Miscibility of polylactide (PLA) and polyhydroxybutyrate (PHB) is studied by the microsecond atomistic molecular-dynamics (MD) simulations for the first time. The model and the simulation protocol were confirmed through comparison of the glass transition temperature (T g ) with experimental data. It was established that PLA and PHB are miscible on the basis of the 2 Flory-Huggins theory. Analysis of the mobilities of PLA and PHB subchains revealed that the blends have two transitions to a glassy state at the length scale of a few Kuhn segments, which is in line with the predictions of the self-concentration model. At the same time at the larger length scale, a single transition to a glassy state was observed suggesting scale-dependence of PLA and PHB miscibility. This scale-dependence was confirmed through the evaluation of the interchain pair correlation functions.

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Polymer bio-nanocomposites are widely used nowadays in various applications. One of the most promising polymers for producing such materials is poly(lactic acid) (PLA) filled with cellulose nanocrystals (CNCs). To increase the compatibility of hydrophobic PLA and hydrophilic cellulose, the surface of the latter can be chemically modified. Among the various surface modifications, the grafting of lactic acid oligomers (OLAs) is of special interest. In this paper, the first all-atom molecular dynamics simulation of nanocomposites of PLA filled with CNCs with surface-grafted OLAs is presented. The influence of the degree of modification of the CNCs with OLA chains on the PLA structure near the CNC surface and on the grafted OLA, as well as on the thermal and mechanical properties of the nanocomposites is studied. It is demonstrated that a 50% modification of CNCs with OLA chains leads to PLA expulsion from the CNC surface.

The structural properties of polymer nanocomposites based on thermoplastic polyimides filled with the surface-modified carbon nanotubes (CNT) have been studied by means of the fully-atomistic molecular-dynamics simulations. The influence of a distibution of functional carboxyl groups over the CNT surface on the polymer-matrix density distribution, and the orientational ordering of polymer chains have been investigated. It was shown that the polymer shifts far away from the nanoparticle surface with increase of the CNT modification degree. The orientational ordering of PI chains was not observed in the case of nanocomposites filled with modified CNTs where carboxyl groups are distributed uniformly on the surface. However, in case of the edge-modified CNTs the polymer can interact with CNT surface; such edge-modified nanoparticle induces orientational ordering of crystallisable polyimide chains that can be considered as an initial stage of the polymer-matrix crystallization.

Atomic-scale computational models of paraffins are critically assessed and compared.

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.

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