Computer Simulation of Cyclic Block Copolymer Microphase Separation

Topology and Sequence-Dependent Micellization and Phase Separation of Pluronic L35, L64, 10R5, and 17R4: Effects of Cyclization and the Chain Ends

The topology effects of cyclization on thermal phase transition behaviors were investigated for a series of amphiphilic Pluronic copolymers of both hydrophilic-hydrophobic-hydrophilic and hydrophobic-hydrophilic-hydrophobic block sequences. The dye solubilization measurements revealed the lowered critical micelle temperatures (TCMT) along with the decreased micellization enthalpy (ΔHmic) and entropy (ΔSmic) for the cyclized species. Furthermore, the transmittance and dynamic light scattering (DLS) measurements indicated a block sequence-dependent effect on the clouding phenomena, where a profound decrease in cloud point (Tc) was only found for the copolymers with a hydrophilic-hydrophobic-hydrophilic block sequence. Thus, the effect of cyclization on these critical temperatures was manifested differently depending on its block sequence. Finally, a comparison of the linear hydroxy-terminated, methoxy-terminated, and cyclized species indicated the effect of cyclization to be unique from a simple elimination of the terminal hydrophilic moieties.

Blending Linear and Cyclic Block Copolymers to Manipulate Nanolithographic Feature Dimensions

Block copolymers (BCPs) consist of two or more covalently bound chemically distinct homopolymer blocks. These macromolecules have emerging applications in photonics, membrane separations, and nanolithography stemming from their self-assembly into regular nanoscale structures. Theory suggests that cyclic BCPs should form features up to 40% smaller than their linear analogs while also exhibiting superior thin-film stability and assembly dynamics. However, the complex syntheses required to produce cyclic polymers mean that a need for pure cyclic BCPs would present a challenge to large-scale manufacturing. Here, we employ dissipative particle dynamics simulations to probe the self-assembly behavior of cyclic/linear BCP blends, focusing on nanofeature size and interfacial width as these qualities are critical to nanopatterning applications. We find that for mixtures of symmetric cyclic and linear polymers with equivalent lengths, up to 10% synthetic impurity has a minimal impact on cyclic BCP feature dimensions and interfacial roughness. On the other hand, blending with cyclic BCPs provides a route to "fine-tune" linear BCP feature sizes. We analyze simulated blend domain spacings within the context of strong segregation theory and find significant deviations between simulation and theory that arise from molecular-level packing motifs not included in theory. These insights into blend self-assembly will assist experimentalists in rationally designing BCP materials for advanced nanolithography applications.

The ENUF method-Ewald summation based on nonuniform fast Fourier transform: Implementation, parallelization, and application

Computer simulations of model systems are widely used to explore striking phenomena in promising applications spanning from physics, chemistry, biology, to materials science and engineering. The long range electrostatic interactions between charged particles constitute a prominent factor in determining structures and states of model systems. How to efficiently calculate electrostatic interactions in simulation systems subjected to partial or full periodic boundary conditions has been a grand challenging task. In the past decades, a large variety of computational schemes has been proposed, among which the Ewald summation method is the most reliable route to accurately deal with electrostatic interactions between charged particles in simulation systems. In addition, extensive efforts have been done to improve computational efficiencies of the Ewald summation based methods. Representative examples are approaches based on cutoffs, reaction fields, multi-poles, multi-grids, and particle-mesh schemes. We sketched an ENUF method, an abbreviation for the Ewald summation method based on the nonuniform fast Fourier transform technique, and have implemented this method in particle-based simulation packages to calculate electrostatic energies and forces at micro- and mesoscopic levels. Extensive computational studies of conformational properties of polyelectrolytes, dendrimer-membrane complexes, and ionic fluids demonstrated that the ENUF method and its derivatives conserve both energy and momentum to floating point accuracy, and exhibit a computational complexity of O N log N with optimal physical parameters. These ENUF based methods are attractive alternatives in molecular simulations where high accuracy and efficiency of simulation methods are needed to accelerate calculations of electrostatic interactions at extended spatiotemporal scales.

Spontaneous Formation of Moiré Patterns through Self-Assembly of Janus Nanoparticles

Two periodic two-dimensional lattices overlap with each other with a twisted angle can result in moiré patterns (MPs). In this in silico study, we show that by using amphiphilic Janus nanoparticles (JNPs) as a building block, the MPs of JNPs emerge spontaneously via direct self-assembly in dilute solution without additional complicated operation. The formation of MPs is attributed to the hydrophobicity of the nanoparticles (and the so-induced "force strings" at the membrane rim) together with suitable grafted hydrophilic and hydrophobic chain lengths. The mass production of MPs with controlled size can be fulfilled by adding stabilizers that effectively reduce the line tension at the rim of membranes with MPs.

Lattice self-consistent field calculations of confined symmetric block copolymers of various chain architectures

The effects of chain architecture and confinement on the structure and orientation of lamellae formed by incompressible and symmetric AB-type block copolymer melts confined between two parallel and identical surfaces are investigated using self-consistent field calculations on a simple cubic lattice. Five systems of various chain architectures (linear, ring, and star) and lengths are studied, with their bulk lamellar period L₀ chosen such that they have comparable L₀/R_g, where R_g denotes the ideal-chain radius of gyration. For thin films of thickness D = L₀ confined between two neutral surfaces, we define the rescaled volume fraction profiles of A, B, chain end, and joint segments in the parallel and perpendicular lamellae such that these profiles can be directly compared among the five systems to quantitatively reveal the interplay between the chain-end enrichment near confining surfaces and the surface-induced A-B compatibilization, and how such interplay is affected by the chain architectures (for example, the chain-crowding effects in the star block copolymers). The effects of D and surface preference for one of the blocks are also investigated.

Microscopic characteristics of Janus nanoparticles prepared via a grafting-from reaction at the immiscible liquid interface

The dynamic process of synthesizing Janus nanoparticles (JNPs) at a water/oil two-phase interface using a grafting-from reaction is investigated via dissipative particle dynamics simulations. We find that the interfacial tension, the initial monomer concentration, and the reaction probability can greatly influence the microscopic characteristics of JNP structure. It is difficult to synthesize a symmetric JNP with an equal volume ratio between hydrophilic and hydrophobic parts by grafting-from methods unless the physical chemical conditions in the two phases are strictly symmetric, and there is always a disordered domain on the JNP at a two immiscible solvents interface. Interestingly, for certain routes for synthesizing JNPs with a grafting-from method, the higher interfacial tension between the water and oil phases may enhance the degree of disorder of the grafted chains. The asymmetric initial monomer concentration in solution and the reaction probability can be used to control the syntheses of asymmetric JNPs.

Alpha-linolenic acid-loaded oil/water microemulsion: Effects of phase behaviour simulation and environmental stress on phase stabilizing and anti-oxidation capacity

α-Linolenic acid (ALA)-loaded microemulsion (ME) was prepared from isoamyl acetate, polyoxyethylene ether 35 (EL-35), ethanol and water. The dynamic phase behaviour was simulated using dissipative particle dynamics (DPD), which showed that spherical ME was formed at water/oil ratios of 1:9 and 9:1, while a lamellar structure with distinctive water-course and oil layer appeared at ratios of 3:7, 5:5, and 7:3. Phase stabilizing and anti-oxidation effect of environmental stresses on ALA-loaded microemulsion were investigated. Results showed that the ME region was large and had good environmental tolerance. Subsequently, the investigation of anti-oxidation stability revealed that more than 60% ALA of ALA-loaded ME could be protected from oxidation under environmental stresses. Furthermore, ALA-loaded ME was applied in aqueous-based foods. The transparency, precipitate, stratification and phase separation were used to evaluate influence of ME on product properties, confirming great feasibility and stability of ALA-loaded ME for practical applications.

Ring and Linear Copolymer Blends under Confinement

The behavior of dense mixtures of two topologically different diblock copolymer (CP) chains, viz., linear (L)-CP and ring (R)-CP of the same molecular weight, which form lamellae is studied under confinement by two nonselective substrates. The effect of varying interaction strength between L-CP and R-CP from purely repulsive (demixed state) to weakly attractive (mixed state) on the morphology, domain size, chain conformations, and distribution of chains in the film is investigated. In the demixed state, collective structure factor S(q) shows a split of the predominant peak indicating the presence of two dominant length scales, while there is only one predominant peak in the mixed state, and hence a lamellar structure with single domain size. We show that the peak position q* of S(q) can be varied with the L/R interaction strength and thus allows one to control domain size by tuning L/R interaction strength without altering the chain size. We further characterize the chain size and illustrate that this domain size variation is a consequence of the variation in the size of L-CPs. Furthermore, results on the average instantaneous shape of R/L-CP reveal that their shapes are very different both in bulk and near the substrate, and R-CP assumes an oblate shape near the substrate. This shape/size difference leads to the segregation of R-CPs near the polymer-substrate interface and hence a relatively higher density of R-CPs at the interface.

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

The formation of an onion shape vesicle.

Dissipative particle dynamics simulation study of poly(2-oxazoline)-based multicompartment micelle nanoreactor

We investigate multicompartment micelles for nanoreactor applications, using the DPD simulation method to characterize the internal structure and the distribution of the reactant.

Dissipative particle dynamics simulation study on self-assembly of amphiphilic hyperbranched multiarm copolymers with different degrees of branching

Hyperbranched multiarm copolymers (HMCs) have shown great potential to be excellent precursors in self-assembly to form various supramolecular structures in all scales and dimensions in solution. However, theoretical studies on the self-assembly of HMCs, especially the self-assembly dynamics and mechanisms, have been greatly lagging behind the experimental progress. Herein, we investigate the effect of degree of branching (DB) on the self-assembly structures of HMCs by dissipative particle dynamics (DPD) simulation. Our simulation results demonstrate that the self-assembly morphologies of HMCs can be changed from spherical micelles, wormlike micelles, to vesicles with the increase of DBs, which are qualitatively consistent with the experimental observations. In addition, both the self-assembly mechanisms and the dynamic processes for the formation of these three aggregates have been systematically disclosed through the simulations. These self-assembly details are difficult to be shown by experiments and are very useful to fully understand the self-assembly behaviors of HMCs.

Monte Carlo simulations on thermodynamic and conformational properties of catenated double-ring copolymers

The thermodynamic and conformational properties of catenated double-ring A-B copolymer melts are investigated through lattice Monte Carlo simulations. The topological constraint on the catenated copolymers is shown to suppress demixing of A and B monomers. This action results in their order-to-disorder transition (ODT) at an increased segregation level and the lamellae below ODT with reduced order, when compared to diblock copolymers of linear or single-ring topology. The A and B rings are pulled closer by catenation in the copolymer, which leads to its smaller gyration radius, lamellar domain spacing, and distance between mass centers of the two rings than for the diblock copolymers. With increasing segregation tendencies, the gyration radii of the A rings of the catenated copolymers stretch along the direction normal to lamellae, while the A-block conformations of the single-ring copolymers change their shapes from ellipsoid to sphere.

Phase diagrams of block copolymer melts by dissipative particle dynamics simulations

Phase diagrams for monodisperse and polydisperse diblock copolymer melts and a random multiblock copolymer melt are constructed using dissipative particle dynamics simulations. A thorough visual analysis and calculation of the static structure factor in several hundreds of points at each of the diagrams prove the ability of mesoscopic molecular dynamics to predict the phase behavior of polymer systems as effectively as the self-consistent field-theory and Monte Carlo simulations do. It is demonstrated that the order-disorder transition (ODT) curve for monodisperse diblocks can be precisely located by a spike in the dependence of the mean square pressure fluctuation on χN, where χ is the Flory-Huggins parameter and N is the chain length. For two other copolymer types, the continuous ODTs are observed. Large polydispersity of both blocks obeying the Flory distribution in length does not shift the ODT curve but considerably narrows the domains of the cylindrical and lamellar phases partially replacing them with the wormlike micelle and perforated lamellar phases, respectively. Instead of the pure 3d-bicontinuous phase in monodisperse diblocks, which could be identified as the gyroid, a coexistence of the 3d phase and cylindrical micelles is detected in polydisperse diblocks. The lamellar domain spacing D in monodisperse diblocks follows the strong-segregation theory prediction, D∕N(1∕2) ~ (χN)(1∕6), whereas in polydisperse diblocks it is almost independent of χN at χN < 100. Completely random multiblock copolymers cannot form ordered microstructures other than lamellas at any composition.

Molecular dynamics and dissipative particle dynamics simulations of the miscibility and mechanical properties of GAP/DIANP blending systems

Prediction of miscibility and mechanical properties of GAP/DIANP blends.

Self-assembly of Au15 into single-cluster-thick sheets at the interface of two miscible high-boiling solvents

Wet (nano)blanket: The self-assembly of Au nanoclusters into single-cluster-thick nanosheets is performed in two miscible high-boiling solvents with a slight polarity difference, which generates microphase separation and acts as a soft template to direct the self-assembly in a two-dimensional orientation.

DISSIPATIVE PARTICLE DYNAMICS SIMULATION STUDY ON CONTROLLING MOLECULAR WEIGHT DISTRIBUTION IN EMULSION POLYMERIZATION

The controlling factors on molecular weight distribution in emulsion polymerization are investigated using dissipative particle dynamics simulations. The propagation and bi-radical termination kinetic steps are taken into account in the simulations by coupling a Monte Carlo type reaction model. We find that monomer concentration can be very efficient on controlling molecular weight distribution and plays a decisive role on the formation of high molecular weight polymers. Increasing initiator concentration can effectively reduce the polymer molecular weight while increasing the polydispersity index. Moreover, increasing polymerization rate can slightly narrow the molecular weight distribution. We also find that, by suitably tuning the surfactant chain length, it may be possible to obtain an optimal molecular weight distribution in emulsion polymerization.

Morphology transformation of hybrid micelles self-assembled from rod-coil block copolymer and nanoparticles

Hybrid polymeric micelles self-assembled from a mixture containing poly(γ-benzyl-L-glutamate)-block-poly(ethylene glycol) (PBLG-b-PEG) block copolymer and gold nanoparticles (AuNPs) were prepared. The effect of AuNPs on the self-assembly behavior of PBLG-b-PEG was studied both experimentally by transmission electron microscopy, scanning electron microscopy, and laser light scattering and computationally using dissipative particle dynamics (DPD) simulations. It was found that, the pure PBLG-b-PEG block copolymer self-assembles into long cylindrical micelles. By introducing AuNPs to the stock block copolymer solution, the formed aggregate morphology transforms to spherical micelles. The DPD simulation results well reproduced the morphological transformations observed in the experiments. And the simulation revealed that the main reason for the aggregate morphology transformation is the breakage of ordered packing of PBLG rods in micelle core by the added nanoparticles. Moreover, from the DPD simulations, the distribution information on nanoparticles was obtained. The nanoparticles were found to prefer to locate near the core/shell interface as well as in the core center of the micelles. The combination of experimental and simulation methods lead to a comprehensive understanding of such a complex self-assembly system.