Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: Based on silica nanoparticle and hydrophobic association

Journal of Colloid and Interface Science 381 (2012) 107–115

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: Based on silica nanoparticle and hydrophobic association Jun Yang a,b, Fu-Kuan Shi a, Cheng Gong a, Xu-Ming Xie a,⇑ a b

Advanced Materials Laboratory, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 12 March 2012 Accepted 23 May 2012 Available online 31 May 2012 Keywords: Hydrogel Dual cross-linked networks Hydrophobic association

a b s t r a c t A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Hydrogels consisting three-dimensional networks structure have attracted many attentions as functional soft materials, such as pharmaceutics, biomedicine, coating, cosmetics, and other water-based fields [1–4]. They form bi-continuous phases through two components: one three-dimensional infinite network (minor component) and one aqueous medium (major component). The networks cross-linked either through permanent covalent bonds, termed chemical hydrogels, or through temporary junctions, termed physical hydrogels [5,6]. As most of the chemically crosslinked hydrogels have shortcomings, such as weak tensile strength at swollen state, toxicity for most chemically cross-linking agents, there is a growing need for physical hydrogels as an attractive alternative [7–11]. Those hydrogels cross-linked by non-covalent usually demonstrate high mechanical strength, biocompatibility, and gel-to-sol reversibility (the transition of hydrogel from solid, jelly-like material to liquid one) [9–11]. Amphiphilic block copolymers composing both hydrophilic and hydrophobic segments serve as attractive building blocks for versatile applications; thus, the studies on the preparation and charac⇑ Corresponding author. Fax: +86 10 62784550. E-mail address: [email protected] (X.-M. Xie). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.05.046

terization of various types of amphiphilic block copolymers have been broadly reported [12–15]. The multi-block copolymers contain a number of hydrophobic short sequences that randomly distribute along hydrophilic backbone chains, and the properties of associative copolymers could be tailored by designing the molecular characteristics, such as molecular weight, hydrophobic block content, nature and length of hydrophilic sequences, as well as hydrophobic block distribution along the backbone chains [16]. Hao and Weiss [17] reported a physical hydrogel based on N,Ndimethylamide and 2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate with excellent mechanical properties, and they found that the hydrophobic associations in aqueous medium formed unique core–shell structures that acted as physical cross-links. Wang et al. [18] present a facile method for synthesis of amphiphilic hydrophilic/hydrophobic cross-linked networks based on perfluoropolyethers (PFPEs) and poly(ethylene glycol)s (PEGs) for fouling-release coating applications, and results indicated that the functionalized PEG served as difunctional cross-linker for the network along with the PFPE. Wu et al. [19] prepared a series of supermolecular hydrogels by tailoring two isomeric units molar ratio in aqueous solutions via hydrogen bonding as driving force. They found that two units distributed homogeneously within the matrix, and the backbone shape could be changed from fiber to sheet under different conditions.

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In most cases, the formation of transient network arises from the self-assembling behavior of associating polymers [12–16]. For water-based systems, these associations are generally formed via hydrophobic interactions between alkyl, perfluoroalkyl, aromatic, or polyacrylate stickers that spread along hydrophilic backbones, that is, the hydrophobic blocks dangle on hydrophilic backbones and form hydrophobic association micro-domains (HAMDs, the micelle-like physical junction points), which construct the polymers networks [16,20,21]. The widely studied amphiphilic associative polymers are ABA triblock copolymers that compose several neutral or charged (polyelectrolyte) chains end-capped by short hydrophobic blocks. As the concentrations of those hydrophobic blocks exceed some critical values, the polymer chains could form micelle aggregates by hydrophobic interaction, and the threedimensional network is formed [16,20,21]. Popescu et al. synthesized heptablock copolymers and observed the ‘‘sol-to-gel’’ transition at 3.5 wt.% and exhibited a three-dimensional network through the extended inter-molecular hydrophobic association [21]. Hourdet and Petit [22] prepared hybrid hydrogels by adsorbing side chains with silica nanoparticles and polymer concentrations above the critical values. Therefore, the nature of this interaction among associative blocks determines the hydrogels in strong (high cross-linked stability with long lifetime) or weak (low cross-linked stability with short lifetime) ones. In our earlier work [23], the monomer of acrylic acid (AA) in situ polymerized from silica nanoparticles (SNs) surface and constructed a cross-linked network by poly(acrylic acid) (PAA) chains entanglements: The long entangled chains randomly cross-linked in the structural network, a fraction of entanglements could be permanently trapped in the network and hence played a similar effect on the modulus as an actual network junction would have (Fig. 1). The hydrogels synthesized by this unique method demonstrated amazing characters that they simultaneously overcame the disadvantages relative with chemically cross-linked ones, such as poor mechanical strength and fragility. Furthermore, the swelling and mechanical properties could be tailored over a wide range by altering the network compositions, such as the concentrations of organic (PAA) and inorganic (SN) component within the solution. In this paper, we extend this line of investigation by incorporation of hydrophobic monomers as side chains to construct hydrophobic association as another part of cross-linking network. For this purpose, we prepare physical hydrogels with dual crosslinked networks: hydrophilic backbone PAA and hydrophobic unit of octylphenol polyoxyethylene acrylate (OP-10-AC) as side chain graft from the SNs surface, where both SNs and HAMDs play as ‘‘multi-functional cross-links’’ to maintain networks stability. Due to this unique network structure, the hydrogels present distinctive swelling behavior and high mechanical strength. The effects of SNs content, molar ratio of OP-10-AC to AA, and molecular weight of

backbone polymer chains on hydrogels swelling behavior, heating induced gel-to-sol phase transition, and mechanical strength are addressed. 2. Materials and method 2.1. Materials Acrylic acid (AA), triethylamine (TEA), tetrahydrofuran (THF), tetraethyl orthosilicate (TEOS), and c-methacryloxypropyl trimethoxy silane were purchased from Beijing Chemical Reagent Company, China. Acryloyl chloride (AC) and octylphenol polyoxyethylene ether (OP-10, where 10 means the number of ethoxy units in each molecule, Mw = 647) were provided by Beijing Hengyezhongyuan Chemical Company, China. Other reagents were analytical grade and used without further purification. Deionized water was used in all experiments, and oxygen in the deionized water was removed by bubbling nitrogen gas for 30 min prior to use. 2.2. Preparation and modification of silica nanoparticles The silica nanoparticles were prepared via the well-known Stöber procedure [24] by hydration TEOS in diluted alkaline solution. The obtained SNs were surface modified by silane coupling treatment of c-methacryloxypropyl trimethoxy silane to obtain double bonds. The SNs with diameter of 232 nm were used in this study, and the specific synthetic procedures could be found in our earlier work [23]. 2.3. Preparation of OP-10-AC The OP-10 (67.4 g), TEA (16.7 mL), and THF (80 mL) were added into a flask under magnetic stirring at 0 °C for 10 min. Then, AC (9.75 mL) was slowly dropped into the flask, and the solution was stirred at 0 °C for 6 h. The THF was removed by vacuum distillation, the solution was precipitated by 150 mL acetone, and then the precipitate (triethylamine hydrochloride) was filtrated. The yellow supernatant was purified by vacuum distillation and dried under vacuum at 30 °C for 48 h. The reaction process is schemed in Fig. 2. 2.4. Preparation of dual cross-linked physical hydrogels The P(AA-b-OP-10-AC) hydrogels were synthesized by adding a certain amount of SNs, AA, OP-10-AC, sodium dodecyl sulfate (SDS, solution of hydrophobic monomer and formation of associated domains), and water (25 mL) into a beaker. The solution was dispersed by ultrasonication in an ice-water bath for 15 min. Next, the solution was heated up to 70 °C by an oil bath for 2–6 h under N2 atmosphere, and the free-radical polymerization was allowed to be initiated by ammonium persulfate (Fig. 3). Then, the physical hydrogels were obtained, and the specific material compositions were shown in Table 1. 2.5. Characterization

Fig. 1. Schematic illustration of hybrid hydrogel network structures and the surface of each silica nanoparticle surrounded by concentrated grafted polymer chains. Two chains cannot pass through one another, which creates topological interactions known as entanglements that form the cross-linked network structure.

2.5.1. Swelling ratio The swelling ratio experiment was performed by immersing dried hydrogels into excessive water (changed the water several times during the measurement), and then the sample was taken out at certain time intervals, blotted with wet filter paper to remove water on the surface and weighted. The swelling ratio was calculated by the ratio of swollen hydrogel weight (Wwet) to the corresponding dried hydrogel (Wdry).

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Fig. 2. Synthesis of OP-10-AC.

Fig. 3. Schematic illustration of hydrophilic monomers (AA) and hydrophobic monomers (OP-10-AC) graft from the SNs surface to synthesize physical hydrogels with dual cross-linked networks structure. The OP-10-AC as building block copolymerize on the hydrophilic backbone and form hydrophobic association micro-domains, which are solubilized by SDS. The SNs and HAMDs both act as cross-links within hydrogels and maintain the three-dimensional network structures integrity.

Table 1 Compositions for hydrogels preparation. Sample

OP-10-AC (wt.%)

AA (wt.%)

SNsa (wt.%)

(NH4)2S2O8b (wt.%)

SDSc (wt.%)

Grafted chain weight–average molecular weight (Da)

HAMD 1.5% HAMD 2.5% HAMD 4% HAMDMW1 4%d HAMDMW2 4%d HAMD 6.5% HAMD 4% + SNs 0.5% HAMD 4% + SNs 1.5% SNs 1.5%

1.5 2.5 4 4 4 6.5 4 4 0

98.5 97.5 96 96 96 93.5 96 96 100

0 0 0 0 0 0 0.5 1.5 1.5

0.07 0.09 0.11 0.15 0.16 0.18 0.21 0.23 0.14

1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 0

19,500 18,400 18,600 45,200 102,700 18,800 18,900 18,200 19,400

a b c d

Weight percentage silica particle relative to AA + OP-10-AC. Weight percentage (NH4)2S2O8 relative to AA + OP-10-AC. Weight percentage SDS relative to OP-10-AC. HAMDMW1 4% and HAMDMW2 4% stand for the same composition but different molecular weight relative with HAMD 4%.

2.5.2. Mechanical properties For mechanical properties measurement, the hydrogel was tested by using an electronic universal material testing machine (Gotech Testing Machines Inc., GT-TS-2000) by following conditions: cross-head speed = 30 mm/min, gauge length = 20 mm, hydrogel size = 10 mm in width  15 mm in length  30 mm in height. The initial cross section (150 mm2) was used to calculate the tensile strength. For any of the conditions, 2–3 samples were used to confirm the reproducibility, and no significant differences (deviation <5%) were used to confirm the reproducibility. 2.5.3. Molecular weight The grafted polymer chains on the SNs were retrieved by etching silica particles in hydrofluoric acid solution (20 wt.%) for 24 h and neutralized by sodium carbonate solution (0.1 mol/L). Then, the molecular weight of graft chain was measured by gel permeation chromatography (GPC) with a column Superose 12 (Pharmacia Biotech Corp., 1 cm  30 cm) at UV 247 nm (LKB 2238, SIIBROMMA Corp.). The solution was filtered by 0.45 lm polypropylene filter and diluted to 5 mg/mL in phosphate buffer (NaH2PO4 0.01 mol/L and Na2HPO4 0.02 mol/L, pH 7). The degassed phosphate buffer was used as mobile phase at flow of 0.2 mL/min.

The column was calibrated by dextran standard sample which was obtained from Pharmacia Biotech Corp. 2.5.4. NMR 1 H NMR spectra measurements were executed on JNM-ECA600, JEOL (Japan) NMR spectrometer operating at 600 MHz, using deuterated chloroform (d-CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. The temperature was 25 °C. 2.5.5. Viscosity measurements Oscillatory shear rheology measurements were conducted by Physica MCR301 (Antor Paar) Rheometer, operating with cone geometry (CC27, 26.667 mm in diameter, 40.026 mm in length) under N2 atmosphere at 70 °C. The values of storage modulus (G0 ) and loss modulus (G00 ) were recorded between the frequency ranged from 0.01 to 100 rad/s at a strain of 0.01. 2.5.6. Thermal analysis Thermogravimetric analysis (TGA) was performed by the Shimadzu DTG (TA-60), and the temperature interval was between 25 and 600 °C at a constant heating rate of 10 °C/min under the N2 atmosphere.

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3. Results and discussion 3.1. Characterization of OP-10-AC According to 1H-NMR spectra of OP-10, 3.5–4 ppm resonance peaks assign to the protons of (ACH2ACH2AOA)10. While some new peaks at 6.4, 6.1, and 5.8 ppm assign to the protons of CH2@CHA in OP-10-AC. In addition, a new peak at 4.3 ppm corresponds to the protons of ACH2ACOOACH@CH2, and the peak at 2.6 ppm disappears, which belongs to the protons of A(OCH2CH2)10OHA. The above 1H-NMR spectra confirm that the hydrophobic macromonomer of OP-10-AC is successfully synthesized (Fig. 4). 3.2. Gel effect onset and critical concentration The in situ polymerization is initiated by ammonium persulfate, and the original low-viscosity solution transfers to rubber-like hydrogel. To monitor the gel formation process, the rheological behavior as a function of gelation time is measured, and the values of G0 and G00 as a function of frequency at three typical time points are shown in Fig. 5. At the initial stage of polymerization (45 min), G0 < G00 , which corresponds to the viscous properties dominated liquid state. As the reaction proceeds, the system gains more elastic properties resulting in a gel-like state of the hydrogel composite. Although both of the moduli increase, the slope of G0 with respect to time is higher than that of G00 . This difference in the rates leads a point of cross-over of G0 and G00 (G0  G00  xn), which represents the gelation time (85 min). After 120 min, G00 > G0 , it shows a progressive shift from solution-like behavior to solid-like behavior, which suggests the formation of a solid-like cross-linked polymer network. We systematically observe the hydrogels gelation time

Fig. 5. Typical evolution of elastic modulus, G0 (open symbols), and viscous modulus, G0 0 (full symbols), as a function of frequency within the formation of physical hydrogels (HAMD 4%).

(Table 2), and it is indicated that HAMD 4% + SNs 1.5% shows the shortest gelation time around 63 min followed by HAMD 4% + SNs 0.5% (78 min), HAMD 2.5% (85 min), and HAMD 1.5% (103 min). These results demonstrate that gelation time is mainly determined by hydrophobic segment concentration, where a hydrogel containing the highest concentration of HAMD/SNs shows the shortest gelation time and could easily to form networks: For the hydrogel with higher hydrophobic fraction, it would be easier for chains to entangle and form integrity network structure, so less time is needed before gel point appears. Thus, this trend can be interpreted that the introduction of hydrophobic architecture contributes to the hydrogel network conformation. Since it is well known that the network built on hydrophobic association is highly depended on the hydrophobic block concentration [7,8], for this purpose, a series of solutions with different OP-10-AC concentrations are prepared to examine the gelation critical concentration (take HAMD 4% and HAMD 4% + SNs 0.5% as examples). It is found that there has no bulk hydrogel formation observed if the OP-10-AC initial solution concentration is lower than 7.8 wt.% for the sample of HAMD 4% (Fig. 6). Below this critical association concentration (Ccrit), the hydrophobic association chains mainly form intra-molecular associations, and no threedimensional network can be formed. While as the concentration of hydrophobic block exceeds the Ccrit, these hydrophobic side chains can form inter-molecular associations, and polymer chains deeply inter-penetrate among different macromolecules. Besides, after another careful observation in Fig. 6, one can note that the addition of SNs into the system is favorable for associative gel formation, thus the shift of Ccrit toward a lower value (4 wt.%) for network formation. Thus, these results strongly suggest the following conditions have to be met before gelation onset: (1) the initial concentration of solution has to be over Ccrit, where the critical concentration of entanglements begin to form physical junctions of a three-dimensional cross-linked network via possible nanometric precursors; (2) the hydrophilic/hydrophobic balance needs to achieve a critical value for these system containing hydrophobic blocks [17,18]; (3) an inter-phase, corresponding to a bi-dimensional sol-to-gel transition, has to be formed within solution uniformly before a percolating gelation onset. 3.3. Swelling behavior of hydrogels

Fig. 4. 1H-NMR spectra of OP-10 (a) and OP-10-AC (b).

The hydrogels with single (HAMD) and dual (HAMD + SNs) cross-linked networks structure are immersed into water at

J. Yang et al. / Journal of Colloid and Interface Science 381 (2012) 107–115 Table 2 Gelation time for hydrogels with different compositions. Sample

HAMD 1.5%

HAMD 2.5%

HAMD 4% + SNs 0.5%

HAMD 4% + SNs 1.5%

Gelation time (min)

103

85

78

63

Fig. 6. Schematic illustration of intra-molecular association (C < Ccrit) and intermolecular association (C > Ccrit) at different hydrophobic block concentrations.

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phase after the swelling ratio initial monotonous growth stage, and this can be regarded as a hydrophilicity dominated with hydrophobic micelles association–disassociation dynamic equilibrium process. As more water molecules further penetrate into the polymer networks, those un-reacted monomers or self-polymerization chains would penetrate out of the network, which lead to the decrease in swelling ratio therefore. After those soluble segments that not actually incorporated into the backbones spread out of network thoroughly, the value of swelling ratio approaches to equilibrium (Fig. 8). However, the incorporation of SNs to the above hydrogel network and building the dual cross-linked network alter the hydrogel swelling behavior in following ways (Fig. 7): (1) the swelling ratio for stage (I) decreases as the SNs can be regarded as additional part of cross-links, and the more rigidity network configuration is less inclined to swell; (2) the movement of hydrophobic blocks in polymer chain is restricted, thus the fluctuation range for stage (II) declines; and (3) the amount of soluble segment decreases and its final equilibrium swelling ratio is higher than that of single HAMDs constructed hydrogel (stages III and IV). Here, comparing with the swelling behavior for hydrogels constructed by single SNs [23], there are two interesting features that need to be noted for the cross-linked networks: (1) the swelling ratio fluctuate stage is only ascribed to the incorporation of hydrophobic units, and there is no such phenomenon observed for the single SNs induced hydrogels; (2) the polymer chains entanglements induced by SNs are more effective than that induced by HAMDs, which indicated by the polymer network dissolution ratio is less than 10% for SNs hydrogels and 50% for HAMD hydrogels, respectively. 3.4. Effect of hydrophobic monomer and SNs content

Fig. 7. Swelling behavior of hydrogels by HAMDs and SNs cross-linked network at 25 °C (average value ± SD, n = 3).

25 °C, respectively, and the values of swelling ratio are measured at certain time intervals (Fig. 7). It is demonstrated that the hydrogel network constructed by single HAMD shows four distinct swelling stages: the swelling ratio increases monotonically at initial 30 h (I), then it fluctuates between 255 and 285 within the following 24 h (II), next it decreases to 150 gradually (III), and the swelling ratio approaches to a plateau value and attains swelling equilibrium thereafter (IV). At the initial swelling period, small water molecules penetrate into hydrogel, and the hydrogel cross-linked network stretching leads to the swelling ratio rise fast. While as the volume of network expansion and polymer chains undergo coilto-extended conformation transition, those hydrophobic branches that grafted on polymer backbones gradually expose to aqueous medium, and this result would produce two effects on hydrogel swelling behavior: (1) the exposed association micelles lead to an increase amount of network apparent cross-links and (2) part of these hydrophobic branches may disassociate and re-arrange due to the associated micelles thermodynamic instability and weak interaction, which can be regarded as the decrease amount of cross-links. Thus, there appears a distinct swelling ratio oscillatory

In order to study further the role of HAMDs and SNs in constructing hydrogels network, we synthesize a series of hydrogels with different cross-linked densities by tailoring the OP-10-AC and SNs fractions (Fig. 9). One can note that as the OP-10-AC content increases from 2 to 6 mol%, the hydrogel equilibrium swelling ratio increases from 88 to 220, next it declines to 61 when the OP10-AC content further increases to 10 mol%. This trend is similar to the effect of generally chemical cross-links on hydrogel swelling ratio [25–27]. As the content of OP-10-AC increases at initial phase, the hydrogel network is gradually constructed by HAMDs, and the swelling ratio rises therefore. However, as the content of OP-10-AC exceeds a threshold, it would be adverse to swelling property instead: (1) the sample hydrophilicity declines as the content of OP-10-AC rise and (2) an excessive level of cross-linking density occurs. While for the hydrogels with dual cross-linked networks (HAMDs + SNs), the swelling ratio exhibits a parallel trend but with the lower value after the incorporation of some amount of SNs, which corresponds to the heavily cross-linked hydrogel networks restrict the stretched polymer chains relaxation. After another careful observation in Fig. 8, there is one point that needs to mention that for the OP-10-AC content of 2 mol%, the equilibrium swelling ratio of hydrogel with dual cross-linked networks is higher than that with single cross-linked network instead. The reason for this unusual finding is that the hydrogel network starts to be constructed at the low content of OP-10-AC, and this cross-linked network is not ‘‘hard’’ enough to maintain the whole network for such a low cross-linked density physical gel, thus some polymer chains would transfer to soluble segments. While after the incorporation of SNs as the second cross-linked network, which reinforces network stability, the equilibrium swelling ratio is higher than that of single cross-linked network therefore. Whereas for the hydrogels with the more mature cross-linked networks, the sample with high density of cross-linking network is a disadvantageous factor to the swelling ratio instead.

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Fig. 8. Schematic illustration of hydrogels based on single HAMDs cross-linked network. It contains four phases within the whole swelling process: Initial coil-conformation polymer chains start preliminary swelling (I); hydrophobic associations expose as polymer chains stretching and hydrophobic association–disassociation dynamic equilibrium appears (II); some soluble segments spread out of hydrogel network (III); swelling equilibrium achieves gradually (IV). In this model, only a small number of polymer chains are depicted for simplicity.

Fig. 9. The equilibrium swelling ratio of hydrogels for HAMDs and SNs as crosslinks (backbone molecular weight = 18,600 Da, average value ± SD, n = 3).

Fig. 10. Effect of temperature on hydrogels swelling ratio for HAMDs and SNs as cross-linking points (average value ± SD, n = 3).

3.5. Thermal induced dissolution

process and increases the amount of soluble segments, which finally presents by the lower equilibrium swelling ratio and a faster bulk hydrogel dissolution process (Fig. 10). This result is attributed to the disturbance of hydrogen bonding based chain entanglements and hydrophobic interactions: Water molecules in hydrogels networks form orderly structure by hydrogen bonding along the hydrophobic side chains, and if the solution is heated, its originally orderly structure would be collapsed, and this would be disadvantageous to swelling process. Thus, the temperature sensitivity of the hydrogel is related to the hydrophobic association directly. For the system at 40 °C (HAMD 4%), the swelling oscillatory phase is shorten, and its final swelling equilibrium is broken; thus, many polymer chains transfer to soluble segments, which causes swelling ratio less than 30. While for the solution further

As mentioned above, those hydrophobic blocks built hydrogels networks are partly constructed by hydrophobic associated micelles, and this non-covalent conjunction is easily suffered to the impact of environmental influences, such as temperature, ionic strength, and pH, which easily disturbs the hydrophobic micelles association–disassociation equilibrium [15,16]. Moreover, the increase in temperature would also be disadvantageous to polymer chains entanglements based on hydrogen bonding junctions [16,20]. Here, the hydrogels are immersed in water with different aqueous temperatures, and the results of swelling behavior indicate that the increase in temperature not only promotes the initial swelling rate, but also speeds the unstable micelles disassociation

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Fig. 11. Schematic illustration of temperature-induced polymer chains disentanglement that leads to dissolution process. The polymer chains present coil-conformation before swelling (a); hydrogel cross-linked by single HAMDs with low network stability and easy to dissolution (b); hydrogel cross-linked by both HAMDs and SNs with high network stability and survives under disentanglement to keep gel-like network (c).

heated to 50 °C (HAMD 4%), almost there is no oscillatory phase, and the bulk hydrogel goes into dissolution phase directly after initial swelling phase. Instead, for the hydrogels with dual crosslinked networks, the degree of temperature-induced dissolution is suppressed: for the hydrogel with the SNs content of 1.5 wt.%, the swelling ratio after 100 h can still up to 110 at 50 °C (HAMD 4% + SNs 1.5%). Therefore, it is concluded that the incorporation of SNs to the hydrophobic associated structure reinforces its network integrity and stability. This temperature-induced gel-to-sol phase transition is depended on the structure of the sensitive blocks, building units of the network, and the roles they play (physical junctions or elastic chains) in the networks [12–15]: (1) the outside stimuli affect associative blocks and their interactions that contribute the physical junctions and (2) dimension or shape changes when the stimuli affect the elastic chains that inter-connect the cross-linking domains. The former effect corresponds to the HAMDs de-association process and the latter effect corresponds to polymer chains de-entanglement process (Fig. 11).

Fig. 12. The gel–sol phase transition of hydrogels with different molecular weight.

3.6. Effect of molecular weight on gel-to-sol transition

3.7. Mechanical strength of hydrogels

A series of hydrogels with different molecular weight are applied to study the temperature-induced gel-to-sol phase transition, and the results are revealed in Fig. 12. As there is an increase in the immersion time for hydrogels in heated water, a great part of polymer chains from the associative gel release fast to the dilute solution, which means the dissolution of the associative gel may occur in an explosive way as the gel disappears and cause the dissolution of associative gel. It is demonstrated that in the case of lower molecular weight of polymer chain, the only lower temperature and shorter time is required before the gel-to-sol phase transition finished. For example, there need to be 165 and 105 h before the gel-to-sol phase transition completed at 45 and 60 °C, respectively, for the hydrogel with molecular weight of 102,700 Da, while the dissolution time decreases to 104 and 65 h, respectively, for the hydrogel with molecular weight of 18,600 Da. This result can be justified that the attraction force becomes weak as the molecular weight of polymer backbone decrease; thus, more grafted chains move freely in dilute solution rather than remaining in their original gel network, and much less energy is needed to disturb this temporary cross-linked entanglement before the polymer chains disentanglement process starts. Furthermore, for the hydrogel with dual cross-linked networks, the even higher energy is required to trigger this chain disentanglement process. For example, for the hydrogel with HAMDs and SNs dual cross-linked networks, approximately 128 and 84 h is needed before the completion of gel-to-sol phase transition (HAMD 4% + SNs 0.5%), for which much longer time is required comparing with the single HAMDs (HAMD 4%) cross-linked network hydrogel.

The stress–strain curves for hydrogels prepared using a wide range of cross-linking levels by altering the content of OP-10-AC and the comparison of network strength that built by individual HAMDs or SNs are shown in Fig. 13. The tensile properties of physical hydrogels strongly depend on SNs/HADMs content, the general tensile behavior is parallel for all the samples, and the deformation process is homogeneous during the test (no necking phenomenon is observed). As the HAMDs play the role of cross-links in physical hydrogels, it is safe to predict that the hydrogel with the higher content of OP-10-AC may endure stronger stretching energy. The results show that when the OP-10-AC content is less than 2 wt.%, it is difficult to measure the stress–strain curves because the sample is too weak and brittle, while as the OP-10-AC content attains 4 wt.%, it can survive 116 kPa stress and that value increases to 140 Kpa after the OP-10-AC content further increases to 6.5 wt.%. This result indicates that the number of cross-linking chains in the network structure increases with rising level of cross-links, which presents the same rule for most chemically cross-linked rubber-like materials [26,28], that is, the basic architecture of hydrogel network is initially formed with increasing concentration of OP-10-AC, and the number of cross-links (cross-linking density) also increases with further increase in the concentration of OP-10AC. Meanwhile, comparing with the hydrogel network constructed by HAMDs, the network constructed by SNs could provide more excellent toughness and deformable properties due to its closer polymer chains entanglements: The strength of hydrogel network constructed by SNs (1.5 wt.%) could attain as high as 399 Kpa and a strain above 500%. Therefore, one can further speculate that the

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Fig. 13. The stress–strain curves of hydrogels for different cross-linking conditions. The hydrogel mechanical strength depends on domains that constructed by hydrophobic associations and SNs, which act as physical cross-links. The hydrogels behave like thermoplastic elastomer because they contain both hydrophobic association as soft domains and SNs as hard domains, which contribute to excellent ultimate elongation and tensile strength.

hydrogel network constructed by both HAMDs and SNs may play the synergistic effect and survive even the higher strength, and the experimental result verifies this expectation: The hydrogel formed by dual cross-linked network could sustain 466 Kpa, which is higher than that of any single cross-linked network. These results allow us to demonstrate that the tough as well as excellent flexibility hydrogels are successfully synthesized and pave a simple way for exploring and improving physically cross-linked systems. Combining the above results may suggest the following process for the formation of dual networks structure, that is, the incorporation of HAMD is considered the first stage of gelation, where the elongation at break increases, corresponding to the process of primary network structure formation, and three-dimensional networks throughout the sample have not formed yet. The addition of SN would be considered as the second stage, where both modulus and strength increase more significantly, corresponding to the increase in the number of cross-linking chains, that is, increase in network density, which produces strong and flexible hydrogels. While the phenomenon that the tensile strength of hydrogels constructed by HAMD (curves c and d) is higher than that of constructed by SNs (curve b) within the very early elongation stage (initial range of 50%) may be ascribed to the fact that hydrogel formed by SN responds slower to the outside stress at early stage, and then it recovers its ‘‘hard’’ building block character at subsequent tensile test and presents strong hydrogel property. Can a hydrogel be both tough and flexible? The idea of this was once a paradox. Gong et al. [29] with a series of dual networks have proposed partially inter-connected inter-penetrated networks (IPNs), and such a highly cross-linked network plus a loosely entangled one produced high modulus systems. Haraguchi et al. [30,31] found that with high strength, nanocomposite hydrogels were synthesized by free-radical polymerization of N-isopropylacrylamide (NIPA) and N,N-dimethylacrylamide (DMA) in the presence of exfoliated clays. The strong adsorption between polymer chains and clays led to a percolating network where clays act as multi-functional physical cross-linking points. Here, it should be noted that (1) some amount of chemically cross-linking agent is used for the IPN hydrogels [29], and it improves the strength by increasing the dissipative volume ahead of the crack tip; thus, these gels may be permanently damaged upon deformation and become softer after reloaded, and (2) interaction between clay and polymer for nanocomposite hydrogels is purely physical

Fig. 14. TGA curves of dry hydrogels cross-linked by HAMDs and SNs, respectively.

adsorption, although no result is reported that the clays escape from network in the free water during the swelling to the equilibrium stage. Although this concept with reinforced mechanical properties has been successfully applied to other polymers, such as poly(acrylamide), the replacement of clay by other inorganic particles like silica or titanium oxide has not presented comparable results yet [31]. Comparing with the above two strategies, the method reported here shows its unique properties: (1) no chemically cross-linking agent is used, the network structure is formed by the concentrations of SNs and HAMDs attaining a percolation threshold, and the infinitely network can swell but cannot dissolve in a solvent at low temperature and (2) the interaction between silica and polymer chains is bridged by covalent bond, the key to attain such high elastic modulus would be reproducible chain entanglements/hydrophobic associations, and redistribution of stress to other polymer chains is then possible when some chains break; thus, this facile method may be extended to other reinforcing fillers, such as cellulose nanocrystals, which is currently investigated in our lab, and they show some similar results like SNs in inorganic/organic network structure. In addition, the hydrogels properties mentioned here could be tailored by the level of network structure entanglements, for example, the molecular weight of grafted polymer chains, contents of HAMDs and SN, which provide facile methods to control the network characterizations according to specific conditions in future field applications. 3.8. Thermal characterization Thermal degradation behavior of dry hydrogels with individual HAMDs or SNs as cross-linking points is measured by TGA measurement (Fig. 14). The results show that the thermal stability of hydrogel that constructed by the SNs is higher than that of constructed by the HAMDs, which could be reflected by the entanglements among polymer chains that grafted on SNs surface is more stable than those HAMDs. 4. Conclusions The unique physical hydrogels constructed by dual cross-linked networks (HADMs and SNs) are successfully synthesized. The hydrogels exhibit excellent mechanical strength, and the physical cross-links constructed by HAMDs and SNs maintain the network integrity. The gelation is triggered as OP-10-AC concentration higher than the critical concentration and bulk hydrogels present thereafter. The effects of hydrophobic monomer content, temperature, and molecular weight of polymer chain on hydrogels swelling

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behavior are studied, and the results indicate that the hydrogel swelling process can be divided into four phases. The mechanical strength results demonstrate that hydrogels networks formed by the SNs present better rigidity and stability than that of HAMDs. As the physical hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength can be tailored by the polymer chains architecture design and composition, these hydrogels may have broad applications in future due to their unique properties. Acknowledgment We are grateful for the funding from the National Natural Science Foundation of China (51073088, 91023027, 50573038, and 20874056). References [1] [2] [3] [4] [5]

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