Synergistic toughening of nanocomposite double network ...

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Cite this: RSC Adv., 2016, 6, 37974

Received 1st February 2016 Accepted 8th April 2016 DOI: 10.1039/c6ra02956f advances

Synergistic toughening of nanocomposite double network hydrogels by physical adsorption and chemical bonding of polymer chains to inorganic nanospheres and nanorods: a comparative study

Guorong Gao,ab Ying Xiao,ab Qiang Wanga and Jun Fu*ab

Previously, we have reported nanocomposite double network (ncDN) hydrogels by introducing bare or reactive inorganic nanospheres or nanorods into double network hydrogels. The obtained ncDN gels showed very high compression strength and toughness. However, the toughening mechanisms remains yet to explore. In this work, a comparative study is presented to provide detailed investigations on the polymer?nanoparticle interactions for ncDN gels with bare or vinyl-grafted nanoparticles. First, the effects of physical adsorption and/or chemical bonding of polymer chains to nanoparticles on the mechanical properties of the parent single network hydrogels of poly(2-acrylamido-2-methyl-propane sulfonic acid) (PAMPS) and polyacrylamide (PAAm) are compared. The nanoparticles showed significant toughening to PAAm gel than to PAMPS gel, due to the strong adsorption of PAAm to nanoparticles. Second, by using PAMPS?nanoparticle hydrogel as a host for in situ polymerization of AAm monomers, the obtained ncDN gels showed outstanding compression strength and toughness, with vinyl-grafted nanoparticles toughening more than bare nanoparticles. Detailed comparative analysis on the initial and ultimate modulus of the ncDN gels suggests that, after PAMPS network fracturing upon compression at high strains, the strong polymer?nanoparticle adsorption/bonding plays a critical role in the mechanical properties of the gels, with silica nanospheres working more effectively than ATP nanorods. TEM images revealed that, ATP nanorods were fractured upon large strain compression, while the silica nanospheres served as energy dissipation center. This study provides toughening mechanisms of nanospheres and nanorods for nanocomposite double network hydrogels.

Introduction

Polymer hydrogels are known as so and wet materials with highly hydrated three-dimensional networks. Conventional polymer hydrogels are limited in their practical applications due to poor mechanical properties, due to the lack of energy dissipation mechanisms of the highly swollen network. Recently, numerous strategies have been demonstrated to develop hydrogels with outstanding strength and toughness by introducing efficient energy dissipation mechanisms. For example, double network hydrogels,1,2 nanocomposite hydrogels,3,4 hybrid hydrogels,5,6 and micelle crosslinked hydrogels7,8 etc., show extraordinary strength, stretchability, or fatigue resistance. Therein, sacricial bonds are utilized to dissipate

aCixi Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Room 1324, 777 Sanbei Road, Cixi, Ningbo, Zhejiang Province 315300, P. R. China. E-mail: fujun@nimte.; Fax: +86 574 86685176; Tel: +86 574 86685176 bPolymers and Composites Division, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang Province 315201, P. R. China

energy through fracturing upon loading. Some sacricial bonds are reversible and thus could entitle a recovery of the fractured network under proper conditions.

Nanoparticles have been widely used to reinforce polymer hydrogels. Different from the compositing effects for polymer/ inorganic composites with weak polymer?ller interactions, the nanoparticles used usually show strong interactions with the hydrophilic polymer chains of hydrogels. As a result, both the strength and toughness are enhanced for most nanoparticle/polymer composite hydrogels. For example, silica nanoparticles show very strong reversible adhesion with poly(dimethylacrylamide) (PDMA) chains,9 and silica suspension could be used as `glue' to rmly adhere two pieces of PDMA hydrogels. Silica nanoparticles have also been introduced into chemically crosslinked PDMA hydrogels to form dual crosslinked hybrid hydrogels.10,11 The modulus, strength, and exibility of these hydrogels increased with nanoparticle content. It is suggested that transient crosslinked networks formed in these hydrogels through silica nanoparticles/PDMA junctions, which could release accumulated mechanical force within polymer chains through detachment under large strains. Yang

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et al.12,13 introduced cellulose nanocrystals into chemically crosslinked hydrophilic polymer networks to form nanocomposite hydrogels. The strength, toughness and effective network chain density increased with nanoparticles content, and effective hysteresis loops were observed in tensile loading? unloading cycles. Therein, nanoparticles were observed to bridge cracks during crack propagation. These results suggest that the strong physical adsorption between polymer chains to nanoparticles plays critical roles in enhancing both the strength and toughness of hydrogels.

In previous studies, we have demonstrated the use of inorganic nanoparticles, e.g., silica nanospheres14 and clay nanorods,15 to reinforce double network (DN) hydrogels. DN gels comprised of two (semi-) interpenetrating contrasting polymer networks exploit the rigid network as sacricial bonds to dissipate energy upon loading, while the ductile network acts as hidden length to sustain large deformation.16 Thus, double network hydrogels show high strength and toughness. We used vinyl-graed nanoparticles to copolymerize with the rigid network, which was subsequently interpenetrated with loosely cross-linked ductile network to form nanocomposite double network (ncDN) hydrogels. Compared to DN gels, these ncDN gels showed enhanced compressive strength, modulus, and toughness.14,15 With the polymer chains bonded to nanoparticles, embedded micro-network porous structures are observed for ncDN gels, presumably due to the microphase separation between the nanoparticles and the polymer network.14 This phenomenological explanation, however, did not consider the polymer?chain interactions. Upon cyclic loading?unloading tests, the ncDN gels show remarkable hysteresis during the second and subsequent cycles, indicating residual energy dissipation aer the fracturing of the rigid network. This energy dissipation decays with more cycles but could be recovered at elevated temperatures. Interestingly, this residual energy dissipation is observed for both bare and vinylgraed nanoparticle composited gels, and increases monotonically with the nanoparticle content.15 Both physical adsorption and chemical bonding are attributed to this phenomenon and, further, to the signicant reinforcement to the gels. However, these results did not provide details on how these nanoparticles interact with each polymer network. Besides, it is not clear how the geometric difference of nanospheres and nanorods inuence the toughening mechanisms of polymer hydrogels.

In order to investigate the toughening mechanisms of nanoparticles to double network hydrogels, this work presents a comparative study on the mechanical behaviors of nanoparticle-composited single and double network hydrogels. Hydrogels composited with bare or modied nanoparticles showed remarkable differences in compression stress?strain curves, as well as the modulus and fracture properties. Besides, the moduli of hydrogels composited with silica nanospheres and clay nanorods at high strains are compared in order to study the polymer?nanoparticle interactions aer the fracture of the rigid PAMPS/nanoparticle composite network. TEM images provide direct evidence to the different behaviors of nanospheres and nanorods under load, suggesting a critical

role played by nanoparticle geometry on the toughening mechanisms.

Experimental

Materials

Tetraethoxysilane (TEOS), vinyltriethoxysilane (VTEOS), acetic acid (HAc), ammonium hydroxide (NH4OH, 25 wt% aqueous solution), ethanol, acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N0-methylene bisacrylamide (MBAA) and potassium persulfate (KPS) were purchased from the Sinopharm Chemical Reagent Co. Ltd., Attapulgite (ATP) was provided by Jiuchuan Nano-Material Technology Co. Ltd. De-ionized water was used for all experiments, with oxygen removed by bubbling nitrogen gas for more than 2 hours before use.

Synthesis of vinyl-graed silica (G-silica) and bare (B-silica) silica nanospheres

Bare and vinyl modied silica nanoparticles were synthesized by in situ hydrolysis and precipitation of precursors. Briey, TEOS (8.5 mL) in ethanol (10 mL) was quickly added to a solution of NH4OH (7 mL), water (23 mL) and ethanol (120 mL) with stirring at room temperature. As the mixture turned turbid, VTEOS was added until the TEOS/VTEOS molar ratio reached 85/15. The solution was dialyzed in a dialysis bag (cut-off molecular weight: 8000?14 000) against water to remove unreacted chemicals. The product, G-silica, was collected by centrifugation and freeze dryed.17 B-silica were also prepared following this procedure without using VTEOS.

Synthesis of vinyl graed attapulgite (G-ATP) nanorods

ATP (2 g) was sonicated in an ethanol/water mixture (75 mL/ 75 mL) for 2 h. Then the VTEOS (5 mL) in water (45 mL) solution was added under vigorous stirring, followed by acidication to pH 3.0 with HAc. The mixture was then transferred into a ask and reuxed at 70 C for 30 min. Subsequently, the suspension was centrifuged and washed with de-ionized water to pH 7.0, followed by washing with ethanol three times. The obtained G-ATP powders were lyophilized before use.18

Synthesis of nanocomposite single network (ncSN) hydrogels

The ncSN gels were synthesized by free radical polymerization of mixed solution of monomers (AMPS or AAm), crosslinker (MBAA), and nanoparticles. The feed ratio of MBAA to AMPS was xed at 4%, and that of MBAA to AAm was 0.01%. The water content of resulted gels was 90 wt%. For example, silica (0.885 g), MBAA (0.265 g), initiator KPS (0.012 g) and AMPS (8.85 g) were mixed in water (90 mL) under stirring. Aer stirring for 30 min at 25 C, the solution was transferred into glass moulds for free radical polymerization at 60 C for 10 h to form hydrogels.

By using bare or graed nanoparticles for the synthesis, a series of ncSN gels, namely, PAMPS-BS1, PAMPS-GS1, PAMPSBA1, PAMPS-GA1, PAAm-BS1, and PAAm-BA1 were obtained, with BS for bare silica, GS for G-silica, BA for bare ATP, and GA for graed ATP. The nanoparticle content was 1 wt% with

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respect to the monomers. For comparison, single network hydrogels of PAMPS (PAMPS-SN) and PAAm (PAAm-SN), free of nanoparticles, were synthesized by using MBAA as crosslinker.

? 5?10%. The fracture energy (U), is calculated by integrating the area under the stress?strain curve:

?

DU ? sd3

(1)

Synthesis of nanocomposite double network (ncDN) hydrogels

The nanocomposite single network hydrogels of PAMPS were used to host the AAm monomers for in situ free radical synthesis of the second network. Briey, the nanocomposite PAMPS gel was swollen in a aqueous solution of 3 mol L?1 AAm, 0.01 mol% MBAA, and 0.01 mol% KPS for 24 h. The swollen gel was sandwiched by two glass plates and sealed, followed by polymerization at 60 C for 10 h. Finally, the gels were immersed in water for 7 days, with water been changed twice a day.

The synthesized ncDN gels were designated as ncDN-BSm, ncDN-GSm, ncDN-BAm and ncDN-GAm gels, where m is the weight percentage (wt%) of nanoparticles to AMPS. On the other hand, the ncDN gels with bare nanoparticles were denoted as ncDN-B gels, the ncDN gels with vinyl graed nanoparticles as ncDN-G gels, the ncDN gels with silica nanospheres (graed or not) as ncDN-silica gels, and the ncDN gels with ATP nanorods (graed or not) as ncDN-ATP gels. For comparison, the PMAPS/ PAAm double network (DN) gels with the formulations mentioned above, free of nanoparticles, were prepared.

Scanning electron microscopy (SEM)

Silica or G-silica dispersions were cast on silicon wafers and then dried for imaging by using a Hitachi S4800 scanning electron microscope (Hitachi Inc., Japan) at 15 kV. On the other hand, gel samples were freeze-fractured in liquid nitrogen, freeze-dried and then sputter coated with platinum for SEM imaging.

Transmission electron microscopy (TEM)

The suspensions of bare ATP or G-ATP were cast on carbon supported copper grids, and then dried for TEM imaging by using a Tecnai F20 transmission electron microscope (FEI Inc., Oregon) at 200 kV. On the other hand, lyophilized gels were embedded in epoxy resin for cryo-microtoming at ?40 C into ca. 70 nm sections, which were collected onto copper grids for imaging operating.

Cyclic compression tests with a maximum strain of 90% were

conducted on specimens with the same size and at the same

crosshead speed. The dissipated energy for each cycle, DU, is

dened as the area encompassed by the loading?unloading

curve:

?

?

DU ?

sd3 ?

sd3

(2)

loading

unloading

Results and discussion

Preparation of nanoparticles modied with vinyl groups

Silica nanoparticles synthesized by in situ precipitation showed well controlled particle size with narrow distributions and smooth surface (Fig. 1a). By controlling the stirring speed and reaction time, silica nanoparticles with an average diameter of 257.9 ? 7.4 nm were used for this study. To gra vinyl groups to silica nanoparticles, VTEOS was used aer the pre-hydrolysis of TEOS, leading to nanoparticles with raspberry like surface (Fig. 1b), likely due to the in situ hydrolysis and polymerization of VTEOS at the nanoparticle surface. On the other hand, vinyl groups were graed onto attapulgite (ATP) nanorods dispersed in aqueous solutions. TEM results showed that bare ATP nanorods were about 600?700 nm long and 20?24 nm in diameter (Fig. 1c). Aer vinyl modication, the nanorods became hairy, and the average length and diameter were not signicantly different from the inact ones (Fig. 1d). The presence of vinyl groups in G-silica and G-ATP were identied through FTIR measurements in previous work.14,15

Mechanical tests

Cylindrical gel samples (9 mm diameter and 5?7 mm height, n ? 5 each) were compression tested by using an Instron 5567 (Instron Inc., MA) instrument equipped with a 3 kN load cell. All the samples were loaded at a crosshead speed of 10% min?1 to

up to 98% strain to protect the load cell. The engineering compression stress (s) was calculated as s ?

F/pR2, where F is the loading force and R is the original radius of specimen. The engineering compression strain (3) was dened as the change in height (h) relative to initial height (h0) of the specimen, 3 ? (h0 ? h)/h0 ? 100%. The initial modulus (Ei) was calculated as the slope of stress?strain curve within the range 3

Fig. 1 SEM images of (a) bare and (b) vinyl-grafted silica nanospheres, and TEM images of (c) pristine, and (d) vinyl-grafted attapulgite (ATP) nanorods.

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Compression properties of nanocomposite single network (ncSN) hydrogels

Previously, nanoparticle-reinforced double network hydrogels showed very high compression strength and toughness.

Therein, the outstanding reinforcing effect was presumably attributed to the physical adsorption and/or covalent bonding of polymer chains to nanoparticles. Herein, we provide evidences to the synergistic contribution of physical adsorption and chemical bonding to nanoparticles to the toughening effect of single network and double network hydrogels. For this purpose, the compression properties of single network (SN) hydrogels of PAMPS and PAAm with the presence of bare or modied nanoparticles have been systematically and comparatively investigated.

Fig. 2 compares representative compression stress?strain curves of these ncSN gels. The PAMPS-SN gel showed a fracture strength (sb) of 59.7 ? 9.6 kPa, fracture strain (3b) of 31.9 ? 1.5%, initial modulus (Ei) of 180 ? 10.4 kPa, and fracture energy (U) of 6.6 ? 0.9 kJ m?3. With the presence of 1 wt% bare silica nanospheres, the sb was increased to 87.7 ? 7.1 kPa, 3b to 35.9 ? 1.2%, Ei to 191.5 ? 12.1 kPa, and U to 10.5 ? 1.8 kJ m?3. These results show a simultaneous enhancement in both compression strength and toughness. This behavior is much different from the conventional reinforcement of polymer materials with nanoparticles, where there are usually no or weak polymer? particle interactions. Herein, this toughening effect, although relatively low, may be related to the adsorption of polymer chains to silica nanoparticles. As the silica nanoparticles were modied with vinyl groups, the compression properties were further enhanced. The PAMPS-GS1 gels with 1 wt% G-silica nanoparticles showed a sb of 121.2 ? 2.9 kPa, 3b of 33.9 ? 0.9%, Ei of 277.7 ? 30.2 kPa, and U of 14.1 ? 1.6 kJ m?3, which

are signicantly higher than those for the PAMPS-BS1 hydrogels except that both 3b values are not signicantly different (p > 0.05). The chemical bonding between polymer chains to silica nanoparticles further improved the strength and toughness of the PAMPS-SN gel.

PAAm chains are well known for their strong adsorption to inorganic particles (e.g., clay19 or silica nanoparticles20,21). In order to investigate the effect of PAAm?nanoparticle adsorption to the mechanical properties of SN gels, nanocomposite PAAm gel was synthesized with the presence of 1 wt% bare silica nanoparticles. Fig. 2b compares representative compression stress?strain curves of the PAAm-BS1 gel and PAAm gel. In comparison to the PAMPS gel, the PAAm gel showed higher compression properties (Fig. 2c). The sb of PAAm-SN gels was 703.6 ? 32.8 kPa, and 3b was 80.2 ? 3.3%. With 1 wt% bare silica nanoparticles, the sb was largely enhanced to 6.7 ? 0.4 MPa with 3b of about 90.7 ? 4.1%. As a result, the U was increased from about 108.3 ? 10.2 kJ m?3 to about 521.7 ? 30 kJ m?3.

As the silica nanoparticles were replaced by attapulgite (ATP) nanorods, similar toughening of PAMPS and PAAm SN gels were also investigated. Bare ATP nanorods slightly improved the compressive strength and toughness of the PAMPS gel, while the gel containing G-ATP nanorods showed a bit higher strength and toughness (Fig. 2d). In contrast, the compression strength and fracture strain of the PAAm-BA1 gel were 8.7 ? 0.7 MPa and 92.1 ? 3.5%, much higher than those for the PAAm gel (Fig. 2e). As a result, the compression toughness of PAAm-BA1 was 583.9 ? 47.2 kJ m?3, which is almost 6 times that of PAAm-SN gel (Fig. 2f).

Such a big increase in toughness is attributed to the strong adsorption of PAAm chains to silica/ATP nanoparticles. In contrast to the small difference in the strength and toughness between PAMPS-BS1 (or PAMPS-BA1) and PAMPS-SN gels, the

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Fig. 2 Representative stress?strain curves of (a) PAMPS-SN, PAMPS-BS1 and PAMPS-GS1 gel, and (b) PAAm-SN and PAAm-BS1 gel. (c) Ei and U of silica composited SN gels. Representative stress?strain curves of (d) PAMPS-SN, PAMPS-BA1 and PAMPS-GA1 gel, and (e) PAAm-SN and PAAm-BA1 gel. (f) Ei and U of ATP composited SN gels.

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remarkable toughening effect of PAAm gel indicates a stronger physical adsorption of PAAm chains to silica/ATP nanoparticles than those for PAMPS chains. The bare silica nanoparticles and bare ATP nanorods contain a lot of hydroxyl groups on surface, which may form hydrogen bonding to the AAm monomers prior to polymerization. Similar adsorption of monomers to inorganic particles has been presumed in nanocomposite hydrogels by Haraguchi et al.,22 who used synthetic hectorite clay nanosheets to adsorb monomers for synthesis of hydrogels with very high strength and toughness. Besides, the surface modication enables chemical bonding, in addition to physical adsorption, of polymer chains to nanoparticles. This is important, as to be shown and discussed below, for the outstanding toughening of double network hydrogels.

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Compression properties of nanocomposite double network (ncDN) hydrogels The nanocomposite single network PAMPS hydrogels were used to host free radical polymerization of AAm monomers into a second network interpenetrating to the rigid rst one, as reported previously,14,15 resulting in ncDN hydrogels. Previous studies have demonstrated outstanding compression strength and toughness of these hydrogels. Herein, a series of ncDN hydrogels with different nanoparticle contents were synthesized in order to further comparatively study the effect of bare and functional nanoparticles on the mechanical properties of the gels.

Fig. 3 compares representative compression stress?strain curves of ncDN gels with 1 wt% silica nanospheres or ATP nanorods. In comparison to DN gels, the ncDN gels showed much higher compression strength and fracture strain. For example, the ncDN-GA1 and ncND-BA1 gels did not fracture at 98% strain, while the sb was 47.5 ? 6 MPa for ncDN-BS1 gel and 73.5 ? 2.6 MPa for ncDN-GS1 gel, which are much higher than 18.6 ? 2.1 MPa for the DN gel. Apparently, the compression toughness is much higher for the ncDN gels. On the other hand, the vinyl graed nanoparticles showed a higher reinforcement effect on the ncDN gels than the bare ones did.

The mechanical properties of ncDN hydrogels with different nanoparticle contents are summarized and compared in Fig. 4 and 5. First of all, all the ncDN gels showed signicantly higher

Fig. 3 Representative compression stress?strain curves of DN, ncDNGS1, ncDN-BS1, ncDN-GA1 and ncDN-BA1 gels.

Fig. 4 Mechanical properties of ncDN-silica gels with various silica nanosphere contents. (a) Fracture strength (sb), (b) fracture strain (3b), (c) initial modulus (Ei), and (d) fracture energy (U).

strength, moduli, and toughness than those for DN gel as control. With high silica nanosphere content, the fracture strain were not necessarily higher than that of DN gel (Fig. 4b). Other than this, the strength, modulus, and toughness of ncDN gels with vinyl-functionalized silica nanospheres are signicantly higher than those for ncDN gels with bare silica nanospheres (Fig. 4a, c and d). As m was increased from 0.5 to 1, 2, 3, and 4, the sb was varied from 32.2 ? 5.3 to 47.5 ? 6, 47.9 ? 10.2, 46.7 ? 3.8, and 42.8 ? 6.5 MPa for ncDN-BSm gels, and from 59 ? 6.6 to 73.5 ? 2.6, 62.0 ? 5.2, 53.1 ? 5.8, and 46.1 ? 8.4 MPa for ncDN-GSm. Moreover, the U were enhanced from 1.1 ? 0.1 MJ m?3 for DN gel to a maximum of 2.9 ? 0.7 MJ m?3 for the ncDN-BSm gels, and further to a maximum of 3.9 ? 0.5 MJ m?3 for the ncDN-GSm gels. These results suggest that, in addition to physical adsorption, chemical bonding of polymer chains to nanospheres provides further reinforcement to the network.

Similar high strength and toughness of the ncDN gels with bare or vinyl-modied ATP nanorods are showed in Fig. 5. With 0.1, 0.5, and 1.0 wt% nanorods (bare or graed), the gels did not fracture at 98% strain, and the sb (or s0.98) was higher than 18.6 ? 2.1 MPa for DN gel, showing a maximum of 65.7 ? 3.7 MPa with 1 wt% nanorods (Fig. 5a). The 3b was increased with the presence of nanorods, but decreased at high ATP contents (1.5 and 2 wt%, Fig. 5b). The Ei was increased monotonically with increasing nanorod content (Fig. 5c). The U of these gels were higher than that of DN gel, showing a maximum of 2.6 ? 0.2 MJ m?3 with 1 wt% G-ATP (Fig. 5d). Gels with vinyl-graed ATP nanorods showed slightly higher strength, modulus, and toughness than those with bare ATP nanorods (p > 0.05 with in nanorod content of 1 wt% or lower). With relatively high bare nanorod content (1.5 and 2 wt%), the strength and toughness was decreased, probably due to the slight precipitation and aggregation of nanorods in the hydrogel matrix. However, the loss mechanical properties became less for gels with vinylgraed ATP nanorods.

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