Self-assembly of cyclic rod-coil diblock copolymers

Topology-Directed Coassembly of Linear and Cyclic Amphiphilic Diblock Copolymers: A Monte Carlo Study

The self-assembly behaviors of the mixtures composed of linear and cyclic AB diblock copolymers in A-selective solvents are investigated by means of Monte Carlo simulation. The simulation results indicate that a typical morphological transition of the aggregate from sphere to cylinder, to lamella, and then to vesicle can be achieved via solely adjusting the molar fraction of the cyclic diblock copolymers in the mixture. Furthermore, the simulation results show that under the condition that the pure cyclic and linear diblock copolymers can both form vesicles, the structure characteristics (e.g., the inner radius and hydrophobic membrane thickness of the vesicle) and the formation pathway of the vesicles formed by the mixtures can also be regulated via solely changing the molar fraction of the cyclic diblock copolymers in the mixture. It is worth noting that the inner radius of the vesicle can be considerably increased by increasing the molar fraction of the cyclic diblock copolymers in the mixture, which results in a remarkable increase in the inner capacity of the vesicle. This phenomenon has a unique significance in the field of drug delivery. Our simulation works can provide a new approach to the preparation of polymer materials with novel properties and functions.

A many-body dissipative particle dynamics parametrisation scheme to study behaviour at air-water interfaces

In this article, we present a general parametrisation scheme for many-body dissipative particle dynamics (MDPD). The scheme is based on matching model components to experimental surface tensions and chemical potentials. This allows us to obtain the correct surface and mixing behaviours of complex, multicomponent systems. The methodology is tested by modelling the behaviour of nonionic polyoxyethylene alkyl ether surfactants at an air/water interface. In particular, the influence of the number of ethylene oxide units in the surfactant head group is investigated. We find good agreement with many experimentally obtained parameters, such as minimum surface area per molecule; and a decrease in the surface tension with increasing surfactant surface density. Moreover, we observe an orientational transition, from surfactants lying directly on the water surface at low surface coverage, to surfactants lying parallel or tilted with respect to the surface normal at high surface coverage. The parametrisation scheme is also extended to cover the zwitterionic surfactant lauryldimethylamine oxide (LDAO), where we provide good predictions for the surface tension at maximum surface coverage. Here, if we exceed this coverage, we are able to demonstrate the spontaneous production of micelles from the surface surfactant layer.

Temperature-responsive morphology formation of a PS-b-PI copolymer: a dissipative particle dynamics simulation study

Self-assembly responsiveness to stimuli of polystyrene-block-polyisoprene (PS-b-PI) diblock copolymer materials is explored by means of classical molecular dynamics (MD) and dissipative particle dynamics (DPD) simulations. A concerted relationship between the parameters achieved from atomistic and DPD simulations is obtained for this molecular recognition as clearly pronounced in a phase transition. Effects of temperature, model size and composition on the morphological formation were systematically investigated for the diblock copolymeric system. Structural changes resulting in the evolution of rheology as well as an equilibrium ordered structure were analyzed in terms of order parameters and radial distribution functions. From our models, various morphologies were observed including discrete clusters (sphere-liked morphology), connected clusters (gyroid-liked morphology), hexagonally packed cylinders (HEX), connected cylinders, irregular cylinders, perfect lamellae, perforated lamellae and defected lamellae. Based on this finding, a bottom-up multi-scale simulation of the PS-b-PI diblock copolymer provides a link between equilibrium copolymeric morphologies and the crucial parameters.

Spontaneous onion shape vesicle formation and fusion of comb-like block copolymers studied by dissipative particle dynamics

The formation of an onion shape vesicle.

Simulation insights into the role of antiparallel molecular association in the formation of smectic A phases

A simple dissipative particle dynamics (DPD) model is introduced, which can be used to represent a broad range of calamitic mesogens. The model allows for antiparallel association that occurs naturally in a number of mesogens with terminal dipoles, including the 4-n-alkyl-4'-cyanobiphenyl (nCB) series. Favourable antiparallel interactions lead to the formation of SmA_d phases in which the layer spacing is intermediate between monolayer and bilayer. The model is easily tuned to vary the strength of antiparallel association and the SmA layer spacing, and to give either isotropic-smectic or isotropic-nematic-smectic phase sequences. The model allows for a range of other smectics: including SmA₁ phases exhibiting microphase separation within layers, and smectics A structures with more complicated repeat units. For large system sizes (≥50 000 molecules) in the nematic phase, we are able to demonstrate the formation of three distinct types of cybotactic domains depending on the local interactions. Cybotactic domains are found to grow in the nematic-smectic pretransitional region as the system moves closer to T_SN.

Liquid crystalline assembly of rod-coil diblock copolymer and homopolymer blends by dissipative particle dynamics simulation

Liquid crystalline assembly of rod-coil diblock copolymers blended with coil or rod homopolymers is investigated by dissipative particle dynamics simulation, considering systematically the effect of the interactions between rods and coils, the volume fraction and length of the added coil or rod homopolymers. The addition of coil or rod homopolymers induces disorder-order or order-liquid crystalline transition. In rod-coil/coil blends, the solubilization of homopolymers will saturate at a certain amount of homopolymers and then the excess homopolymers will be segregated into the central regions of coil block domains, forming "wet-dry mixture" lamellae. The solubility capacity decreases with increasing homopolymer length, determined by the competition between the mixing entropy and the elastic entropy. In rod-coil/rod blends, due to the orientational interactions between rods, the length matched rod homopolymers directly interdigitate with rod blocks with less entropy loss, thus prompting the formation of a bilayer liquid crystalline phase. The rod domain spacing Dr remains unchanged and conversely the coil domain spacing Dc becomes thin, to occupy more interfacial area. With the addition of shorter rod homopolymers, the overall lamellar spacing D of blends monotonically increases with the volume fraction of homopolymers, similar to the case of rod-coil/coil blends. Generally, rod homopolymers have a more significant impact on the liquid crystalline assembly of the blends, compared with the coil homopolymers. Our results indicate that blending with coil or rod homopolymers into a rod-coil system is an effective method to induce liquid crystal phase transition and control the phase spacing of the ordered structure.

Self-assembly and mesophase formation in a non-ionic chromonic liquid crystal system: insights from dissipative particle dynamics simulations

Results are presented from a dissipative particle dynamics (DPD) simulation of a model non-ionic chromonic system, TP6EO2M, composed of a poly(ethylene glycol) functionalised aromatic (triphenylene) core.