Conformational Switching of Molecular Brushes in Response to the Energy of Interaction with the Substrate

Sun, Frank C.; Sheiko, Sergei S.; Möller, Martin; Beers, Kathryn L.; Matyjaszewski, Krzysztof

doi:10.1021/jp047929g

Cited by 60 publications

(64 citation statements)

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“…Such forces have been generated using extensional flow [7][8][9] , ultrasonic irradiation 10 , receding meniscus 11 and by directly stretching a single molecule with nanoprobes [12][13][14][15][16] . Here we show that simple adsorption of brush-like macromolecules with long side chains on a substrate can induce not only conformational deformations 17 , but also spontaneous rupture of covalent bonds in the macromolecular backbone. We attribute this behaviour to the fact that the attractive interaction between the side chains and the substrate is maximized by the spreading of the side chains, which in turn induces tension along the polymer backbone.…”

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confidence: 85%

“…2). Here, we consider only the dominant term in S: that is, the difference between the surface free energies of substrate-gas, liquid-gas, and substrate-liquid-gas interfaces ðS ¼ g s 2 g l 2 g sl Þ: Previous measurements for the substrates that were used in this study found S ø 20 mN m 21 on graphite 19 and water/alcohol mixtures 17 . Therefore, a brush macromolecule with short side chains (n ¼ 12) and a width of d ¼ 11 nm (ref.…”

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confidence: 99%

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Adsorption-induced scission of carbon–carbon bonds

Sheiko

Sun

Randall

et al. 2006

Nature

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346

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Covalent carbon-carbon bonds are hard to break. Their strength is evident in the hardness of diamonds 1,2 and tensile strength of polymeric fibres [3][4][5][6] ; on the single-molecule level, it manifests itself in the need for forces of several nanonewtons to extend and mechanically rupture one bond. Such forces have been generated using extensional flow [7][8][9] , ultrasonic irradiation 10 , receding meniscus 11 and by directly stretching a single molecule with nanoprobes [12][13][14][15][16] . Here we show that simple adsorption of brush-like macromolecules with long side chains on a substrate can induce not only conformational deformations 17 , but also spontaneous rupture of covalent bonds in the macromolecular backbone. We attribute this behaviour to the fact that the attractive interaction between the side chains and the substrate is maximized by the spreading of the side chains, which in turn induces tension along the polymer backbone. Provided the side-chain densities and substrate interaction are sufficiently high, the tension generated will be strong enough to rupture covalent carbon-carbon bonds. We expect similar adsorption-induced backbone scission to occur for all macromolecules with highly branched architectures, such as brushes and dendrimers. This behaviour needs to be considered when designing surface-targeted macromolecules of this typeeither to avoid undesired degradation, or to ensure rupture at predetermined macromolecular sites.A series of brush-like macromolecules with the same number average degree of polymerization of a poly(2-hydroxyethyl methacrylate) backbone, N n ¼ 2,150^100, and different degrees of polymerization of poly(n-butyl acrylate) (pBA) side chains ranging from n ¼ 12^1 to n ¼ 140^12 were synthesized by atom transfer radical polymerization (see 'Polymer Characterization' in the Methods) 18 . Owing to the high grafting density, the side chains repel each other and thereby stretch the backbone into an extended conformation. Placing these macromolecules on a surface enhances the steric repulsion between the side chains, which results in both an extension of the polymer backbone and an increase of the persistence length.The effect is illustrated in Fig. 1, which shows atomic force microscopy (AFM) micrographs of monolayers of pBA brushes with short ( Fig. 1a) and long side chains (Fig. 1b). Measurements on both types of molecules yielded a number average contour length per monomeric unit of the backbone of l ¼ L n =N n ¼ 0:23^0:02 nm (see 'Atomic Force Microscopy' in Methods), which is close to l 0 ¼ 0.25 nm, the length of the tetrahedral C-C-C section. This means that even for short side chains (n ¼ 12), the backbone is already fully extended and adopts an all-trans conformation. As the side chains become longer, we observe global straightening of the backbone reflected in the increase of the persistence length (Fig. 1c).Chain extension requires a substantial amount of force, which we estimate using simple spreading arguments (Fig. 2). Just as in normal liquids, the polymer...

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Adsorption-induced scission of carbon–carbon bonds

Sheiko

Sun

Randall

et al. 2006

Nature

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346

350

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“…Indeed, the prediction of the theory was confirmed in the next paper. [259] The topic of this paper was the behaviour of the same comb-like macromolecules in Langmuir monolayers at the surface of liquid subphase with compositions being varied. As methanol was added into the aqueous subphase, the surface energy was thus reduces and consequently the spreading coefficient of the macromolecules was reduced also.…”

Section: Vapour-induced Conformational Transitions Of Comb-like Macromentioning

confidence: 99%

“…[258] Consequently, the conformational transitions into a globular state were observed by SFM. [259] In the above mentioned works the SFM-study of conformational transitions of comb-like macromolecules were performed following ex situ scheme, i.e., after transitions of monolayers onto a hard substrate. In our works we aimed at direct study of similar processes with analogous macromolecules but in real time regime using in situ scheme.…”

Section: Vapour-induced Conformational Transitions Of Comb-like Macromentioning

confidence: 99%

Scanning Force Microscopy as Applied to Conformational Studies in Macromolecular Research

Gallyamov

2011

Macromol. Rapid Commun.

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The modern state of SFM research on polymer nano-objects including single chains is discussed in comparison with other similar high-resolution microscopy techniques. The range of problems to be solved preferentially by SFM is highlighted. Promising methodology to describe quantitatively the morphology of macromolecular objects is proposed. The main benefits of this algorithm seem to be the apparent mathematical correctness as well as the possibility to estimate errors and the confidence of the numbers obtained. Special attention is paid to the dynamic observations of conformational transitions on a substrate in real time regime. This approach allows one to realise direct control of the adsorbed macromolecules by means of exposure to different vapours. Driving forces of the vapour-induced reorganisation are discussed.

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“…The characteristic property of these macromolecules is their ability to change shape upon lateral compression on a substrate. [16][17][18] If the film pressure increases, the number of side chains adsorbed to the surface decreases allowing the backbone to coil. This causes the macromolecules to become more compact and occupy less area on the substrate.…”

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confidence: 99%

Molecular Pressure Sensors

Sun

Shirvanyants

et al. 2007

Advanced Materials

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Flow properties of molecularly thin films are at the foundation of many practical applications such as lithography, microfluidics, coatings, and lubrication. [1][2][3][4][5][6][7][8][9][10] Further advances in these fields depend on understanding the mechanisms that control the kinetics of flow. [11][12][13][14] However, one of the problems in flowing monolayers is the independent characterization of the driving and frictional forces that are intimately coupled through the molecular interactions between the fluid and the substrate. In this regard, the visualization of compressible macromolecules during flow provides an exceptional opportunity to study these forces.[15] Here we report on the monitoring of brush-like macromolecules as they change their shape in response to variations in the film pressure during flow. After appropriate calibration, these molecular sensors can be used to gauge both the pressure gradient and the friction coefficient at the substrate. We anticipate the utilization of such miniature sensors for probing flow properties on nanometer length scales. The design of the pressure-responsive macromolecules is based on brush-like polymer architectures comprised of a flexible backbone surrounded by a dense shell of side chains (Fig. 1a). The characteristic property of these macromolecules is their ability to change shape upon lateral compression on a substrate. [16][17][18] If the film pressure increases, the number of side chains adsorbed to the surface decreases allowing the backbone to coil. This causes the macromolecules to become more compact and occupy less area on the substrate. Therefore, the area per molecule can be used as a pressure sensitive parameter to gauge the variations of film pressure within flowing monolayers. Molecular brushes (Fig. 1a) with the same degree of polymerization of a poly(2-hydroxyethyl methacrylate) backbone (n = 570 ± 50) and different degrees of polymerization of poly(n-butyl acrylate) (pBA) side chains (n = 35 ± 5 and n = 51 ± 5) were synthesized by atom transfer radical polymerization. [19,20] At room temperature, the materials are fluid melts that spontaneously spread when placed on higher surface energy substrates such as mica and graphite.[21] Small drops of molecular brushes (volume ∼ 1 nl, radius ∼ 100 lm) were deposited on a substrate inside an environmental chamber under controlled temperature (T = 25°C) and relative humidity (RH = 30-99 %). Like many other fluids, the drops first spread by generating a molecularly thin precursor film moving ahead of the macroscopic drop (Fig. 1b). [22] Using an atomic force microscope (Multimode, Nanoscope 3A, Veeco Metrology Group), we monitored the spreading process over a broad range of length scales ranging from the motion of the film front all the way down to the movements of the individual molecules within the film.[15,23] Figure 1c shows the time dependence of the film length L= R -R 0 observed on mica at a high relative humidity (RH = 99 %), where R 0 = 63 lm is the initial drop radius and R is the total radiu...

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Conformational Switching of Molecular Brushes in Response to the Energy of Interaction with the Substrate

Cited by 60 publications

References 20 publications

Adsorption-induced scission of carbon–carbon bonds

Adsorption-induced scission of carbon–carbon bonds

Scanning Force Microscopy as Applied to Conformational Studies in Macromolecular Research

Molecular Pressure Sensors

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