Semi-Crystalline Hydrophobically Associated Hydrogels with

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Semi-Crystalline Hydrophobically Associated Hydrogels with Integrated High Performances Dandan Wei, Jia Yang, Lin Zhu, Feng Chen, Ziqing Tang, Gang Qin, and Qiang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15843 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Semi-Crystalline Hydrophobically Associated Hydrogels with Integrated High Performances Dandan Wei1, Jia Yang1, *, Lin Zhu1, Feng Chen1, Ziqing Tang1, Gang Qin1, and Qiang Chen1* 1

School of Materials Science and Engineering Henan Polytechnic University, Jiaozuo, China, 454003 * Corresponding Author: [email protected] and [email protected]


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ABSTRACT Hydrophobically associated hydrogels (HA gels) are one of most extensively investigated high strength hydrogels. Semi-crystalline HA gels, prepared by micellar copolymerization, show high strength and notable functionalities of self-healing and shape-memory. However, the hydrophobic comonomers in these semi-crystalline HA gels are usually limited to the long alkyl length monomers (18-alkyl(meth)acrylates). In the present work, N-acryloyl 11-aminoundecanoic acid (A11AUA), consisting of 10 –CH2 groups and a -COOH group at the end of alkyl chain, was used as hydrophobic comonomer to prepare physical A11AUA-based HA gels in the presence of high concentration cetyl trimethyl ammonium bromide (CTAB) or sodium dodecyl sulfate (SDS). Differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) experiments had identified the A11AUA-based HA gels possessed crystalline domains and clusters of crystalline domains, while lauryl methacrylate (C12M)-based HA gels were amorphous. As a result, A11AUA-based HA gels displayed much better tensile properties than those of C12M-based HA gels. At the optimal condition, the A11AUA-CTAB HA gel demonstrated integrated high performances, including high stiffness (E of 1016 kPa), high strength (σf of 0.75 MPa), high toughness (T of 7540 J/m2), rapid self-recovery (94% recovery after heat treatment at 60 oC for 2 min), outstanding shape memory (fully recovered to the permanent shape only 2-14 s) and excellent self-healing properties (as healed at 60 oC for 2 h, stress and strain healing efficiency reached to 64 % and 85%, respectively). We believe this work provides a new insight for HA gels, which is benefit to design new hydrogels with integrated high performances, such as high strength, high toughness, large extensibility, shape-memory and self-healing properties. Keywords: Hydrophobically associated hydrogels; High Strength; High Toughness, Shape Memory; Self-Healing.



1.

INTRODUTION

Hydrogels have been extensively studied as a unique smart material for several decades and have already been used in biological materials, such as tissue engineering, bio-adhesives, biosensors, drug delivery and protein carriers. 1-4 Unlike living tissue, conventional hydrogels are particularly weak or brittle which has limited their potential applications in the high mechanical performances demanded fields such as load-bearing soft tissues repairment and regeneration,5-6 Therefore, to improve the mechanical properties of hydrogels, different types of hydrogels with various network structures have been synthesized, such as topological gels (TG gels),7 double network hydrogels (DN gels),8-9, 10 nanocomposite hydrogels (NC gels),11-13 macromolecular microsphere composite hydrogels (MMC gels),14-15 hydrogen bonds or dipole-dipole interactions enhanced hydrogels16-18, polyampholyte19-20 and polyelectrolyte complex hydrogels.21-22 Although these hydrogels have achieved very good mechanical properties, the synthesis of hydrogels with integrated high performances, including highly mechanical strength and toughness, large extensibility, shape-memory and self-healing properties, is still a big challenge. Hydrophobically associated hydrogels (HA gels), which are cross-linked by the physically hydrophobic associations, have been comprehensively studied in recent years.23-27 Upon deformation, the hydrophobically associated cross-linking points in HA gels can reversibly disassemble and dissipate a large amount of energies to resist crack propagation.28 Therefore, HA gels are often considered as one of high strength hydrogels. In recent years, many efforts have been made to improve the mechanical strength of HA gels. Wang’s group23, 29 replaced conventional hydrophobic comonomer with anionic or cationic surface active monomer and the concentration of hydrophobic comonomer could reach to 20 mol%. The ionic surface active monomer could self-assemble into polymerizable micelles. Therefore, the preparation of the HA gels could be absent of surfactant. The obtained HA gels achieved better mechanical properties (σf of 0.64 MPa, εf of 13.25 mm/mm and compression stress of 22.5 MPa). Osada et al. was the first to prepare semi-crystalline HA gels via solution polymerization of acrylic acid (AAc) and stearyl acrylate (S18A) in the ethanol.30 They found the gel underwent a dramatic change in its Young’s modulus at a certain temperature. Oguz Okay and co-workers28 had successfully prepared supramolecular semi-crystalline HA gels via bulk polymerization method using long alkyl length monomer as hydrophobic comonomer (~18 carbons).28, 31 The semi-crystalline HA gels exhibited extremely high stiffness of 4-308 MPa, high tensile strength of 1.5-6.7 MPa and high toughness of 3.6-20 kJ/m2. Moreover, the semi-crystalline HA gels also demonstrated self-healing and shape-memory properties.31 Bilici et al.32 also reported the preparation of semi-crystalline HA gels via micellar copolymerization of acrylic acid (AAc) and n-octadecyl acrylate (C18A) in the high concentration surfactant solution, and the semi-crystalline PAAc/C18A HA gels showed melt-processable, high strength (compressive strength of 90 MPa), shape-memory and self-healing properties. However, the hydrophobic comonomers in these semi-crystalline HA gels are limited to the long alkyl length monomers (18-alkyl(meth)acrylates).28, 30-32 It was found the HA gels prepared with high lauryl methacrylate (C12M) content didn’t show any


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crystallinity. The crystallinity of HA gels, using C12M and n-octadecyl acrylate (C18A) as hydrophobic comonomers, decreased as the increase of C12M, and no crystalline domains existed above 8 % C12M.33 Therefore, there is still a big challenge to develop semi-crystalline HA gels with novel hydrophobic monomer and integrated high performances. Herein, we synthesized novel semi-crystalline HA gels in the absence of chemical crosslinker using N-acryloyl 11-aminoundecanoic acid (A11AUA) as hydrophobic monomer. A11AUA, consisting of 10 –CH2 groups and a -COOH group at the end of alkyl chain, had been reported by Phadke et al.34 They found the gel prepared by acryloyl-6-aminocaproic acid (A6ACA) displayed strong and rapid self-healing properties at acid condition, while the gel prepared by A11AUA didn’t heal at the same condition. In the present of work, A11AUA was used as hydrophobic comonomer to copolymerize with acrylamide (AAm) in the presence of high concentration cetyl trimethyl ammonium bromide (CTAB) or sodium dodecyl sulfate (SDS), and the semi-crystalline A11AUA-based HA gels could be successfully prepared by UV-initiated micellar copolymerization under ~50 oC. The existence of crystalline domains and cluster of crystalline domains in our HA gels was identified by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). At the optimal condition, the A11AU-CTAB HA gel achieved E of 1016 kPa, σf of 0.75 MPa, εf of 34.30 mm/mm and work of extension (W) of 17.53 MJ/m3. Moreover, the gel also exhibited high tearing energy (T) of 7540 J/m2, which is comparable to unfilled rubbers (102-103 J/m2), articular cartilage (102-103 J/m2) and double network hydrogels (102-104 J/m2). In addition, it also showed an excellent self-recovery, self-healing, and shape memory performances as triggered by heating above the melting temperature. We believe this work provides a new insight for HA gels, which is benefit to design new hydrogels with integrated high performances, such as high strength and toughness, large extensibility, shape-memory and self-healing properties. 2.

EXPERIMENTAL SECTION

2.1 Materials Acrylamide (AAm, 98%), and 2-Hydroxy-4’-(2-hydoxyethoxy)-2-methylpropiophenone (Irgacure 2959) were obtained from TCI China Inc. Cetyl trimethyl ammonium bromide (CTAB) was purchased from Sigma-aldrich Inc. Sodium dodecyl sulfate (SDS), 11-aminoundecanoic acid, acryloyl chloride, ethyl acetate, tetrahydrofuran (anhydrous), sodium sulfate, petroleum ether and lauryl methacrylate (C12M) were all obtained from Aladdin (shanghai) Inc. All the reagents are used as received without further purification. 2.2 Synthesis of A11AUA N-acryloyl11-aminoundecanoic acid (A11AUA) was synthesized from 11-aminoundecanoic acid as reported in literature (Figure S1 and S2).34 Briefly, 0.055 mol 11-aminoundecanoic acid and 0.11mol NaOH were dissolved in 80 mL deionized



water in ice bath under vigorous stirring. Then 0.05 mol acryloyl chloride and tetrahydrofuran in 7.5 ml were added dropwise. The pH was maintained at 7.5–7.8 until the reaction was completed. The reaction mixture was then extracted with ethyl acetate. The clear aqueous layer was acidified to pH 3.0 and then extracted again with ethyl acetate. The organic layers were collected, combined, and dried with sodium sulfate. Then the solution was condensed by a rotating evaporator and the concentrated solution was precipitated by petroleum ether. Further purification was achieved by repeated precipitation and then centrifuged. At last, the precipitate was collected and dried. The yield of A11AUA was about 63%. The characterization A11AUA was carried out through FTIR spectroscopy and Proton nuclear magnetic resonance spectrum (1H NMR) as shown in Figure S3 and S4. 2.3 Preparation of HA Gels The HA gel was prepared by UV-initiated micellar copolymerization of the hydrophilic monomer AAm and the hydrophobic monomer A11AUA in the presence of cationic surfactant CTAB (A11AUA-CTAB HA gel) under ~50 oC (Figure S2). The content of hydrophobic monomer A11AUA was varied between 5 and 25 mol %. The total monomer and CTAB concentration were tuned between 10 and 30 wt %. Briefly, 2 g CTAB was added to 10 ml deionized water and heated at 90 °C to dissolve the surfactant, obtaining a transparent solution. Then, the 80 mol % AAm, 20 mol % A11AUA at the total monomers of 30 wt% and Irgacure 2959 with 1.5 mol % of the total monomers were added to the solution. After deoxygenization and nitrogen protection, the mixture was heated again at 90°C to dissolve all the reactants and the original transparent solution of CTAB was transformed into a translucent yellowish solution. The resulted translucent yellowish solution was quickly transferred into several plastic syringes of 8.5 mm in diameter, followed by initiate the copolymerization under UV lamp (8 w) for 1 hour with a wavelength of 365 nm to. The temperature was maintained about 50 oC during the whole polymerization process. The HA gels with anionic surfactant SDS (A11AUA-SDS HA gel), or the HA gels using C12M as hydrophobic comonomer (C12M-CTAB HA gel and C12M-SDS HA gel), were all prepared by the same process. 2.4 Characterizations DSC Measurements. Differential Scanning Calorimetry (DSC) was conducted on a Perkin Elmer Diamond DSC under a nitrogen atmosphere to monitor the phase transition property. The as-prepared HA gel samples were sealed in aluminum pans scanned between 30 °C and 80 °C at a heating and cooling rate of 0.5 °C /min. WAXD. Wide-angle X-ray diffraction (WAXD) of CTAB power and frozen-dried as-prepared and swollen HA gel thin films were collected on a Rigaku Miniflex diffractometer using a high power Cu Kα source (λ = 0.154 nm), operating at 30 kV/15 mA. The range of diffraction angle (2θ) was 5−80° at a scanning speed of 5° min−1. WAXS and SAXS. Wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) experiments of as-prepared A11AUA-CTAB HA gel film (thickness of 1 mm) were performed on a modified Xeuss system of Xenocs France


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equipped with a semiconductor detector (Pilatus 100K, DECTRIS, Swiss) attached to a multilayer focused Cu Kα X-ray source (GeniX3D Cu ULD, Xenocs SA, France), generated at 50 kV and 0.6 mA. The wavelength (λ) of the X-ray radiation was 0.154 nm. The distances of sample-to-detector were 39.8 mm and 1080 mm; acquisition times were 30 min and 60 min in WAXS and SAXS measurements, respectively. The effective scattering vector range of SAXS (q, q= (4πsinθ)/λ, where 2θ is the scattering angle) was 0.11-2.14 nm-1. The as-prepared gel film was loaded a cell in X-ray transmission direction. The patterns were obtained in the center of the sample, which were then background corrected and normalized using a standard procedure. One dimensional scattering intensity distributions were obtained by integrating the two-dimensional scattering pattern and an 180o integration of intensity at each scattering vector q was performed. 2.5 Mechanical characterization Tensile Test. Uniaxial tensile test was carried out at a speed of 100 mm min−1 using a universal tensile tester equipped with a 100 N load cell and two spring clamps. The shape of as-prepared gel was cylindrical (8.5 mm in diameter and 60 mm in length). The fracture strain (εf) was determined as εf = ∆l / l0 (mm/mm), λ was determined as λ= l / l0 (mm/mm), where ∆l is the difference value of elongation length (l) and initial length (l0). The fracture stress (σf) was defined as σ = F /A0, where F is the load force and A0 is the original specimen cross sectional area. The elastic modulus (E) of gel was calculated by fitting the initial linear regime of stress−strain curve. The work of extension (W) was calculated by the area under the tensile stress-stain curve. The self-healing experiments were also measured by the same tensile machine. The cylindrical gel was cut into two pieces, and the two gel pieces were physically contacted and healed at various times or temperatures. The mechanical properties of the healed gels were also tested at a speed of 100 mm min−1. Tearing Test. Tearing test was performed on the same tensile machine. The gel film (thickness of 1 mm) was cut into a trousers shape (80 mm in length, 10 mm in width, and 1 mm in thickness) with an initial notch of 20 mm. The two arms of sample was clamped, one arm was fixed while the other one was pulled upward at a crosshead speed of 50 mm min−1. The tearing energy (T) is defined as the work required tearing a unit area, as estimated by35: T=

ଶிೌೡ೐ ௪

Where Fave is the average force of peak values during steady-state tearing and w is the thickness of specimen. Cycle test. The dissipated energy or hysteresis of the gel was also measured by above-mentioned tensile tester. The gel specimen (diameter of 8.5 mm and length of 60 mm) was firstly loaded to various extension ratios with the speed of 100 mm min−1 and then unloaded with the same speed. The dissipated energy (Uhys) was estimated by the area between loading-unloading curves. For self-recovery test, the gel was firstly loaded to λ=10, and then unloaded. The loaded gel was allowed to heat at 60 oC for various times and then cooled. The cooled gel was performed the same



loading-unloading cycle. The recovery time is defined by the heat treatment time, and the recovery rate is defined by the ratio of Uhys at various heating times to Uhys of original ones. Rheology. The viscoelasticity of the gel film (diameter of 25 mm and thickness of 1 mm) was investigated by rheological experiment and measured by a MCR302 rheometer with C-PTD200 temperature control system (Anton Paar China Inc.). The gel was first heated to 80 °C, and then was cooled to 4 °C with a rate of 2 °C /min. The temperature was maintained at 4 °C for 2 min. Then, the gel was heated to 80°C with a rate of 2 °C /min. The heating-cooling cycle was performed five times to evaluate the shape-memory property. The storage modulus (G’), loss modulus (G’’) and loss factor (tan δ) of gels were recorded as a function of temperature during this cooling-heating process with constant shear strain of 0.1% and angular frequency of 6.28 rad/s. 3.

RESULTS and DISCUSSION

3.1 Synthesis of HA gels Scheme 1 showed the preparation process of HA gel via UV-initiated micellar polymerization method. The surfactant of CTAB was dissolved in deionized water at 90 °C, and then the AAm, A11AUA and Irgacure 2959 were dissolved in the CTAB solution at 90 °C. At this stage, A11AUA was dissolved in the CTAB micelles to form polymerizable micelles. After deoxygenization and nitrogen protection, UV-photopolymerization was conducted to form HA gels. As CTAB would separate out from water as the temperature lower than 28 °C, the polymerization was maintained at ~ 50 °C during the whole process. It should be noted that the concentration of surfactant used in the presence of work (~ 20 wt%) was much higher than the conventional micellar polymerization. Due to the high concentration of surfactant, the molar ratio of hydrophobic comonomor (A11AUA or C12M) can reach to 25 mol%, which also was much larger than the conventional micellar polymerization. The chemical structure of A11AUA and C12M can be found in the Scheme 1, and there is a hydrophilic head of carboxylic acid group in the A11AUA, however, the alkyl chain of C12M is totally hydrophobic. The C12M used to compare with A11AUA was because of the similar alkyl chain length between C12M and A11AUA. Different from C12M-based HA gels without crystallinity, A11AUA-based HA gels were semi-crystalline. The crystalline domains in A11AUA-based HA gels were served as physical cross-linkers and “sacrificial bonds” to dissipate energy during deformation.


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Scheme 1. Preparation process and network structure of A11AUA-CTAB HA gels via UV-initiated micellar polymerization. 3.2 Network Structure Figure 1a illustrated the DSC curves of four HA gels with A11AUA or C12M as hydrophobic comonomer and CTAB or SDS as surfactant (the four HA gels were represented as A11AUA-CTAB, A11AUA-SDS, C12M-CTAB and C12M-SDS, respectively.) It was clearly found that no peaks could be detected for C12M-CTAB HA gel and C12M-SDS HA gel during heating and cooling process, indicating C12M-based HA gels were amorphous. The results were consistent with the literatures33. However, the A11AUA-CTAB HA gel and A11AUA-SDS HA gel demonstrated obvious melting peaks as the gels heated and the melting temperatures (Tm) were 47.3 and 45.5 oC, respectively. For the cooling process, the A11AUA-CTAB HA gel and A11AUA-SDS HA gel also displayed crystallization peaks and the crystallization temperatures (Tc) are 43.5 and 41.7 oC, respectively. As shown in Figure 1b, many characteristic peaks of CTAB could be found on the WAXD curve of the frozen-dried as-prepared HA gel, which was difficult to reveal the crystalline structure of HA gel itself. After the removal of CTAB by soaking the HA gel in a large excess of water for 10 days, the frozen-dried swollen HA gel showed a single peak (2θ=20.5o) corresponding to the Bragg d-spacing of 0.43 nm, indicating the side chains of A11AUA were packed into a paraffin-like hexagonal lattice. 28, 30 Figure 1c showed the WAXS curve of as-prepared A11AUA-CTAB HA gel. The gel exhibited a broad peak at 2θ=22.3o, which was ascribed to the diffraction of water36. Moreover, the gel also demonstrated a sharp peak at 2θ =16.4o,



corresponding to 0.54 nm (d1) side-by-side spacing between the long alkyl chains of A11AUA. Compared to Figure 1b, the results also infer the lattice spacing of alkyl chains of A11AUA becomes smaller after swelling of the HA gel. Figure 1d illustrated the SAXS curve of the as-prepared A11AUA-CTAB HA gel. The HA gel exhibited two peaks at qmax of 0.29 and 1.09 nm-1, respectively. The peak at qmax =1.09 nm-1 indicated a long-range ordering with lattice spacing d2 of 5.76 nm. The d2 spacing revealed tail-to-tail alignment of the A11AUA side chains perpendicularly to the main chain, which was similar to other HA hydrogels33, 36-37. Because the length lmax of the fully extended alkyl chain of A11AUA is 2.085 nm, and the d2 spacing (5.76 nm) of the A11AUA-CTAB HA gel is larger than twice of the fully extended alkyl chain of A11AUA (2lmax = 4.17 nm). The results indicate the alkyl lamellar crystals are separated by the polymer backbond (amorphous domain, Scheme 1), which is similar to the microstructure of semicrystalline hydrogels reported by Bilici et al33. The peak at qmax =0.29 nm-1 indicated another long-range ordering of 21.66 nm, which might be the correlation length from the clusters of the A11AUA crystalline domains38-40. Owing to the inhomogeneities in spatial distribution of crystalline domains, some A11AUA domains could form nanodomain clusters in the gel network (Scheme 1)40. A11AUA-CTAB HA gel A11AUA-SDS HA gel C12M-CTAB HA gel C12M-SDS HA gel

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Figure 1. (a) DSC curves of various HA gels; (b) WAXD of CTAB, as-prepared and swollen A11AUA-CTAB HA gel; (c) WAXS of A11AUA-CTAB HA gel; (d) SAXS of A11AUA-CTAB HA gel. A11AUA % of 20 mol%, CCTAB of 20 wt% and C0 of 30 wt%.


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It was surprising that the shorter alkyl side chains (11 carbons) in A11AUA-CTAB HA gel were also packing into hexagonal lattice, which was similar to the long alkyl side chain (18 carbons) in C18A-based HA gels. However, compared to the A11AUA-based HA gels, the C12M-based HA gels with the similar alkyl side chain length didn’t show any crystallinity. The results infer the hydrogen bonding interactions among –COOH groups in the A11AUA are important to the formation of crystalline domains. Combination with DSC, WAXS and SAXS results, it could be identified that our HA gels with shorter A11AUA side chains were semi-crystalline HA gels, which were similar to the C18M-based HA gels with long alkyl side chain. 3.3 Tensile Properties The tensile properties of A11AUA-CTAB, A11AUA-SDS, C12M-CTAB and C12M-SDS HA gels were compared in Table 1 and Figure S5. It was found the A11AUA-CTAB HA gel and A11AUA-SDS HA gel exhibited higher tensile properties than those of C12M-CTAB HA gel and C12M-SDS HA gel. Specifically, A11AUA-CTAB HA gel and A11AUA-SDS HA gel displayed σf of 0.75 and 0.89 MPa, respectively. In contrast, C12M-CTAB HA gel and C12M-SDS HA gel showed σf of 0.21 and 0.31 MPa, respectively. The high tensile properties of A11AUA-based HA gels were attributed to the existence of crystalline domains in the gels. However, the physical cross-linking points of C12M-based HA gels were hydrophobic associations, inferring the crystalline domains in the A11AUA-based HA gels were important to achieve high strength HA gels. Because the –COOH groups of A11AUA could be deprotonated at high pH, there were electrostatic interactions between cationic CTAB and –COO- groups, which might influence the mechanical properties of A11AUA-based HA gels. At pH= 7.0, the carboxylic acid of A11AUA side group was deprotonated and became hydrophilic. Although there were electrostatic interactions between A11AUA-COO- and CTAB, the HA gel prepared at pH= 7.0 still exhibited very weak tensile properties (Table 1 and Figure S5d). The fact indicates the electrostatic interactions between A11AUA-COO- and CTAB are not strong enough to form high mechanical HA gels if A11AUA side groups are deprotonated and hydrophilic. However, at pH=3.0, the carboxylic acid of A11AUA was protonated and hydrophobic. The main interactions between A11AUA and CTAB were hydrophobic associations. After polymerization, the HA gel prepared at pH= 3.0 demonstrated much better mechanical properties compared to the HA gel prepared at pH=7.0. In our present work, deionized water was used, and the pH value of the deionized water was ~5.3. We didn’t tune the pH of deionized water to prepare most of HA gels. At pH = 5.3, partial carboxylic acid of A11AUA might be deprotonated. Therefore, we prepared A11AUA HA gels with CTAB or SDS as surfactant to investigate the effect of the charge of surfactant on the mechanical properties. As shown in Table 1, both A11AUA-CTAB HA gel and A11AUA-SDS HA gel showed high tensile properties, which were similar to the HA gel prepared at pH=3.0. Therefore, we concluded the electrostatic interactions between A11AUA and CTAB were not the main drive force to achieve the highly mechanical properties of our HA gels. Instead, as identified by DSC, WAXS and SAXS (Figure 1), the highly



mechanical properties of our A11AUA-based HA gels are attributed to form the crystalline domains. Table 1. Mechanical properties for HA gels with different hydrophobic monomers and surfactantsa. E σf εf W H2 O (wt%) (kPa) (MPa) (mm/mm) (MJ/m3) A11AUA/CTAB 1016±14.849 0.75±0.066 34.3±3.273 17.53±1.725 66 A11AUA/SDS 726±16.263 0.89±0.064 46.38±1.796 25.57±0.750 66 C12M/CTAB 225±19.092 0.31±0.0212 18.64±0.445 3.64±0.205 66 C12M/SDS 183±12.727 0.2±0.028 27.93±1.952 3.42±0.919 66 a A11AUA/CTAB 207±6.928 0.71±0.0264 38.06±3.850 19.42±2.943 66 A11AUA/CTABb 50±4.950 0.017±0.002 85.08±1.888 0.011±0.071 66 a: A11AUA % or C12M % = 20 mol%, C0 = 30 wt%, Csurfactant=20 wt%; b: pH = 3.0; c: pH = 7.0 HA gels

Our A11AUA-based HA gels were totally different from Wang’s work. In their work, the anionic reactive surfactant of sodium 9 or 10-acrylamidostearic acid (NaAAS) was a twin-tail reactive surfactant, in which one of tails was hydrophobic alkyl chain and another tail was similar to A11AUA but with –COO- group at the head of the tail23. NaAAS could self-assemble into reactive micelles in the absence of conventional surfactant (CTAB or SDS), which were acted as the hydrophobically physical cross-linking points. In contrast, at pH = 7.0, the head of –COO- made the side chain of our A11AUA be hydrophilic, which couldn’t form effective hydrophobic associated interactions between the side chains of A11AUA and surfactant as well as the crystalline domains. The results further identify that the highly mechanical properties of our A11AUA-based HA gels are attributed to the crystalline domains. Unless otherwise stated, below we mainly focused on A11AUA-CTAB HA gels to further reveal the mechanical behaviors and mechanisms of such HA gels. We further conducted a series of tensile tests to investigate the effects of A11AUA %, CTAB concentration (CCTAB) and total monomer concentration (C0) on the mechanical properties of HA gels. As shown in Figure 2a and 2b, the E, σf and W all increased as the increase of A11AUA % from 5 to 20 mol%, while the mechanical properties deceased as A11AUA % > 20 mol% (Table S1). The increase of mechanical properties as the A11AUA increased was attributed to the increase of crystalline domains in HA gels. However, as A11AUA % > 20 mol%, the mechanical properties of HA gel became worse, which might be caused by the increase of heterogeneous network structure as the dissolution of A11AUA decreased at the high concentration of A11AUA.The mechanical properties of HA gels were also significantly influenced by CCTAB. As shown in Figure 2c and 2d, there was an optimal value at the concentration of 20 wt% to achieve the best mechanical properties. As CCTAB < 20 wt%, the mechanical enhancement of HA gels contributed to the increase of effective cross-linking density (N1). As shown in Table S2, the N1, calculated from the E of gels assuming the gel could be treated by classical rubber


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elasticity theory (i.e. E= 3G’ and G’= N1RT, where G’ is the storage modulus, R is gas constant and T is absolute temperature), increased as CCTAB increased from 10 to 20 wt%. However, as CCTAB > 20 wt%, the concentration of CTAB was over introduced, the crystalline domains in our HA gels could also be partially destroyed by the over introduced CTAB. The phenomena were similar to Wang’s report41. Different from A11AUA % and CCTAB with an optimal value, the mechanical parameters of E, σf and W all solely increased as the increase of C0 (Figure 3e and 3f). The remarkable improvement in mechanical properties could be ascribed to the increase of the crystalline domains in HA gels. Unless otherwise stated, below we mainly focused on A11AUA-CTAB HA gels prepared at the optimal polymerization conditions (A11AUA % of 20 mol%, CCTAB of 20 wt% and C0 of 30 wt%).

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Figure 2. (a) Stress-strain curves of A11AUA-CTAB HA gels at different A11AUA %. C0 = 20 wt %, CCTAB = 15 wt%; (b) Effect of A11AUA % on the E, σf and W of HA gels; (c) Stress-strain curves of A11AUA-CTAB HA gels at different CCTAB. C0 = 20 wt%, A11AUA % = 20 mol%; (d) Effect of CCTAB on the E, σf and W of HA gels; (e) Stress-strain curves of A11AUA-CTAB HA gels at different C0. CCTAB = 20 wt%, A11AUA % = 20 mol%; (f) Effect of C0 on the E, σf and W of HA gels. 3.4 Tearing Properties We also conducted tearing experiments to estimate the toughness of our A11AUA-based HA gels. As shown in Figure 3a, a trouser shape gel specimen could be tore to 3.77 N, corresponding to tearing energies of 7540 J/m2 at a tearing rate of 50 mm/min. The toughness of our A11AUA-based HA gels was comparable to rubbers (102-103 J/m2), articular cartilage (102-103 J/m2) and double network hydrogels (102-104 J/m2). In contrast, the tearing energies of the conventional HA gels with low hydrophobic monomer content (2 mol%) were only 300 to 400 J/m2, which were much lower than that of our A11AUA-based HA gels42. Kurt et al.31 also found their semi-crystalline HA gels, prepared by bulk polymerization, exhibited high toughness of 3600~20000 J/m2. Li et al.43 also reported Poly(vinyl



alcohol)/Polyacrylamide (PVA/PAAm) hybrid gels showed high tearing energies of 14000 J/m2 due to the PVA crystallites served as the cross-linking points. Therefore, the presence of crystalline domains in hydrogels could significantly improve the toughness of the hydrogels. The toughness of HA gels was also strain rate-dependent, and the tearing energies linearly increased from 6400 to 8560 J/m2 as the strain rate increased from 0.01 to 0.208 s-1 (Figure 3b).The strain rate-dependent toughness of physical gels was also detected by Luo et al.20 They found the tearing energies of polyampholyte hydrogels increased from 860 to 3100 J/m2 as the strain rate changed from 0.002 to 1.04 s-1. The results basically reveal the physical nature of our A11AUA-based HA gels. We further examined the notch-sensitivity of our A11AUA-based HA gels. As shown in Figure 3c, a notched gel specimen (~10 mm notch) could stretch to 9 times its original length, and the notch was strongly blunted. The initial of crack advancing could be clearly detected at ε = 2.7 mm/mm, and the crack angle (θ) was maintained an obtuse angle during crack propagation. At ε = 9 mm/mm, before the fracture of the gel specimen, the θ was 155o. The notch-insensitive property of the A11AUA-CTAB HA gel was consistent with its high toughness. Tearing energy, T (J/m 2)

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3.5 Energy Dissipation and Self-Recovery Figure S6a illustrated the cyclic loading-unloading of A11AUA-based HA gels at various maximum extension ratios (λmax), and it was found all the gels demonstrated distinct hysteresis loops. The dissipated energies (Uhys) significantly increased from 0.52 to 11.92 MJ/m3 as the λmax increased from 5 to 40 (Figure S6b). The hysteresis phenomenon was also observed in other HA gels prepared by conventional micellar polymerization. The HA gel with 1 mol% of C18M (PAAm/C18M0.1 HA gel) exhibited the Uhys of 86 kJ/m3 at λ=10,42 which was much lower than that of our A11AUA-based HA gel at the same extension ratio (Uhys of 1320 kJ/m3). The large hysteresis of A11AUA-based HA gels indicated the destruction of crystalline domains during the deformation could effectively dissipate energies. It was interesting to find our A11AUA-based HA gels displayed very large residual strain after the unloading (Figure S6a and S6c), and the residual strain increased as the increase of the λmax. At λ=40, the residual strain even had extension ratio of 20, and the residual strain couldn’t be recovered at room temperature. The results indicate A11AUA-based HA gels show very poor shape-recovery at room temperature. Because HA gels prepared by conventional micellar polymerization (2 mol% of C18M) exhibited very good shape-recovery42, the large residual strain and poor shape-recovery of A11AUA-based HA gels were caused by the destruction of crystalline domains. We further conducted five successively cyclic loading on the same gel specimen of our HA gel, and the diameter of the gel needed to be measured before each loading. As shown in Figure S6d, our HA gel became more and more strong and tougher as the loading cycles increased. The σλ=10 and Uhys was 0.3 MPa and 1.48 kJ/m3 at the 1st loading, respectively. However, these values increased to 1.79 MPa and 5.78 kJ/m3 at the 5th loading (Table S4). The results infer A11AUA-based HA gels also show self-reinforcement during cyclic loading. Although A11AUA-based HA gels showed large residual strain and poor shape-recovery at room temperature, they displayed very excellent self-recovery properties after heat treatment at 60 °C. As shown in Figure 4a, the residual strain of HA gel disappeared and the shape of the gel was recovered as heating at 60 °C for 2 min. We further examined the self-recovery properties of the HA gels at various heat treatment times (recovery times). In Figure 4b, the hysteresis loops at each time were almost completely recovered compared to the original loop. The recovery rates, defined by the ratio of Uhys at various heating times to Uhys of original ones, achieved 94%, 90 %, 97% and 100% at the recovery time of 2, 4, 6 and 8 min (Figure 4c), respectively. Rheological experiments were also conducted to examine the self-recovery of the HA gels. The gel was cooled from 80 oC to 4 oC, and then was heated from 4 oC to 80 oC. As shown in Figure 4d, the storage modulus (G’) and loss modulus (G’’) of the gel increased as the temperature decreased from 80 oC to 4 oC, and one transition could be detected at ~46 oC, which was consistent with the Tc of the A11AUA-CTAB HA gel. As heating the gel from 4 oC to 80 oC, the G’ and G’’ decreased as the temperature increased, and another transition at 45 oC was also



observed, which was consistent with the Tm of the A11AUA-CTAB HA gel. Moreover, the G’ was always larger than G” and the tan δ was also larger than 0.1 during the whole cooling-heating process, inferring our A11AUA-base HA gel was an elastic gel and there was no gel-sol or sol-gel transition during the whole cooling-heating process. In addition, the G’ and G’’ were almost the same at a specific temperature in spite of cooling or heating process. The results indicate A11AUA-based HA gels demonstrate excellent self-recovery as T > Tm. (a)

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Figure 4. (a) The shape-recovery of A11AUA-CTAB HA gel after heat treatment at 60 °C for 2 min; (b) Loading-unloading curves of the original and recovered hydrogels at various recovery times; (c) Recovery rate (%) of HA gel at 60 °C at various recovery times; (d) G’, G’’ and tan δ of HA gel as cooled from 80 oC to 4 oC and then heated from 4 oC to 80 oC. 3.6 Shape Memory Behavior As shown in Figure 4d, the G’ of our HA gel was 0.026 and 8.75 MPa at the temperature of 80 and 4 °C, respectively, indicating the gel became 336 times stiffer at 4 °C than the gel at 80 °C. We also conducted five cooling-heating cycles to estimate the reversible properties of the HA gel (Figure S7). The values of G’ was almost the same for five cycles at 80 and 4 °C, respectively. The results infer that our HA gels exhibit excellent shape-memory properties. To further identify the shape-memory properties of our HA gels, the rectangular gel sheet (2D) was firstly heated to 60 °C, and then the gel was folded to a complex 3D temporary shape which


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was fixed at the room temperature as shown in Figure 5a. The 3D gel was immersed into 80 °C water, and the gels could rapidly recover to the permanent rectangular shape (2D, ~ 2 s) (Figure 5a). Similarly, a 3D gel sample (1 cm*1cm*0.4 cm) was deformed and fixed to a 2D dumbbell shape, and the dumbbell shape gel was also immersed into 80 °C water. It was found the shape recovery was very interesting and a two-stage process was observed. At 2 s, a “V” shape was found, and the shape was changed into “U” shape at 3 s. The “U” shape gel could be stood in the water for 2 s, and then the gel was recovered to the 3D permanent shape for ~9 s. The total recovery time was about 14 s from 2D dumbbell shape to 3D shape (Figure 5b). The results further indicate A11AUA-CTAB HA gels demonstrate excellent shape memory properties.

Figure 5. Shape memory behavior of A11AUA-CTAB HA gels: (a) from 2D to 3D to 2D; (b) from 3D to 2D to 3D. The gel sample was heated to 60 oC and then folded or stretched to two different temporary shapes. The temporary shapes were fixed at 25 oC, and the gels with temporary shape were immersed in 80 °C to recover. 3.7 Self-Healing Properties Considering the reversible physically cross-linked nature of A11AUA-based HA gel, it was expected the HA gels demonstrated self-healing properties. To investigate the self-healing properties, the gel was cut into two pieces, and then the two pieces were physically contacted together in a plastic syringe. The gel was healed at various temperatures for 2 h. Figure 6a showed the healed gel could bear its own weight and sustain bending without separation. Moreover, the healed gel could stretch to 3 times its original length without breakage. We also conducted tensile tests to evaluate the healing efficiency of the HA gel at various conditions. In Figure 6b, the healed gel



exhibited very poor tensile properties if the healing temperature was lower than the Tm of HA gel. At T = 40 oC (< Tm =47.3 oC), the σf and εf of healed gel were 0.12 MPa and 2.5 mm/mm, respectively. However, at T = 60 oC (> Tm =47.3 oC), the σf and εf were 0.48 MPa and 29.2 mm/mm, respectively. We also found the tensile properties of healed gel couldn’t be improved even the healing temperature up to 80 oC. The self-healing efficiency of HA gel at different healing times was also shown in Figure S8. The healing efficiency is defined by the ratio of σf /σf, 0 and εf /εf, 0, where the σf and εf are the tensile stress/strain of healed gel, and the σf, 0 and εf, 0 are tensile stress/strain of original HA gel, respectively. The healing efficiency increased as the healing times increased. At 60 oC for 2 h, the strength could be healed to 64% and the strain could be healed to 85 % (Figure 6c and Figure S8b). The results indicate A11AUA-CTAB HA gels not only demonstrate high strength and toughness, but also display excellent self-healing properties.

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Figure 6. (a) Healed A11AUA-CTAB HA gels could bear its weight and sustain bending without separation, and also could be stretched to 3 times its original length without breakage as the gel healed at 60 °C for 2h; (b) Strain-stress curves of original and healed A11AUA-CTAB HA gels at various healing temperatures; (c) Healing efficiency of A11AUA-CTAB HA gels at various healing temperatures. 3.8 Mechanisms It’s particularly important to reveal the toughening, shape memory and self-healing mechanisms of the semi-crystalline HA gels. As identified by DSC, WAXS and SAXS experiments (Figure 1a, 1c and 1d), our A11AUA-based HA gels are semi-crystalline HA gels. The crystalline domains in the A11AUA-based HA gels provided to the effective elastic chains, which led to the high stiffness (E~ 1 MPa) of the gels. It was found most of semi-crystalline HA gels exhibited high stiffness. The semi-crystalline HA gels prepared by bulk polymerization (DMA/C17.3M0.2 HA gel, DMA/C18A0.2 HA gel and AAc/C18A0.2 HA gel) showed E of 4~ 24 MPa (Table S5), which could be even higher at a larger content of hydrophobic monomer.


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Upon deformation, the crystalline domains were fractured in our A11AUA-based HA gels and the fracture process was accompanied with a large amount of energy dissipation, resulting in large hysteresis loop in the cyclic loading-unloading tests. A11AUA-CTAB HA gel showed the Uhys of 1320 kJ/m3 at λ=10, which was 15.35 times larger than that of PAAm/C18M0.1 HA gel prepared at a low surfactant concentration and low content of hydrophobic monomer (Uhys of 86 kJ/m3 at λ=10)42. Owing to the effective energy dissipation, the crack tip at the notched gel sample was strongly blunted and the crack angle (θ) was larger than 90o (Figure 3c), indicating A11AUA-CTAB HA gel could resist the crack propagation during deformation. Consequently, the toughness of A11AUA-CTAB HA gel was 6.4-8.56 kJ/m2, which was comparable to other semi-crystalline HA gels (3.6-18 kJ/m2, Table S5), but the value was much larger than that of PAAm/C18M0.1 HA gel (350 kJ/m2)42. Although A11AUA-CTAB HA gel demonstrated high strength and toughness, there was large residual strain after stretching (Figure S6a and S6c), and the residual strain couldn’t be recovered at room temperature (R.T.). The shape-recovery could be only achieved after heat treatment (T > Tm). At T > Tm, the crystalline domains were destroyed, and the gel became soft (Figure 4d). At this state, the residual strain decreases and the gel was recovered to the original state with crystalline domains after cooling at R.T. (T< Tc). Therefore, the gel was almost completely recovered after heat treatment at 60 oC for 2 min (94%, Figure 4c). If A11AUA-based HA gel was heated at T > Tm, the gel sample could be deformed to arbitrary temporary shapes and the temporary shapes could be fixed at R.T. The fixation of the gel at R.T. contributed to the recrystallization of A11AUA side chains as T < Tc. As shown in Figure 5, the gel could be deformed from 2D to 3D or from 3D to 2D. As the gel with temporary shape was immersed into the hot water (80 oC), the crystalline domains in the HA gel were destroyed and the shape of the gel was rapidly changed from temporary shape to the permanent shape. The shape memory properties were also found in other semi-crystalline HA gels (Table S5), however, we couldn’t detect such properties for HA gels prepared by conventional micellar polymerization. Owing to the low hydrophobic monomer content, these HA gels were amorphous and the shape of the gels couldn’t be effectively fixed as the temperature come down to R.T. As shown in Figure 6b and 6c, the self-healing of our A11AUA-CTAB HA gel was only pronounced as the healing temperature higher than Tm. At T= 40 oC (< Tm= 47.3 oC), the healed gel shows poor tensile properties, which was caused by the low movement ability of chain segment in the presence of crystalline domains. On the contrary, at T= 60 oC (> Tm= 47.3 oC), the healed gel achieves much better tensile properties. At 60 oC, the crystalline domains were destroyed, and the chain movement ability was enhanced, and the chains are easier to diffuse and interact with each other at the interface. The self-healing properties were also found in other semi-crystalline HA gels (Table S5)31-32. In the presence of high concentration SDS, the PAAc/C18A0.2 HA gel (C0= 1 M) even possessed a gel-sol transition as heated above the Tm, indicating it displayed very good self-healing properties32. However, compared to semi-crystalline HA gels, HA gels prepared at a low surfactant concentration and low content of hydrophobic monomer showed



limited self-healing ability, especially at high monomer concentration44. At C0= 30 %, the stress of PAAm/C18M0.2 HA gel and PAAm/C18M0.1 HA gel could only be healed to 22 and 50% (Table S5) 42, 44, respectively. Improved by hydrogen bonding interactions, the healing efficiency of PAAc/C18M0.2 HA gel could reach to 65%, which was comparable to our A11AUA-CTAB HA gel (64%)44. 4.

Conclusions In the present of work, N-acryloyl 11-aminoundecanoic acid (A11AUA) was used as the hydrophobic comonomer to synthesize hydrophobic associated hydrogels (HA gels) without any chemical cross-linkers via micellar polymerization in the presence of high concentration surfactant (CTAB or SDS). DSC, WAXS and SAXS experiments have identified A11AUA-based HA gels were semi-crystalline HA gels though the alkyl chain of A11AUA was only 11 carbons. In contrast, C12M-based HA gels, in which C12M had similar alkyl chain length compared to A11AUA, were amorphous. The results indicate the –COOH groups at the end of the alkyl chain of A11AUA are important to form the crystalline domains in the A11AUA-based HA gels. Consistently, A11AUA-based HA gels also showed much better tensile properties than C12M-based HA gels. At the optimal condition, the A11AUA-CTAB HA gel achieved E of 1016 kPa, σf of 0.75 MPa, εf of 34.30 mm/mm and W of 17.53 MJ/m3. Moreover, the gel also exhibited high tearing energy of 7540 J/m2, which is comparable to unfilled rubbers (102-103 J/m2), articular cartilage (102-103 J/m2) and double network hydrogels (102-104 J/m2). Moreover, A11AUA-based HA gels also demonstrated excellent self-recovery (94% recovery as heat treatment at 60 oC for 2 min), shape memory (fully recovered to the permanent shape only 2-14 s) and self-healing properties (as healed at 60 oC for 2 h, stress and strain healing efficiency reached to 64 % and 85%, respectively). We believe this work provides a new insight for HA gels, which is benefit to design new hydrogels with integrated high performances, such as high strength, large extensibility, shape-memory and self-healing properties. Notes The authors declare no competing financial interest. Acknowledgment. We thank Prof. Zhiyong Jiang at Changchun Institute of Applied Chemistry (CAS) and Prof. Pengju Pan at Zhejiang University for the valuable discussion about the WAXS and SAXS results in the present work. Q.C. is also grateful for financial support, in part, from National Nature Science Foundation of China (21504022), the Joint Fund for Fostering Talents of NSFC-Henan Province (U1304516), Henan Province (NSFRF1605, 2016GGJS-039 and 17HASTIT006) and Henan Polytechnic University (72105/001 and 672517/005).


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Figure of Content


Semi-Crystalline Hydrophobically Associated Hydrogels with

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