Tension amplification in molecular brushes in solutions and on substrates.
Tension amplification in molecular brushes in solutions and on substrates.
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DOI：
10.1021/jp807671b
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发表时间：
2009-03-26
期刊：
影响因子：
--
通讯作者：
Rubinstein M
中科院分区：
文献类型：
--
作者：
Panyukov S;Zhulina EB;Sheiko SS;Randall GC;Brock J;Rubinstein M
分子瓶刷是高度支化的大分子，侧链密集地接枝在长的聚合物主干上。刷状结构允许将侧链张力聚焦到主链上，并将其放大到皮牛顿到纳米牛顿范围。主链张力取决于整体分子构象和周围环境。在这里，我们研究了分子刷在溶液、熔体和衬底上的张力和构象之间的关系。在溶液中，我们发现侧链连接在每个主链单体上的致密刷子的主链张力在非热溶剂中为f0N3/8，在θ-溶剂中为f0N1/3，在贫溶剂和熔体中为f0，其中N是侧链的聚合度，f0≃kBt/b是侧链中的最大张力，b是库恩长度，kB是玻尔兹曼常数，T是绝对温度。根据侧链长度和溶剂质量的不同，分子刷在溶液中产生的张力约为10-100皮牛顿，足以打破氢键。当刷子吸附到衬底上时，张力就会显著放大。在具有很强吸引力的衬底上，刷子主链上的最大张力为~f0N，达到几个纳米牛顿的数量级，超过了典型的共价键的强度。在低接枝密度和高铺展参数下，吸附分子刷的截面轮廓近似为矩形，厚度较大，其中A为Hamaker常数，S为铺展参数。在很高的铺展参数(S和GT；A)下，分子刷的厚度在单层~b处饱和。当铺展参数很低时，吸附分子刷的截面轮廓呈三角形帐篷状。在这两种相反情况的交叉中，覆盖了很大范围的参数空间，吸附的分子刷由两层组成。由于与底物的直接相互作用，较低层中的侧链获得表面能量，而第二层在第一层的顶部扩散。标度理论预测这第二层具有宽度为R~n3/5、高度为h~n2/5的三角形截面。利用自洽场理论，我们计算了帽形y(X)=h(1−x2/R2)2，其中x是到主干的横向距离。预测的帽形与计算机模拟和实验结果吻合较好。
Molecular bottle-brushes are highly branched macromolecules with side chains densely grafted to a long polymer backbone. The brush-like architecture allows focusing of the side-chain tension to the backbone and its amplification from the picoNewton to nanoNewton range. The backbone tension depends on the overall molecular conformation and the surrounding environment. Here we study the relation between the tension and conformation of the molecular brushes in solutions, melts, and on substrates. In solutions, we find that the backbone tension in dense brushes with side chains attached to every backbone monomer is on the order of f0N3/8 in athermal solvents, f0N1/3 in θ-solvents, and f0 in poor solvents and melts, where N is the degree of polymerization of side chains, f0≃ kBT/b is the maximum tension in side chains, b is the Kuhn length, kB is Boltzmann constant, and T is absolute temperature. Depending on the side chain length and solvent quality, molecular brushes in solutions develop tension on the order of 10–100 picoNewtons, which is sufficient to break hydrogen bonds. Significant amplification of tension occurs upon adsorption of brushes onto a substrate. On a strongly attractive substrate, maximum tension in the brush backbone is ~ f0N, reaching values on the order of several nanoNewtons which exceed the strength of a typical covalent bond. At low grafting density and high spreading parameter the cross-sectional profile of adsorbed molecular brush is approximately rectangular with thicknes , where A is the Hamaker constant and S is the spreading parameter. At a very high spreading parameter (S > A), the brush thickness saturates at monolayer ~ b. At a low spreading parameter, the cross-sectional profile of adsorbed molecular brush has triangular tent-like shape. In the cross-over between these two opposite cases, covering a wide range of parameter space, the adsorbed molecular brush consists of two layers. Side chains in the lower layer gain surface energy due to the direct interaction with the substrate, while the second layer spreads on the top of the first layer. Scaling theory predicts that this second layer has a triangular cross-section with width R ~ N3/5 and height h ~ N2/5. Using self-consistent field theory we calculate the cap profile y (x) = h (1 − x2/R2)2, where x is the transverse distance from the backbone. The predicted cap shape is in excellent agreement with both computer simulation and experiment.
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