Daria V. Guseva

Simulation of phase separation in melts of reacting multiblock copolymers

The dissipative particle dynamics method is employed to conduct the computer simulation of the phase separation occurring in a melt of a polymer containing monomer units of two types under the conditions of reversible polycondensation or interchain exchange. The chemical reactions are simulated via the Monte Carlo method. It is shown that growth in the Flory-Huggins parameter leads to macrophase separation, irrespective of the rate and type of the reactions and the spatial structure of an initial system. Relative changes in the probabilities of elementary reactions between units of different types shift the phase-transition point. Different levels of refinement are considered for description of the stationary states being developed. By the example of interchain exchange, it is shown that the structure of the polymer melt in the initial state can substantially affect the dynamics of the phase separation.

Simulation of heterogeneous end-coupling reactions in polydisperse polymer blends

The influence of polydispersity on the interfacial kinetics of end-coupling and microstructure formation in the melt of immiscible polymers was studied using dissipative particle dynamics simulations. The irreversible reaction started at a flat interface between two layers, each of which contained polymer chains of two different lengths with functionalized or unreactive end groups. As in the case of fully functionalized monodisperse reactants [A. V. Berezkin and Y. V. Kudryavtsev, Macromolecules 44, 112 (2011)], four kinetic regimes were observed: linear (mean field coupling at the initial interface), saturation (decreasing the reaction rate due to the copolymer brush formation or reactant depletion near the interface), autocatalytic (loss of the initial interface stability and formation of a lamellar microstructure), and terminal (microstructure ripening under diffusion control). The interfacial instability is caused by overcrowding the interface with the reaction product, and it can be kinetically suppressed by increasing chain length of the reactants. Main effects of polydispersity are as follows: (i) the overall end-coupling rate is dominated by the shortest reactive chains; (ii) the copolymer concentration at the interface causing its instability can be not the same as in the lamellas formed afterwards; (iii) mean length of the copolymer product considerably changes with conversion passing through a minimum when a microstructure is just formed.

Monte Carlo simulation of the interchain exchange reaction in a blend of incompatible polymers

The interchain exchange reaction in a blend composed of two contacting layers of incompatible A and B homopolymers was simulated by means of the dynamic off-lattice Monte Carlo method. The evolution of local molecular-mass and block mass distributions, depending on the effective temperature and the reaction rate, was studied for the first time. It was shown that the components interpenetrate as the copolymer forms in the interphase layer and the average block length decreases below a certain, temperature-dependent value. The state of dynamic equilibrium, whose characteristics are determined mainly by temperature, is established in the system. The time of establishment of equilibrium and the intensity of compatibilization at the early steps of the process are controlled by the rate of the reaction. The results of the study allow the contribution of the reaction to the interchange processes to be evaluated.

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.

Monte Carlo simulation of a polymer-analogous reaction in a polymer blend

The autocatalytic polymer-analogous reaction A → B in a blend composed of two contacting layers of compatible homopolymers A and B is studied by numerical simulation using the dynamic continuum Monte Carlo method. The evolution of the numerical density of units A and units initially belonged to the chains of homopolymer A is investigated in the course of the reaction and interdiffusion. Local characteristics of the distribution of the homopolymer with respect to its composition and blocks A and B with respect to their length are calculated at different times. The dispersions of the above distributions are appreciably higher than the corresponding dispersion of the Bernoullian copolymer of the same average composition, despite the random character of the reaction. This effect can be provided by changes in the composition of the blend on the scale of the reacting chain as well as by the diffusive mixing of the above chains. For the products of the polymer-analogous reaction, the broadening of the compositional distribution is predicted also by the theoretical model, which describes interdiffusion in the reacting system on scales that are markedly greater than the size of a polymer chain.

Dynamic and Static Mechanical Properties of Crosslinked Polymer Matrices: Multiscale Simulations and Experiments

We studied the static and dynamic mechanical properties of crosslinked polymer matrices using multiscale simulations and experiments. We continued to develop the multiscale methodology for generating atomistic polymer networks, and applied it to the case of phthalonitrile resin. The mechanical properties of the resulting networks were analyzed using atomistic molecular dynamics (MD) and dissipative particle dynamics (DPD). The Young’s and storage moduli increased with conversion, due both to the appearance of a network of covalent bonds, and to freezing of degrees of freedom and lowering of the glass transition temperature during crosslinking. The simulations’ data showed good quantitative agreement with experimental dynamic mechanical analysis measurements at temperatures below the glass transition. The data obtained in MD and DPD simulations at elevated temperatures were conformable. This makes it possible to use the suggested approach for the prediction of mechanical properties of a broad range of polymer matrices, including ones with high structural heterogeneity.