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Anti-Arrhenius Cleavage of Covalent Bonds in Bottlebrush Macromolecules on Substrate

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Anti-Arrhenius Cleavage of Covalent Bonds in Bottlebrush Macromolecules on Substrate Anti-Arrhenius cleavage of covalent bonds in bottlebrush macromolecules on substrate Natalia V. Lebedevaa, Alper Neseb, Frank C. Suna, Krzysztof Matyjaszewskib, and Sergei S. Sheikoa,1 aDepartment of Chemistry, CB 3290, University of North Carolina, Chapel Hill, NC 27599-3290; and bDepartment of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 Edited by Stephen L. Craig, Duke, Durham, NC, and accepted by the Editorial Board April 13, 2012 (received for review November 9, 2011) Spontaneous degradation of bottlebrush macromolecules on and temperature. The exponential increase of the rate constant aqueous substrates was monitored by atomic force microscopy. with bond tension (4–7) is vital for a wide range of applications Scission of C─C covalent bonds in the brush backbone occurred due ranging from lithography and self-healing materials to drug de- to steric repulsion between the adsorbed side chains, which gen- livery and mechanocatalysis (8–11). erated bond tension on the order of several nano-Newtons. Unlike In this paper, we report a third unique feature of the adsorp- conventional chemical reactions, the rate of bond scission was tion-induced mechanochemistry: an anti-Arrhenius decrease of shown to decrease with temperature. This apparent anti-Arrhenius the scission rate with temperature. Observations of anti- or non- behavior was caused by a decrease in the surface energy of the Arrhenius kinetics are rare. They are usually ascribed to an underlying substrate upon heating, which results in a correspond- entropic contribution to the free energy of activation (e.g., in bio- ing decrease of bond tension in the adsorbed macromolecules. molecular processes that involve chain folding) (12–16). It is Even though the tension dropped minimally from 2.16 to 1.89 nN, more typical to observe an apparent anti-Arrhenius behavior that this was sufficient to overpower the increase in the thermal energy is caused by a decrease of the equilibrium constant of a multistep k T ( B ) in the Arrhenius equation. The rate constant of the bond- reaction or changes in the surrounding environment (e.g., scission reaction was measured as a function of temperature decrease in viscosity) (17–19). In our case, the anti-Arrhenius and surface energy. Fitting the experimental data by a perturbed behavior results from a net reduction of the bond activation −βx V ¼ V ð − e Þ2 − fx CHEMISTRY Morse potential 0 1 , we determined the depth energy caused by the heating-induced decrease of the substrate V ¼ Æ ∕ β−1 ¼ and width of the potential to be 0 141 19 kJ mol and surface energy. The goal of this paper is to demonstrate that the Æ V 0.18 0.03 Å, respectively. Whereas the 0 value is in reasonable temperature-induced variation in surface energy can overpower E ¼ – ∕ k T agreement with the activation energy a 80 220 kJ mol of me- the thermal energy ( B ) and effectively slow down the bond chanical and thermal degradation of organic polymers, it is signif- scission. Independent control of the temperature and bond ten- icantly lower than the dissociation energy of a C─C bond De ¼ sion enabled quantitative analysis of the potential energy of a ∕ K ¼ β2V ¼ Æ 350 kJ mol. Moreover, the force constant x 2 0 1.45 covalent bond within the bottlebrush backbone. 0.36 kN∕m of a strained bottlebrush along its backbone is markedly larger than the force constant of a C─C bond Kl ¼ 0.44 kN∕m, Results and Discussion which is attributed to additional stiffness due to deformation of Our results are presented in two sections. First, we report the rate the side chains. constant of bond scission as a function of temperature and bond tension. Second, we describe fitting analysis of the measured data mechanochemistry ∣ molecular brushes ∣ molecular imaging points and evaluation of the shape of the bond potential. he phenomenon of molecular “fatal adsorption,” previously Experimental Data. Every bond rupture within a brush backbone Treported by us in 2006 (1), is a unique mechanochemical yields an extra bottlebrush molecule of a shorter length. As shown process attributed to spontaneous scission of covalent bonds in in Fig. 1, the scission processes are evidenced by two concurrent i brush-like macromolecules upon their adsorption onto a sub- observations: ( ) shortening of the molecular contour length and ii strate (Fig. 1). This molecular self-destruction is caused by ( ) the corresponding increase of the number of molecules. remarkably strong tension of the order of several nano-Newtons, Molecular imaging allows accurate measurements of both para- which is developed in the polymer backbone due to steric repul- meters (i.e., number-average contour length and the number of sion between densely grafted side chains. The crowdedness of severed bonds per unit area) as a function of time under con- the side chains and, hence, the backbone tension are strongly trolled temperature. Furthermore, molecular imaging allows sta- enhanced upon spreading of the side chains on a high-energy sub- tistical averaging for a large ensemble of molecules measured at strate. The backbone tension increases with the grafting density, the same time and under identical conditions (constant force). the length of the side chains, and the strength of their adhesion to These features distinguish our experimental setup from molecu- the substrate (2). lar force probes that typically create a series of consecutive single- – This unimolecular bond-scission process exhibits two distinct molecule extensions under a controlled loading rate (20 24). features. First, strong covalent bonds rupture spontaneously with- The inherently strained macromolecules can be viewed as “ ” out applying any external force. Significant bond tension is gen- molecular tensile machines that are able autonomously to gen- erated within adsorbed macromolecules as they opt to rearrange erate and control mechanical tension in their covalent bonds. In their conformations in order to maximize the number of contacts this paper, we use bottlebrush macromolecules to interrogate the between the side chains and the substrate (1,2). Second, the rate constant of the bond-scission reaction exhibits extraordinary Author contributions: N.V.L., A.N., and F.C.S. performed research; K.M. contributed new sensitivity to minute variations of the surface energy (γ) of the reagents/analytic tools; and N.V.L. and S.S.S. wrote the paper. underlying substrate (3). The rate constant can change by two The authors declare no conflict of interest. γ orders of magnitude with only a 3% change in . In other words, This article is a PNAS Direct Submission. S.L.C. is a guest editor invited by the the lifetime of a covalent bond may increase from 1 min to 1 h Editorial Board. upon an incremental decrease of the substrate surface energy by 1To whom correspondence should be addressed. E-mail: [email protected]. ca 1 ∕ . mN m, which routinely occurs in surface chemistry due to This article contains supporting information online at www.pnas.org/lookup/suppl/ incidental variations in chemical composition, vapor pressure, doi:10.1073/pnas.1118517109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1118517109 PNAS Early Edition ∣ 1of5 Downloaded by guest on October 1, 2021 Fig. 1. Significant tension of the order of several nano-Newtons is devel- oped in the polyacrylate backbone of molecular bottlebrushes due to steric repulsion between the densely grafted PBA side chains (A). This intramole- cular tension leads first to nearly full extension of the backbone adopting an all-trans conformation (B) and then scission of its C─C bonds (C, D). The AFM Fig. 3. The scission rate constant (ln k) decreases with temperature. The data micrographs (Bottom) document the random fracturing of the covalent back- points (•) were obtained from the fitting analysis of the kinetic traces in Fig. 2 bone as evidenced by systematic decrease of the brush contour length. with Eq. 1 using k as a single fitting parameter. The solid line is a linear fit 3 with ln k ∼ B1∕T [Eq. 10] and B1 ¼ð6.1 Æ 0.4Þ · 10 K. ─ C C bonds in the brush backbone with respect to the effect of 1∕L − 1∕L temperature on the scission rate. Fig. 2A shows molecular images ∞ ¼ e−kt; [1] 1∕L − 1∕L of the poly(n-butyl acrylate) (PBA) brushes captured at different 0 ∞ temperatures, after being adsorbed on the surface of pure water L t ¼ 0 L ¼ for 10 min. The molecules captured at lower temperatures are where 0 is the initial chain length at and ∞ 65 Æ 10 nm is the chain length at t ¼ ∞ (3). For each tempera- markedly shorter—i.e., they undergo faster bond scission—than ture dataset, the initial time (t ¼ 0) was assigned to the first col- molecules at higher temperatures. The decrease of the number- lected data point. The L∞ was measured in two ways: (i) the average contour length was measured as a function of time for ii B average length at long times (days) and ( ) the length of a short- different temperatures (Fig. 2 ). To ensure the standard devia- est molecule observed. Both approaches gave similar values with- tion of the mean is below 10%, at least 500 molecules were ana- in the range of 65 Æ 10 nm. Eq. 1 was used to fit the experimental B lyzed for every data point in Fig. 2 . data points with the rate constant k as a single fitting parameter Assuming that the observed bond scission is a first-order (Fig. 2B, Inset).The rate constants so obtained are plotted in Fig. 3 unimolecular reaction, the time dependence of the molecular as a function of reciprocal temperature (1∕T). The semi-log plot length can be written as: ln kð1∕TÞ clearly demonstrates the anti-Arrhenius behavior as the rate constant decreases with temperature.
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