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.

Complex coacervation, a liquid-liquid phase separation that occurs when two oppositely charged polyelectrolytes are mixed in a solution, has the potential to be exploited for many emerging applications including wet adhesives and drug delivery vehicles. The ultra-low interfacial tension of coacervate systems against water is critical for such applications, and it would be advantageous if molecular models could be used to characterize how various system properties (e.g., salt concentration) affect the interfacial tension. In this article we use field-theoretic simulations to characterize the interfacial tension between a complex coacervate and its supernatant. After demonstrating that our model is free of ultraviolet divergences (calculated properties converge as the collocation grid is refined), we develop two methods for calculating the interfacial tension from field-theoretic simulations. One method relies on the mechanical interpretation of the interfacial tension as the interfacial pressure, and the second method estimates the change in free energy as the area between the two phases is changed. These are the first calculations of the interfacial tension from full fieldtheoretic simulation of which we are aware, and both the magnitude and scaling behaviors of our calculated interfacial tension agree with recent experiments. Complex coacervation, a liquid-liquid phase separation that occurs when two oppositely charged polyelectrolytes are mixed in a solution, has the potential to be exploited for many emerging applications including wet adhesives and drug delivery vehicles. The ultra-low interfacial tension of coacervate systems against water is critical for such applications, and it would be advantageous if molecular models could be used to characterize how various system properties (e.g., salt concentration) affect the interfacial tension. In this article we use field-theoretic simulations to characterize the interfacial tension between a complex coacervate and its supernatant. After demonstrating that our model is free of ultraviolet divergences (calculated properties converge as the collocation grid is refined), we develop two methods for calculating the interfacial tension from field-theoretic simulations. One method relies on the mechanical interpretation of the interfacial tension as the interfacial pressure, and the second method estimates the change in free energy as the area between the two phases is changed. These are the first calculations of the interfacial tension from full fieldtheoretic simulation of which we are aware, and both the magnitude and scaling behaviors of our calculated interfacial tension agree with recent experiments.

We present a detailed analysis of coarse-grained molecular dynamics simulations of semiflexible polymer melts in contact with a strongly adsorbing substrate. We have characterized the segments in the interfacial layer by counting the number of trains, loops, tails, and unadsorbed segments. For more rigid chains, a tail and an adsorbed segment (a train) dominate while loops are more prevalent in more flexible chains. The tails exhibit a nonuniformly stretched conformation akin to the "polydisperse pseudobrush" originally envisioned by Guiselin. To probe the dynamics of the segments, we computed the layer z-resolved collective intermediate dynamic structure factor, S(q,t,z), mean-square displacement of segments, and the second Legendre polynomial of the time autocorrelation of unit bond vectors, ⟨P 2 [n⃗ i (t,z)·n⃗ i (0,z)]⟩. Our results show that segmental dynamics is slower for stiffer chains, and there is a strong correlation between the structure and dynamics in the interfacial layer. There is no "glassy layer", and the slowing down in dynamics of stiffer chains in the adsorbed region can be attributed to the densification and a more persistent layering of segments. ■ INTRODUCTIONThe adsorption of polymers on surfaces is a fundamental problem in polymer physics 1,2 which has been extensively examined experimentally, 3−12 theoretically, 13−22 and through computer simulations. 23−35 Particularly of technological interest are systems involving polymer nanocomposites and polymer− electrolyte systems for organic electronic applications. The importance of a thorough understanding of polymer chain behavior in contact with a substrate cannot be overstated. It is known that in the presence of interfaces the structure and dynamics of polymers are influenced by the interface where dynamics differ from bulk and polymer conformations are perturbed within an interfacial layer. 7,8,36−47 There is now growing experimental evidence of the formation of the "reduced mobility interface" (RMI) layer in between unadsorbed chains in a matrix and the adsorbed layer where the dynamics is intermediate and distinguishable from those of the bulk and the adsorbed layer. 6,37,39,47,48 This leads in particular to the observed shift in the glass transition temperature, T g . 49−51 However, there are still considerable gaps in the understanding of the underlying physics and dependencies on polymer properties and its interaction with the substrate.In general, a fully flexible chain model is inadequate in describing experimental results 52 since the backbone of a real polymer chain is not completely flexible. A more accurate description of experimental results can be achieved when another length scale, representing the stiffness of the chain along the molecular backbone, is introduced. 24,52−54 This highlights the importance of chain stiffness in polymer− substrate contacts and interactions. In this article we attempt to describe the structure and dynamics of semiflexible polymer melts at the segment level. We believe that in this framework, co...

A family of four new DA polymers, in which the acceptor moiety benzobisthiadiazole was paired with four different donor moieties, has been synthesized. Surpri-singly, all members of the family exhibit balanced ambipolar behavior, despite polymer to polymer mobilities varying from 10(-4) cm(2) V(-1) s(-1) to 10(-1) cm(2) V(-1) s(-1). Applications in single component CMOS integrated circuits are envisioned.

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