Polydisperse Polymer Brushes: Internal Structure, Critical Behavior, and Interaction with Flow

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• 作者：Shuanhu Qi ; Leonid I. Klushin ; Alexander M. Skvortsov ; Friederike Schmid
• 刊名：Macromolecules
• 出版年：2016
• 出版时间：December 27, 2016
• 年：2016
• 卷：49
• 期：24
• 页码：9665-9683
• 全文大小：771K
• ISSN：1520-5835

文摘

We study the effect of polydispersity on the structure of polymer brushes by analytical theory, a numerical self-consistent field approach, and Monte Carlo simulations. The polydispersity is represented by the Schulz–Zimm chain-length distribution. We specifically focus on three different polydispersities representing sharp, moderate, and extremely wide chain length distributions and derive explicit analytical expressions for the chain end distributions in these brushes. The results are in very good agreement with numerical data obtained with self-consistent field calculations and Monte Carlo simulations. With increasing polydispersity, the brush density profile changes from convex to concave, and for given average chain length N_n and grafting density σ, the brush height H is found to scale as (H/H_mono – 1) ∝ (N_w/N_n – 1)^1/2 over a wide range of polydispersity indices N_w/N_n (here H_mono is the height of the corresponding monodisperse brush). Chain end fluctuations are found to be strongly suppressed already at very small polydispersity. On the basis of this observation, we introduce the concept of the brush as a near-critical system with two parameters (scaling variables), (N_nσ^2/3)⁻¹ and (N_w/N_n – 1)^1/2, controlling the distance from the critical point. This approach provides a good description of the simulation data. Finally, we study the hydrodynamic penetration length l_p for brush-coated surfaces in flow. We find that it generally increases with polydispersity. The scaling behavior crosses over from l_p ∼ N_n^1/2σ^–1/6 for monodisperse and weakly polydisperse brushes to l_p ∼ N_n^2/3 for strongly polydisperse brushes.