Molecular theory of the tilting transition and computer simulations of the tilted lamellar phase of rod-coil diblock copolymers

Symmetric rod-coil diblock copolymers have been simulated using the method of dissipative particle dynamics in the broad range of the Flory-Huggins parameter. It has been found that the tilted lamellar phase appears to be the most stable one at strong segregation. The rod-coil copolymer tilt angle and orientational order parameters have been determined as functions of the segregation strength. The density functional theory of rod-coil diblock copolymers has been generalized to the case of the tilted lamellar phase and used to study the stability of the orthogonal lamellar phase with respect to tilt. The orthogonal phase indeed appears to be unstable in the broad region of the parameter space in the case of relatively strong segregation. It has also been shown that the transition into the tilted lamellar phase is determined by a strong coupling between two independent tilt order parameters.

Phase behavior and orientational ordering in block copolymers doped with anisotropic nanoparticles

A molecular field theory and coarse-grained computer simulations with dissipative particle dynamics have been used to study the spontaneous orientational ordering of anisotropic nanoparticles in the lamellar and hexagonal phases of diblock copolymers and the effect of nanoparticles on the phase behavior of these systems. Both the molecular theory and computer simulations indicate that strongly anisotropic nanoparticles are ordered orientationally mainly in the boundary region between the domains and the nematic order parameter possesses opposite signs in adjacent domains. The orientational order is induced by the boundary and by the interaction between nanoparticles and the monomer units in different domains. In simulations, sufficiently long and strongly selective nanoparticles are ordered also inside the domains. The nematic order parameter and local concentration profiles of nanoparticles have been calculated numerically using the model of a nanoparticle with two interaction centers and also determined using the results of computer simulations. A number of phase diagrams have been obtained which illustrate the effect of nanoparticle selectivity and molar fraction of the stability ranges of various phases. Different morphologies have been identified by analyzing the static structure factor and a phase diagram has been constructed in coordinates' nanoparticle concentration-copolymer composition. Orientational ordering of even a small fraction of nanoparticles may result in a significant increase of the dielectric anisotropy of a polymer nanocomposite, which is important for various applications.

Vertical vs Lateral Macrophase Separation in Thin Films of Block Copolymer Mixtures: Computer Simulations and GISAXS Experiments

Mixtures of two diblock copolymers of very different lengths may feature both macro- and microphase separation; however, not much is known about the mechanisms of separation in diblock copolymer thin films. In the present work, we study thin films of mixtures of two compositionally symmetric block copolymers, both in the one-phase and in the two-phase state, combining coarse-grained molecular simulations (dissipative particle dynamics, DPD) with scattering experiments (grazing-incidence small-angle X-ray scattering, GISAXS). We reveal that the film thickness and selective adsorption of different blocks to the substrate control the distribution of macrophases within the film as well as the orientation of the lamellae therein. In thick films, the mixtures separate in the vertical direction into three layers: Two layers being rich in short copolymers are formed near the film interfaces, whereas a layer being rich in long copolymers is located in the film core. The lamellar orientation in the layers rich in short copolymers is dictated by the surface selectivity, and this orientation only weakly affects the vertical orientation of lamellae in the film core. This provides the opportunity to control the domain orientation in the copolymer films by mixing block copolymers with low-molecular additives instead of relying on a more complicated chemical modification of the substrate. In thinner films, a lateral phase separation appears.

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

Template copolymerization near a patterned surface: Computer simulation

We perform a Monte Carlo simulation of irreversible template copolymerization near a chemically heterogeneous surface with a regular distribution of discrete adsorption sites that selectively adsorb from solution one of the two polymerizing monomers and the corresponding chain segments. In the polymerization model, the chain propagation process is simulated by adding individual monomers to the end of growing macroradical. We focus in this paper on the influence of polymerization rate, adsorption energy, and the distance between adsorption sites on the chain conformation and the primary sequence of the resulting two-letter (AB) copolymers and, specifically, on the coupling between polymerization and adsorption. The conditions for the realization of conformation-dependent copolymerization are formulated. For this regime, we observe the formation of a quasiregular copolymer with two types of alternating sections. One of them contains randomly distributed A and B segments. The second one consists mainly of strongly adsorbed A segments. It is found that the average length of the random sections is proportional to the distance between the nearest neighbor adsorption sites. The average length of the A-rich sections is determined by the "adsorption capacity" of adsorption site. By varying the strength of the effective monomer-substrate interaction and the distribution of adsorption sites on the substrate, the copolymers with different surface-induced primary sequences can be designed and synthesized in a controlled fashion.