Letter Cite This: ACS Macro Lett. 2019, 8, 800−805
pubs.acs.org/macroletters
Quantifying Solvent Effects on Polymer Surface Grafting Lukas Michalek,†,‡ Kai Mundsinger,†,‡ Leonie Barner,*,†,§ and Christopher Barner-Kowollik*,†,∥ †
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School of Chemistry, Physics and Mechanical Engineering, Institute for Future Environments, Queensland University of Technology (QUT), 2 George Street, QLD 4000, Brisbane, Australia § Institut für Biologische Grenzflächen (IBG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie (ITCP), Karlsruhe Institute of Technology (KIT), Engesserstrasse 18, 76128 Karlsruhe, Germany S Supporting Information *
ABSTRACT: When grafting polymers onto surfaces, the reaction conditions critically influence the resulting interface properties, including the grafting density and molar mass distribution (MMD) on the surface. Herein, we show theoretically and experimentally that the application of poor solvents is beneficial for the “grafting-to” approach. We demonstrate the effect by grafting poly(methyl methacrylate) chains on silica nanoparticles in different solvents and compare the MMD of the polymer in solution before and after grafting via size exclusion chromatography (SEC). The shorter polymer chains are preferentially grafted onto the surface, leading to a distortion effect between the MMD in solution and on surfaces. The molecular weight distortion effect is significantly higher for ethyl acetate (good solvent quality, difference in Mw surface to solution 14%) than for N,Ndimethylacetamide (poor solvent quality, 6%). The difference in MMD on the surface to the solution significantly affects both the surface properties (e.g. the grafting densities) and their determination.
D
tion of the preferred attachment of short polymer chains on surfaces is depicted in Scheme 1 on the example of silica nanoparticles, which we based the herein introduced method on. Our method allows for the simple and instrumentally nondemanding determination of MMD shifts during “graftingto” of polymers. Earlier, the preferred attachment of shorter polymer chains was quantified via quartz crystal microbalance (QCM) measurements and correlated with the difference in radius of gyration, Rg. The quantification of this shift, corroborated by atomic force microscopy (AFM) based single-molecule force spectroscopy (SMFS) measurements, led to the establishment of a preferential grafting factor, κ. The preferential grafting factor applied on a number based molar mass distribution predicts the shift of the distribution upon surface attachment. The shift is more pronounced for higher molar masses and broad distributions.33 In addition, a solvent dependency of the shift is expected due to the power law of the preferential grafting factor, κ, with the solvent interaction exponent, n*,
esigning and tailoring functional surfaces is one of the key endeavors in soft matter materials science. Applications for functional interfaces range from 3D cell scaffolds,1,2 optoelectronics,3−5 and coatings6−8 to sensors.9−11 One way to tailor properties such as hydrophobicity,12−14 tribology,15,16 or antibiofouling17−22 is to covalently tether polymers onto surfaces. The surface attached polymers can be additionally equipped with specific functional groups to finetune the desired surface properties.23,24 Two important properties of surface grafted polymers are the grafting density (tethering distance between individual polymeric chains) and the molar mass distribution (MMD) of the surface attached polymers, as these determine the surface performance. The main approaches to achieve polymer-functionalized surfaces are the “grafting-to” and “grafting-from” methods.25−30 In the latter approach, the polymers are synthesized in situ, growing from the surface, and giving access to high grafting densities. The “grafting-to” approach uses polymer chains that are synthesized prior to grafting and are equipped with end groups that easily react with the surface, generally leading to lower grafting densities compared to the “grafting-from” approach.31 However, the “grafting-to” method allows for a plethora of characterization methods of the polymer chains to be applied prior to surface attachment. Polymer properties determined as such are often assumed to reflect the polymer properties on the surface and are even used as factual data in further characterization.25,32 Recently, we demonstrated this assumption to be incorrect.33 Due to a preferred attachment of shorter polymer chains, a significant shift of the MMDs to lower molar masses was observed on the surface. A schematic representa© 2019 American Chemical Society
ij M yz κ = jjj n zzz jM z k i{
n*
(1)
and the ratio of number-average molecular weight, Mn, and single molar mass, Mi, at a certain point of the polymer Received: May 7, 2019 Accepted: June 13, 2019 Published: June 18, 2019 800
DOI: 10.1021/acsmacrolett.9b00336 ACS Macro Lett. 2019, 8, 800−805
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ACS Macro Letters Scheme 1. Schematic and Simplified Presentation of the Preferred Attachment of Shorter PMMA Polymer Chains onto SiO2 Nanoparticles (NPs)a
a
The colored lines represent the short (red) and long (blue) polymer chains.
distribution. In the case of n* = 0.5 (θ-conditions), the polymer chain behaves like an ideal chain following a random walk. At θ-conditions, the polymer−polymer interactions are energetically equal to polymer−solvent interactions, and therefore, the excess energy of mixing is zero. In good solvents (n* > 0.5), the polymer−solvent interactions are advantageous, causing the chain to expand. Whereas for poor solvents (n* < 0.5), the polymer chain contracts due to favorable polymer− polymer interactions.34 To evaluate and quantify the effect of preferred surface attachment (difference of MMDs from solution to surface) with varying solvent quality, we herein conduct a calculation showing a larger difference for good solvents. Symmetrical Gaussian distributions are employed as an approximation for MMDs to simplify the calculations. Furthermore, the distributions employed in the calculations are narrow (dispersity of Đ = 1.05 for a number-average molar mass of Mn = 5.00 × 105 g·mol−1) to illustrate the relevance for polymers synthesized via controlled polymerization techniques. The prediction of the MMD on a surface is conducted by multiplying the preferential grafting factor (eq 1), κ, with the MMD in solution. The calculated shifts of MMD on surfaces for the described distribution with a solvent interaction parameter from n* = 0.1 (very poor solvent) to n* = 1.0 (very good solvent) are depicted in Figure 1. The dotted lines in Figure 1A show the normalized MMD of the calculated surface distributions. An increase of the solvent quality results in a stronger shift of the MMD toward lower molar masses (difference in percentage of Mn plotted in Figure 1B), whereas poor solvents show a negligible shift. The difference in radius of gyration, Rg, of polymer coils with low versus high molar mass is distinctly lower for contracted chains. In a good solvent, the increased size difference of the expanded polymer coils results in a preferred attachment of the lower molar mass fraction of the MMD and therefore results in a more pronounced shift of the distribution. For an increase of the solvent quality the impact on the surface distribution is
Figure 1. (A) Calculated shift of normalized MMD on surfaces (dotted lines) for different solvent interaction parameters, n* (ranging from 0.1 “poor solvent”, over 0.5 “θ-condition”, to 1.0 “good solvent”). Starting with a narrow Gaussian MMD (Đ = 1.05) with a number and weight-average molecular weight of Mn = 5.00 × 105 g· mol−1 and Mw = 5.24 × 105 g·mol−1 (solid black line). (B) Percentage shift of Mn on surface MMD depending on the solvent quality extracted from the MMDs in (A).
noticeably stronger, suggesting that preferably poor solvents should be employed for the “grafting-to” approach to guarantee a similar surface MMD in comparison to the solution MMD. We herein demonstrate this effect. A shift in MMD from higher to lower molar masses on the surface implies that a shift of the same magnitude but toward higher molar masses has to occur in the solution. The sum of the shifted MMD on the surface and the MMD of the remaining reaction solution has to be equal to the distribution of the starting reaction solution (principle of mass conservation). The above raises a critical question: Why are there no reports for such MMD shifts of the reaction solution? Typically in the “grafting-to” approach, a large excess of polymer chains to reactive surface sites is employed to drive the reactions to high grafting densities.35,36 Especially for smooth planar substrates, the excess is commonly several orders of magnitude. In the case of our previous QCM study, the experiments were conducted in a flow setup (very high 801
DOI: 10.1021/acsmacrolett.9b00336 ACS Macro Lett. 2019, 8, 800−805
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ACS Macro Letters excess of end-functionalized polymers) to ensure homogeneous and full coverage of the surface.33 The enormous excess of polymer guaranteed exclusion of any concentration related effects on the grafting density, however, it is also the reason why a shift in the MMD of the remaining reaction solution cannot be detected. The change in the polymer concentration is therefore infinitesimal small and thus virtually nonobservable. To be able to detect a change in the MMD of the remaining polymer concentration either the number of end-functionalized polymers in the reaction solution has to be strongly decreased or the number of reactive surface sites has to be sufficiently high. In an ideal case, the concentration of polymer chains in the reaction solution will be reduced by 50 percent after grafting. In other words, half of the total amount of polymers is tethered to the surface and the other half remains in solution. The sum of the two MMDs must result in the distribution of the reaction solution before grafting. For a symmetrical Gaussian distribution (same as in Figure 1 with a solvent interaction parameter of n* = 0.9) the surface and solution MMD would look like the deconvoluted peak in Figure 2A. The area of the surface distribution (red curve) and the remaining reaction solution distribution (blue curve) is equal and the sum is resulting in the start solution distribution (black curve). The different shape of the two distributions can be explained by the power law relation (see eq 1) of the preferential grafting factor, κ. The increased weighting of smaller molar masses will lead to slight peak broadening of the surface distribution. In Figure 2B, the normalized MMDs on surfaces (red curves) and in solution (blue curves) for varying solvent quality can be seen. The sum of the surface and remaining reaction solution distribution always results in the solution distribution before grafting (black curve). To elucidate the complementary shift of MMD in solution, a sufficient amount of polymer must be tethered to the surface to induce measurable changes in the remaining reaction solution. Theoretically, this may be achieved by very low sample concentrations, yet in practice one would have to use such low concentrations that most characterization methods would not yield meaningful data. Instead, we chose to use common procedures and instruments. A concentration of 2 mg·mL−1 of polymer is common and can be readily analyzed via SEC. To observe a change in this solution, sufficient polymer must be removed from the solution, requiring a large surface for grafting-onto. Spherical nanoparticles (NPs) are ideal substrates due to their ratio of surface to volume that increases with decreasing particle size. Poly(methyl methacrylate)s with hydrolyzable silane end groups were used as this enables the application of silica nanoparticles as substrate. The minimal nanoparticle size is limited by the radius of gyration of the grafted polymers. The particles diameter needs to be sufficiently large to rule out excluded volume effects.37 Therefore, a difference between the diameter of a silica NP before and after grafting should be as low as possible. With a radius of gyration of Rg ≈ 7.1 nm (refer to section 7 of the Supporting Information), the difference between a 460 nm silica NP before and after grafting is less than 3%. Silica NPs of 460 nm diameter were synthesized according to a published procedure.38 The next consideration is the amount of NPs to be used as substrate, ideally 50% of the polymer in solution should be immobilized. In our hands solutions containing 4 mg·mL−1 polymer and dispersions containing 400 mg·mL−1 silica particles result in the desired 50% reduction. The solutions were prepared in different
Figure 2. (A) Deconvoluted peak for a starting reaction solution MMD (reduction in polymer concentration of 50% in arbitrary units, a.u.) with the surface distribution (red curve) and solution distribution). (B) Calculated shift of normalized MMD on surfaces (red dotted lines) and solutions (blue dotted lines) for different solvent interaction parameters, n* (ranging from 0.1 “poor solvent”, over 0.5 “θ-condition”, to 1.0 “good solvent”). Starting with a narrow Gaussian MMD (Đ = 1.05) with a number and weight-average molecular weight of Mn = 5.00 × 105 g·mol−1 and Mw = 5.24 × 105 g· mol−1 (solid black line).
solvents (S1 ethyl acetate (EA), S2 N,N-dimethylacetamide (DMAc) containing 0.08 wt.-% LiBr). Subsequently, 2.5 mL of polymer solution and 2.5 mL of nanoparticle dispersion were combined and stirred for 7 d at 50 °C. The particles were separated via centrifugation and washed twice with THF to remove physisorbed polymers. Prior to SEC analysis, the supernatants were combined and the solvent evaporated under reduced pressure. The SEC traces before and after grafting indicate a change in concentration of 50 ± 3% (peak area), which is the desired range and equal to the shift assumed in the calculated data (see Figure 2A). DMAc and EA were chosen as they represent a near θ-solvent (DMAc) and a good solvent (EA) under the current conditions. The difference in the apparent hydrodynamic radii was determined via dynamic light scattering to be 0.3 nm (7.3 nm in DMAc, 7.6 nm in EA, see Figure S7 in the 802
DOI: 10.1021/acsmacrolett.9b00336 ACS Macro Lett. 2019, 8, 800−805
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ACS Macro Letters
(tethering density of polymer chains on surfaces), a precise knowledge of the molar mass and MMD on the surface is critical. The general used procedures for such grafting density calculations are the dry thickness method, the gravimetric assessment, and swelling experiments. The precise knowledge of MMD on the surface is significantly increasing the exact quantification of surface grafting for the first two methods.39 By changing from good solvents to θ-conditions, the precision of these quantification methods can be improved and the error of the calculated value can be reduced by a factor of 2 (difference in molecular weight in Figure 3B). Additionally, previous studies conducted via X-ray photoelectron spectroscopy (XPS) and AFM showed that the grafting density itself can be improved by decreasing the solvent quality,40 showing that a decrease in solvent quality (by changing the ratio of a bad and good solvent in a binary solvent mixture) is increasing the grafting densities. In summary, we demonstrate that solvent−polymer interaction has a critical impact on the resulting surface functionalization via the “grafting-to” approach. Using DMAc (near θ-condition) instead of EA (good solvent), the preferential grafting of shorter polymer chains was reduced by a factor of 2. If surfaces with a similar MMD in solution and on surface are desired, a rather poor solvent must be employed, accompanied by a concomitant increase of the grafting density.40 The choice of a poor solvent is in contrast to the usual choice of good solvents for surface functionalization, because the polymer must remain soluble to achieve surface functionalization. Solvent systems with low solvent interaction parameters (n* ≪ 0.5) are unable to sufficiently solvate the polymer and can lead to agglomeration and precipitation. The ideal conditions of surface grafting by the “grafting-to” approach are therefore close to θ-conditions, as demonstrated by the grafting of poly(methyl methacrylate) (PMMA) chains on silica NPs in EA at 50 °C. We submit that the herein introduced simple nanoparticle and SEC based method for determining shifts in MMDs during surface grafting is applicable to other polymer−solvent systems.
Supporting Information). All SEC samples were run multiple times to exclude errors due to shifts in retention volume caused by potential fluctuations in temperature or eluent composition. Retention volumes were referenced to an internal standard. Per sample three virtually identical traces were averaged to simplify the further calculations. Figure 3 illustrates
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Figure 3. (A) Experimentally observed shift of normalized MMD, recorded via SEC, on surfaces (red curves) and solutions (blue curves) for grafting in DMAc (dashed lines) and EA (dotted line). (B) Weight-averaged molecular weight Mw of the MMD and the difference between surface and solution for DMAc and EA functionalization.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00336.
the shift of the MMD in solution from the dissolved polymer before grafting in black (solution start) to the MMD after grafting (solution DMAc/EA) in blue and the complementary MMD on the particle surface in red (Surface DMAc/EA). The surface distributions were calculated by the difference of the MMD in solution before and after grafting (principle of mass conservation). Clearly evident is a distinct difference in the observed shift depending on the solvent quality, with the shift being significantly larger for good solvents. Especially for the higher molar mass region the effect is clearly visible. The distortion effect results in a difference of the weight-averaged molar mass Mw between in solution (after grafting) and on surfaces of 14% for EA and 6% for DMAc (shown in Figure 3B). Using DMAc, the impact on the preferential grafting of shorter polymer chains on surfaces was reduced, and therefore, the MMD on the surface is closer to the MMD in solution. Why are these information of relevance and how does the preferential grafting change the properties of the functionalized surface? For an accurate calculation of grafting densities
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Detailed experimental procedures, calculations, and characterization data (SEC, NMR, and TEM); Materials and methods (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Leonie Barner: 0000-0002-6034-0942 Christopher Barner-Kowollik: 0000-0002-6745-0570 Author Contributions ‡
These authors contributed equally. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. 803
DOI: 10.1021/acsmacrolett.9b00336 ACS Macro Lett. 2019, 8, 800−805
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ACS Macro Letters
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ACKNOWLEDGMENTS C.B.-K. acknowledges receipt of an Australian Laureate Fellowship from the Australian Research Council (ARC) as well as key support from the Queensland University of Technology (QUT). L.B. acknowledges support from QUT and the Institute for Future Environments (IFE). Some of the data reported in the current study were obtained at the Central Analytical Research Facility (CARF) operated by the IFE. L.M. acknowledges the support by his QUTPRA Ph.D. scholarship.
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