Direct Mapping of RAFT Controlled ... - ACS Publications

Mar 30, 2016 - ABSTRACT: Single molecule force spectroscopy (SMFS) is employed to gain insight into reversible addition−fragmentation chain transfer...
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Direct Mapping of RAFT Controlled Macromolecular Growth on Surfaces via Single Molecule Force Spectroscopy Thomas Tischer,† Robert Gralla-Koser,† Vanessa Trouillet,‡ Leonie Barner,†,§ Christopher Barner-Kowollik,*,∥,§ and Cornelia Lee-Thedieck*,† †

Institute of Functional Interfaces and §Soft Matter Synthesis Laboratory, Institute for Biological Interfaces, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT) and Karlsruhe Nano Micro Facility (KNMF), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: Single molecule force spectroscopy (SMFS) is employed to gain insight into reversible addition−fragmentation chain transfer (RAFT) polymerization processes with living characteristics on glass surfaces. Surface-initiated (SI)-RAFT was selected to grow poly(hydroxyethyl methacrylate) (PHEMA). After aminolysis of the RAFT chain termini, thiol moieties serve as anchoring points for the gold tip of an atomic force microscope. The results allow to directly monitor the macromolecular growth of the surface-initiated polymerization. The obtained SMFS-based molecular weight distribution data of the polymers present on the surface indicate that the RAFT chain extension proceeds linearly with time up to high conversions. The current study thus adds SMFS as a valuable tool for the investigation of SI-RAFT polymerizations.

T

surface bound macromolecule with an AFM cantilever tip can reveal intramolecular information such as the contour length or the conformation of molecules, as well as intermolecular interactions.23 For macromolecules, the molar mass of a single polymer chain can be calculated from the contour length and the length and mass of the respective monomer.24−29 Statistical analysis offers insights into the mass distribution and the polydispersity of the polymer on the surface. Recently, we investigated poly(hydroxyethyl methacrylate) (PHEMA) grafts on Si surfaces generated via SI-RAFT polymerization.30 A functional RAFT silane was immobilized on the surface to graft HEMA from the surface following an Rgroup approach. The resulting substrates were investigated employing ellipsometry, X-ray photoelectron spectroscopy (XPS), AFM imaging, and contact angle measurements. All these characterization techniques focus on the chemical composition or the morphology of the generated films. An in-depth study of the generated polymer film is only possible in bulk, while investigations of the polymer chains are mainly carried out in solution. Herein, we introduce SMFS as a powerful technique, which allows monitoring the growth of

he use of surface-initiated (SI) polymerization techniques is constantly increasing, as they represent important tools for functional interface design.1 In recent years, SI polymerization processes were, for example, employed for the functionalization of Au surfaces,2 biosurfaces,3−10 steel,11 or particles.12 Frequently employed control processes are reversible addition−fragmentation chain transfer (RAFT)13 polymerization or atom transfer radical polymerization (ATRP).14 Thus, an in-depth understanding of the processes occurring during polymerization directly at the surface is mandatory. Substantial efforts have been made to obtain information on the SI polymerization process: For example, surface termination kinetics have been investigated via time-resolved electron spin resonance spectroscopy,15 the polymer generated on the surfaces was cleaved and analyzed, for example, from nanoparticles16 and cellulose surfaces,17 as well as the polymer generated in solution via size-exclusion chromatography (SEC).18 As the amount of detachable polymer is, in general, relatively low, its characterization is challenging. In other studies, the addition of monomer units to the polymer chains at the surface was followed employing surface plasmon resonance or quartz crystal microbalance.19,20 Atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) is a versatile method in biology21 and chemistry.22 Stretching single molecules by pulling on a single © XXXX American Chemical Society

Received: February 6, 2016 Accepted: March 22, 2016

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DOI: 10.1021/acsmacrolett.6b00106 ACS Macro Lett. 2016, 5, 498−503

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Scheme 2. (Top) Reaction Sequencea; (Bottom) C 1s XP Spectra of Substratesb

single polymer chains on the surface generated via the RAFT process in a nondestructive fashion. SMFS furthermore allows an analysis in aqueous/solvated environment and of solvated polymer chains. Scheme 1 displays the general setup of a SMFS experiment. Surface tethered polymer chains are elongated employing an Scheme 1. General Scheme for the Elongation of Polymer Chains of Different Lengths During an SFMS Experimenta

a

Top: The gold tip of the AFM cantilever binds to the thiol terminus of a single polymer chain and when retracted the cantilever bends/ deflects in dependence of the force needed to stretch the polymer chain. Bottom: Schematic representation of force curves resulting from chains with different length. The occurring force is plotted vs the displacement of the cantilever. Red: Force curve resulting from stretching a shorter polymer; Blue: Force curve resulting from a longer polymer.

a

(A) Immobilization of RAFT-silane on activated substrate; 4-(3(triethoxysilyl) propylcarbamoyl)-2-cyanobutan-2-yl benzo dithioate, toluene, 50 °C to ambient temperature, 16 h; (B) SI-RAFT polymerization; HEMA, 4-cyano-4-(phenylcarbonothioylthio)-pentanoic acid (sacrificial RAFT-agent), AIBN in dioxane/water, 80 °C; [HEMA]/[RAFT]/[AIBN] 1735:1:1 (C) Aminolysis of the RAFT end group; triethylamine, hexyl amine, ethanol, ambient temperature, 5 h. The generated thiol terminus can be exploited for the characterization via SMFS. b(from Bottom to Top) RAFT-silane functional substrate; substrate after 1 h RAFT polymerization of HEMA; substrate after 6 h RAFT polymerization of HEMA. All spectra are normalized to the highest intensity.

AFM cantilever tip. Fitting of the resulting force curve leads to information on the contour length (lc) and the stiffness (via the persistence length lp) of the investigated polymer chain. By varying the polymerization time, samples with polymers that differ in length are generated. The contour length of a multitude of polymer chains is recorded by SMFS and the distribution of the contour length of the polymers on the different samples is compared. SI-RAFT polymerization was employed to generate polymer chains with high end-group fidelity, which allows for post modification of the polymer chains, i.e. the thiocarbonylthio end-group of the polymer can be transferred into a thiol endgroup via aminolysis (Scheme 2, top). PHEMA was selected because of its good biocompatibility with regard to future applications in stem cell biology, where we wish to employ these functional interfaces. The mechanical properties of the microenvironment are well-known to impact stem cell behavior and may even allow directing stem cell differentiation.31 For this purpose, we aim to generate a synthetic biomimetic analog of one of the most elastic biomolecules known, tropoelastin.32 An established procedure reported earlier by us30 was adapted to generate PHEMA strands on glass surfaces. The overall strategy is depicted in Scheme 2. Activated glass substrates are functionalized with a tailor-made RAFT-silane

anchor in a straightforward manner. In a subsequent polymerization step, HEMA is polymerized in the presence of sacrificial RAFT agent via a SI polymerization procedure to generate polymer chains in varying density on the surfaces. The resulting surfaces are analyzed employing AFM imaging, XPS, and ellipsometry. Finally, an aminolysis is carried out to transform the terminal RAFT moieties into thiol groups, which readily tether to the Au surface of an AFM cantilever and therefore allow for an analysis via SMFS. For the SMFS experiments, a very low concentration of the RAFT-silane anchor is employed to generate a low density polymer array. Specifically, a 2 μmol· L−1 RAFT-silane solution was used that corresponds to a thousandth of the concentration employed for the surfaces generated for the ellipsometry and XPS measurements. XPS was chosen as an additional surface characterization method. Due to its high surface sensitivity it is a valuable tool to determine the changes in thin layer coatings. The XP spectrum of the substrate with the RAFT functional silane (Scheme 2, bottom) shows the distinctive signal at C 1s = 285.0 eV 499

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ACS Macro Letters ⎛ k T⎜ 1 F (x ) = b ⎜ lp ⎜⎜ 4 1− ⎝

corresponding to C−C/C−H groups. In addition, the expected C 1s peaks at 286.7 and 288.5 eV can be attributed to the C− N/C−O and the OC−N structural motifs of the RAFTsilane anchor, respectively.33,34 In the subsequent polymerization step, the XP spectrum changes in favor of the C−O and OC−O (C 1s = 288.9 eV) groups, which correspond to the attached HEMA monomer units (Scheme 2, 1 h middle)30 and resembles the one after a 6 h polymerization time (top). Since XPS, for organic materials, has a sampling depth of approximately 10 nm, after 1 h of polymerization, only bulk polymer can be detected as the thickness of the grafted surface exceeds 10 nm (see Figure 2, bottom). Therefore, the presence of the RAFT end group can only be shown on the silane functional substrate before polymerization. Nevertheless, it can be assumed that the RAFT moiety is still intact after the polymerization, as can be shown via SMFS. The number of rupture events is almost 15-fold higher after aminolysis (Figure S1), which transforms the thiocarbonylthio terminus of the PHEMA chains into a thiol moiety allowing a better tethering to the Au cantilever than would be possible for the RAFT chain terminus. Therefore, SMFS provides indirect proof of the presence of the RAFT group. For SMFS, only force curves showing unfolding of a single polymer chain followed by a clear rupture event of the tip from the polymer are selected for analysis as shown in Figures 1 and S2. Approximately 3% of the total number of recorded force curves show unfolding and subsequent rupture. The obtained force−distance curves are fitted to the worm-like chain (WLC) model,35

(

x lc

2

)

⎞ 1 x⎟ − + ⎟ 4 lc ⎟⎟ ⎠

where x is the distance of the AFM tip from the surface, kb is the Boltzmann constant, T is the absolute temperature, lp is the persistence length, and lc is the contour length. As described before, the AFM cantilever tip specifically attaches to thiol moieties of the free (not surface-tethered) end groups of the polymer chains. The specificity of the binding is shown by the above-noted strong increase in the number of observed unfolding events when comparing the measurements of polymer chains before (no free thiol groups available) and after aminolysis of the RAFT agent, which results in free thiol moieties at the surface-distant polymer ends (Figure S1). Thus, the polymer chains are bound at their ends when stretched between the surface and the AFM cantilever tip, which is a prerequisite to ensure that the full length of the polymer chain contributes to the measured contour length l c . The representative force curves of single polymer chains synthesized with different polymerization times in Figure 1 show a distinct change in the measured contour length over time. Figure 1 also includes a representative force curve of a chain extension experiment (black curve) proving the livingness of the RAFT polymerization. The persistence length lp is in the range of 0.2 ± 0.1 nm for all samples, which is in the range of the value described for tropoelastin (0.36 ± 0.14 nm).32 To study the roughness and nanotopography of the generated PHEMA surfaces, AFM imaging is applied. Figure 2 displays the resulting AFM mapping micrographs of representative, dried surfaces (refer to Figure S3 for additional images). The displayed surfaces are synthesized using a higher concentration (2 mmol·L−1) of the RAFT-silane anchor (refer to the SI) to generate a fully covered polymer film allowing for a characterization via techniques such as ellipsometry. The PHEMA surfaces show a very low roughness in the range of 1− 2 nm, which can be attributed to a homogeneous distribution of the polymer chains on the surface. Ellipsometry measurements reveal that the polymer layer thickness ranges from 12 to 70 nm and is increasing linearly with longer polymerization time (Figure 2, bottom, and Figure S4), as expected, and are in the range reported earlier.30 However, at higher polymerization times, the increase in the layer thickness decelerates. In addition, it is noteworthy that an initial sudden increase in layer thickness occurs. This observation can be attributed to a socalled “hybrid” behavior of the polymerization system at early reaction times when the system displays a combination of a “conventional” chain transfer and a controlled free-radical polymerization.36 Figure 3 depicts the average contour lengths determined by SMFS of surface-anchored PHEMA chains that were polymerized for different periods of time. As expected, the polymer chains elongate with progressing polymerization time. Moreover, a linear increase of the polymeric chain length with polymerization time can be observed, suggesting a polymerization mechanism with controlled/living characteristics occurring at the surface bound RAFT moieties. Similar to the thickness evolution (Figure 2), an initial increase in chain length occurs at early reaction times. Figure 3, bottom, displays the dispersity of the contour length of polymer chains on four different representative glass surfaces with varying polymerization times. After 1 h polymerization time, the polymers

Figure 1. Representative force−distance curves of different PHEMA strands that were grown for varying periods of time and, therefore, differ in their chain length. [HEMA]/[RAFT]/[AIBN] 1735:1:1 in dioxane/H2O (1:1) at 80 °C (for further details, refer to the Supporting Information). For the time point 6 + 4 h, the surface was removed after 6 h from the polymerization solution and transferred to a new reaction mixture to show the possibility of chain extension. 500

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Figure 3. Contour length lc of the PHEMA chains on the surface correlated with the polymerization time (top). The time point at 10 h of polymerization was obtained by adding the surface after 6 h of polymerization ([HEMA]/[RAFT]/[AIBN] 1735:1:1) to a new reaction mixture to evidence the living character of the polymer chains on the surface. Distribution of the contour length lc at different polymerization times and the corresponding polydispersity, Đ, and number n of analyzed force curves (bottom). The contour length increases with increasing polymerization time and the polymer chains on the surface show a narrow dispersity.

Figure 2. AFM images of the investigated surfaces at different polymerization times with the corresponding roughness (root-mean squared) (top). Results of ellipsometry measurements (bottom). [HEMA]/[RAFT]/[AIBN] 1735:1:1 in dioxane/H2O (1:1) at 80 °C (for further details, refer to the Supporting Information).

exhibit an average contour length of close to 110 nm, while after 6 and 10 h, a length of approximately 300 and 390 nm, respectively, is achieved. The increase in polymer chain length is linear, although the conversion in solution calculated via NMR indicates limited growth, as does the molecular weight distribution evolution acquired via SEC (refer to Figures S5 and S6). The polymerization on the surface based on RAFT chain termini still shows a dynamic behavior, observed as a linear growth over time, while the free polymer chains generated in solution already cease macromolecular growth. In contrast to conventional state-of-the-art surface characterization techniques, SMFS can be carried out in solution, allowing for the characterization of surface bound polymers in their solvated state. It is thus possible to directly compare chain length or molecular weight of polymers on the surface and in solution, without the need to take the effect of dry versus solvated state of polymers into account. Figure 4 shows a comparison between the molecular weight of the obtained PHEMA polymer in solution measured via SEC and the molecular weight calculated from the length of the RAFT polymer chains bound on the surface determined via SMFS. The molar mass of a polymer chain Mi is calculated from the contour length lc as

Figure 4. Comparison of the results obtained via SEC of the polymer in solution and the molecular weight calculated from the contour length of the polymer chains on the surface determined via SMFS (conditions: [HEMA]/[RAFT]/[AIBN] 1735:1:1). The results were obtained at different time points of the polymerization. Horizontal error bars represent the standard deviation of the averaging over different SMFS experiments. Vertical error bars represent the standard error of a SEC experiment.

Mi = M momo 501

lc lmono DOI: 10.1021/acsmacrolett.6b00106 ACS Macro Lett. 2016, 5, 498−503

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ACS Macro Letters with the molar mass of the monomer HEMA as Mmono = 130.13 g·mol−1 and the length of the monomer lmono = 0.25 nm as the projected C−C−C distance in the polymer backbone. Using the number-averaged molar mass Mn

Mn =

1 n

shorter chains (Figure S6) and thinner layers (refer to Figure S4), yet showing the same overall plateauing effect as the lower RAFT concentration experiments. Interestingly, Brooks and colleagues found good agreement of the SMFS results with the ones obtained from SEC on latex particles functionalized via SIATRP for poly(N-isopropylacrylamide).26 These authors observed a higher Đ for the tethered polymer than in solution, yet their system is different in terms of the controlling process, the monomer, and the SFMS attachment process. In summary, we introduce a SFMS method that allows for the analysis of the SI-RAFT polymerization of HEMA using thiocarbonylthio aminolysis, which is applicable to potentially all SI-RAFT systems. We envision that the further systematic utilization of SMFS as a versatile and direct characterization technique for SI-RAFT polymerizations under a wide range of conditions and monomers will lead to an in-depth understanding of the associated polymerization processes on surfaces.

∑ Mi

and the mass average molar mass Mw Mw =

∑ Mi2 ∑ Mi

the polydispersity Đ can be calculated. The above calculation was already employed by other groups.25,26 The comparison of the different molecular weights of polymers in solution and on the surface shows that the RAFT polymerization on the surface still proceeds, even at high conversion, while in solution the observed average molecular weight is reaching a plateau (see also Figure S5). It should also be noted that the molecular weight values of the polymers in solution measured via SEC are not absolute values, but values relative to PMMA standards, while the values obtained by SMFS are absolute. Thus, the comparison between SMFS and by SEC data allows for conclusions on the relative polymer growth in solution and on the surface, yet not for an absolute correlation of the molecular weight values obtained with the different methods. A correlation between the determined contour length and the conversion calculated via NMR is shown in Figure S7. The RAFT terminus capped polymer chains are growing on the substrate up to high conversions of ∼71% after 6 h, as estimated from the measured contour length and the targeted degree of polymerization. The surface bound thiol (i.e., former RAFT) terminal polymer is characterized by a narrow distribution of Đ ≈ 1.1 (Figure 3), yet the dispersity of the polymer generated in solution is increasing with longer polymerization time, reaching Đ ≈ 1.8 after 6 h (Figure S5). In solution, a hybrid behavior is observed (Figure S5),36 that is, an initial rapid increase in molecular weight at early reaction times congruent with what is observed from both the ellipsometry and SMFS data. Further, in solution, additional high molecular weight material forms at long reaction times (Figures S6 and S7). More importantly, the molecular weight distributions of polymers in solution show a significant tailing to low molecular weights suggesting that a considerable fraction of non-chain extendable material is generated, possibly by disproportionation processes during bimolecular termination events early on in the RAFT processes (hybrid behavior) and during the course of the polymerization. In this light, the disparate behavior of RAFT chain growth on the surface mapped by SMFS and the one observed in solution can be understood: The SMFS technique has only access to chains that carry a RAFT functionality and not those that are terminated by radical processes. Thus, it comes at no surprise that (i) the SMFS observed dispersities are lower than those observed in solution and (ii) that the SMFS deduced molecular weights do not show a leveling off at increased reaction times. The ellipsometry data, on the other hand, also capturing the entire distribution, including chains generated by continued disproportionation events, leaving nonfunctional chains behind, show a similar plateauing in their thickness evolution as the molecular weight evolution in the solution process. Not surprisingly, an increase in RAFT and initiator concentration improves the dispersity in solution, but results, as expected, in

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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/acsmacrolett.6b00106. Contains all syntheses, experimental characterization methods, and instrument data, as well as the spectra that where referred to in the main text (and Figures S1− S7; PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.L.-T. acknowledges funding from the BMBF NanoMatFutur program (FKZ 13N12968) and the program BioInterfaces in Technology and Medicine (BIFTM) of the Helmholtz Association. C.B.-K. acknowledges continued funding from the Karlsruhe Institute of Technology (KIT) and the Helmholtz Association via the BIFTM program, the German Research Council (DFG), and the Ministry of Science and Arts of the state of Baden-Württemberg supporting the current project. The authors acknowledge Dr. Kerstin Blank and Dr. Ruby Sullan (both Max Planck Institute of Colloids and Interfaces, Potsdam, Germany) for advice on SMFS experiments and fruitful discussions.

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REFERENCES

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