- Email: [email protected]
Molecular dynamics modeling of the structure, dynamics, energetics and mechanical properties of cement-polymer nanocomposite
Composites Part B 162 (2019) 433–444
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Molecular dynamics modeling of the structure, dynamics, energetics and mechanical properties of cement-polymer nanocomposite
T
Dongshuai Houa,b,∗, Jiao Yua, Pan Wanga,b,∗∗ a b
Department of Civil Engineering, Qingdao University of Technology, Qingdao, China Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao, 266033, China
A B S T R A C T
Efforts to tune the performance of organic/inorganic composites are hindered owing to a lack of knowledge related to the interfacial interaction mechanisms. Here we investigated the interfacial structure, dynamics, energetics and mechanical properties between calcium silicate hydrates (C-S-H) and polymers by molecular dynamics (MD) simulation. In this work, polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyacrylic acid (PAA) are intercalated into nanometer channel of CS-H sheets to construct the model of polymer/C-S-H composite. In the interfacial region, the calcium ions near the surface of C-S-H play mediating role in bridging the functional groups in the polymers and oxygen in the silicate chains by forming Os-Ca-Op bond. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provide plenty oxygen sites to form H-bonds with silicate hydroxyl, interlayer water and calcium hydroxyl in C-S-H substrate. The interfacial binding energy is dependent on polarity of functional groups in the polymers, the stability of the H-bond and CaO bond, ranking in the following order: E(PAA) > E(PVA) > E(PEG). The PVA with small number of H-bonds formed between oxygen in PVA and water molecules, resulting in increasing the mobility of confined water in the interlayer region. On the other hand, PAA and PVA, with strong polarity, can provide more number of non-bridging oxygen sites that widely distributed along the polymer chains to associate with more calcium ions and H-bonds. Furthermore, uniaxial tensile test is utilized to study the mechanical behavior of the composites. The incorporation of polymers, strengthening the H-bonds in the interfacial region and healing the defective silicate chains, can inhibit the crack growth during the loading process, which both enhance the cohesive strength and ductility of the C-S-H gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively. The toughening mechanism in this organic/inorganic system can provide useful guidelines for polymer selection, design, and fabrication of C-S-H/polymer nanocomposites, and help eliminate the brittleness of cement-based materials from the genetic level.
1. Introduction The advantage in the application of inorganic-organic composite has promoted the development in numerous fields, such as industrial, biological, aviation, medicine, engineering and geological systems [1–8]. The first step to enhance the performance of nanocomposites is to understand the complex interaction mechanism between inorganic and organic composite [2,9–11]. In recent years, calcium silicate hydrates (C-S-H)/polymer nanocomposites have attracted substantial academic and industrial interest [12,13]. C-S-H accounts for 60%–70% of cement hydration production, which is the main binding phase of concrete [14]. However, cement-based concrete is a brittle material with low tensile and flexural strength [15,16], which results in easily deterioration of material when it subjects to external loading and limits the application of cement-based materials in engineering. C-S-H/ polymer nanocomposites combine the advantages of the organic polymer (e.g., flexibility, dielectric, ductility, and processability) and the C-S-H (e.g., rigidity, thermal stability) [17–20]. Because C-S-H gels
∗
are an amorphous structure, the doping of the polymer further increases the complexity of the structure [21]. And the study of the interfacial interactions between these phases at nanoscale is still incomplete. Therefore, it is necessary to study the interfacial interaction mechanisms between C-S-H and polymers. The role of intercalated polymer has been investigated by various experimental techniques [13,22]. The average length of silicon chains of C-S-H and C-S-H/polymer have been obtained by NMR [23]. And the change of silica polymerization degree of pure C-S-H modified by polyethylene glycol (PEG), polyvinylalcohol (PVA) and polyacrylic acid (PAA) [24] are reported by 29Si MAS NMR [25]. With a combination of attenuated total reflectance–Fourier transform infrared (ATR–FTIR) and X-ray diffraction (XRD), it has been found that specific surface area increased when the intercalation of polymers into the C-S-H gels [22]. Based on advanced experimental techniques, some interfacial information can be captured and the possible interaction mechanisms have been concluded [26]. However, investigating the interaction mechanisms of inorganic-organic composite by experiment alone is
Corresponding author. Department of Civil Engineering, Qingdao University of Technology, Qingdao, China. Corresponding author. Department of Civil Engineering, Qingdao University of Technology, Qingdao, China. E-mail addresses: [email protected] (D. Hou), [email protected] (P. Wang).
∗∗
https://doi.org/10.1016/j.compositesb.2018.12.142 Received 1 August 2018; Received in revised form 23 December 2018; Accepted 30 December 2018 Available online 02 January 2019 1359-8368/ © 2019 Elsevier Ltd. All rights reserved.
Composites Part B 162 (2019) 433–444
D. Hou et al.
challenging due to the certain limitations, such as the resolution of the detectors and the material purity [27]. Computational methods are still necessary to understand the structure and dynamic properties of nanocomposites and interpret the interaction mechanisms [28]. Molecular dynamics have been used to elucidate and reveal the comprehensive interfacial mechanisms between polymers and C-S-H. The interlayer stiffening mechanism in the z direction when C-S-H samples subjected to hydrostatic pressure has been explained [29]. Zhou et al. evaluate the influence of polymer intercalation by Reaxff. Moreover, the influence of strain rate and tensile direction on the ductility and other mechanical properties is explored [30]. There are also many researches on clay-polymer systems [31–33]. Zeng et al. investigate the density distribution, conformation and polymer chain title angle of clay-polymer system [34–36]. And the interlayer spacing of clay largely depends on the number of intercalated polymer [34,37]. Debashis et al. [38] reveal that the critical factor of increasing the nanomechanical properties of polymer/clay nanocomposites is the interactions between polymers, intercalated clay and organic modifiers. However, the difference of molecular structure between clay and C-S-H will lead to the different strengthening mechanism. For C-S-H/polymer nanocomposites, the simulation of structural evolution, dynamics properties are still missing, the mechanical behaviors and mechanism interpretation are still incomplete. Therefore, the primary purpose of this study is to investigate the microstructure evolution and mechanical properties of C-S-H/polymer nanocomposites. Moreover, uniaxial tension testing was performed on the C-S-H/polymer models to obtain the mechanical properties, such as Young's modulus and bulk modulus. And the intrinsic mechanism of the nanocomposites' extraordinary mechanical properties is revealed. Understanding the origin of the improved mechanical properties [39,40] of C-S-H/polymer nanocomposites is of great significance to the design and development for engineering applications. This makes it possible to select polymer that compatible with C-S-H to produce materials with desirable structures. 2. Simulation method 2.1. Force field In this simulation, the empirical force field is employed to model the interactions between C-S-H and polymer. CSHFF force field (a ClayFFlike potential) [41] was unitized to simulate the C-S-H gel. CSHFF force field has been widely used in the C-S-H gel simulation and successfully applied to describe the structure, energy and mechanical properties of various calcium silicate phases [42–47]. Potential parameters for polymers were taken from the consistent valence force field (CVFF) [48], which was widely used in the proteins, peptides, and other organic system. And it has been proved to be quite suitable for modeling organic crystals with low molecular weight, such as carboxylic acids and amides [49–51]. CVFF could also describe the potential states related to bond lengths, bond angles, torsion angles and out-of-plane interactions. More details and parameters of the force fields are available in Ref. [52]. 2.2. Model construction The C-S-H model and nanocomposites are constructed based on the 11 Å tobermorite structure [53,54]. Proper amounts of bridging site silicate tetrahedral were removed to modify the Ca/Si ratio to 1.3 [55,56]. After establishing the C-S-H with Ca/Si ratio 1.3, structures of polymer chains (PEG, PVA and PAA) are added into the interlayer regions of the anhydrous C-S-H. As shown in Fig. 1, each kind of polymer chain is composed of 8 monomers. There are two different oxygen atoms and hydrogen atoms in PEG. Ofp denote oxygen atoms in the C−O−C functional groups and Hcp denote the hydrogen atoms in the –CH2−, while Ohp and Hop denote the oxygen, hydrogen atoms in the
(caption on next page)
434
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 1. Structures of polymers: (a) PEG (C16O8H34) (Ofp denote oxygen atoms in the C−O−C functional groups, Hcp denote the hydrogen atoms in the –CH2−,Ohp and Hop denote the oxygen, hydrogen atoms in the hydroxyl groups); (b) PVA (C16O8H34) (Hcp denote the hydrogen atoms in the –CH2−,Ohp and Hop denote the oxygen, hydrogen atoms in the hydroxyl groups and to be separated from the hydroxyl group in the system); (c) PAA (C23O16H32) (Osp, Odp and Hsp, denote single-bonded oxygen, double-bonded oxygen and hydrogen atoms in COOH carboxyl groups, respectively, Hcp denote the hydrogen atoms in the –CH2− and H–C−C (grey balls represents the carbon, white balls correspond to the hydrogen, and red represents the oxygen), (d) finally C-S-H gel structure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
hydroxyl groups. There are seven Ofp and one Ohp in PEG, thus Ofp atoms play a major role in the process of connecting with the C-S-H. To the contrary, there are only one oxygen atoms (Ohp) in PVA. While in PAA, there are also two different oxygen atoms, Osp, Odp denote singlebonded oxygen, double-bonded oxygen in -COOH carboxyl groups respectively. Hsp denote hydrogen atoms in -COOH carboxyl groups and Hcp is hydrogen atom connected with carbon atoms. PAA has the most branches in the three kinds of polymers. In order to be consistent with the experimental results [57], eight polymer chains are intercalated into the C-S-H [27]. Subsequently, the grand canonical Monte Carlo (GCMC) method was utilized to model the pure C-S-H and C-S-H/ploymer with water adsorption [58]. The GCMC simulation describes well the water adsorption in the microporous and mesoporous materials [59], which fix the water chemical potential to a certain value that corresponds to the bulk liquid phase with 1g/cm3 density at 300k. 1000 attempts were carried out to insert, delete, displace and rotate the water molecules in the constant volume [60]. The simulation process include equilibrium run for 108 circles and production run for 2 × 108 circles to obtain 4 samples. After hydroxylation, each of the models has a dimension of around 42 × 43 × 47 Å3 along x, y, and z directions, with total 7200-7400 atoms. The relatively large number of atoms involved in each model can guarantee the accuracy of the data for mechanisms analysis. As shown in Fig. 1 (d), the calcium sheets and silicate tetrahedra lie in the xy plane, where the defective silicate chains line up along the y direction. Along the z direction, the intralayer and interlayer regions are alternately arranged. Molecular dynamics simulations were then carried out in an isothermalisobaric (NPT) ensemble to give the structure equilibrium states. A further 1000ps NVT run was utilized to obtain the equilibrium atomic trajectories for structural and dynamic analysis. 2.3. Uniaxial tension tests The pure C-S-H gel and the three C-S-H/polymer were obtained from previous manipulation and the uniaxial tensile simulation was performed in the LAMMPS software [61]. At the beginning, the C-S-H model was first relaxed for 1000 ps by NPT ensemble at 300 K and 0 Par along x, y and z direction by using a Nose-Hoover thermostat and the Verlet integration scheme with time step of 1 fs. Subsequently, taking the tension along z direction for example, the C-S-H model was subjected to uniaxial tensile strain through gradually elongation of the C-SH model along z direction at strain rates of 0.008/ps. During the tensile process, the stress in the x and y directions was coupled to zero to consider the Poisson effect. The stress-strain curves were plotted. In addition, Young's modulus, the failure strength describing the stiffness and the interlayer cohesive force are obtained by the uniaxial tensile test. (caption on next page)
3. Results and discussions polymer nanocomposites are presented here to briefly introduce the molecular structural feature of polymer intercalated into the interlayer region of the C-S-H gel.
3.1. Molecular structures of C-S-H structure and nanocomposites The equilibrium structures of the pure C-S-H and three C-S-H/ 435
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 2. Structure of (a) calcium silicate hydrates with PEG, (b) calcium silicate hydrates with PVA and (c) calcium silicate hydrates with PAA. (Red and yellow sticks represents the silicate tetrahedra, green and purple balls correspond to the interlayered calcium atoms (Cas) and the interlayer calcium atoms (Caw) respectively, white and red lines represents water molecules and hydroxy, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, and red for oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.1.1. C-S-H molecular structures transformation The layered structure of the calcium silicate hydrates with polymers intercalated can be clearly observed in Fig. 2 (a). On the basis of the local chemical environment, the calcium atoms in the pure C-S-H gel and nanocomposites are categorized into calcium atoms in the sheets (Cas) and calcium atoms in the interlayer (Caw). The nearest neighboring oxygen atoms of the two types Ca atoms include Ow (oxygen atoms in water), Os (oxygen atoms in silicate chains), Oh (oxygen atoms in hydroxyl), and Op (the oxygen atoms from polymers). Defective silicate chains are grafted on both sides of the calcium sheet that is constructed by the calcium-oxygen polyhedral. The interlayer region is between the two adjacent calcium silicate layers, where the Caw, water molecules, hydroxyl groups and polymers are present. As shown in Fig. 3, the length in the z direction is 47.88 Å, 49.52 Å, 47.72 Å, 47.92 Å respectively for pure C-S-H gel, C-S-H/PEG, C-S-H/ PVA and C-S-H/PAA composites. The intercalated PEG polymer can increase the length in the z direction, while the length remains almost unchanged with the incorporation of PVA and PAA polymers. The intensity profiles of different atoms perpendicular to substrate plane are plotted in Fig. 4. As shown in Fig. 4 (a), the pure C-S-H has a sandwichlike structure. Sharper density distributions of Cas and Si indicate a much more ordered arrangement of the calcium silicate sheets. Additionally, Caw is only distributed between the neighboring calcium silicate sheets. However, this kind of orderly arrangement of C-S-H gel is disturbed when the polymers are incorporated into the C-S-H. The intensity of Op is distributed from 11 Å to 18 Å across the interlayer direction, which means the PEG can impregnated into the defective region of C-S-H gel. Even though the polymers are initially intercalated into the interlayer region, the polymer chains can twine the silicate skeleton, and interact with the calcium silicate sheet. As compared with that in the pure C-S-H gel, the binomial distribution for Cas becomes broadening in the intensity profile of C-S-H/PEG as depicted in Fig. 4 (b). The small shoulder of Si intensity appeared in the interlayer region represents the protruded bridging silicate tetrahedron in the silicate chains. It means that the “dier” arrangement of the silicate skeleton lose the order of bridging-pair tetrahedron distribution. The penetrated polymers, forming chemical bonds with interlayer water and calcium ions, give rise to the rearrangement of atoms in the calcium silicate sheet, which results in the transformation from order to disorder state
Fig. 4. The density profile of Cas, Si, Ow, Caw in the (a) C-S-H and Cas, Si, Ow, Caw, Op in the (b) C-S-H/PEG; (c) C-S-H/PVA; (d) C-S-H/PAA. Fig. 3. Length in the z direction for C-S-H and nanocomposites. 436
Composites Part B 162 (2019) 433–444
D. Hou et al.
Table 1 The number of the coordination calcium ions of each polyer in the nanocomposites. CN
C-S-H/PEG
C-S-H/PVA
C-S-H/PAA
Cas Caw Total
3.20 2.56 5.76
5.08 2.44 7.52
6.17 4.31 10.48
of the layered structure. The influence of other polymers on the structure of calcium silicate sheets in the interlayer region can also be found in the intensity distributions of C-S-H/PVA and C-S-H/PAA in Fig. 4 (c) and (d).
with hydrogen, the deprotonated Odp, with high polarity, occupies strong charge negativity, which can easily attract the surrounding calcium ions. It should be noted that the interfacial bonding greatly depends on the polarity of functional groups for organic-inorganic composite. In previous study, due to formation of a stronger Ca-O bond, the cohesive strength and binding energy in the graphene oxide and cement composite are significantly enhanced as the carboxyl groups in GO sheets are partly deprotonated to –COO- groups [62,63]. The average number of coordination calcium atoms of each polymer are calculated and shown in Table .1 (All the Ca-O cutoff distance is set on the basis of the location of the first minimum in RDF). In all three CS-H/polymer nanocomposites, polymers can be both coordinated by Cas and Caw. All the polymers have more Cas atoms than Caw species as their nearest neighbor. It confirms that the polymers invade into the defective regions of the silicate chains and connect with the calcium ions in the silicate sheets. The interaction between Cas-Op results in the dissociation of Cas from the calcium silicate skeleton, disturbing the layered structure. It explains the order-disorder transformation observed in Fig. 4. The total coordinate number (CN) of different polymers rank in the following order: CN (PAA) > CN (PVA) > CN (PEG). In particular, the CN for Cas atoms in C-S-H/PAA is almost twice as that of C-S-H/PEG, implying stronger interfacial chemical connection between PAA and the calcium silicate sheets. It is worth noting that the number of coordination Caw in PAA is also the most among all the polymers. This is mainly attributed to the fact that the PAA chains have large number of carboxyl branches, which are easily protruded in the silicate sheets and capture the neighboring calcium ions. On the other hand, it is hard for the calcium ions to associate with the bridging oxygen atoms in C−O−C of single PEG chains. It can be observed in Fig. 6 that the PEG, PVA and PAA polymers, with different conformation, connect with neighboring calcium silicate sheets by forming Ca-Op ionic bonds. In addition to the intrinsic structure of the polymers such as the chain length, the branch and cross-linking structure, and structure of the single chain, the flexibility of the polymers ultra-confined in the nanometer channel are influenced by the highly solvated calcium ions and the neighboring water molecules.
3.1.2. Interfacial Ca-O bond and local structure of polymers The Ca-O bond is characterized by the radial distribution function (RDF). Fig. 5 (a) and (b) illustrate the RDFs between oxygen atoms from polymer (PEG, PVA, PAA) and the Caw and Cas atoms of C−S−H. In the composite of PEG/C-S-H, the RDF of Caw-Ohp and Caw-Ofp have the first peak peak positioned at 2.53 Å and 2.55 Å, respectively. The pronounced peaks mean that the inserted PEG polymer can associate with the neighboring calcium ions by bridging oxygen in the middle chains and the OH groups at the end of chain. In addition to Caw-O connection, the oxygen atoms in the polymer can also form bond with calcium ions in the silicate sheet. The RDF of Cas-Ohp and Cas-Ofp have the first peak located at 2.39 Å and 2.50 Å, respectively. Similarly, the sharp peaks of Cas-Ohp and Caw-Ohp also indicate the stable chemical bond formation between polymer and C-S-H gel in the C-S-H/PVA composite. In the composite of C-S-H/PAA, the oxygen atoms in the polymer exhibit different affinity with the neighboring calcium ions. While the RDF of Ca-Odp has sharp peak with high intensity located at 2.50 Å, the RDF of Ca-Osp only has one small shoulder at 2.63 Å. It means that both the Cas and Caw ions energetically prefer to adsorb on the Odp sites in the carboxyl groups of PAA polymer. As compared with Osp protonated
3.1.3. H-bond network Besides the Ca-O ionic bond, the H-bonds between C-S-H and polymers also determine the interfacial structure of the nanocomposite. The H-bond formation requires that the acceptor atom and the hydrogen atom connected to the donor atom is less than 2.45 Å. As listed from Table .2, after the polymers intercalated in interlayer region of CS-H, there are four kinds of H-bonds for polymer connecting with the calcium silicate substrates. On average, each PEG polymer can donate 1.11 H-bonds to neighboring silicate oxygen atoms, accept 0.47 Hbonds from Si-OH or Ca-OH, and accept 0.34 H-bonds to surrounding water molecules. And the number of Op-d-Os H-bond have predominated ratio of all the H-bonds in C-S-H/PEG. It implies that the hydrogen atoms (Hcp) in the –CH2− of the silicate chains that are more likely to form H-bonds with oxygen atoms in the silicon chain. For all the polymers, the H-bond formed between polymer and calcium silicate substrate occupies more than 80% of the total H-bonds, while H-bonds connected between polymer and water only take account of less than 20%. It reflects the hydrophobic nature of the polymers that have strong repulsive force with confined water molecules [64]. It should be
Fig. 5. Radial distribution function of (a) Cas-Op (c) Caw-Op in the nanocomposites.
437
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 6. Overall view of calcium silicate hydrates with (a) PEG (b) PVA (c) PAA intercalated and enlarged snapshots of the Ca-Op connection. Table 2 Average number of hydrogen bonds per polymer (1Op-a-Os,Op accept Ho from hydroxyl groups; 2 Op-d-Os, Op donate Hp to Os from the silicate chains and Ho from hydroxyl groups; 3 Op-a-Ow, Op accept Hw from water molecule; 4 Op-dOw, Op donate Hp to Ow from the water molecule.). Number
C-S-H/PEG
C-S-H/PVA
C-S-H/PAA
1Op-a-Os 2Op-d-Os 3Op-a-Ow 4Op-d-Ow Op Total
0.47 1.11 0.34 0 1.92
0.21 1.19 0.06 0.01 1.47
0.58 0.86 0.19 0.07 1.70
Fig. 7. Overall view of calcium silicate hydrates with (a) PEG (b) PVA (c) PAA intercalated and enlarged snapshots of the Os-Hp connection.
PVA < C-S-H/PAA < C-S-H/PEG. The relative smaller H-bond number in C-S-H/PVA is due to that almost no H-bonds are formed between oxygen in PVA and water molecules. It can be observed in Fig. 7 that the H-bonds are widely connected between hydroxyl groups in C-S-H and oxygen atoms in the polymer chains.
noted that the interactions between water and polymer mainly come from polymer oxygen accepting H-bonds from neighboring water. There is almost no polymer donating their hydrogen to water molecules. In all three C-S-H/polymer nanocomposites, the average total Hbond number of each polymer is ranked in the following order: C-S-H/ 438
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 9. The MSD evolution for (a) Caw, Cas, Si, Ow, Oh, Op in C-S-H/PVA and (b) Ow in 1000Ps.
Fig. 8. TCF of (a) Ca-Op and (b) Os-Hp bonds in the nanocomposites.
3.2. Dynamics behavior of C-S-H structure and nanocomposites
the main link between organic and inorganic components. Fig. 8 (b) demonstrates the TCF evolution of Os-Hp in the nanocomposites. As compared with ionic Ca-O bonds, the TCF of H-bond degrades faster, implying the relative weaker H-bond strength. All the Os-Hp bonds from the weakest to the strongest in this simulation rank in the following order: C-S-H/PVA < C-S-H/PAA < C-S-H/PEG. It should be noted that the H-bond number of PEG is the highest in three composites. Although H-bonds are relative weaker interactions, they still change the distribution of interlayer region of C-S-H, and help enhance mechanical properties in nanocomposite of C-S-H/PEG. In addition, the stability of the chemical bonds is strongly related with the mobility of atoms in the system. The mean square displacement (MSD), a parameter to evaluate the translational motion behavior of the atoms in the C-S-H gel, is defined as the following equation.
In previous section, the Ca-O bonds and H-bonds are considered as important interfacial connections between C-S-H gel and different polymers. The stability of various chemical bonds, described by the time correlated function (TCF), is calculated by the following equation [65]:
Where takes either one or zero, at time t, if the pairs are connected, the value is equal to one, otherwise equal to zero. The chemical bonds break down and reform as time passes, which results in the value variation of the TCF from 1 to 0. In other words, C(t) represents the degree of bond pairs that remained bonded as the system evolves. The fast decay of TCF indicates the weak bonding. In view of Cas-Op bond TCF in Fig. 8 (a), the decay of Ca-O TCF remains very slow during, suggesting the high strength of Ca-O ionic bonds. The bond strength of Ca-O is ranked in the following order: C-S-H/PEG < C-S-H/ PVA < C-S-H/PAA. It has been known from the previous analysis that the number of coordination calcium atoms of PAA is the most among all the polymer composites. Considering large number of Ca-O connections and high bond strength, it can be concluded that Ca-Op bonds are the primary source of cohesion between PAA and silicate chains, acting as
MSD (t ) = (|ri (t ) − ri (0)|2 )
(2)
Where ri (0) is the original position for atom i and ri (t) represents the position for atom i at time t. MSD (t) describes that atoms deviate from their initial position as the function of time. Fig. 9 (a) shows the MSD evolution as a function of time for Cas, Caw, Ow and Oh. The MSD of Cas and Si atoms remain at relative lower value, implying the stable skeleton role in the C-S-H gel. In the polymer composite, the MSD for the oxygen atoms in polymers and the Ow atoms indicates faster movement of interlayer species, including the rotation and vibration of the polymer chains and their branch structures. In Fig. 9 (b), the mobility of 439
Composites Part B 162 (2019) 433–444
D. Hou et al.
Table 3 Young's modulus and bulk modulus of the pure calcium silicate hydrates and nanocomposites.
Young's modulus(GPa)-z Tensile strength(GPa) Failure strain
C-S-H/PEG
C-S-H/PVA
C-S-H/PAA
C-S-H
37.83 1.80 0.25
41.52 1.97 0.30
45.96 2.11 0.35
37.59 1.77 0.21
Fig. 11. Nonbond interaction energies Etotal, van der waals potentials Evdw and electrostatic potentials Eelec between each polymer chain(PEG, PVA and PAA) and substrate.
Fig. 12. Nonbond interaction energies Etotal evolution between each polymer chain(PEG, PVA and PAA) and substrate in the tensile process of z directions.
3.3. Mechanical properties Based on the structural and dynamical analysis described in previous sections, the mechanical properties of the C-S-H gel and nanocomposites are systematically investigated by uniaxial tension test. 3.3.1. Stress-strain relation and Young's modulus Stress-strain relation in the whole tensile loading process is used to access the mechanical performance of the C-S-H structure. Stress-strain curves for tension loading in the z direction of the four models are plotted in Fig. 10. It is well known that due to the weak H-bond connections, the C-S-H gel is more likely to be broken in the z direction when it is subjected to tensile loading [16]. During the tension process along z direction, the stress-strain relation includes three stages: the stress increases to maximum value of 2.0 Gpa at the strain of 0.06, and slowly decreases to nearly 1.5 GPa at 0.14, and finally rapidly degrade
Fig. 10. Stress-strain relations of (a) C-S-H with C-S-H/PEG (b) C-S-H with C-SH/PVA (c) C-S-H with C-S-H/PAA under tension loading along z direction.
water molecules confined in C-S-H/PVA nanocomposite is slightly faster than that in the other two composites. As discussed in previous section, there are almost no H-bonds formed between oxygen in PVA and water molecules. Hence, the moving ability of water is enhanced in the interlayer region with more space.
440
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 13. Snapshots of (a) C-S-H/PEG nanocomposites and (b) pure C-S-H in z direction tensile process (green balls are calcium ions, red and yellow sticks are silicate tetrahedra, white and red lines are water molecules, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, and red for oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
strength.
to zero at the strain of 0.2. The short post-failure regime indicates the brittle feature of the C-S-H gel in the interlayer direction. The incorporation of PEG in the C-S-H gel pronouncedly changes the third stage of the stress-strain relation and results in the rebound of the stress, significantly elongating the ductility of the material. It should be noted that the ductility is improved in all the three nanocomposites, with the C-S-H/PAA slightly changing. In previous study, a series of cementitious composites with high toughness and flexural strength was obtained by melt-dispersing ultra-high molecular weight PEG into a cement matrix. The tensile test of the PEG/cement composite also showed the slow ladder-like decreasing stage in the post-failure stage [66]. It can be explained by the enhanced adhesion between polymers and cement matrix. The ductility enhancement mechanism at molecular level will be further discussed in the following section. The Young's modulus and tensile strength are important parameters for the mechanical properties for C-S-H gels that can be obtained from the stress-strain curves. As shown in Table 3, the Young's modulus is improved in all the three nanocomposites, and the C-S-H/PAA exhibits the highest value. Moreover, the tensile strength and failure strain are also slightly improved in the nanocomposites compared with pure C-SH. Similarly, the C-S-H/PAA and C-S-H/PVA exhibit the higher tensile
3.4. Driving forces of conformational and mechanical properties change The different configurations and dynamics behavior of nanocomposites could be ascribed to the changing interactions of polymer and substrate. Here, the interactions between polymer and substrate are calculated, to rationalize the driving forces controlling conformational change of polymer. 3.4.1. Interactions between polymer and substrate The nonbond interaction (Etotal) including electrostatic (Eelec) and van der waals (Evdw) interactions [67]. The interaction energy is calculated in Fig. 11. The average adsorption energy of PEG, PVA and PAA is −104.35 kcal/mol, −159.53 kcal/mol, −199.86 kcal/mol respectively, which confirms that PAA has the strongest interaction with the substrate surface. And the maximum adsorption energy of C-S-H/PAA is consistent with the higher tensile strength. As for each polymer, it can be found that the Etotal is approximately equal to the Evdw, and the Eelec is very small, which implicates that the interaction between the polymer and substrate is mainly contributed by van der waals potential. 441
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 14. Snapshots of (a) C-S-H/PVA and (b) C-S-H/PAA nanocomposites in z direction tensile process (green balls are calcium ions, red and yellow sticks are silicate tetrahedra, white and red lines are water molecules, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, and red for oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
tensioned along interlayer direction. As the strain proceeds to 0.4 Å/Å, the polymer bridge across the interlayer region in spite of some cracks growing along interface between polymers and C-S-H gel. As shown in Fig. 14, the configurations of C-S-H/PVA and C-S-H/ PAA nanocomposites at strain levels of 0.15, 0.25, 0.40, 0.45 Å/Å are compared in panels. It can be observed in Fig. 14 (a) that when strain is 0.15 Å/Å, local bond breakage happens between polymer and C-S-H gel, initiating the small cracks growth at the interfacial region between C-S-H and polymers. When strain reaches 0.25 Å/Å, bond breakage frequently happens in the Ca-O ionic bonds and H-bonds, and the cracks grow rapidly and coalesce to huge crack through the interlayer region. As the strain proceeds to 0.40 Å/Å, C-S-H/PAA model is completely fractured. It can be observed that even though the major percentage of the PVA chains are pulled out from the silicate surface, the terminals of the PVA chain still bridges the surface calcium ions in the neighboring calcium silicate layers, reflecting the strong bond between C-S-H and polymers. Due to the flexibility of the polymer chains, the rearrangement of the atoms in the chains can contribute to the ductility enhancement due to movement of polymers such as rotation and torsion.
The nonbond interaction evolution between the polymer and substrate in the tensile process of z direction is demonstrated in Fig. 12. It can be observed that the absolute value of Etotal in three kinds of nanocomposites gradually decrease before the strain arrives the 0.2, which means a desorption effect occurs between the polymer and substrate. On the one hand, the evolution of Etotal of C-S-H/PEG is due to the alternation of debond-stick motion. When the C-S-H/PEG is stretched in the z direction, a series of bonds breakage between the C-SH and PEG, result in the reduction of Etotal (debond). Subsequently, the fluctuation of Etotal (stick) happens due to the reformation of chemical bonds. This motioned-bond and stick behaviors increase the toughness of the nanocomposites. 3.4.2. Mechanical properties enhanced mechanisms The mechanisms on the mechanical responses with respect to the influence of polymer intercalation can be interpreted by analyzing the molecular structural evolution in the tensile process along z direction. The configurations of pure C-S-H and C-S-H/PEG nanocomposites at strain levels of 0.10, 0.25, 0.40 Å/Å are shown in Fig. 13. At the beginning, in both pure C-S-H and C-S-H/PEG nanocomposites, there are Ca-O ionic bonds and H-bonds that can connect the neighboring silicate layers. When strain reaches 0.25 Å/Å, the relatively weaker interlayer region is completely fractured in pure C-S-H. While the C-S-H/PEG model still maintains intact, with the composite structure is slightly
4. Conclusions In this work, molecular dynamics was utilized to investigate the interfacial structure, dynamics, energetics and mechanical properties of 442
Composites Part B 162 (2019) 433–444
D. Hou et al.
calcium silicate hydrates (C-S-H)-polymer nanocomposite. The conclusions can be summarized as following:
[12]
(1) In the interfacial region of the nanocomposites, Os-Ca-Op bonds play mediating roles in bridging the polymers and C-S-H. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provides plenty oxygen sites for the formation of the H-bonds, which also strengthens the connectivity of the interlayer region. (2) The interfacial binding energy is ranked in the following order: E (PAA) > E(PVA) > E(PEG). This is due to the strong polarity of PAA and PVA that can provide a lot of non-bridging oxygen sites widely distributed along the polymer chains, which has strong connection with C-S-H sheets. On the other hand, the small number of non-bridging oxygen sites of PEG, resulting in disjoining of the calcium silicate sheets and increasing the mobility of confined water in the interlayer region. (3) Because of the strengthening of H-bonds in the interfacial region and the healing of the defective silicate chains, the incorporation of polymers can inhibit the crack growth during the loading process, which both enhances the cohesive strength and ductility of the C-SH gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively.
[13]
[14] [15]
[16]
[17] [18]
[19]
[20] [21] [22]
[23]
This work sheds new light on the structural evolution and dynamics behavior of the C-S-H/polymer nanocomposites and can cause broad interest among scientists and engineers in manufacturing high-performance composite materials. And the mechanism of concrete materials toughened by polymers is revealed on the molecular scale, which can provide solutions to removing the brittleness of concrete from the genetic level.
[24] [25] [26] [27]
[28]
Acknowledgement [29]
Financial support from National Natural Science Foundation of China under Grant 51508292, 51678317, 51420105015, the China Ministry of Science and Technology under Grant 2015CB655100, Natural Science Foundation of Shandong Province under Grant ZR2017JL024, State Key Laboratory of High Performance Civil Engineering Materials under Grant 2018CEM012 are gratefully acknowledged.
[30]
[31] [32] [33]
References [34] [1] Balazs AC, Emrick T, Russell TP. Nanoparticle polymer composites: where two small worlds meet. Science 2006;314:1107–10. [2] 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 Nov 22 2017;9:41014–25. [3] Macfarlane RJ, Lee B, Jones MR, Harris N, Schatz GC, Mirkin CA. Nanoparticle superlattice engineering with DNA. Science 2011;334:204–8. [4] Sanchez C, Belleville P, Popall M, Nicole L. Applications of advanced hybrid organic–inorganic nanomaterials: from laboratory to market. Chem Soc Rev 2011;40:696–753. [5] Lu N, Li L, Sun P, Wang M, Liu Q, Lv H, Long S, Liu M. A novel method of identifying the carrier transport path in metal oxide resistive random access memory. J Phys Appl Phys 2015;48:065101. [6] Li L, Lu N, Liu M, Bässler H. General Einstein relation model in disordered organic semiconductors under quasiequilibrium. Phys Rev B 2014;90:214107. [7] Milionis A, Giannuzzi R, Bayer IS, Papadopoulou EL, Ruffilli R, Manca M, Athanassiou A. Self-cleaning organic/inorganic photo-sensors. ACS Appl Mater Interfaces 2013;5:7139–45. [8] Sun G, Chen G, Liu Z, Chen M. Preparation, crystallization, electrical conductivity and thermal stability of syndiotactic polystyrene/carbon nanotube composites. Carbon 2010;48:1434–40. [9] Hillmyer MA, Lipic PM, Hajduk DA, Almdal K, Bates FS. Self-assembly and polymerization of epoxy resin-amphiphilic block copolymer nanocomposites. J Am Chem Soc 1997;119:2749–50. [10] Laine RM, Choi J, Lee I. Organic–inorganic nanocomposites with completely defined interfacial interactions. Adv Mater 2001;13:800–3. [11] Zhou H, Chen Q, Li G, Luo S, Song T-b, Duan H-S, Hong Z, You J, Liu Y, Yang Y.
[35] [36]
[37]
[38]
[39] [40] [41] [42]
[43] [44]
443
Interface engineering of highly efficient perovskite solar cells. Science 2014;345:542–6. Minet J, Abramson S, Bresson B, Franceschini A, Van Damme H, Lequeux N. Organic calcium silicate hydrate hybrids: a new approach to cement based nanocomposites. J Mater Chem 2006;16:1379–83. Matsuyama H, Young JF. Synthesis of calcium silicate hydrate/polymer complexes: Part II. Cationic polymers and complex formation with different polymers. J Mater Res 1999;14:3389–96. Allen AJ, Thomas JJ, Jennings HM. Composition and density of nanoscale calcium–silicate–hydrate in cement. Nat Mater 2007;6:311. Hou D, Zhao T, Wang P, Li Z, Zhang J. Molecular dynamics study on the mode I fracture of calcium silicate hydrate under tensile loading. Eng Fract Mech 2014;131:557–69. Hou D, Zhang J, Li Z, Zhu Y. Uniaxial tension study of calcium silicate hydrate (C–S–H): structure, dynamics and mechanical properties. Mater Struct 2014;48:3811–24. Singh V, Khatri S, Singh R. Hydration and some other properties of polyethylene glycol modified cement products. Trans Indian Ceram Soc 2002;61:152–61. Ha J, Chae S, Chou KW, Tyliszczak T, Monteiro PJM. Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study. J Mater Sci 2011;47:976–89. Lee BY, Kim J-K, Kim J-S, Kim YY. Quantitative evaluation technique of Polyvinyl Alcohol (PVA) fiber dispersion in engineered cementitious composites. Cement Concr Compos 2009;31:408–17. Ma H, Li Z. Microstructures and mechanical properties of polymer modified mortars under distinct mechanisms. Constr Build Mater 2013;47:579–87. Monteiro PJ, Miller SA, Horvath A. Towards sustainable concrete. Nat Mater 2017;16:698. Ha J, Chae S, Chou K, Tyliszczak T, Monteiro P. Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study. J Mater Sci 2012;47:976–89. Cong X, Kirkpatrick RJ. 29Si MAS NMR study of the structure of calcium silicate hydrate. Adv Cem Base Mater 1996;3:144–56. Beaudoin JJ, Raki L, Alizadeh R. A 29Si MAS NMR study of modified C–S–H nanostructures. Cement Concr Compos 2009;31:585–90. Hu X-S, Liang R, Sun G. Super-adsorbent hydrogel for removal of methylene blue dye from aqueous solution. J Mater Chem 2018;6:17612–24. Zahn DR, Gavrila GN, Salvan G. Electronic and vibrational spectroscopies applied to organic/inorganic interfaces. Chem Rev 2007;107:1161–232. Pelisser F, Gleize P, Mikowski A. Structure and micro-nanomechanical characterization of synthetic calcium–silicate–hydrate with Poly (Vinyl Alcohol). Cement Concr Compos 2014;48:1–8. Duque-Redondo E, Manzano H, Epelde-Elezcano N, Martínez-Martínez V, LópezArbeloa I i. Molecular forces governing shear and tensile failure in clay-dye hybrid materials. Chem Mater 2014;26:4338–45. Geng G, Myers RJ, Qomi MJA, Monteiro PJM. Densification of the interlayer spacing governs the nanomechanical properties of calcium-silicate-hydrate. Sci Rep Sep 8 2017;7:10986. Zhou Y, Hou D, Geng G, Feng P, Yu J, Jiang J. Insights into the interfacial strengthening mechanisms of calcium-silicate-hydrate/polymer nanocomposites. Phys Chem Chem Phys Mar 28 2018;20:8247–66. Bitinis N, Hernández M, Verdejo R, Kenny JM, Lopez‐Manchado M. Recent advances in clay/polymer nanocomposites. Adv Mater 2011;23:5229–36. Sikdar D, Katti DR, Katti KS, Bhowmik R. Insight into molecular interactions between constituents in polymer clay nanocomposites. Polymer 2006;47:5196–205. Katti KS, Sikdar D, Katti DR, Ghosh P, Verma D. Molecular interactions in intercalated organically modified clay and clay–polycaprolactam nanocomposites: experiments and modeling. Polymer 2006;47:403–14. Zeng Q, Yu A, Lu G, Standish R. Molecular dynamics simulation of the structural and dynamic properties of dioctadecyldimethyl ammoniums in organoclays. J Phys Chem B 2004;108:10025–33. Paul D, Zeng Q, Yu A, Lu G. The interlayer swelling and molecular packing in organoclays. J Colloid Interface Sci 2005;292:462–8. Zeng Q, Yu A, Lu G, Standish R. Molecular dynamics simulation of organic− inorganic nanocomposites: layering behavior and interlayer structure of organoclays. Chem Mater 2003;15:4732–8. Greenwell HC, Harvey MJ, Boulet P, Bowden AA, Coveney PV, Whiting A. Interlayer structure and bonding in nonswelling primary amine intercalated clays. Macromolecules 2005;38:6189–200. Sikdar D, Katti DR, Katti KS. The role of interfacial interactions on the crystallinity and nanomechanical properties of clay–polymer nanocomposites: a molecular dynamics study. J Appl Polym Sci 2008;107:3137–48. 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. Hu X-S, Liang R, Li J, Liu Z, Sun G. Highly stretchable self-healing nanocomposite hydrogel reinforced by 5 nm particles. ES Materials & Manufacturing; 2018. Shahsavari R, Pellenq RJ, Ulm FJ. Empirical force fields for complex hydrated calcio-silicate layered materials. Phys Chem Chem Phys Jan 21 2011;13:1002–11. Pellenq RJ, Kushima A, Shahsavari R, Van Vliet KJ, Buehler MJ, Yip S, Ulm FJ. A realistic molecular model of cement hydrates. Proc Natl Acad Sci U S A Sep 22 2009;106:16102–7. Bonnaud PA, Ji Q, Coasne B, Pellenq RJ, Van Vliet KJ. Thermodynamics of water confined in porous calcium-silicate-hydrates. Langmuir Aug 7 2012;28:11422–32. Youssef M, Pellenq RJ, Yildiz B. Glassy nature of water in an ultraconfining disordered material: the case of calcium-silicate-hydrate. J Am Chem Soc Mar 2 2011;133:2499–510.
Composites Part B 162 (2019) 433–444
D. Hou et al.
[45] Manzano H, Moeini S, Marinelli F, Van Duin AC, Ulm F-J, Pellenq RJ-M. Confined water dissociation in microporous defective silicates: mechanism, dipole distribution, and impact on substrate properties. J Am Chem Soc 2012;134:2208–15. [46] Ji Q, Pellenq RJM, Van Vliet KJ. Comparison of computational water models for simulation of calcium–silicate–hydrate. Comput Mater Sci 2012;53:234–40. [47] Bonnaud PA, Ji Q, Coasne B, Pellenq RJM, Van Vliet KJ. Thermodynamics of water confined in porous calcium-silicate-hydrates. Langmuir 2012;28:11422–32. [48] Randall J-JL, Cygan T, Kalinichev Andrey G. Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. J Phys Chem B 2003;108:1255–66. [49] 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. [50] 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. Protein Struct Func Bioinf 1988;4:31–47. [51] Greathouse JAA, Allendorf MD. Force field validation for molecular dynamics simulations of IRMOF-1 and other isoreticular zinc carboxylate coordination polymers. J Phys Chem C 2008;112:5795–802. [52] Shahsavari R. Hierarchical modeling of structure and mechanics of cement hydrate. Massachusetts Institute of Technology; 2011. [53] Hou D, Ma H, Zhu Y, Li Z. Calcium silicate hydrate from dry to saturated state: structure, dynamics and mechanical properties. Acta Mater 2014;67:81–94. [54] Murray S, Subramani V, Selvam R, Hall K. Molecular dynamics to understand the mechanical behavior of cement paste. Transportation research record: journal of the transportation research board. 2010. p. 75–82. [55] Hamid S. The crystal structure of the 11 Ä natural tobermorite Ca2. 25 [Si3O7. 5 (OH) 1.5]· 1H2O. Z für Kristallogr - Cryst Mater 1981;154:189–98. [56] Matsuyama H, Young JF. Intercalation of polymers in calcium silicate hydrate: a new synthetic approach to biocomposites? Chem Mater 1999;11:16–9.
[57] Beaudoin JJ, Dramé H, Raki L, Alizadeh R. Formation and properties of CSH–PEG nano-structures. Mater Struct 2009;42:1003–14. [58] Hou D, Zhao T, Ma H, Li Z. Reactive molecular simulation on water confined in the nanopores of the calcium silicate hydrate gel: structure, reactivity, and mechanical properties. J Phys Chem C 2015/01/22 2015;119:1346–58. [59] Hou D, Ma H, Li Z, Jin Z. Molecular simulation of “hydrolytic weakening”: a case study on silica. Acta Mater 2014;80:264–77. [60] Hou D, Yu J, Jin Z, Hanif A. Molecular dynamics study on calcium silicate hydrate subjected to tension loading and water attack: structural evolution, dynamics degradation and reactivity mechanism. Phys Chem Chem Phys 2018;20:11130–44. [61] Plimpton S, Thompson A, Crozier P, Kohlmeyer A. LAMMPS molecular dynamics simulator. 2001. 2014. [62] Hou D, Yang T, Tang J, Li S. Reactive force-field molecular dynamics study on graphene oxide reinforced cement composite: functional group de-protonation, interfacial bonding and strengthening mechanism. Phys Chem Chem Phys 2018;20:8773–89. [63] Hou D, Yang T. A reactive molecular dynamics study of graphene oxide sheets in different saturated states: structure, reactivity and mechanical properties. Phys Chem Chem Phys 2018;20:11053–66. [64] Zhou Y, Hou D, Jiang J, She W, Li J. Molecular dynamics study of solvated aniline and ethylene glycol monomers confined in calcium silicate nanochannels: a case study of tobermorite. Phys Chem Chem Phys 2017;19:15145–59. [65] Hou D, Li Z, Zhao T. Reactive force field simulation on polymerization and hydrolytic reactions in calcium aluminate silicate hydrate (C–A–S–H) gel: structure, dynamics and mechanical properties. RSC Adv 2015;5:448–61. [66] Sun G, Liang R, Zhang J, Li Z, Weng L-T. Mechanism of cement paste reinforced by ultra-high molecular weight polyethylene powder and thermotropic liquid crystalline copolyester fiber with enhanced mechanical properties. Cement Concr Compos 2017;78:57–62. [67] Zhong J, Wang X, Du J, Wang L, Yan Y, Zhang J. Combined molecular dynamics and quantum mechanics study of oil droplet adsorption on different self-assembly monolayers in aqueous solution. J Phys Chem C 2013;117:12510–9.
444
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Molecular dynamics modeling of the structure, dynamics, energetics and mechanical properties of cement-polymer nanocomposite
T
Dongshuai Houa,b,∗, Jiao Yua, Pan Wanga,b,∗∗ a b
Department of Civil Engineering, Qingdao University of Technology, Qingdao, China Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao, 266033, China
A B S T R A C T
Efforts to tune the performance of organic/inorganic composites are hindered owing to a lack of knowledge related to the interfacial interaction mechanisms. Here we investigated the interfacial structure, dynamics, energetics and mechanical properties between calcium silicate hydrates (C-S-H) and polymers by molecular dynamics (MD) simulation. In this work, polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyacrylic acid (PAA) are intercalated into nanometer channel of CS-H sheets to construct the model of polymer/C-S-H composite. In the interfacial region, the calcium ions near the surface of C-S-H play mediating role in bridging the functional groups in the polymers and oxygen in the silicate chains by forming Os-Ca-Op bond. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provide plenty oxygen sites to form H-bonds with silicate hydroxyl, interlayer water and calcium hydroxyl in C-S-H substrate. The interfacial binding energy is dependent on polarity of functional groups in the polymers, the stability of the H-bond and CaO bond, ranking in the following order: E(PAA) > E(PVA) > E(PEG). The PVA with small number of H-bonds formed between oxygen in PVA and water molecules, resulting in increasing the mobility of confined water in the interlayer region. On the other hand, PAA and PVA, with strong polarity, can provide more number of non-bridging oxygen sites that widely distributed along the polymer chains to associate with more calcium ions and H-bonds. Furthermore, uniaxial tensile test is utilized to study the mechanical behavior of the composites. The incorporation of polymers, strengthening the H-bonds in the interfacial region and healing the defective silicate chains, can inhibit the crack growth during the loading process, which both enhance the cohesive strength and ductility of the C-S-H gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively. The toughening mechanism in this organic/inorganic system can provide useful guidelines for polymer selection, design, and fabrication of C-S-H/polymer nanocomposites, and help eliminate the brittleness of cement-based materials from the genetic level.
1. Introduction The advantage in the application of inorganic-organic composite has promoted the development in numerous fields, such as industrial, biological, aviation, medicine, engineering and geological systems [1–8]. The first step to enhance the performance of nanocomposites is to understand the complex interaction mechanism between inorganic and organic composite [2,9–11]. In recent years, calcium silicate hydrates (C-S-H)/polymer nanocomposites have attracted substantial academic and industrial interest [12,13]. C-S-H accounts for 60%–70% of cement hydration production, which is the main binding phase of concrete [14]. However, cement-based concrete is a brittle material with low tensile and flexural strength [15,16], which results in easily deterioration of material when it subjects to external loading and limits the application of cement-based materials in engineering. C-S-H/ polymer nanocomposites combine the advantages of the organic polymer (e.g., flexibility, dielectric, ductility, and processability) and the C-S-H (e.g., rigidity, thermal stability) [17–20]. Because C-S-H gels
∗
are an amorphous structure, the doping of the polymer further increases the complexity of the structure [21]. And the study of the interfacial interactions between these phases at nanoscale is still incomplete. Therefore, it is necessary to study the interfacial interaction mechanisms between C-S-H and polymers. The role of intercalated polymer has been investigated by various experimental techniques [13,22]. The average length of silicon chains of C-S-H and C-S-H/polymer have been obtained by NMR [23]. And the change of silica polymerization degree of pure C-S-H modified by polyethylene glycol (PEG), polyvinylalcohol (PVA) and polyacrylic acid (PAA) [24] are reported by 29Si MAS NMR [25]. With a combination of attenuated total reflectance–Fourier transform infrared (ATR–FTIR) and X-ray diffraction (XRD), it has been found that specific surface area increased when the intercalation of polymers into the C-S-H gels [22]. Based on advanced experimental techniques, some interfacial information can be captured and the possible interaction mechanisms have been concluded [26]. However, investigating the interaction mechanisms of inorganic-organic composite by experiment alone is
Corresponding author. Department of Civil Engineering, Qingdao University of Technology, Qingdao, China. Corresponding author. Department of Civil Engineering, Qingdao University of Technology, Qingdao, China. E-mail addresses: [email protected] (D. Hou), [email protected] (P. Wang).
∗∗
https://doi.org/10.1016/j.compositesb.2018.12.142 Received 1 August 2018; Received in revised form 23 December 2018; Accepted 30 December 2018 Available online 02 January 2019 1359-8368/ © 2019 Elsevier Ltd. All rights reserved.
Composites Part B 162 (2019) 433–444
D. Hou et al.
challenging due to the certain limitations, such as the resolution of the detectors and the material purity [27]. Computational methods are still necessary to understand the structure and dynamic properties of nanocomposites and interpret the interaction mechanisms [28]. Molecular dynamics have been used to elucidate and reveal the comprehensive interfacial mechanisms between polymers and C-S-H. The interlayer stiffening mechanism in the z direction when C-S-H samples subjected to hydrostatic pressure has been explained [29]. Zhou et al. evaluate the influence of polymer intercalation by Reaxff. Moreover, the influence of strain rate and tensile direction on the ductility and other mechanical properties is explored [30]. There are also many researches on clay-polymer systems [31–33]. Zeng et al. investigate the density distribution, conformation and polymer chain title angle of clay-polymer system [34–36]. And the interlayer spacing of clay largely depends on the number of intercalated polymer [34,37]. Debashis et al. [38] reveal that the critical factor of increasing the nanomechanical properties of polymer/clay nanocomposites is the interactions between polymers, intercalated clay and organic modifiers. However, the difference of molecular structure between clay and C-S-H will lead to the different strengthening mechanism. For C-S-H/polymer nanocomposites, the simulation of structural evolution, dynamics properties are still missing, the mechanical behaviors and mechanism interpretation are still incomplete. Therefore, the primary purpose of this study is to investigate the microstructure evolution and mechanical properties of C-S-H/polymer nanocomposites. Moreover, uniaxial tension testing was performed on the C-S-H/polymer models to obtain the mechanical properties, such as Young's modulus and bulk modulus. And the intrinsic mechanism of the nanocomposites' extraordinary mechanical properties is revealed. Understanding the origin of the improved mechanical properties [39,40] of C-S-H/polymer nanocomposites is of great significance to the design and development for engineering applications. This makes it possible to select polymer that compatible with C-S-H to produce materials with desirable structures. 2. Simulation method 2.1. Force field In this simulation, the empirical force field is employed to model the interactions between C-S-H and polymer. CSHFF force field (a ClayFFlike potential) [41] was unitized to simulate the C-S-H gel. CSHFF force field has been widely used in the C-S-H gel simulation and successfully applied to describe the structure, energy and mechanical properties of various calcium silicate phases [42–47]. Potential parameters for polymers were taken from the consistent valence force field (CVFF) [48], which was widely used in the proteins, peptides, and other organic system. And it has been proved to be quite suitable for modeling organic crystals with low molecular weight, such as carboxylic acids and amides [49–51]. CVFF could also describe the potential states related to bond lengths, bond angles, torsion angles and out-of-plane interactions. More details and parameters of the force fields are available in Ref. [52]. 2.2. Model construction The C-S-H model and nanocomposites are constructed based on the 11 Å tobermorite structure [53,54]. Proper amounts of bridging site silicate tetrahedral were removed to modify the Ca/Si ratio to 1.3 [55,56]. After establishing the C-S-H with Ca/Si ratio 1.3, structures of polymer chains (PEG, PVA and PAA) are added into the interlayer regions of the anhydrous C-S-H. As shown in Fig. 1, each kind of polymer chain is composed of 8 monomers. There are two different oxygen atoms and hydrogen atoms in PEG. Ofp denote oxygen atoms in the C−O−C functional groups and Hcp denote the hydrogen atoms in the –CH2−, while Ohp and Hop denote the oxygen, hydrogen atoms in the
(caption on next page)
434
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 1. Structures of polymers: (a) PEG (C16O8H34) (Ofp denote oxygen atoms in the C−O−C functional groups, Hcp denote the hydrogen atoms in the –CH2−,Ohp and Hop denote the oxygen, hydrogen atoms in the hydroxyl groups); (b) PVA (C16O8H34) (Hcp denote the hydrogen atoms in the –CH2−,Ohp and Hop denote the oxygen, hydrogen atoms in the hydroxyl groups and to be separated from the hydroxyl group in the system); (c) PAA (C23O16H32) (Osp, Odp and Hsp, denote single-bonded oxygen, double-bonded oxygen and hydrogen atoms in COOH carboxyl groups, respectively, Hcp denote the hydrogen atoms in the –CH2− and H–C−C (grey balls represents the carbon, white balls correspond to the hydrogen, and red represents the oxygen), (d) finally C-S-H gel structure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
hydroxyl groups. There are seven Ofp and one Ohp in PEG, thus Ofp atoms play a major role in the process of connecting with the C-S-H. To the contrary, there are only one oxygen atoms (Ohp) in PVA. While in PAA, there are also two different oxygen atoms, Osp, Odp denote singlebonded oxygen, double-bonded oxygen in -COOH carboxyl groups respectively. Hsp denote hydrogen atoms in -COOH carboxyl groups and Hcp is hydrogen atom connected with carbon atoms. PAA has the most branches in the three kinds of polymers. In order to be consistent with the experimental results [57], eight polymer chains are intercalated into the C-S-H [27]. Subsequently, the grand canonical Monte Carlo (GCMC) method was utilized to model the pure C-S-H and C-S-H/ploymer with water adsorption [58]. The GCMC simulation describes well the water adsorption in the microporous and mesoporous materials [59], which fix the water chemical potential to a certain value that corresponds to the bulk liquid phase with 1g/cm3 density at 300k. 1000 attempts were carried out to insert, delete, displace and rotate the water molecules in the constant volume [60]. The simulation process include equilibrium run for 108 circles and production run for 2 × 108 circles to obtain 4 samples. After hydroxylation, each of the models has a dimension of around 42 × 43 × 47 Å3 along x, y, and z directions, with total 7200-7400 atoms. The relatively large number of atoms involved in each model can guarantee the accuracy of the data for mechanisms analysis. As shown in Fig. 1 (d), the calcium sheets and silicate tetrahedra lie in the xy plane, where the defective silicate chains line up along the y direction. Along the z direction, the intralayer and interlayer regions are alternately arranged. Molecular dynamics simulations were then carried out in an isothermalisobaric (NPT) ensemble to give the structure equilibrium states. A further 1000ps NVT run was utilized to obtain the equilibrium atomic trajectories for structural and dynamic analysis. 2.3. Uniaxial tension tests The pure C-S-H gel and the three C-S-H/polymer were obtained from previous manipulation and the uniaxial tensile simulation was performed in the LAMMPS software [61]. At the beginning, the C-S-H model was first relaxed for 1000 ps by NPT ensemble at 300 K and 0 Par along x, y and z direction by using a Nose-Hoover thermostat and the Verlet integration scheme with time step of 1 fs. Subsequently, taking the tension along z direction for example, the C-S-H model was subjected to uniaxial tensile strain through gradually elongation of the C-SH model along z direction at strain rates of 0.008/ps. During the tensile process, the stress in the x and y directions was coupled to zero to consider the Poisson effect. The stress-strain curves were plotted. In addition, Young's modulus, the failure strength describing the stiffness and the interlayer cohesive force are obtained by the uniaxial tensile test. (caption on next page)
3. Results and discussions polymer nanocomposites are presented here to briefly introduce the molecular structural feature of polymer intercalated into the interlayer region of the C-S-H gel.
3.1. Molecular structures of C-S-H structure and nanocomposites The equilibrium structures of the pure C-S-H and three C-S-H/ 435
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 2. Structure of (a) calcium silicate hydrates with PEG, (b) calcium silicate hydrates with PVA and (c) calcium silicate hydrates with PAA. (Red and yellow sticks represents the silicate tetrahedra, green and purple balls correspond to the interlayered calcium atoms (Cas) and the interlayer calcium atoms (Caw) respectively, white and red lines represents water molecules and hydroxy, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, and red for oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.1.1. C-S-H molecular structures transformation The layered structure of the calcium silicate hydrates with polymers intercalated can be clearly observed in Fig. 2 (a). On the basis of the local chemical environment, the calcium atoms in the pure C-S-H gel and nanocomposites are categorized into calcium atoms in the sheets (Cas) and calcium atoms in the interlayer (Caw). The nearest neighboring oxygen atoms of the two types Ca atoms include Ow (oxygen atoms in water), Os (oxygen atoms in silicate chains), Oh (oxygen atoms in hydroxyl), and Op (the oxygen atoms from polymers). Defective silicate chains are grafted on both sides of the calcium sheet that is constructed by the calcium-oxygen polyhedral. The interlayer region is between the two adjacent calcium silicate layers, where the Caw, water molecules, hydroxyl groups and polymers are present. As shown in Fig. 3, the length in the z direction is 47.88 Å, 49.52 Å, 47.72 Å, 47.92 Å respectively for pure C-S-H gel, C-S-H/PEG, C-S-H/ PVA and C-S-H/PAA composites. The intercalated PEG polymer can increase the length in the z direction, while the length remains almost unchanged with the incorporation of PVA and PAA polymers. The intensity profiles of different atoms perpendicular to substrate plane are plotted in Fig. 4. As shown in Fig. 4 (a), the pure C-S-H has a sandwichlike structure. Sharper density distributions of Cas and Si indicate a much more ordered arrangement of the calcium silicate sheets. Additionally, Caw is only distributed between the neighboring calcium silicate sheets. However, this kind of orderly arrangement of C-S-H gel is disturbed when the polymers are incorporated into the C-S-H. The intensity of Op is distributed from 11 Å to 18 Å across the interlayer direction, which means the PEG can impregnated into the defective region of C-S-H gel. Even though the polymers are initially intercalated into the interlayer region, the polymer chains can twine the silicate skeleton, and interact with the calcium silicate sheet. As compared with that in the pure C-S-H gel, the binomial distribution for Cas becomes broadening in the intensity profile of C-S-H/PEG as depicted in Fig. 4 (b). The small shoulder of Si intensity appeared in the interlayer region represents the protruded bridging silicate tetrahedron in the silicate chains. It means that the “dier” arrangement of the silicate skeleton lose the order of bridging-pair tetrahedron distribution. The penetrated polymers, forming chemical bonds with interlayer water and calcium ions, give rise to the rearrangement of atoms in the calcium silicate sheet, which results in the transformation from order to disorder state
Fig. 4. The density profile of Cas, Si, Ow, Caw in the (a) C-S-H and Cas, Si, Ow, Caw, Op in the (b) C-S-H/PEG; (c) C-S-H/PVA; (d) C-S-H/PAA. Fig. 3. Length in the z direction for C-S-H and nanocomposites. 436
Composites Part B 162 (2019) 433–444
D. Hou et al.
Table 1 The number of the coordination calcium ions of each polyer in the nanocomposites. CN
C-S-H/PEG
C-S-H/PVA
C-S-H/PAA
Cas Caw Total
3.20 2.56 5.76
5.08 2.44 7.52
6.17 4.31 10.48
of the layered structure. The influence of other polymers on the structure of calcium silicate sheets in the interlayer region can also be found in the intensity distributions of C-S-H/PVA and C-S-H/PAA in Fig. 4 (c) and (d).
with hydrogen, the deprotonated Odp, with high polarity, occupies strong charge negativity, which can easily attract the surrounding calcium ions. It should be noted that the interfacial bonding greatly depends on the polarity of functional groups for organic-inorganic composite. In previous study, due to formation of a stronger Ca-O bond, the cohesive strength and binding energy in the graphene oxide and cement composite are significantly enhanced as the carboxyl groups in GO sheets are partly deprotonated to –COO- groups [62,63]. The average number of coordination calcium atoms of each polymer are calculated and shown in Table .1 (All the Ca-O cutoff distance is set on the basis of the location of the first minimum in RDF). In all three CS-H/polymer nanocomposites, polymers can be both coordinated by Cas and Caw. All the polymers have more Cas atoms than Caw species as their nearest neighbor. It confirms that the polymers invade into the defective regions of the silicate chains and connect with the calcium ions in the silicate sheets. The interaction between Cas-Op results in the dissociation of Cas from the calcium silicate skeleton, disturbing the layered structure. It explains the order-disorder transformation observed in Fig. 4. The total coordinate number (CN) of different polymers rank in the following order: CN (PAA) > CN (PVA) > CN (PEG). In particular, the CN for Cas atoms in C-S-H/PAA is almost twice as that of C-S-H/PEG, implying stronger interfacial chemical connection between PAA and the calcium silicate sheets. It is worth noting that the number of coordination Caw in PAA is also the most among all the polymers. This is mainly attributed to the fact that the PAA chains have large number of carboxyl branches, which are easily protruded in the silicate sheets and capture the neighboring calcium ions. On the other hand, it is hard for the calcium ions to associate with the bridging oxygen atoms in C−O−C of single PEG chains. It can be observed in Fig. 6 that the PEG, PVA and PAA polymers, with different conformation, connect with neighboring calcium silicate sheets by forming Ca-Op ionic bonds. In addition to the intrinsic structure of the polymers such as the chain length, the branch and cross-linking structure, and structure of the single chain, the flexibility of the polymers ultra-confined in the nanometer channel are influenced by the highly solvated calcium ions and the neighboring water molecules.
3.1.2. Interfacial Ca-O bond and local structure of polymers The Ca-O bond is characterized by the radial distribution function (RDF). Fig. 5 (a) and (b) illustrate the RDFs between oxygen atoms from polymer (PEG, PVA, PAA) and the Caw and Cas atoms of C−S−H. In the composite of PEG/C-S-H, the RDF of Caw-Ohp and Caw-Ofp have the first peak peak positioned at 2.53 Å and 2.55 Å, respectively. The pronounced peaks mean that the inserted PEG polymer can associate with the neighboring calcium ions by bridging oxygen in the middle chains and the OH groups at the end of chain. In addition to Caw-O connection, the oxygen atoms in the polymer can also form bond with calcium ions in the silicate sheet. The RDF of Cas-Ohp and Cas-Ofp have the first peak located at 2.39 Å and 2.50 Å, respectively. Similarly, the sharp peaks of Cas-Ohp and Caw-Ohp also indicate the stable chemical bond formation between polymer and C-S-H gel in the C-S-H/PVA composite. In the composite of C-S-H/PAA, the oxygen atoms in the polymer exhibit different affinity with the neighboring calcium ions. While the RDF of Ca-Odp has sharp peak with high intensity located at 2.50 Å, the RDF of Ca-Osp only has one small shoulder at 2.63 Å. It means that both the Cas and Caw ions energetically prefer to adsorb on the Odp sites in the carboxyl groups of PAA polymer. As compared with Osp protonated
3.1.3. H-bond network Besides the Ca-O ionic bond, the H-bonds between C-S-H and polymers also determine the interfacial structure of the nanocomposite. The H-bond formation requires that the acceptor atom and the hydrogen atom connected to the donor atom is less than 2.45 Å. As listed from Table .2, after the polymers intercalated in interlayer region of CS-H, there are four kinds of H-bonds for polymer connecting with the calcium silicate substrates. On average, each PEG polymer can donate 1.11 H-bonds to neighboring silicate oxygen atoms, accept 0.47 Hbonds from Si-OH or Ca-OH, and accept 0.34 H-bonds to surrounding water molecules. And the number of Op-d-Os H-bond have predominated ratio of all the H-bonds in C-S-H/PEG. It implies that the hydrogen atoms (Hcp) in the –CH2− of the silicate chains that are more likely to form H-bonds with oxygen atoms in the silicon chain. For all the polymers, the H-bond formed between polymer and calcium silicate substrate occupies more than 80% of the total H-bonds, while H-bonds connected between polymer and water only take account of less than 20%. It reflects the hydrophobic nature of the polymers that have strong repulsive force with confined water molecules [64]. It should be
Fig. 5. Radial distribution function of (a) Cas-Op (c) Caw-Op in the nanocomposites.
437
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 6. Overall view of calcium silicate hydrates with (a) PEG (b) PVA (c) PAA intercalated and enlarged snapshots of the Ca-Op connection. Table 2 Average number of hydrogen bonds per polymer (1Op-a-Os,Op accept Ho from hydroxyl groups; 2 Op-d-Os, Op donate Hp to Os from the silicate chains and Ho from hydroxyl groups; 3 Op-a-Ow, Op accept Hw from water molecule; 4 Op-dOw, Op donate Hp to Ow from the water molecule.). Number
C-S-H/PEG
C-S-H/PVA
C-S-H/PAA
1Op-a-Os 2Op-d-Os 3Op-a-Ow 4Op-d-Ow Op Total
0.47 1.11 0.34 0 1.92
0.21 1.19 0.06 0.01 1.47
0.58 0.86 0.19 0.07 1.70
Fig. 7. Overall view of calcium silicate hydrates with (a) PEG (b) PVA (c) PAA intercalated and enlarged snapshots of the Os-Hp connection.
PVA < C-S-H/PAA < C-S-H/PEG. The relative smaller H-bond number in C-S-H/PVA is due to that almost no H-bonds are formed between oxygen in PVA and water molecules. It can be observed in Fig. 7 that the H-bonds are widely connected between hydroxyl groups in C-S-H and oxygen atoms in the polymer chains.
noted that the interactions between water and polymer mainly come from polymer oxygen accepting H-bonds from neighboring water. There is almost no polymer donating their hydrogen to water molecules. In all three C-S-H/polymer nanocomposites, the average total Hbond number of each polymer is ranked in the following order: C-S-H/ 438
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 9. The MSD evolution for (a) Caw, Cas, Si, Ow, Oh, Op in C-S-H/PVA and (b) Ow in 1000Ps.
Fig. 8. TCF of (a) Ca-Op and (b) Os-Hp bonds in the nanocomposites.
3.2. Dynamics behavior of C-S-H structure and nanocomposites
the main link between organic and inorganic components. Fig. 8 (b) demonstrates the TCF evolution of Os-Hp in the nanocomposites. As compared with ionic Ca-O bonds, the TCF of H-bond degrades faster, implying the relative weaker H-bond strength. All the Os-Hp bonds from the weakest to the strongest in this simulation rank in the following order: C-S-H/PVA < C-S-H/PAA < C-S-H/PEG. It should be noted that the H-bond number of PEG is the highest in three composites. Although H-bonds are relative weaker interactions, they still change the distribution of interlayer region of C-S-H, and help enhance mechanical properties in nanocomposite of C-S-H/PEG. In addition, the stability of the chemical bonds is strongly related with the mobility of atoms in the system. The mean square displacement (MSD), a parameter to evaluate the translational motion behavior of the atoms in the C-S-H gel, is defined as the following equation.
In previous section, the Ca-O bonds and H-bonds are considered as important interfacial connections between C-S-H gel and different polymers. The stability of various chemical bonds, described by the time correlated function (TCF), is calculated by the following equation [65]:
Where takes either one or zero, at time t, if the pairs are connected, the value is equal to one, otherwise equal to zero. The chemical bonds break down and reform as time passes, which results in the value variation of the TCF from 1 to 0. In other words, C(t) represents the degree of bond pairs that remained bonded as the system evolves. The fast decay of TCF indicates the weak bonding. In view of Cas-Op bond TCF in Fig. 8 (a), the decay of Ca-O TCF remains very slow during, suggesting the high strength of Ca-O ionic bonds. The bond strength of Ca-O is ranked in the following order: C-S-H/PEG < C-S-H/ PVA < C-S-H/PAA. It has been known from the previous analysis that the number of coordination calcium atoms of PAA is the most among all the polymer composites. Considering large number of Ca-O connections and high bond strength, it can be concluded that Ca-Op bonds are the primary source of cohesion between PAA and silicate chains, acting as
MSD (t ) = (|ri (t ) − ri (0)|2 )
(2)
Where ri (0) is the original position for atom i and ri (t) represents the position for atom i at time t. MSD (t) describes that atoms deviate from their initial position as the function of time. Fig. 9 (a) shows the MSD evolution as a function of time for Cas, Caw, Ow and Oh. The MSD of Cas and Si atoms remain at relative lower value, implying the stable skeleton role in the C-S-H gel. In the polymer composite, the MSD for the oxygen atoms in polymers and the Ow atoms indicates faster movement of interlayer species, including the rotation and vibration of the polymer chains and their branch structures. In Fig. 9 (b), the mobility of 439
Composites Part B 162 (2019) 433–444
D. Hou et al.
Table 3 Young's modulus and bulk modulus of the pure calcium silicate hydrates and nanocomposites.
Young's modulus(GPa)-z Tensile strength(GPa) Failure strain
C-S-H/PEG
C-S-H/PVA
C-S-H/PAA
C-S-H
37.83 1.80 0.25
41.52 1.97 0.30
45.96 2.11 0.35
37.59 1.77 0.21
Fig. 11. Nonbond interaction energies Etotal, van der waals potentials Evdw and electrostatic potentials Eelec between each polymer chain(PEG, PVA and PAA) and substrate.
Fig. 12. Nonbond interaction energies Etotal evolution between each polymer chain(PEG, PVA and PAA) and substrate in the tensile process of z directions.
3.3. Mechanical properties Based on the structural and dynamical analysis described in previous sections, the mechanical properties of the C-S-H gel and nanocomposites are systematically investigated by uniaxial tension test. 3.3.1. Stress-strain relation and Young's modulus Stress-strain relation in the whole tensile loading process is used to access the mechanical performance of the C-S-H structure. Stress-strain curves for tension loading in the z direction of the four models are plotted in Fig. 10. It is well known that due to the weak H-bond connections, the C-S-H gel is more likely to be broken in the z direction when it is subjected to tensile loading [16]. During the tension process along z direction, the stress-strain relation includes three stages: the stress increases to maximum value of 2.0 Gpa at the strain of 0.06, and slowly decreases to nearly 1.5 GPa at 0.14, and finally rapidly degrade
Fig. 10. Stress-strain relations of (a) C-S-H with C-S-H/PEG (b) C-S-H with C-SH/PVA (c) C-S-H with C-S-H/PAA under tension loading along z direction.
water molecules confined in C-S-H/PVA nanocomposite is slightly faster than that in the other two composites. As discussed in previous section, there are almost no H-bonds formed between oxygen in PVA and water molecules. Hence, the moving ability of water is enhanced in the interlayer region with more space.
440
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 13. Snapshots of (a) C-S-H/PEG nanocomposites and (b) pure C-S-H in z direction tensile process (green balls are calcium ions, red and yellow sticks are silicate tetrahedra, white and red lines are water molecules, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, and red for oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
strength.
to zero at the strain of 0.2. The short post-failure regime indicates the brittle feature of the C-S-H gel in the interlayer direction. The incorporation of PEG in the C-S-H gel pronouncedly changes the third stage of the stress-strain relation and results in the rebound of the stress, significantly elongating the ductility of the material. It should be noted that the ductility is improved in all the three nanocomposites, with the C-S-H/PAA slightly changing. In previous study, a series of cementitious composites with high toughness and flexural strength was obtained by melt-dispersing ultra-high molecular weight PEG into a cement matrix. The tensile test of the PEG/cement composite also showed the slow ladder-like decreasing stage in the post-failure stage [66]. It can be explained by the enhanced adhesion between polymers and cement matrix. The ductility enhancement mechanism at molecular level will be further discussed in the following section. The Young's modulus and tensile strength are important parameters for the mechanical properties for C-S-H gels that can be obtained from the stress-strain curves. As shown in Table 3, the Young's modulus is improved in all the three nanocomposites, and the C-S-H/PAA exhibits the highest value. Moreover, the tensile strength and failure strain are also slightly improved in the nanocomposites compared with pure C-SH. Similarly, the C-S-H/PAA and C-S-H/PVA exhibit the higher tensile
3.4. Driving forces of conformational and mechanical properties change The different configurations and dynamics behavior of nanocomposites could be ascribed to the changing interactions of polymer and substrate. Here, the interactions between polymer and substrate are calculated, to rationalize the driving forces controlling conformational change of polymer. 3.4.1. Interactions between polymer and substrate The nonbond interaction (Etotal) including electrostatic (Eelec) and van der waals (Evdw) interactions [67]. The interaction energy is calculated in Fig. 11. The average adsorption energy of PEG, PVA and PAA is −104.35 kcal/mol, −159.53 kcal/mol, −199.86 kcal/mol respectively, which confirms that PAA has the strongest interaction with the substrate surface. And the maximum adsorption energy of C-S-H/PAA is consistent with the higher tensile strength. As for each polymer, it can be found that the Etotal is approximately equal to the Evdw, and the Eelec is very small, which implicates that the interaction between the polymer and substrate is mainly contributed by van der waals potential. 441
Composites Part B 162 (2019) 433–444
D. Hou et al.
Fig. 14. Snapshots of (a) C-S-H/PVA and (b) C-S-H/PAA nanocomposites in z direction tensile process (green balls are calcium ions, red and yellow sticks are silicate tetrahedra, white and red lines are water molecules, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, and red for oxygen). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
tensioned along interlayer direction. As the strain proceeds to 0.4 Å/Å, the polymer bridge across the interlayer region in spite of some cracks growing along interface between polymers and C-S-H gel. As shown in Fig. 14, the configurations of C-S-H/PVA and C-S-H/ PAA nanocomposites at strain levels of 0.15, 0.25, 0.40, 0.45 Å/Å are compared in panels. It can be observed in Fig. 14 (a) that when strain is 0.15 Å/Å, local bond breakage happens between polymer and C-S-H gel, initiating the small cracks growth at the interfacial region between C-S-H and polymers. When strain reaches 0.25 Å/Å, bond breakage frequently happens in the Ca-O ionic bonds and H-bonds, and the cracks grow rapidly and coalesce to huge crack through the interlayer region. As the strain proceeds to 0.40 Å/Å, C-S-H/PAA model is completely fractured. It can be observed that even though the major percentage of the PVA chains are pulled out from the silicate surface, the terminals of the PVA chain still bridges the surface calcium ions in the neighboring calcium silicate layers, reflecting the strong bond between C-S-H and polymers. Due to the flexibility of the polymer chains, the rearrangement of the atoms in the chains can contribute to the ductility enhancement due to movement of polymers such as rotation and torsion.
The nonbond interaction evolution between the polymer and substrate in the tensile process of z direction is demonstrated in Fig. 12. It can be observed that the absolute value of Etotal in three kinds of nanocomposites gradually decrease before the strain arrives the 0.2, which means a desorption effect occurs between the polymer and substrate. On the one hand, the evolution of Etotal of C-S-H/PEG is due to the alternation of debond-stick motion. When the C-S-H/PEG is stretched in the z direction, a series of bonds breakage between the C-SH and PEG, result in the reduction of Etotal (debond). Subsequently, the fluctuation of Etotal (stick) happens due to the reformation of chemical bonds. This motioned-bond and stick behaviors increase the toughness of the nanocomposites. 3.4.2. Mechanical properties enhanced mechanisms The mechanisms on the mechanical responses with respect to the influence of polymer intercalation can be interpreted by analyzing the molecular structural evolution in the tensile process along z direction. The configurations of pure C-S-H and C-S-H/PEG nanocomposites at strain levels of 0.10, 0.25, 0.40 Å/Å are shown in Fig. 13. At the beginning, in both pure C-S-H and C-S-H/PEG nanocomposites, there are Ca-O ionic bonds and H-bonds that can connect the neighboring silicate layers. When strain reaches 0.25 Å/Å, the relatively weaker interlayer region is completely fractured in pure C-S-H. While the C-S-H/PEG model still maintains intact, with the composite structure is slightly
4. Conclusions In this work, molecular dynamics was utilized to investigate the interfacial structure, dynamics, energetics and mechanical properties of 442
Composites Part B 162 (2019) 433–444
D. Hou et al.
calcium silicate hydrates (C-S-H)-polymer nanocomposite. The conclusions can be summarized as following:
[12]
(1) In the interfacial region of the nanocomposites, Os-Ca-Op bonds play mediating roles in bridging the polymers and C-S-H. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provides plenty oxygen sites for the formation of the H-bonds, which also strengthens the connectivity of the interlayer region. (2) The interfacial binding energy is ranked in the following order: E (PAA) > E(PVA) > E(PEG). This is due to the strong polarity of PAA and PVA that can provide a lot of non-bridging oxygen sites widely distributed along the polymer chains, which has strong connection with C-S-H sheets. On the other hand, the small number of non-bridging oxygen sites of PEG, resulting in disjoining of the calcium silicate sheets and increasing the mobility of confined water in the interlayer region. (3) Because of the strengthening of H-bonds in the interfacial region and the healing of the defective silicate chains, the incorporation of polymers can inhibit the crack growth during the loading process, which both enhances the cohesive strength and ductility of the C-SH gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively.
[13]
[14] [15]
[16]
[17] [18]
[19]
[20] [21] [22]
[23]
This work sheds new light on the structural evolution and dynamics behavior of the C-S-H/polymer nanocomposites and can cause broad interest among scientists and engineers in manufacturing high-performance composite materials. And the mechanism of concrete materials toughened by polymers is revealed on the molecular scale, which can provide solutions to removing the brittleness of concrete from the genetic level.
[24] [25] [26] [27]
[28]
Acknowledgement [29]
Financial support from National Natural Science Foundation of China under Grant 51508292, 51678317, 51420105015, the China Ministry of Science and Technology under Grant 2015CB655100, Natural Science Foundation of Shandong Province under Grant ZR2017JL024, State Key Laboratory of High Performance Civil Engineering Materials under Grant 2018CEM012 are gratefully acknowledged.
[30]
[31] [32] [33]
References [34] [1] Balazs AC, Emrick T, Russell TP. Nanoparticle polymer composites: where two small worlds meet. Science 2006;314:1107–10. [2] 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 Nov 22 2017;9:41014–25. [3] Macfarlane RJ, Lee B, Jones MR, Harris N, Schatz GC, Mirkin CA. Nanoparticle superlattice engineering with DNA. Science 2011;334:204–8. [4] Sanchez C, Belleville P, Popall M, Nicole L. Applications of advanced hybrid organic–inorganic nanomaterials: from laboratory to market. Chem Soc Rev 2011;40:696–753. [5] Lu N, Li L, Sun P, Wang M, Liu Q, Lv H, Long S, Liu M. A novel method of identifying the carrier transport path in metal oxide resistive random access memory. J Phys Appl Phys 2015;48:065101. [6] Li L, Lu N, Liu M, Bässler H. General Einstein relation model in disordered organic semiconductors under quasiequilibrium. Phys Rev B 2014;90:214107. [7] Milionis A, Giannuzzi R, Bayer IS, Papadopoulou EL, Ruffilli R, Manca M, Athanassiou A. Self-cleaning organic/inorganic photo-sensors. ACS Appl Mater Interfaces 2013;5:7139–45. [8] Sun G, Chen G, Liu Z, Chen M. Preparation, crystallization, electrical conductivity and thermal stability of syndiotactic polystyrene/carbon nanotube composites. Carbon 2010;48:1434–40. [9] Hillmyer MA, Lipic PM, Hajduk DA, Almdal K, Bates FS. Self-assembly and polymerization of epoxy resin-amphiphilic block copolymer nanocomposites. J Am Chem Soc 1997;119:2749–50. [10] Laine RM, Choi J, Lee I. Organic–inorganic nanocomposites with completely defined interfacial interactions. Adv Mater 2001;13:800–3. [11] Zhou H, Chen Q, Li G, Luo S, Song T-b, Duan H-S, Hong Z, You J, Liu Y, Yang Y.
[35] [36]
[37]
[38]
[39] [40] [41] [42]
[43] [44]
443
Interface engineering of highly efficient perovskite solar cells. Science 2014;345:542–6. Minet J, Abramson S, Bresson B, Franceschini A, Van Damme H, Lequeux N. Organic calcium silicate hydrate hybrids: a new approach to cement based nanocomposites. J Mater Chem 2006;16:1379–83. Matsuyama H, Young JF. Synthesis of calcium silicate hydrate/polymer complexes: Part II. Cationic polymers and complex formation with different polymers. J Mater Res 1999;14:3389–96. Allen AJ, Thomas JJ, Jennings HM. Composition and density of nanoscale calcium–silicate–hydrate in cement. Nat Mater 2007;6:311. Hou D, Zhao T, Wang P, Li Z, Zhang J. Molecular dynamics study on the mode I fracture of calcium silicate hydrate under tensile loading. Eng Fract Mech 2014;131:557–69. Hou D, Zhang J, Li Z, Zhu Y. Uniaxial tension study of calcium silicate hydrate (C–S–H): structure, dynamics and mechanical properties. Mater Struct 2014;48:3811–24. Singh V, Khatri S, Singh R. Hydration and some other properties of polyethylene glycol modified cement products. Trans Indian Ceram Soc 2002;61:152–61. Ha J, Chae S, Chou KW, Tyliszczak T, Monteiro PJM. Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study. J Mater Sci 2011;47:976–89. Lee BY, Kim J-K, Kim J-S, Kim YY. Quantitative evaluation technique of Polyvinyl Alcohol (PVA) fiber dispersion in engineered cementitious composites. Cement Concr Compos 2009;31:408–17. Ma H, Li Z. Microstructures and mechanical properties of polymer modified mortars under distinct mechanisms. Constr Build Mater 2013;47:579–87. Monteiro PJ, Miller SA, Horvath A. Towards sustainable concrete. Nat Mater 2017;16:698. Ha J, Chae S, Chou K, Tyliszczak T, Monteiro P. Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study. J Mater Sci 2012;47:976–89. Cong X, Kirkpatrick RJ. 29Si MAS NMR study of the structure of calcium silicate hydrate. Adv Cem Base Mater 1996;3:144–56. Beaudoin JJ, Raki L, Alizadeh R. A 29Si MAS NMR study of modified C–S–H nanostructures. Cement Concr Compos 2009;31:585–90. Hu X-S, Liang R, Sun G. Super-adsorbent hydrogel for removal of methylene blue dye from aqueous solution. J Mater Chem 2018;6:17612–24. Zahn DR, Gavrila GN, Salvan G. Electronic and vibrational spectroscopies applied to organic/inorganic interfaces. Chem Rev 2007;107:1161–232. Pelisser F, Gleize P, Mikowski A. Structure and micro-nanomechanical characterization of synthetic calcium–silicate–hydrate with Poly (Vinyl Alcohol). Cement Concr Compos 2014;48:1–8. Duque-Redondo E, Manzano H, Epelde-Elezcano N, Martínez-Martínez V, LópezArbeloa I i. Molecular forces governing shear and tensile failure in clay-dye hybrid materials. Chem Mater 2014;26:4338–45. Geng G, Myers RJ, Qomi MJA, Monteiro PJM. Densification of the interlayer spacing governs the nanomechanical properties of calcium-silicate-hydrate. Sci Rep Sep 8 2017;7:10986. Zhou Y, Hou D, Geng G, Feng P, Yu J, Jiang J. Insights into the interfacial strengthening mechanisms of calcium-silicate-hydrate/polymer nanocomposites. Phys Chem Chem Phys Mar 28 2018;20:8247–66. Bitinis N, Hernández M, Verdejo R, Kenny JM, Lopez‐Manchado M. Recent advances in clay/polymer nanocomposites. Adv Mater 2011;23:5229–36. Sikdar D, Katti DR, Katti KS, Bhowmik R. Insight into molecular interactions between constituents in polymer clay nanocomposites. Polymer 2006;47:5196–205. Katti KS, Sikdar D, Katti DR, Ghosh P, Verma D. Molecular interactions in intercalated organically modified clay and clay–polycaprolactam nanocomposites: experiments and modeling. Polymer 2006;47:403–14. Zeng Q, Yu A, Lu G, Standish R. Molecular dynamics simulation of the structural and dynamic properties of dioctadecyldimethyl ammoniums in organoclays. J Phys Chem B 2004;108:10025–33. Paul D, Zeng Q, Yu A, Lu G. The interlayer swelling and molecular packing in organoclays. J Colloid Interface Sci 2005;292:462–8. Zeng Q, Yu A, Lu G, Standish R. Molecular dynamics simulation of organic− inorganic nanocomposites: layering behavior and interlayer structure of organoclays. Chem Mater 2003;15:4732–8. Greenwell HC, Harvey MJ, Boulet P, Bowden AA, Coveney PV, Whiting A. Interlayer structure and bonding in nonswelling primary amine intercalated clays. Macromolecules 2005;38:6189–200. Sikdar D, Katti DR, Katti KS. The role of interfacial interactions on the crystallinity and nanomechanical properties of clay–polymer nanocomposites: a molecular dynamics study. J Appl Polym Sci 2008;107:3137–48. 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. Hu X-S, Liang R, Li J, Liu Z, Sun G. Highly stretchable self-healing nanocomposite hydrogel reinforced by 5 nm particles. ES Materials & Manufacturing; 2018. Shahsavari R, Pellenq RJ, Ulm FJ. Empirical force fields for complex hydrated calcio-silicate layered materials. Phys Chem Chem Phys Jan 21 2011;13:1002–11. Pellenq RJ, Kushima A, Shahsavari R, Van Vliet KJ, Buehler MJ, Yip S, Ulm FJ. A realistic molecular model of cement hydrates. Proc Natl Acad Sci U S A Sep 22 2009;106:16102–7. Bonnaud PA, Ji Q, Coasne B, Pellenq RJ, Van Vliet KJ. Thermodynamics of water confined in porous calcium-silicate-hydrates. Langmuir Aug 7 2012;28:11422–32. Youssef M, Pellenq RJ, Yildiz B. Glassy nature of water in an ultraconfining disordered material: the case of calcium-silicate-hydrate. J Am Chem Soc Mar 2 2011;133:2499–510.
Composites Part B 162 (2019) 433–444
D. Hou et al.
[45] Manzano H, Moeini S, Marinelli F, Van Duin AC, Ulm F-J, Pellenq RJ-M. Confined water dissociation in microporous defective silicates: mechanism, dipole distribution, and impact on substrate properties. J Am Chem Soc 2012;134:2208–15. [46] Ji Q, Pellenq RJM, Van Vliet KJ. Comparison of computational water models for simulation of calcium–silicate–hydrate. Comput Mater Sci 2012;53:234–40. [47] Bonnaud PA, Ji Q, Coasne B, Pellenq RJM, Van Vliet KJ. Thermodynamics of water confined in porous calcium-silicate-hydrates. Langmuir 2012;28:11422–32. [48] Randall J-JL, Cygan T, Kalinichev Andrey G. Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. J Phys Chem B 2003;108:1255–66. [49] 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. [50] 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. Protein Struct Func Bioinf 1988;4:31–47. [51] Greathouse JAA, Allendorf MD. Force field validation for molecular dynamics simulations of IRMOF-1 and other isoreticular zinc carboxylate coordination polymers. J Phys Chem C 2008;112:5795–802. [52] Shahsavari R. Hierarchical modeling of structure and mechanics of cement hydrate. Massachusetts Institute of Technology; 2011. [53] Hou D, Ma H, Zhu Y, Li Z. Calcium silicate hydrate from dry to saturated state: structure, dynamics and mechanical properties. Acta Mater 2014;67:81–94. [54] Murray S, Subramani V, Selvam R, Hall K. Molecular dynamics to understand the mechanical behavior of cement paste. Transportation research record: journal of the transportation research board. 2010. p. 75–82. [55] Hamid S. The crystal structure of the 11 Ä natural tobermorite Ca2. 25 [Si3O7. 5 (OH) 1.5]· 1H2O. Z für Kristallogr - Cryst Mater 1981;154:189–98. [56] Matsuyama H, Young JF. Intercalation of polymers in calcium silicate hydrate: a new synthetic approach to biocomposites? Chem Mater 1999;11:16–9.
[57] Beaudoin JJ, Dramé H, Raki L, Alizadeh R. Formation and properties of CSH–PEG nano-structures. Mater Struct 2009;42:1003–14. [58] Hou D, Zhao T, Ma H, Li Z. Reactive molecular simulation on water confined in the nanopores of the calcium silicate hydrate gel: structure, reactivity, and mechanical properties. J Phys Chem C 2015/01/22 2015;119:1346–58. [59] Hou D, Ma H, Li Z, Jin Z. Molecular simulation of “hydrolytic weakening”: a case study on silica. Acta Mater 2014;80:264–77. [60] Hou D, Yu J, Jin Z, Hanif A. Molecular dynamics study on calcium silicate hydrate subjected to tension loading and water attack: structural evolution, dynamics degradation and reactivity mechanism. Phys Chem Chem Phys 2018;20:11130–44. [61] Plimpton S, Thompson A, Crozier P, Kohlmeyer A. LAMMPS molecular dynamics simulator. 2001. 2014. [62] Hou D, Yang T, Tang J, Li S. Reactive force-field molecular dynamics study on graphene oxide reinforced cement composite: functional group de-protonation, interfacial bonding and strengthening mechanism. Phys Chem Chem Phys 2018;20:8773–89. [63] Hou D, Yang T. A reactive molecular dynamics study of graphene oxide sheets in different saturated states: structure, reactivity and mechanical properties. Phys Chem Chem Phys 2018;20:11053–66. [64] Zhou Y, Hou D, Jiang J, She W, Li J. Molecular dynamics study of solvated aniline and ethylene glycol monomers confined in calcium silicate nanochannels: a case study of tobermorite. Phys Chem Chem Phys 2017;19:15145–59. [65] Hou D, Li Z, Zhao T. Reactive force field simulation on polymerization and hydrolytic reactions in calcium aluminate silicate hydrate (C–A–S–H) gel: structure, dynamics and mechanical properties. RSC Adv 2015;5:448–61. [66] Sun G, Liang R, Zhang J, Li Z, Weng L-T. Mechanism of cement paste reinforced by ultra-high molecular weight polyethylene powder and thermotropic liquid crystalline copolyester fiber with enhanced mechanical properties. Cement Concr Compos 2017;78:57–62. [67] Zhong J, Wang X, Du J, Wang L, Yan Y, Zhang J. Combined molecular dynamics and quantum mechanics study of oil droplet adsorption on different self-assembly monolayers in aqueous solution. J Phys Chem C 2013;117:12510–9.
444