A.V. Subbotin

Comb copolymer cylindrical brushes containing semiflexible side chains : a Monte Carlo study

The off-lattice Monte Carlo method is applied to investigate the equilibrium conformations of isolated comb copolymer cylindrical brushes in an athermal solution. The molecules considered consist of a flexible backbone, which is densely grafted with semiflexible side chains. The study focuses on the influence of the degree of intrinsic stiffness, λside, of the side chains on the conformational behavior of the molecules. It is demonstrated that with a fixed side chain length, M, the local length scale conformational fluctuations of the backbone increase as a function of λside. However, the persistence length, λ, of the cylindrical brush increases considerably with the side chain stiffness, indicating that the backbone becomes more extended at the large length scale. Moreover, as a function of λside, there is an increase in the ratio λ/D of the persistence length and the diameter, D, of the brush. This behavior is in good agreement with recent theoretical predictions and provides important new insight in the latest experimental observations.

ADSORPTION-DESORPTION KINETICS IN NANOSCOPICALLY CONFINED OLIGOMER FILMS UNDER SHEAR

The method of molecular dynamics computer simulations is employed to study oligomer melts confined in ultra-thin films and subjected to shear. The focus is on the self-diffusion of oligomers near attractive surfaces and on their desorption, together with the effects of increasing energy of adsorption and shear. It is found that the mobility of the oligomers near an attractive surface is strongly decreased. Moreover, although shearing the system forces the chains to stretch parallel to the surfaces and thus increase the energy of adsorption per chain, flow also promotes desorption. The study of chain desorption kinetics reveals the molecular processes responsible for the enhancement of desorption under shear. They involve sequences of conformations starting with a desorbed tail and proceeding in a very fast, correlated, segment-by-segment manner to the desorption of the oligomers from the surfaces.

Lamellar-in-lamellar self-assembled C–b–(B–b–A)m–b–B–b–C multiblock copolymers: Alexander–de Gennes approach and dissipative particle dynamics simulations

A simple theoretical analysis of the lamellar-in-lamellar self-assembled state of ternary C–b–(B–b–A)m–b–B–b–C multiblock copolymer melts in the strong segregation limit is presented using the Alexander–de Gennes approximation. For a given value of m, the influence of the chain length of the various blocks and the value of the Flory–Huggins χAB and χBC interaction parameters on the number k of internal domains is discussed in detail. The theoretically predicted tendencies are corroborated by computer simulations using the dissipative particle dynamics technique.

Phase behavior and structure formation of hairy-rod supramolecules.

Abstract.Phase behavior and microstructure formation of rod and coil molecules, which can associate to form hairy-rod polymeric supramolecules, are addressed theoretically. Association induces considerable compatibility enhancement between the rod and coil molecules and various microscopically ordered structures can appear in the compatibility region. The equilibria between microphase-separated states, the coil-rich isotropic liquid and the rod-rich nematic are discussed in detail. In the regime where hairy-rod supramolecules with a high grafting density appear as a result of the association, three phase diagram types are possible depending on the value of the association energy. In the low grafting density regime only the lamellar microstructure is proven to be stable.

Parallel versus Perpendicular Lamellar-within-Lamellar Self-Assembly of A-b-(B-b-A)(n)-b-C Ternary Multiblock Copolymer Melts

Different types of lamellar-within-lamellar structure formations in A-b-(B-b-A)(n)-b-C terpolymer melts, with a volume fraction of components A, B, and C in the ratio of 1:1:2, are analyzed in the strong segregation limit using a simple theoretical approach. We consider the lamellar, parallel lamellar-within-lamellar, and perpendicular lamellar-within-lamellar self-assembled states. The influence of the copolymer chain length N, the value of the Flory-Huggins interaction parameters chi(AB), chi(AC), and chi(BC), and the number of blocks n in the AB multiblock chain on the phase behavior is discussed. We show that in the limiting case of n >> 1, the perpendicular lamellar-within-lamellar state becomes stable when the interaction parameters satisfy the relation 0 < chi(BC) < 0.22 chi(AC).

Nonlinear elasticity and friction of liquid-crystalline polymer monolayers

In the present paper we consider nonlinear elasticity and friction of grafted persistent chains, which are highly stretched in the normal to the surface direction due to orientational interactions. We examine the normal and the lateral forces both in equilibrium and under shear sliding when the monolayer is confined by a bare surface. We show that in the confined monolayer in equilibrium the tilted orientation of the director becomes stable. In the sliding regime the friction force passes through a maximum value. The additional normal force in the sliding regime, when the distance between the surfaces is fixed, is also considered. We show that this force is attractive for small velocities and becomes repulsive for high velocities after the friction force passes through the maximum value.