Collective chain packing of flexible linear amorphous polymers at polymer-solid interfaces

Linear homopolymers are the simplest and most common macromolecular chemical structures. The processing and end-use properties of these polymers are contingent on the spatial conformation and chain packing in the amorphous phase. It is noteworthy that amorphous flexible polymers with side groups, such as polystyrene, exhibit the so-called polymerization peak in addition to a common "amorphous halo" in X-ray scattering data. The polymerization peak is primarily attributed to the intermolecular backbone-backbone correlation distance. This indicates that the packing of backbone chain is loose, resulting from the presence of "voids" that are sparsely filled by bulky side groups. A fundamental question arises: what is the mechanism by which the collective chain packing of backbone chains and side groups occurs at the polymer-solid interface? To address this, we have chosen model flexible polymer chains, including isotactic polypropylene (iPP), atactic and isotactic polystyrene (aPS and iPS), and poly(2-vinylpyridine) P2VP, and prepared strongly adsorbed chains on planar silicon substrates using established solvent leaching protocols. The chain packing was investigated by using grazing incidence wide-angle X-ray scattering. Concurrently, atomistic molecular dynamics simulations were employed to complement the experimental results and to further elucidate the packing process. We will discuss the generality and roles of side groups during the packing process on impenetrable solid surfaces.