Monte Carlo simulations of copolymers at homopolymer interfaces: Interfacial structure as a function of the copolymer density

By means of extensive Monte Carlo simulations of the bond fluctuation model, we study the effect of adding AB diblock copolymers on the properties of an interface between demixed homopolymer phases. The parameters are chosen such that the homopolymers are strongly segregated, and the whole range of copolymer concentrations in the two phase coexistence region is scanned. We compare the ``mushroom'' regime, in which copolymers are diluted and do not interact with each other, with the ``wet brush'' regime, where copolymers overlap and stretch, but are still swollen by the homopolymers. A ``dry brush'' regime is never entered for our choice of chain lengths. ``Intrinsic'' profiles are calculated using a block analysis method introduced by us in earlier work. We discuss density profiles, orientational profiles and contact number profiles. In general, the features of the profiles are similar at all copolymer concentrations, however, the profiles in the concentrated regime are much broader than in the dilute regime. The results compare well with self-consistent field calculations.Comment: to appear in J. Chem. Phy

Effect of long range forces on the interfacial profiles in thin binary polymer films

We study the effect of surface fields on the interfacial properties of a binary polymer melt confined between two parallel walls. Each wall attracts a different component of the blend by a non-retarded van der Waals potential. An interface which runs parallel to the surfaces is stabilized in the center of the film. Using extensive Monte Carlo simulations we study the interfacial properties as a function of the film thickness, the strength of the surface forces and the lateral size over which the profiles across the film are averaged. We find evidence for capillary wave broadening of the apparent interfacial profiles. However, the apparent interfacial width cannot be described quantitatively by a simple logarithmic dependence on the film thickness. The Monte Carlo simulations reveal that the surface fields give rise to an additional reduction of the intrinsic interfacial width and an increase of the effective interfacial tension upon decreasing the film thickness. These modifications of the intrinsic interfacial properties are confirmed by self-consistent field calculations. Taking account of the thickness dependence of the intrinsic interfacial properties and the capillary wave broadening, we can describe our simulation results quantitatively.Comment: to appear in J.Chem.Phy

Reactions at polymer interfaces: A Monte Carlo Simulation

Reactions at a strongly segregated interface of a symmetric binary polymer blend are investigated via Monte Carlo simulations. End functionalized homopolymers of different species interact at the interface instantaneously and irreversibly to form diblock copolymers. The simulations, in the framework of the bond fluctuation model, determine the time dependence of the copolymer production in the initial and intermediate time regime for small reactant concentration $\rho_0 R_g^3=0.163 ... 0.0406$. The results are compared to recent theories and simulation data of a simple reaction diffusion model. For the reactant concentration accessible in the simulation, no linear growth of the copolymer density is found in the initial regime, and a $\sqrt{t}$-law is observed in the intermediate stage.Comment: to appear in Macromolecule

Molecularly imprinted conductive polymers for controlled trafficking of neurotransmitters at solid–liquid interfaces

We realize a molecularly imprinted polymer (MIP) which is imprinted with the retinal neurotransmitter glutamate. The films prepared by electrochemical deposition have a smooth surface with a granular morphology as observed using an atomic force microscope. Multiple reflection attenuated total reflection infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to chemically confirm the imprint of a neurotransmitter in the MIP at the solid–liquid and the solid–air interface, respectively. Fluorescence spectroscopy using the dye fluorescamine is used to monitor the changes in neurotransmitter concentration in various solvents induced by application of voltage to the MIP. By controlling neurotransmitter trafficking across a solid–liquid interface with voltage, we suggest the possibility of using such a neurotransmitter imprinted MIP for chemical stimulation of retinal neurons. The current state of the art approach to restore sight in certain cases of blindness is the replacement of damaged photoreceptors by a subretinal implant consisting of light-sensitive photodiodes. Thus a future perspective of our work would be to chemically stimulate the neurons by replacing the photodiodes in the subretinal implant by the neurotransmitter imprinted polymer film

Intrinsic profiles and capillary waves at homopolymer interfaces: a Monte Carlo study

A popular concept which describes the structure of polymer interfaces by ``intrinsic profiles'' centered around a two dimensional surface, the ``local interface position'', is tested by extensive Monte Carlo simulations of interfaces between demixed homopolymer phases in symmetric binary (AB) homopolymer blends, using the bond fluctuation model. The simulations are done in an LxLxD geometry. The interface is forced to run parallel to the LxL planes by imposing periodic boundary conditions in these directions and fixed boundary conditions in the D direction, with one side favoring A and the other side favoring B. Intrinsic profiles are calculated as a function of the ``coarse graining length'' B by splitting the system into columns of size BxBxD and averaging in each column over profiles relative to the local interface position. The results are compared to predictions of the self-consistent field theory. It is shown that the coarse graining length can be chosen such that the interfacial width matches that of the self-consistent field profiles, and that for this choice of B the ``intrinsic'' profiles compare well with the theoretical predictions.Comment: to appear in Phys. Rev.

Spinodal Decomposition in a Binary Polymer Mixture: Dynamic Self Consistent Field Theory and Monte Carlo Simulations

We investigate how the dynamics of a single chain influences the kinetics of early stage phase separation in a symmetric binary polymer mixture. We consider quenches from the disordered phase into the region of spinodal instability. On a mean field level we approach this problem with two methods: a dynamical extension of the self consistent field theory for Gaussian chains, with the density variables evolving in time, and the method of the external potential dynamics where the effective external fields are propagated in time. Different wave vector dependencies of the kinetic coefficient are taken into account. These early stages of spinodal decomposition are also studied through Monte Carlo simulations employing the bond fluctuation model that maps the chains -- in our case with 64 effective segments -- on a coarse grained lattice. The results obtained through self consistent field calculations and Monte Carlo simulations can be compared because the time, length, and temperature scales are mapped onto each other through the diffusion constant, the chain extension, and the energy of mixing. The quantitative comparison of the relaxation rate of the global structure factor shows that a kinetic coefficient according to the Rouse model gives a much better agreement than a local, i.e. wave vector independent, kinetic factor. Including fluctuations in the self consistent field calculations leads to a shorter time span of spinodal behaviour and a reduction of the relaxation rate for smaller wave vectors and prevents the relaxation rate from becoming negative for larger values of the wave vector. This is also in agreement with the simulation results.Comment: Phys.Rev.E in prin