title: Controlling Interfacial Dynamics: Covalent Bonding versus Physical Adsorption in Polymer Nanocomposites.
abstract: 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 (RNP = 12.5 nm) for three different molecular weight (MW) systems. Interestingly, when the MW of the matrix is as low as 6 kg/mol (RNP/Rg = 5.4) or as high as 140 kg/mol (RNP/Rg= 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 (RNP/Rg = 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.
authors.forename: Adam P
authors.initials: AP
authors.lastname: Holt
authors.forename: Vera
authors.initials: V
authors.lastname: Bocharova
authors.forename: Shiwang
authors.initials: S
authors.lastname: Cheng
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Kisliuk
authors.forename: B Tyler
authors.initials: BT
authors.lastname: White
authors.forename: Tomonori
authors.initials: T
authors.lastname: Saito
authors.forename: David
authors.initials: D
authors.lastname: Uhrig
authors.forename: J P
authors.initials: JP
authors.lastname: Mahalik
authors.forename: Rajeev
authors.initials: R
authors.lastname: Kumar
authors.forename: Adam E
authors.initials: AE
authors.lastname: Imel
authors.forename: Thusitha
authors.initials: T
authors.lastname: Etampawala
authors.forename: Halie
authors.initials: H
authors.lastname: Martin
authors.affiliation: Department of Chemistry, Columbus State University , Columbus, Georgia 33232, United States.
authors.forename: Nicole
authors.initials: N
authors.lastname: Sikes
authors.forename: Bobby G
authors.initials: BG
authors.lastname: Sumpter
authors.forename: Mark D
authors.initials: MD
authors.lastname: Dadmun
authors.forename: Alexei P
authors.initials: AP
authors.lastname: Sokolov
keywords: glass transition; interfacial dynamics; polymer nanocomposites; polymer-grafted nanoparticles; segmental dynamics; self-consistent field theory