Controlling Interfacial Dynamics: Covalent Bonding versus Physical Adsorption in Polymer Nanocomposites

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• 作者：Adam P. Holt ; Vera Bocharova ; Shiwang Cheng ; Alexander M. Kisliuk ; B. Tyler White ; Tomonori Saito ; David Uhrig ; J. P. Mahalik ; Rajeev Kumar ; Adam E. Imel ; Thusitha Etampawala ; Halie Martin ; Nicole Sikes ; Bobby G. Sumpter ; Mark D. Dadmun ; Alexei P. Sokolov
• 刊名：ACS Nano
• 出版年：2016
• 出版时间：July 26, 2016
• 年：2016
• 卷：10
• 期：7
• 页码：6843-6852
• 全文大小：482K
• 年卷期：0
• ISSN：1936-086X

文摘

It is generally believed that the strength of the polymer–nanoparticle interaction controls the modification of near-interface segmental mobility in polymer nanocomposites (PNCs). However, little is known about the effect of covalent bonding on the segmental dynamics and glass transition of matrix-free polymer-grafted nanoparticles (PGNs), especially when compared to PNCs. In this article, we directly compare the static and dynamic properties of poly(2-vinylpyridine)/silica-based nanocomposites with polymer chains either physically adsorbed (PNCs) or covalently bonded (PGNs) to identical silica nanoparticles (R_NP = 12.5 nm) for three different molecular weight (MW) systems. Interestingly, when the MW of the matrix is as low as 6 kg/mol (R_NP/R_g = 5.4) or as high as 140 kg/mol (R_NP/R_g= 1.13), both small-angle X-ray scattering and broadband dielectric spectroscopy show similar static and dynamic properties for PNCs and PGNs. However, for the intermediate MW of 18 kg/mol (R_NP/R_g = 3.16), the difference between physical adsorption and covalent bonding can be clearly identified in the static and dynamic properties of the interfacial layer. We ascribe the differences in the interfacial properties of PNCs and PGNs to changes in chain stretching, as quantified by self-consistent field theory calculations. These results demonstrate that the dynamic suppression at the interface is affected by the chain stretching; that is, it depends on the anisotropy of the segmental conformations, more so than the strength of the interaction, which suggests that the interfacial dynamics can be effectively tuned by the degree of stretching—a parameter accessible from the MW or grafting density.