Effect of molecular architecture on the morphology diversity of the multicompartment micelles: A dissipative particle dynamics simulation study

Dissipative particle dynamics simulation is performed to study the sensitive influence of the molecular architecture and/or segment sequence on the morphology diversity of the multicompartment micelles. The multicompartment micelle morphologies formed by ABC triblock copolymers with various molecular architectures, such as the linear, the pentalinear, the cyclic, the star-like, the tetra-arm, and the π-shape are investigated, and different morphologies of the multicompartment micelles, for example, worm-like, “hamburger”, sheet-like with pores, “sweet potato” with alternating layers, sheet-like with cylinder-inclusion, and three-dimensional network are observed in this work. The density profiles and the radial distribution functions are calculated to characterize the structures of the multicompartment micelles. The preparation of complex multicompartment micelles can be fulfilled by simply changing the segment sequence and molecular architecture such as adding new bonds and grafting points.     

Pickering emulsion transport in skeletal muscle tissue: A dissipative particle dynamics simulation approach

Lymph node targeting is a commonly used strategy for particulate vaccines, particularly for Pickering emulsions. However, extensive research on the internal delivery mechanisms of these emulsions,especially the complex intercellular interactions of deformable Pickering emulsions, has been surprisingly sparse. This gap in knowledge holds significant potential for enhancing vaccine efficacy. This study aims to address this by summarizing the process of lymph-node-targeting transport and introducing a dissipative particle dynamics simulation method to evaluate the dynamic processes within cell tissue.The transport of Pickering emulsions in skeletal muscle tissue is specifically investigated as a case study.Various factors impacting the transport process are explored, including local cellular tissue environmental factors and the properties of the Pickering emulsion itself. The simulation results primarily demonstrate that an increase in radial repulsive interaction between emulsion particles can decrease the transport efficiency. Additionally, larger intercellular gaps also diminish the transport efficiency of emulsion droplet particles due to the increased motion complexity within the intricate transport space compared to a single channel. This study sheds light on the nuanced interplay between engineered and biological systems influencing the transport dynamics of Pickering emulsions. Such insights hold valuable potential for optimizing transport processes in practical biomedical applications such as drug delivery. Importantly, the desired transport efficiency varies depending on the specific application. For instance, while a more rapid transport might be crucial for lymph-node-targeted drug delivery, certain applications requiring a slower release of active components could benefit from the reduced transport efficiency observed with increased particle repulsion or larger intercellular gaps.     

Phase behavior of an amphiphilic molecule in the presence of two solvents by dissipative particle dynamics

We examine the phase behavior of A_mB_n amphiphilic molecules in the presence of two solvents X₂ and Y₂, which are strongly selective for A and B, respectively, by dissipative particle dynamics (DPD). We find that increasing the immiscibility parameter between the two solvents not only drives a macrophase separation into two phases X₂-rich and Y₂-rich for systems at less concentrated regimes, but also expands the ordered microphase region at more concentrated regimes. It even induces a sequential transition of various ordered structures. This is not surprising since increasing the solvent immiscibility parameter enhances the preferentiality of X₂ for A and Y₂ for B, and thus qualitatively varies the degree of molecular asymmetry in the amphiphilic molecules. In general, our current results reveal that the DPD simulation method has successfully captured the phase separation behavior of an amphiphilic molecule in the presence of two solvents. However, we find that the packing order of the spherical micelles is greatly affected by the finite size of the simulation box. As such, it becomes difficult to examine the most stable packing array of spheres via the DPD method. Still, DPD reveals a possible spherical order of A15, which has been observed in some amphiphilic molecule systems.     

Multicompartment micelles formed from star-dendritic triblock copolymers in selective solvents: A dissipative particle dynamics study

Dissipative particle dynamics method was used to study the multicompartment micelles formed from star-dendritic triblock copolymers in selective solvents, in which particular attention was paid to the effects of dendritic structure. The simulations show that the dendritic structure not only influences the morphology and the formation process of multicompartment micelles formed, but also the response of the micelle structure to solvent quality. The information obtained is useful for the future design of multicompartment micelles for practical applications, especially in the field of drug delivery.     

Microphase separation and molecular conformation of AB2 miktoarm star copolymers by dissipative particle dynamics

We simulate the microphase separation behavior and analyze the molecular conformation of AB₂ miktoarm star copolymers via dissipative particle dynamics (DPD). The phase diagram is constructed by varying the composition and interaction parameter. Through a mapping of the interaction parameter for a finite chain length, we find that the phase diagram via DPD is in near quantitative agreement with that predicted by the self-consistent mean-field (SCMF) theory. However, when the B composition is small, AB₂ is not able to form the ordered microstructure as easily as SCMF has predicted. Instead, only a tube-like phase is formed. This aggregated micelle-like phase via DPD, which is ignored in the SCMF study, has been frequently observed in experiments. In the analysis of the radius of gyration (R_g), when the interaction parameter increases, the R_g values of each A and B arm remain relatively unchanged; while the overall radius of gyration of AB₂ significantly increases. Furthermore, the angle between A and B arms shows an increasing trend while the angle between B and B arms shows a decreasing behavior with the interaction parameter. These results reveal that in order to reduce the contacts between A and B, the A and B arms tend to separate from each other, and the two B arms are squeezed onto the same side.     

Dissipative particle dynamics simulation study on complex structure transitions of vesicles formed by comb-like block copolymers

Vesicles are membrane-enclosed capsules that can store or transport substances. Their structures and the corresponding structural transitions are important to fulfill specific functions. Using dissipative particle dynamics method, we study the complex structure transitions of vesicles that are spontaneously formed by A₆(B₂)₃ type comb-like block copolymers. In the simulations, the interaction parameters between different components are tuned to mimic the variations of amphiphilicity of the block copolymers and the selectivity of the solvent which are experimentally tractable by, for example, a temperature quench. Complex vesicle structures are found in this research; their transitions, such as fission and reversal, are studied in detail with this dynamic simulation method. We find that the line tension plays a decisive role on the vesicle fission pathways. Moreover, the tube-like vesicles tend to transform to a special layered micelle structure, whereas the onion-shape vesicles tend to transform to reverse onion-shape vesicles when vesicle reversal takes place due to the variation of solvent selectivity.     

复杂物系的介观结构和DPD模拟

耗散粒子动力学(DPD)模拟方法是研究复杂物系介观结构的一种重要手段。首先简要介绍了DPD模拟方法,然后介绍了实际物系与DPD模型映射的2种方法,即Groot和Warren方法以及Pagonabarraga和Frenkel方法。最后对DPD模拟的理论和应用研究进展作了回顾。     

Morphologies of diblock copolymer confined in a slit with patterned surfaces studied by dissipative particle dynamics

Diblock copolymers with ordered mesophase structures have been used as templates for nano-fabrication. Unfortunately, the ordered structure only exists at micromete rscale areas, which precludes its use in many advanced applications. To overcome this disadvantage, the diblock copolymer confined in a restricted system with a patterned surface is proved to be an effective means to prohibit the formation of defects and obtain perfect ordered domains. In this work, the morphologies of a thin film of diblock copolymer confined between patterned and neutral surfaces were studied by dissipative particle dynamics. It is shown that the morphology of the symmetric diblock copolymer is affected by the ratio of the pattern period on the surface to the lamellar period of the symmetric diblock copolymer and by the repulsion parameters between blocks and wall particles. To eliminate the defects in the lamellar phase, the pattern period on the surface must match the lamellar period. The difference in the interface energy of different compartments of the pattern should increase with increasing film thickness. The pattern period on the surface has a scaling relationship with the chain length, which is the same as that between the lamellar period and the chain length. The lamellar period is also affected by the polydispersity of the symmetric diblock copolymer. The total period is the average of the period of each component multiplied by the weight of its volume ratio. The morphologies of asymmetric diblock copolymers are also affected by the pattern on the surface, especially when the matching period of the asymmetric diblock copolymer is equal to the pattern period, which is approximately equal to the lamellar period of a symmetric diblock copolymer with the same chain length.     

Mesoscale simulation on the shape evolution of polymer drop and initial geometry influence

The process experimentally measuring the interfacial tension of polymers was simulated by means of dissipative particle dynamics (DPD) simulation method in the present study. It is a retraction of ellipsoid drop of a polymer within the other polymer bulk. Effect of the initial geometry of the ellipsoid on the finally obtained interfacial tension was focused, since it was found the ellipsoid might not have the same length for both short axes, which is against original assumption. Simulated results show that different initial short axes of b and c do influence eventual measurement of the tension. However, a smaller deviation between b and c was found to be leading to an acceptable measurement. For this judgment, we found a criterion is γ=(b−c)/a<0.3.     

Dissipative particle dynamics study on the phase morphologies of the ultrahigh molecular weight polyethylene/polypropylene/poly(ethylene glycol) blends

The dissipative particle dynamics (DPD) simulation method has been used to study mesophase formation of the binary UHMWPE/PP and ternary UHMWPE/PP/PEG blends. The effects of shear rates and volume fractions of each of the blend components on end-to-end distances of UHMWPE, diffusivities and mesoscale morphologies of the blends have been investigated in detail. As compositions of the UHMWPE/PP and UHMWPE/PP/PEG blends vary, the mesoscale simulations have predicted the ordered structures with defined morphologies of lamellas, perforated lamellas, hexagonal spheres, and body-centered-cubic spheres. Micelle-like melted structures between totally disordered and the ordered phases have also been found in the UHMWPE/PP (10/90) blends. Immiscibility property of UHMWPE, PP and PEG induces the phase separation and exhibits different mesoscpic morphologies at different shear rates and volume fractions. Taking the shear rates dependence of mesophase into account, the change in morphology of the UHMWPE/PP/PEG blends with shear rate is also well studied in this work. As a function of PP concentration, the end-to-end distances of UHMWPE are found to decrease with the increase of PP concentration. This effect is more prominent for a high amount of PP.     

The application of lateral force microscopy to particle removal in aqueous polymer solutions

Abstraet-An atomic force microscope (AFM) has been used to examine the effect of a typical polymeric dispersant on the adhesion between an iron oxide sphere and a silicon wafer in the presence and absence of shear. Two separate methods for the determination of the lateral spring constant (k₁) of AFM cantilevers were employed. Determination of k₁ allows the absolute, rather than relative, shear force to be extracted from the lateral force output of the AFM. A comparison is made between the pull-off force (no shear) and the lateral force as the dispersant concentration and loading force are varied. While in both cases the magnitude of the forces decrease with increasing dispersant concentration, the effect is much less marked for the lateral force. A linear increase in removal forces with increasing loading force was observed. For a given load, the removal force is typically an order of magnitude smaller in the presence of shear.     

The concentration dependence of diffusion in semi-dilute polymer solutions

Dynamic light-scattering data for polystyrene fractions in a good solvent (benzene) and a theta solvent (cyclopentane, 20.4°C) are presented. They encompass a broad concentration interval in the semi-dilute range. These data are compared with those for the systems poly(ethylene oxide)/water and hydroxyethylcellulose/water. The entanglement network in these systems is reflected in part by the nature of the concentration dependence of the mutual diffusion coefficient and in part by the presence of a slow relaxation, information on which may be derived from the non-exponential autocorrelation function.     

Diffusion of single alkane molecule in carbon nanotube studied by molecular dynamics simulation

Full atomistic molecular dynamics simulations have been used to study the diffusion of alkane molecule in single wall carbon nanotube (SWCNT), with different alkane chain lengths and nanotube diameters. In this paper, we calculated the self-diffusion coefficient, mean-square gyration and bond-orientation order parameter of alkane molecule and the average intermolecular interaction energy per segment between SWCNT and alkane. Furthermore, structure of alkane in SWCNT was characterized through the radial distribution function, with results showing that the self-diffusion coefficient is related to the nanotube diameter. The component of mean-square gyration in z-direction scales with alkane chain length in SWCNT(9,9) like N1.07±0.04, which is in good agreement with the prediction from scaling theory for polymers. The obtained results show that nanotube diameter and alkane chain length are important factors affecting the behavior of one-dimensional confined alkanes.     

Molecular dynamics simulation of a flexible polymer network in a liquid crystalline solvent; dynamical properties

Molecular dynamics simulation was performed for the systems consisting of a flexible regular tetrafunctional polymer network and a low molecular liquid crystal (LC) solvent. The LC solvent comprises of anisotropic rod-like semiflexible linear molecules composed of beads bonded by a FENE potential. Rigidity was induced by a bending potential, proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard–Jones potential. For comparison the simulations of the system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. The influence of the network and linear chain polymer on the translational and rotational mobility of low molecular LC solvent was studied. The influence of the LC solvent ordering on the local translational mobility of the polymer chains was also observed.     

Molecular dynamics simulation of a flexible polymer network in a liquid crystal solvent; structure and equilibrium properties

A flexible regular tetrafunctional polymer network containing a low molecular liquid crystal (LC) solvent was simulated with molecular dynamics. The LC solvent comprises of anisotropic rod-like semi-flexible linear molecules composed of beads bonded by a FENE potential. Flexibility was induced by a bending potential proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard-Jones potential. The average length of the network chain was chosen to be close to the length of a mesogen. The number of network cells was constant and the simulated systems differ from each other by the number of LC layers. The simulations of a system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. Increasing the density of the composite system the LC solvent experiences the same phase transition as the pure LC: isotropic, nematic and smectic. The presence of the network shifts the isotropic-nematic transition to higher densities but does not significantly change the position of nematic-smectic transition. Transition of the LC solvent into the smectic state changes the morphology of the network. The periodicity of LC phase determines the number of network layers. The presence of linear chains in the LC solvent decreases the number of LC layers in the smectic phase.The LC order induces some stretching of the network chains along the direction of orientation and at the same time causes shrinkage in the perpendicular direction especially in the smectic phase.     

Molecular dynamics (MD) simulation on the collision of a nano-sized particle onto another nano-sized particle adhered on a flat substrate

Adhesion of a nano particle on a flat substrate both with and without deformation and also the behaviors of bullet and target particles after collision are simulated using the MD technique. The bullet particle, a low-temperature solid Argon, is modeled by a Lennard–Jones (LJ) potential, and the target particle is modeled by a strong LJ potential. Parameters varied are the size of bullet and contaminant particles, adhesion force between the target particle and the substrate, and the velocity and collision angle of the bullet particle. Removal characteristics are different between weakly adhered and strongly adhered particles. For soft target particles high velocity at small angle favors removal. For hard particles the particle–substrate adhesion is the determining factor, and the ineffective angle is observed.     

Dissipative particle dynamics study on the multicompartment micelles self-assembled from the mixture of diblock copolymer poly(ethyl ethylene)-block-poly(ethylene oxide) and homopolymer poly(propylene oxide) in aqueous solution

Dissipative particle dynamics (DPD) method is applied to model the self-assembly of diblock copolymer poly(ethyl ethylene)-block-poly(ethylene oxide) (PEE-b-PEO) and homopolymer poly(propylene oxide) (PPO) in aqueous solution. In this study, several segments are coarse-grained into a single simulation bead based on the experimental density. For the self-assembly of pure diblock copolymer PEE-b-PEO in dilute solution, the DPD simulation results are in good agreement with experimental data of micelle morphologies and sizes. The chain lengths of the block copolymers and the volume ratios between PPO and PEE-b-PEO are varied to find the conditions of forming multicompartment micelles. The micelles with core-shell-corona structure and the micelles with two compartments are both formed from the mixture of PEE-b-PEO and PPO in aqueous solution.     

Diffusion of linear macromolecules and spherical particles in semidilute polymer solutions and polymer networks

The dynamics of fluorescently labeled linear macromolecules and spherical particles that are enclosed in semidilute polymer matrixes was studied by fluorescence recovery after photobleaching. The experiments were designed such that the transition from a semidilute solution to a permanent network could be covered. This was achieved by employing a matrix polymer, polyacrylamide, carrying pendant dimethylmaleimide groups. Stepwise irradiation of such samples in the presence of a triplet sensitizer causes successive dimerization of the maleimides leading to progressive crosslinking.Studies were performed with varying concentrations of matrix polymer (20-80 g L⁻¹) as well as different molar masses (200,000-1,300,000 g mol⁻¹) and particle radii (17 and 36 nm) of enclosed labeled probes. Results show notable differences between the behavior of linear and spherical tracers: while the mobility of flexible linear chains remains nearly unaffected by the transition from a semidilute polymer solution into a chemically crosslinked network, spherical tracers get completely immobilized when the degree of crosslinking exceeds a certain threshold.     

Molecular dynamics simulation of miscibility in several polymer blends

The miscibility in several polymer blend mixtures (polymethylmethacrylate/polystyrene, (1,4-cis) polyisoprene/polystyrene, and polymethylmethacrylate/polyoxyethylene) has been investigated by using Molecular Dynamics simulations performed for fully atomistic representations of short chains. The trajectories obtained from simulation boxes representing the mixtures have been analyzed in terms of the collective scattering structure function. The Flory-Huggins parameter is determined from fits of the simulation results for this function to the random phase approximation expression. The numerical values of this parameter and its variation with temperature obtained with this procedure show a general qualitative and semi-quantitative agreement with existing experimental data for the different systems, though with significant error bars. These results together with those previously obtained for the polyvinylmethylether/polystyrene blends with the same method are compared with data yielded by other computational simpler approaches, which are considerably more sensitive to different parameter choices.