Polymer nano-doplets forming liquid bridges in chemically structured slit pores: A computer simulation

Using a coarse-grained bead-spring model of flexible polymer chains, the structure of a polymeric nanodroplet adsorbed on a chemically decorated flat wall is investigated by means of Molecular Dynamics simulation. We consider sessile drops on a lyophilic (attractive for the monomers) region of circular shape with radius R_D while the remaining part of the substrate is lyophobic. The variation of the droplet shape, including its contact angle, with R_D is studied, and the density profiles across these droplets also are obtained. In addition, the interaction of droplets adsorbed on two walls forming a slit pore with two lyophilic circular regions just opposite of one another is investigated, paying attention to the formation of a liquid bridge between both walls. A central result of our study is the measurement of the force between the two substrate walls at varying wall separation as well as the kinetics of droplet merging. Our results are compared to various phenomenological theories developed for liquid droplets of mesoscopic rather than nanoscopic size.Comment: 8 pages, 9 figures, accepted in J. Chem. Phys. 200

Non-Fickian Interdiffusion of Dynamically Asymmetric Species: A Molecular Dynamics Study

We use Molecular Dynamics combined with Dissipative Particle Dynamics to construct a model of a binary mixture where the two species differ only in their dynamic properties (friction coefficients). For an asymmetric mixture of slow and fast particles we study the interdiffusion process. The relaxation of the composition profile is investigated in terms of its Fourier coefficients. While for weak asymmetry we observe Fickian behavior, a strongly asymmetric system exhibits clear indications of anomalous diffusion, which occurs in a crossover region between the Cases I (Fickian) and II (sharp front moving with constant velocity), and is close to the Case II limit.Comment: to appear in J. Chem. Phy

The Electrostatic Persistence Length of Polymers beyond the OSF Limit

We use large scale Monte Carlo simulations to test scaling theories for the electrostatic persistence length $l_e$ of isolated, uniformly charged polymers with \DH intrachain interactions in the limit where the screening length $\kappa^{-1}$ exceeds the intrinsic persistence length of the chains. Our simulations cover a significantly larger part of the parameter space than previous studies. We observe no significant deviations from the prediction $l_e\propto\kappa^{-2}$ by Khokhlov and Khachaturian which is based on applying the Odijk-Skolnick-Fixman theory to the stretched de Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear or sublinear dependence of the persistence length on the screening length can be ruled out. We argue that previous numerical results pointing into this direction are probably due to a combination of excluded volume and finite chain length effects. The paper emphasizes the role of scaling arguments in the development of useful representations for experimental and simulation data.Comment: 11 pages, 7 figure

Dynamic Compression of in situ Grown Living Polymer Brush: Simulation and Experiment

A comparative dynamic Monte Carlo simulation study of polydisperse living polymer brushes, created by surface initiated living polymerization, and conventional polymer monodisperse brush, comprising linear polymer chains, grafted to a planar substrate under good solvent conditions, is presented. The living brush is created by end-monomer (de)polymerization reaction after placing an array of initiators on a grafting plane in contact with a solution of initially non-bonded segments (monomers). At equilibrium, the monomer density profile \phi(z) of the LPB is found to decline as \phi(z) ~ z^{-\alpha} with the distance from the grafting plane z, while the distribution of chain lengths in the brush scales as c(N) ~ N^{-\tau}. The measured values \alpha = 0.64 and \tau = 1.70 are very close to those, predicted within the framework of the Diffusion-Limited Aggregation theory, \alpha = 2/3 and \tau = 7/4. At varying mean degree of polymerization (from L = 28 to L = 170) and effective grafting density (from \sigma_g = 0.0625 to \sigma_g = 1.0), we observe a nearly perfect agreement in the force-distance behavior of the simulated LPB with own experimental data obtained from colloidal probe AFM analysis on PNIPAAm brush and with data obtained by Plunkett et. al., [Langmuir 2006, 22, 4259] from SFA measurements on same polymer