Effect of Polymer Grafting Density on Mechanophore Activation at

Letter pubs.acs.org/macroletters

Effect of Polymer Grafting Density on Mechanophore Activation at Heterointerfaces Jun Li,† Bin Hu,‡ Ke Yang,† Bin Zhao,‡ and Jeffrey S. Moore*,† †

Beckman Institute Beckman Institute for Advanced Science and Technology, Department of Materials Science and Engineering, Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Silica nanoparticles grafted with poly(methyl acrylate) chains whose anchor points are maleimide-anthracene cycloadducts were prepared at various grafting densities to demonstrate fundamental characteristics of mechanophore activation at heterointerfaces. The monotonically decreasing correlation between polymer grafting density and surface-bound maleimide-anthracene mechanophore activation was quantitatively elucidated and discussed. Presumably as a result of polymer−polymer interactions, polymer grafting density plays a significant role in heterogeneous mechanophore activation. The findings are a valuable guide in the design of efficient force-sensitive, damage-reporting polymer composites, where damage is often localized to the interface between the matrix and the reinforcing phase.

P

We previously reported the interfacial activation of an anthracene-maleimide (MA) mechanophore located between poly(methyl acrylate) (PMA) grafted onto silica nanoparticles (SiO2NPs). The MA mechanophore was demonstrated previously to undergo a mechanically induced [4 + 2] retro

olymer mechanochemistry represents an interesting way to probe relations between macroscopic mechanical forces and chemical reactivity.1 The past decade witnessed rapid development of mechanophore design and incorporation of mechanophores in functional polymeric materials.2−5 Various mechanophores were developed to enable activation of catalysts,6 depolymerization,7 mechanochromism,8 mechanoluminescence,9 and production of reactive functional groups10 for further usage. We previously reported the first demonstration of mechanophore activation at a heterointerface to address the behavior of interfacial mechanophore activation,11 as it is relevant to composite materials where damage-reporting and self-healing functions are needed.12−14 In general, mechanochemical characteristics of mechanophores at heterointerfaces exhibit behavior similar to mechanophores located at the center of homopolymers activated by elongational flow of dilute solutions or suspensions, for example, threshold molecular weight and a linear increase in activation rate accompany an increasing polymer molecular weight. However, in other respects, mechanochemical activation at heterointerfaces differs sharply from mechanophore-centered homopolymers where degree of polymerization is the determining factor of activation rate.15,16 For example, at heterointerfaces the local concentration of polymers is determined by the grafting density, a parameter that in return often influences the chemical reactivity,17,18 biological activity,19,20 mechanical properties,21 and so on of polymer grafted materials. Herein, we demonstrate that polymer grafting density at the heterointerface is a key determining factor of mechanophore activation. We believe this work will further the understanding of mechanochemical behavior of polymer at interfaces and foster the development of self-sensing and self-repairing composite materials. © XXXX American Chemical Society

Scheme 1. Illustration of Mechanophore Activation at SiO2NPs−PMA Interfacea

a

Note: polymer brushes are drawn with lighter color indicating farther from paper plane. The sizes of polymer and SiO2NPs are chosen for visualization and do not convey actual dynamics.

Received: May 18, 2016 Accepted: June 22, 2016

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DOI: 10.1021/acsmacrolett.6b00389 ACS Macro Lett. 2016, 5, 819−822

Letter

ACS Macro Letters Diels−Alder reaction.22,23 The mechanophore-anchored polymer grafted silica nanoparticles (SiO2NPs-MA-PMA) were prepared by initiator immobilization and subsequent surfaceinitiated polymerization (Scheme 1). The mechanochemically selective activation of MA mechanophore was triggered by ultrasound and characterized using gel permeation chromatography (GPC) with a UV detector set at 254 nm to monitor the generation of anthracene-containing PMAs. In mechanophore-centered homopolymers, attachment of polymer chains is a prerequisite to transduce mechanical energy to the mechanophore.24,25 We recently reported the distinction between molecular weight (MW) and degree of polymerization (DP) on their effect on mechanophore activation.15 The length, rather than the absolute MW, of the attached polymer was elucidated to be the fundamental property underlying the kinetics of mechanical transduction in mechanophore-centered homopolymers. Given the complexity of the mechanophore anchored interface, we wonder if parameters related to the properties of the interface and how polymer is grafted may have an effect on activation. Grafting density, depicting the surface crowdedness of a polymer-grafted interface, is a well-known crucial parameter in composite materials design.26−28 Based on the MA-mechanophore model system, SiO2NPs-MA-PMA series with three different grafting densities (0.27 chain/nm2 (GD027), 0.18 chain/nm2 (GD-018), and 0.05 chain/nm2 (GD005)) were prepared. The MWs of PMA grafted on the SiO 2 NPs with the corresponding grafting density are summarized in Table 1 (see the Supporting Information for details).

Figure 1. GPC trace of SiO2NPs-MA-PMA-40k of 0.27 chain/nm2 (red), 0.18 chain/nm2 (blue), and 0.05 chain/nm2 (green); 2 mg/mL in THF after 120 min ultrasonication.

as a result a molecular weight (degree of polymerization) dependence of mechanophore activation is observed.29,30 To be more specific, threshold molecular weight is typically observed in mechanophore-anchored polymer grafted nanoparticles, and the activation rate of mechanophore increases linearly along with MW increase. Similarly, by subjecting aliquots to GPC analysis with increasing sonication time (Figure 2), the amount

Table 1. Summary of Grafting Density and MW of Grafted PMA; Polydispersity of the Polymers Are below 1.2 GD-027

GD-018

GD-005

MW (kDa)

grafting density (chain/nm2)

MW (kDa)

grafting density (chain/nm2)

MW (kDa)

grafting density (chain/nm2)

59.8 51.2 40.0 22.0 12.3

0.268 0.272 0.267 0.269 0.274

49.0 41.2 27.2 17.1 9.1

0.181 0.179 0.182 0.182 0.178

60.0 51.2 38.7 29.6 10.2

0.0495 0.0490 0.0509 0.0511 0.0523

Figure 2. GPC trace of GD-018 SiO2NPs-MA-PMA-50k, 2 mg/mL in THF at 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, and 120 min ultrasonication.

Differences in activation rate are apparent from an inspection of the primary sonication data for different grafting density samples having similar degrees of polymerization. For example, shown in Figure 1 is the comparison between cleaved anthracene-containing PMAs of SiO2NPs-MA-PMA-40k series of three grafting densities after 120 min sonication. The integrated areas of cleaved anthracene-containing PMAs are normalized to the initial amount of grafted PMAs and, thus, as they are shown, directly compare the amount of activated, surface-bound MA mechanophores for different grafting densities. The GD-005 SiO2NPs-MA-PMA 40k exhibited the most intense cleaved anthracene-containing PMA (i.e., highest mechanochemically induced activation) upon sonication as the GD-027 SiO2NPs-MA-PMA 40k displayed the lowest activation rate. The intriguing result strongly suggests that lower grafting density enhances the activation efficiency of the MA mechanophore. In mechanophore-centered homopolymers, longer chains transduce greater mechanical force to the mechanophores and

of the PMA cleaved was quantified to give the first order kinetic coefficient for the retro cycloaddition (see Supporting Information for kinetic coefficient calculations). These kinetic coefficients were plotted against MW of the grafted PMA as shown in Figure 3. Among the similar MW samples, the ones with lower grafting density activated faster (GD-005 > GD-018 > PMA-MA-PMA > GD-027) consistent with the trend that was noted above. Remarkably, decreasing grafting density by 37% (from 0.27 to 0.18 chain/nm2) accelerated the mechanochemically activated retro cycloaddition to an equivalent extent to MW increase of 10 kDa. The underlying relationship between activation of mechanophore and polymer grafting density is therefore of great interest. Moreover, the x-axis intercept in the plot of activation rate coefficient against MW (i.e., the threshold MW of activation) 820

DOI: 10.1021/acsmacrolett.6b00389 ACS Macro Lett. 2016, 5, 819−822

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ACS Macro Letters

cally activation. On the other hand, in a highly stretched regime, further increasing grafting density leads to stronger interaction between neighboring chains including interchain entanglements, reducing the concentration of force on any given chain. As most of the grafted polymers are above the critical entanglement MW of PMA (11 kDa), 35 the entanglement between neighboring chains dominates the competition and this collective or shared load-bearing characteristic at high grafting density overall hinders cleavage of polymer chain.36 To conclude, polymer grafting density is intriguingly a determining factor of mechanophore activation at heterointerfaces. The model anthracene-maleimide mechanophore was employed at the interface of SiO2NPs and PMA with a series of grafting densities and activated in sonication-generated elongational flows. Comparison of the normalized amount of the mechanochemically cleaved anthracene-containing PMA in samples with different grafting densities clearly indicates that lower grafting density facilitates the mechanophore activation. The discrepancies in activation rates were further demonstrated using a systematic variation of PMA MW and compared with that of mechanophore-centered PMAs. The different activation behaviors were attributed to the stronger interchain interactions in higher grafting density samples. Simulations of molecular dynamics of tethered polymer chains may be helpful to further elucidate the mechanism of the grafting density effect and thus better guide the design of mechanical-sensitive and selfrepairing composite materials with a precise control of activation characteristics.

Figure 3. Molecular weight dependence of first order kinetic coefficient for reactions conducted at 5 °C in GD-027 (black square, slope = 2.38 × 10−5, r2 = 0.96), GD-018 (blue hexagonal, slope = 6.93 × 10−5, r2 = 0.99), GD-005 (green diamond, slope = 1.00 × 10−4, r2 = 0.99), SiO2NPs-MA-PMA series and PMA-MA-PMA series (red circle, slope = 4.51 × 10−5, r2 = 0.99). The error bars were obtained with three parallel experiments.

was also found to depend on grafting density. In general, the data suggest that higher grafting density samples exhibit a higher threshold MW. We conclude from the data in Figure 3 that at heterointerfaces, grafting density is a significant factor in polymer mechanochemistry. As shown in Figure 4, a majority of samples prepared in this work fall in highly stretched regime (see Supporting

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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.6b00389. (1) Synthetic routes and characterizations of SiO2NPsMA-PMA; (2) Quantification methods and eqs; (3) Other miscellaneous information (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The author would like to acknowledge U.S. Office of Naval Research Award N0004-13-1-0170 for financial support. The authors would like to thank Professor Anna Balazs, Professor Kenneth S. Suslick, and Dr. Maxwell Robb for useful discussion. The authors would like to thank Professor Jonathan V. Sweedler lab for courtesy usage of ultracentrifuges. B.Z. thanks NSF for the support (DMR-1206385).

Figure 4. Schematic illustration of polymer nanoenvironment (red triangle) in grafted SiO2NPs with different grafting densities.

Information),31 where grafted polymers are stretched compared to their random coil conformation and form polymer brushes at the interfaces. Several groups reported mechanochemical activity of polymer brushes in solution and at interfaces.32,33 On one hand, formation of polymer brushes generates tension along polymer backbone.34 Higher grafting densities lead to higher internal tension and thus facilitates the mechanochemi-

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DOI: 10.1021/acsmacrolett.6b00389 ACS Macro Lett. 2016, 5, 819−822