel films are uniform with tunable thickness (40-80 µm) and diameter (5-12 cm), and possess robust mechanical properties with tensile breaking stress, εb, and breaking strain, εb, being 0.3-1.1 MPa and 30-290%, respectively. The mechanical properties of SN gel films can be further improved by introducing ductile poly(N-isopropylacrylamide) (PNIPAm) into the preformed gel matrix, which forms strong hydrogen bonds with the first network. The obtained physical double-network (DN) hydrogel films are transparent and show excellent mechanical performances with σb of 3-5 MPa and εb of 100-500%. Due to the ultrathin thickness of gel films 1 ACS Paragon Plus Environment


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and response of PNIPAm to saline solutions, the tough DN gel films exhibit fast response (≤ 60 s) and large stroke force (0.5 MPa) after switching the environment from water bath to saline solution, making them an ideal material to design artificial muscles, soft actuators, and chemomechanical devices.

Keywards: hydrogel films; Marangoni effect; high toughness; fast response, chemomechanical systems

Introduction Hydrogel films have received increasing attentions due to their promising applications in molecular separation, medical dressings, flexible electronics, etc.1-4 The primary challenge is how to improve the mechanical performances of hydrogel films for load bearing and easy handling. In recent years, several kinds of tough hydrogels have been developed with different network structure and toughening mechanism,5-18 which merit the fabrication of robust hydrogel thin films.19-24 For example, Gong et al. have synthesized tough hydrogel films with thickness of 30-100 µm by three-step free radical polymerization based on the double-network (DN) technology.19,20 The tensile breaking stress (σb) and strain (εb) of DN gel films were 2 MPa and 1000%, respectively. However, this approach is complex and time consuming, and the mechanical performances of gel films will be dramatically reduced due to the permanent breakage of the first network.20 Shull et al. have prepared tough physical hydrogel films with thickness of several hundred micrometers by casting the solution of triblock polymer (midblock: poly(methyl methacrylate) (PMMA); end blocks: poly(methacrylic acid) (PMAA)) and then swelling it in an aqueous solution of divalent metallic ions, where PMMA blocks segregated to form hydrophobic domains and PMAA blocks were ionically crosslinked by the metallic ions.21 The gel films showed good mechanical properties (σb~0.4-1 2 ACS Paragon Plus Environment

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MPa, εb~100-500%), yet the thickness and its uniformity were difficult to control. A novel approach is really desired to prepare ultrathin and tough hydrogel films. We demonstrate here a facile approach to prepare hydrogel films based on Marangoni effect. It is well-known that ethanol and water are miscible at arbitrary ratio. However, their surface tensions are quite different (ethanol: 22 mN/m; water: 72 mN/m, at 25 oC).25 Such huge difference in surface tension leads to energetically mixing when one drop of ethanol is dripped into water, where the ethanol quickly spreads on the water surface. This phenomenon is termed as Marangoni effect26,27 and has been widely used for the designing of self-propel and chemomechanical systems.28-30 In the present work, one drop ethanol solution of poly(stearyl acrylate-co-acrylic acid) (P(SA-co-AAc)) was dripped onto water surface, which quickly spread to form a thin film due to the Marangoni effect. The solvent exchange led to sol-gel transition, in which long alkyl chains of SA units segregated to form crosslinking junctions of the gel matrix (Figure 1).31,32 The obtained hydrogel films was uniform with tunable thickness in the range of 40-80 µm by controlling the polymer composition and concentration. These single-network (SN) gel films had robust mechanical performances (σb~0.3-1.1 MPa, εb~30-290%), which can be further improved by incorporating a ductile and responsive poly(N-isopropylacrylamide) (PNIPAm). The resultant double-network (DN) physical hydrogel films have σb of 3-5 MPa and εb of 100-500%. Owing to the ultrathin thickness, the response of the DN gel films is very fast (≤ 60 s) with a large stroke of output force (0.5 MPa). These tough and responsive hydrogel films should be ideal materials to develop soft actuators, chemomechanical devices, etc.

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Figure 1. (a) Chemical structures of P(SA-co-AAc) and PNIPAm. (b) Schematic illustration of the preparation of tough physical hydrogel film. After dripping one drop ethanol solution of P(SA-co-AAc) onto water (i), it quickly spread to form a thin film (ii). Meanwhile, solvent exchange led to segregation of alkyl chain of SA unit and formation of single-network hydrogel (iii). The gel film was swelled in a precursor solution of NIPAm (iv) and followed by photopolymerization (v) to produce tough double-network hydrogel film (vi).

Experimental Section Materials: Stearyl acrylate (SA, Tokyo Chemical Industry Development Co., Ltd.) and 2,2'-azobis(isobutyronitrile) (AIBN, Sigma-Aldrich) were recrystallized from ethanol. Acrylic acid (AAc, Aladdin Industrial Corporation) was purified by vacuum distillation at 90 oC. Ethanol was used as received from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B and pyrene were purchased from Aladdin Industrial Corporation. N-isopropylacrylamide (NIPAm) and 2,2'-azobis(2-methyl-propionamidine)dihydrochloride (V-50) were used as received from Sigma-Aldrich.


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Synthesis of P(SA-co-AAc): P(SA-co-AAc) with various molar fractions of SA, f, in the total monomers (4-12 mol%) was synthesized by free radical copolymerization of SA and AAc in ethanol in the presence of AIBN as the initiator. The product was precipitated in ether and dried in vacuo at 50 oC.32 The purified copolymers were dissolved in ethanol to obtain transparent and homogeneous solutions. Preparation of SN gel films: As shown in Figure 1b, one droplet of P(SA-co-AAc) ethanol solution with prescribed concentration and volume was dripped onto water surface. The polymer solution quickly spread to form a thin film at the interface. The solvent exchange led to sol-gel transition; thus obtained gel film was swelled in pure water to achieve the equilibrium state. The single network (SN) gel films are coded as SN-f. To fabricate fluorescent and ferromagnetic films, 0.01 wt% pyrene and 0.6 wt% ferromagnetic iron oxide nanocubes (synthesized according to reported protocol33) were added to 37 wt% P(SA-co-AAc) ethanol solution, respectively. Via a similar process, hybrid gel films were prepared. Preparation of DN gel films: The preformed SN gel film was swelled in a precursor solution containing NIPAm (different concentration, Cn) and V-50 (0.5 mol%, relative to the monomer) for 12 h. The gel film was sandwiched between two glass substrates and irradiated under UV light (365 nm, 5 mW/cm2) for 2 min. The photopolymerization produced PNIPAm as the second network in the P(SA-co-AAc) gel matrix. The double-network (DN) gel film was immersed in a large amount of water to remove the residuals and achieve the equilibrium state. The DN gel films are coded as DN-f-Cn, in which f and Cn are the SA fraction of P(SA-co-AAc) in mol% and the monomer concentration of NIPAm in M used for the synthesis of the second network, respectively. Characterizations: The spreading process was recorded by a digital camera and analyzed with image data analyzer (Potplayer). For better visualization, the ethanol solution was dyed with 0.2 wt% (relative to the mass of solution) of rhodamine B. The thickness of gel films was examined by a digital stereo microscope (VH-Z100R, 5 ACS Paragon Plus Environment


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Keyence Corporation). The water content of hydrogels, q, was calculated by q = ws/wd, in which ws and wd are the masses of gels in the swollen and dried state, respectively. Tensile tests were conducted to the gel films at room temperature by using a universal testing machine (Instron 3343) with a tensile rate of 100 mm/min. Rectangle specimen with the length of 3 cm and width of 2 cm was cut from the gel film for tensile test; the gauge length was 2 cm. Five parallel tests were performed for each gel film. The contractile stress of DN gel film was measure on the universal testing machine at room temperature. The sample (DN-4-1; length: 3 cm, width: 1 cm, gauge length: 2 cm) was first stretched in water bath to a strain of 100%, which was kept in the following switching of the environment. After a period of interval time, the water bath was periodically exchanged to 3 M NaCl solution and then switched back to water bath. The variation of stress with time was recorded. Results and Discussion P(SA-co-AAc) with different composition was synthesized by free radical copolymerization of SA and AAc in ethanol, as described in our previous work.31 The preparation of SN gel film based on Marangoni effect is shown in Figures 2a and 2b. After one drop ethanol solution of P(SA-co-AAc) was dripped into water, it quickly spread on the surface of water. The spreading was usually accompanied by spontaneous translation movement (Movie S1), a characteristic of Marangoni effect.30 As the ethanol spread into water, the viscous solution of P(SA-co-AAc) was gradually stretched to form a thin disc film. Simultaneously, solvent exchange occurred, leading to sol-gel transition and formation of a uniform and transparent hydrogel film (Figure S1), in which long alkyl chains of SA units segregated to form hydrophobic domains as the physical crosslinking junctions.31,32 As ethanol was exhausted, the gel film ceased moving and the diameter became constant. This process was very fast; a gel film with diameter of 12 cm was prepared within 1 s.


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Figure 2. (a,b) Top view (a) and side view (b) snapshots of Marangoni effect assisted preparation of SN gel film. A small amount of rhodamine B was added to the ethanol solution for better visualization. (c-e) Effects of SA fraction, f (c), concentration, CP (d), and volume, VP (e), of the ethanol solution on the diameter, D, and thickness, T, of resultant gel films. (c) CP = 37 wt%, VP = 60 µL; (d) f = 8 mol%, VP = 60 µL; (e) f = 8 mol%, CP = 37 wt%.

The diameter and thickness of the resulting SN gel films depended on the SA fraction of P(SA-co-AAc), f, the concentration, CP, and the volume, VP, of ethanol solution. In a wide range of f and CP, the obtained gel films had uniform thickness. As shown in Figure S2, the films prepared from ethanol solution of P(SA-co-AAc) with different f (CP = 37 wt%, VP = 60 µL) possessed uniform thickness. However, the thickness, T, increased from 40 to 80 µ m and the diameter, D, decreased from 11.5 to 5.2 cm, as f increased from 4 to 12 mol% (Figure 2c), due to the increase in viscosity of pregel solution that resisted the spreading of polymer solution to some extent. When f = 8 mol% and VP = 60 µL, T increased from 48 to 70 µm and D decreased from 10.5 to 6 cm, as CP increased from 33 to 43 wt% (Figure 2d). This was also due to the increase in viscosity with CP. When CP = 37 wt% and f = 8 mol%, T was a constant value of 61 µm and D increased from 6.7 to 11.5 cm, as VP increased from 20 to 140



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µL (Figure 2e). Therefore, uniform hydrogel thin films with controllable thickness and diameter can be obtained by this facile approach. The obtained ultrathin SN gel films are coded as SN-f, which had water content of 52-78 wt% (Figure S3a) and robust mechanical properties (Figure 3a). The tensile breaking stress,σb, breaking strain, εb, and Young's modulus, E, of the SN gel films were 0.3-1.1 MPa, 30-290%, and 0.2-4.7 MPa, respectively (Figure S4), which are comparable to those of bulk P(SA-co-AAc) hydrogels prepared by solution casting.31 σb and E increased, yet εb decreased with the increase in f. As previously revealed, the variation of gel films from soft and ductile to rigid and brittle is related to the increase in crosslinking density and the enhanced rigidity of gel matrix.32

Figure 3. (a) Tensile stress-strain curves of SN-f gel films (CP = 37 wt%, VP = 60 µL). (b) Photo of SN-8 gel films with and without pyrene under 365 nm UV light irradiation. (c) Photos of SN-8 gel film with incorporation of magnetic nanocubes placed atop a magnetic stirrer. A piece of paper was placed atop the gel film as an indicator of the rotation. Dot arrows indicate the rotation direction. Based on this facile approach to prepare gel films, functional molecules or nanoparticles can be easily incorporated into the pregel solution to fabricate hybrid gel films with specific functionality. Pyrene, a commonly used 8 ACS Paragon Plus Environment

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fluorescent probe, is soluble in ethanol yet insoluble in water. A small amount of pyrene was dissolved in the ethanol solution of P(SA-co-AAc) (CP = 37 wt%), which was dripped into water to prepare the hybrid gel film. As shown in Figure 3b, the pyrene-containing gel film showed strong fluorescence under 365 nm UV light irradiation. In contrast, the gel film without pyrene had no fluorescence. By a similar way, ferrimagnetic iron oxide nanocubes (Figure S5) were introduced into the pregel solution to prepare magnetic hybrid hydrogel film.33 As shown in Figure 3c, the magnetic gel film rotated when the magnetic stirrer was turned on. Functionalization of these hydrogel films should expand their applications in different fields. In addition, the mechanical properties of SN gel films can be further improved by introducing a second ductile and responsive network of poly(N-isopropylacrylamide) (PNIPAm) into the preformed gel matrix (Figure 1b). The SN gel film was swelled in a precursor solution of NIPAm and then irradiated under UV light for photopolymerization.11,34,35 The equilibrated P(SA-co-AAc)/PNIPAm double-network (DN) gel films were transparent (Figure S1) and coded as DN-f-Cn, in which f and Cn are SA fraction of P(SA-co-AAc) in mol% and monomer concentration of NIPAm in M used for the synthesis of the second network, respectively. The mass ratio of the two networks measured by elemental analysis was shown in Table S1. The DN gel films (DN-f-1) had water content of 43-73 wt% (Figure S3) and thickness of 38-68 µm (Figure S6), both of which were slightly less than those of SN gel films. As shown in Figure 4a, the DN gel films with different f and Cn possessed excellent mechanical properties with σb of several MPa and εb of several hundred percent. Yielding appeared in all DN gels at a small tensile strain. As f increased from 4 to 10 mol%, σb and E increased from 3.5 and 20 MPa to 5 and 60 MPa, respectively, yet εb decreased from 480% to 130% (Figure 4b). This tendency was similar to that of SN gel films (Figure S4), indicating the contribution of the first network to the mechanical performances 9 ACS Paragon Plus Environment


of DN gels. On the other hand, as Cn increased from 0.25 to 2 M, σb and E increased from 1.7 and 11 MPa to 7.9 and 51 MPa, respectively; εb achieved the maximum value of 480% at Cn = 1 M (Figures 4c and 4d). The dramatic improvement of mechanical properties are related to the typical DN structure, as well illustrated by Gong et al.34

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Figure 4. Tensile stress-strain curves (a,c) and measured mechanical properties (b,d) of DN-f-1 (a,b) and DN-4-Cn (c,d) gel films.

Another remarkable point is the extremely high E (10-60 MPa) of these DN gel films, which is rooted in the strong hydrogen bonding between the acrylic acid groups of the first network and the amino groups of the second network.36-38 The existence of hydrogen bonds was confirmed by FTIR (Figure S7) and reflected in the decrement of film thickness after incorporation of PNIPAm (Figure S6). Moreover, the DN gel film was quickly dissolved in urea solution (Figure 5), indicating the supramolecular nature of the DN structure. This is because that concentrated urea solution can destroy both the hydrophobic association in the first network (Figure S8) and the hydrogen bonding between the 10 ACS Paragon Plus Environment

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two networks of DN gel film.15,39 The dynamic nature of noncovalent bonds as the physical crosslinks also rendered the DN gel films with self-recovery ability (Figure S9) and thus high fatigue resistance against dynamic deformation.14,40,41

Figure 5. Photos to show the gel-to-sol transition of the DN-4-1 gel film in 6 M urea solution for 10 min. (a) The gel film before immersion; (b) part of the gel film just immersed in urea solution; (c) the gel after the gel-to-sol transition process.

Due to the responsiveness of PNIPAm, the tough DN gel films also responded to external stimuli. The DN gel films contracted their volumes after being swelled in concentrated saline solution due to the dehydration of PNIPAm (Figure S10). It is probably because the addition of NaCl enhances the hydrophobic interaction between the isopropyl side chains and thus leads to the collapse of PNIPAm chains at a relatively low temperature.42,43 For instance, the DN-4-1 gel film shrank to 70% of its original length after being incubated in 3 M NaCl solution. Swelling of DN gel films in saline solution also leads to the pronounced variation of mechanical properties. As shown in Figure 6, σb and E of the DN gels increased from 3.5 and 19.5 MPa to 6.1 and 32.8 MPa, respectively, yet εb decreased from 480% to 190%, as the NaCl concentration, CNaCl, increased from 0 to 4 M. The contraction of gel volume and variation of mechanical properties were reversible, making these DN gels ideal candidates of artificial muscles. 11 ACS Paragon Plus Environment


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As shown in Figure 7a, the DN-4-1 gel strip was stretched to a strain of 200% when loaded with 200 g weight in water bath at room temperature. When the water bath was switched to 3 M NaCl solution, the weight was readily lifted up 2.6 cm and the strain was decreased to 16% due to the volume contraction and stiffening of the DN gel film. This process was fully reversible for at least three cycles (Figure 7b), indicating that the responsive tough gel film is an ideal chemomechanical system with the capacity to convert chemical energy to mechanical work.44 To quantitatively analyze the response speed and output force, stress change of the DN-4-1 gel film at a constant strain of 100% was investigated under periodic switch of the environment from pure water to 3 M NaCl solution (Figures 7c and 7d). The DN gel film maintained a constant stress, σ0, of 2.1 MPa in water, which quickly increased to a stress of 2.6 MPa and maintained this value, σp, after switching the bath from water to saline solution. The stress was readily back to 2.1 MPa, when the bath was switched back from saline solution to water. The stroke force, ∆σ = σp – σ0, was 0.5 MPa, superior to the value (~0.35 MPa) of real muscles.45 The response time of the stress drop and rise after switching the environment was 60 and 44 s, respectively. Such fast response was rooted in the micron-scaled thickness; further decrease in the thickness should further speed up the response of these 12 ACS Paragon Plus Environment

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DN gel films. The fast response and large stroke make these tough gel films with integrated performances superior than most synthetic hydrogels and comparable to liquid crystalline elastomers,46-50 which should be an ideal material to design soft actuators and chemomechanical devices.

Figure 7. (a) Photos of the gel strip (DN-4-1; width: 1 cm, thickness: 90 µm) reversibly lifting up 200 g weight after switching the bath from water to 3 M NaCl solution. (b) Variation of the strain after periodic switch the environment. The strain was calculated based on the length of unloaded gel strip in water. (c,d) Response of isometric stress of the DN-4-1 gel film periodically triggered by 3 M NaCl solution and pure water. The gel film was stretched to a strain of 100% before the cyclic solvent exchange.

Conclusions In conclusion, we have developed a facile approach to prepare robust hydrogel films based on Marangoni effect. The different surface tension drove the spreading of ethanol solution of P(SA-co-AAc) on water surface to form a uniform single-network (SN) hydrogel film within 1 s. The thicknesses and 13 ACS Paragon Plus Environment


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diameter in the range of 40-80 µm and 5-12 cm, respectively, can be well tuned by varying the composition of P(SA-co-AAc), concentration and volume of the ethanol solution. The SN gel films possessed good mechanical performances, with σb, εb, and E of 0.3-1.1 MPa, 30-290%, and 0.2-4.7 MPa, respectively. To further improve the mechanical properties and impart responsiveness to the SN gel films, a ductile PNIPAm network was incorporated to form a double-network (DN) structure. The obtained DN gel films showed excellent mechanical performances, with σb, εb, and E being 3.5-5 MPa, 130-500%, and 20-60 MPa, respectively. These DN gel films responded to concentrated saline solutions due to the dehydration of PNIPAm. The ultrathin thickness made the response of DN gel film with a high speed (40-60 s) and a large stroke force (0.5 MPa), which should find applications in artificial muscles, chemomechanical systems, and soft actuators. The approach to prepare ultrathin gel films should be applicable to other systems toward versatile materials with specific properties and functions.

Supporting Information The Supporting Information is available available free of charge on the ACS Publications website at DOI:xxx Elemental analysis results, transmittance spectra, FTIR spectra, water contents, and mechanical properties of the SN and DN gel films, TEM iamge of iron oxide nanocubes, gel-to-sol transition of SN gel film, dimension change of DN gel film as a function of CNaCl (PDF) Vedio of the preparation of SN gel film (AVI)

Acknowledgments This work was supported by the National Natural Science Foundation of China (51403184, 51773179), Scientific Research Foundation for the Returned Overseas Chinese Scholars (J20141135), and Thousand Young Talents Program of China. 14 ACS Paragon Plus Environment

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A Facile Approach to Prepare Tough and Responsive Ultrathin Physical Hydrogel Films as Artificial Muscles Ye Tian, Xiaoshuang Wei, Zhi Jian Wang, Pengju Pan, Fangyuan Li, Daishun Ling, Zi Liang Wu,* and Qiang Zheng


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A Facile Approach To Prepare Tough and Responsive Ultrathin

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