Star Block Copolymers Research Articles

Tuning the morphology of silicon containing copolymers via macromolecular architecture effect

We demonstrate the successful synthesis of linear and non-linear star block copolymers consisting of the highly segregated polystyrene (PS) and poly(dimethylsiloxane) (PDMS) blocks through sequential anionic polymerization combined with chlorosilane chemistry. To study the macromolecular architecture effect on the adopted morphology star block copolymers with six equivalent diblock arms [(PS-b-PDMS)6] were synthesized and compared with their linear diblock copolymer precursors as well as with the corresponding star block copolymers with smaller arm number (2, 3, 4) but identical molecular characteristics. After verifying the molecular and thermal characteristics, bulk studies were carried out through transmission electron microscopy and small angle X-ray scattering. Well-defined structures with enhanced order and similar domain spacing values, irrespective of the architecture complexity of the samples, were observed. The better order of star block copolymers suggests an alternative way to further reduce the lower limit of repeating period by tuning the architecture which is important for the potential future use of the copolymers for nanotechnology related applications. The findings of the specific study are related to the effects of the increased conformational restriction of the inner block. Herein, we show morphological transformations in the (PS-b-PDMS)6 copolymers having non symmetric volume fraction ratios when compared to their linear diblock precursors and triblock copolymers which has not been observed in previous relevant (PS-b-PDMS)n studies that involved stars with less than six arms. To establish the structure/property relationship as a function of arm number, a series of samples involving the diblock precursor, the triblock, the three-, four- and six- arm star block copolymers was prepared, and a systematic comparison was conducted.

Preparation and controllable heavy oil viscosity reduction performance of pH-responsive star block copolymers

In this study, a series of S-PHEDM star block copolymers of hydrophilic N-hydroxyethyl acrylamide (HEAm) and pH-responsive N,N-dimethylaminoethyl methacrylate (DMAEMA) were synthesized by reversible addition-fragment chain transfer (RAFT) copolymerization for the controllable viscosity reduction of heavy oil. The structures and average molecular weights of the star polymers were characterized by FT-IR spectra, 1H NMR spectra and GPC measurements. The aggregation behavior and oil/water interfacial properties of S-PHEDM were studied using DLS, TEM and IFT measurements in aqueous solutions with various pH values. The properties of heavy oil emulsions formed by S-PHEDM were monitored under different pH conditions using Turbiscan Lab measurements and optical microscopy at ambient temperature, and cyclic regulation of emulsion stability was successfully achieved through regular adjustments of pH values in oil/water mixing systems. Then, S-PHEDM was applied to the field of heavy oil viscosity reduction. Therein, S-PHEDM-2 achieved a degree of viscosity reduction of 99.88% at a dosage of 1000 mg·L−1 for heavy oil (ηapp, 50 °C = 85820 mPa·s) under alkaline conditions. The dehydration rate of the formed emulsion was 25.58% after standing for 1 h. When the heavy oil mixing system was adjusted to acidic, the heavy oil emulsion demulsified rapidly and achieved a dehydration rate of 90.7% within 1 h. The flexible transformation between viscosity reduction and oil–water separation for the heavy oil/water mixing system by adjusting the pH values provided a new idea to enhance efficiency and reduce costs during heavy oil recovery.

Self-assembly modulation in star block copolymers by amphiphilic diol: A scattering insight

The present study offers a better insight into the alteration in the aggregation characteristics of ethylene oxide-propylene oxide (EO-PO)-based star block copolymers Tetronics® T1304 and T1307 induced by an amphiphilic diol Surfynol® 104 (hereafter C14 diol) using scattering (dynamic light scattering and small angle neutron scattering) with complementary physical property measurement viz. solution viscosity and cloud point (CP). It is the hydrophobic interaction that drives C14 diol molecules to penetrate inside copolymer micelles resulting in dehydration that lowered the CP and markedly increase in solution viscosity. Quite interestingly, initial lowering in CP is followed by sudden increase at higher level of solubilization. A significant increase in the apparent hydrodynamic diameter (Dh) divulges the growth of micelles which is equally supported by SANS measurements. The micellar parameters obtained from SANS analysis for T1304 and T1307 in the presence of C14 diol are described. The preferential partitioning of C14 diol into the copolymer micelles is the driving force for morphological changes from spherical to unilamellar vesicles. Also, the effect of temperature and NaCl was examined with the aim to observe various micellar transitions. The observed changes are clarified in terms of the hydrophobic interaction of C14 diol with Tetronic® micelles and HLB value of copolymers. The perception of C14 diol stabilised Tetronic® micelles is anticipated and correlated with optimal parameters obtained from computational simulation approach, providing a clear understanding of the correlation between the molecular orbital energy levels of Tetronic® and C14 diol. The present manuscript thus sheds light on C14 diol induced dehydration causing the micelle growth for both Tetronics® with varied hydrophobicity. Such hydrophobic diol-induced spherical to vesicular transition is observed for the first time in Tetronic®-diol mixed system.

Synthesis and morphological characterization of linear and miktoarm star poly(solketal methacrylate)-block-polystyrene copolymers

The synthesis, molecular characterization, and morphological evaluation of ABn, (n = 2,3) miktoarm star block copolymers consisting of poly(glycerol monomethacrylate) (PGMA) and polystyrene (PS) with varying molecular weights and compositions is described. The system is known to demonstrate a remarkably high Flory-Huggins interaction parameter. The corresponding linear diblock copolymer analogues, were synthesized as well, and comparisons with regard to feature dimensions and morphologies are provided. Well-ordered nanostructures of various morphologies were formed with domain spacing as low as 7.2 nm. Different morphologies were attained by some of the topological isomers indicating that in miktoarm star block copolymers the phase boundaries were strongly shifted. Additionally, a triblock ABA analogue was studied to investigate the effect of triblock copolymer conformations. Noteworthy is that for copolymers with different macromolecular architecture leading to similar morphology; different domain spacings were obtained. The synthesis of all the samples was carried out by high-vacuum anionic polymerization techniques. Molecular characterization with Size Exclusion Chromatography (SEC) and Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR) confirmed well-defined copolymers obtained. The morphological characterization was accomplished by Small-Angle X-ray Scattering (SAXS). The observations from this study highlight the potential of incorporating macromolecular architecture in the self-assembly of strongly immiscible block copolymers to attain ultra-small nanofeatures with desired morphologies.

Improved Solid Electrolyte Conductivity via Macromolecular Self-Assembly: From Linear to Star Comb-like P(S-co-BzMA)-b-POEGA Block Copolymers.

Star block copolymer electrolytes with a lithium-ion conducting phase are investigated in the present work to assess the influence of this complex architecture compared to that of the linear one, on both, bulk morphology and ionic conductivity. For that purpose, the controlled synthesis of a series of poly(styrene-co-benzyl methacrylate)-b-poly[oligo(ethylene glycol) methyl ether acrylate] [P(S-co-BzMA)-b-POEGA] block copolymers (BCPs) by reversible addition-fragmentation transfer polymerization was performed from either a monofunctional or a tetrafunctional chain transfer agent containing trithiocarbonate groups. We emphasized how a small amount of styrene (6 mol %) drastically improved the control of the RAFT polymerization of benzyl methacrylate mediated by the tetrafunctional chain transfer agent. Transmission electron microscopy and small-angle X-ray scattering demonstrated a clear segregation of the BCPs in the presence of lithium salt. Interestingly, the star BCPs gave rise to highly ordered lamellar structures as compared to that of the linear analogues. Consequently, the reduced lamellae tortuosity of self-assembled star BCPs improved the lithium conductivity by more than 8 times at 30 °C for ∼30 wt % of the POEGA conductive phase.

Assembly of polyelectrolyte star block copolymers at the oil-water interface.

To understand and resolve adsorption, reconfiguration, and equilibrium conformations of charged star copolymers, we carried out an integrated experimental and coarse-grained molecular dynamics simulation study of the assembly process at the oil-water interface. This is important to guide development of novel surfactants or amphiphiles for chemical transformations and separations. The star block copolymer consisted of arms that are comprised of hydrophilic-hydrophobic block copolymers that are covalently tethered via the hydrophobic blocks to one point. The hydrophobic core represents polystyrene (PS) chains, while the hydrophilic corona represents quaternized poly(2-vinylpyridine) (P2VP) chains. The P2VP is modeled to become protonated when in contact with an acidic aqueous phase, thereby massively increasing the hydrophilicity of this block, and changing the nature of the star at the oil-water interface. This results in a configurational change whereby the chains comprising the hydrophilic corona are significantly stretched into the aqueous phase, while the hydrophobic core remains solubilized in the oil phase. In the simulations, we followed the kinetics of the anchoring and assembly of the star block copolymer at the interface, monitoring the lateral assembly, and the subsequent reconfiguration of the star via changes in the interfacial tension that varies as the degree-of-protonation increases. At low fractions of protonation, the arm cannot fully partition into the aqueous side of the interface and instead interacts with other arms in the oil phase forming a network near the interface. These insights were used to interpret the non-monotonic dependence of pH with the asymptotic interfacial tension from pendant drop tensiometry experiments and spectral signatures of aromatic stretches seen in vibrational sum frequency generation (SFG) spectroscopy. We describe the relationship of interfacial tension to the star assembly via the Frumkin isotherm, which phenomenologically describes anti-cooperativity in adsorbing stars to the interface due to crowding. Although our model explicitly considers long-range electrostatics, the contribution of electrostatics to interfacial tension is small and brought about by strong counterion condensation at the interface. These results provide key insights into resolving the adsorption, reconfiguration, and equilibrium conformations of charged star block copolymers as surfactants.

Model dynamic covalent thermoresponsive amphiphilic polymer co-networks based on acylhydrazone end-linked Tetronic T904 star block copolymers

Hydrogels based on acylhydrazone end-linked Tetronic T904 four-armed amphiphilic star block copolymers show thermally-induced aqueous self-assembly and toughening, and cross-link dissociation and association traceableviasimple1H NMR spectroscopy.

Amphiphilic tetra-PCL-b-PEG star block copolymers using benzoxazinone-based linking groups

Two well-defined amphiphilic tetra-arm star block copolymers with a poly(ε-caprolactone) core and poly(ethylene glycol) arms were prepared by a hetero-complementary linkage reaction and studied with respect to their solution behavior in water.

Structural Engineering of Star Block Biodegradable Polymer Unimolecular Micelles for Drug Delivery in Cancer Cells.

The present investigation reports the structural engineering of biodegradable star block polycaprolactone (PCL) to tailor-make aggregated micelles and unimolecular micelles to study their effect on drug delivery aspects in cancer cell lines. Fully PCL-based star block copolymers were designed by varying the arm numbers from two to eight while keeping the arm length constant throughout. Multifunctional initiators were exploited for stepwise solvent-free melt ring-opening polymerization of ε-caprolactone and γ-substituted caprolactone to construct star block copolymers having a PCL hydrophobic core and a carboxylic PCL hydrophilic shell, respectively. A higher arm number and a higher degree of branching in star polymers facilitated the formation of unimolecular micelles as opposed to the formation of conventional multimicellar aggregates in lower arm analogues. The dense core of the unimolecular micelles enabled them to load high amounts of the anticancer drug doxorubicin (DOX, ∼12-15%) compared to the aggregated micelles (∼3-4%). The star unimolecular micelle completely degraded leading to 90% release of the loaded drug upon treatment with the lysosomal esterase enzyme in vitro. The anticancer efficacies of these DOX-loaded unimolecular micelles were tested in a breast cancer cell line (MCF-7), and their IC50 values were found to be much lower compared to those of aggregated micelles. Time-dependent cellular uptake studies by confocal microscopy revealed that unimolecular micelles were readily taken up by the cells, and enhancement of the drug concentration was observed at the intracellular level up to 36 h. The present work opens new synthetic strategies for building a next-generation biodegradable unimolecular micellar nanoplatform for drug delivery in cancer research.

Dual-Crosslinked Degradable Elastomeric Networks With Self-Healing Properties: Bringing Multi(catechol) Star-Block Copolymers into Play

In the biomedical field, degradable chemically crosslinked elastomers are interesting materials for tissue engineering applications, since they present rubber-like mechanical properties matching those of soft tissues and are able to preserve their three-dimensional (3D) structure over degradation. Their use in biomedical applications requires surgical handling and implantation that can be a source of accidental damages responsible for the loss of properties. Therefore, their inability to be healed after damage or breaking can be a major drawback. In this work, biodegradable dual-crosslinked networks that exhibit fast and efficient self-healing properties at 37 °C are designed. Self-healable dual-crosslinked (chemically and physically) elastomeric networks are prepared by two methods. The first approach is based on the mix of hydrophobic poly(ethylene glycol)-poly(lactic acid) (PEG-PLA) star-shaped copolymers functionalized with either catechol or methacrylate moieties. In the second approach, hydrophobic bifunctional PEG-PLA star-shaped copolymers with both catechol and methacrylate on their structure are used. In the two systems, the supramolecular network is responsible for the self-healing properties, thanks to the dynamic dissociation/reassociation of the numerous hydrogen bonds between the catechol groups, whereas the covalent network ensures mechanical properties similar to pure methacrylate networks. The self-healable materials display mechanical properties that are compatible with soft tissues and exhibit linear degradation because of the chemical cross-links. The performances of networks from mixed copolymers versus bifunctional copolymers are compared and demonstrate the superiority of the latter. The biocompatibility of the materials is also demonstrated, confirming the potential of these degradable and self-healable elastomeric networks to be used for the design of temporary medical devices.

Tailored Polypeptide Star Copolymers for 3D Printing of Bacterial Composites Via Direct Ink Writing.

Hydrogels hold much promise for 3D printing of functional living materials; however, challenges remain in tailoring mechanical robustness as well as biological performance. In addressing this challenge, the modular synthesis of functional hydrogels from 3-arm diblock copolypeptide stars composed of an inner poly(l-glutamate) domain and outer poly(l-tyrosine) or poly(l-valine) blocks is described. Physical crosslinking due to ß-sheet assembly of these star block copolymers gives mechanical stability during extrusion printing and the selective incorporation of methacrylate units allows for subsequent photocrosslinking to occur under biocompatible conditions. This permits direct ink writing (DIW) printing of bacteria-based mixtures leading to 3D objects with high fidelity and excellent bacterial viability. The tunable stiffness of different copolypeptide networks enables control over proliferation and colony formation for embedded Escherichia coli bacteria as demonstrated via isopropyl ß-d-1-thiogalactopyranoside (IPTG) induction of green fluorescent protein (GFP) expression. This translation of molecular structure to network properties highlights the versatility of these polypeptide hydrogel systems with the combination of writable structures and biological activity illustrating the future potential of these 3D-printed biocomposites.

Self-assembly and complex formation of amphiphilic star and bottle-brush block copolymers.

We use molecular dynamics computer simulations to investigate complexation and clustering of branched polymers. In this study, we focus on star and bottle-brush polymers. In our investigation, we identify key factors governing cluster formation of branched amphiphilic polymers and provide guidance for designing and preparing various types of polymer clusters for applications, e.g., in drug delivery or materials science. We observe different kinds of clustering in the polymer systems. Our star polymers consist of several arms of hydrophilic core particles with hydrophobic particles attached to the end of each star arm. We observe that amphiphilic star polymers generally tend to form spherical complexes. In contrast to this, bottle-brush polymers exhibit a larger variety of complex structures. With large grafting density and large side arms, we also observe spherical polymer clusters; however, for low grafting density and shorter side chains, distinct clusters connected by bridging particles are formed. Furthermore, we observe membrane-like clustering of bottle-brush polymers. We employ two different clustering algorithms for further analysis of the obtained structures with respect to shape factors, pair correlation functions, and radii of gyration. We find that the hydrophobic parts of polymers play a crucial role in the formation of the resulting structures during self-assembly. The hydrophilic core parts in star polymers along with steric hindrance lead to a screening effect for the hydrophobic parts of the polymers. With bottle-brush copolymers, the hydrophilic parts of the polymers exhibit a screening effect that is sensitive to the grafting density and side chain lengths along the backbone.

The Effect of Topology on Block Copolymer Nanoparticles: Linear versus Star Block Copolymers in Toluene.

Linear and star block copolymer (BCP) nanoparticles of (polystyrene-block-poly(4-vinylpyridine))n (PS-b-P4VP)n with arm numbers of 1, 2, 3, and 4 were prepared by two methods of polymerization-induced self-assembly (PISA) and general self-assembly of block copolymers in the low-polar organic solvent, toluene. The effect of the arm number on the size and/or morphology of the (PS-b-P4VP)n nanoassemblies synthesized by the two methods in toluene and on the polymerization kinetics was investigated in detail. Our results show that in toluene, a low-polar solvent, the topology not only affected the morphology of the BCP nanoparticles prepared by PISA, but also influenced the BCP nanoparticles synthesized through general self-assembly.

A meticulous study on the interaction of bile salts with star block copolymeric micelles

Here we report, the interaction of a star block copolymer T1304 and biologically important additive, bile salts (BS) in aqueous condition. Bile salts used for the system are sodium cholate (NaC), sodium taurocholate (NaTC) and sodium deoxycholate (NaDC). Mixed micelles of T1304-BS have been assessed by employing cloud point (CP), scattering [dynamic light scattering (DLS), small-angle neutron scattering (SANS)] and spectral techniques [UV–visible, 1H NMR, and fluorescence spectroscopy]. As the system is comprising of two important bio amphiphiles, so the solubilization of drug Quercetin (QCT) in this mixed micellar environment is also studied to give it a broader spectrum. An increased trend in CP along with a decrease of micellar size is observed with the addition of BSs. With increasing concentration of bile salts, the mixed micellar aggregates are becoming surfactant rich and maximum replacement of T1304 molecule is done by NaDC due to its lower CMC and more hydrophobicity.

Linear and Star Block Copolymer Nanoparticles Prepared by Heterogeneous RAFT Polymerization Using an ω,ω-Heterodifunctional Macro-RAFT Agent.

Herein, an ω,ω-heterodifunctional macromolecular reversible addition-fragmentation chain transfer (macro-RAFT) agent containing two different RAFT end groups was synthesized and employed to mediate aqueous photoinitiated RAFT dispersion polymerization of a methacrylic monomer. Because of the different RAFT controllability of two RAFT end groups toward methacrylic monomers, the RAFT end group with good controllability dominated the polymerization while the other RAFT end group with poor controllability was unreacted, leading to the formation of linear block copolymers. Because of the unique structure of the linear block copolymers, a diverse set of block copolymer nanoparticles with rich RAFT groups at the interface of the hydrophilic corona/the hydrophobic core were successfully prepared. Finally, μ-A(BC)C miktoarm star block copolymer nanoparticles were prepared by RAFT seeded emulsion polymerization of an acrylic monomer, which enables the further morphological control over polymer nanoparticles. We believe that the utilization of an ω,ω-heterodifunctional macro-RAFT agent in heterogeneous RAFT polymerization will offer considerable opportunities for the rational synthesis of well-defined molecular architectures and polymer nanoparticles.

Influence of CNT Length on Dispersion, Localization, and Electrical Percolation in a Styrene-Butadiene-Based Star Block Copolymer.

This study followed the approach of dispersing and localizing carbon nanotubes (CNTs) in nanostructured domains of block copolymers (BCPs) by shortening the CNTs via ball milling. The aim was to selectively tune the electrical and mechanical properties of the resulting nanocomposites, e.g., for use as sensor materials. Multiwalled carbon nanotubes (MWCNTs) were ground into different size fractions. The MWCNT length distribution was evaluated via transmission electron microscopy and dynamic light scattering. The nanostructure of the BCPs and the glass transition temperatures of the PB-rich and PS phases were not strongly affected by the addition of CNTs up to 2 wt%. However, AFM and TEM investigations indicated a partial localization of the shortened CNTs in the soft PB-rich phase or at the interface of the PB-rich and PS phase, respectively. The stress-strain behavior of the solution-mixed composites differed little from the mechanical property profile of the neat BCP and was largely independent of CNT amount and CNT size fraction. Significant changes could only be observed for Young’s modulus and strain at break and may be attributed to CNT localization and small changes in morphology. For nanocomposites with unmilled CNTs, the electrical percolation threshold was less than 0.1 wt%. As the CNTs were shortened, the resistivity increased and the percolation threshold shifted to higher CNT contents. Composites with CNTs ground for 7.5 h and 13.5 h showed no bulk conductivity but significantly decreased surface resistivity on the bottom side of the films, which could be attributed to a sedimentation process of the grind and thereby highly compressed CNT agglomerates during evaporation.

Influence of sodium salts on the phase and gelation behaviour of T1107 to be used as proposed polymer gel electrolyte

Aqueous electrolyte presents in batteries have high ionic conductivity. They are economic, non-flammable and offer high energy density. However, restricted thermodynamic electrochemical stable potential window of water and its spilling suppresses the use of these batteries. To address these problems, development of a polymer-based electrolyte system is an upcoming area of research. Even, the uses of sodium salts in battery electrolytes are gaining importance over lithium salts due to their abundance and economic feasibility. The present study examines the self-aggregation of star block copolymer Tetronic® 1107 in aqueous medium in the presence of sodium salts viz. sodium chloride, sodium fluoride, sodium nitrate, sodium hexafluoro phosphate and disodium hydrogen phosphate with different anions by cloud point (CP), gelation, dynamic light scattering (DLS) and small angle neutron scattering (SANS). Conductivity and DC polarization measurements are also performed for polymeric systems in the absence and also in the presence of these salts. The gelation behaviour of the block copolymer to reduce leakage problems and proper ionic conductivity of it in the presence of sodium salts make the system a credible choice as a polymer gel electrolyte (PGE).

Thermo/pH dual-responsive micelles based on the host\u2013guest interaction between benzimidazole-terminated graft copolymer and \u03b2-cyclodextrin-functionalized star block copolymer for smart drug delivery

Novel temperature and pH dual-sensitive amphiphilic micelles were fabricated exploiting the host–guest interaction between benzimidazole-terminated PHEMA-g-(PCL-BM) and β-CD-star-PMAA-b-PNIPAM. The fabricated graft copolymer had a brush-like structure with star side chains. The micelles were utilized as dual-responsive nanocarriers and showed the LCST between 40 and 41 °C. The acidic pH promoted the dissociation of the PHEMA-g-(PCL-BM: β-CD-star-PMAA-b-PNIPAM) micelles. DOX.HCl was loaded into the core of the micelles during self-assembly in an aqueous solution with a high encapsulation efficacy (97.3%). The average size of the amphiphilic micelles was about 80 nm, suitable size for the enhanced permeability and retention effect in tumor vasculature. In an aqueous environment, these micelles exhibited very good self-assembly ability, low CMC value, rapid pH- and thermo-responsiveness, optimal drug loading capacity, and effective release of the drug. The biocompatibility was confirmed by the viability assessment of human breast cancer cell line (MCF-7) through methyl tetrazolium assay. DOX-loaded micelles displayed excellent anti-cancer activity performance in comparison with free DOX.Graphical

Is the “Bricks-and-Mortar” Mesophase Bicontinuous? Dynamic Simulations of Miktoarm Block Copolymer/Homopolymer Blends

A new mesophase in binary blends of A–b–(BA′)3 miktoarm star block copolymers and A homopolymers has recently been discovered experimentally and explored with field-theoretic simulations. This mesophase has been reported to consist of aperiodic discrete domains of A embedded in a continuous matrix of B up to very high concentrations of A. Because of the material’s potential as a thermoplastic elastomer, a deeper understanding of its structural and dynamic-mechanical properties, including its domain connectivity, linear rheological behavior, response to shear, and response to uniaxial tension, is warranted. These properties are explored here using dissipative particle dynamics in three dimensions, for the first time. These simulations establish that the so-called “bricks-and-mortar” phase, while appearing discrete in two dimensions, is bicontinuous. The simulations focusing on dynamics establish that the role of molecular bridging dominates the mechanical behavior and outweighs the influence of microphase segregation (contributions from the interfacial tension alone) even at the highest homopolymer concentrations we study. Additionally, it appears that the bricks-and-mortar phase is sensitive to the application of sufficiently high shear, leading to nonisotropic mechanical responses, which has ramifications for the processability of such materials. We find that upon application of shear the phase becomes closer in structure to its speculated discrete nature. Molecular simulations on our longest accessible timescales show that the material is unable to relax back to its original structure, suggesting that the morphology observed depends heavily on the material process pathway.

Correction: Double hydrophilic poly(ethylene glycol)-block-poly(dehydroalanine) four-arm star block copolymers: synthesis and solution behavior

Correction for ‘Double hydrophilic poly(ethylene glycol)-block-poly(dehydroalanine) four-arm star block copolymers: synthesis and solution behavior’ by Peter J. Mons et al., Polym. Chem., 2022, 13, 4298–4308, https://doi.org/10.1039/D2PY00579D.