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Insights into the molecular structure and reinforcement mechanism of the hydrogel-cement nanocomposite: An experimental and molecular dynamics study
Composites Part B 177 (2019) 107421
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Insights into the molecular structure and reinforcement mechanism of the hydrogel-cement nanocomposite: An experimental and molecular dynamics study Dongshuai Hou a, b, Jianyu Xu c, Yu Zhang a, *, Guoxing Sun c, ** a
Department of Civil Engineering, Qingdao University of Technology, Qingdao, China Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao, China Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyacrylamide hydrogel Ca(OH)2 nano-spherulite Reinforcement mechanism Tricalcium silicate Molecular dynamics
The biomedical and industrial application of hydrogels are strongly limited by their poor mechanical properties. In this paper, non-aggregated Ca(OH)2 nano-spherulites (CNS) with diameters <5 nm is synthesized and used to reinforce polyacrylamide (PAM) hydrogel. The CNS/PAM hydrogel obtained possesses super stretchable prop erty, high toughness, and strength with low CNS concentration. Molecular dynamics (MD) is employed to study the reinforced mechanism of the CNS to promote the further application of CNS in the field of hydrogel. In the network structure of hydrogel, the interaction between CNS and PAM contributes to the formation of cross-linked nodes around CNS, in which PAM chains play roles in reinforcement and connection, respectively. Furthermore, the introduction of CNS leads to more chemical bonds and cross-linked nodes formed in the structure, which significantly improves the tensile strength and elastic modulus of the hydrogel, but decreases the stretchable properties to some extent. Interestingly, CNS also has a beneficial side to improve the stretchable properties via the division into relatively small CNSs under high stress. Both experimental results and theoretical simulations deepen the understanding of nanocomposite hydrogels and can promote the application of CNS to other poly meric hydrogel for property enhancement.
1. Introduction Hydrogels are three-dimentionally cross-linked network composed of hydrophilic polymer and are capable of absorbing and holding a large amount of water. Generally, their inherent mechanical weakness, poor deformability and brittleness are inevitable problems that seriously limit their biomedical and industrial applications. For example, the high local shear forces between backwashing steps [1], and deformation and debris entering the hydrogel during installation, operation, and maintenance [2] prevent the hydrogel filtration membrane with poor mechanical properties from long-life service for wastewater purification. The application of artificial skin and muscles in biomedical field have higher requirement for the strength, toughness, and fatigure resistance of hydrogel [3]. In the textile field [4], the filamentous hydrogels are required to ensure no degradation of mechanical properties under cycles
of swelling and shrinkage due to the absorption and loss of water. Introducing nanoparticles into hydrogels is a typical method to reinforce hydrogel that relies on polymer chains crosslinking on the surface of the nanoparticles to form a strong network to enhance me chanical properties of nanocomposite (NC) hydrogels. Until now, a va riety of nanoparticles have been introduced into NC hydrogels, such as montmorillonite [5], hectorite [6–9], layered double hydroxide (LDH) [10], titanites (IV) nanosheet (TiNS) [11], and graphene and derivatives [12]. But there are several defects in these NC hydrogels, such as the nanoparticles aggregation due to too large nanoparticles (gener ally > 60 nm), low transparency caused by high content of nano particles, and high strength and toughness that cannot be achieved simultaneously. Recently, our previous work [13] overcame these problems. We proposed a new strategy to fabricate calcium hydroxide (Ca(OH)2) non-aggregated spherulite nanoparticles (CNS) with
* Corresponding author. ** Corresponding author., E-mail addresses: [email protected] (Y. Zhang), [email protected] (G. Sun). https://doi.org/10.1016/j.compositesb.2019.107421 Received 25 June 2019; Received in revised form 24 August 2019; Accepted 7 September 2019 Available online 7 September 2019 1359-8368/© 2019 Published by Elsevier Ltd.
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Fig. 1. Schematic diagram of the preparation of PAM/CNS hydrogel. (a) Ca2þ and OH released from the surface of tricalcium silicate; (b) CNS with di ameters < 5 nm formed from the crystallization of calcium hydroxide at 0 � C; (c) S2O28 from the initiator APS adsorbed on the surface of CNS; (d) introduction of monomer acrylamide (AM); (e) in situ polymerization of PAM on the surfaces of CNS.
diameters < 5 nm to reinforce hydrogel, and the synthesized hydrogels possess high transparency and excellent mechanical properties at very low inorganic content. However, reinforced mechanism of such prom ising CNS in the hydrogels is still vague, which greatly limits the further application of CNS in the field of hydrogel. Herein, molecular dynamics (MD) is proposed in our study to solve these confusions. In the past decades, MD has been applied in the simulation studies of hydrogel, including poly (vinyl alcohol) [14], poly (N-isopropylacrylamide) [15,16], poly (vinyl methyl ether) [15], epoxy-amine [17], polyacrylamide [18] network and double networks [19–21]. For constructing the MD model of hydrogel, two methods are extensively utilized: one is self-assembly that several carbon chains are placed into the box, and they run and interact under the selected force field at ordinary or elevated temperature [22,23]; the other one is an ideal model that is constructed directly, in which polymer chains are arranged in an orderly manner to form a diamond-like or square network [19–21,24–26]. However, neither method is true enough. The initial position of polymer chains and particles have a great influence on the final structure and performances of the hydrogel. In addition, the reinforced roles of nanoparticles in the entire structure of hydrogel have not been fully studied so far. There are only a few simulation studies that investigate the local interactions between nanoparticles and polymers, such as a sandwich model with two GO sheets and several polymer chains [27]. In this work, a polyacrylamide hydrogel was fabricated by intro ducing non-aggregated Ca(OH)2 nano-spherulites with di ameters < 5 nm. Combined with experiments, molecular dynamics method was utilized to study the self-cross-linking process of the NC hydrogel, and construct a more realistic NC hydrogel model. In order to explore the reinforced mechanisms of CNS, the chemical bonding be tween CNS and polymer chains, the structure and morphology of poly mer chains, and the tensile process of the whole structure were investigated on the molecular level. The combination of experiments and theoretical simulations would deepen the understanding of NC hydrogels, and provide a theoretical basis for the design of next gener ation NC hydrogels with CNS.
2. Methodology 2.1. Experiment Fabrication process of CNS and PAM/CNS NC hydrogel is shown in Fig. 1. The hydrogel containing n ppm CNS (named Cn as sample identification) were fabricated by in situ free-radical polymerization of acrylamide (AM) in CNS suspension. 2.1.1. Preparation of CNS suspension The hydration of tricalcium silicate (Ca3SiO5), a main component of Portland cement, was utilized to produce CNS, during which both cal cium silicate hydrate (C–S–H) gels and portlandite were rapidly formed at normal temperature [28–31]. In this work, 0 � C was proved to be the optimal temperature because the releasing speed of Ca2þ from the tri calcium silicate at this temperature was just enough to form CNS, and meanwhile the formation of C–S–H gel and the size of CNS were sup pressed [13]. Ca3SiO5 with synthesis method of Ref. [13] was dispersed in the deionized water at 0 � C in an ice bath, and the system was stirred under ultrasonication for 10 min to yield a homogeneous dispersion. 2.1.2. Preparation of PAM/CNS NC hydrogels The PAM/CNS hydrogel was prepared by in situ free-radical poly merization. AM was introduced into the mixed solution with CNS, ammonium peroxydisulfate (APS) and N,N,N0 ,N0 - tetramethyl - ethyl enediamine (TEMED). The concentration of CNSs/AM/APS/TEMED was 60 g/15 g/0.03 g/48 μL. The polymerization process proceeded at an ice bath (0 � C) and vacuum environment (0.01 atm) for at least 72 h. 0 � C and vacuum environment is necessary to inhibit the redox of S2O28 and remove NH3. S2O28 can be adsorbed on the CNS surface by electrostatic force. 2.1.3. Characterization of morphology and structure of the NC hydrogels A high-resolution scanning electron microscope (JEOL, model JSM6700F) was used to characterize the microscopic of xerogels. The con centration of CNSs in suspension was measured by inductively coupled 2
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Fig. 2. Model construction process of PAM/CNS aqueous mixture with homogeneous dispersion. (a) and (b) are schematic diagrams of the distribution of PAM chains represented by colored long arrows; (c) is the ultimate model of PAM/CNS aqueous mixture with homogeneous dispersion, where Oc and Hc represent the oxygen and hydrogen of carboxyl in PAM chains, and Os is the oxygen bonded to sulfur, and Ow and Hw are the oxygen and hydrogen of water.
plasma mass spectrometry. Time-of-flight secondary ion mass spec trometry (ToF-SIMS) was performed to demonstrate characteristic groups of PAM and CNS. Static ToF-SIMS spectra of hydrogels were received from ToF-SIMS V spectrometer (IONTOF GmbH, Münster, Germany). The hydrogel samples were bombarded with Biþ 3 primary ions, and the Biþ 3 primary ions were accelerated at 25 kV with an average pulsed current of 0.3 pA. Each spectrum was acquired for 40 s, and the raster was 200 � 200 μm.
bonds are formed between layers playing roles in layer connection. For this box, only one CNS can be placed in the center. Thus if a box is filled with a CNS, it can be called 100% filling. The box was enlarged 2 � 2 � 2 times and 2 � 3 � 4 times along x, y, and z axis to obtain supercells for the analysis of structure and mechanical properties, respectively. There are more than 100000 atoms in the supercell used to model mechanical properties. Five samples, CNS 1, CNS 3/4, CNS 1/2, CNS 1/4, and CNS 0, were chosen in this study, in which CNS filling rate is 100%, 75%, 50%, 25%, and 0, respectively.
2.1.4. Mechanical tests The MTS (model E44, EXCEED) testing machine was used to evaluate the mechanical properties of hydrogels. For tensile tests, hydrogels were prepared as a rod-like shape with 30 mm in length and 3.2 mm in diameter. The loading rate was 50 mm/min, and tensile tests was con ducted at 25 � C.
2.2.2. Force field and simulation procedure Empirical force field was conducted to model CNSs and PAMs. For PAMs, consistent valence force (CVFF) was chosen. It was extensively used in the simulation of the structure and dynamics of peptides, pro teins, and other organic systems, and has been confirmed to be accurate to simulate organics with low molecular weight (e.g., carboxylic acids and amides) [32–34]. Previously published paper confirms that the bond lengths, bond angles, torsion angles, and dihedral of carbon chains can be accurately described under CVFF [35–37]. As to the description of PAMs, ClayFF force field [38] was utilized in this study. It has already been used to successfully model oxide and hydroxide materials [39–41]. Using ClayFF force field, Hou [42] and Kalinichev [39] have investi gated the structure and dynamics of portlandite, and the interaction of aqueous species with portlandite surface. Both of them achieved admi rable results. Lennard-jones function and Coulomb formula, in which parameters were determined from the density function theory, is used in ClayFF to describe the interaction between calcium, oxygen, hydrogen in portlandite. Furthermore, the combination of CVFF and ClayFF force filed has already been applied in similar systems [35,36]. Therefore, CVFF and ClayFF force filed were combined in this study, more details and parameters of the force fields are available in Refs. [39,43]. The simulation was performed on LAMMPS, a large-scale atomic/ molecular massively parallel simulator. Nose-Hoover thermostat and Verlet algorithm were applied in the simulation. The cutoff distance of long-rang force is 10 Å in this simulation work. Time step was set as 1 fs. Before the simulation begins, energy minimization was conducted to optimize initial structure. Then the simulation is performed as the following procedure. First, NPT run was conducted at 1 atm and 300 K to compress the supercell to 57% volume. This is to ensure that PAM chains
2.2. Molecular dynamics simulation 2.2.1. Model construction Construction process of the model is shown in Fig. 2. This model approximately represents PAM/CNS aqueous mixture with homoge neous dispersion. In order to the homogeneous dispersion of PAMs in the box, each plane (xy, yz, and xz plane) selected two PAM chains perpendicular to each other and these six chains were placed as shown in Fig. 2a. Various forms of PAM chains will be generated after a period of motion. In order to allow the boxes to cross each other, one chain was selected from each plane and moved out of the box. The yellow chain in Fig. 2b is taken as an example. Finally, Grand Canonical Monte Carlo (GCMC) method was utilized to adsorb water molecules into box to achieve a solution environment. This process proceeded 300 million steps to reach 1 g/m3 of water density. The final model is shown in Fig. 2c that was constructed using Ma terials Studio 7.0. Periodic boundaries were set in three directions of xyz. PAM chains were functionalized by carboxyl groups and sulfate according to the structure characterization in the ToF-SIMS experi ments. The degree of polymerization of PAM in the model was 42 (42 monomers). CNS with diameters of around 4 nm was constructed, where the coordination number of calcium was eight and its coordinated hy droxyl groups were located between calcium layers. H-bonds and ionic 3
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Fig. 3. Scanning electron microscope image of C200 (NC hydrogel with 200 ppm CNS concentration) xeogel with scale bar of (a) 1 mm, (b) 100 μm, and (c) (d) 10 μm.
and CNSs can interact, meanwhile the substances in the box do not collapse together due to the excessive shrinkage of the box. Compression to 50%–60% volume can well meet the above conditions. Second, NVT run was performed at 300 K for 1 ns to obtain an equilibrated system, where PAMs and CNSs had fully interacted and the structure reached stable. Then, it was followed by another 1 ns under NVT for structure analysis. Finally, supercells were subject to uniaxial tensile strain through progressive elongation at the strain rate of 0.08/ps.
42 X
Dn
(1)
D Dstraight
(2)
D¼ n¼1
r¼
where n is the monomer order and the numbering sequence is from one end of the chain to the other end; one carbon is chosen from each monomer and Dn is the distance between the carbon in nth monomer and the first carbon; Dstraight is the unfolded distance of the straight chain. Mobility of PAM can be described quantitatively by threedimensional mean square displacement (MSD) [44], calculated ac cording to following equation. � MSDðtÞ ¼ < �ri ðtÞ ri ð0Þj2 > (3)
2.2.3. Data analysis method One frame is output per 1000 steps and 100 steps for the analysis of structure and tensile properties, respectively. Unfolded degree, r is uti lized to estimate the structure of PAM chains. 100% unfolded degree means that the chain is completely straight. Under the interaction with neighboring environment, the morphology of PAM chains evolves and becomes curled to some extent, which can be reflected by a decrease in the unfolded degree r. The r is calculated from unfolded distance, D.
where ri (t) is the position of atom i at time t. The MSD of carbon atoms in PAM is calculated here to represent the mobility of the PAM, which is controlled by surrounding constraints. Interatomic spatial correlation is calculated here based on radial distribution function (RDF) [45], determined by following equation, to estimate the interaction between atoms. g(r) ¼ dN / (4πr2ρ)
(4)
where r is the distance between the selected atom 1 and its surrounding atom 2; dN represents the number of atom 2 within an annular region of dr; ρ represents the average density of the whole system. 3. Results and discussions 3.1. Structure and mechanical properties of PAM/CNS NC hydrogel SEM images of freeze-dried PAM/CNS xerogel with 200 ppm CNS concentration are presented in Fig. 3. PAM/CNS xerogel possesses a uniform porous structure with diameter of 100–400 μm as shown in
Fig. 4. Tensile stress-stretch ratio curves of PAM/CNS hydrogels with different concentration of CNS. λ represents the stretch ratio. 4
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concentrations are performed at room temperature. The tensile stressstrain curves plotted in Fig. 4 shows that the introduction of CNS dramatically enhances the mechanical properties of PAM hydrogels. As compared with the hydrogel with CNS, the maximum stress and stretch ratio of pure PAM hydrogel can be neglected. The maximum stress and stretch ratio at rupture of PAM/CNS hydrogel with 40 ppm CNS (C40 sample) achieve 420 kPa and 120, respectively. Its toughness achieves 33.92 MJ/m3 that is 170 times more than that of pure PAM hydrogel. It achieves high stress and stretch at the same time. As CNS concentration increases to 200 ppm (C200 sample), the maximum stress continues to increase and reaches 530 kPa at a rupture stretch ratio of 62. The frac ture toughness was 26.2 MJ/m3, which is lower than that of C40, because of the decreased deformation ability of the crosslinked network of C200 sample. As listed in Table 1, the overall mechanical properties of NC hydrogel achieved in this study has been improved significantly compared to the hydrogels reported so far. The mechanical properties of PAM/CNS hydrogels are significantly influenced by the cross-linked network structure controlled by amount of CNS. In order to investigate the structure of hydrogel to support MD simulation, ToF-SIMS is employed to characterize the chemical bonding in the hydrogel. The peaks of negative ions in Fig. 5a and b represent the side (-CN－and -CNO－) and end (-SO4CH－and –SO4CH2－) functional groups of PAM in the sample of pure PAM and C200. The decreased peak intensity of –SO4CH－and –SO4CH2－ in C200 indicates a decrease in the end groups. Similarly, the amount of side groups (-CN－and -CNO－) also reduces due to the reaction with OH , during which gaseous NH3 is formed and the side groups are replaced by COO- and COOH. In Fig. 5c, the peaks of -CaSO4CH－and -CaSO4CH2－ in C200 means the ionic bonding between the PAM end groups and CNS. Their peak intensity
Table 1 Maximum stress and stretch ratio at rupture of CN hydrogel with different crosslinkers. Matrix
Nanoparticle
Content (ppm)
Max. stress (KPa)
Stretch ratio at rupture
PAM
Graphene oxide [12]
80 480 500
282 385 300
31 34 15
8000
34
62
8700 80800 110000 330000 40 200
100 175 150 230 420 530
118 85 23 4 120 62
vinyl hybrid silica nanoparticle [46] Layered double hydroxide [10] Montmorillonite [47] Alginate [48] CNS (this work)
Fig. 3a. As the amplification increases, it can be observed that the pore wall also contains numerous smaller pores. Fig. 3d shows pores with diameter less than 1 μm. In Fig. 3b, a large number of parallel and crosslinked filamentous structure are regularly distributed inside the pores. Embossments at the junction between the filaments and pore walls demonstrate the strong connection between them. Fig. 3c and d show a crosslinked network constituted by filaments with diameter of 50 nm ~ 4 μm. Bumps are formed on the filaments and a shish-kebab structure can be observed in the structure. The bumps may be CNSs coated with PAM chains. To study the roles of CNS on the mechanical properties of PAM hydrogel, elongation tests on the NC hydrogels with different CNS
Fig. 5. ToF-SIMS spectra of the hydrogel. (a)(b) Peaks of negative ions representing the side (-CN－and -CNO－) and end (-SO4CH－and –SO4CH2－) functional groups of pure PAM and C200, and (c) peaks of -CaSO4CH－and -CaSO4CH2－ in pure PAM and C200. C200 represents PAM/CNS hydrogel with 200 ppm CNS concentration. 5
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Fig. 6. Schematic diagram of the interacted process between PAM chains and CNS. (a) Initial and (b) ultimate configuration of PAM and CNS, in which connected PAM (c-PAM) and reinforced PAM (r-PAM) can be categorized.
Fig. 7. (a) Cross-linked node formed near CNS (CNS node), (b) node formed by the entanglement between PAM chains (PAM node).
does not change before and after being stretched for 60 times, implying the high strength of the ionic bond.
reinforcement to wrap CNS to reduce its dissolution, and to strengthen the connection between c-PAM and CNS. The generation process and detailed structure of these two types of chains are illustrated in Fig. 6. As a cross-linked node, CNS node can connect 6–7 c-PAMs. In addition to CNS node, another type of node can be observed in the structure, as shown in Fig. 7b, which is formed by the entanglement between PAM chains and named PAM node. The PAM node can only connect 2–4 cPAMs. Such a cross-linked structure generates numerous ring-shaped structure filling the hydrogel, which is consistent with the SEM image in Fig. 3. Mechanically, such a ring-shaped structure can resist and transfer stress well when it is loaded. In order to better show the structure of these two types of node and their functions, one frame of tensile process is taken as an example (Fig. 8a). Several nodes can be observed in Fig. 8a. Some are relatively big, like CNS nodes, and some are relatively small, such as PAM nodes. The nodes play roles in transferring stress and are the key to the me chanical properties and swelling of the NC hydrogel. In the experiment, shish-kebab structures on the filaments can be observed in the SEM image of C200. Amount of the bump decreases with reducing CNS concentration. Actually, the bump here is the enlargement of the CNS
3.2. Generation process of PAM/CNS NC hydrogel structure During the gelation process of PAM/CNS mixture, PAMs and CNSs are intertwined and overlapped with each other by hydrogen-bond, electrostatic force and Van der Waals’s force, constructing a crosslinked network of PAM/CNS NC hydrogel. In the network structure, CNSs act as cross-linkers and greatly improving the mechanical prop erties of the CN hydrogel. The interacted process between PAM and CNS is illustrated in Fig. 6. PAMs are attracted by CNS and connected to it gradually. As a result, three types of structure are formed between CNS and PAMs that is winding, lapping, and docking. Which structure is generated depends on the relative position between PAM and CNS. As a result, as shown in Fig. 7a, cross-linked nodes are formed around CNS due to the interaction between PAM and CNS, which is named CNS node. Two types of PAM chains, connected PAM (c-PAM) and reinforced PAM (r-PAM) should be noted in this node structure. c-PAM plays roles in connecting neighboring nodes or chains. r-PAM acts as a 6
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Fig. 8. (a) Illustration of PAM/CNS NC hydrogel structure, (b) zoomed image of the box region ofIin a, (c) zoomed image of the box region of IIin a, and (d) radial distribution function (RDF) of the structure. Please refer to Fig. 2 for the meaning of Oc/Os/Hc.
node that CNSs are coated with PAM chains. As the system grows from nanoscale to microscale, local concentration unevenness begins to appear, resulting in serious aggregation of PAMs around CNSs and the formation of such bumps. Fig. 8b and c exhibit the details of PAM node and CNS node,
respectively. PAM node is formed by the entanglement between chains, which is controlled by two types of H-bond (Os-Hc and Oc-Hc). For CNS node (Fig. 8c), CNS is winded by r-PAM chains and linked to seven c-PAM chains. Os/Oc-Ca ionic bonds and H-bonds control the node, where the ionic bonds link PAMs and CNS together and the H7
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Fig. 9. (a) Elongation stress-strain curve of CN 1 sample, and evolution of (b) mean unfolded degree of carbon chains, (c) bond number per carbon chain, and (d) CNS number as function of tensile strain in CN 1 sample.
bonds mainly act to connect r-PAM and c-PAM. Fig. 8c clarifies the reinforcement effect of the r-PAM. One end of the r-PAM is bound to CNS, and the other end is linked to c-PAM. Such a structure strengthens the connection between c-PAM and CNS, and also contributes to the transfer and dispersion of the stress around node when it is loaded. Morphology of the r-PAMs near CNS node explains the embossments observed in SEM image that is formed in the connection point between filaments and inner walls of pores. Radial distribution function (RDF) is utilized to estimate interatomic spatial correlation and further reflect chemical bonding in the structure, as shown in Fig. 8d. The first and second RDF peaks of Os–Ca are the highest, meaning the strong short-range and medium-range spatial correlations between the SO4 at the end of PAM and the Ca in CNS. It indicates the existence of a great many Os–Ca ionic bonds, with bong length of 2.5 Å, connecting PAMs to CNS. It is consistent with the chemical bonding characterization given by above ToF-SIMS spectra. By contrast, the spatial correlation of Oc-Ca is much weaker. In addition, there are H-bonds of Os-Hc and Oc-Hc with O–H length of 1.55 Å and 1.75 Å, respectively, which connects PAM chains together. Os-Hc connection with less bond length and stronger spatial correlation shows higher chemical bond strength than Oc-Hc. Overall, it is clear that the terminal sulfate on PAM plays a key role in this cross-linked struc ture. Additionally, there are also H-bonds formed between carboxyl groups of PAM and hydroxyl groups of CNS, but it is ignored in our analysis because there have already been numerous ionic bonds between PAM and CNS.
3.3. Structure evolution during tensile process Structure evolution of the hydrogel during tensile process is inves tigated in this part by means of stress-strain relation, and the statistics of chemical bond number and unfolded degree of PAM chains. Elongation stress-strain curve of CN 1 sample and related structure characterizations are shown in Fig. 9. The stress-strain curve can be roughly divided into three stages, elastic stage, yield stage, and failure stage. In the elastic stage, the cross-linked network structure is stretched to take up tensile strain and the stress reaches maximum (~123 MPa) at the strain of 1.2. During this process, unfolded degree of PAM chains increases with tensile strain, indicating that curled PAM chains are gradually unfolded, stretched, and tightened. This unfolded process of PAM chains leads to the breakage of H-bond inside and between chains. As shown in Fig. 9c, the H-bond formed between carboxyl groups is broken most, and reduces from 11 to around 8 per PAM. By contrast, the number of ionic bond and the H-bond formed between carboxyl and terminal sulfate keeps stable before the strain of 1 and then reduces slightly. In the yield stage, the stress sharply drops to around 36 MPa, and then reaches a plateau and fluctuates until the structure breaks. In the stage of stress drop, the chemical bonds break largely, resulting in the breakage of some connection points. The number of ionic bond with relatively high bond strength also reduces from 10.5 to 9.9 as the strain increases from 1.2 to 1.7. Subsequently, the stress reaches a plateau and the number of chemical bond remain stable as well. It is related to the constant rebirth of H-bond caused by the relative slip between PAM 8
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Fig. 11. Structure evolution of the CN hydrogel at tensile strain (a) 0.5, (b) 1.5, and (c) 2.5. Fig. 10. Local structure of CN hydrogel at the strain of (a) 1.5 and (b) 2. (c) Zoomed image of the left one of two blue regions in b. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
is slightly deformed at the strain of 0.5 (Fig. 11a). As the stress increases, damage of some nodes can be observed, where the connection on one end of chains is stretched to be broken and this end of the chains sways in the structure. Meanwhile, the structure of chains gradually changes from curl to straight, among which c-PAMs are predominant and a few rPAMs are also included. When CNS nodes are divided, their r-PAM chains are stretched to some extent or even become c-PAMs. But there are still numerous r-PAMs maintaining winding structure around CNS.
chains during the tensile process, which contributes to the release of energy. In the yield stage, the unfolded degree of PAM chains still lin early increases with strain, and it only depends on the stretching degree of the structure. During the whole tensile process, ionic bond number only declines by around 0.5, one tenth of the reduced number of H-bond. It is partly due to the relatively high bond strength of ionic bond, and partly because of the connection reinforcement of r-PAMs. Under high tensile stress, as shown in Fig. 10, CNSs are dragged by PAM chains, causing some CNSs to be cracked and divided into two even three relatively small CNSs with diameters less than 2 nm. New nodes are quickly formed surrounding these divided CNSs, and yet there is few r-PAM. As plotted in Fig. 9d, the number of CNS increases from 8 to around 17.8. The climax of CNS division starts when the stress reaches around the maximum, and ends at the strain of 3. The division of CNSs contributes to the release of tensile stress, which is one of the reasons why the stress-strain curve can enter the plateau stage (strain 2–3) and show a slight strengthening peak (strain around 3.4). Moreover, it im proves the stretchable property of the hydrogel, but it is irreversible deformation. The CNS size has an impact on its reinforcement to the hydrogel structure, such as the amount of CNS connecting PAM chains, but interestingly the feature of CNS division weakens the effect of CNS size on the hydrogel reinforcement to some extent. Snapshots of the hydrogel configuration at the tensile strain of 0.5, 1.5 and 2.5 are presented in Fig. 11. It shows a complex network structure, in which PAM chains are parallel and cross-connected. Numerous nodes are clearly visible, especially the CNS nodes. They play critical roles in transferring and bearing tensile stress. The structure
3.4. Effect of CNS on the structure and tensile property of PAM/CNS CN hydrogel As mentioned above, CNS nodes not only show relatively high con nectivity and strength, but also can be divided under high tensile stress to release stress. It seems very beneficial to the hydrogel. In this part the effect of CNS on the structure and tensile property of the hydrogel is investigated by changing CNS amount in the structure. Variation of chemical bond number per PAM chain with CNS amount is shown in Fig. 12a. Total bond number per PAM decreases from 32.1 to 24.5 with CNS amount decreases from 100% filling (CNS 1) to 0 (CNS 0). As to the sample of CNS 1, the number of each type of chemical bonding is basically the same, around 10 per PAM (Fig. 12a). Among the 10 ionic bonds, there are around 8 ionic bonds formed between terminal sulfate and calcium of CNS, implying that each sulfate is coordinated with around four calcium ions. Such high concentrations of Os–Ca ionic bond guarantee the high strength of the connection between CNS and the end of PAM. This is one of the reasons why CNS can be divided by the dragging of PAM chains, rather than the connection point between them breaks. As CNS amount declines, ionic bond number reduces correspond ingly and reaches 0 when there is no CNS in the structure. In the region without CNS, therefore, sulfate can only interact with carboxyl groups 9
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Fig. 12. Evolution of (a) the number of chemical bonds and (b) nodes with CNS amount; unfolded degree of PAM chains in (c) CNS 1 and (d) CNS 1/4; (e) examples of unfolded degree; (f) evolution of mean square displacement (MSD) with CNS amount.
on PAM to form H-bond with it, resulting in an increase of Os-Hc H-bond from 9.7 to 17 per PAM. Moreover, Oc-Hc H-bond number decreases from 12 to 7 as 100% filling of CNS decreases to 0. It means that the reduction of CNS amount decreases the H-bond number formed inside and between PAM chains, which also implies a decrease in PAM nodes. With CNS amount decreasing, not only are CNS nodes reduced, but also PAM nodes decline. Therefore, as shown in Fig. 12b, mean node number per PAM progressively decreases from 8.8 to 5.6 as 100% filling of CNS reduces to 0. In addition to an influence on the number of chemical bonds and
cross-linked nodes, introduction of CNS also greatly changes the local structure of the hydrogel. Unfolded degree of PAM chains in the samples of CNS 1 and CNS 1/4 is shown in Fig. 12c and d. For CNS 1, the intensity profile of unfolded degree is bimodal distribution and the two peaks are located at around 20% and 45%, respectively. Such a distribution con firms the ubiquity of c-PAM and r-PAM in the structure. Unfolded degree of 20% illustrates the curled structure of PAM chains, which is related to the r-PAM. Unfolded degree of 45% corresponds to the c-PAM. Ac cording to the statistics in Fig. 12c, around 65% of PAM chains are cPAMs and remaining 35% belong to r-PAMs in the structure of CNS 1. 10
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Fig. 13. (a) Stress-strain curves, and variations of (b) tensile strength, elastic modulus, (c) strain at break, and (d) change of unfold degree as function of ten sile strain.
Fig. 12e illustrates the structure of these two types of chains by means of unfolded distance profiles. The chains with unfolded degree of 40% and 23% are taken as examples. If unfolded distance linearly in creases with monomer order all along, it means that the PAM chain is completely straight, which is 100% unfolded. Specific morphology of the chains with unfolded degree of 23% and 40% can refer to the r-PAM and c-PAM in Fig. 6, respectively. For CNS 1/4, unfolded degree of PAM chains falls in the range from 10% to 65%, but its intensity is distributed disorderly. In the absence of CNS, PAM chains are randomly crossed and overlapped. Hence it can be concluded that the presence of CNS makes the structure of PAM chains more orderly. Fig. 12f investigates the dynamics of PAM chains and exhibits the amplitude and displacement of the chains in different structure. Mean square displacement (MSD) gradually increases over time and fluctuates simultaneously, meaning the displacement and oscillation of PAM chains in the structure. Obviously, the amplitude of CNS 1/4 is much larger than the other two samples. Also, the MSD values of CNS 1/4 at 150 ps are 460 Å2, around 4.6 and 7.6 times larger than that of CNS 1/2 and CNS 1, respectively. It can be seen that the mobility of PAM chains is influenced by CNS amount in the structure. Adding CNS into the struc ture increases chemical bonds and cross-linked nodes, geometrically restricting PAM chains. Effect of CNS on the tensile property of the hydrogel is illustrated in Fig. 13. Fig. 13a plots elongation stress-strain curves of the samples with different CNS amount. For CNS 1, stress increases to failure strength of 123 MPa at strain 1.2, and followed by a sharp drop to around 36 MPa at
strain 1.7. Subsequently, the stress steps into a plateau and maintains unchanged until the strain reaches 3.5. Finally, the stress drops to 0 meaning the complete damage of the structure. As CNS amount reduces from 100% filling to 0, the failure strength and elastic modulus decline linearly, and decrease by 84.5% and 94.2% when CNS amount is 0. Besides, the peak of stress-strain curve is getting weaker with CNS amount decreasing, meaning that the skeleton of carbon chains become weaker and weaker as well. However, as shown in Fig. 13a, the stretchable property of CNS 1 is not the best. The variation of the strain at break has a turning point, and the maximum (~8) occurs at around 50% filling of CNS (CNS 1/2). Furthermore, it can be observed that the stress of CNS 1/4 fluctuates greatly in the stress-strain curve as compared with CNS 1. It is related to the relative slip between PAM chains, during which the H-bonds formed between chains are constantly broken and regenerated. Such a structure rearrangement is beneficial to improving the stretchable property of hydrogel. In addition to it and the division of CNS mentioned in Section 3.3, the stiffness of the structure is also an important factor affecting the stretchable property. The stiffness is characterized by unfolded speed of PAM chains during the tensile process. As shown in Fig. 13d, the unfolded degree of PAM chains increases with tensile strain, implying that the chains are gradually stretched from curled shape. An increase in unfolded degree with strain is approximately linear and can be linearly fitted. The slope of the fitting line is unfolded speed of PAM chains during the tension process. The unfolded speed decreases with CNS amount increasing as shown in Fig. 13d, and reaches the minimum of 11.11% per strain in the sample of CNS 1. Compared with samples with less CNS amount, there 11
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are more chemical bonds and cross-linked nodes in CNS 1, providing more confinement to the mobility and flexibility of PAM chains. For CNS 1, this linear increase of unfolded degree ends at the strain of 3.5, while the strain when the linear growth ends in CNS 1/2 and CNS 1/ 4 is 2.3 and 1.5, respectively. This unfolded process of PAM chains is also the process of releasing tensile stress and energy. Furthermore, it is interesting that the total growth of unfolded degree of PAM is almost the same for the three samples in Fig. 13d, and is close to 50%. After a linear growth of unfolded degree with strain, the unfolded degree enters a plateau with large fluctuations.
[10] Hu Z, Chen G. Novel nanocomposite hydrogels consisting of layered double hydroxide with ultrahigh tensibility and hierarchical porous structure at low inorganic content. Adv Mater 2014;26:5950–6. [11] Liu M, Ishida Y, Ebina Y, Sasaki T, Hikima T, Takata M, Aida T. An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets. ON Nat 2015;517:68–72. [12] Liu R, Liang S, Tang X-Z, Yan D, Li X, Yu Z-Z. Tough and highly stretchable graphene oxide/polyacrylamide nanocomposite hydrogels. J Mater Chem 2012;22: 14160–7. [13] Sun G, Li Z, Liang R, Weng L-T, Zhang L. Super stretchable hydrogel achieved by non-aggregated spherulites with diameters <5 nm. Nat Commun 2016;7:12095. [14] Chiessi E, Cavalieri F, Paradossi G. Water and polymer dynamics in chemically cross-linked hydrogels of poly (vinyl alcohol): a molecular dynamics simulation study. J Phys Chem B 2007;111:2820–7. [15] Tamai Y, Tanaka H. Permeation of small penetrants in hydrogels. 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J Mech Phys Solids 2016; 94:372–87. [26] Hu C, Li Z. Property investigation of individual phases in cementitious composites containing silica fume and fly ash. Cement Concr Compos 2015;57:17–26. [27] Compton OC, Cranford SW, Putz KW, An Z, Brinson LC, Buehler MJ, Nguyen ST. Tuning the mechanical properties of sraphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding. ACS Nano 2012;6:2008–19. [28] Hu C, Ruan Y, Yao S, Wang F, He Y, Gao Y. Insight into the evolution of the elastic properties of calcium-silicate-hydrate (C-S-H) gel. Cement Concr Compos 2019; 104:103342. [29] Hu C, Yao S, Zou F, Nie S, Liu Z, Wang F. Insights into the influencing factors on the micro-mechanical properties of calcium-silicate-hydrate gel. J Am Ceram Soc 2019;102:1942–52. [30] Gao Y, Hu C, Zhang Y, Li Z, Pan J. Investigation on microstructure and microstructural elastic properties of mortar incorporating fly ash. Cement Concr Compos 2018;86:315–21. [31] Hu C, Hou D, Li Z. Micro-mechanical properties of calcium sulfoaluminate cement and the correlation with microstructures. Cement Concr Compos 2017;80:10–6. [32] Gaedt K, H€ oltje H-D. Consistent valence force-field parameterization of bond lengths and angles with quantum chemical ab initio methods applied to some heterocyclic dopamine D3-receptor agonists. J Comput Chem 1998;19:935–46. [33] Asensio JL, Martin-Pastor M, Jimenez-Barbero J. The use of CVFF and CFF91 force fields in conformational analysis of carbohydrate molecules. Comparison with AMBER molecular mechanics and dynamics calculations for methyl α-lactoside. Int J Biol Macromol 1995;17:137–48. [34] Greathouse JA, Allendorf MD. Force field validation for molecular dynamics simulations of IRMOF-1 and other isoreticular zinc carboxylate coordination polymers. J Phys Chem 2008;112:5795–802. [35] Zhou Y, Hou D, Manzano H, Orozco CA, Geng G, Monteiro PJM, Liu J. Interfacial connection mechanisms in calcium–silicate–hydrates/polymer nanocomposites: a molecular dynamics study. ACS Appl Mater Interfaces 2017;9:41014–25. [36] zhang Y, Yang T, Jia Y, Hou D, Li H, Jiang J, Zhang J. Molecular dynamics study on the weakening effect of moisture content on graphene oxide reinforced cement composite. Chem Phys Lett 2018;708:177–82. [37] Zhang Y, Zhang J, Jiang J, Hou D, Zhang J. The effect of water molecules on the structure, dynamics, and mechanical properties of sodium aluminosilicate hydrate (NASH) gel: a molecular dynamics study. Constr Build Mater 2018;193:491–500. [38] Cygan RT, Liang J, Kalinichev AG. Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. J Phys Chem B 2004; 108:1255–66.
4. Conclusions Hydrogel achieved by non-aggregated spherulites with diameters <5 nm is reported in this paper. Its properties can be tuned through the concentration of CNS, and reach very high values for stretchable prop erty, toughness, and strength. Molecular dynamics simulations are performed to study the reinforced mechanisms of CNS in the hydrogel and related conclusions are as follows. Structurally, two types of cross-linked nodes (CNS nodes and PAM node) are formed in the structure by the interaction between CNSs and PAMs and the entanglement between PAM chains, respectively. By contrast, CNS node not only exhibits higher strength and connectivity, but also makes the structure of PAM more orderly. In the CNS node, two types of PAMs can be categorized, and they possess curly and stretched structure playing roles in reinforcement and connection, respectively. Moreover, the number of chemical bonds and nodes increases with CNS amount, providing more confinement to the mobility and flexibility of PAM chains. Mechanically, the introduction of CNS significantly improves the tensile strength and elastic modulus of the hydrogel but decreases stretchable properties to some extent. The decrease in the stretchable properties is also shown by the slowdown of unfolded speed of PAM chains during the tension. Interestingly, CNS also has a beneficial side to improve the stretchable properties via the division into relatively small CNSs under high stress. Acknowledgements Financial support from National Natural Science Foundation of China under Grant 51678317, 51420105015, the China Ministry of Science and Technology under Grant 2015CB655100, Natural science foundation of Shandong Province under Grant ZR2017JL024, The Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (Grant No. 161069), Science and Tech nology Development Fund from Macau (0074/2018/A2). References [1] Huisman IH, Williams K. Autopsy and failure analysis of ultrafiltration membranes from a waste-water treatment system. Desalination 2004;165:161–4. [2] Ayala DF, Ferre V, Judd SJ. Membrane life estimation in full-scale immersed membrane bioreactors. J Membr Sci 2011;378:95–100. [3] Pal Kunal, Banthia AK, M DK. Starch based hydrogel with potential biomedical application as artificial skin. Afr J Biomed Res 2006;9:23–9. [4] Hu J, Meng H, Li G, Ibekwe SI. A review of stimuli-responsive polymers for smart textile applications. Smart Mater Struct 2012;21:053001. [5] Guorong G, Gaolai D, Yuanna S, Jun F. Self-healable, tough, and ultrastretchable nanocomposite hydrogels based on reversible polyacrylamide/montmorillonite adsorption. ACS Appl Mater Interfaces 2015;7. 5029-2037. [6] Haraguchi K, Ebato M, Takehisa T. Polymer-clay nanocomposites exhibiting abnormal necking phenomena accompanied by extremely large reversible elongations and excellent transparency. Adv Mater 2006;18:2250–4. [7] Haraguchi K, Li H-J. Control of the coil-to-globule transition and ultrahigh mechanical properties of PNIPA in nanocomposite hydrogels. Angew Chem, Int Ed 2005;44:6500–4. [8] Haraguchi K, Li H-J, Matsuda K, Takehisa T, Elliott E. Mechanism of forming organic/inorganic network structures during in-situ free-radical polymerization in PNIPA-clay nanocomposite hydrogels. Biol Macromol 2005;38:3482–90. [9] Haraguchi K, Li H-J. Mechanical properties and structure of polymer-clay nanocomposite gels with high clay content. Biol Macromol 2006;39:1898–905.
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[39] Kalinichev AG, Kirkpatrick RJ. Molecular dynamics modeling of chloride binding to the surfaces of calcium hydroxide, hydrated calcium aluminate, and calcium silicate phases. Chem Mater 2002;14:3539–49. [40] Kalinichev AG, Wang J, Kirkpatrick RJ. Molecular dynamics modeling of the structure, dynamics and energetics of mineral–water interfaces: application to cement materials. Cement Concr Res 2007;37:337–47. [41] Zhang Y, Li T, Hou D, Zhang J, Jiang J. Insights on magnesium and sulfate ions’ adsorption on the surface of sodium alumino-silicate hydrate (NASH) gel: a molecular dynamics study. Phys Chem Chem Phys 2018;20:18297–310. [42] Dongshuai H, Zeyu L, Peng Z, Qingjun D. Molecular structure and dynamics of an aqueous sodium chloride solution in nano-pores between portlandite surfaces: a molecular dynamics study. Phys Chem Chem Phys 2016;18:2059–69. [43] Dauber-Osguthorpe P, Roberts VA, Osguthorpe DJ, Wolff J, Genest M, Hagler AT. Structure and energetics of ligand binding to proteins: Escherichia coli dihydrofolate reductase-trimethoprim, a drug-receptor system. Proteins 2010;4: 31–47.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Insights into the molecular structure and reinforcement mechanism of the hydrogel-cement nanocomposite: An experimental and molecular dynamics study Dongshuai Hou a, b, Jianyu Xu c, Yu Zhang a, *, Guoxing Sun c, ** a
Department of Civil Engineering, Qingdao University of Technology, Qingdao, China Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao, China Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyacrylamide hydrogel Ca(OH)2 nano-spherulite Reinforcement mechanism Tricalcium silicate Molecular dynamics
The biomedical and industrial application of hydrogels are strongly limited by their poor mechanical properties. In this paper, non-aggregated Ca(OH)2 nano-spherulites (CNS) with diameters <5 nm is synthesized and used to reinforce polyacrylamide (PAM) hydrogel. The CNS/PAM hydrogel obtained possesses super stretchable prop erty, high toughness, and strength with low CNS concentration. Molecular dynamics (MD) is employed to study the reinforced mechanism of the CNS to promote the further application of CNS in the field of hydrogel. In the network structure of hydrogel, the interaction between CNS and PAM contributes to the formation of cross-linked nodes around CNS, in which PAM chains play roles in reinforcement and connection, respectively. Furthermore, the introduction of CNS leads to more chemical bonds and cross-linked nodes formed in the structure, which significantly improves the tensile strength and elastic modulus of the hydrogel, but decreases the stretchable properties to some extent. Interestingly, CNS also has a beneficial side to improve the stretchable properties via the division into relatively small CNSs under high stress. Both experimental results and theoretical simulations deepen the understanding of nanocomposite hydrogels and can promote the application of CNS to other poly meric hydrogel for property enhancement.
1. Introduction Hydrogels are three-dimentionally cross-linked network composed of hydrophilic polymer and are capable of absorbing and holding a large amount of water. Generally, their inherent mechanical weakness, poor deformability and brittleness are inevitable problems that seriously limit their biomedical and industrial applications. For example, the high local shear forces between backwashing steps [1], and deformation and debris entering the hydrogel during installation, operation, and maintenance [2] prevent the hydrogel filtration membrane with poor mechanical properties from long-life service for wastewater purification. The application of artificial skin and muscles in biomedical field have higher requirement for the strength, toughness, and fatigure resistance of hydrogel [3]. In the textile field [4], the filamentous hydrogels are required to ensure no degradation of mechanical properties under cycles
of swelling and shrinkage due to the absorption and loss of water. Introducing nanoparticles into hydrogels is a typical method to reinforce hydrogel that relies on polymer chains crosslinking on the surface of the nanoparticles to form a strong network to enhance me chanical properties of nanocomposite (NC) hydrogels. Until now, a va riety of nanoparticles have been introduced into NC hydrogels, such as montmorillonite [5], hectorite [6–9], layered double hydroxide (LDH) [10], titanites (IV) nanosheet (TiNS) [11], and graphene and derivatives [12]. But there are several defects in these NC hydrogels, such as the nanoparticles aggregation due to too large nanoparticles (gener ally > 60 nm), low transparency caused by high content of nano particles, and high strength and toughness that cannot be achieved simultaneously. Recently, our previous work [13] overcame these problems. We proposed a new strategy to fabricate calcium hydroxide (Ca(OH)2) non-aggregated spherulite nanoparticles (CNS) with
* Corresponding author. ** Corresponding author., E-mail addresses: [email protected] (Y. Zhang), [email protected] (G. Sun). https://doi.org/10.1016/j.compositesb.2019.107421 Received 25 June 2019; Received in revised form 24 August 2019; Accepted 7 September 2019 Available online 7 September 2019 1359-8368/© 2019 Published by Elsevier Ltd.
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Fig. 1. Schematic diagram of the preparation of PAM/CNS hydrogel. (a) Ca2þ and OH released from the surface of tricalcium silicate; (b) CNS with di ameters < 5 nm formed from the crystallization of calcium hydroxide at 0 � C; (c) S2O28 from the initiator APS adsorbed on the surface of CNS; (d) introduction of monomer acrylamide (AM); (e) in situ polymerization of PAM on the surfaces of CNS.
diameters < 5 nm to reinforce hydrogel, and the synthesized hydrogels possess high transparency and excellent mechanical properties at very low inorganic content. However, reinforced mechanism of such prom ising CNS in the hydrogels is still vague, which greatly limits the further application of CNS in the field of hydrogel. Herein, molecular dynamics (MD) is proposed in our study to solve these confusions. In the past decades, MD has been applied in the simulation studies of hydrogel, including poly (vinyl alcohol) [14], poly (N-isopropylacrylamide) [15,16], poly (vinyl methyl ether) [15], epoxy-amine [17], polyacrylamide [18] network and double networks [19–21]. For constructing the MD model of hydrogel, two methods are extensively utilized: one is self-assembly that several carbon chains are placed into the box, and they run and interact under the selected force field at ordinary or elevated temperature [22,23]; the other one is an ideal model that is constructed directly, in which polymer chains are arranged in an orderly manner to form a diamond-like or square network [19–21,24–26]. However, neither method is true enough. The initial position of polymer chains and particles have a great influence on the final structure and performances of the hydrogel. In addition, the reinforced roles of nanoparticles in the entire structure of hydrogel have not been fully studied so far. There are only a few simulation studies that investigate the local interactions between nanoparticles and polymers, such as a sandwich model with two GO sheets and several polymer chains [27]. In this work, a polyacrylamide hydrogel was fabricated by intro ducing non-aggregated Ca(OH)2 nano-spherulites with di ameters < 5 nm. Combined with experiments, molecular dynamics method was utilized to study the self-cross-linking process of the NC hydrogel, and construct a more realistic NC hydrogel model. In order to explore the reinforced mechanisms of CNS, the chemical bonding be tween CNS and polymer chains, the structure and morphology of poly mer chains, and the tensile process of the whole structure were investigated on the molecular level. The combination of experiments and theoretical simulations would deepen the understanding of NC hydrogels, and provide a theoretical basis for the design of next gener ation NC hydrogels with CNS.
2. Methodology 2.1. Experiment Fabrication process of CNS and PAM/CNS NC hydrogel is shown in Fig. 1. The hydrogel containing n ppm CNS (named Cn as sample identification) were fabricated by in situ free-radical polymerization of acrylamide (AM) in CNS suspension. 2.1.1. Preparation of CNS suspension The hydration of tricalcium silicate (Ca3SiO5), a main component of Portland cement, was utilized to produce CNS, during which both cal cium silicate hydrate (C–S–H) gels and portlandite were rapidly formed at normal temperature [28–31]. In this work, 0 � C was proved to be the optimal temperature because the releasing speed of Ca2þ from the tri calcium silicate at this temperature was just enough to form CNS, and meanwhile the formation of C–S–H gel and the size of CNS were sup pressed [13]. Ca3SiO5 with synthesis method of Ref. [13] was dispersed in the deionized water at 0 � C in an ice bath, and the system was stirred under ultrasonication for 10 min to yield a homogeneous dispersion. 2.1.2. Preparation of PAM/CNS NC hydrogels The PAM/CNS hydrogel was prepared by in situ free-radical poly merization. AM was introduced into the mixed solution with CNS, ammonium peroxydisulfate (APS) and N,N,N0 ,N0 - tetramethyl - ethyl enediamine (TEMED). The concentration of CNSs/AM/APS/TEMED was 60 g/15 g/0.03 g/48 μL. The polymerization process proceeded at an ice bath (0 � C) and vacuum environment (0.01 atm) for at least 72 h. 0 � C and vacuum environment is necessary to inhibit the redox of S2O28 and remove NH3. S2O28 can be adsorbed on the CNS surface by electrostatic force. 2.1.3. Characterization of morphology and structure of the NC hydrogels A high-resolution scanning electron microscope (JEOL, model JSM6700F) was used to characterize the microscopic of xerogels. The con centration of CNSs in suspension was measured by inductively coupled 2
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Fig. 2. Model construction process of PAM/CNS aqueous mixture with homogeneous dispersion. (a) and (b) are schematic diagrams of the distribution of PAM chains represented by colored long arrows; (c) is the ultimate model of PAM/CNS aqueous mixture with homogeneous dispersion, where Oc and Hc represent the oxygen and hydrogen of carboxyl in PAM chains, and Os is the oxygen bonded to sulfur, and Ow and Hw are the oxygen and hydrogen of water.
plasma mass spectrometry. Time-of-flight secondary ion mass spec trometry (ToF-SIMS) was performed to demonstrate characteristic groups of PAM and CNS. Static ToF-SIMS spectra of hydrogels were received from ToF-SIMS V spectrometer (IONTOF GmbH, Münster, Germany). The hydrogel samples were bombarded with Biþ 3 primary ions, and the Biþ 3 primary ions were accelerated at 25 kV with an average pulsed current of 0.3 pA. Each spectrum was acquired for 40 s, and the raster was 200 � 200 μm.
bonds are formed between layers playing roles in layer connection. For this box, only one CNS can be placed in the center. Thus if a box is filled with a CNS, it can be called 100% filling. The box was enlarged 2 � 2 � 2 times and 2 � 3 � 4 times along x, y, and z axis to obtain supercells for the analysis of structure and mechanical properties, respectively. There are more than 100000 atoms in the supercell used to model mechanical properties. Five samples, CNS 1, CNS 3/4, CNS 1/2, CNS 1/4, and CNS 0, were chosen in this study, in which CNS filling rate is 100%, 75%, 50%, 25%, and 0, respectively.
2.1.4. Mechanical tests The MTS (model E44, EXCEED) testing machine was used to evaluate the mechanical properties of hydrogels. For tensile tests, hydrogels were prepared as a rod-like shape with 30 mm in length and 3.2 mm in diameter. The loading rate was 50 mm/min, and tensile tests was con ducted at 25 � C.
2.2.2. Force field and simulation procedure Empirical force field was conducted to model CNSs and PAMs. For PAMs, consistent valence force (CVFF) was chosen. It was extensively used in the simulation of the structure and dynamics of peptides, pro teins, and other organic systems, and has been confirmed to be accurate to simulate organics with low molecular weight (e.g., carboxylic acids and amides) [32–34]. Previously published paper confirms that the bond lengths, bond angles, torsion angles, and dihedral of carbon chains can be accurately described under CVFF [35–37]. As to the description of PAMs, ClayFF force field [38] was utilized in this study. It has already been used to successfully model oxide and hydroxide materials [39–41]. Using ClayFF force field, Hou [42] and Kalinichev [39] have investi gated the structure and dynamics of portlandite, and the interaction of aqueous species with portlandite surface. Both of them achieved admi rable results. Lennard-jones function and Coulomb formula, in which parameters were determined from the density function theory, is used in ClayFF to describe the interaction between calcium, oxygen, hydrogen in portlandite. Furthermore, the combination of CVFF and ClayFF force filed has already been applied in similar systems [35,36]. Therefore, CVFF and ClayFF force filed were combined in this study, more details and parameters of the force fields are available in Refs. [39,43]. The simulation was performed on LAMMPS, a large-scale atomic/ molecular massively parallel simulator. Nose-Hoover thermostat and Verlet algorithm were applied in the simulation. The cutoff distance of long-rang force is 10 Å in this simulation work. Time step was set as 1 fs. Before the simulation begins, energy minimization was conducted to optimize initial structure. Then the simulation is performed as the following procedure. First, NPT run was conducted at 1 atm and 300 K to compress the supercell to 57% volume. This is to ensure that PAM chains
2.2. Molecular dynamics simulation 2.2.1. Model construction Construction process of the model is shown in Fig. 2. This model approximately represents PAM/CNS aqueous mixture with homoge neous dispersion. In order to the homogeneous dispersion of PAMs in the box, each plane (xy, yz, and xz plane) selected two PAM chains perpendicular to each other and these six chains were placed as shown in Fig. 2a. Various forms of PAM chains will be generated after a period of motion. In order to allow the boxes to cross each other, one chain was selected from each plane and moved out of the box. The yellow chain in Fig. 2b is taken as an example. Finally, Grand Canonical Monte Carlo (GCMC) method was utilized to adsorb water molecules into box to achieve a solution environment. This process proceeded 300 million steps to reach 1 g/m3 of water density. The final model is shown in Fig. 2c that was constructed using Ma terials Studio 7.0. Periodic boundaries were set in three directions of xyz. PAM chains were functionalized by carboxyl groups and sulfate according to the structure characterization in the ToF-SIMS experi ments. The degree of polymerization of PAM in the model was 42 (42 monomers). CNS with diameters of around 4 nm was constructed, where the coordination number of calcium was eight and its coordinated hy droxyl groups were located between calcium layers. H-bonds and ionic 3
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Fig. 3. Scanning electron microscope image of C200 (NC hydrogel with 200 ppm CNS concentration) xeogel with scale bar of (a) 1 mm, (b) 100 μm, and (c) (d) 10 μm.
and CNSs can interact, meanwhile the substances in the box do not collapse together due to the excessive shrinkage of the box. Compression to 50%–60% volume can well meet the above conditions. Second, NVT run was performed at 300 K for 1 ns to obtain an equilibrated system, where PAMs and CNSs had fully interacted and the structure reached stable. Then, it was followed by another 1 ns under NVT for structure analysis. Finally, supercells were subject to uniaxial tensile strain through progressive elongation at the strain rate of 0.08/ps.
42 X
Dn
(1)
D Dstraight
(2)
D¼ n¼1
r¼
where n is the monomer order and the numbering sequence is from one end of the chain to the other end; one carbon is chosen from each monomer and Dn is the distance between the carbon in nth monomer and the first carbon; Dstraight is the unfolded distance of the straight chain. Mobility of PAM can be described quantitatively by threedimensional mean square displacement (MSD) [44], calculated ac cording to following equation. � MSDðtÞ ¼ < �ri ðtÞ ri ð0Þj2 > (3)
2.2.3. Data analysis method One frame is output per 1000 steps and 100 steps for the analysis of structure and tensile properties, respectively. Unfolded degree, r is uti lized to estimate the structure of PAM chains. 100% unfolded degree means that the chain is completely straight. Under the interaction with neighboring environment, the morphology of PAM chains evolves and becomes curled to some extent, which can be reflected by a decrease in the unfolded degree r. The r is calculated from unfolded distance, D.
where ri (t) is the position of atom i at time t. The MSD of carbon atoms in PAM is calculated here to represent the mobility of the PAM, which is controlled by surrounding constraints. Interatomic spatial correlation is calculated here based on radial distribution function (RDF) [45], determined by following equation, to estimate the interaction between atoms. g(r) ¼ dN / (4πr2ρ)
(4)
where r is the distance between the selected atom 1 and its surrounding atom 2; dN represents the number of atom 2 within an annular region of dr; ρ represents the average density of the whole system. 3. Results and discussions 3.1. Structure and mechanical properties of PAM/CNS NC hydrogel SEM images of freeze-dried PAM/CNS xerogel with 200 ppm CNS concentration are presented in Fig. 3. PAM/CNS xerogel possesses a uniform porous structure with diameter of 100–400 μm as shown in
Fig. 4. Tensile stress-stretch ratio curves of PAM/CNS hydrogels with different concentration of CNS. λ represents the stretch ratio. 4
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concentrations are performed at room temperature. The tensile stressstrain curves plotted in Fig. 4 shows that the introduction of CNS dramatically enhances the mechanical properties of PAM hydrogels. As compared with the hydrogel with CNS, the maximum stress and stretch ratio of pure PAM hydrogel can be neglected. The maximum stress and stretch ratio at rupture of PAM/CNS hydrogel with 40 ppm CNS (C40 sample) achieve 420 kPa and 120, respectively. Its toughness achieves 33.92 MJ/m3 that is 170 times more than that of pure PAM hydrogel. It achieves high stress and stretch at the same time. As CNS concentration increases to 200 ppm (C200 sample), the maximum stress continues to increase and reaches 530 kPa at a rupture stretch ratio of 62. The frac ture toughness was 26.2 MJ/m3, which is lower than that of C40, because of the decreased deformation ability of the crosslinked network of C200 sample. As listed in Table 1, the overall mechanical properties of NC hydrogel achieved in this study has been improved significantly compared to the hydrogels reported so far. The mechanical properties of PAM/CNS hydrogels are significantly influenced by the cross-linked network structure controlled by amount of CNS. In order to investigate the structure of hydrogel to support MD simulation, ToF-SIMS is employed to characterize the chemical bonding in the hydrogel. The peaks of negative ions in Fig. 5a and b represent the side (-CN－and -CNO－) and end (-SO4CH－and –SO4CH2－) functional groups of PAM in the sample of pure PAM and C200. The decreased peak intensity of –SO4CH－and –SO4CH2－ in C200 indicates a decrease in the end groups. Similarly, the amount of side groups (-CN－and -CNO－) also reduces due to the reaction with OH , during which gaseous NH3 is formed and the side groups are replaced by COO- and COOH. In Fig. 5c, the peaks of -CaSO4CH－and -CaSO4CH2－ in C200 means the ionic bonding between the PAM end groups and CNS. Their peak intensity
Table 1 Maximum stress and stretch ratio at rupture of CN hydrogel with different crosslinkers. Matrix
Nanoparticle
Content (ppm)
Max. stress (KPa)
Stretch ratio at rupture
PAM
Graphene oxide [12]
80 480 500
282 385 300
31 34 15
8000
34
62
8700 80800 110000 330000 40 200
100 175 150 230 420 530
118 85 23 4 120 62
vinyl hybrid silica nanoparticle [46] Layered double hydroxide [10] Montmorillonite [47] Alginate [48] CNS (this work)
Fig. 3a. As the amplification increases, it can be observed that the pore wall also contains numerous smaller pores. Fig. 3d shows pores with diameter less than 1 μm. In Fig. 3b, a large number of parallel and crosslinked filamentous structure are regularly distributed inside the pores. Embossments at the junction between the filaments and pore walls demonstrate the strong connection between them. Fig. 3c and d show a crosslinked network constituted by filaments with diameter of 50 nm ~ 4 μm. Bumps are formed on the filaments and a shish-kebab structure can be observed in the structure. The bumps may be CNSs coated with PAM chains. To study the roles of CNS on the mechanical properties of PAM hydrogel, elongation tests on the NC hydrogels with different CNS
Fig. 5. ToF-SIMS spectra of the hydrogel. (a)(b) Peaks of negative ions representing the side (-CN－and -CNO－) and end (-SO4CH－and –SO4CH2－) functional groups of pure PAM and C200, and (c) peaks of -CaSO4CH－and -CaSO4CH2－ in pure PAM and C200. C200 represents PAM/CNS hydrogel with 200 ppm CNS concentration. 5
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Fig. 6. Schematic diagram of the interacted process between PAM chains and CNS. (a) Initial and (b) ultimate configuration of PAM and CNS, in which connected PAM (c-PAM) and reinforced PAM (r-PAM) can be categorized.
Fig. 7. (a) Cross-linked node formed near CNS (CNS node), (b) node formed by the entanglement between PAM chains (PAM node).
does not change before and after being stretched for 60 times, implying the high strength of the ionic bond.
reinforcement to wrap CNS to reduce its dissolution, and to strengthen the connection between c-PAM and CNS. The generation process and detailed structure of these two types of chains are illustrated in Fig. 6. As a cross-linked node, CNS node can connect 6–7 c-PAMs. In addition to CNS node, another type of node can be observed in the structure, as shown in Fig. 7b, which is formed by the entanglement between PAM chains and named PAM node. The PAM node can only connect 2–4 cPAMs. Such a cross-linked structure generates numerous ring-shaped structure filling the hydrogel, which is consistent with the SEM image in Fig. 3. Mechanically, such a ring-shaped structure can resist and transfer stress well when it is loaded. In order to better show the structure of these two types of node and their functions, one frame of tensile process is taken as an example (Fig. 8a). Several nodes can be observed in Fig. 8a. Some are relatively big, like CNS nodes, and some are relatively small, such as PAM nodes. The nodes play roles in transferring stress and are the key to the me chanical properties and swelling of the NC hydrogel. In the experiment, shish-kebab structures on the filaments can be observed in the SEM image of C200. Amount of the bump decreases with reducing CNS concentration. Actually, the bump here is the enlargement of the CNS
3.2. Generation process of PAM/CNS NC hydrogel structure During the gelation process of PAM/CNS mixture, PAMs and CNSs are intertwined and overlapped with each other by hydrogen-bond, electrostatic force and Van der Waals’s force, constructing a crosslinked network of PAM/CNS NC hydrogel. In the network structure, CNSs act as cross-linkers and greatly improving the mechanical prop erties of the CN hydrogel. The interacted process between PAM and CNS is illustrated in Fig. 6. PAMs are attracted by CNS and connected to it gradually. As a result, three types of structure are formed between CNS and PAMs that is winding, lapping, and docking. Which structure is generated depends on the relative position between PAM and CNS. As a result, as shown in Fig. 7a, cross-linked nodes are formed around CNS due to the interaction between PAM and CNS, which is named CNS node. Two types of PAM chains, connected PAM (c-PAM) and reinforced PAM (r-PAM) should be noted in this node structure. c-PAM plays roles in connecting neighboring nodes or chains. r-PAM acts as a 6
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Fig. 8. (a) Illustration of PAM/CNS NC hydrogel structure, (b) zoomed image of the box region ofIin a, (c) zoomed image of the box region of IIin a, and (d) radial distribution function (RDF) of the structure. Please refer to Fig. 2 for the meaning of Oc/Os/Hc.
node that CNSs are coated with PAM chains. As the system grows from nanoscale to microscale, local concentration unevenness begins to appear, resulting in serious aggregation of PAMs around CNSs and the formation of such bumps. Fig. 8b and c exhibit the details of PAM node and CNS node,
respectively. PAM node is formed by the entanglement between chains, which is controlled by two types of H-bond (Os-Hc and Oc-Hc). For CNS node (Fig. 8c), CNS is winded by r-PAM chains and linked to seven c-PAM chains. Os/Oc-Ca ionic bonds and H-bonds control the node, where the ionic bonds link PAMs and CNS together and the H7
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Fig. 9. (a) Elongation stress-strain curve of CN 1 sample, and evolution of (b) mean unfolded degree of carbon chains, (c) bond number per carbon chain, and (d) CNS number as function of tensile strain in CN 1 sample.
bonds mainly act to connect r-PAM and c-PAM. Fig. 8c clarifies the reinforcement effect of the r-PAM. One end of the r-PAM is bound to CNS, and the other end is linked to c-PAM. Such a structure strengthens the connection between c-PAM and CNS, and also contributes to the transfer and dispersion of the stress around node when it is loaded. Morphology of the r-PAMs near CNS node explains the embossments observed in SEM image that is formed in the connection point between filaments and inner walls of pores. Radial distribution function (RDF) is utilized to estimate interatomic spatial correlation and further reflect chemical bonding in the structure, as shown in Fig. 8d. The first and second RDF peaks of Os–Ca are the highest, meaning the strong short-range and medium-range spatial correlations between the SO4 at the end of PAM and the Ca in CNS. It indicates the existence of a great many Os–Ca ionic bonds, with bong length of 2.5 Å, connecting PAMs to CNS. It is consistent with the chemical bonding characterization given by above ToF-SIMS spectra. By contrast, the spatial correlation of Oc-Ca is much weaker. In addition, there are H-bonds of Os-Hc and Oc-Hc with O–H length of 1.55 Å and 1.75 Å, respectively, which connects PAM chains together. Os-Hc connection with less bond length and stronger spatial correlation shows higher chemical bond strength than Oc-Hc. Overall, it is clear that the terminal sulfate on PAM plays a key role in this cross-linked struc ture. Additionally, there are also H-bonds formed between carboxyl groups of PAM and hydroxyl groups of CNS, but it is ignored in our analysis because there have already been numerous ionic bonds between PAM and CNS.
3.3. Structure evolution during tensile process Structure evolution of the hydrogel during tensile process is inves tigated in this part by means of stress-strain relation, and the statistics of chemical bond number and unfolded degree of PAM chains. Elongation stress-strain curve of CN 1 sample and related structure characterizations are shown in Fig. 9. The stress-strain curve can be roughly divided into three stages, elastic stage, yield stage, and failure stage. In the elastic stage, the cross-linked network structure is stretched to take up tensile strain and the stress reaches maximum (~123 MPa) at the strain of 1.2. During this process, unfolded degree of PAM chains increases with tensile strain, indicating that curled PAM chains are gradually unfolded, stretched, and tightened. This unfolded process of PAM chains leads to the breakage of H-bond inside and between chains. As shown in Fig. 9c, the H-bond formed between carboxyl groups is broken most, and reduces from 11 to around 8 per PAM. By contrast, the number of ionic bond and the H-bond formed between carboxyl and terminal sulfate keeps stable before the strain of 1 and then reduces slightly. In the yield stage, the stress sharply drops to around 36 MPa, and then reaches a plateau and fluctuates until the structure breaks. In the stage of stress drop, the chemical bonds break largely, resulting in the breakage of some connection points. The number of ionic bond with relatively high bond strength also reduces from 10.5 to 9.9 as the strain increases from 1.2 to 1.7. Subsequently, the stress reaches a plateau and the number of chemical bond remain stable as well. It is related to the constant rebirth of H-bond caused by the relative slip between PAM 8
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Fig. 11. Structure evolution of the CN hydrogel at tensile strain (a) 0.5, (b) 1.5, and (c) 2.5. Fig. 10. Local structure of CN hydrogel at the strain of (a) 1.5 and (b) 2. (c) Zoomed image of the left one of two blue regions in b. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
is slightly deformed at the strain of 0.5 (Fig. 11a). As the stress increases, damage of some nodes can be observed, where the connection on one end of chains is stretched to be broken and this end of the chains sways in the structure. Meanwhile, the structure of chains gradually changes from curl to straight, among which c-PAMs are predominant and a few rPAMs are also included. When CNS nodes are divided, their r-PAM chains are stretched to some extent or even become c-PAMs. But there are still numerous r-PAMs maintaining winding structure around CNS.
chains during the tensile process, which contributes to the release of energy. In the yield stage, the unfolded degree of PAM chains still lin early increases with strain, and it only depends on the stretching degree of the structure. During the whole tensile process, ionic bond number only declines by around 0.5, one tenth of the reduced number of H-bond. It is partly due to the relatively high bond strength of ionic bond, and partly because of the connection reinforcement of r-PAMs. Under high tensile stress, as shown in Fig. 10, CNSs are dragged by PAM chains, causing some CNSs to be cracked and divided into two even three relatively small CNSs with diameters less than 2 nm. New nodes are quickly formed surrounding these divided CNSs, and yet there is few r-PAM. As plotted in Fig. 9d, the number of CNS increases from 8 to around 17.8. The climax of CNS division starts when the stress reaches around the maximum, and ends at the strain of 3. The division of CNSs contributes to the release of tensile stress, which is one of the reasons why the stress-strain curve can enter the plateau stage (strain 2–3) and show a slight strengthening peak (strain around 3.4). Moreover, it im proves the stretchable property of the hydrogel, but it is irreversible deformation. The CNS size has an impact on its reinforcement to the hydrogel structure, such as the amount of CNS connecting PAM chains, but interestingly the feature of CNS division weakens the effect of CNS size on the hydrogel reinforcement to some extent. Snapshots of the hydrogel configuration at the tensile strain of 0.5, 1.5 and 2.5 are presented in Fig. 11. It shows a complex network structure, in which PAM chains are parallel and cross-connected. Numerous nodes are clearly visible, especially the CNS nodes. They play critical roles in transferring and bearing tensile stress. The structure
3.4. Effect of CNS on the structure and tensile property of PAM/CNS CN hydrogel As mentioned above, CNS nodes not only show relatively high con nectivity and strength, but also can be divided under high tensile stress to release stress. It seems very beneficial to the hydrogel. In this part the effect of CNS on the structure and tensile property of the hydrogel is investigated by changing CNS amount in the structure. Variation of chemical bond number per PAM chain with CNS amount is shown in Fig. 12a. Total bond number per PAM decreases from 32.1 to 24.5 with CNS amount decreases from 100% filling (CNS 1) to 0 (CNS 0). As to the sample of CNS 1, the number of each type of chemical bonding is basically the same, around 10 per PAM (Fig. 12a). Among the 10 ionic bonds, there are around 8 ionic bonds formed between terminal sulfate and calcium of CNS, implying that each sulfate is coordinated with around four calcium ions. Such high concentrations of Os–Ca ionic bond guarantee the high strength of the connection between CNS and the end of PAM. This is one of the reasons why CNS can be divided by the dragging of PAM chains, rather than the connection point between them breaks. As CNS amount declines, ionic bond number reduces correspond ingly and reaches 0 when there is no CNS in the structure. In the region without CNS, therefore, sulfate can only interact with carboxyl groups 9
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Fig. 12. Evolution of (a) the number of chemical bonds and (b) nodes with CNS amount; unfolded degree of PAM chains in (c) CNS 1 and (d) CNS 1/4; (e) examples of unfolded degree; (f) evolution of mean square displacement (MSD) with CNS amount.
on PAM to form H-bond with it, resulting in an increase of Os-Hc H-bond from 9.7 to 17 per PAM. Moreover, Oc-Hc H-bond number decreases from 12 to 7 as 100% filling of CNS decreases to 0. It means that the reduction of CNS amount decreases the H-bond number formed inside and between PAM chains, which also implies a decrease in PAM nodes. With CNS amount decreasing, not only are CNS nodes reduced, but also PAM nodes decline. Therefore, as shown in Fig. 12b, mean node number per PAM progressively decreases from 8.8 to 5.6 as 100% filling of CNS reduces to 0. In addition to an influence on the number of chemical bonds and
cross-linked nodes, introduction of CNS also greatly changes the local structure of the hydrogel. Unfolded degree of PAM chains in the samples of CNS 1 and CNS 1/4 is shown in Fig. 12c and d. For CNS 1, the intensity profile of unfolded degree is bimodal distribution and the two peaks are located at around 20% and 45%, respectively. Such a distribution con firms the ubiquity of c-PAM and r-PAM in the structure. Unfolded degree of 20% illustrates the curled structure of PAM chains, which is related to the r-PAM. Unfolded degree of 45% corresponds to the c-PAM. Ac cording to the statistics in Fig. 12c, around 65% of PAM chains are cPAMs and remaining 35% belong to r-PAMs in the structure of CNS 1. 10
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Fig. 13. (a) Stress-strain curves, and variations of (b) tensile strength, elastic modulus, (c) strain at break, and (d) change of unfold degree as function of ten sile strain.
Fig. 12e illustrates the structure of these two types of chains by means of unfolded distance profiles. The chains with unfolded degree of 40% and 23% are taken as examples. If unfolded distance linearly in creases with monomer order all along, it means that the PAM chain is completely straight, which is 100% unfolded. Specific morphology of the chains with unfolded degree of 23% and 40% can refer to the r-PAM and c-PAM in Fig. 6, respectively. For CNS 1/4, unfolded degree of PAM chains falls in the range from 10% to 65%, but its intensity is distributed disorderly. In the absence of CNS, PAM chains are randomly crossed and overlapped. Hence it can be concluded that the presence of CNS makes the structure of PAM chains more orderly. Fig. 12f investigates the dynamics of PAM chains and exhibits the amplitude and displacement of the chains in different structure. Mean square displacement (MSD) gradually increases over time and fluctuates simultaneously, meaning the displacement and oscillation of PAM chains in the structure. Obviously, the amplitude of CNS 1/4 is much larger than the other two samples. Also, the MSD values of CNS 1/4 at 150 ps are 460 Å2, around 4.6 and 7.6 times larger than that of CNS 1/2 and CNS 1, respectively. It can be seen that the mobility of PAM chains is influenced by CNS amount in the structure. Adding CNS into the struc ture increases chemical bonds and cross-linked nodes, geometrically restricting PAM chains. Effect of CNS on the tensile property of the hydrogel is illustrated in Fig. 13. Fig. 13a plots elongation stress-strain curves of the samples with different CNS amount. For CNS 1, stress increases to failure strength of 123 MPa at strain 1.2, and followed by a sharp drop to around 36 MPa at
strain 1.7. Subsequently, the stress steps into a plateau and maintains unchanged until the strain reaches 3.5. Finally, the stress drops to 0 meaning the complete damage of the structure. As CNS amount reduces from 100% filling to 0, the failure strength and elastic modulus decline linearly, and decrease by 84.5% and 94.2% when CNS amount is 0. Besides, the peak of stress-strain curve is getting weaker with CNS amount decreasing, meaning that the skeleton of carbon chains become weaker and weaker as well. However, as shown in Fig. 13a, the stretchable property of CNS 1 is not the best. The variation of the strain at break has a turning point, and the maximum (~8) occurs at around 50% filling of CNS (CNS 1/2). Furthermore, it can be observed that the stress of CNS 1/4 fluctuates greatly in the stress-strain curve as compared with CNS 1. It is related to the relative slip between PAM chains, during which the H-bonds formed between chains are constantly broken and regenerated. Such a structure rearrangement is beneficial to improving the stretchable property of hydrogel. In addition to it and the division of CNS mentioned in Section 3.3, the stiffness of the structure is also an important factor affecting the stretchable property. The stiffness is characterized by unfolded speed of PAM chains during the tensile process. As shown in Fig. 13d, the unfolded degree of PAM chains increases with tensile strain, implying that the chains are gradually stretched from curled shape. An increase in unfolded degree with strain is approximately linear and can be linearly fitted. The slope of the fitting line is unfolded speed of PAM chains during the tension process. The unfolded speed decreases with CNS amount increasing as shown in Fig. 13d, and reaches the minimum of 11.11% per strain in the sample of CNS 1. Compared with samples with less CNS amount, there 11
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are more chemical bonds and cross-linked nodes in CNS 1, providing more confinement to the mobility and flexibility of PAM chains. For CNS 1, this linear increase of unfolded degree ends at the strain of 3.5, while the strain when the linear growth ends in CNS 1/2 and CNS 1/ 4 is 2.3 and 1.5, respectively. This unfolded process of PAM chains is also the process of releasing tensile stress and energy. Furthermore, it is interesting that the total growth of unfolded degree of PAM is almost the same for the three samples in Fig. 13d, and is close to 50%. After a linear growth of unfolded degree with strain, the unfolded degree enters a plateau with large fluctuations.
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4. Conclusions Hydrogel achieved by non-aggregated spherulites with diameters <5 nm is reported in this paper. Its properties can be tuned through the concentration of CNS, and reach very high values for stretchable prop erty, toughness, and strength. Molecular dynamics simulations are performed to study the reinforced mechanisms of CNS in the hydrogel and related conclusions are as follows. Structurally, two types of cross-linked nodes (CNS nodes and PAM node) are formed in the structure by the interaction between CNSs and PAMs and the entanglement between PAM chains, respectively. By contrast, CNS node not only exhibits higher strength and connectivity, but also makes the structure of PAM more orderly. In the CNS node, two types of PAMs can be categorized, and they possess curly and stretched structure playing roles in reinforcement and connection, respectively. Moreover, the number of chemical bonds and nodes increases with CNS amount, providing more confinement to the mobility and flexibility of PAM chains. Mechanically, the introduction of CNS significantly improves the tensile strength and elastic modulus of the hydrogel but decreases stretchable properties to some extent. The decrease in the stretchable properties is also shown by the slowdown of unfolded speed of PAM chains during the tension. Interestingly, CNS also has a beneficial side to improve the stretchable properties via the division into relatively small CNSs under high stress. Acknowledgements Financial support from National Natural Science Foundation of China under Grant 51678317, 51420105015, the China Ministry of Science and Technology under Grant 2015CB655100, Natural science foundation of Shandong Province under Grant ZR2017JL024, The Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (Grant No. 161069), Science and Tech nology Development Fund from Macau (0074/2018/A2). References [1] Huisman IH, Williams K. Autopsy and failure analysis of ultrafiltration membranes from a waste-water treatment system. Desalination 2004;165:161–4. [2] Ayala DF, Ferre V, Judd SJ. Membrane life estimation in full-scale immersed membrane bioreactors. J Membr Sci 2011;378:95–100. [3] Pal Kunal, Banthia AK, M DK. Starch based hydrogel with potential biomedical application as artificial skin. Afr J Biomed Res 2006;9:23–9. [4] Hu J, Meng H, Li G, Ibekwe SI. A review of stimuli-responsive polymers for smart textile applications. Smart Mater Struct 2012;21:053001. [5] Guorong G, Gaolai D, Yuanna S, Jun F. Self-healable, tough, and ultrastretchable nanocomposite hydrogels based on reversible polyacrylamide/montmorillonite adsorption. ACS Appl Mater Interfaces 2015;7. 5029-2037. [6] Haraguchi K, Ebato M, Takehisa T. Polymer-clay nanocomposites exhibiting abnormal necking phenomena accompanied by extremely large reversible elongations and excellent transparency. Adv Mater 2006;18:2250–4. [7] Haraguchi K, Li H-J. Control of the coil-to-globule transition and ultrahigh mechanical properties of PNIPA in nanocomposite hydrogels. Angew Chem, Int Ed 2005;44:6500–4. [8] Haraguchi K, Li H-J, Matsuda K, Takehisa T, Elliott E. Mechanism of forming organic/inorganic network structures during in-situ free-radical polymerization in PNIPA-clay nanocomposite hydrogels. Biol Macromol 2005;38:3482–90. [9] Haraguchi K, Li H-J. Mechanical properties and structure of polymer-clay nanocomposite gels with high clay content. Biol Macromol 2006;39:1898–905.
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