Self-assembly of amphiphilic polymers of varying architectures near attractive surfaces

Effect of polymer architecture on the adsorption behaviour of amphiphilic copolymers: A theoretical study

HYPOTHESIS

Polymer architecture is known to have significant impact on its adsorption behaviour. Most studies have been concerned with the more concentrated, "close to surface saturation" regime of the isotherm, where complications such as lateral interactions and crowding also additionally affect the adsorption. We compare a variety of amphiphilic polymer architectures by determining their Henry's adsorption constant (k_H), which, as with other surface active molecules, is the proportionality constant between surface coverage and bulk polymer concentration in a sufficiently dilute regime. It is speculated that not only the number of arms or branches, but also the position of adsorbing hydrophobes influence the adsorption, and that by controlling the latter the two can counteract each other.

METHODOLOGY

The Self-consistent field calculation of Scheutjens and Fleer was implemented to calculate the adsorbed amount of polymer for many different polymer architectures including linear, star and dendritic. Using the adsorption isotherms at very low bulk concentrations, we determined the value of k_H for these.

FINDINGS

It is found that the branched structures (star polymers and dendrimers) can be viewed as analogues of linear block polymers based on the location of their adsorbing units. Polymers containing consecutive trains of adsorbing hydrophobes in all cases showed higher level of adsorption compared to their counterparts, where the hydrophobes were more uniformly distributed on the chains. While increasing the number of branches (or arms for star polymers) also confirmed the known result that the adsorption decreased with the number of arms, this trend can be partially offset by the appropriate choice of the location of anchoring groups.

Self-Assembled Cationic Nanoparticles Combined with Curcumin against Multidrug-Resistant Bacteria

The overuse of antibiotics exacerbates the development of antibiotic-resistant bacteria, threatening global public health, while most traditional antibiotics act on specific targets and sterilize through chemical modes. Therefore, it is a desperate need to design novel therapeutics or extraordinary strategies to overcome resistant bacteria. Herein, we report a positively charged nanocomposite PNs-Cur with a hydrodynamic diameter of 289.6 nm, which was fabricated by ring-opening polymerization of ε-caprolactone and Z-Lys-N-carboxyanhydrides (NCAs), and then natural curcumin was loaded onto the PCL core of PNs with a nanostructure through self-assembly, identified through UV-vis, and characterized by scanning electron microscopy (SEM) and dynamic light scattering (DLS). Especially, the self-assembly dynamics of PNs was simulated through molecular modeling to confirm the formation of a core-shell nanostructure. Biological assays revealed that PNs-Cur possessed broad-spectrum and efficient antibacterial activities against both Gram-positive and Gram-negative bacteria, including drug-resistant clinical bacteria and fungus, with MIC values in the range of 8-32 μg/mL. Also, in vivo evaluation showed that PNs-Cur exhibited strong antibacterial activities in infected mice. Importantly, the nanocomposite did not indeed induce the emergence of drug-resistant bacterial strains even after 21 passages, especially showing low toxicity regardless of in vivo or in vitro. The study of the antibacterial mechanism indicated that PNs-Cur could indeed destruct membrane potential, change the membrane potential, and cause the leakage of the cytoplasm. Concurrently, the released curcumin further plays a bactericidal role, eventually leading to bacterial irreversible apoptosis. This unique bacterial mode that PNs-Cur possesses may be the reason why it is not easy to make the bacteria susceptible to easily produce drug resistance. Overall, the constructed PNs-Cur is a promising antibacterial material, which provides a novel strategy to develop efficient antibacterial materials and combat increasingly prevalent bacterial infections.

Influences of the Periodicity in Molecular Architecture on the Phase Diagrams and Microphase Transitions of the Janus Double-Brush Copolymer with a Loose Graft

The backbone of the Janus double-brush copolymer may break during long-term service, but whether this breakage affects the self-assembled phase state and microphase transitions of the material is still unknown. For the Janus double-brush copolymers with a periodicity in molecular architecture ranging from 1 to 10, the influences of the architectural periodicity on their phase diagrams and order–disorder transitions (ODT) were investigated by the self-consistent mean field theory (SCFT). In total, nine microphases with long-range order were found. By comparing the phase diagrams between copolymers of different periodicity, a decrease in periodicity or breakage along the copolymer backbone had nearly no influence on the phase diagrams unless the periodicity was too short to be smaller than 3. For copolymers with neutral backbones, a decrease in periodicity or breakage along the copolymer backbone reduced the critical segregation strengths of the whole copolymer at ODT. The equations for the critical segregation strengths at ODT, the architectural periodicity, and the volume fraction of the backbone were established for the Janus double-brush copolymers. The theoretical calculations were consistent with the previous theoretical, experimental, and simulation results.

Synthesis of a hydrophobic associating polymer and its application in plugging spacer fluid

The high temperature of formation and multiple stages of leakage zone seriously affect the efficiency and safety of drilling and cementing operations. To improve leakage plugging quality before the cementing process, the hydrophobic associating polymer PHAAO was synthesized from acrylamide (AM), 2-acrylamide-2-methyl propane sulfonic acid (AMPS), and the long side-chain hydrophobic monomer octadecyl dimethyl allyl ammonium chloride (ODAAC) in this study. The structure and molecular weight of the polymer were characterized, and it was proved that the polymer has strong association properties and excellent heat resistance. Utilizing the bridge plugging principle, the polymer PHAAO was used with 36-mesh walnut shells and lignin fiber to form a compound plugging agent. This agent was added to spacer fluid to become a plugging spacer. API water loss tests and loading capacity tests under high temperatures show that the filter cake formed by the spacer fluid is dense. The sealing pressure of the spacer fluid on a 1 mm crack can reach 6.5 MPa at 160 °C, and it has good compatibility with cement slurry. A scanning electron microscopy (SEM) test was conducted to explore the membrane formation mechanism of the polymer. An ultra-low permeability membrane is formed on the surface of the filter cake from the spacer fluid due to the hydrophobic association and hydrogen bonding between the polymer and lignin fiber, thereby greatly reducing the loss of spacer fluid.

Interaction potential for coarse-grained models of bottlebrush polymers

Bottlebrush polymers are a class of highly branched macromolecules that show promise for applications such as self-assembled photonic materials and tunable elastomers. However, computational studies of bottlebrush polymer solutions and melts remain challenging due to the high computational cost involved in explicitly accounting for the presence of side chains. Here, we consider a coarse-grained molecular model of bottlebrush polymers where the side chains are modeled implicitly, with the aim of expediting simulations by accessing longer length and time scales. The key ingredients of this model are the size of a coarse-grained segment and a suitably coarse-grained interaction potential between the non-bonded segments. Prior studies have not focused on developing explicit forms of such potentials, instead, relying on scaling arguments to model non-bonded interactions. Here, we show how to systematically calculate an interaction potential between the coarse-grained segments of bottlebrush from finer grained explicit side chain models using Monte Carlo and Brownian dynamics and then incorporate it into an implicit side chain model. We compare the predictions from our coarse-grained implicit side chain model with those obtained from models with explicit side chains in terms of the potential of mean force, the osmotic second virial coefficient, and the interpenetration function, highlighting the range of applicability and limitations of the coarse-grained representation. Although presented in the context of homopolymer bottlebrushes in athermal solvents, our proposed method can be extended to other solvent conditions as well as to different monomer chemistries. We expect that our implicit side chain model will prove useful for accelerating large-scale simulations of bottlebrush solutions and assembly.

Structural properties of cationic surfactant-fatty alcohol bilayers: insights from dissipative particle dynamics

Bilayers, self-assembled by cationic surfactants and fatty alcohols in water, are the basic units of lamellar gel networks - creamy formulations extensively used in cosmetics and pharmaceutics. Mesoscopic modelling and study of the bilayers formed by single- or double-tail cationic surfactants (CTAC or DHDAC), and fatty alcohols (FAs) in the lamellar fluid and gel phases were employed. Fatty alcohols with alkyl tail equal to or greater than the surfactant alkyl tail, i.e., C₁₆FA or C₁₈FA and C₂₂FA, were considered. A model formulation was explored with the FA concentration greater than that of the surfactant and the structure of the fluid and gel bilayers in tensionless state characterised via the density profiles across the bilayers, orientational order parameters of the surfactant and FA chains, intrinsic analysis of the bilayer interfaces, and bending rigidity. The intrinsic analysis allows identification and quantification of the coexistence of the interdigitated and non-interdigitated phases present within the gel bilayers. The FA chains were found to conform the primary scaffolding of the bilayers while the surfactant chains tessellate bilayer monolayers from their water-hydrophobic interface. Further, the overlap of the FA chains from the apposed monolayers of the fluid bilayers rises with increasing FA length. Finally, the prevalence of the non-interdigitated phase over the interdigitated phase within the gel bilayers becomes enhanced upon the FA length increase with a preference of the surfactant chains to reside in the non-interdigitated phase rather than the interdigitated phase.

Kinetically-arrested single-polymer nanostructures from amphiphilic mikto-grafted bottlebrushes in solution: a simulation study

Solution self-assembly of molecular bottlebrushes offers a rich platform to create complex functional organic nanostructures. Recently, it has become evident that kinetics, not just thermodynamics, plays an important role in defining the self-assembled structures that can be formed. In this work, we present results from extensive molecular dynamics simulations that explore the self-assembly behavior of mikto-grafted bottlebrushes when the solvent quality for one of the side blocks is changed by a rapid quench. We have performed a systematic study of the effect of different structural parameters and the degree of incompatibility between side chains on the final self-assembled nanostructures in the low concentration limit. We found that kinetically-trapped complex nanostructures are prevalent as the number of macromonomers increases. We performed a quantitative analysis of the self-assembled morphologies by computing the radius of gyration tensor and relative shape anisotropy as the different relevant parameters were varied. Our results are summarized in terms of non-equilibrium morphology diagrams.