Self-Assembling a Polyoxometalate–PEG Hybrid into a Nanoenhancer

Article pubs.acs.org/Macromolecules

Self-Assembling a Polyoxometalate−PEG Hybrid into a Nanoenhancer To Tailor PEG Properties Jing Tang, Chi Ma, Xue-Ying Li, Li-Jun Ren, Han Wu, Ping Zheng, and Wei Wang* Center for Synthetic Soft Materials, Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: The unique performance of natural materials stems from their hierarchical hybrid structures formed through self-assembly. The self-assembly principles of natural materials have been exploited to create artificial materials. Herein, we demonstrate a bottom-up approach that produces polymer nanocomposites as well as a self-assembled nanoenhancer for tailoring the polymer properties. The polymer is a poly(ethylene glycol) (PEG), and the nanoenhancer is aggregates formed by self-assembly of a hybrid. The hybrid is prepared through covalent bonding of a surfactant-encapsulated polyoxometalate (S-POM) complex with a PEG chain and can form aggregates composed of an SPOM complex bilayer sandwiched by two PEG layers. The lateral size of aggregates changes, depending on the conditions used in the sample preparation. Hence, we examined four nanostructures in the solid samples of nanocomposites: hybrid self-assembled nanosheets, PEG crystallized lamellae, PEG/hybrid cocrystallized lamellae, and hybrid crystallized lamellae. Because of a strong interaction among the S-POM complexes as well as good miscibility of the PEG layers with the PEG matrix, the stable aggregate homogeneously disperses in the melted PEG matrix, and hence it can enhance the performance of the melted PEG. For instance, the shear storage moduli of nanocomposites are adjustable over many orders of magnitude at temperatures above the PEG melting point. These findings provide a novel approach to generate synthetic nanocomposites with self-assembled enhancers that can tailor the polymer properties.

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INTRODUCTION

composites may help the community realize the immense potential of man-made composites. A number of supramolecular nanomaterials or nano-objects with desired functions have been made through molecular selfassembly.9−13 Normally, they are soft and have a structural hierarchy. Softness stems from the fact that the forces responsible for organizing and maintaining supramolecular assemblies are weak and reversible noncovalent interactions including hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π−π interactions, and electrostatic effects. Structural hierarchy is associated with hierarchical self-assembly across the molecular−nano−micro−macro scales. The fundamental principles of self-assembly have been exploited to design and generate naturally inspired composites with hierarchical structures and properties that mimic natural composites.14−19 It is critical to rationally design building blocks that can be used for programmable self-assembly. From this aspect, inorganic− organic hybrids with structural diversity and functional versatility are a class of important components used to construct natural and bioinspired composites.16,17

Natural or man-made composites are often formed by combining two or more materials with significantly different physical and/or chemical properties together to achieve superior performances.1−7 Notably, natural composites differ greatly from manmade ones in their performance mainly due to the different hierarchical structures constructed through varying approaches. Nearly all man-made composites such as fiber- or clay-reinforced plastics are made by mechanically mixing reinforcements (or enhancers) with plastic matrices. The property, size, and shape of the fillers are predetermined, and thus the structure of composites is relatively simple.1−3 On the contrary, natural composites, like bones, are grown in nature or developed by natural processes.4−7 More precisely, their rich hierarchical structures grow through a unique biomineralization process of inorganic precursors and biomolecules. For example, bones have a very complex hierarchical structure ranging from the molecular level up to the whole bone level. At the nanoscale, a two-phase structure is composed of a flexible organic matrix (collagen) and an inorganic enhancer, i.e., carbonated apatite mineral phase.8 It is well-known that optimization of the structural hierarchy plays a major role in achieving a remarkable mechanical performance of bones. That is, lessons from previous studies on natural © XXXX American Chemical Society

Received: January 31, 2015 Revised: March 17, 2015

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molecular weight has a hydroxyl end-group. Using an acid anhydride esterification followed by an amidation with tris(hydroxymethyl)aminomethane (Tris), the hydroxyl end-group of the PEG was converted into three hydroxyl groups. Finally, the hybrid was prepared through an esterification between the three vanadium atoms in the S-POM complex and the three hydroxyl groups in the Tris-modified PEG.26,29 The details of hybrid preparation and characterization have been reported in our previous work.26 Here, we emphasize that we did not detect any changes or degradation of the PEG block due to the chemically active S-POM complex. Importantly, this hybrid has following features: (i) a covalent bond linking the SPOM complex with the PEG chain, (ii) strong interactions among the S-POM complexes, and (iii) a crystallizable PEG block.26 These features are crucial factors in the formation of selfassembled aggregates of the hybrid in the nanocomposites. Water Solubility. It is well-known that PEG is soluble in water due to hydrogen bonds.31,32 In this work, we prepared composites from mixed aqueous solutions of the hybrid and PEG. Before this, we first studied the dissolution of the hybrid in water. The hybrid can dissolve in water to form a yellowish solution at room temperature. A Tyndall effect (Figure 1A) suggests that the hybrid molecules form aggregates in the aqueous solution. Figure 1B shows aqueous solutions of five mixtures with molar fractions, f h, of the hybrid from 2 to 50% at a total nanocomposites concentration of c = 2 mg/mL. For the sake of brevity, the nanocomposites obtained from the solutions are labeled as NC-2, NC-5, NC-10, NC-20, and NC-50, corresponding to f h = 2, 5, 10, 20, and 50%. These solutions also display

Here, we report a new strategy for preparation of nanocomposites with hierarchical structures and improved performance by adding an inorganic−organic hybrid into the polymer matrix. This type of hybrids is composed of a surfactantencapsulated polyoxometalate (S-POM) complex covalently bound to a polymer chain. They can self-assemble into diverse nano-objects.20−28 In this work, we selected a 5000 Da poly(ethylene glycol) (PEG) and a derivative of a Wells− Dawson-type polyanion cluster that was encapsulated by cationic surfactants to build up an S-POM−PEG hybrid.26 Nanocomposites of the hybrid and pure 5000 Da PEG were prepared from their mixed aqueous solutions. The large size and property differences between the S-POM complex and the PEG result in the hybrid self-assembling into nanosized aggregates during solvent evaporation and/or PEG crystallization. In the solid nanocomposite samples, the hybrid aggregates have good miscibility with the PEG component and hence are homogeneously dispersed in the crystallized PEG matrix. At temperatures above the PEG melting point, the strong interactions among the S-POM complexes enable these stable aggregates to markedly improve the performance of the melted PEG matrix. For instance, the shear storage modulus, G′, of the nanocomposites varies over a wide range spanning several orders of magnitude as a function of the hybrid content above the PEG melting point.

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RESULTS AND DISCUSSION Structure of the Hybrid Molecule. The structure of the hybrid molecule containing an S-POM complex and a PEG chain is shown in Scheme 1. The S-POM complex was fabricated by Scheme 1. Structure of the Hybrid Molecule of an S-POM Complex and a PEG Chain

combining an anionic POM cluster and six cationic surfactants. The POM cluster is a trivanadium-substituted derivative of a Wells−Dawson-type polyoxotungstate with two phosphorus heteroatoms in the center.29 It has an ellipsoidal shape with a long axis of 1.2 nm and a short axis of 1.0 nm.30 The cationic surfactant is cationic tetrabutylammoniums (Bu4N+). Thus, the chemical structure of the S-POM complex is (Bu4N+)6H3(P2W15V3O62)9− with a calculated molecular weight of 5422 Da. The linear monomethyl ether PEG with a 5000 Da

Figure 1. (A) Tyndall scattering of the aqueous solution of the hybrid. (B) Aqueous solutions of the five mixtures with f h = 2, 5, 10, 20, and 50% at a total nanocomposite concentration of c = 2 mg/mL. B

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Figure 2. (A) Picture of the NC-20 nanocomposite. (B) WAXD profiles and (C) DSC curves of the PEG, five nanocomposites, and hybrid. (D) Plots of ϕPEG and Tm versus f h. The dashed lines show the calculations according to linear relations between the ϕPEG and Tm of the pure PEG and hybrid versus f h.

Dynamic Rheology Characterization. The dynamic shear storage modulus, G′, in Figure 3A (or complex viscosity, η*, in Figure S3 in Supporting Information) as well as loss tangent, tan δ (Figure S4 in Supporting Information), of the seven samples were obtained via dynamic rheology characterization from 30 to 90 °C. Importantly, 90 °C is lower than the degradation temperature (180 °C) of the hybrid and pure PEG measured using thermogravimetric analysis26 (note TGA of seven samples in Figure S5 and Table S2 in the Supporting Information). The values of G′ and tan δ clearly change at about 60 °C (the melting temperature of PEG). Below 60 °C, G′ ≈ 105−107 Pa and tan δ ≈ 10−1−100 suggest that the samples are hard plastics or tough solids. Above 60 °C, the values of G′ and tan δ show a large component dependence. Figure 3B presents plots of G′ and tan δ at 80 °C versus f h (the plot of η* − f h is shown in Figure S6 of the Supporting Information). At 80 °C, G′ = 6.2 × 10−2 Pa for the pure PEG and G′ = 5.5 × 105 Pa for the pure hybrid. The corresponding values of tan δ are 12.9 to 0.21. These values indicate that the PEG is a liquid, and the hybrid is still a tough solid at 80 °C. For the nanocomposites, the G′ (or η*) increases and tan δ decreases with increasing f h. The G′ and tan δ values changed rapidly when a small amount of the hybrid was added. When f h = 2 and 10%, for instance, G′ = 2.7 × 100 and 1.1 × 102 Pa and tan δ = 2.4 and 0.8, respectively. Versus the values for PEG only, G′ rapidly increase 43- and 1774-fold, which indicates a clear reinforcement effect. The tan δ values dropped to 0.19 and 0.06, which means that there is a reduction in the chain mobility. More precisely, a significant proportion of the polymer chains have become immobilized in the nanocomposite in the presence of the hybrid, and thus the composites become viscous liquids or solids even after all the PEG have melted. This rapid change indicates the efficiency of reinforcement. That is, adding even a

Tyndall scattering, and thus the aggregate diameters, da, were determined using dynamic light scattering (DLS) (see Figure S1 in Supporting Information). With increasing f h, there is no significant difference in the aggregate diameter, da = 105.9 ± 4.9 nm. These studies indicate that the hybrid aggregates in aqueous solution of the hybrid alone or when mixed with the pure PEG. Crystal Structures and Melting and Crystallization Behaviors. Figure 2A shows a solid sample of NC-20 that was prepared by drying the solution under vacuum (0.01 MPa) at room temperature for 24 h. The sample’s color comes from the yellow S-POM complex. The crystal structures as well as the melting and crystallization behaviors of the PEG, five nanocomposites, and hybrid were determined using wide-angle X-ray diffraction (WAXD) (Figure 2B) and differential scanning calorimetry (DSC) (Figure 2C and Figure S2 in Supporting Information). The reflection peaks at 2θ = 19.1° and 23.3° in Figure 2B as well as the melting and crystallization temperatures, Tm and Tc, in Table S1 of the Supporting Information suggest that the PEG can crystallize in the hybrid and nanocomposites.33 The crystallization degree and melting temperature of the PEG block in the hybrid are ϕPEG = 0.55 and Tm = 53.1 °C, which is lower than the ϕPEG = 0.94 and Tm = 59.3 °C of the pure PEG. These results highlight the effect of the S-POM complex on the crystallizability of the PEG block. For the five nanocomposites, their ϕPEG and Tm values decrease from 0.91 to 0.73 and 58.9 to 57.5 °C with increasing f h (Figure 2D). The ϕPEG values are lower than the calculations according to the linear relationship between the ϕPEG of the pure PEG and hybrid versus f h. Notably, the Tm values are slightly lower than the calculations for f h < 0.2 and higher than the calculations for f h ≥ 0.2. These indicate that the S-POM complex in the hybrid has an impact on the crystallization degree of the PEG component. However, its impact on the melting temperature of the PEG component is limited because the thickness of most lamellae remains constant. C

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Spherulitic Morphology and Nanostrctures. The nanocomposite structures were further investigated using the film specimens prepared by casting their aqueous solutions onto a glass slide and drying under vacuum at room temperature for 24 h. We observed the resulting films and noted spherulites via polarized light microscopy (PLM) as shown in Figure 4. The sizes of the spherulites are on the range of tens to hundreds of micrometers. Clearly, the crystallized PEG in the seven samples further organizes into a spherulitic morphology during the drying process of their aqueous solutions. This indicates that the crystallization behavior of the hybrid molecules is similar to that of pure PEG macromolecules. Meanwhile, the diffraction peaks and shoulder peaks in 5° ≤ 2θ ≤ 8° (see WAXD profiles in Figure 2B) indicate an ordered packing of the S-POM complexes in these samples.26 The d-spacing between the S-POM complexes is dS‑POM ≈ 1.5 nm and was determined by the WAXD measurements. We also prepared ultrathin film specimens of the nanomopposites and hybrids and characterized them with transmission electron microscopy (TEM) at 200 kV. Energydispersive X-ray spectroscopy (EDX) was also applied to confirm the chemical composition of the specimens. It is impossible to observe any structures related to crystallized lamellar morphology of the pure PEG sample under such conditions. In this work, we focused on nanostructures within the spherulites of the five nanocomposites and hybrid. Figure 5 shows their TEM images and the same images but in A4 size can be found in Figures S7−12 of the Supporting Information for a better presentation of the fine structures. The EDX spectrum (Figure S13 in Supporting Information) shows the signals of tungsten, vanadium, and phosphorus from the S-POM complex. The S-POM complex scatters more electrons than PEG because it is made of transition metals. Hence, the dark areas in the TEM micrographs indicate the S-POM complexes. This enables us to

Figure 3. (A) Shear storage modulus, G′, of the seven samples as a function of temperature. (B) Plots of G′ and tan δ at 80 °C versus f h.

small amount of the hybrid into the pure PEG can markedly tailor the PEG material performance.

Figure 4. PLM micrographs of NC-2 (A), NC-5 (B), NC-10 (C), NC-20 (D), NC-50 (E) and the S-POM−PEG hybrid (F) showing spherulites. Their average diameters are 105 μm for NC-2, 85 μm for NC-5, 75 μm for NC-10, 60 μm for NC-20, 40 μm for NC-50, and 30 μm for the hybrid. D

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Figure 5. TEM micrographs of the ultrathin film specimens of the five nanocomposites and hybrid: (A) NC-2, (B) NC-5, (C) NC-10, (D) NC-20, (E) NC-50, and (F) the S-POM−PEG hybrid. In images A−C, red arrows indicate the flat nanosheets and yellow arrows indicate wrinkled parts of the nanosheets. In image D, bright green arrows indicate the lamellae.

directly visualize the structural characteristics in the nonstaining thin specimens under bright-field conditions. The TEM micrographs in Figure 5A−C display many dark areas (indicated by red arrows) plus some darker stripes (indicated by yellow arrows) randomly distributed in the view area. These are nanosheets of the hybrid formed through selfassembly of the hybrid in the aqueous solutions and/or the drying process of the solutions. This implies that their formation is independent of crystallization. For convenience, they are called hybrid self-assembled (HSA) nanosheets. Their size decreases when f h increases from 2% to 10%. The difference in darkness between the dark areas and the darker stripes may be due to the direction of observation. The darker stripes should correspond to an edge-on image of the wrinkled parts of the nanosheets. The thickness of the stripes is 3.2 ± 0.4 nm (Figure S14 in Supporting Information), corresponding to 2−3 times that of the POM cluster length. In the NC-2 nanocomposite, the HSA nanosheets were the only nanostructure that can be seen with TEM (see Figure 5A and Figure S7 in Supporting Information in a large size). Because there are only 2% hybrids in this nanoscomposite, the PEG crystallized (PEGC) lamellae should exist in this nanocomposite although we cannot directly visualize them at 200 kV accelerating voltage. For the NC-5 and NC-10 nanocomposites, new nanostructures were found in addition to the HSA nanosheets and PEGC lamellae. The TEM micrographs in Figure 5B,C show nanostructures made of alternatively arranged dark and white layers. Because the S-POM complex scatters more electrons, this nanostructure should be the lamellae consisting of the PEG-rich white layers and the S-POM-rich dark layers. Figures S8 and S9 in the Supporting Information are the large images of Figure 5B,C, which make the nanostructures easy to identify. Comparing Figure 5B to Figure 5C, we can see that the lamellae become darker and more prominent with increasing hybrid component.

Figure 5C obviously shows that the lamellae radiates upward from the bottom of the TEM image. This means that it is a sector of the spherulites (Figure 4) in which lamellae radiate out from the center of the spherulites.34 Hence, their formation is associated with cocrystallization of the pure PEG and PEG block of the hybrid. Thus, they are called PEG/hybrid cocrystallized (PEG-HCC) lamellae. Because of high immiscibility between the S-POM complex and the PEG chain, the cocrystallization of the pure PEG and the PEG block results in a nanoscale phase separation into the SPOM-rich layers and the PEG-rich layers. We do not see any microscale domains because of the strong covalent bond between the S-POM complex and the PEG chain in the hybrid as well as the good compatibility between the pure PEG and the PEG block of the hybrid. Interestingly, the POM-rich dark layer is “dotted” or formed through the arrangement of dotted aggregates of the S-POM complexes. The size of the dotted aggregates is 3.4 ± 0.4 nm by TEM (see Figure S15 in Supporting Information). Surprisingly, we can barely see pure PEG domains although the molar contents of PEG are 95 and 90% in NC-5 and NC-10 nanocomposites. This suggests that the hybrid selfassembles into the aggregates that homogeneously distribute in the concentrated aqueous solutions as well as the solid samples. It also implies that the PEG crystallization does not have any selection for the pure PEG and the PEG block of the hybrid. At f h = 20% the nanostructures begin to dramatically change. In Figure 5D (and Figure S10 in Supporting Information in a large size) the orientated PEG-HCC lamellae are obvious. Many darker stripes can also be seen, as indicated by bright green arrows. Compared to the stripes found in Figure 5A−C, the stripes in Figure 5D are short and ill-shaped. Seemingly, they are linking the PEG-HCC lamellae together to form a network. When f h = 50%, the nanostructure feature of NC-50 shows dark stripes connected together to form a large 3D network (Figure E

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The NC-2 and NC-5 nanocomposites show three scattering peaks with a weak intensity. Their ratio is q1/q2/q3 = 1/2/3 and indicates the formation of a loose lamellar structure. The corresponding d-spacing values are d = 18.1 nm for NC-2 and 17.5 nm for NC-5. The disappearance of these peaks at 80 °C indicates their relationship to the PEG lamellae. Interestingly, the scattering in q < 0.8 nm−1 at 80 °C is much stronger than that of the pure PEG sample. This suggests the existence of a heterogeneous phase at 80 °C. This finding explains the improvement of the G′ (or η*) values of these two nanocomposites at 80 °C. In the SAXS profiles of NC-10 it is difficult to identify any scattering peaks at the two temperatures, but there is strong scattering in q < 0.8 nm−1. We speculate that the periodicity and orientation of the lamellar nanostructure, which has been already revealed in Figure 5C, are poor. It is interesting to see that the scattering peaks reappeared in NC-20 and NC-50 even at 80 °C. The peak positions are q1 = 0.24 nm−1 and q2 = 0.48 nm−1 at 20 °C and q1 = 0.29 nm−1 and q2 = 0.56 nm−1 at 80 °C for NC-20, and q1 = 0.24 nm−1 and q2 = 0.51 nm−1 at 20 °C and q1 = 0.29 nm−1 and q2 = 0.57 nm−1 at 80 °C for NC-50. The q1/q2 ≈ 1/2 ratio indicates a lamellar structure. The d-spacing values are d = 26.2 nm at 20 °C and d = 21.5 nm at 80 °C for NC-20 and d = 26.2 nm at 20 °C and d = 21.5 nm at 80 °C for NC-50. These values are larger than that of the pure PEG. We believe that the PEG and the hybrid coassemble into the lamellae through cocrystallization of the pure PEG and the PEG block, and thus they are stable at temperatures above the PEG melting point. The SAXS profiles of the S-POM−PEG hybrid show two wide scattering peaks at q1 = 0.32 nm−1 and q2 = 0.67 nm−1 at 20 °C and q1 = 0.35 nm−1 and q2 = 0.70 nm−1 at 80 °C, respectively. The q1/q2 = 1/2 ratio indicates the formation of the lamellar structure with d = 19.6 nm and d = 18.0 nm. These values are larger than that of pure PEG because the lamellae are composed of alternating S-POM complexes and PEG layers (also see the TEM image in Figure 5F). In comparison to the SAXS profiles of pure PEG, the wider scattering peaks of the hybrid at 20 and 80 °C indicate that a stable hybrid lamellae has formed with poor periodicity and orientation. Compared with nanocomposites, however, the periodicity and orientation of the hybrid lamellae are better. Suggested Nanostructure Models. In Scheme 2 we propose four ideal models to depict the nanostructures of nanosheets, pure PEG, nanocomposites, and hybrid. Scheme 2A shows a piece of a bilayer in which two layers of hybrid arranged in such a way that their S-POM complexes are projected inward while their PEG chains stretch on the outer surfaces. In other words, the S-POM complex bilayer is sandwiched by two PEG layers. This model describes the hybrid self-assembled nanosheets found in NC-2, NC-5, and NC-10. This bilayer structure is based on our data. The thickness of its edge-on wrinkled stripe is 2−3 times that of the POM cluster length (see Figure S14 in Supporting Information). Scheme 2B shows a typical chain-folded model of polymer lamellar crystals.33 In this model, the ∼16 nm d-spacing corresponds to the sum of a ∼15 nm thick once-folded lamella in the 5000 Da PEG chain and a ∼1.0 nm thick amorphous layer.35,36 The periodicity and orientation of the lamellar structure are almost perfect. This is the reason that we detected the multiple scattering peaks in our SAXS characterization. Scheme 2C presents a model of the nanocomposites in which dotted or platy aggregates of the S-POM complexes are embedded in between the PEG lamellae. In this model we still

5E). For NC-20 and NC-50, it is difficult to see the HSA nanosheets. The orientation of the PEG-HCC lamellae suggests that the view area is a part of the spherulites. The nanostructures of the S-POM−PEG hybrid (Figure 5F) are very interesting. At first, it is difficult to see whether the nanosheets are present in the solid sample of the pure hybrid. The good contrast in this image clearly shows well-defined alternating white and black layers that correspond to S-POM complex and PEG layers. These layers form a hybrid crystallized (HC) lamellae, the fourth nanostructure that can be found in this study. On a larger scale, the lamellar orientation and arrangement presented in this image are better than those formed in the nanocomposites with f h = 20 and 50% (see Figure 5D,E). More importantly, it reveals a typical lamellar morphology within a part of spherulites that is identical to those found in spherulites of most crystalline polymers.34 In summary, we found four nanostructures in the nanocomposites: hybrid self-assembled (HSA) nanosheets, PEG crystallized (PEGC) lamellae, PEG/hybrid cocrystallized (PEGHCC) lamellae, and hybrid crystallized (HC) lamellae. We also noted a conversion from the nanosheets or lamellae to the 3D network of lamellae with increasing hybrid component. SAXS Characterization. We also used small-angle X-ray scattering (SAXS) to study the samples’ nanostructures to avoid false appearances possibly caused by selecting a wrong area for TEM imaging. In this work, the SAXS analyses were executed at 20 and 80 °C, which correspond to the solid state and melt state of the PEG matrix. Figure 6 shows the SAXS profiles of these

Figure 6. SAXS scattering profiles of the seven samples at 20 °C (A) and 80 °C (B).

samples at 20 °C (A) and 80 °C (B) in which the logarithmic scattering intensity, log I, was plotted versus the scattering vector q. Here, q = 4π sin θ/λ, in which 2θ is the scattering angle and λ = 0.154 nm is the wavelength. The scattering peaks are indicated by red arrows. The scattering vector of the peak positions and the corresponding d-spacing values are summarized in Table S3 of the Supporting Information. At 20 °C, the PEG sample presents a SAXS profile with four clear scattering peaks at q1 = 0.38 nm−1, q2 = 0.75 nm−1, q3 = 1.12 nm−1, and q4 = 1.49 nm−1. The ratio of q1/q2/q3/q4 = 1/2/3/4 indicates the formation of a perfect lamellar structure with a d-spacing value of d ≈ 16.6 nm, corresponding to a one folded-chain lamellae.35,36 At 80 °C, the PEG lamellae melted and these scattering peaks disappeared. F

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Scheme 2. Four Ideal Models Depicting the Nanostructures of the Nanosheets (A), Pure PEG (B), Nanocomposites (C), and Hybrid (D)

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CONCLUSIONS In summary, we have successfully prepared a series of nanocomposites from an aqueous solution of PEG and the hybrid consisting of the S-POM complex and the PEG chain. Both the PEG crystallization and the self-assembly of the hybrid during the concentration of their solutions form four nanostructures: hybrid self-assembled nanosheets, PEG crystallized lamellae, PEG/hybrid cocrystallized lamellae, and hybrid crystallized lamellae. Within the hybrid nanosheets or lamellae the S-POM complex self-assembles into dotted or platy aggregates with a bilayer structure. Because aggregates of the SPOM complexes are covalently covered by the PEG layers, they have good miscibility with the PEG and homogeneous dispersion in the melted PEG matrix. Above the melting point of the PEG matrix, the stable aggregates increase the shear storage modulus of the nanocomposites by many orders of magnitude as seen in our dynamic rheology experiment. Therefore, the nanosized aggregates, formed via self-assembly of the hybrid, act as a nanoenhancer to efficiently improve the performance of the nanocomposites. Overall, this work opens up new opportunities for generating nanocomposites using conventional polymers and self-assembled nanoenhancers. We expect that the nanocomposites could be used as adhesives for improving tape stability at higher temperatures.

suggest an S-POM complex bilayer because the determined size of the dark dots or layers is close to the double long axis of the POM cluster. This model represents the structure of the PEGHCC lamellae. In this case, the d-spacing values are larger and the periodicity and orientation of the lamellar structure become worse compared to pure PEG samples. When the hybrid content increases, the S-POM complex dots will gradually expand laterally into two-dimensional S-POM complex plates. Finally, its branching can build up a network in the high hybrid fraction. Scheme 2D shows a model of nanostructures formed in the pure hybrid. In this model, the hybrid lamellae are constructed by alternatively arranging the S-POM complex and PEG layers. The formation of the hybrid lamellae also indicates that the crosssectional area occupied by the S-POM complexes should be identical to that occupied by the PEG blocks.26 Formation and Reinforcement of the Nanoenhancer. The formation of the nanostructures described in these models is associated with the self-assembly of the S-POM complex in aqueous solutions and/or in PEG matrix. In these models we suggest that the S-POM complex bilayer can be maximized because of the interactions between the ellipsoidally shaped complexes. The nanosheet formation in the aqueous solutions and/or the drying process of the solutions is due to poor water solubility of the complex. In concentrated solutions, crystallization and cocrystallization of the pure PEG and the PEG block in the hybrid also assemble the S-POM complex into the bilayers. During PEG crystallization, the S-POM complexes cannot join the PEG lamellae, and thus they aggregate to form bilayers embedded in the PEG lamellae. The S-POM complex bilayers are stable at temperatures above the PEG melting point because of the strong interactions among the S-POM complexes. Meanwhile, the two PEG layers, covalently coated on the S-POM complex bilayers, have a good interaction with the melted PEG matrix. Moreover, these hybrid self-assembled aggregates are nanosized and homogeneously dispersed or form a network in the PEG matrix. Because of these favorable factors, the stable aggregates will efficiently improve the performance of the melted PEG matrix. In our experiment, we found that the rapid increase in G′ values at f h ≤ 10% indicates that a small amount of the aggregates have a profound influence on the performance of the melted PEG.37 When f h > 10%, further increases in G′ causes further improvements due to the formation of the networked S-POM layers.38 Hence, we can conclude that the hybrid aggregates enhance the PEG matrix. Because of their nanoscale size and formation as well as growth via self-assembly, we consider the aggregates to be a selfassembled nanoenhancer.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional experimental results. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-22-23498126 (W.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support of the National Natural Science Foundation of China for grants (Grants NSFC 21274069 and 21334003), PCSIRT (IRT1257), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry. We also thank Ms. Ellen Gao and Mr. Eric Zhang for their assistance with the rheometrical experiments at Anton Paar Experimental Station (Beijing). G

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DOI: 10.1021/acs.macromol.5b00214 Macromolecules XXXX, XXX, XXX−XXX