Polymer Nanocomposite Cluster

Article pubs.acs.org/JPCC

Interaction of Silica Nanoparticle/Polymer Nanocomposite Cluster Network Structure: Revisiting the Reinforcement Mechanism Jun Yang,*,†,‡ Chun-Rui Han,‡ Jiu-Fang Duan,‡ Feng Xu,†,‡ and Run-Cang Sun†,‡ †

Beijing Key Laboratory of Lignocellulosic Chemistry and ‡College of Materials Science and Technology, Beijing Forestry University, Beijing, China S Supporting Information *

ABSTRACT: The understanding of nanoparticle−polymer interaction is a key element to demystify the structure−property relationship for nanocomposites. The clusters composing poly(acrylamide) (PAM) grafted from silica nanoparticles (SNPs) were prepared according to the developed synthetic platform and analyzed by transmission electron microscopy and dielectric relaxation analysis. The morphological evolution of clusters was observed and correlated to the mechanical behaviors of nanocomposites. Dynamic dielectric analysis was conducted to examine the nature of the constrained polymer region in view of a reinforcement mechanism. The modulus enhancement of the nanocomposite hydrogels was found to correlate with the volume of constrained polymer chains, and a constrained region model for SNP/polymer nanocomposites was proposed. The strong interactions between SNPs and polymer chains affect the modulus of the nanocomposites that was predicted by the percolation model and contributed to the mechanical reinforcement. The interplay between filler−polymer interaction and network rearrangement during the deformations should be considered to propose the reinforcement mechanism.

■

INTRODUCTION Nanocomposites, composed inorganic and organic building blocks, could combine the properties from the parent constituents and generate new properties.1−4 To obtain the targeted functional nanocomposites, it is requisite to select the situate nanoparticles as building blocks as well as control the spatial arrangement of these nanoparticles over multiple length scales.5−7 Depending on the structural characteristics and modes of interaction among different nanoparticles, the selfassembly approach provides an opportunity to arrange the interparticle ordering precisely and to control the coupling process in colloidal nanoparticle clusters.8−10 The forces involved in the nanoparticle clustering process, including covalent and noncovalent interactions,11,12 can be tailored by various parameters (such as temperature,13 pH,14 ionic strength,15 and concentration16), which provide an unlimited platform to remarkably improve the physicochemical properties of resulting systems like optical property,17 viscoelasticity,18 electronics,19 and mechanical strength.20 In our recent work,21−24 a versatile synthetic platform capable of synthesizing highly extensible, tough, and elastomeric nanocomposite hydrogels from silica nanoparticles (SNPs) and polymer chains has been pursued with tunable properties. The networks described here based on hydrophilic monomers (such as acrylic acid,21,22 poly(ethylene glycol) methyl ether,23 and acrylamide24) in situ graft from the multifunctional cross-links of SNP surfaces and build SNP/ polymer nanoparticle clusters with core−shell structures © 2013 American Chemical Society

(Figure 1). By approaching the critical concentration, these initial free-moving clusters with random coil polymer chains

Figure 1. Schematic illustration of the preparation strategy for silica nanoparticle/polymer colloidal nanocomposite clusters.

could further associate to form a stable cross-linked network that sustains its own weight (sol−gel transition). It is expected that if energetically favorable interactions exist between neighboring nanoparticle clusters the polymer chains would generate strong physical entanglements and form high strength hydrogels in the absence of potentially harmful chemical crosslinking agents.25 An understanding of the network structure and chain dynamic motion is critical to interpret macroscopic properties and to design the nanocomposites with high performance.26,27 In current SNP/polymer systems, the restrictive environment of polymer chains close to the SNP surface would affect the Received: January 7, 2013 Revised: March 21, 2013 Published: April 2, 2013 8223

dx.doi.org/10.1021/jp400200s | J. Phys. Chem. C 2013, 117, 8223−8230

The Journal of Physical Chemistry C

Article

−140 °C. The bubble-free and homogeneous strips with dimension of 30 mm in length and 5 mm in width were cut and applied to dynamic mechanical analysis using a TA Instruments DMA 2980 at a fixed oscillation frequency of 0.5 Hz and at 25 °C. Dielectric spectra were collected using a computercontrolled dielectric spectrometer (ALPHA analyzer, Novocontrol) operating over a frequency range from 10−1 to 105 Hz at 25 °C, where silver electrodes were coated onto the polymer samples to provide maximum electrical contact.

polymer chain’s relaxation dynamics as well as mobility, where the mechanical properties largely depend on the stress transfer efficiency between rigid fillers and soft polymer.24 According to molecular dynamics simulations,28 it has been indicated that the mobility of the nanofillers in a polymer matrix depends on their ability to dissipate energy, which would enhance the strength of the nanocomposites in the case of proper thermodynamic state of the matrix. Therefore, what distinguishes SNP-loaded nanocomposite hydrogels from conventional chemically crosslinked hydrogels is that the SNP role might not be limited to only add stiffness to the polymer but also to direct morphology, as well as introduce an additional energy dissipation process, leading to the enhanced strength in nanocomposite hydrogels. The preliminary data suggest that it is possible to form large clusters and tough gels by bridging neighboring SNPs.21−24 However, more studies are necessary to study the underlying reasons of how these “permanent” networks are built and to present a more complete physical picture of the SNP−polymer interactions. Therefore, as a continuation of previous work,24 the aim of this paper is to (1) observe the SNP/polymer nanoparticle cluster evolution during the gelation process and investigate the role of constrained polymer chains in the mechanical reinforcement of polymer nanocomposites, (2) tentatively propose a model to describe the nature of the hybrid network structure that correlates with the morphology and percolation effect of the SNP network, and (3) demonstrate that the concept of nanoparticle mobility is valid for toughening the SNP/polymer mechanism if both preconditions of reinforced polymer/nanoparticle interactions and intercluster interactions are met. Overall, the results obtained here would promote us to further interpret the primary factors determining the mechanical properties of nanocomposite hydrogels that were observed previously.

■

RESULTS AND DISCUSSION Dispersion of Nanoparticles. In principle, the welldispersed nanoparticles in the polymer matrix are a precondition for achieving pronounced mechanical reinforcement, where fracture strength and Young’s modulus of nanocomposites can be obtained with increasing interfacial interaction.2,3 For nanoparticle/polymer systems, both experiment and theoretical simulation showed that the entropic and energetic factors dominated the dispersion of particles in the polymer matrix. The entropic penalty of grafting polymer chains on the particle surface would be compensated by the raised conformational arrangement, thus the dispersion state of particles in the matrix is largely determined by the affinity of polymer chains.30 According to this viewpoint, the homogeneous dispersion of SNPs in the polymer matrix stems from great affinity of grafting molecules to situate within the interSNP spacing. The nanocomposite networks reported here consist of poly(acrylamide) (PAM) chains covalently grafted from the SNP surface in an aqueous solution, so that the SNP and AM are homogeneously mixed in the precursor solution during the network formation. The silane pretreatment of SNPs is analogous to the nanoparticle put on a “slippery coating”, where silane bridges promote relative sliding of the grafted nanoparticles under applied stress. The nanocomposite hydrogels were washed and then swollen to equilibrium in distilled water. By simply tailoring the weight fraction of the SNP in the matrix, the mechanical properties (Young’s modulus, fracture stress, and fracture strain) of the nanocomposite hydrogels were controlled, while maintaining high elasticity. The development of high-performance nanocomposites requires the control of interactions between nanoscale fillers and polymer chains.3−5 The flexible and elastomeric features of the nanocomposite hydrogels can be largely ascribed to long and flexible PAM chains as well as to noncovalent interactions between the SNP and polymer. According to size exclusion chromatography measurement,24 the molecular weight of PAM attained a 104 Da magnitude. These polymer chains within the network can be viewed as highly entangled, long, and flexible in concentrated solutions, which appear to be a permanent structure if the relaxation time of the network is much longer than the application time of stress.28 Except for the covalent bonding between SNPs and PAM via silane bridges, there may also exist some noncovalent interactions between SNPs and polymer chains, which contribute to hydrogen bonding between Si−OH groups on the SNP surface and amide side groups (−CONH) of polymer chains (Figure 2). Thus, combined with covalent bonds and some reversible physical bonds, that damage upon deformation and recover back on unloading, these physical interactions serve as reversible sacrificial bonds and temporary cross-links between polymer chains, providing localized regions of enhanced strength, which in turn can delay the growth of cracks.31

■

EXPERIMENTAL SECTION The silica nanoparticles (SNPs) with average diameter of 64 nm were prepared by the well-known Stöber procedure through hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in diluted alkaline solution.29 Then the pristine SNPs were chemically modified by organosilane (γ-methacryloxypropyl trimethoxy silane) to anchor double bonds that initiate the free radical polymerization. The homogeneous precursor aqueous solutions containing modified SNPs, acrylamide (AM), and photoinitiator (2,2′-diethoxyacetophenone) were prepared and subject to UV radiation (365 nm) for 40 min. The resulting nanocomposite hydrogels were purified by immersing in distilled water for 72 h to remove unreacted monomer and unlinked polymers. The specific procedures could be found in an earlier report.24 The SNP/polymer nanocomposite cluster morphological evolution was characterized by transmission electron microscopy (TEM). The specimens under different gelation stages were collected and dried. The thin sections of approximately 80 nm in thickness were cut and examined using a JEM-101 (JEOL) at an acceleration voltage of 80 kV. The SNP/polymer nanocomposite hydrogels were stretched uniaxially using Zwick Z005 under different stains and “fixed” with binder clips to maintain their elongated state. The stretched samples were then cryo-fractured by immersing into liquid nitrogen. Ultrathin sections (∼100 nm) were cut using a Leica cold knife at −80 °C. The sections were collected on carbon grids and transferred to a cryoholder (D626, Gatan Inc.) without any further modification, and then the vitrified sample was observed and imaged using a JEOL-2010 microscope at 8224



Article

network. Generally, the grafted amount and layer thickness depend on the average effective functionality of each SNP and the molecular weight of polymer chains. Combined with an earlier report, the average effective functionality of each SNP could reach 28,24 which agrees with the density of polymer chains around the SNP surface in TEM images. It is known that entanglement coupling is not likely to produce pronounced strength improvement for the polymer matrix, especially for the swollen gel network where polymer chains are highly mobile and may slip past the entanglements,32 whereas in this study, the gels exhibited excellent mechanical properties, which were ascribed to their unique network structures: the grafted polymer chains on the SNP surface entangled with the matrix polymer in the course of polymerization, leading to improved nanoparticle/matrix interactions. Elastomeric Properties. By generating the homogeneously dispersed SNP/polymer nanoparticle clusters with the polymer matrix, the tensile strength of the hydrogels is dramatically improved. According to uniaxial tension results,24 there was a clear yielding at low strain of the stress−strain curves (Figure 4). This phenomenon infers that there likely exists a fraction of clusters undergoing partial dissociation at a lower stress than that required to cause the completed network fracture. That is, the dissociation of interconnected clusters into isolated ones may consume a large amount of energy before complete fracture. Therefore, this yielding indicates network structural change of network arrangement beyond a certain strain, which may be caused by the plastic deformation and dissociation of interpenetrated clusters. In fact, the feature of the sacrificial bonds in the nanocomposites that has been visualized by the apparent hysteresis loop from the loading−unloading cycle tests on the hydrogels upon tension to a predefined tensile strain could also be justified by this stepwise cluster dissociation model.24 Before the yield point, the hydrogels experience the plastic deformation under tensile stretching. Above the yield point, the network arrangements begin to be extensively elongated, and those initially interpenetrated clusters are forced to gradually dissociate. In contrast, a chemically cross-linked gel

Figure 2. Schematic illustration of SNP/polymer nanoparticle cluster structure. Covalent bonds between SNP and PAM via silane bridges induce the formation of an elastic network, whereas physical interactions lead to viscoelastic properties. The constraint phase (dashed line in A) distributes around the SNP, and the interpenetrated zone (dash line in B) builds the entangled network.

Gelation Observation. To better understand the structure−property relationships for SNP/polymer nanocomposite clusters, the morphological evolution within the gelation process is obtained from selected specimens at different stages through TEM observation (Figure 3). At the initial stage of polymerization (t = 5 min), the typical distance between two neighboring SNPs is 50−200 nm, where the viscous property dominates the liquid state. The neighboring particles were linked by a grafted chain, either by the mutual termination of two growing grafted chains that initiated at two SNPs or by the termination of a grafted chain from one SNP by a radical of a second SNP. This speculation was based on the fact that the nanocomposite hydrogels swelled but did not dissolve with the expansion of network. As the reaction proceeded (t = 15 min), the system gradually gained elastic property resulting in a gellike state, where the diameter of the SNP/polymer cluster nanocomposite increased to 70−85 nm. After 30 min, the dimension of nanocomposites leveled off (∼100 nm), and the layer of “polymer shell” became thick, indicating the completion of gelation and the formation of an elastic polymer

Figure 3. TEM images and schematic illustration of SNP/polymer nanoparticle cluster network structure evolution at different gelation stages (SNP concentration of 0.1 wt %, bar = 50 nm). Those originally loose distributed clusters become a close entangled network structure where the bulk cross-linked network is formed, leading to shortened distance between neighboring particles and pronounced strength enhancement. 8225



Article

Figure 4. Schematic illustration of SNP/polymer nanoparticle cluster deformation response to stress, where interpenetrated clusters undergo stepwise dissociation and become isolated ones. The cryo-TEM images of vitrified nanocomposite hydrogels exhibit cluster morphological changes under strain. (The stress−strain curve was given as ref 24 for samples with SNP concentration of 0.1 wt %; bar = 100 nm for (a) and (b), and bar = 250 nm for (c)).

The mechanical behaviors of hydrogels formed by two types of cross-links can also be explained from the view of interactions between the polymer chain and its cross-links. For nanocomposite hydrogels, the critical stress required to dissociate each SNP/polymer junction can be expressed as the summation of the stresses to dissociate each interacted SNP/ polymer cluster. For this process, one may imagine that there exists a fraction of SNP/polymer residues undergoing dissociation even at a lower stress than that required to cause the complete dissociation. The partial deformation may reduce the stress concentration at the local crack tips and significantly increase the fracture stress. Therefore, the complete dissociation of the SNP/polymer junction can be restrained by the stepwise energy dissipation process within the neighboring SNP/polymer clusters. Besides, this partial dissociation in SNP/polymer clusters may be followed by a reassociative process, which recovers the original cross-linked structure. In sharp contrast, the dissociation process in the chemically crosslinked hydrogels occurs in a more homogeneous manner, where the stress localized at a single cross-link before critical stress was achieved. Consequently, the area of deformation zone is limited in the process zone, and the crack propagation rate is much faster than the nanocomposite hydrogels. The larger fracture work (area under the stress−strain curves) of nanocomposite hydrogels also indicates the formation of a large dissociation zone which needs more energy to completely unlock the plastic zone, which supports the above-proposed mechanism. Meanwhile, the morphological features of cluster evolution support the above-proposed reinforcement model. The addition of SNPs significantly enhances the strength of the matrix and resists the propagation of cracks due to the presence of covalent bonds at the interface, so that the nanocomposites could tolerate the higher fracture stress, and a dense layer of polymer shell is observed. The polymer chain entanglement

can be viewed as an elastic one, and the minor hysteresis loop is ascribed to the presence of elastically ineffective chains. Thus, a large amount of stress may accumulate on the extended network due to the absence of an energy-dissipative process, which leads to low fracture strength and poor resilience. To better understand the network rearrangement at different stretched levels, the cryo-TEM observation was applied (Figure 4). At the initial low strain state, the SNP/polymer clusters present an interpenetrated network with a thick wall (polymer layer) on the SNP surface. Then, the interconnected structure reorganizes, and the pore wall becomes thinner at higher stress, where the polymer chains are pulled away from the SNP surface into the matrix, leading to a stretched and isolated network. This stepwise dissociation process could dissipate a large amount of energy and increase the resistance against crack propagation: as the material is subjected to stress, sufficiently high interfacial adhesion between the polymer and nanoparticles impulses the mobility of the polymer chains and transfers to nanoparticles, and then, these nanoparticles are allowed to move and align, providing an additional energydissipating mechanism which is absent in the neat polymer. This dynamic process might give rise to a more efficient energy dissipation mechanism in the nanocomposites, thereby delaying crack propagation. Besides, this hysteresis loop relates to a residual deformation after tensile loading: the sample does not immediately relax to its original shape, and the width of the sample remains somewhat narrower when compared to the original one,24 which is a clear indication of network rearrangement after stretching. Therefore, the pronounced increase in strength of the SNP-loaded nanocomposite hydrogels is largely ascribed to the rigid but viscous nature of cluster interactions, where the covalent bonds promote the plastic deformation and reversible and noncovalent hydrogen associations serve as sacrificial bonds. 8226



Article

Figure 5. Constrained polymer chains in SNP/polymer nanoparticle clusters. (a) Variation of the dielectric loss factor ε″ as a function of frequency for nanocomposites loaded with (■) 0 wt %, (●) 0.02 wt %, (▲) 0.04 wt %, and (▼) 0.055 wt % of SNP. (b) Volume of the constrained chain as a function of SNP percentage.

a small amount of SNP could immobilize a large amount of polymer chains during the cluster growth,23,24 where SNPs acted as multifunctional cross-links to anchor dozens of polymer chains. Thus, the effect of constrained chain volume on segmental dynamic of nanocomposite hydrogels promotes us to further look into the reinforcement mechanism. The interpenetrated clusters and the constrained polymer chains develop a network where the closely stacked cluster is analogous to a “crystalline” zone and the polymer matrix is similar to the “amorphous” domain.35,36 It is speculated that during the radical polymerization polymer chains gradually grow on the SNP surface, and the SNP/polymer nanoparticle clusters self-assemble into the interconnected secondary structures. A small volume of constrained region is composed of the proximal polymer chains encircling nanoscale particles. The key here is the nanoparticle−polymer interaction: stronger interaction leads to better dispersion, larger amount of constrained region, larger modulus of the constrained region, and ultimately larger nanocomposite modulus.37,38 Besides, there may exist an associative equilibrium between the polymer chain and SNP during this process, where a polymer chain may form bonds with another SNP. At a low SNP content, one layer of polymer more likely anchors on the surface of a single SNP. When the SNP content increases, the anchored polymer chain is more likely to associate with another SNP rather than with chains in a matrix, and the distance between neighboring SNPs becomes smaller, leading to a denser layer of polymer shell and a higher level of cross-linked network. Percolation Network. According to our earlier work,24 the nanocomposite hydrogels exhibited enhanced mechanical properties with a 3.7-fold increase in modulus and 9-fold increase in fracture strength, respectively, at a 0.15 wt % loading of SNP, suggesting the interaction between the SNP and polymer chain is a critical factor to affect the network structure. It is well-known that for the reinforcement of nanoparticles to the polymer matrix one should consider (1) a strong interaction between the clusters and (2) the percolation effect where the SNP concentration should exceed a critical threshold.5,6 If we correlate the fracture stress of the composite with the above constrained polymer volume fraction, the following power law relation can be attained

introduced by interpenetrated clusters increases to a higher level, resulting in the formation of an efficient energy dissipation process during deformation. As a consequence, covalent bonding and attractive interface physical interactions between the clusters are critical for enhancing the strength as well as for resisting the propagation of cracks. Constrained Chain. The uniaxial tension data have implied that the incorporation of SNP would significantly reinforce the mechanical strength of nanocomposites; naturally, it is of interest to learn the mobility of the polymer chains around the SNP surface and to interpret how the presence of SNPs affects their mobility. An ideal way of probing such changes is to study the isothermally dielectric relaxation spectra, where the dipolar relaxation level is inversely proportional to the peak area and the decrease in the dielectric constant (ε″) characterizes the restriction of polymer chains.33,34 Dielectric measurements were performed on SNP/polymer nanocomposites, and the variation of the loss peak for specimens with different SNP contents is shown in Figure 5(a). The shape of these dependences is similar for all curves and indicates that in the observed frequency intervals one overlapping relaxation process exists. The spectrum corresponds to a simple dipole relaxation and reflects the chain mobility in the system. After another careful observation, it is noted that the addition of more SNPs results in a decrease in the magnitude of dielectric loss curve ε″ (maximum) and broadens its peak width, suggesting the increased fraction of constrained chains per unit volume and the broader distribution of relaxation times in the nanocomposite hydrogels. Recent reports have shown that the polymer chain mobility depended not only on the extent of nanoparticle dispersion but also on the nature of interactions between the filler and polymer matrix.4,5 As SNP content increases, more polymer chains are confined (immobilized) among SNP galleries and leads to the reduction of the level of mobility. To interpret the role of the constrained region in enhancing the mechanical properties of the nanocomposite hydrogels, it is critical to determine the volume fraction of these regions (Figure 5(b)). The relationship between the constrained chain volume and the amount of SNP can be fitted by the following equation (assuming density of the SNP is 1.6 g/cm3) InVcon = 0.46 × InVSNP + 1.22

lg σ = 0.85Vcon + 2.32

(1)

(2)

where σ is the fracture stress of the nanocomposite hydrogels. Here, we attempt to elucidate the increase in σ in light of

where Vcon and VSNP are the volume fraction of constrained polymer chains and SNP, respectively. It was noted earlier that 8227



Article

ascribed to the randomly formed bonds in a three-dimensional lattice.41,42 Combined with Gersappe’s results,28 the ability of a nanofiller to increase the toughness of the material results from the equivalence of time scales of motion for the polymer and filler. If the filler is of the same size magnitude as the hydrodynamic radius of the chain, the time scales for motion of the filler and the polymer are comparable. Consequently, during the deformation process, the filler can generate temporary cross-links between the polymer chains, thereby forming a local region of enhanced strength and retarding the growth of the cracks. Taken together, a nanoparticle concentration and filler/ polymer matrix interaction dependent multimodel reinforcement mechanism of SNP/polymer clusters is proposed to explain the experimental results. For a well-dispersed nanoparticle/polymer system, the particle reinforcement mechanism is generally related to the interfacial interaction, percolated particle network, and polymer chains free mobility. As the cluster concentration is lower than the macroscopic percolation concentration (φ < Xc), the clusters are homogeneously dispersed in the polymer matrix. The mechanical strength of nanocomposite hydrogels is ascribed to the interfacial interaction between the SNP and polymer chains. The multifunctional cross-links of SNPs and close entanglement of long grafted polymer chains between interfacial layers and the polymer matrix promote improvement of the stress dissipation. At this junction, the increase of cluster concentration is equivalent to an increase in the volume of crosslinking density, where the interfacial interaction is a vital factor to determine the strength of the nanocomposite hydrogels. As the cluster concentration approaches the critical threshold (φ ∼ Xc), the macroscopic percolated clusters to contribute to the strength of the nanocomopsite hydrogels and the polymer segments increase their mobility. Because the toughness of a material is related to the energy dissipation, resistance of crack propagation, and cross-linked density, the fracture strength increases with increasing cluster concentration.43 In the rubbery state of the network, the movement of the SNP is promoted, which forms temporary bonds between the surrounding polymer chains that allow it to dissipate energy. That is, for the rubbery polymer matrix, the improved interaction between the polymer and nanoparticles would facilitate the fillers to move upon the elongation.44 Meanwhile, the deformation and alignment of agglomerates in turn leave space for arrangement of polymer chains.

covalent bonding between polymer chains and SNPs via damping spectra. Figure 5(a) indicates the effect of SNP loading on the dielectric loss factor ε″ of the nanocomposite hydrogels. It is noted that the relative peak height is inversely proportional to the volume fraction of confined segments at the particle surface, indicating the increased covalent bonding between the SNP and the matrix. With increasing volume fraction of confined segments, the relaxation of segments at different sites within the matrix would span a wider range. Those segments in the immediate vicinity of the SNP experience greater confinement, and thus their relaxation occurs at higher frequency. This perspective is consistent with the results shown in Figure 5(a), where a gradual broadening peak with increasing SNP loading is observed. The reinforcement observed in the SNP loaded nanocomposites relates to the formation of a percolating SNP network where stress release is promoted by noncovalent interactions between cluster nanoparticles.30 In this regard, the mechanical properties of the network can be described by using

Figure 6. Shear modulus (E′c) of nanocomposite hydrogel plot against volume fraction of SNP.

a percolation model (Figure 6), where the shear modulus of the composite E′c is expressed as39,40 Ec′ =

■

(1 − 2ψ + ψX r)Es′Er′ + (1 − X r)ψEr′ 2 (1 − X r)Er′ + (X r − ψ )Es′

CONCLUSIONS We have examined the structure−property relationship of the SNP/PAM nanocomposite system and the picture that begins to emerge for the role and nature of the constrained polymer region in a nanocomposite reinforcement mechanism. The SNP/polymer interactions determine the volume of constrain region and contribute to the enhancement in the mechanical properties of nanocomposite hydrogels. The reinforcement role of the SNP to polymer matrix is believed to be three primary factors, i.e., percolation filler network, covalent bonding via silane bridge and noncovalent interactions, and chain flexibility. With increasing SNP content to the percolation threshold, the percolated particle network begins to act on the strength enhancement of nanocomposite hydrogels. The experimental results indicate that the energy dissipation mechanism induced by nanoparticle mobility works in reinforcing the strength of SNP/polymer composites. From the microstructure viewpoint,

with ⎛ X r − Xc ⎞0.4 ψ = X r⎜ ⎟ ⎝ 1 − Xc ⎠

where ψ is the volume fraction of the SNPs that participate in the load transfer; Xr is the volume fraction of the randomly reinforcing phase; Xc is the critical SNP percolating volume fraction; and E′s and E′r refer to the shear modulus of the neat polymer matrix and reinforcing phase, respectively. According to previous reports based on percolation concepts,24 Xc is the percolation threshold; here, Xc = 0.13% (v/v) for this study. Flory and Stockmayer et al. applied percolation theory to describe the critical phenomena near the gel point and predicted universal scaling behavior for structural and dynamical properties, where the connectivity properties were 8228



Article

(14) Fielding, L. A.; Armes, S. P. Preparation of Pickering Emulsions and Colloidosomes Using Either a Glycerol-Functionalised Silica Sol or Core−Shell Polymer/Silica Nanocomposite Particles. J. Mater. Chem. 2012, 2, 11235−11244. (15) Azzam, F.; Heux, L.; Putaux, J. L.; Jean, B. Preparation By Grafting Onto, Characterization, and Properties of Thermally Responsive Polymer-Decorated Cellulose Nanocrystals. Biomacromolecules 2010, 11, 3652−3659. (16) Lasater, K. L.; Thostenson, E. T. In Situ Thermoresistive Characterization of Multifunctional Composites of Carbon Nanotubes. Polymer 2012, 53, 5367−5374. (17) Villari, V.; Mazzaglia, A.; Trapani, M.; Castriciano, M. A.; Luca, G.; Romeo, A.; Scolaro, L. M.; Micali, N. Optical Enhancement and Structural Properties of a Hybrid Organic−Inorganic Ternary Nanocomposite. J. Phys. Chem. C 2011, 115, 5435−5439. (18) Filippone, G.; Luna, M. S. A Unifying Approach for the Linear Viscoelasticity of Polymer Nanocomposites. Macromolecules 2012, 45, 8853−8860. (19) Bose, S.; Kuila, T.; Mishra, A. K.; Rajasekar, R.; Kim, N. H.; Lee, J. H. Carbon-Based Nanostructured Materials and Their Composites as Supercapacitor Electrodes. J. Mater. Chem. 2012, 22, 767−784. (20) Han, J. B.; Dou, Y. B.; Yan, D. P.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Biomimetic Design and Assembly of Organic−Inorganic Composite Films with Simultaneously Enhanced Strength and Toughness. Chem. Commun. 2011, 47, 5274−5276. (21) Yang, J.; Wang, X. P.; Xie, X. M. In Situ Synthesis of Poly(acrylic acid) Physical Hydrogels From Silica Nanoparticles. Soft Matter 2012, 8, 1058−1063. (22) Yang, J.; Shi, F. K.; Gong, C.; Xie, X. M. Dual Cross-Linked Networks Hydrogels with Unique Swelling Behavior and High Mechanical Strength: Based on Silica Nanoparticle and Hydrophobic Association. J. Colloid Interface Sci. 2012, 381, 107−115. (23) Yang, J.; Gong, C.; Shi, F. K.; Xie, X. M. High Strength of Physical Hydrogels Based on Poly(acrylicacid)-g-poly(ethylene glycol) Methyl Ether: Role of Chain Architecture on Hydrogel Properties. J. Phys. Chem. B 2012, 116, 12038−12047. (24) Yang, J.; Deng, L. H.; Han, C. R.; Duan, J. F.; Ma, M. G.; Zhang, X. M.; Xu, F.; Sun, R. C. Synthetic and Viscoelastic Behaviors of Silica Nanoparticle Reinforced Poly(acrylamide) Core−Shell Nanocomposite Hydrogels. Soft Matter 2013, 9, 1220−1230. (25) Watanabe, H. Viscoelasticity and Dynamics of Entangled Polymers. Prog. Polym. Sci. 1999, 24, 1253−1403. (26) Chen, B. Q.; Evans, J. R. G.; Greenwell, H. C.; Boulet, P.; Coveney, P. V.; Bowden, A. A.; Whiting, A. A Critical Appraisal of Polymer−Clay Nanocomposites. Chem. Soc. Rev. 2008, 37, 568−594. (27) Noto, V. D.; Gliubizzi, R.; Negro, E.; Pace, G. Effect of SiO2 on Relaxation Phenomena and Mechanism of Ion Conductivity of [Nafion/(SiO2)x] Composite Membranes. J. Phys. Chem. B 2006, 110, 24972−24986. (28) Gersappe, D. Molecular Mechanisms of Failure in Polymer Nanocomposites. Phys. Rev. Lett. 2002, 89, 058301. (29) Stö ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62−69. (30) Pan, Y. Z.; Xu, Y.; An, L.; Lu, H. B.; Yang, Y. L.; Chen, W.; Nutt, S. Hybrid Network Structure and Mechanical Properties of Rodlike Silicate/Cyanate Ester Nanocomposites. Macromolecules 2008, 41, 9245−9258. (31) Haque, M. A.; Kurokawa, T.; Kamita, G.; Gong, J. P. Lamellar Bilayers as Reversible Sacrificial Bonds To Toughen Hydrogel: Hysteresis, Self-Recovery, Fatigue Resistance, and Crack Blunting. Macromolecules 2011, 44, 8916−8924. (32) Zhang, L.; Wang, Z. P.; Xu, C.; Li, Y.; Gao, J. P.; Wang, W.; Liu, Y. High Strength Graphene Oxide/Polyvinyl Alcohol Composite Hydrogels. J. Mater. Chem. 2011, 21, 10399−10406. (33) Lopes, A. C.; Silva, M. P.; Gonçalves, R.; Pereira, M. F. R.; Botelho, G.; Fonseca, A. M.; Mendez, S. L.; Neves, I. C. Enhancement of the Dielectric Constant and Thermal Properties of r-Poly-

the key issues to provide nanocomposite hydrogels with high strength lie in the high mobility of the matrix and strong polymer−filler interactions. Substantially, the structure−property relationships developed in this work would greatly merit the understanding of tough hydrogels and the significance of nanocomposite systems.

■

ASSOCIATED CONTENT

* Supporting Information S

TEM image of a pristine SNP, SEM picture of SNP/PAM nanocomposites, viscoelastic properties evolution of SNP/PAM nanocomposites, and TGA curves of silane-modified SNP. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-10-62338152. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by Fundamental Research Funds for the Central Universities (TD2011-10), Beijing Forestry University Young Scientist Fund (BLX2011010), and Research Fund for the Doctoral Program of Higher Education of China (20120014120006).

■

REFERENCES

(1) Zou, H.; Wu, S. S.; Shen, J. Polymer/Silica Nanocomposites: Preparation, Characterization, Properties, and Applications. Chem. Rev. 2008, 108, 3893−3957. (2) Pavlidou, S.; Papaspyrides, C. D. A Review on Polymer−Layered Silicate Nanocomposites. Prog. Polym. Sci. 2008, 33, 1119−1198. (3) Tan, J. C.; Cheetham, A. K. Mechanical Properties of Hybrid Inorganic−Organic Framework Materials: Establishing Fundamental Structure−Property Relationships. Chem. Soc. Rev. 2011, 40, 1059− 1080. (4) Sayari, A.; Hamoudi, S. Periodic Mesoporous Silica-Based Organic−Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13, 3151−3168. (5) Lu, Z. D.; Yin, Y. D. Colloidal Nanoparticle Clusters: Functional Materials by Design. Chem. Soc. Rev. 2012, 41, 6874−6887. (6) Loizou, E.; Butler, P.; Porcar, L.; Schmidt, G. Dynamic Responses in Nanocomposite Hydrogels. Macromolecules 2006, 39, 1614−1619. (7) Lydon, M. E.; Unocic, K. A.; Bae, T. H.; Jones, C. W.; Nair, S. Structure−Property Relationships of Inorganically Surface-Modified Zeolite Molecular Sieves for Nanocomposite Membrane Fabrication. J. Phys. Chem. C 2012, 116, 9636−9645. (8) Liu, Y. B.; Wang, X. S. Recent Advances in Block CopolymerAssisted Synthesis of Supramolecular Inorganic/Organic Hybrid Colloids. Polym. Chem. 2011, 2, 2741−2757. (9) Wei, S. Y.; Wang, Q.; Zhu, J. H.; Sun, L. Y.; Lin, H. F.; Guo, Z. H. Multifunctional Composite Core−Shell Nanoparticles. Nanoscale 2011, 3, 4474−4502. (10) Ishida, H.; Campbell, S.; Blackwell, J. General Approach to Nanocomposite Preparation. Chem. Mater. 2000, 12, 1260−1267. (11) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. Toward Functional Nanocomposites: Taking the Best of Nanoparticles, Polymers, and Small Molecules. Chem. Soc. Rev. 2013, 42, 2654−2678. (12) Das, D.; Kar, T.; Das, P. K. Gel-Nanocomposites: Materials with Promising Applications. Soft Matter 2012, 8, 2348−2365. (13) Tan, W. S.; Cohen, R. E.; Rubner, M. F.; Sukhishvili, S. A. Temperature-Induced, Reversible Swelling Transitions in Multilayers of a Cationic Triblock Copolymer and a Polyacid. Macromolecules 2010, 43, 1950−1957. 8229



Article

(vinylidene fluoride)/Zeolite Nanocomposites. J. Phys. Chem. C 2010, 114, 14446−14452. (34) Dimopoulos, A.; Wietor, J. L.; Wübbenhorst, M.; Napolitano, S.; Benthem, R. A. T. M.; With, G. de; Sijbesma, R. P. Enhanced Mechanical Relaxation below the Glass Transition Temperature in Partially Supramolecular Networks. Macromolecules 2010, 43, 8664− 8669. (35) Keledi, G.; Hári, J.; Pukánszky, B. Polymer Nanocomposites: Structure, Interaction, and Functionality. Nanoscale 2012, 4, 1919− 1938. (36) Hu, X. L.; Hou, G. M.; Zhang, M. Q.; Rong, M. Z.; Ruan, W. H.; Giannelis, E. P. A New Nanocomposite Polymer Electrolyte Based on Poly(vinyl alcohol) Incorporating Hypergrafted Nano-Silica. J. Mater. Chem. 2012, 22, 18961−18967. (37) Zheng, J. R.; Ozisik, R.; Siegel, R. W. Phase Separation and Mechanical Responses of Polyurethane Nanocomposites. Polymer 2006, 47, 7786−7794. (38) Chen, B. Q.; Evans, J. R. G. Impact Strength of Polymer−Clay Nanocomposites. Soft Matter 2009, 5, 3572−3584. (39) Fox, J.; Wie, J. J.; Greenland, B. W.; Burattini, S.; Hayes, W.; Colquhoun, H. M.; Mackay, M. E.; Rowan, S. J. High-Strength, Healable, Supramolecular Polymer Nanocomposites. J. Am. Chem. Soc. 2012, 134, 5362−5368. (40) Favier, V.; Chanzy, H.; Cavaillé, J. Y. Polymer Nanocomposites Reinforced by Cellulose Whiskers. Macromolecules 1996, 28, 6365− 6367. (41) Flory, P. J. Molecular Size Distribution in Three Dimensional Polymers. I. Gelation1. J. Am. Chem. Soc. 1941, 63, 3083−3090. (42) Stockmayer, W. H. Theory of Molecular Size Distribution and Gel Formation in Branched-Chain Polymers. J. Chem. Phys. 1943, 11, 45−55. (43) Bergström, J. S.; Boyce, M. C. Mechanical Behavior of Particle Filled Elastomers. Rubber Chem. Technol. 1999, 72, 633−656. (44) Treloar, L. R. G. The Physics of Rubbery Elasticity; Clarendon Press, London, 2005; Chapter 2.

8230


Polymer Nanocomposite Cluster

Recommend Documents