Thickness dependent self assembly in ultra-thin polymer films: Formation of self-aligned nano-strips and nano-fibres

Abstract

Ultra thin short-chain polymer films near melt under strong confinement have been investigated, using NVT-Monte Carlo simulation method. Bottom surface of the simulation box, has affinity to monomers. The upper surface has hard wall interaction with monomers. The pattern formations in polymer films with thickness of the order of few tens of nanometers i.e. <100 nm, are generally explained on the basis of spinodal mechanism. We report spinodal like tearing mechanism of linear chain ultra-thin polymer films with intra-chain monomers connected via finite-extensible non-linear elastic (FENE) potential. Morse potential is taken for inter-chain bead-bead interactions. It is shown that the influence of Morse potential can propagated via FENE potential and vice-versa. The free end segments of the chains are observed to be more dynamic. Free end monomers show coalescence, initiating tearing of the films. So, one may scientifically speculate that the long-range van-der Waal type force no longer seems to be the lonely cause of rupturing of thin nano-films. Rather than interfacial effects and the more dynamic free end chain configurations of short chains interplay with the earlier one significantly. Formation of self-aligned quasi-equilibrium nano- structures as; strip-like and fibrous ones is reported for different thicknesses and interaction parameters of the films. Polymer chains with 30 monomers are taken with three different film thicknesses. Total 160, 200 and 240 chains are taken for three different cases of studies. The pattern formations appear to be of replicating nature. It makes this work noteworthy, especially for bio-physicists and chemists, working with macromolecules. Rupturing of film is also reported on chemically patterned substrates with variations in surface affinity to monomers in order to reveal the delicate interplay of molecular forces and interfacial effects, in case of short chain free end polymer films under strong confinement.