Molecular bottle brushes in thin films: An off-lattice Monte Carlo study

The effect of intramolecular excluded volume interactions on the conformati ons of isolated comb-shaped copolymers-"molecular bottle brushes''-in ather mal solvents confined between two hard impenetrable parallel plates is stud ied by off-lattice Monte Carlo simulations. Equilibrium conformational prop erties of molecules consisting of a freely jointed backbone of 100 hard sph eres (beads), where 50 equally flexible side chains of lengths M up to 30 b eads are connected, are determined as a function of the width of the slit, including the pure two-dimensional (2D) case. Besides general properties su ch as the size and the shape of the chains and the orientation of the chain s with respect to the confining plates, the persistence length of the bottl e-brush backbone lambda is considered in particular. It is demonstrated tha t due to the side chains, the backbone is, in fact, effectively confined be tween two soft plates, i.e., the side chains act like softening springs bet ween the backbone and plates. The most striking result is that, unlike the three-dimensional (3D) situation, in the 2D case the ratio between lambda a nd the diameter D, lambda/D, of the bottle brush starts to increase as a fu nction of the side chain length for M greater than or equal to 10. For 3D, an increase of lambda/D (the essential parameter for lyotropic behavior) as a function of M is predicted by theory but has so far not been observed nu merically for the regime studied (i.e., M less than or equal to 30). These results suggest that due to the weaker excluded volume effect in 3D, a poss ible upturn of lambda/D may only be observed for longer side chains than st udied so far. (C) 1999 American Institute of Physics. [S0021-9606(99)70801- 9].