Theoretical Study of Microgel Functional Groups' Mobility.

Polymer microgels, micrometer-sized cross-linked polymer particles, are considered to be a promising type of advanced materials for a wide range of applications. To enhance the microgels' applicability, it is essential to incorporate various functional groups into a microparticle polymer network. Yet, the availability of functional groups for the interaction with surroundings depends strongly on the properties of the polymer network and has a great impact on further effective usage. In this theoretical study, we address this question and, with the help of coarse-grained molecular dynamics computer simulations, assess the segmental mobility and accessibility of functional groups bound to polymer network depending on microgel architecture and solvent quality. Additionally, we evaluate the minimum number of functional groups needed to facilitate the hopping mechanism between the functional groups (i.e., charge transfer). As an example of practical implementation of the obtained results, we estimate the optimal network topology for redox-active microgels to provide the maximum charge capacity for the dispersion electrolyte in redox-flow batteries.

Microphase separation in random multiblock copolymers.

Microphase separation in random multiblock copolymers is studied with the mean-field theory assuming that long blocks of a copolymer are strongly segregated, whereas short blocks are able to penetrate into "alien" domains and exchange between the domains and interfacial layer. A bidisperse copolymer with blocks of only two sizes (long and short) is considered as a model of multiblock copolymers with high polydispersity in the block size. Short blocks of the copolymer play an important role in the microphase separation. First, their penetration into the "alien" domains leads to the formation of joint long blocks in their own domains. Second, short blocks localized at the interface considerably change the interfacial tension. The possibility of penetration of short blocks into the "alien" domains is controlled by the product χNsh (χ is the Flory-Huggins interaction parameter and Nsh is the short block length). At not very large χNsh, the domain size is larger than that for a regular copolymer consisting of the same long blocks as in the considered random copolymer. At a fixed mean block size, the domain size grows with an increase in the block size dispersity, the rate of the growth being dependent of the more detailed parameters of the block size distribution.

Dissipative particle dynamics for systems with high density of charges: Implementation of electrostatic interactions.

In this work, we study the question of how to introduce electrostatic interactions in dissipative particle dynamics (DPD) method in order to correctly reproduce the properties of systems with high density of charges, including those with inhomogeneous charge distribution. To this end, we formulate general requirements for the electrostatic force in DPD and propose a new functional form of the force which suits better for satisfying these requirements than the previously used ones. In order to verify the proposed model, we study the problem of a single polyelectrolyte chain collapse and compare the results with molecular dynamics (MD) simulations in which the exact Coulomb force is used. We show that an excellent quantitative agreement between MD and DPD models is observed if the length parameter D of the proposed electrostatic force is chosen properly; the recommendations concerning the choice of this parameter value are given based on the analysis of a polyelectrolyte chain collapse behavior. Finally, we demonstrate the applicability of DPD with the proposed electrostatic force to studying microphase separation phenomenon in polyelectrolyte melts and show that the same values of D as in the case of single chain collapse should be used, thus indicating universality of the model. Due to the charge correlation attraction, a long-range order in such melts can be observed even at zero Flory-Huggins parameter.

Systematic study of glass transition in low-molecular phthalonitriles: Insight from computer simulations.

Phthalonitrile compounds with Si bridges were recently suggested for producing thermosetting polymer composites with reduced Tg and thus expanded processing range. The detailed experimental investigation of this class of phthalonitriles is still difficult due to development time and costs limitations and the need to take into account the structural changes during the crosslinking. In this paper, we try to overcome these limitations using computer simulations. We performed full-atomistic molecular dynamics simulations of various phthalonitrile compounds to understand the influence of molecular structure on the bulk glass temperature Tg. Two molecular properties affect Tg of the resulting bulk compound: the size of the residue and the length of the Si bridge. The larger residues lead to higher Tgs, while compounds with longer Si bridges have lower Tgs. We have also studied relaxation mechanisms involved in the classification of the samples. Two different factors influence the relaxation mechanisms: energetic, which is provided by the rigidity of molecules, and entropic, connected with the available volume of the conformational space of the monomer.

Multiblock copolymers prepared by patterned modification: Analytical theory and computer simulations.

We describe a special type of multiblock copolymers which are synthesized by a hypothetic procedure of the modification of monomer units in a polymer melt according to a certain geometrical criterion. In particular, we explore the case of lamellar-like structures: the sequence statistics of the resulting multiblock copolymers is described and their ability to self-assemble is studied. It is found that the block-size distribution P(k) for such random copolymers contains a large fraction of short blocks with the asymptotic dependence ∼k(-3/2), where k is the block size. A characteristic feature of such multiblock copolymers is their extremely high block-size polydispersity with the polydispersity index being proportional to the space period of the modification. The morphological behavior of such copolymers is simulated by means of dissipative particle dynamics. A stable self-assembled lamellar structure is observed, but the domain size appears to be sufficiently larger than the initial pattern period.

Anomalous diffusion in fractal globules.

The fractal globule state is a popular model for describing chromatin packing in eukaryotic nuclei. Here we provide a scaling theory and dissipative particle dynamics computer simulation for the thermal motion of monomers in the fractal globule state. Simulations starting from different entanglement-free initial states show good convergence which provides evidence supporting the existence of a unique metastable fractal globule state. We show monomer motion in this state to be subdiffusive described by ⟨X(2)(t)⟩∼t(αF) with αF close to 0.4. This result is in good agreement with existing experimental data on the chromatin dynamics, which makes an additional argument in support of the fractal globule model of chromatin packing.

Conformation-dependent sequence design: evolutionary approach.

A new modification of evolutionary approach to sequence design of copolymers has been proposed. A model of step-by-step evolution of a two-letter ( HP) copolymer sequence has been studied by means of a coarse-grained Monte Carlo algorithm. The conditions for accepting a change in the primary sequence depend on the spatial conformation of HP-copolymer chain. This leads to a coupling between sequence and conformation and to formation of protein-like conformations and primary sequences (for some values of parameters of the model) independently of initial sequence and/or conformation. Simple theory describing these computer simulation observations is developed.