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...

Organic photovoltaics (OPVs) are a topic of extensive research because of their potential application in solar cells. Recent work has led to the development of a coarse-grained model for studying poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular simulations. Here we provide further validation of the force field and use it to study the thermal annealing process of P3HT:PCBM blends. A key finding of our study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported. Rather, we find that the self-assembly of the blends is characterized by three rate dependent stages that require much longer simulations to approach convergence. Using state-of-the-art high performance computing, we are able to study annealing at length and time scales commensurate with devices used in experiments. Our simulations show different phase segregated morphologies dependent on the P3HT chain length and PCBM volume fraction in the blend. For short chain lengths, we observed a smectic morphology containing alternate P3HT and PCBM domains. In contrast, a phase segregated morphology containing domains of P3HT and PCBM distributed randomly in space is found for longer chain lengths. Theoretical arguments justifying stabilization of these morphologies due to shape anisotropy of P3HT (rod-like) and PCBM (sphere-like) are presented. Furthermore, results on the structure factor, miscibility of P3HT and PCBM, domain spacing and kinetics of phase segregation in the blends are presented in detail. Qualitative comparison of these results with published small-angle neutron scattering experiments in the literature is presented and an excellent agreement is found.

This article reviews current experimental observations and theoretical calculations devoted towards understanding micro-phase separation in charged block copolymer systems. We discuss bulk morphologies in melt and in solution, as well as some of the new emerging research directions. Overall, a comprehensive picture is beginning to emerge on the fundamental role of electrostatics in the microphase separation of charged block copolymers. This understanding provides exciting new insight that may be used to direct targeted structures that endow the materials with desired properties that can have tremendous potential in technological applications.

Frustration in chain packing has been proposed to play an important role in thermodynamic and dynamic properties of polymeric melts and glasses. Based on a quantitative analysis using Voronoi tessellations and large scale molecular dynamics simulations of flexible and semi-flexible polymers, we demonstrate that the rigid polymer chains have higher averaged Voronoi polyhedral volumes and significantly wider distribution of the volume due to frustration in the chain packing. Using these results, we discuss the advantage of the rigid polymers for possible enhancement of transport properties, e.g. for enhancing ionic conductivity in solid polymer electrolytes.