Using a coarse-grained bead-spring model for semi-flexible macromolecules forming a polymer brush, structure and dynamics of the polymers is investigated, varying chain stiffness and grafting density. The anchoring condition for the grafted chains is
chosen such that their first bonds are oriented along the normal to the substrate plane. Compression of such a semi-flexible brush by a planar piston is observed to be a two-stage process: for small compressions the chains contract by buckling deformation whereas for larger compression the chains exhibit a collective (almost uniform) bending deformation. Thus, the stiff polymer brush undergoes a 2-nd order phase transition of collective bond reorientation. The pressure, required to keep the stiff brush at a given degree of compression, is thereby significantly smaller than for an otherwise identical brush made of entirely flexible polymer chains! While both the brush height and the chain linear dimension in the z-direction perpendicular to the substrate increase monotonically with increasing chain stiffness, lateral (xy) chain linear dimensions exhibit a maximum at intermediate chain stiffness. Increasing the grafting density leads to a strong decrease of these lateral dimensions, compatible with an exponential decay. Also the recovery kinetics after removal of the compressing piston is studied, and found to follow a power-law / exponential decay with time. A simple mean-field theoretical consideration, accounting for the buckling/bending behavior of semi-flexible polymer brushes under compression, is suggested.
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 substrat
e 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.
When a very thin capillary is inserted into a liquid, the liquid is sucked into it: this imbibition process is controlled by a balance of capillary and drag forces, which are hard to quantify experimentally, in particularly considering flow on the na
noscale. By computer experiments using a generic coarse-grained model, it is shown that an analysis of imbibition forced by a controllable external pressure quantifies relevant physical parameter such as the Laplace pressure, Darcys permeability, effective pore radius, effective viscosity, dynamic contact angle and slip length of the fluid flowing into the pore. In determining all these parameters independently, the consistency of our analysis of such forced imbibition processes is demonstrated.
