Diffusion of Molecular and Macromolecular Polyolefin Probes in

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

Diffusion of Molecular and Macromolecular Polyolefin Probes in Cylindrical Block Copolymer Structures As Observed by High Temperature Single Molecule Fluorescence Microscopy Moritz Baier,† Dominik Wöll,*,‡ and Stefan Mecking*,† †

Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52074 Aachen, Germany

‡

S Supporting Information *

ABSTRACT: Highly sensitive fluorescence microscopy methods allow for the observation of single bright fluorescent probes. Analysis of their trajectories gives access to the mode of diffusion and the heterogeneity in motion of individual probes. Especially for structured soft materials, this information is of paramount importance for a multitude of possible applications such as nanoelectronics, nanophotonics, or nanomembrane technology. Compared to biological systems, utilization for materials research faces the challenge that relevant processes occur at elevated temperature, often above 100 °C, and that fluorescence labeling procedures are yet less evolved. We investigated the motion of single probes in block copolymer morphologies from room temperature to over 100 °C with a custom-made heating device to allow for such high temperatures without damaging the optics of a commercial optical microscope and also with the possibility to measure under a nitrogen atmosphere to reduce photobleaching of the dyes. Apart from tracking single perylenediimide derivative as a molecular probe, we labeled polyolefin chains with this chromophore and observed their diffusion. For the synthesis of the polyolefins, as the most important class of polymeric materials in general, we present a protocol that provides high quality samples in terms of molecular weights, molecular weight distributions, and proven degree of dye functionalization. The dependency of temperature, block copolymer composition, and probe size on the diffusion behavior is elaborated.

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INTRODUCTION The self-assembly of block copolymers1 to functional materials with well-controlled morphology is highly relevant for various fields such as nanophotonics,2 nanopatterning3 for nanoelectronics,4 solar cell applications,5 and biomimetic membranes.6 A broad spectrum of mesoscopic morphologies is offered by block copolymers. 7−9 Depending on their composition, block copolymers can form various microphaseseparated structures, such as lamellae, spheres, cylinders, or gyroid, but also even more sophisticated morphologies can be realized and the quest for exciting novel structures is still on.10 Self-assembled block copolymers can be investigated by different methods such as atomic (or scanning) force microscopy,11,12 cryogenic transmission electron microscopy,13,14 transmission electron microtomography,15 and recently also super-resolution microscopy16−21 beyond the diffraction limit of light. These methods, primarily address structural aspects. However, apart from the morphology of the selfassembly, understanding the dynamics of embedded molecules within self-assembled block copolymer structures or of single block copolymer chains themselves is of fundamental interest to promote the ongoing miniaturization of devices. Molecular sieving and membrane technologies, for example, strongly © XXXX American Chemical Society

depend on detailed knowledge of molecular transport. Also, envisioned catalytic reactions become diffusion-controlled in the rather dense environments of polymeric structures. Furthermore, an understanding of dynamics is essential for a controlled ordering of the structures obtained by selfassembling block copolymers which are initially often rich in defects.22 Self-assembling can be achieved by solvent vapor annealing23 or heating. Both strategies enhance the chain mobility and, thus, promote the self-assembly process. Addressing the dynamics within block copolymers on the single molecule level has significant advantages to bulk methods. The observation of single molecule motion can reveal the structures along which mobility occurs and elucidate inhomogeneities within these structures. Thus, also jumps between different compartments or channels become accessible which has already convincingly been demonstrated for the mobility of single dye molecules within nanoporous materials.24,25 Similar experiments were also reported on microporous coordination polymers26 and surfactant mesophases.27 YorulReceived: January 12, 2018 Revised: February 15, 2018

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DOI: 10.1021/acs.macromol.8b00071 Macromolecules XXXX, XXX, XXX−XXX

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and passed through columns of Al2O3 and R3-11 (BASF) before it was used as a polymerization medium. NMR spectra were recorded on a Varian Unity INOVA 400, a Bruker Avance III 400, or a Bruker Avance III 600 spectrometer. 1H and 13C chemical shifts were referenced to the solvent signal. NMR spectra of polymers were recorded on a Varian Unity INOVA 400 spectrometer at 130 °C in 1,1,2,2-tetrachloroethane-d2. For the measurement of polymer 13C spectra 1 mg mL−1 Cr(acac)3 was added to the sample. All chemical shifts are given in ppm. All coupling constants J are given in Hertz. Multiplicities are given as follows (or combinations thereof): s: singlet; d: doublet; t: triplet; q: quartet; hept: heptet; m: multiplet; br: broad. Fluorescence spectra and quantum yields were measured on a Hamamatsu Absolute PL Quantum Yield Measurement System C9920-02 CCD spectrometer equipped with an additional cuvette holder (qpod 2e, Quantum Northwest). Absorption spectra were recorded on a Varian Cary 50 spectrometer. Differential scanning calorimetry (DSC) was performed on a Netzsch Phoenix 204 F1 at a heating/cooling rate of 10 K min−1. Gel permeation chromatography (GPC) was carried out in 1,2,4trichlorobenzene at 160 °C at a flow rate of 1 mL min−1 on a Polymer Laboratories PL-GPC 220 instrument equipped with three PLgel Olexis columns with differential refractive index, viscosity, and light scattering (15° and 90°) detectors. Molecular weights were calculated by the triple detection method of the Cirrus GPC multidetector software or by linear calibration against narrow polyethylene standards. Detector calibration for triple detection was performed with a narrow polystyrene standard (Mp 2.05 × 105 g mol−1, Mn 1.91 × 105 g mol−1, Mw/Mn 1.05, IV 0.709 dL g−1). The SBS triblock copolymer SBS30 with a polystyrene content of 30 wt % and a molecular weight Mw of 140 kg mol−1 is a commercially available thermoplastic elastomer and was purchased from Aldrich (Mw 21−98−21 kg mol−1). It possesses a degree of 1,2-butadiene enchainment of 9%. The SBS triblock copolymer SBS72 (Mn 19−15−19 kg mol−1, Mw/ Mn = 1.07) with a polystyrene content of 72 wt % was purchased from Polymer Source Inc. (P8410-SBdS). It possesses an overall molecular weight of Mn 53 kg mol−1 and a 1,2-butadiene enchainment degree of 7%. The polystyrene fraction exhibits a glass transition temperature of 88 °C and the polybutadiene of −49 °C. Toluene (spectroscopic grade) for sample preparation was purchased from Aldrich. ITO-coated glass coverslips were cleaned by ultrasonication in isopropanol (spectroscopic grade), which was obtained from Aldrich. Atomic force microscopy (AFM) was performed on a JPK NanoWizard instrument in intermittent contact mode using a silicon tip with a force constant of 40 N m−1 and a resonance frequency of about 300 kHz. Height and phase images were recorded simultaneously. General Polymerization Procedure. Olefin polymerization was carried out in a 250 or 500 mL glass reactor (Büchi). The reactor was equipped with a mechanical stirrer and a heating/cooling jacket, which was connected to a thermostat. The monomer pressure was controlled by a Bronkhorst mass-flow control unit consisting of two mass-flow sensors (EL-FLOW select F-111CM, 150 mLn min−1 (milliliters normal per minute) and 500 mLn min−1) and a pressure sensor (ELPRESS select P-502CM) with a control valve. Under an inert gas atmosphere, toluene (150 mL) was added to the reactor and saturated at 25 °C with ethylene at the desired polymerization pressure. Dry MAO (300 mg, 5 mmol) was dissolved in 2−3 mL of toluene and injected into the reactor by a pressure buret together with ∼50 mL of toluene. Subsequently, a solution of the catalyst precursor 1 (6.4 mg, 10 μmol) in ∼5 mL of toluene together with ∼50 mL of toluene was injected to initiate the polymerization. The polymerization was carried out under continuous ethylene mass flow at constant pressure. After the desired polymerization time the ethylene feed was stopped, ethanol was injected to quench the polymerization, and the pressure was released. The polymerization mixture was poured into MeOH/conc HCl (ca. 100:1 v/v), and the mixture was shaken for at least 1 h. The polymer was filtered off with a

maz et al. studied the diffusion of terrylene molecules in a poly(butadiene-b-ethylene oxide) block copolymer film by single-molecule fluorescence imaging.28 Confined motion of the dye molecules in the amorphous polybutadiene channels embedded in a crystalline poly(ethylene oxide) matrix was observed. More detailed investigations were conducted by Ito and co-workers on aligned cylinder structures of a poly(styreneb-ethylene oxide) block copolymer.29−31 In addition, single molecule investigations of the morphology and mass transport dynamics in nanostructured materials have been reviewed.32 Single molecule fluorescence microscopy in polymer science has so far mainly been conducted in the temperature regime below 50 °C, although many important phenomena (e.g., glass transition, melting, and crystallization) of relevant polymeric materials occur at significantly higher temperatures. The reason for this restriction is that optics is rather sensitive to the expansion of materials which is unavoidably accompanied by heating. Thus, severe aberrations are introduced and microscopy parts can get destroyed at elevated temperatures above approximately 70 °C. We developed a device to overcome this problem with indium tin oxide (ITO)-covered glass coverslips which we heated resistively.33,34 In combination with the usage of an air objective the heating keeps restricted to the coverslip in this case. Despite the less efficient collection of photons with respect to an oil immersion objective, we can track single rylene dye molecules35 due to their outstanding photostability. Here, we report the temperature-dependent mobility of free perylenediimide dyes and polymer chains labeled with this dye in self-assembled block copolymer structures. We focused our research on poly(styrene-b-butadiene-b-styrene) triblock copolymers (SBS) since they are among the most widely studied block copolymers and find application for instance as thermoplastic elastomers.36 The mobility of the probes was analyzed from single molecule tracking of their fluorescence signal. In order to investigate the diffusion of polyolefins in the block copolymer structures, an attached fluorescence marker is required. Thus, hydroxyl end-functionalized ethylene/1-butene copolymers with low polydispersity (80%) of the copolymer molecules became mobile,

which was retained also during cooling below the glass transition temperature of polystyrene before all molecules become immobile at around 25 °C again. Therefore, the measurements presented in Figure 6 were started at 110 °C, and the temperature was slowly reduced to room temperature. It can be shown that the motion of labeled chains is in most cases not entirely confined to the polybutadiene cylinders. The trajectories of the dye-labeled polymer chains mainly look like random motion trajectories. This becomes obvious from the rather equally distributed angles in Figure 6 (left) and from the step length distributions which can be fitted with a normal diffusion model (see red fit lines in Figure 6 (right)). In a few cases, however, for some periods of time poly(E-co-B)-PDI seem to follow a predetermined path (for two such examples see Figure S12). For some trajectories, this is quite obvious (Figure S12, top), but for others only segments seem to follow the cylinder structure (Figure S12, bottom). Our observations indicate that the mobility of the poly(E-co-B)-PDI chains in the polybutadiene channels within SBS72 at temperature below Tg of PS was too low to be resolved with our experimental settings. Above Tg of PS, defects of the structure appear which increase mobility and also cause that the motion of single chains is mostly not restricted to the channels, similar to the situation as it was observed for the molecular fluorescent probe 3 in SBS72 (see above). However, even after cooling down to room temperature, the copolymer structures were not destroyed but could be still imaged by AFM. One strength of the single molecule approach is that it enabled us to observe trajectories in very close proximity of ca. 1 μm. As shown in Figure 7 very distrinct behavior could be I

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Figure 7. Trajectories of two single molecules in a SBS72 film at 60 °C with the corresponding angle and step length distributions (see also Movie S12 in the Supporting Information). The fit on the top step length distribution corresponds to normal diffusion with a diffusion coefficient D of 7.1 × 10−15 m2 s−1 (yellow trajectory). In contrast, the data of the green trajectory can be better fitted with a 1D diffusion model (eq 2; solid black fit; D1 = 1.6 × 10−13 m2 s−1) than with a 2D diffusion model (eq 1; dash-dotted gray fit; D = 3.0 × 10−14 m2 s−1). The diffusion coefficients indicate that the yellow trajectory was caused by a single perylenediimide-labeled ethylene/1-butene copolymer poly(E-co-B)-PDI chain, whereas the green trajectory belongs to a free PDI molecule which apparently was not separated during the purification of the labeled polymer.

here, we ascertained that this observation was an exception and the number of free dyes was so low that they did not significantly alter the interpretation of our results. The temperature dependence D(T) of the diffusion coefficients of the systems investigated is plotted in an Arrhenius-type way in Figure 8. The data can be well fitted by the following Arrhenius function

observed for two single probes. A detailed analysis of the diffusion behavior (see the yellow and the green filled symbols in Figure 8) showed that the yellow trajectory is typical for

⎛ E ⎞ D(T ) = D0 exp⎜ − A ⎟ ⎝ RT ⎠

(3)

with the formal diffusion coefficient at infinite temperature D0, the activation energy EA and the ideal gas constant R. The corresponding fitted values are also given in Figure 8. Note that the pink data of probe 3 diffusion in SBS72 originate from poor fits assuming normal 2D diffusion and, thus, have high uncertainties. As already shown above, probe diffusion in this system can be better described by a 1D diffusion model for temperatures below the Tg of polystyrene. The corresponding diffusion coefficients D1 are also plotted and fitted linearly in Figure 8. It is surprising that diffusion in the systems investigated here follows an Arrhenius-type behavior and not the Vogel−Fulcher−Tammann relationship. Therefore, the polymers in the structures investigated here seem not to be in the typical glassy state comparable to the one of the bulk polymer which follow the Vogel−Fulcher−Tammann temperature dependence. The reason for this behavior is presumably due to the morphology of the material, which was not further elaborated here.

Figure 8. Arrhenius plot for the diffusion of PDI molecules 3 in SBS30 (filled blue circles), PDI molecules 3 in SBS72 fitted with an 1D diffusion model (eq 2; filled black stars; open black star not included in the black fitting line) and a 2D diffusion model (eq 1; open pink diamonds), and PDI-labeled ethylene/1-butene copolymer poly(E-coB)-PDI in SBS72 (filled red squares). The activation energies EA and the formal diffusion coefficients at infinite temperature D0 as determined from the linear fits of the data according to Arrhenius are noted on the right side of the figure. Because of the inappropriate fitting of the diffusion data of PDI molecules 3 in SBS72 with a 2D diffusion model, the symbols are presented in open form and no fit is shown. The diffusion coefficients obtained from the two trajectories of Figure 7 are also plotted filled with the corresponding color (the green filled square corresponds to 2D fitted data, the green hexagon to 1D fitted data). They fit well to the diffusion coefficients of the corresponding species in SBS72.

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CONCLUSION We demonstrate the feasibility and limitations of hightemperature single molecule fluorescence microscopy in the studies of diffusion of molecular dyes and dye-labeled macromolecular polyolefin chains in block copolymer morphologies. The motion of individual molecules in cylindrical poly(styrene-b-butadiene-b-styrene) triblock copolymer morphologies was observed by single molecule tracking. In order to address the question how the diffusion changes when

poly(E-co-B)-PDI in SBS72, whereas the green trajectory exhibits the behavior observed for the molecular probe 3 in SBS72. Therefore, the latter trajectory with high probability belongs to a single perylenediimide molecule which has not been attached to a polymer chain and which was not separated off in the purification procedure of the labeled polymer poly(Eco-B)-PDI. However, even though we present this example J

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proceeding from room temperature to temperatures above the glass transition of polystyrene, we had to ensure to reach such temperatures without damage of the optical equipment and to reduce photobleaching of the dyes at elevated temperatures. Thus, we modified an existing heating device to enable single molecule measurements at temperatures above 100 °C under a nitrogen atmosphere. For the measurement of the diffusion of single polyolefin chains via following the fluorescence label attached to them, we have elaborated a protocol that provides high quality samples in terms of molecular weights, molecular weight distributions, and proven degree of dye functionalization. This was achieved by the extremely well controlled living (co)polymerization of ethylene with 1 and oxidation of the Ti− polymeryl bond. Subsequently, the end-functionalized polyolefins were labeled with appropriate perylenediimide derivatives. It could be shown that the type of diffusion as well as the diffusion coefficient changes for different compositions of the block copolymers and the resulting morphology. In particular, 1D diffusion for the molecular probe in polybutadiene channels within a polystyrene matrix was demonstrated in the system with a polystyrene content of 72%. In contrast, this behavior was not as pronounced for the diffusion of labeled polyolefin chains. In general, for all systems significant heterogeneity in motion was observed which reflect the complexity of the structure and the huge difference of the glass transition temperatures of both polymer blocks. Concerning future prospects for the tracking of mobility in block copolymer morphologies, our investigations demonstrate how well the diffusion of polyolefins labeled with highly photostable and bright fluorescence dyes can be studied.

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Dominik Wöll: 0000-0001-5700-4182 Stefan Mecking: 0000-0002-6618-6659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the BMBF (project 03X3565D) is gratefully acknowledged. M.B. thanks the Fonds der Chemischen Industrie for support with a stipend. We thank Lars Bolk for GPC measurements, Dr. Inigo Göttker gen. Schnetmann for DOSY measurements, and Dr. Beate Stempfle and Dr. Maren Dill for the assistance on fluorescence microscopy. Additionally, we thank the reviewers for helpful discussions on the step length distribution analysis.

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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.8b00071. Sample holder for microscopy, gel permeation chromatography (GPC); NMR spectra for the synthesis of the functionalized polymers; single molecule tracks; synthesis of the ortho-fluorinated enolatoimine titanium complex; synthesis of the perylenediimide-labeled polymer (PDF) Movie S1 (MPG) Movie S2 (MPG) Movie S3 (MPG) Movie S4 (MPG) Movie S5 (MPG) Movie S6 (MPG) Movie S7 (MPG) Movie S8 (MPG) Movie S9 (MPG) Movie S10 (MPG) Movie S11 (MPG) Movie S12 (MPG)

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AUTHOR INFORMATION

Corresponding Authors

*(S.M.) Tel +49 (0)7531 88-5151; Fax +49 (0)7531 88-5152; e-mail [email protected]. *(D.W.) Tel +49 (0)241 80 98624; e-mail [email protected]. K

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