Molecular dynamics simulations of oligoester brushes: the origin of unusual conformations

Abstract

We present results from all-atom molecular dynamics simulations for the structural properties of oligomeric lactic acid chains (OLA) grafted to the surface of cellulose nanocrystals (CNCs) and immersed in the melt of polylactic acid (PLA). Earlier, we have found that the distribution of free ends of OLA molecules is bimodal [Glova et al., Polym. Int., 2016, 65(8), 892]. The results cannot be explained within the standard picture of uncharged polymer brushes exposed to the melt of a chemically identical polymer. Although the oligomeric brushes of the OLA chains are uncharged, they have partial polarization charges producing a non-zero dipole moment of the monomeric chain unit. We study the influence of partial charges on the structure of the layer of OLA chains grafted to the CNC surface. A detailed analysis of the conformations of the grafted chains shows that interaction of partial charges in the models causes bending of the OLA molecules toward the cellulose surface, forming a hairpin structure. The observed separation of the grafted chains into two populations increases with grafting density. We demonstrate that hydrogen bonds can be formed between the free ends of the grafted chains and the CNC surface, but they do not affect the brush structure significantly. Thus, dipole-dipole interactions turn out to be the key factor governing the unusual conformations of grafts.

Grafting-Induced Structural Ordering of Lactide Chains

“…Both the backfolding and the structural ordering may be viewed as the initial stage of the crystallization of the layer of grafted lactide chains. In contrast to the known behavior for conventional polymer brushes in the "dry brush" regime, the structure of the grafted lactide chains can be either amorphous or ordered, depending on the graft length N and the grafting density σ when their product Nσ is fixed.of the grafts into populations of backfolded and stretched chains was most pronounced at this σ in 96 comparison with the other grafting densities [19,20]. Having fixed σ, the length of the grafted chains 97 N was varied.…”

“…Despite their dipolar nature, the lactide chains, being aliphatic ester chains, may initially seem to be the conventional grafts that were considered earlier [10][11][12][15][16][17]. A separation of the grafted lactide chains into two populations was observed in the melt of the chemically similar polymer, although only a strong stretching of the grafts was expected [18,19]. Importantly, the segregation was found to be particularly pronounced in the "dry brush" regime [19].…”

“…of the grafts into populations of backfolded and stretched chains was most pronounced at this σ in 96 comparison with the other grafting densities [19,20]. Having fixed σ, the length of the grafted chains 97 N was varied.…”

“…In our previous studies, the unusual behavior of the grafted lactide chains was observed by using atomistic molecular dynamics (MD) simulation when attempting to reproduce the known influence of the grafting density on the grafted layer structures [18,19]. Despite their dipolar nature, the lactide chains, being aliphatic ester chains, may initially seem to be the conventional grafts that were considered earlier [10][11][12][15][16][17].…”

“…A separation of the grafted lactide chains into two populations was observed in the melt of the chemically similar polymer, although only a strong stretching of the grafts was expected [18,19]. Importantly, the segregation was found to be particularly pronounced in the "dry brush" regime [19]. Only a fraction of the grafts was stretched from the surface, and, in contrast to the previous results, the other grafts were backfolded to the surface and adopted a loop-like conformation [10,11,16].…”