A possible strategy for generating polymer chains with an entanglement-free structure.

We propose a possible strategy that may experimentally generate long polymeric chains with an entanglement-free structure. The basic idea is designing the conditions to restrict polymer chains from growing along the surface with an obviously concave curvature. This strategy is proved to effectively reduce the chance of forming both inter- and intra-molecular entanglements, which is quite similar to the self-avoiding random walking of chains on a two dimensional plane. We believe that this kind of chain growth strategy may supply a kind of possible explanation on the formation of the entanglement-free structure of chromosomes, which also have tremendously large molecular weight. Besides, this study also guides experimentalists on synthesizing specific entanglement-free functional polymeric or biological materials.

Understanding of supramolecular solution polymerization and interfacial polymerization via forming multiple hydrogen bonds: a computer simulation study.

By employing dissipative particle dynamics (DPD) simulations combined with stochastic polymerization models, we have conducted a detailed simulation study of supramolecular solution polymerization as well as interfacial polymerization employing a coarse-grained model which is closer to the real monomer structure. By adding bending angle potentials to coarse-grained models representing supramolecular reactive monomers, we achieved monomer model simulations for different kinds of multiple hydrogen bonds. Our simulation results indicated that for the interfacial polymerization system, the volume of the monomer caused a strong steric hindrance effect, which in turn led to a low average degree of polymerization of the product. Therefore, by appropriately reducing the volume of the reaction monomer (corresponding to different confinement ascribed to the multiple hydrogen bonds), the average polymerization degree, the degree of reaction and the polymerization rate of the monomer can be effectively improved. For the solution polymerization system and the interfacial polymerization system, a certain proportion of rigid monomers and flexible monomers (60% rigid monomers and 40% flexible monomers) are mixed. High molecular weight products can thus be obtained via the polymerization reaction. The simulation strategy proposed in this study can not only provide theoretical guidance for better design of new supramolecular systems, but also provide ideas for the further synthesis of higher molecular weight supramolecular polymers.

Polymerization-induced polymer aggregation or polymer aggregation-enhanced polymerization? A computer simulation study.

In this study, using dissipative particle dynamics simulations coupled with the stochastic reaction model, we investigate the polymerization-induced polymer aggregation process and the polymer aggregation-enhanced polymerization process in a binary solution, by simply tuning the solubility of the solvent to one species of copolymerization. Our simulations indicate that it is a complicated interplay of the copolymerization on the formation of aggregates, namely, on one hand the polymerization may induce the aggregation of one species; on the other hand it has an effect of mixing the two species together. We also find that the polymerization process basically follows the first order reaction kinetics. With the increase of insolubility of B species in the solution, it continuously deviates from the first order reaction kinetics. In the symmetric copolymerization system, we find that the dispersity of copolymers monotonically decreases with the increase of reaction probability. This counterintuitive result can be understood via the comparison of diffusion-controlled kinetics and reaction-controlled kinetics. In the asymmetric system, for systems with preferential copolymerization, the mass distribution shapes are Gaussian-like with certain peaks. For comparison, for systems with preferential homopolymerization, the mass distribution shape shows an obviously bimodal form. This study helps to better understand the cooperative competition between the reaction dynamics and the diffusion dynamics during the preparation of copolymer materials, and could act as a guide to better design and improve the copolymerization technologies in laboratories and in industry.

Simultaneous polymer chain growth with the coexistence of bulk and surface initiators: insight from computer simulations.

By Brownian dynamics simulations we study the simultaneous polymer chain growth process with the coexistence of bulk and surface initiators. We find that when the surface initiator density is low enough, the practical experimental way to estimate the dispersity (D) of surface-initiated chains on the basis of the dispersity of bulk-initiated chains remains valid as long as the conformations of grafted chains remain within the mushroom regime (i.e., the grafted chains are sparsely distributed). On the other hand, although the average chain lengths of surface and bulk polymers could be equivalent when certain conditions are met, their mass distributions are still different. We also find that increasing the fraction of surface initiators leads to an enlarged disparity in D and average length between surface and bulk chains, which is inconsistent with previous studies. This study helps in better understanding the cooperative competition and suppressing effect of bulk chains on surface grown chains, as well as the cause of the dispersity of the surface grown chains as compared to their bulk counterparts with the coexistence of bulk and surface initiators.

Polymer-grafted nanoparticles prepared via a grafting-from strategy: a computer simulation study.

Nanoparticles (NPs) grafted with polymer chains prepared via a grafting-from strategy are studied through coarse-grained molecular dynamics simulations combined with our stochastic reaction model. A system involving multiple individual NPs, with grafting-from processes for all the NPs induced simultaneously, is simulated, so that chain growth competition on the same NP, as well as between neighbouring NPs, are both naturally considered. Our results imply that there should be an optimized range of NP sizes, as compared to monomer size, in which initiator sites are most easily induced. Besides, when the initiator density is high, a shielding effect from the sparse long chains on the most short chains or initiators evidently yields an extremely unbiased distribution of chains. We also adopt a representative polymer-tethered NP prepared via a grafting-from strategy to study the potential of mean force between NPs, so that the dispersion and stabilization abilities of such polymer-grafted NPs in a polymer matrix can be generally predicted during the preparation of polymer nanocomposite materials. Our study helps to elucidate the cause of chain dispersity during the grafting-from process and could act as a guide for better design and to improve the performance of polymer nanocomposites.

Kinetic step-growth polymerization: A dissipative particle dynamics simulation study.

Kinetic step-growth polymerization is studied by dissipative particle dynamics coupled with our previously developed reaction algorithm on a coarse-grained level. The simulation result proves that this step-growth polymerization obeys the second-order reaction kinetics. We apply this algorithm to study the step-growth polymerization using the subunits with different flexibilities or within confinement. Good agreement of the number fraction distributions with the Flory distribution is obtained, implying that this algorithm is reasonable to describe such a kind of step-growth polymerization. This algorithm can further supply a convenient platform for simulating typical step-growth polymerization in reactive polymer systems.

The selectivity of nanoparticles for polydispersed ligand chains during the grafting-to process: a computer simulation study.

By constructing a grafting-to reaction model of polydispersed polymer chains to bind onto nanoparticles (NPs), we elucidate the changes of grafting density, polydispersity index and chain length distribution of grafted ligand chains as a dependent of the feeding polymer chains. Our study shows a linear dependence of the grafting density on the average chain length of the feeding polymers. We also clearly demonstrate the NP's selectivity of short chains in the later stage of the reaction. Our results also show that the polydispersity of the ligand chains on each individual NP is commonly higher than that of the feeding polymer chains. Furthermore, the simulations imply that polydispersed polymer chains are more easily bound during the grafting-to process compared to monodispersed chains. Thus, relatively higher polydispersity of feeding polymer chains are beneficial to promoting the grafting density of NPs. Our study helps to elucidate the cause of chain polydispersity and could act as a guide to finely regulate the polydispersity of ligand chains on nanoparticles.

Self-assembly of block copolymers on lithographically patterned template with ordered posts.

Dissipative particle dynamics simulations are employed to study the self-assembly of block copolymers on a template modified with ordered posts. Templates with hexagonally arranged and rectangularly arranged posts are both studied. For the systems with hexagonally arranged posts, morphologies with bending alignments are seen most often. We find that the different kinds of patterns, which can be directly observed in experiments, are substantially induced by the pattern of the bottom layer. In the simulations with a template modified with rectangularly arranged posts, by finely adjusting the distances between neighboring posts in both x and y directions, mesh-shaped structures with different angles between the bottom and the sub-bottom layers can be obtained. These results shed light on the better design of lithographically patterned materials on the scale of 10 nm via the directed self-assembly of BCPs by templating.

Polymer-grafted nanoparticles prepared by surface-initiated polymerization: the characterization of polymer chain conformation, grafting density and polydispersity correlated to the grafting surface curvature.

The polymer-grafted nanoparticles prepared by the surface-initiated polymerization induced from the spherical surface is studied by coarse-grained molecular dynamics simulations combined with the stochastic reaction model. The coupling effect of the initiator density and the grafting surface curvature is mainly investigated. The confinement degree greatly changes with the grafting surface curvature, thus the initiation efficiency, the grafted chain polydispersity, as well as the chain mass distribution show great dependence on the surface curvature. The results reveal that preparing the nanoparticle with desired size (i.e., grafting surface curvature) is crucial for control of the grafted chain polydispersity and even its dispersion in the polymer matrix. Our studies shed light on better design of grafted nanoparticles and better control of dispersion in polymer matrices for improving the performance of polymer nanocomposite materials.

Layered structure in compatible binary polymer brushes with high graft density: A computer simulation study.

We focus on highly grafted binary polymer brushes with compatible components in the cases of different chain lengths. Layered structures parallel to the surface that indicating "phase separation" are observed in a series of dissipative particle dynamics simulations. The stretch parameters indicate that the short chains are suppressed in the lower layer of the film, whereas the longer chains are much stretched in the region dominated by the short chains (lower layer) but possess relaxed conformations in the upper layer. By slightly changing the solvent selectivity to prefer the short chains, we find a reversion of the layered structure. Such a sensitive switch of film property implies its potential application as tuning the wettability and adhesion of the surface in industry.

A practical method to avoid bond crossing in two-dimensional dissipative particle dynamics simulations.

Dissipative particle dynamics (DPD) simulation technique is an effective method targeted on mesoscopic simulations in which the interactions between particles are soft. As a result, it inevitably causes bond crossing and interpenetration between particles. Here we develop a practical method based on the two-dimensional DPD model which can extremely reduce the possibility of bond crossing. A rigid core is added to each particle by modifying the form of the conservative force in DPD so that the particles cannot penetrate each other. Then by adjusting the spring constant of the bond, we can impose a simple geometry constraint so that the bond crossing can hardly take place. Furthermore, we take into account an analytic geometry constraint in the polymerization model of DPD by which we can successfully avoid the severe bond crossing problem during bond generation in two dimensions. A parabola fitting between the pressure and the particle number density shows that our modified DPD model with small rigid cores can still be mapped onto the Flory-Huggins model, and the mesoscopic length scale of our simulations does not change. By analyzing the mean-square displacement of the innermost monomer and the center of mass of the chains, we find a t(8/15) power law of the polymer dynamics in our model instead of the Rouse prediction supporting the recent results in literature.