Uniform Distance Scaling Behavior of PlanetâSatellite Nanostructures

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Uniform Distance Scaling Behavior of Planet– Satellite Nanostructures made by Star Polymers Christian Rossner, Qiyun Tang, Otto Glatter, Marcus Mueller, and Philipp Vana Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04473 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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Uniform Distance Scaling Behavior of Planet−Satellite Nanostructures made by Star Polymers Christian Rossner,*,† Qiyun Tang,*,‡ Otto Glatter,§ Marcus Müller, ‡ and Philipp Vana† †

Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstraße 6, D-

37077 Göttingen, Germany. ‡

Institut für Theoretische Physik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1,

D-37077 Göttingen, Germany. §

Institut für Anorganische Chemie, Technische Universität Graz, Stremayrgasse 9/V, A-8010

Graz, Austria. KEYWORDS. nanoscale materials; nanostructures; star polymers; gold nanoparticles; colloids; RAFT; computer simulation.

ABSTRACT

Planet−satellite nanostructures from RAFT star polymers and larger (planet) as well as smaller (satellite) gold nanoparticles are analyzed in experiments and computer simulations regarding the influence of arm number of star polymers. A uniform scaling behavior of planet−satellite

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distances as a function of arm length was found both in the dried state (via transmission electron microscopy) after casting the nanostructures on surfaces and in the colloidally dispersed state (via simulations and small-angle X-ray scattering) when 2-, 3-, and 6-arm star polymers were employed. This indicates that the planet−satellite distances are mainly determined by the arm length of star polymers. The observed discrepancy between TEM and simulated distances can be attributed to the difference of polymer configurations in dried and dispersed state. Our results also show that these distances are controlled by the density of star polymers end groups, and the number of grabbed satellite particles is determined by the magnitude of the corresponding density. These findings demonstrate the feasibility to precisely control the planet−satellite structures at the nanoscale.

1 Introduction Because of their chemical stability and facile surface modification, gold nanoparticles (AuNPs) are attractive building blocks for the exploration of strategies that assemble these nanoscale objects into hierarchical higher-order structures. Such structures can take a generic form in which one AuNP acts as a central unit with specific surface functionalization as scaffold architecture.1,2 For example, smaller AuNPs can be adsorbed directly on the surface of a larger central AuNP with specific surface chemistry3 or they can be covalently attached to the central AuNP using “click” chemistry.4 These approaches are very straightforward to implement, however, they do not allow the tuning of the spatial relations of the two types of particles, which strongly influence the physical properties of the resulting nanoobject.5 These interparticle distances can be modulated within a few nanometers by varying the length of linking alkyl chains.6–8 For controlling interparticle separations within larger length scales, it appears logical to employ well-


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defined polymers as particle linkers. In the past decade, the most significant advances in this field mostly relied on supramolecular recognition via complementary ssDNA, either alone9–21 or in conjunction with rigid particle-linking bundles assembled via DNA origami.22 Although these results are intriguing, they are limited by their complicated design and synthesis procedures, offer only restricted flexibility with regard to the chemical properties of the particle linker, and are hardly expected to be suited for providing larger quantities of material. Therefore, much research is devoted to using tailored synthetic polymers as key components in the development of efficient and flexible routes to structurally ordered nanoarchitectures.23–26 RAFT polymers are particularly well-suited in this respect: The inherently contained thiocarbonyl thio group can be exploited as an anchor site for chemisorption on gold surfaces.27– 31

Grafting of linear RAFT polymers with trithiocarbonate (TTC) groups at the ω-chain end to

AuNPs leads to core−shell particles with a well-defined polymer shell.32–34 By controlling the distribution of these TTC moieties over the produced macromolecules, nanostructures with polymer loops on AuNP surfaces34 as well as cross-linked AuNP superstructures35,36 can be achieved. When inorganic-core−polymer-shell particles are decorated with TTC groups on the exterior of the polymer layer, it is possible to use these nanoobjects for the patterning of flat gold surfaces,37 or for the attachment of smaller AuNPs to the polymer shell, forming planet−satellite nanostructures.31,38,39 In recent work,39 we described the formation of such planet−satellite nanostructures by using scaffold architectures involving AuNPs with a shell of RAFT 4-arm star polymers. These structures could possibly be applied to form nanopatterned surfaces, which can form the central part in biophysical investigations.40–42 For such investigations, the possibility to arrange two


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distinct building units in a distance range from several nm up to ~30 nm is very tempting. For making our unique nanostructures ready for applications, it is necessary to thoroughly resolve their structure and behavior. This challenge we tried to tackle with this contribution. In particular, we probed the influence of the number of arms in RAFT star polymers on the formation of planet−satellite nanostructures. We were particularly interested in the resulting scaling behavior of the planet−satellite distance, both in the dried and colloidal dispersed state. By using simulation approaches, we obtain direct, real-space information of the planet−satellite nanostructures in the colloidally dispersed state and we also analyzed how the distribution of star polymer end groups in the polymeric shell around planet particles affects the number of attached satellite particles and their positions. 2 Results Synthesis of core− −shell particles and planet− −satellite nanostructures from RAFT star polymers RAFT star polymers can be produced via two principally different strategies, i.e. the Z group43 and R group approach. While sulfur-containing RAFT moieties will remain at the core in Z star polymers, these functional units will be expressed as end groups at the exterior of the star-shaped macromolecules following the R group approach, and thus may enable surface functionalization. We prepared RAFT star polymers with six, three, and (if considered a star polymer) two arms following the core-first R group approach. This strategy implies that, in addition to the formation of well-defined star polymers, there will inevitably be contaminations from star-star couples formed via recombination of macro-star radicals, and also from linear initiator-derived chains which may be both irreversibly terminated and dormant.44 The fraction of these side-products


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can be reduced to a small number, when only a low amount of radical initiator is decomposed under the polymerization conditions applied. Table 2 lists polymerization conditions for the preparation of star polymer samples used in this study. In SEC characterization of macromolecules with star topology, the fact that their hydrodynamic radii are systematically smaller as compared with polymers of linear topology with the same molar mass needs to be considered. In earlier work, our lab determined correction factors to compensate for this topological effect45 and average molar mass values for star polymers were thus corrected by multiplication with these experimental factors. The measured SE chromatograms (Figures S5−S7) display only small shoulders at higher molar masses and only reasonably low tailing toward lower molar masses, suggesting that contaminating side products mentioned above are present to only a small extent in the star polymer materials produced. The produced macromolecules were subsequently used to cover gold nanocrystals (13 nm, see Figure S17 for diameter histogram) from citrate-reduction in a ‘grafting-to’ approach, forming core−shell nanohybrids. Upon addition of Brust−Schiffrin AuNPs to these nanohybrids, followed by addition of stabilizing linear31 poly(N,N-dimethylacrylamide) (see Supporting information Figure S8 for details) planet−satellite particles were formed. The preparation of the inorganic particle building blocks is described elsewhere.39


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Scheme 1. Synthetic scheme illustrating the formation of RAFT star polymers exemplarily for the 3-arm star case. RAFT end groups are highlighted in red color. Formation of planet− −satellite nanostructures and distance scaling behavior in the dried state The nanomaterials formed from 2-, 3-, and 6-arm star polymers were characterized using TEM after drop-casting of the respective colloidal dispersion on carbon-coated copper grids. The formation of planet−satellite nanostructures (see Figure 1) from precursor core−shell particles (see Figure S15) with star polymer shells can be considered as an indication that free, not surfacebound TTC end groups are present in the polymer shell of this precursor. This claim can be substantiated by control experiments: The small satellite AuNPs do not attach to hybrid particles with a shell of monofunctional linear polymer (see Figure S18). In the case telechelic polymer (the 2-arm case), it is interesting to compare the scaling behavior of interparticle distances in self-assembled hexagonal arrangements of core−shell particles with earlier data34 for linear monofunctional RAFT polymer of NiPAAM. Between both types of polymers, the only distinction is the presence of TTC end groups on both or only one terminal of


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the linear macromolecules. Such comparison is shown in Figure S26 and reveals significantly smaller particle separations when the monofunctional RAFT polymer of NiPAAM is replaced by its telechelic analog. This can be considered as an indication for the formation of loops of the telechlic polymer on the AuNP surface, i.e., a bidentate binding mode; such loops had already been found for multifunctional linear RAFT polymers of NiPAAM on AuNP surfaces.34 However, as planet−satellite nanostructure formation is also observed with telechlic polymers, but not with monofunctional RAFT polymer (see above), we can conclude that not all end groups of the telechelic polymer are permanently confined to the planet particle surface. Hence, there is a fraction of looped telechelic polymer on the planet particle surface and a corresponding fraction attaches with one end group only to the planet particle and provides free TTC end groups for the attachment of satellite particles. As we were particularly interested in the distance scaling behavior in planet−satellite nanostructures in dependence of both star polymer arm number and number average molar mass, Mn, we systematically determined planet−satellite edge-to-edge distances in isolated nanostructures from several TE micrographs. We plotted the experimentally obtained data as a function of the average molar mass of one star polymer arm for 2-, 3-, and 6-arm polymers, see Figure 2. Interestingly, we find that the three curves approach one master curve, thus indicating a uniform distance scaling behavior, demonstrating that the distances are mainly determined by the arm length of star polymers. In other words: An increase of arm number does not change the planet−satellite distance. Thus, varying the arm number of star polymers provides an additional degree of freedom to this system. From an inspection of TE micrographs, we find that more satellite particles are grabbed as star polymer arm number increases (for example, from the star polymer samples 2-24k, 3-36k and 6-69k distances of 9.2 ± 2.5nm, 8.4 ± 2.4nm, and 9.2 ± 2.4nm


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are generated, which are in the same range within the error bar. However, the average number of grabbed satellite particles increases as the star polymer arm number increases, see Table S1). This phenomenon could also be observed in simulations, which will be discussed later.

Figure 1. TE micrographs of planet−satellite structures formed via 2-, 3-, and 6-arm star polymers. From top to bottom, the star polymers average molar mass increases as labeled.


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planet−satellite distance / nm

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2-arm star polymer (telechelic) 3-arm star polymer 6-arm star polymer

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Mn / arm number / (10 g⋅mol ) Figure 2. Scaling of planet−satellite edge-to-edge distances as a function of the number average molar mass per star polymer arm as derived from an analysis of TE micrographs. Scaling of planet− −satellite distance in the colloidally dispersed state Here we use Monte Carlo simulation based on a coarse-grained bead-spring model and SAXS experiments to explore the distance scaling behavior of planet−satellite structures in the colloidally dispersed state.


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Figure 3. Simulation snapshots of attached 3-arm star polymers and the adsorbed satellite particles with the total monomer number in star polymers Ntotal = (a) 49, (b) 70, (c) 79, (d) 88, (e) 130, and (f) 139. Blue color shows the bond between neighboring monomers in polymers and red color shows the star polymer end groups. The yellow spheres indicate the adsorbed satellite particles. In simulations, we prepared and relaxed the planet−satellite structures using the experimentally determined amount of star polymers on the big planet AuNP. For further details see the Supporting Information. Given the large negative attachment free energy, the association of AuNPs with arm ends is virtually permanent on the time scale of simulation. Figure 3 depicts typical snapshots of attached 3-arm star polymers and attached satellite AuNPs obtained from simulations. It is shown that the 3-arm star polymers form a spherical shell where the free end groups (red spheres) distribute at the surface. This distribution of chain ends (see blue curves in


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Figure 4) is rather broad and not highly concentrated at the outer edge of the polymer shell, thus indicating that the star polymers are not highly stretched (see also Figure S14 for the radial density of total monomers of star polymers). The satellite particles (illustrated as yellow spheres) are grabbed by the free end groups, from which we can obtain the distribution of the positions of these satellite particles, see black curves in Figure 4. This distribution can be compared with the density of free end groups of star polymers, and we find that the maximum of the distribution of planet−satellite distances coincides with the maximum of the free end group density of star polymers. This can be considered as an indication for the claim that the position of satellite particles in planet−satellite nanostructures is controlled by the location of the free star polymer end groups in the precursor AuNP-core−star-polymer-shell scaffold structures. The more narrow distribution of the planet−satellite edge-to-edge distance stems from the fact that the satellite particles experience the excluded volume of the multiple star polymers and are thereby influenced by an averaged density profile that fluctuates less. We can also find from Figures 3 and 4 that as the arm length (molar mass or total monomer number) increases, the satellite AuNPs locate farther from the planet particle. The planet−satellite spacing data approach one master curve (see Figure 5) when they are plotted as a function of arm length, which is similar to the result obtained from the analysis of TEM images (Figure 3). However, we notice a markedly different scaling behavior here as compared with the TEM results (Figure 5). At the lower arm molar mass, the simulated planet−satellite distances are larger than the TEM ones, while at larger arm molar mass, the situation reverses. This discrepancy is due to the fact that the planet−satellite structures in simulations reflect the colloidally dispersed state, in which the configurations of star polymers are influenced by good solvents.


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Figure 4. Comparison of the distribution of the edge-to-edge distances of planet-satellite AuNPs in simulation (black curves) and the end monomer densities of star polymers on planet particles (blue curves) at different Ntotal, (a) 49, (b) 70, (c) 79, (d) 88, (e) 130, and (f) 139.


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planet−satellit distance / nm

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Mn / arm number / (10 g⋅mol ) Figure 5. Edge-to-edge distances of planet−satellite nanostructures obtained from TEM measurements and simulation data in dependence of number average molar mass per arm for different star polymer arm numbers. To support the simulation data, we performed exemplary SAXS measurements for planet−satellite nanostructures derived from 3-arm star polymers in colloidal dispersion. Due to the much higher contrast of gold to the solvent compared with the organic polymers, it can be assumed that only the inorganic part of the nanostructures contributes to the scattered intensity. Experiments were performed under dilute conditions, such that the intensity profile reflects the particle form factor. The scattering curves for the six samples derived from 3-arm star polymers are shown in Figure S27. Höller et al. recently performed SAXS measurements of core−satellite structures with very small particle distances and analyzed the form factor directly.2 Here, to obtain real-space information about the planet−satellite structures, we transformed these scattering curves into the corresponding pair-distance distribution functions (PDDFs),46,47


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p(r)48,49 (see Figure 6), via indirect Fourier Transformation (IFT)50 of the intensity profiles. The function p(r) is a measure for the number of distances within a scattering object of homogeneous electron density; since only the gold part of the nanostructures contributes to the scattering to a good approximation here, the p(r) function represents the number of distances within this inorganic part of the here studied nanostructures. As these PDDFs have a clear geometrical definition, PDDFs for model structures can be judged relatively easily in comparison to the experimental data, and from a fitting model structure information about planet−satellite distances can be obtained. (This is explained in more detail in the supporting information.) The SAXS results clearly confirm the presence of planet−satellite structures also in colloidal dispersion. The numbers of satellite particles in the model structures used to fit the experimental SAXS data (Table 1) are in reasonable agreement with the values determined from microscopy (Table S1). The planet−satellite edge-to-edge distances from simulations are in striking agreement with the SAXS results for the three samples with smallest molar masses (Table 1), thus proving that our simulations are perfectly suited to provide solution data for planet−satellite separations, and confirming the scaling of planet−satellite distances also experimentally for the colloidally dispersed state. For the remaining three samples with higher molar masses, there is a discrepancy between the experimental values and the simulation data (Table 1). We can only speculate about the reasons for this discrepancy, which appears to be more pronounced as the star polymer average molar mass increases. It may be that the structures with star polymers of higher molar masses are more prone to aging effects, possibly because of the decreasing grafting density with increasing molar masses (Table S2). In addition, the excluded volume of a segment may differ between the experimental conditions and the simulations. Also, the dispersity of synthetically prepared star polymer is neglected in simulations. It is expected that the influence


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of the molar mass dispersity on the planet−satellite separation would be higher, as average molar mass increases. Consistent with that, the deviation between experimental and simulated planet−satellite separations becomes more pronounced with increasing star polymer average molar mass.

3−32k

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Figure 6. Experimental PDDFs (gray circles) for planet−satellite nanostructures derived from 3arm star polymer and PDDFs of models (red data points, see supporting information, section 4 for informations about how model PDDFs had been obtained). Insets in the upper right of the single plots schematically illustrate the arrangement of satellites around the planet particles in the structure models.


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Table 1. Characteristics of model structures displayed in Figure 6 and comparison to simulation results. Polymer

Experimental (SAXS) average planet−satellite edge-to-edge distance / nm

3-23k

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Range of distances employed in model

Number of surrounding satellite particles

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2−22 nm

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9±5

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3 Discussion From the TEM experiments and simulation results, we find that the scaling of edge-to-edge distances of the planet−satellite structures for 2-arm, 3-arm, and 6-arm star polymers approaches one master curve determined by the arm length of the star polymers. Obviously, the grafting density of the adsorbed star polymers also contributes to the planet−satellite distance. However, the grafting density in the experimental system is not an equilibrium property but it is dictated by the adsorption kinetics. Thus its influence on the scaling behavior cannot be a simple normalization coefficient. Here the observed uniform scaling behavior, confirmed by both experiments and simulations, is based on the experimental grafting densities, which were obtained from the same preparation history. In this sense, it implies that one can precisely control the distance between planet−satellite structures by simply adjusting the arm length of star polymers. The data from TEM characterization (in the dried state after casting on an amorphous carbon surface) differ from the simulation results and SAXS data (colloidal dispersion state). We can give the following explanation for the discrepancy between simulation and TEM results shown in Figure 5: At low arm molar mass, the simulated planet−satellite structures are in a dispersed state and the star polymers are swollen in good solvents. In contrast, TEM measurements are performed in the dried state and require the deposition of the nanostructures on a carbon film support, which results in a deformation of the soft polymer canopy.51 In this case, the deposited satellite particles stay closer to the planet particles (see illustration in Figure 7(a)). This results in the smaller TEM planet−satellite distances as shown in Figure 5. At high arm molar masses, the configurations of the star polymers are extended after casting the particles on the amorphous carbon film, as a result of the wetting of this film (see supporting information, discussion in section 3 and especially Figure S16). However, for the situation in the dispersed


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state, the simulated polymer configurations favor less perturbed conformations at the outer shell due to the small monomer density, which lowers the planet−satellite distances, see the illustration in Figure 7(b). Concerning the influence of star polymer arm number, one may ask: what is the advantage of varying the arm number of star polymers here? One advantage is that varying the arm number provides an additional degree of freedom to tune the free end group densities at fixed grafting density. The uniform distance scaling behavior of planet−satellite structures demonstrated in this paper tells us that the distance is mainly determined by the arm length, not arm number. From this observation, one may argue that the polymer shell is not dense, see also Figure S14 for the radial density of total monomers of star polymers. However, increasing the arm number could increase the free end monomer density, and grab more satellites without changing the particle distance in planet−satellite structures. In fact, in simulations we find that 6-arm star polymers can generate relatively high end group densities in the polymer shell around planet AuNPs. The ratio between the maximum of free end group density and the star polymer grafting density on planet AuNPs is shown in Figure 8. This ratio is largest for six-arm star polymers and smallest for telechelic (2-arm star) polymers, and generally decreases within each star polymer series with increasing degree of polymerization. As a consequence of that, we observe experimentally in TEM measurements that the average number of satellite particles per planet particle increases with increasing star polymer arm number (around 4 for the 2-arm case, an exception being the sample with the highest molar mass, around 7 for the 3-arm case, exceptions being the two samples with highest molar masses, and around 9 for the 6-arm case, see Table S1). Hence, this experimentally observed topological effect reflects, again, the fact that satellite particle attachment is caused by the interaction with the TTC end groups: A higher number of end groups


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in the polymer shell leads to the attachment of more satellite particles under otherwise equal conditions.

Figure 7. Illustration of star polymer configurations leading to the planet−satellite distances obtained via TEM measurements and simulations for (a) low star polymers arm length, (b) high star polymer arm length. [Note that we do not mean to imply that satellites are attached by single star polymers.]


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0.12

2-arm star polymer 3-arm star polymer 6-arm star polymer

0.10 0.08

ρmax / Σgraft

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Narm Figure 8. Simulated ratio between the maximum density of free arm end monomers and the grafting density of star polymers as a function of arm length. 4 Conclusion We employed TEM, computer simulations, and SAXS measurements to analyze in detail planet−satellite nanostructures formed from RAFT star polymers regarding the influence of star polymer arm number and average molar mass. Because of the strong attractive interactions between arm end groups (TTC) and gold surfaces, the distribution of satellite particles around larger planet particles is determined by the distribution of TTC end groups in the polymeric shell, as revealed by simulations. Moreover, both TEM measurements (in dried state) and simulation results (in colloidally dispersed state, confirmed by SAXS measurements) demonstrate a uniform distance scaling behavior of planet−satellite structures for the 2-arm, 3-arm, and 6-arm star polymers (which results from the polymer molar mass and the grafting density that is achieved by grafting the star polymers to the planet particle), that is, the distance as a function of arm


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length could be collapsed to one master curve describing the collective behavior of the different star polymer topologies. These results imply that one can control the edge-to-edge distance of planet−satellite structures via precisely adjusting the arm length of the star polymers. Increasing the arm number of star polymers provides a possible rationale to increase the density of free arm end groups, and therefore to grab more satellite particles. Our findings show the feasibility to precisely control the structural features of higher order nanostructures by targeted macromolecular design of the polymeric key components. 5 Methods Transmission Electron Microscopy (TEM). TEM measurements were conducted on a Philips CM 12 transmission electron microscope equipped with a LaB6 cathode applying an acceleration voltage of 120 kV. A 50 µm aperture was used in the condenser lens, and scattered electrons were blocked with a 20 µm aperture in the focal plane of the objective lens. The instrument was equipped with an Olympus 1376 × 1032 pixel CCD-camera. All samples were applied on Plano 200 mesh carbon-coated copper grids by drop-casting.

Small-Angle X-ray Scattering (SAXS). SAXS measurements were conducted employing a SAXSess camera (Anton Paar, Graz, Austria) and sealed tube X-ray generator (DebyeFlex3000) operating at 40 kV and 50 mA. The divergent polychromatic X-rays were focused into a line-shaped X-ray beam (λ = 1.54 Å, Cu Kα radiation) using a Goebel mirror. Scattering profiles were recorded with a CCD camera (2084 × 2084 pixels, pixel size 24 µm × 24 µm, PI-SCX, Roper Scientific, Germany). For every sample,


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measurements were performed in methanol dispersion four times for 10 minutes at 25 °C and averaged. Scattering patterns were edited by subtracting solvent background and integrated into one-dimensional scattering functions. Absolute intensities were determined from measurements of water as a standard.52

Modeling of Star Polymers Here we use the coarse-grained model to describe the adsorption of star polymers at gold nanoparticle surfaces (see Figure S9(a)). The non-bonded interactions between all monomers are treated via a truncated and shifted Lennard-Jones potential: LJts =

4 ⁄ − ⁄ + 1⁄4, ≤ 2⁄ 0, > 2⁄

and the bonded interactions between neighbor monomers along polymers are modeled by a FENE potential:53 1 − max ln 1 − ⁄ max , ≤ b = 2 ∞, >

max

max

where the parameters lmax = 1.5σ, k = 30.0ε/σ2, and kbT = 1.0ε are selected to prevent the chain crossings. This model is suitable for modeling the properties of polymers in good solvents. In the synthesis of RAFT star polymers, each arm is polymerized with nearly the same rate and the arm length can thus be assumed equal within one star polymer. To map the star polymers with different molar and arm number into our simulations, we chose the following mapping procedure: The initiator for 2-arm, 3-arm, and 6-arm polymers holds different molecular weights


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(0.6 kDa, 0.7 kDa, and 1.5 kDa). The repeat unit (NiPAAM) has a molecular weight of 0.113 kDa. In our simulations, we consider the initiator as one core monomer for star polymers and each monomer along each arm takes 4 repeat units. Thus, for the 3-arm star polymer with molar mass of 23 kDa, we can calculate the monomer number of each arm as (23 kDa – 0.7 kDa)/3/(4×0.113 kDa) ~ 16, and the total number of monomers of 3-arm star polymer (23 kDa) in simulations is 16×3+1 = 49 [Here, the core monomer and arm monomer correspond to groups with different molecular weights (0.6 kDa, 0.7 kDa, and 1.5 kDa for the initiator and 0.476 kDa for one monomer along the star polymer arms), indicating distinct volume. However, for simplicity, we neglect this molecular weight (or volume) difference in our simulation and assume the volume of the core and arm monomers are the same.] The length unit of σ is mapped in simulations as 1 nm, corresponding to the length of 4 repeat units of NiPAAM. The mapping of synthesized star polymers with distinct molar masses and the corresponding simulation arm length and overall monomer number is shown in Table 2. Table 2. Polymerization conditions, characterization results for the different polymer samples, and number of monomer segments per arm, Narm, as well as total number of monomer segments, Ntotal, used in simulations.a Polymerb

t/ min 150

Mn / 3 (10 g mol−1)c 19

Đc

Narm

Ntotal

2-19k

[DMF]/[NiPAAM]0/ [RAFT]0/[AIBN]0 1000 : 250 : 1 : 0.1

1.12

20

41

2-24k

1000 : 250 : 1 : 0.1

240

24

1.13

25

51

2-33k

1800 : 400 : 1 : 0.1

240

33

1.13

35

71

2-36k

1800 : 400 : 1 : 0.1

300

36

1.15

39

79

2-40k

1800 : 400 : 1 : 0.1

420

40

1.16

43

87

2-53k

6000 : 1000 : 1 : 0.1

420

53

1.15

58

117


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a

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3-23k

1000 : 250 : 1 : 0.1

240

23

1.13

16

49

3-32k

1800 : 400 : 1 : 0.1

255

32

1.11

23

70

3-36k

1800 : 400 : 1 : 0.1

290

36

1.10

26

79

3-41k

1800 : 400 : 1 : 0.1

420

41

1.11

29

88

3-59k

6000 : 1000 : 1 : 0.1

420

59

1.13

43

130

3-63k

6000 : 1000 : 1 : 0.1

540

63

1.17

46

139

6-29k

1800 : 400 : 1 : 0.1

240

29

1.10

10

61

6-33k

1800 : 400 : 1 : 0.1

300

33

1.13

12

73

6-42k

1800 : 400 : 1 : 0.1

420

42

1.11

15

91

6-50k

3000 : 600 : 1 : 0.1

420

50

1.14

18

109

6-59k

6000 : 1000 : 1 : 0.1

420

59

1.14

21

127

6-69k

6000 : 1000 : 1 : 0.1

540

69

1.10

25

151

All Polymerizations were conducted at 60 °C. bThe preparation of the telechelic RAFT agent

used to prepare polymers 2-19k – 2-53k is described in our earlier publication;36 the syntheses and characterization for the three- and six-arm star RAFT agents can be found in the supporting information. cApparent average molar masses and dispersity values measured by DMAc SEC (PMMA calibration, refractive index detection). For 3-arm and 6-arm star polymer samples, Mn values were corrected by an experimentally determined form factor, taking into account the polymer topology (see below). Interaction between the star polymer end groups and nanoparticles Blakey et al. developed a UV/visible spectroscopic experiment for the analysis of the Langmuir isotherm of adsorption and determined a free enthalpy a adsorption of −36 kJ/mol for dithioesters on gold nanoparticle surfaces.30 Since RAFT polymers of the TTC type are treated in this work,


24

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the interaction of TTCs with AuNP surfaces was evaluated here following the method of Blakey et al. (see supporting information Figure S9−S11 and explanations there). This is important, as the phenyl group is in direct conjugation with the thiocarbonyl group (through which surface attachment occurs) in phenyldithioesters studied by Blakey et al.; TTCs are structurally different in this regard, and we used the model TTC S,S’-bis(α,α’-dimethyl-α’’-acetic acid)trithiocarbonate, synthesized according to a published procedure.54 Nevertheless, a similar enthalpy of adsorption of −36 kJ/mol was obtained at 21 °C, see Figure S9b, corresponding to −14.52 kBT. In our simulations, we chose the energy between end groups and the AuNP surface as the following: ∞, < -. + 5 − -. '( = / ∙ exp 3 5 , -. ≤ ≤ -. + 2 2 * )0, > -. + 2 Here, Rc= 6.8σ for big and 1.9σ for small particles, corresponding to the AuNP sizes of 13.6 nm and 3.8 nm obtained in earlier experiments.39 The pre-exponential A = −14.52 kBT, obtained from the experimental measurements indicates a strong attraction between the end groups of the star polymers and the AuNP surfaces. More detailed information about the simulations are given in the supporting information. ASSOCIATED CONTENT


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Details about the chemicals employed, synthetic procedures, more detailed information about the performed simulations, NMR spectra, SEC traces of all polymers, additional TEM, SAXS, and simulation data are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *(C.R.) E-mail: [email protected]. *(Q.T) E-mail: [email protected]. Notes The authors declare no competing financial interest.

Funding Sources Fonds der Chemischen Industrie; Deutsche Forschungsgemeinschaft (DFG, project numbers VA226-10/1, Mu1674/15-1) ACKNOWLEDGMENT The authors thank the TU Graz for access to the SAXS setup and Dr. Manfred Kriechbaum for support with the measurements. Generous financial support by the Fonds der Chemischen Industrie (Ph.D. Fellowship granted to CR) and by the Deutsche Forschungsgemeinschaft (DFG, Project Nos. VA226-10/1 and Mu1674/15-1) is gratefully acknowledged. The simulations have been performed at the GWDG Göttingen, the HLRN Hannover/Berlin, and the von-Neumann Institute for Computing, Jülich, Germany.


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Langmuir

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Uniform Distance Scaling Behavior of PlanetâSatellite Nanostructures

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