Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

Abstract

Atomistically detailed models of free-standing thin films of random poly(ethylene-co-propylene) were produced using a method recently applied to atactic polystyrene (Clancy, et al. J. Phys. Chem. B 2001, 105, 11493). Monte Carlo simulations of the copolymer were carried out at ethylene fractions that cover the entire range of composition investigated in a recent experimental work (Opdahl, et al. J. Phys. Chem. B 2002, 106, 5212) based on sum-frequency generation (SFG) spectroscopy. We find that there is a minor enrichment of methyl groups on the surface, as compared to the bulk composition. Interestingly, the orientation distribution of methyl and methylene units is not unimodal. For methyl units, there is about 50% probability of orientation perpendicular to the surface. The other 50% are randomly oriented. For the methylene bisector, the simulation shows that the distribution has two broad peaks at opposite angles (at 180°apart) to the surface normal. More conclusive results may be obtained by using larger sample sizes and analysis of a larger number of snapshots, both of which increase the computation time significantly. The opposite angle pairs nearly cancel each other in the computation of an average order parameter and give us a misleading net result that the bisectors are randomly oriented. With increasing ethylene fraction, the orientation distribution of methyl and methylene units tends to become more uniform; however, this change is small. The strong average orientation of methyl groups and weak average orientation of methylene groups is in agreement with experiments. However, the simulation does not show the significant relative increase in the surface methyl groups contributing to the SFG signal as compared to the bulk concentration or an increase in average orientation of methylene units, with an increase in the ethylene fraction, as inferred from the experimental results. We find that the raw experimental data obtained with an increase in ethylene fraction can be explained by the changes in the number of methyl and methylene groups on the surface rather than the increase in surface excess of the methyl groups contributing to the SFG signal, trans cancellation, and increase in orientation of methylene bisectors as deduced from the experiments.

Some mechanisms for subtle influences of stereochemical composition on the physical properties of macromolecules

“…In the surface region, the simulations find a strong average orientation of methyl groups and a weak average orientation of methylene groups, in agreement with the SFG experiments 47. In contrast to the original interpretation of the experiments,47 the simulation finds no significant relative increase in the surface methyl groups contributing to the SFG signal or an increase in the average orientation of methylene units with increasing ethylene content 22. These features in the simulation could be shown to be consistent with the primary SFG data, although they are somewhat different from the previous structural conclusions derived from that data.…”

“…The simulation, therefore, not only reproduces the subtle effects of stereochemical composition on the mixing of PP melts but also provides a molecular mechanism for these effects and shows that the same mechanism is operative in other polymers. The simulation also reproduces local structural details at the surfaces of atactic PS21 and random copolymers of ethylene and propylene 22…”

“…Similar simulations were performed for the surfaces of random copolymers of ethylene and propylene in which the ethylene fractions spanned the range 0.00–0.75 22. The coarse‐grained chains had one bead for each ethylene unit and one bead for each propylene unit.…”

“…The successful capture of local structural effects by suitably designed coarse‐grained chains is also illustrated by studies of the surface structures of amorphous polymers, in which the results of the simulations are compared21, 22 with conclusions derived from infrared–visible sum frequency generation (SFG) spectroscopy 45–47. The symmetry requirements in the selection rules for the SFG technique cause the observed signal to be generated only by that portion of the sample in the extreme outer region of a free surface or very close to a hidden surface (interface) 48–50…”

Prediction of Structure and Energy of Trans-1,4-Polybutadiene Glassy Surface by Atomistic Simulations of Free-Standing Ultrathin Films

“…[16,17] The molecular modeling approach was later extended to that having a full atomistic description of real chains in continuum space using approaches such as the EM of the one-sided free-surface (for atactic polypropylene), [18] MD simulations of chain collapse into a thin film, [19] and the two-dimensional (2D) thin film nonbonded interactions methodology in molecular mechanics and MD simulations by removal of periodicity in one direction, [20][21][22][23][24] for chemically different nonpolar and slightly polar polymers and copolymers. More recently, high coordination lattice MC simulations have also found utility being implemented to successfully simulate structure and thermodynamics of amorphous polyethylene, [25] atactic polystyrene, [26] and atactic poly(ethylene-co-propylene) [27] surfaces. In the context of polybutadienes, the molecular simulation technique of creating a thin film Downloaded by [Temple University Libraries] at 03:00 21 November 2014 having 2D periodicity from a bulk phase of 3D periodicity was further extended to look at surface properties of amorphous glassy 1,2-polybutadiene, [28] poly(tetrafluoroethylene) melt, [29] and freely-standing (thin film) polyimide membrane.…”