Cooperative Soft-Cluster Glass in Giant Molecular Clusters

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Cooperative Soft-Cluster Glass in Giant Molecular Clusters Yuchu Liu,†,‡,§,# GengXin Liu,*,†,# Wei Zhang,‡ Chen Du,‡ Chrys Wesdemiotis,‡ and Stephen Z. D. Cheng*,†,‡,§ †

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Center for Advanced Low-Dimension Materials, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, China ‡ South China Advanced Institute for Soft Matter Science and Technology, School of Molecular Science and Engineering, South China University of Technology, Guangzhou 510640, China § College of Polymer Science and Polymer Engineering, Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Three-dimensional giant molecular clusters, OPOSS16 and OPOSS24, have been designed and precisely synthesized. They have 16 or 24 octyl polyhedral oligomeric silsesquioxane (OPOSS) building blocks chemically linked by short, flexible chains. Despite the small difference in their molecular weight, 25 and 38 kg/mol respectively, they show different dynamics above the conventional glass transition temperature (Tg). OPOSS16 in the bulk quickly turns to viscous. In contrast, OPOSS24 shows a long-lasting elastic plateau even far above the Tg, corresponding to confinements on individual OPOSS. To achieve any confinements, clusters have to be immobile, although individual OPOSS possesses mobility. It directly confirms that in the bulk giant molecular clusters possess a cooperative soft-cluster glass in addition to conventional glass if they are larger than the critical diameter. This critical diameter is close to the estimated diameter of cooperative rearranging regions during glass transition. Giant molecular clusters present unique dynamics different from colloidal caging in solution or polymer entanglements.

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INTRODUCTION Chainlike polymers possess a distinct one-dimensional (1D) curvilinear dynamics as “reptation” within “tubes” formed by confining surrounding chains.1 One effective way to alter the dynamics of polymers is to deviate from the linear architecture, e.g., by using multiarm star,2 long-chain branching,3 and so on. Molecular nanoparticles, such as polyhedral oligomeric silsesquioxane (POSS), fullerene, polyoxometalates, and others, have also been incorporated within or dangling on the polymer linear backbone.4,5 However, the fundamental dynamics is still dominated by entanglement. Without chain ends, a topologically driven glass has also been proposed for infinitely long ring polymers.6,7 The extreme deviation from linear architecture could be achieved when the connectivity leads to a three-dimensional (3D) cluster instead of a 1D chain.2 Modularly synthesized giant molecular clusters containing rigid molecular nanoparticles as building blocks are a new class of unconventional macromolecules.8−14 Unlike polydisperse polymers, they have exact molecular weights, chemical compositions, sequences, and topological structures. They possess virtual shape persistency and fulfill the requirement of forming soft-clusters. They are named “soft-cluster” to emphasize that they are not a rigid aggregate,15 thus different from colloidal hard spheres. The model 3D soft-clusters contain many chemically linked molecular nanoparticles. It is realized as a cluster by using POSS with bulky octyl side groups (OPOSS) as the building © XXXX American Chemical Society

blocks and linking them to a central core POSS (Figure 1). This approach achieves the noncrystalline nature and relatively low Tg of the clusters. Only weak van der Waals forces exist among clusters. Within one cluster, short and flexible chemical linkers constrain molecular nanoparticles not to move far apart from each other, as a cooperative soft-cluster. Soft colloids,16−28 notably dendrimers, multiarm stars, and cross-linked nanogels in the bulk states, were developed in recent years. With varying elasticity, they are a unique model system. Soft-clusters with well-defined chemical structures are orders of magnitude smaller than typical soft colloids and are not interfered by entanglements or cross-linking. We ask what dynamics these soft-clusters can be. Because there is no entanglement, the dynamics should be different from the behavior of reptation. Because they are smaller and softer and contain well-defined building blocks, the dynamics in the bulk might be different from colloidal caging. Indirectly supported by our previous experimental results,10 we hypothesized that above the conventional glass transition giant molecular clusters might have dynamics analogous to the cooperative rearrangement in glasses.29−34 Conventional glass transition can be revealed by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and so on. If this Received: March 18, 2019 Revised: May 4, 2019

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DOI: 10.1021/acs.macromol.9b00549 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Scheme of the modular synthesis of OPOSS16 and OPOSS24.

Figure 2. (a) Modular synthesis routes of OPOSS16 and OPOSS24 and chemical structure of OPOSS. (b) Chemical structure of OPOSS16. (c) 1H NMR spectrum. (d) 29Si NMR spectrum. (e) MALDI-TOF-MS. (f) GPC trace of OPOSS16. (g) Chemical structure of OPOSS24. (h) 1H NMR spectrum. (i) 29Si NMR spectrum. (j) MALDI-TOF-MS. (k) GPC trace of OPOSS24.

OPOSS24 (containing 16 or 24 OPOSS, respectively) as shown in Figure 1. They are different from our previous work in which no specific interactions (such as hydrogen bonding or crystallization) are involved. After verification of their chemical structures and molecular characters, their dynamics will be revealed by rheological measurements and analysis.

speculation is correct, the study of giant molecular clusters may reveal a new cooperative soft-cluster glass and lead to unexplored material properties. Thus, the question is at what size of these clusters this new glass behavior appears. To achieve large enough clusters, we carefully design and synthesize the two giant molecular clusters OPOSS16 and B


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RESULTS AND DISCUSSION Synthesis and Characterization of Giant Molecular Clusters. The modular syntheses of OPOSS16 and OPOSS24 by two chemical reactions are shown in Figure 2a and Figure S3. During the first reaction, all eight hydroxyl groups on hydroxyl-functionalized POSS are converted by esterification with mono- or triple-alkyne groups. The results are POSS cores surrounded by 8 or 24 alkyne groups. The structures of POSS-8yne and POSS-24yne are confirmed by 1H, 13C, and 29 Si NMR as well as MALDI-TOF MS (Figures S14−S22). During the second reaction, N3-2OPOSS and N3-OPOSS, as shown in Figure 2a and Figure S1 are “clicked” onto POSS8yne and POSS-24yne, respectively.11−13 The results are giant molecular clusters with 16 or 24 OPOSS building blocks. Their analyses and characterizations are summarized in Figure 2b−k. In 1H NMR spectra (Figures S22 and S27, comparing to Figures S14 and S18), the disappearance of the chemical shift around 4.7−4.8 ppm confirms the full conversion of triple bonds and the completion of “click” reactions. All protons can be clearly assigned, and the integration ratio matches well with designed structures. In 29Si NMR spectra, four chemical shifts are found in both molecules and assigned (Figures S24 and S29). The major m/z peaks in MALDI-TOF-MS (Figure 2e,j, Figures S25 and S30, and Table 1) agree with the theoretically

Figure 3. (a) Space-filling models show that OPOSS24 is larger than OPOSS16. (b) SAXS patterns of OPOSS16 and OPOSS24 in solutions show the characteristic diameters of OPOSS16 and OPOSS24 as well as individual OPOSS. Lines correspond to scattering intensity I on the left vertical axis; dotted lines correspond to Iq4 on the right vertical axis.

patterns at 4.8 and 6.2 nm, respectively, as shown in Figure 3b. These values are slightly larger than dM values estimated in the bulk. It is reasonable considering the inclusion of solvent molecules within the clusters. SAXS patterns in the bulk (Figure S32) show almost identical broad correlation scattering peak at low q around 4.4 nm. From a dendrimer point of view, OPOSS16 and OPOSS24 are of the same first generation. The conformation of OPOSS16 in the bulk is not an ideal sphere-like one. The characteristic scattering may be an average of many different detecting angles and resulted in this observation.26 Table 2 summarizes the molecular and cluster parameters that are important for following discussions, including the

Table 1. Molecular Weights of OPOSS16 and OPOSS24 (MAg = 107.9 g/mol) OPOSS16 OPOSS24

Mn (calcd)

Mn (MALDI-TOF-MS)

24922.8 38416.9

25030.7 (24922.8 + MAg) 38525.3 (38417.4 + MAg)

calculated molecule masses. Traces given by gel permeation chromatography (GPC) show narrow peaks, indicating each sample is one substance (Figure 2f,k and Figures S26 and S31). From all of the experimental evidence collected, we can conclude that OPOSS16 and OPOSS24 are obtained as designed with no traceable impurities. Structures and Diameters of Giant Molecular Clusters. The two samples have identical chemical linkers between the core POSS and peripheral OPOSS building blocks. With eight more peripheral OPOSS building blocks, OPOSS24 should be larger than OPOSS16 as illustrated in Figure 3a. For the ease of analysis, we estimated a characteristic diameter dM of giant molecular clusters in the bulk via 3 (6M )/(πρN ) , assuming they are densely packed spheres. w A Although they may not be truly spherical, this governing factor helps to quantify their dynamics. With densities measured to be around 1.07 g/cm3, dM would be approximately 1.5, 4.2, and 4.8 nm for OPOSS01 (single OPOSS as a cluster), OPOSS16, and OPOSS24, respectively. Scattering results corroborate these characteristic diameter sizes. In agreement with the estimated dM of OPOSS01, one broad correlation scattering peaks (dS) at a center of 1.26 nm can be observed in a small-angle X-ray scattering (SAXS) pattern in the bulk (Figure S32). It has also been observed at 1.45 nm in the solution and at 1.21 nm in the bulk of SAXS patterns of OPOSS16 or OPOSS24 (Figure 3b and Figure S32). In agreement with the estimated dM of OPOSS16 and OPOSS24, the characteristic diameters dS of OPOSS16 and OPOSS24 in the solution can be observed from Iq4 in SAXS

Table 2. Summary of Parameters from Characterizations and Analysis

OPOSS01 OPOSS16 boundary, ζ OPOSS24

Mw [g/mol]

dM [nm]

dS [nm]

1326 24923

1.5 4.2 ∼4.5 4.8

1.5 4.8

38417

6.2

dM/rb

Gpl [Pa]

d0 [nm]

N.A. 5.6 ∼6 6.4

N.A. N.A.

N.A. N.A.

4.2 × 105

1.9

exact molecular weight (Mw), estimated diameters in the bulk (dM), and diameters obtained by SAXS in the solution (dS). The concepts of boundary ζ and bead diameter will be defined in the following section. Cooperative Glass: A Long-Lasting Elastic Plateau and a Drastic Viscous-to-Elastic Transition. DSC thermodiagrams in Figure 4a show that the conventional glass transition temperatures Tg of OPOSS16 and OPOSS24 are almost identical at around −8 °C; that is about 20 °C higher than the Tg of OPOSS01. We measure linear viscoelasticity at 10 rad/s during temperature ramp and plot storage G′ and loss G″ modulus against temperature as in Figure 4b. Master curves of G′ and G″ in the frequency domain are given in Figure 5a at the reference temperature of 30 °C and complementary to the temperature ramp. They are constructed by time−temperature superposition of the small-amplitude oscillatory shear (SAOS) data as shown in Figures S34 and S35. The horizontal shifting factor aT is given in Figure 5b. C


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Figure 4. (a) DSC thermodiagrams show OPOSS16 and OPOSS24 have almost identical glass transition temperature. (b) Linear viscoelasticity of OPOSS16 and OPOSS24 during temperature ramp shows storage G′ and loss G″ modulus at different temperatures. (c) Enlarged plot of G′ vs temperature of OPOSS24 and linear fit using data between 100 and 140 °C.

Figure 5. (a) Master curves of G′ and G″ in the frequency domain at a reference temperature of 30 °C constructed from SAOS data, between 0 and 70 °C for OPOSS24 and between 0 and 50 °C for OPOSS16. (b) The shifting factor aT between 0 and 70 °C for OPOSS24 and between 0 and 50 °C for OPOSS16. WLF fitting of OPOSS24 extrapolates to 140 °C shows aT between 140 and 70 °C for OPOSS24 is 10−4.

Above 30 °C or below 10 rad/s, the rheological behavior of OPOSS16 is dominated by its viscous component. In contrast, with only eight more OPOSS (13 kg/mol), OPOSS24 shows an elastic plateau even above 50 °C or below 0.01 rad/s. Evidently, this plateau would not end anytime soon, so we stop at 140 °C and do not go to higher temperatures risking degradation of the sample. Figure 5a does not reach lower frequencies because it is hard to determine aT as SAOS gives plateau at temperatures higher than 70 °C. If we extrapolate aT to 140 °C (Figure 5b) by the WLF equation (log aT =

C1(Tg − T ) (T − Tg + C 2)

plateau, their typical molecular weight ranges from a few thousand kg/mol to 10000 kg/mol.18−22 Giant molecular clusters are quite different from typical soft colloids in the absent of solvent and in their much smaller molecular weight and diameter. This drastic transition from viscous to elastic behaviors happens within a window of only eight OPOSS or 13 kg/mol. This dynamics slowdown of giant molecular clusters with respect to the diameter of clusters has to be of cooperative nature, like a glass transition. To differentiate from conventional glasses, such state of soft-clusters is named “cooperative glass”. We observe it experimentally, evidenced by the longlasting elastic plateau for a molecular weight of 38 kg/mol and the drastic viscous to elastic transition over a difference of only 13 kg/mol. This viscous-to-elastic transition occurs, with respect to the diameter of clusters, somewhere between 4.2 and 4.8 nm (the diameter dM of OPOSS24 and OPOSS16). We propose that a critical diameter ζ governs the dynamics of giant molecular clusters. Within this region of eight OPOSS, 0.6 nm and 13 kg/ mol, the exact value of ζ is less important than the existence of such divergent dynamics slowdown. When clusters contain a large number of OPOSS building blocks and have a large diameter, the mobility of each cluster requires many clusters to move cooperatively to clear a path, which becomes impossible under thermal fluctuation.

+ C ), the elastic plateau in Figure 5a

would extend to at least 10−9 rad/s. That is 15 orders of magnitude lower than the high-frequency crossover. These giant molecular clusters do not have any entanglement or specific association (such as hydrogen bonding). The interactions between OPOSS and clusters are only weak van der Waals force. At temperatures far above Tg (50−140 °C) or frequencies (below 0.1 rad/s at 30 °C), individual OPOSS should have sufficient mobility. Yet, we observe this distinct long-lasting elastic plateau far above Tg. With a molecular weight of 38 kg/mol, OPOSS24 in the bulk cannot relax even a small perturbation at ∼150 °C above its Tg. In other words, OPOSS24 remains immobile to prevent relaxation. If thousands of POSS are arranged in chain-like manner,5 such polymers can still reptate and relax. Although soft colloids may also reach colloidal glass state with an elastic D


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Figure 6. Nonlinear rheological measurements on OPOSS24 at 70 °C. (a) Strain sweep experiments with increasing oscillatory strain amplitude at 10 rad/s. (b) Stress relaxations after different amounts of step strains. (c) Creep test under different amounts of stress; the increase of strain is monitored.

cluster. We now observe that giant molecular clusters that are smaller but close to the boundary ζ have this intermediate region and then turn to viscous. The chemistry of the linker may also be a contributing factor besides diameter alone, especially in this intermediate region, and will be examined in the future. Cooperative Glass under Large Perturbation. Large clusters, for instance OPOSS24, are in a cooperative soft-cluster glass state, with a long-lasting elastic plateau Gpl. Thermal energy (before reaching degradation) is not enough to cooperatively activate more than the critical number of molecular nanoparticles. However, external activations, such as large strain or stress, should be able to drive them into a fluid state. The strain sweep in Figure 6a shows linear responses end at a strain amplitude of 20%. The crossover from elastic to viscous response, critical strain γc, is at 60%, above which OPOSS24 becomes a fluid. Figure 6b shows, upon larger strains, for instance 100%, relaxation happens immediately, and the stress can fully relax. After a sudden step strain of 10 or 30%, relaxation does not take place until a few seconds later. A characteristic relaxation time on the order of 20 s is revealed. It equals to10−5 rad/s in Figure 5a (aT from 30 to 70 °C is 2.3 × 10−4). We suspect this is the breaking of confinement on individual OPOSS. Finally, after thousands of seconds, 8% of the stress still remains unrelaxed. In Figure 6c, constant stress is applied instead of strain. A stress of 20 kPa is not enough to activate OPOSS24 into flow, while 40 kPa is sufficient. If we take the critical stress σc as around 30 kPa, then σc ≈ Gpl × γc. To further study the dynamics of cooperative soft-cluster glass, such nonlinear rheological measurements, as widely used in soft glassy materials in general, would provide more information besides linear rheology.20,42−44 Theoretical Connections of Soft-Cluster. Claiming as a new glass state, we provide the following analysis to rationalize our findings. The smallest mobile unit in glassy state is commonly referred to as the “bead”.45,46 It would also be the basic mobile unit at higher temperatures. For giant molecular clusters, we can roughly estimate the bead diameter rb to be 0.75 nm based on πrb3/6 = 35.6kBTg/G (G is the glassy state shear modulus of ∼6 × 108 Pa).10,46 Assuming Gh ∼ kBT/ (dh)3, the characteristic modulus at high frequency Gh of OPOSS24 might correlate to a mobile unit with Mh of 420 g/ mol and dh of 1 nm, roughly corresponding to the size of the estimated bead. With rb, the giant molecular cluster can then

A minor issue is that although we made OPOSS24 and OPOSS16 with identical linker from core POSS and peripheral OPOSS, the intrabranch distance between adjacent OPOSS has not been the same. This will be perfected in future work and may minor influence the exact boundary. Confined Dynamics. The long-lasting elastic plateau suggests confined dynamics and that the cluster as a whole is not diffusing. Yet it is already 150 °C above Tg; logically, the giant molecular cluster should not be a hard sphere at such condition. In other words, different OPOSS within a cluster should have quasi-independent motion, given that they are linked by short and flexible chains. In the plateau region, G′ increases linearly with temperatures as enlarged in Figure 4c. The slope from 100 to 140 °C is 3.82 × 103 Pa/°C. The elastic plateau modulus might reveal the rattling unit in confinement agitated by thermal energy,35−41 with the characteristic mass M0 of 2328 g/mol via Gpl/T ≈ ρR/ M0 and the diameter d0 of 1.9 nm via 3 (6M 0)/(πρNA ) . Numerically, those values suggest that this elastic plateau originates from confinement on the level of individual OPOSS, not on the level of the whole cluster. Would such confinement on individual OPOSS break at a finite or an infinite time? We hypothesize it should be finite, but the breaking might not cause relaxation because not all confinements break at the same time. Thus, it does not interrupt the elastic plateau. Future experiments likely with labeled samples would provide a definite answer. Nonetheless, the clusters must be immobile for confinements to be effective. There are two levels of structures and dynamics: the level of cluster and the level of constituting OPOSS building block. This feature is different from colloidal glass, which dominated by one level of mobility−the colloidal particles, in hard or soft colloids.2,28 In the previous section, we have discussed the lowfrequency, long-time, high-temperature behaviors of the two giant molecular clusters. Between the low-frequency region and high-frequency (conventional glass) region, there is an intermediate region. OPOSS24 shows a plateau-like behavior at those intermediate frequencies (Figure 5a, between 0.1 and 105 rad/s) or temperatures (Figure 4b, between 5 and 45 °C). A characteristic value Gh of 6.35 × 106 Pa can be taken from the local minimum of G″/G′.39 As a comparison, OPOSS16 shows no plateau in this narrow intermediate region, and its G′ and G″ values are very closed with scaling exponent around 0.5. This is also different from the previous Zimm-like behavior on a smaller giant molecular E


Macromolecules be abstracted as a physical model of soft cluster without chemical details. According to experimental measurements and simulation on the cooperative rearranging region, as well as random firstorder transition (RFOT) theory, the critical diameter ζ of cooperative rearranging regions to reach glassy state would be ζ ∼ 6rb.29−34 This value may not be exact but at least very close. For the soft-clusters here, 6rb is thus 4.5 nm, between the diameter dM of OPOSS24, 4.8 nm, and dM of OPOSS16, 4.2 nm. Therefore, the experimental boundary seems in agreement with the critical diameter of cooperative rearranging regions at the conventional glass transition, hinting a connection to its glassy nature. Glassy dynamics diverges quickly, so does the viscous-toelastic transition with respect to cluster diameter. This boundary would be in the range, if not numerically exact. The numerical coincidence would be supportive to analogize soft clusters as the cooperative rearranging cluster, thus revealing a new glass state: cooperative glass. As of the difference between OPOSS16 and previous T10M in the intermediate region, we hypothesize if two clusters are too large to move together cooperatively, confinement and immobility would be effective in a narrow temperature window above Tg. If the diameter of two clusters combined equals ζ, the boundary would be 2−1/3ζ/rb = 4.7. T10M contains 11 POSS, and dM/rb is 3.9, while dM/rb of OPOSS16 is 5.6. It agrees with current experimental results and will be examined in the future. Of course, there may be other ways to interpret the results, for instance in the framework of soft colloids23,25 or in the framework of mode coupling theory.47−49 Similarities and differences with soft colloids have been illustrated in previous sections. Figure 7 makes a summary: (1) the gray dashed circle depicts the confinement on individual OPOSS and its rattling; (2) the cluster of OPOSS building blocks as a whole is immobile, i.e., the center of mass of the cluster should not move. Giant molecular clusters indeed exhibit dynamics different from colloidal suspensions or chains of nanoparticles.5

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CONCLUSION

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MATERIALS AND METHODS

Giant molecular clusters used in this study are chemically linked multiple OPOSS in three dimensions. They do not possess entanglement or strong interaction/association. A critical diameter ζ divides the dynamics of clusters based on their diameters. This ζ can approximate to the cooperative rearranging length scale at glass transition. Clusters larger than ζ exhibit a long-lasting elastic plateau Gpl, even at far above Tg. For instance, OPOSS24 has a molecular weight of 38 kg/mol, much lower than the typical soft colloids. The value of Gpl correlates with individual OPOSS in confinement. Such confinement is possible because large giant molecular clusters are still immobile. Thus, there are two levels of structures and dynamics: cluster and constituting OPOSS building block. The 3D correlation of molecular nanoparticles within clusters causes this immobilitya new kind of dynamics different from polymer entanglement or colloidal caging. The viscous-to-elastic transition happens at a narrow region of no more than 8 OPOSS, 0.6 nm, and 13 kg/mol. It has to be of cooperative nature. Thus, we claim it as a cooperative glass. We also find that large strain or stress can drive cooperative softcluster glass into flow and the critical strain and stress is linked by Gpl. We expect this phenomenon to be general and not specific to any particular type of nanoparticles to form the cluster. Detailed mapping of diameters, conformations, and mixtures as well as examining the rigidity and length of linkers are on schedule as future work.

Synthesis. Details are provided in the Supporting Information. Density. The density was measured by adjusting the density of KI solution so that a small piece of sample can be suspended in the solution. The density of the solution was then measured by weighting its weight in a 5 mL volumetric flask. Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q200, with the heating/cooling rate of 10 °C/min. SAXS. Measurements were taken on a Rigaku MicroMax 002+ instrument with a voltage of 50 kV and a current of 0.6 mA. The detector was a Dectris Pilatus 300K at 2 by 2 movable configurations. The samples in the bulk were self-adhered to thin aluminum foil. Scattering patterns in the bulk are shown in Figure S32. The samples in THF solutions at 10 mg/mL were flame-sealed in 1.5 mm quartz capillary. The raw data of the solution were presented in Figure S33 and smoothed by the LOWESS method with a span of 0.1 to reduce noise (Figure 3b). Rheology. The samples were dissolved in THF and dropwise cast on polyimide film to form a dome larger than 8 mm and higher than 1 mm. The solvent, THF, was slowly evaporated first at ambient conditions, then low vacuum, and finally high vacuum at elevated temperature (∼60 °C). The samples were detached from the film and transferred to 8 mm parallel plates on an ARES-G2 (TA Instruments). The linear viscoelasticity was measured under an oscillatory strain typically of 0.1 or 0.3%. Autostrain adjustment and axial force adjustment were active for temperature ramp experiments with calibrated thermal expansion of the fixture. Control stress experiments in Figure 6c was performed with 15 mm 2° cone and plate on a MCR301 (Anton-Paar).

Figure 7. Scheme of giant molecular clusters (d > ζ) in the bulk. The center of each cluster would not move. The long-lasting Gpl originates from the confinement, as shown by the gray dashed circle, on individual OPOSS. F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00549.

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Materials and characterizations of synthesis, synthetic procedures; Figures S1−S35 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.Z.D.C.) E-mail: [email protected]. *(G.L.) E-mail: [email protected]; [email protected]. ORCID

Yuchu Liu: 0000-0001-9780-8724 GengXin Liu: 0000-0002-2998-8572 Wei Zhang: 0000-0002-9321-6411 Chrys Wesdemiotis: 0000-0002-7916-4782 Stephen Z. D. Cheng: 0000-0003-1448-0546 Author Contributions #

Y.L. and G.L. contributed equally. G.L., Y.L., and S.Z.D.C designed the research; Y.L. and W.Z. (synthesis), C.D. and C.W. (mass spectroscopy), and G.L. (structure and rheology) performed the research; G.L. and Y.L. analyzed data; and G.L., Y.L., and S.Z.D.C wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.L. is supported by Donghua University. This work is also supported by NSF DMR-1408872.

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REFERENCES

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