Kinetically Arrested Assemblies of Architecturally Distinct Block Copolymers

100th Anniversary of Macromolecular Science Viewpoint: The Role of Hydrophobicity in Polymer Phenomena

The seemingly simple notion of the hydrophobic effect can be viewed from multiple angles involving theory, simulation, and experiments. This viewpoint examines five attributes of predictive models to enhance synthetic efforts as well as experimental methods to quantify hydrophobicity. In addition, we compare existing predictive models against experimental data for polymer surface tension, lower critical solution temperature, solution self-assembly morphology, and degradation behavior. Key conclusions suggest that both the Hildebrand solubility parameters (HSPs) and surface area-normalized Log P (Log P SA-1) values provide unique and complementary insights into polymer phenomena. In particular, HSPs appear to better describe bulk polymer phenomena for thermoplastics such as surface tension, while Log P SA-1 values are well-suited for describing and predicting the behavior of polymers in solution.

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.

Molecular environment and reactivity in gels and colloidal solutions under identical conditions

A PEG-Tyr block copolymer forms a kinetically stable colloidal solution in water at room temperature which undergoes an irreversible conversion to a gel phase upon heating. A micellar solution and a gel can therefore be studied under identical experimental conditions. This made it possible to compare physical properties and chemical reactivity of micelles and gels in identical chemical environments and under identical conditions. EPR spectra of the spin-labelled copolymer showed that tyrosine mobility in gels was slightly reduced compared to micelles. Chemical reactivity was studied using photochemical degradation of tyrosine and tyrosine dimerization, in the absence and in the presence of an Fe(iii) salt. The reactivity trends were explained by reduced tyrosine mobility in the gel environment. The largest reactivity difference in gels and micelles was observed for bimolecular dityrosine formation which was also attributed to the reduction in molecular mobility.

Exploring with Molecular Dynamics the Structural Fate of PLGA Oligomers in Various Solvents

This study focuses on the solvent effects that promote preferred solvated structures of polylactic-co-glycolic acid (PLGA) oligomers of molecular weight 278, 668, and 1449 u in ethyl acetate, water, and a mixture of both solvents. Our methodology consists of all-atom, explicit solvent molecular dynamics simulations for inspection of the solvated oligomer structures at ambient conditions. Parameters for the generalized Amber force field are developed in this work for the ethyl acetate liquid and the PLGA oligomers. Energetics, oligomer radius of gyration, end-to-end distance, orientational order parameter, flexibility coefficient, and backbone dihedral angles are reported along with a size scaling property yielding a power law for PLGA oligomers in each of the three solvents considered. It is found that the PLGA oligomer has two characteristic states identified by a set of extended structures and a set of collapsed structures, the former being energetically preferred in ethyl acetate and its mixture with water. The two types of PLGA structures occur in the three solvents and although they flip from one to the other in a sporadic fashion, in ethyl acetate, the extended structures may persist for more than 20 ns. The collapsed structures are significantly more frequent in water, occurring seldom in the mixed ethyl acetate-water solvent. PLGA is a biodegradable polymer approved for use in pharmaceutical and biomedical applications. Insights provided therein are of importance for the polymer aggregation process and its glassy state in condensed phases.

Facile Screening of Various Micellar Morphologies by Blending Miktoarm Stars and Diblock Copolymers

A time-saving phase-diagram screening is introduced for the self-assembly of miktoarm star polymers with different arm numbers for the insoluble part. Agreeing with theory, all conventional micellar morphologies (spherical star-like micelles, cylindrical micelles and vesicles) can be accessed by adjusting the average arm number when blending miktoarm stars with diblock copolymers (at constant arm/block lengths). Additionally, a rare clustered vesicle phase is detected. Hence, this approach permits an easy tuning of the equilibrium morphology and the size of the solvophobic domain. Such screening by scattering, ultracentrifugation, and electron microscopy techniques assists the targeted synthesis of miktoarm stars with a well-defined arm number, aimed at the morphology control of the nanostructures without blending. Specifically, we demonstrate a systematic variation of all classical micellar morphologies based on interpolyelectrolyte complexes (IPECs), consisting of a water-insoluble part formed by electrostatically coupled poly(styrenesulfonate) chains/quaternized poly(2-(dimethylamino)ethyl methacrylate) blocks, being stabilized by hydrophilic poly(ethylene oxide) blocks.

Chemical and mechanical modulation of polymeric micelle assembly

Recently, polymeric micelles self-assembled from amphiphilic polymers have been studied for various industrial and biomedical applications. This nanoparticle self-assembly typically occurs in a solvent-exchange process. In this process, the quality of the resulting particles is uncontrollably mediated by polymeric solubility and mixing conditions. Here, we hypothesized that improving the solubility of an amphiphilic polymer in an organic solvent via chemical modification while controlling the mixing rate of organic and aqueous phases would enhance control over particle morphology and size. We examined this hypothesis by synthesizing a poly(2-hydroxyethyl)aspartamide (PHEA) grafted with controlled numbers of octadecyl (C₁₈) chains and oligovaline groups (termed "oligovaline-PHEA-C₁₈"). The mixing rate of DMF and water was controlled either by microfluidic mixing of laminar DMF and water flows or through turbulent bulk mixing. Interestingly, oligovaline-PHEA-C₁₈ exhibited an increased solubility in DMF compared with PHEA-C₁₈, as demonstrated by an increase of mixing energy. In addition, increasing the mixing rate between water and DMF using the microfluidic mixer resulted in a decrease of the diameter of the resulting polymeric micelles, as compared with the particles formed from a bulk mixing process. Overall, these findings will expand the parameter space available to control particle self-assembly while also serving to improve existing nanoparticle processing techniques.

Continuous Production of Discrete Plasmid DNA-Polycation Nanoparticles Using Flash Nanocomplexation

Despite successful demonstration of linear polyethyleneimine (lPEI) as an effective carrier for a wide range of gene medicine, including DNA plasmids, small interfering RNAs, mRNAs, etc., and continuous improvement of the physical properties and biological performance of the polyelectrolyte complex nanoparticles prepared from lPEI and nucleic acids, there still exist major challenges to produce these nanocomplexes in a scalable manner, particularly for lPEI/DNA nanoparticles. This has significantly hindered the progress toward clinical translation of these nanoparticle-based gene medicine. Here the authors report a flash nanocomplexation (FNC) method that achieves continuous production of lPEI/plasmid DNA nanoparticles with narrow size distribution using a confined impinging jet device. The method involves the complex coacervation of negatively charged DNA plasmid and positive charged lPEI under rapid, highly dynamic, and homogeneous mixing conditions, producing polyelectrolyte complex nanoparticles with narrow distribution of particle size and shape. The average number of plasmid DNA packaged per nanoparticles and its distribution are similar between the FNC method and the small-scale batch mixing method. In addition, the nanoparticles prepared by these two methods exhibit similar cell transfection efficiency. These results confirm that FNC is an effective and scalable method that can produce well-controlled lPEI/plasmid DNA nanoparticles.

Tweaking dendrimers and dendritic nanoparticles for controlled nano-bio interactions: potential nanocarriers for improved cancer targeting

Nanoparticles have shown great promise in the treatment of cancer, with a demonstrated potential in targeted drug delivery. Among a myriad of nanocarriers that have been recently developed, dendrimers have attracted a great deal of scientific interests due to their unique chemical and structural properties that allow for precise engineering of their characteristics. Despite this, the clinical translation of dendrimers has been hindered due to their drawbacks, such as scale-up issues, rapid systemic elimination, inefficient tumor accumulation and limited drug loading. In order to overcome these limitations, a series of reengineered dendrimers have been recently introduced using various approaches, including: (i) modifications of structure and surfaces; (ii) integration with linear polymers and (iii) hybridization with other types of nanocarriers. Chemical modifications and surface engineering have tailored dendrimers to improve their pharmacokinetics and tissue permeation. Copolymerization of dendritic polymers with linear polymers has resulted in various amphiphilic copolymers with self-assembly capabilities and improved drug loading efficiencies. Hybridization with other nanocarriers integrates advantageous characteristics of both systems, which includes prolonged plasma circulation times and enhanced tumor targeting. This review provides a comprehensive summary of the newly emerging drug delivery systems that involve reengineering of dendrimers in an effort to precisely control their nano-bio interactions, mitigating their inherent weaknesses.

Nanoparticle stability in biologically relevant media: influence of polymer architecture

We have contrasted the behavior of nanoparticles formed by the self-assembly of polymers based on poly(ethylene glycol) (PEG) and poly(D,L-lactide), with linear, linear-dendritic and bottle-brush architectures in biologically relevant media. Polymer PEG content ranged between 14% and 46% w/w, and self-assembly was triggered by a rapid and large change in solvent quality inside a four-stream vortex mixer. We examined nanoparticle interaction with human serum albumin (HSA), and solute release in the presence of fetal bovine serum. Dynamic light scattering data showed that PEG surface brushes of all nanoparticles provided effective steric stabilization, thus limiting their interaction with human serum albumin. Calorimetric experiments revealed that nanoparticle-HSA interaction was relatively weak and enthalpically driven, whereas dynamic light scattering results of incubated nanoparticles showed the absence of larger aggregates for most of the polymers examined. Solute core partitioning was examined by the loss of Forster resonance energy transfer (FRET) from a core-loaded donor-acceptor pair. The rate and magnitude of FRET efficiency loss was strongly dependent on the polymer architecture, and was found to be lowest for the bottle-brush, attributed to its covalent nature. Collectively, these findings are expected to impact the molecular design of increasingly stable polymeric carriers for drug delivery applications.

Conjugated polymer composite nanoparticles by rapid mixing

Composite nanoparticles from poly[(9,9-di-n-octylfluoren-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) and poly(9,9-di-n-hexylfluoren-2,7-diyl) (PF) with embedded inorganic nanoparticles (TiO2 , CdSe, and CdSe/CdS) are prepared through kinetic trapping by rapid turbulent mixing in a multi-inlet vortex mixer without the need for polymer functionalization. High contents of inorganic materials up to 50-60 wt% are realized for all composites. The influence of flow ratios, sodium dodecyl sulfate (SDS) concentration, and absolute flow rates on the particle size and morphology is studied. High water-to-THF ratios and high total flow rates around 2 m s(-1) yield particle sizes below 50 nm. By adjusting these parameters, controlled particle sizes between 30 to several hundred nanometers are obtained. Composite particles from CdSe/CdS and F8BT or PF show a strong quenching of the polymer emission and near exclusive emission from the inorganic nanocrystal, which indicates an efficient energy transfer with fluorescence quantum yields of 23% for the F8BT/CdSe/CdS composites and 21% for the PF/CdSe/CdS composites. The dispersions are colloidally stable for several months.