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Register a new userSelf-consistent field predictions for quenched spherical biocompatible triblock copolymer micelles
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We have used the Scheutjens-Fleer self-consistent field (SF-SCF) method to predict the self-assembly of triblock copolymers with a solvophilic middle block and sufficiently long solvophobic outer blocks. We model copolymers consisting of polyethylene oxide (PEO) as solvophilic block and poly(lactic-co-glycolic) acid (PLGA) or poly({ko}-caprolactone) (PCL) as solvophobic block. These copolymers form structurally quenched spherical micelles provided the solvophilic block is long enough. Predictions are calibrated on experimental data for micelles composed of PCL-PEO-PCL and PLGA-PEO-PLGA triblock copolymers prepared via the nanoprecipitation method. We establish effective interaction parameters that enable us to predict various micelle properties such as the hydrodynamic size, the aggregation number and the loading capacity of the micelles for hydrophobic species that are consistent with experimental finding.
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Photon correlation spectroscopy and rheological measurements are performed to investigate the microscopic dynamics and mechanical responses of aqueous solutions of triblock copolymers and aqueous mixtures of triblock copolymers and anionic surfactants. Increasing the concentration of triblock copolymers results in a sharp increase in the magnitude of the complex moduli characterising the samples. This is understood in terms of the changes in the aggregation and packing behaviours of the copolymers and the constraints imposed upon their dynamics due to increased close packing. The addition of suitable quantities of an anionic surfactant to a strongly elastic copolymer solution results in a decrease in the complex moduli of the samples by several decades. It is argued that the shape anisotropy and size polydispersity of the micelles comprising mixtures cause dramatic changes in the packing behaviour, resulting in sample unjamming and the observed decrease in complex moduli. Finally, a phase diagram is constructed in the temperature-surfactant concentration plane to summarise the jamming-unjamming behaviour of aggregates constituting triblock copolymer-anionic surfactant mixtures.
The self-consistent field theory is a popular and highly successful theoretical framework for studying equilibrium (co)polymer systems at the mesoscopic level. Dynamic density functionals allow one to use this framework for studying dynamical processes in the diffusive, non-inertial regime. The central quantity in these approaches is the mobility function, which describes the effect of chain connectivity on the nonlocal response of monomers to thermodynamic driving fields. In a recent study [Mantha et al, Macromolecules 53, 3409 (2020)], we have developed a method to systematically construct mobility functions from reference fine-grained simulations. Here we focus on melts of linear chains in the Rouse regime and show how the mobility functions can be calculated semi-analytically for multiblock copolymers with arbitrary sequences without resorting to simulations. In this context, an accurate approximate expression for the single-chain dynamic structure factor is derived. Several limiting regimes are discussed. Then we apply the resulting density functional theory to study ordering processes in a two-length scale block copolymer system after instantaneous quenches into the ordered phase. Different dynamical regimes in the ordering process are identified: At early times, the ordering on short scales dominates; at late times, the ordering on larger scales takes over. For large quench depths, the system does not necessarily relax into the true equilibrium state. Our density functional approach could be used for the computer-assisted design of quenching protocols in order to create novel nonequilibrium materials.
Starting from a microscopic lattice model, we investigate clustering, micellization and micelle ordering in semi-dilute solutions of AB diblock copolymers in a selective solvent. To bridge the gap in length scales, from monomers to ordered micellar structures, we implement a two-step coarse graining strategy, whereby the AB copolymers are mapped onto ``ultrasoft dumbells with monomer-averaged effective interactions between the centres of mass of the blocks. Monte Carlo simulations of this coarse-grained model yield clear-cut evidence for self-assembly into micelles with a mean aggregation number n of roughly 100 beyond a critical concentration. At a slightly higher concentration the micelles spontaneously undergo a disorder-order transition to a cubic phase. We determine the effective potential between these micelles from first principles.
Synperonic F-108 (generic name, pluronic) is a micelle forming triblock copolymer of type ABA, where A is polyethylene oxide (PEO) and B is polypropylene oxide (PPO). At high temperatures, the hydrophobicity of the PPO chains increase, and the pluronic molecules, when dissolved in an aqueous medium, self-associate into spherical micelles with dense PPO cores and hydrated PEO coronas. At appropriately high concentrations, these micelles arrange in a face centred cubic lattice to show inverse crystallization, with the samples exhibiting high-temperature crystalline and low-temperature fluidlike phases. By studying the evolution of the elastic and viscous moduli as temperature is increased at a fixed rate, we construct the concentration-temperature phase diagram of Synperonic F-108. For a certain range of temperatures and at appropriate sample concentrations, we observe a predominantly elastic response. Oscillatory strain amplitude sweep measurements on these samples show pronounced peaks in the loss moduli, a typical feature of soft solids. The soft solid-like nature of these materials is further demonstrated by measuring their frequency dependent mechanical moduli. The storage moduli are significantly larger than the loss moduli and are almost independent of the applied angular frequency. Finally, we perform strain rate frequency superposition (SRFS) experiments to measure the slow relaxation dynamics of this soft solid.
The escape transition of a polymer mushroom (a flexible chain grafted to a flat non-adsorbing substrate surface in a good solvent) occurs when the polymer is compressed by a cylindrical piston of radius $R$, that by far exceeds the chain gyration radius. At this transition, the chain conformation abruptly changes from a two-dimensional self-avoiding walk of blobs (of diameter $H$, the height of the piston above the substrate) to a flower conformation, i.e. stretched almost one-dimensional string of blobs (with end-to-end distance $approx R$) and an escaped part of the chain, the crown, outside the piston. The extension of this problem to the case of star polymers with $f$ arms is considered, assuming that the center of the star is grafted to the substrate. The question is considered whether under compression the arms escape all together, or whether there occurs an arm by arm escape under increasing compression. Both self-consistent field calculations and Molecular Dynamics simulations are found to favor the latter scenario.
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