Nanorod-Based Supramolecular Nanocomposites: Effects of Nanorod

Aug 30, 2016 - 180 nm NRs (Figure 3) do not demonstrate the good ordering seen by smaller NRs, though some small well-ordered regions composed of near...
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Nanorod-Based Supramolecular Nanocomposites: Effects of Nanorod Length Kari Thorkelsson,† Noah Bronstein,‡ and Ting Xu*,†,‡,§ †

Department of Materials Science and Engineering and ‡Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Nanorods (NRs) have unique anisotropic properties that are desirable for various applications. Block copolymerbased supramolecules present unique opportunities to control inter-rod ordering and macroscopic alignment of NRs to fully take advantage of their unique anisotropic properties. Here, we studied the effects of NR aspect ratio where the NR length is in the range of 20−180 nm on the assemblies of NRs in supramolecular framework. At a moderate loading (∼3 vol %), well-ordered assemblies of 37−90 nm NRs embedded in the supramolecular framework were formed. Shorter NRs (∼22 nm) coassemble with the supramolecule but were not well-ordered and displayed little orientational control within the microdomains. In contrast, longer NRs (∼180 nm) formed kinetically trapped states that restricted the formation of well-ordered coassemblies in NR/ supramolecule blends. Additionally, the NRs are shown to be capable of kinetically trapping the system after normally reversible morphological transitions triggered by the thermal dissociation of the supramolecule, arresting the system away from a stable morphology. These studies shed light on the effects of NR-induced kinetic arrest on the self-assembly of a supramolecular nanocomposite.

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INTRODUCTION Nanorods (NRs) have unique anisotropic properties that are desirable in various applications, such as photovoltaics, plasmonic devices, magnetic storage, and sensors.1−5 Achieving control over inter-rod ordering and macroscopic alignment of NRs is necessary to fully take advantage of their unique anisotropic properties. While significant progress has been made in the generation of ordered assemblies of spherical particles, this has not been the case for anisotropic nanoparticles. A variety of methods have been developed to understand and manipulate the assembly of NRs in solution, in thin film, and in bulk, including DNA-guided assembly,6−8 the application of external electric,9,10 magnetic,11 solvent,12 or shear fields,13,14 and template-assisted assembly.15,16 There have also been significant developments in blends of NRs and polymer that combine the advantages of polymer processing and templated guided NR assembly.17−20 Polymers alone have been shown to be capable of both dispersing NRs or aggregating them into networks,21 while block copolymers (BCPs) can extend this to more precisely controlling the dispersion and alignment of the NRs.18 By changing the periodicity and morphology of the BCP, a particular spatial © 2016 American Chemical Society

distribution of NRs can be selected. Furthermore, extensive research has been done on macroscopic alignment of BCPs,15,22−24 and the bulk of this research would likely remain valid for BCP-NR composites. BCP-based supramolecules offer more advantages in the fabrication of hierarchical structures of inorganic nanoparticles.25,26 By attaching a small molecule to one block of a BCP, the polymer−particle interaction can be mediated and the geometric confinement of the NRs can be significantly improved.25 This approach was used to control the assembly of 32 nm CdS NRs using a supramolecule, polystyrene-blockpoly(4-vinylpyridine)(3-pentadecylphenol) (PS-b-P4VP(PDP)).25 In bulk, the incorporation of NRs depends on the interactions between the NR and supramolecules.27 The system under study here, alkylphosphonic acid-passivated CdSe/CdS seeded NRs in PS-b-P4VP(PDP), has a favorable ligand−comb block interaction that encourages the incorporation of nanorods into the comb block of the supramolecule. Upon NR Received: May 29, 2016 Revised: July 26, 2016 Published: August 30, 2016 6669

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used. Spectra were collected on a Pilatus 1M detector. For in situ scattering studies, the sample was heated from room temperature to 150 °C and then cooled down. The sample was annealed for 10 min at each temperature before the SAXS profile was collected and remained at that temperature for an additional 25 min.

incorporation, it is entropically costly to deform the supramolecular comb block, leading to geometric confinement of the NRs within the microdomain. The translational and orientational entropy of the NRs favors random distribution and orientation of the NRs, while the inter-NR interaction encourages particles to assemble adjacent to each other. The competition between these two contributions also affects the NR assembly. In addition to these energetic factors, the processing conditions and kinetics of assembly are also important and often determine the final morphology.19,28−31 The presence of NRs affects the viscosity of the nanocomposites and can cause kinetic arrest of morphology at elevated temperature.19,32 High aspect ratio NRs and high loading of NRs will increase the probability of kinetic jamming, leading to increased defect densities and decreased grain sizes.29,31 In light of this, it is important to study a range of NR lengths to acquire a better understanding of the NR-induced kinetic arrest. With knowledge of the limits on ordered assembly this phenomenon imposes, an appropriate NR length for a given concentration can be selected to maximize loading without sacrificing ordering. Here, we studied the assemblies of 22, 37, 47, 90, and 180 nm NRs in a supramolecular framework with a periodicity of 26 nm at loading rates of ∼1, ∼3, and ∼6 vol %. At a moderate loading (∼3%), well-ordered assemblies of 37−90 nm NRs embedded in the supramolecular framework were formed. Shorter NRs (∼22 nm) coassemble with the supramolecule but were not well-ordered and displayed little orientational control within the microdomains. In contrast, longer NRs (∼180 nm) formed kinetically trapped states that restricted the formation of well-ordered coassemblies in NR/supramolecule blends. The kinetics were then explored further at lower (∼1 vol %) and higher (∼6 vol %) loading rates to investigate the onset of kinetic arrest. Additionally, the NRs are shown to be capable of kinetically trapping the system after normally reversible morphological transitions triggered by the thermal dissociation of the supramolecule, arresting the system away from a stable morphology. These studies shed light on the effects of NRinduced kinetic arrest on the self-assembly of a supramolecular nanocomposite.

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RESULTS AND DISCUSSION The supramolecule used here is composed of polystyrene(19 kDa)-block-poly(4-vinylpyridine)(5.2 kDa) (PS-b-P4VP) and 3pentadecylphenol (PDP) in a 1:1.7 ratio between the vinylpyridine subunits in the P4VP and the PDP molecules, with the PDP forming hydrogen bonds to the P4VP and forming a comb block as shown previously.33,34 In short, the P4VP(PDP)1.7 block forms a stiff comb-like block where the P4VP chain acts as a backbone with the hydrogen-bonded PDP molecules acting as side chains. Additional PDP beyond a 1:1 vinylpyridine:PDP equivalent mostly intercalates between PDP that has hydrogen bonded to the P4VP, further lengthening and stiffening the comb block. The PS block remains in a random coil conformation. In bulk, this supramolecule forms a cylindrical morphology where PS cylinders are surrounded by a matrix of P4VP(PDP)1.7. The PS cylinders pack into a hexagonal lattice, and 32 nm NRs passivated with alkyl ligands have been shown to preferentially segregate to the P4VP(PDP)1.7 matrix, at the interstitial sites between three PS cylinders, due to a combination of the favorable enthalpic interaction between the P4VP(PDP)1.7 and the NR ligands, ΔHligand−polymer, and multiple entropic factors (see Figure 1a,b).27 The use of P4VP(PDP)r comb blocks instead of a typical coil−coil BCP more strongly confines the NRs by increasing the energetic penalty associated with deforming the

EXPERIMENTAL SECTION

Materials. PS(19000)-b-P4VP(5200) was purchased from Polymer Source, Inc. Chloroform was purchased from Fisher. All other chemicals were purchased from Sigma-Aldrich. All chemicals were used as received. Synthesis of Seeded Nanorods. CdSe/CdS seeded nanorod synthesis was adapted from the literature with size control.35−38 Details can be found in the Supporting Information. Sample Preparation. PS-b-P4VP and PDP were dissolved separately in chloroform, and the polymer solution was added dropwise to the PDP solution while stirring. The solution was stirred for 24 h, after which an appropriate amount of NRs dissolved in chloroform was added to the solution while stirring. The solution was then placed in a Teflon beaker and allowed to dry over 24 h, except for the samples depicted in Figure 4, which were placed in a sealed container and allowed to dry over approximately 4 weeks. TEM. Samples were embedded in resin (Araldite 502, Electron Microscopy Sciences) and cured at 60 °C overnight, before being microtomed on an RMC MT-X ultramicrotome (Boeckler Instruments) and collected onto copper TEM grids from water. Sections were imaged using an FEI Tecnai 12 at an accelerating voltage of 120 kV. SAXS. SAXS studies were carried out at the Advanced Light Source at beamline 7.3.3. X-rays with a wavelength of 1.240 Å (10 keV) were

Figure 1. Schematic showing potential locations of NRs within PS-bP4VP(PDP)1.7 microdomains. In the schematics, PS is red, P4VP(PDP)1.7 is blue, and the NRs are yellow. (a) NRs assembled at interstitial sites between three PS cylinders, with annotated dimensions. (b) An off-axis view of the assembly in (a). (c) NRs assembled perpendicular to the PS cylinders at sites requiring the least deformation of the P4VP(PDP)1.7 comb block. (d) An off-axis view of the assembly in (c). 6670

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Macromolecules polymer chain, ΔScon, and forcing the nanorods to assemble in the center of the P4VP(PDP)r microdomain.25−27 Furthermore, the hydrogen bond connecting the P4VP subunits and the PDP molecules is sensitive to temperature, and the PDP is soluble in the PS block at elevated temperatures,34 allowing for dynamic control of the comb block stiffness and even the morphology of the supramolecule. As the supramolecule is heated, it goes through a number of states: first the P4VP(PDP)r block melts (∼60 °C), then the PS block gains mobility (∼85 °C), and then above 100 °C the fraction of PDP that maintains a hydrogen bond with the P4VP decreases and above 130 °C PDP becomes soluble in PS.34 At 150 °C, a majority of the hydrogen bonds in the system have broken, and a morphological change to a lamellar or inverse (P4VP) cylinder morphology may have occurred due to PDP moving from the P4VP domain to the PS domain.26 Though in the supramolecule alone, this change is fully reversible, here this property is leveraged to characterize the level of kinetic trapping in the nanocomposite. CdSe/CdS seeded NRs 22, 37, 47, 90, and 180 nm in average length capped with alkylphosphonic acids were used. All NRs were 4−6 nm in diameter, with the exception of the 180 nm NRs, which had an average diameter of 13 nm. Thus, the aspect ratio ranges from ∼5 to 15. The uniformity of NR length and width is less than that used in previous studies to reduce energetic contribution from inter-NR interaction (see Supporting Information) The 22 nm NRs are comparable in length to the edge-to-edge distance between two PS cylinders, i.e., the width of the P4VP(PDP)1.7 domain, dP4VP(PDP) (approximately 17 nm). These NRs can be accommodated by the supramolecule at a wide range of angles relative to the interdomain interface without significant deformation of the supramolecule and thus have a relatively large degree of rotational freedom within the P4VP(PDP)1.7 matrix. Two possible extremes are shown in Figure 1: the NRs can rest parallel to the PS cylinders, forming a hexagonal lattice of parallel rods, or the NRs can fit perpendicular to the PS cylinders, as shown in Figure 1c,d, or anywhere in between these two states. In contrast, 37 nm NRs are significantly longer than dP4VP(PDP) and are therefore much more strongly confined by the stiff P4VP(PDP)1.7 comb blocks, making it energetically favorable for them to align parallel to the PS cylinders as shown in Figure 1a,b. However, some deviation from the state shown in Figure 1a,b is still possible. The same is true, with decreasing room for deviation, for the 47, 90, and 180 nm NRs. As the NRs become longer, they are more closely confined to the interstitial sites, since any deviation requires an increasing amount of deformation of the supramolecule. Also, as the NRs become longer, the probability of kinetic trapping increases, to the point where the 180 nm NRs are extremely unlikely to assemble into any sort of nonkinetically trapped morphology. In addition, the particle− particle interaction strength, ΔGp−p, will also increase with the length of the NRs, as the intrinsic dipole of the particles scales with their volume. However, unlike in previous studies, the depletion attraction is not expected to play a very large role here due to a variation of the NR diameter along their length and the polydispersity in NR length (see Supporting Information). Therefore, comparison of composites incorporating these varied lengths of NRs will provide greater insights into the supramolecular confinement of the particles and the kinetic trapping for a weaker, nondominating inter-NR interaction.

Figure 2. TEM images of nanocomposites annealed at 100 °C. NR lengths are (a) 22, (b) 37, (c) 47, and (d) 90 nm. Samples are stained with iodine to darken the P4VP(PDP)1.7 domain. Scale bars are 100 nm.

Figure 2 shows TEM images of nanocomposites containing 22, 37, 47, and 90 nm NRs annealed at 100 °C and allowed to cool to room temperature over 8 h. 100 °C is above the glass transition temperature of the supramolecule, and the hydrogen bonding between the P4VP and the PDP remains fairly stable at this temperature. In each of the composites, regions with good supramolecular ordering can be seen, though it is only in some cases that the ordering of the supramolecular framework extends to distribution and orientational control of the NRs. The 22 nm NRs (Figure 2a) are selectively sequestered in the P4VP(PDP)1.7 microdomains, which are selectively stained through exposure to I2 vapor, but the NRs do not appear to be localized in the intersitial region of the P4VP(PDP)1.7 matrix. As shown in previous studies,25,27 NRs that have comparable lengths to the P4VP(PDP)r microdomain size do not have to lie parallel to the PS cylinder axes in order to minimize deformation of the P4VP(PDP)1.7 matrix, but rather can adopt other orientations as well. The 37 nm NRs (Figure 2b), in contrast, are selectively localized in the intersitial region of the matrix between PS cylinders and aligned parallel to the PS cylinder axes as has been seen in previous work.25 Similar behavior is observed with 47 nm NRs (Figure 2c), indicating that there is no significant kinetic trapping with the present treatment. In fact, 90 nm NRs (Figure 2d) are also wellconfined and align with the supramolecule, though the persistence length of the supramolecule appears to be reduced. This is likely due to the long NRs slowing the assembly kinetics of the system, preventing large ordered regions from forming. The effects of this kinetic slowdown manifest more prominently with longer NRs. 180 nm NRs (Figure 3) do not demonstrate the good ordering seen by smaller NRs, though some small well-ordered regions composed of nearly NR-free supramolecule do exist. Though some NRs occupy the interstitial sites as described for shorter NRs, these NRs are not only longer than the NRs in Figure 2 but also larger in 6671

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fraction of NRs increases. At 1 vol % NRs, the supramolecule order persists over hundreds of nanometers, but at 6 vol % NRs, the order can at best be said to persist over a few adjacent microdomains. However, the 37 nm NRs remain evenly dispersed throughout the composite even at high loading rates, in contrast to the 180 nm NRs (Figures 4d−f). With these larger NRs, large, well-ordered and nearly NR-free supramolecular regions are evident at 1 and 3 vol % NR loading rates, but the NRs are not correspondingly well-ordered and seem to concentrate at defect sites.39 Yet, the NRs remain dispersed in the supramolecular matrix and do not phase separate or form aggregates. This behavior persists even at a higher 6 vol % loading rate, where the NRs are dispersed in a supramolecule with no evident long-range order. This suggests that these large NRs are difficult to incorporate into the supramolecule due to their increased diameter and length. Regions that exclude most of the NRs are able to assemble into well-ordered supramolecular microdomains with few defects, but regions containing a significant amount of NRs are kinetically hindered during assembly and the NRs may substantially increase the volume of the microdomain they reside in causing local distortions in the microdomain size, resulting in defects. At 1 and 3 vol % NR loading, there are few enough NRs that regions excluding them can form, but at 6 vol %, the number of NRs has grown to the point where this is not possible and the poorly ordered, defect-rich structure is prevalent. A lower magnification view of the sample depicted in Figure 4f can be seen in Figure 5, further illustrating the lack of longrange ordering and the even dispersal of the NRs. The lack of phase separation of the NRs, even under these conditions, suggests that the enthalpic interaction between the NR ligands and the comb block of the supramolecule are quite favorable, overcoming the driving force of the NRs to assemble side-toside and the driving forces that typically cause assembly into periodic microdomains to minimize the free energy of the polymer chains. As we hypothesize that kinetic arrest is a large contribution to the structures seen in the previous figures, thermally

Figure 3. TEM image of 180 nm NR-containing composite annealed at 100 °C. Sample is stained with iodine to darken the P4VP(PDP)1.7 domain.

diameter, and so are more difficult to accommodate within the supramolecule, in addition to having a larger excluded volume and a correspondingly slower diffusion constant. This leads to poor NR ordering and small well-ordered supramolecular regions with a lower concentration of NRs. Though NR dimensions have a significant effect on assembly, they are not the only factor influencing it. The concentration of the NRs also is an important factora higher concentration than depicted previously may be required for some applications, yet it results in a greater probability of kinetic arrest. Figure 4 depicts composites with NR volume fractions ranging from 1 to 6%. The composites containing 37 nm (Figures 4a−c) display a significant reduction in long-range ordering as the volume

Figure 4. TEM of composites with varying volume fractions of NRs. Composites depicted in (a−c) contain (a) 1, (b) 3, and (c) 6 vol % 37 nm NRs. Composites depicted in (d−f) contain (d) 1, (e) 3, and (f) 6 vol % 180 nm NRs. Samples are stained with iodine to darken the P4VP(PDP)1.7 domain. Scale bars are 100 nm. 6672

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transition occurs, the supramolecule is still well-defined, as the PDP must come out of solution in the PS domain and re-form the comb block as the sample cools, but evidence of at least a partial transition to lamellae is apparent in some samples. The composite containing 22 nm NRs (Figure 6a) has regions of local disorder within larger ordered regions, suggesting that those regions were transitioning to a lamellar morphology, perhaps nucleating from a defect in the ordered structure. In the composite containing 37 nm NRs (Figure 6d), this transition appears to have been more complete, with the NRs following the supramolecular reorganization and forming sheets within the newly formed lamellae. These lamellae display a better long-range order than the other cases examined, exhibited in the SAXS profile as a relatively strong secondorder peak forming and persisting as the composite is heated and cooled. Upon return to a cylindrical morphology, a large fraction of these NRs have remained in these sheets, creating bands of NRs following the P4VP(PDP)1.7 domains between the PS cylinders. The composite containing 47 nm NRs displays substantially less evidence of this transition, remaining in a morphology largely similar to that seen in Figure 2c. Though the NRs appear to have remained in the interstitial sites between PS cylinders as expected for a cylindrical morphology, the cylindrical supramolecule microdomains themselves have shifted to a morphology which could potentially be considered close to a lamellar morphology. Upon annealing at 150 °C, however, a more complete transition is apparent, particularly in the TEM images (Figures 6b,e,h). Here, a lamellar morphology is apparent postannealing for all three composites, as expected, but there is some evidence that the composites underwent a transition beyond a lamellar morphology to an inverse cylinder morphology as well. Some regions are evident where the NRs are bunched into discrete spots rather than a continuous line. Thus, it is likely that the composites transitioned to this inverse cylinder morphology at an elevated temperature and then moved back to a lamellar morphology as the composite cooled. Composites containing 90 nm NRs (Figure 7) displayed similar behavior as those containing 47 nm NRs at 125 °C (Figure 7b). Though the composite adopted a morphology similar to a cylindrical morphology, it is clearly not the same cylindrical morphology seen in the sample prior to annealing (Figure 7a). Rather, these cylinders are arranged such that the P4VP(PDP)1.7 form unbroken planes (horizontal in Figure 7b) that are interconnected between the PS cylinders, suggesting that the composite transitioned to a lamellar morphology during annealing and then moved back to a cylinder morphology as the composite was cooled. The morphological transition is again more drastic for this composite at 150 °C than at 125 °C. Here (Figure 7c), the composite has transitioned completely to an inverse (P4VP) cylinder state, but unlike the composites containing shorter NRs, it did not return to a lamellar morphology upon cooling. Instead, it remained, likely in a kinetically arrested state, in this inverted morphology. In contrast to composites containing smaller NRs, composites containing 180 nm NRs (Figure 8) do not show any evidence of a cylinder-to-lamellae transition upon annealing at 125 °C (Figure 8b). Rather, they appear almost identical to the unannealed composite (Figure 8a), suggesting that even at this elevated temperature the NRs are capable of kinetically arresting the system in this state. Upon heating to 150 °C (Figures 8c,d), however, a transition to an inverse cylinder

Figure 5. Larger-area view of 6 vol % 180 nm NR-containing composite depicted in Figure 4f. Sample is stained with iodine to darken the P4VP(PDP)1.7 domain.

annealing the composites presents a potential opportunity for bypassing the kinetically arrested state and allowing the composite to adopt a more thermodynamically favored morphology. However, as the supramolecule is heated beyond 100 °C, the hydrogen bonding between the pyridine and the PDP begins to break down, and the PDP becomes soluble in the PS domain.34 All but a small fraction of the PDP returns to the P4VP phase upon cooling, so this process is to a large extent reversible but can, during the annealing, cause a morphological transition due to the changing volume fractions of the PS and P4VP phases. Annealing at high temperatures (150 °C) can cause a permanent morphological transition to a lamellar phase due to the change in block volume fractions caused by the dissolution of PDP in the PS phase (see Supporting Information). Because of these changes in supramolecule morphology (both temporary and permanent) and the increased mobility granted by an elevated temperature, it is not unreasonable to expect significant changes in the ordering and distribution of the NRs upon annealing. Using a combination of TEM and SAXS, composites containing NRs 22, 37, and 47 nm in length were more fully investigated (Figure 6). Though there are some differences, the SAXS profiles (Figures 6c,f,i) show a number of similarities. Most important is at the transition from 120 to 130 °C, when the SAXS indicates that the composites transition away from a cylindrical morphology, which occurs because of dissolution of PDP in the PS domain increasing the volume fraction of the PS domain to the point where a cylindrical morphology is unstable. In the case of the 37 and 47 nm NRs, this is to a lamellar morphology, but the second-order peak does not appear for the 22 nm NRs, making it impossible to determine the morphology through SAXS. Before this transition point, however, the improvement in ordering can be seen in the steady sharpening of the first- and second-order peaks. These transitions can also be seen in the TEM images. After annealing at 125 °C (Figures 6a,d,g), which is approximately where the cylinder to lamellae 6673

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Figure 6. TEM and SAXS of composites at various stages of annealing. (a−c) TEM images of 3 vol % 22 nm NR-containing composites annealed at (a) 125and (b) 150 °C as well as (c) a SAXS profile taken in situ during annealing at 150 °C. SAXS spectra are collected during heating at 30, 60, 90, 100, 110, 120, 130, 140, and 150 °C, then again at the same temperatures during cooling. (d−f) TEM images and SAXS spectra of 3 vol % 37 nm NR-containing composites. (g−i) TEM images and SAXS spectra of 3 vol % 47 nm NR-containing composites. All NR-containing nanocomposites went through cylinder-to-lamellae morphological transition upon heating above 120 °C, and the process is not reversible upon cooling. Scale bars on TEM images are 100 nm.

morphology can be seen. As with the composites containing 90 nm NRs, this transition was not reversed upon cooling. The TEM data here, coupled with the SAXS data, allow for a more complete synthesis of the temperature-dependent behavior of these composites. First, the composites go through two morphological transitions upon heating. The first occurs between 120 and 130 °C, when enough PDP has dissolved in the PS domain to trigger a transition to the lamellar phase. The second occurs at a higher temperature not easily determined by SAXS, but between 125 and 150 °C, when enough PDP has dissolved in the PS domain to push the morphology to an inverse cylinder transition. Since PDP solubilization in PS is partially reversible, upon cooling the composites annealed at 125 °C should return to a cylindrical morphology and the composites annealed at 150 °C should return to a lamellar morphology.

That these transitions do not reverse as much as expected allows a number of conclusions regarding the kinetics of the system assembly to be drawn. Foremost, the presence of NRs can stabilize nonequilibrium morphology through kinetic trapping. The normally reversible morphological transition to lamellar or inverse cylinder morphologies upon heating can be preserved upon cooling in this way, as the inclusion of NRs can cause the system to not have enough mobility upon cooling to reverse the transitions triggered at higher temperatures. The level of kinetic trapping and the temperature at which the mobility of the system is significantly restricted can be controlled by the length of the NRs used. Shorter NRs (22 and 37 nm NRs) cannot kinetically trap the intermediate lamellar or inverse cylinder structures, while longer NRs (90 and 180 nm NRs, as well as to a lesser extent 47 nm NRs) can trap the intermediate structures that form upon the breakdown of the P4VP−PDP hydrogen bond and subsequent partial 6674

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Figure 7. TEM and SAXS of 3 vol % 90 nm NR-contaning composites at various stages of annealing. (a) TEM image of composite as cast (before annealing). (b) TEM image of composite annealed at 125 °C. (c) TEM image of composite annealed at 150 °C. (d) a SAXS profile taken in situ during annealing at 150 °C. SAXS spectra are collected during heating at 30, 60, 90, 100, 110, 120, 130, 140, and 150 °C and then again at the same temperatures during cooling. The nanocomposite went through cylincer-to-lamellae morphological transition upon heating and the process is not reversible upon cooling as seen in (c), where the NRs form cluster. The black dots in the TEM image corresponding to the end-view of NR. Scale bars on TEM images are 100 nm.

dissolution of the PDP in the PS domain. NRs as short as 47 nm in length can accomplish this kinetic trapping in an intermediate state, as seen in Figure 6h, though their ability to do so is more limited than that of longer NRs. In contrast, longer NRs cause kinetic trapping at higher temperatures, leading to morphologies that are generally stable only at elevated temperatures, such as the inverse cylinder morphology demonstrated by the 90 and 180 nm NR-containing composites, or preventing significant diffusion of the NRs upon annealing, such as in Figure 8b. The viscosity of the system during the preparation or annealing of the composite is thus strongly influenced by the amount and length of the NRs incorporated into it. However, ΔHligand−polymer is favorable enough that even in the case where the large, 180 nm NRs cannot easily assemble within the supramolecular framework without significant deformation of the P4VP(PDP)1.7 comb block and corresponding losses in ΔScon and ΔSgeometric, the NRs are still incorporated into the composite rather than phase separating from the supramolecule. This strongly favorable ΔHligand−polymer also keeps the supramolecular framework from moving easily past kinetically jammed NRs, kinetically arresting the structure of the entire nanocomposite upon arrest of the NRs. For certain applications, this complete kinetic arrest may provide a useful structure, but for applications that require a well-defined array of particles, this limits the size and loading of the incorporated NRs.

Figure 8. TEM of 3 vol % 180 nm NR-contaning composites at various stages of annealing. (a) TEM image of composite as cast (before annealing). (b) TEM image of composite annealed at 125 °C. (c) TEM image of composite annealed at 150 °C. (d) Lower magnification image of composite shown in (c). Scale bars on TEM images are 100 nm.

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CONCLUSION In summary, we have investigated the coassembly of a BCPbased supramolecule and NRs as a function of NR length in bulk. NRs, 4−6 nm in diameter with an aspect ration between 6 and 15, could be well-distributed and oriented within supramolecular structural framework. The NRs with a lower aspect ratio can be nanoscopically dispersed without orientational control, and NRs with a higher aspect ratio disrupt supramolecular assemblies due to kinetic trap. Kinetically trapped assemblies become more prominent as the NR length and concentration increased. The jamming effects from the NR incorporation also impede the long-range diffusion of both NRs and supramolecules required for morphological transitions and can be used to stabilize supramolecular morphologies that were formed at elevated temperatures.

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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.6b01145. Synthesis and characterization of NR and supramolecule (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract DE-AC02-05CH11231 through the “Organic/ inorganic Nanocomposite Program”. We thank A. Paul Alivsatos for providing nanorods. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231.

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