Preparation and Cross-Linking of All-Acrylamide Diblock Copolymer Nano-Objects via Polymerization-Induced Self-Assembly in Aqueous Solution

Modern Trends in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) is a powerful and versatile technique for producing colloidal dispersions of block copolymer particles with desired morphologies. Currently, PISA can be carried out in various media, over a wide range of temperatures, and using different mechanisms. This method enables the production of biodegradable objects and particles with various functionalities and stimuli sensitivity. Consequently, PISA offers a broad spectrum of potential commercial applications. The aim of this review is to provide an overview of the current state of rational synthesis of block copolymer particles with diverse morphologies using various PISA techniques and mechanisms. The discussion begins with an examination of the main thermodynamic, kinetic, and structural aspects of block copolymer micellization, followed by an exploration of the key principles of PISA in the formation of gradient and block copolymers. The review also delves into the main mechanisms of PISA implementation and the principles governing particle morphology. Finally, the potential future developments in PISA are considered.

The Production of Polysarcosine-Containing Nanoparticles by Ring-Opening Polymerization-Induced Self-Assembly

N-carboxyanhydride ring-opening polymerization-induced self-assembly (NCA ROPISA) offers a convenient route for generating poly(amino acid)-based nanoparticles in a single step, crucially avoiding the need for post-polymerization self-assembly. Most examples of NCA ROPISA make use of a poly(ethylene glycol) (PEG) hydrophilic stabilizing block, however this non-biodegradable, oil-derived polymer may cause an immunological response in some individuals. Alternative water-soluble polymers are therefore highly sought. This work reports the synthesis of wholly poly(amino acid)-based nanoparticles, through the chain-extension of a polysarcosine macroinitiator with L-Phenylalanine-NCA (L-Phe-NCA) and Alanine-NCA (Ala-NCA), via aqueous NCA ROPISA. The resulting polymeric structures comprise of predominantly anisotropic, rod-like nanoparticles, with morphologies primarily influenced by the secondary structure of the hydrophobic poly(amino acid) that enables their formation.

Low-Viscosity Route to High-Molecular-Weight Water-Soluble Polymers: Exploiting the Salt Sensitivity of Poly(N-acryloylmorpholine)

We report a new one-pot low-viscosity synthetic route to high molecular weight non-ionic water-soluble polymers based on polymerization-induced self-assembly (PISA). The RAFT aqueous dispersion polymerization of N-acryloylmorpholine (NAM) is conducted at 30 °C using a suitable redox initiator and a poly(2-hydroxyethyl acrylamide) (PHEAC) precursor in the presence of 0.60 M ammonium sulfate. This relatively low level of added electrolyte is sufficient to salt out the PNAM block, while steric stabilization is conferred by the relatively short salt-tolerant PHEAC block. A mean degree of polymerization (DP) of up to 6000 was targeted for the PNAM block, and high NAM conversions (>96%) were obtained in all cases. On dilution with deionized water, the as-synthesized sterically stabilized particles undergo dissociation to afford molecularly dissolved chains, as judged by dynamic light scattering and 1H NMR spectroscopy studies. DMF GPC analysis confirmed a high chain extension efficiency for the PHEAC precursor, but relatively broad molecular weight distributions were observed for the PHEAC-PNAM diblock copolymer chains (Mw/Mn > 1.9). This has been observed for many other PISA formulations when targeting high core-forming block DPs and is tentatively attributed to chain transfer to polymer, which is well known for polyacrylamide-based polymers. In fact, relatively high dispersities are actually desirable if such copolymers are to be used as viscosity modifiers because solution viscosity correlates closely with Mw. Static light scattering studies were also conducted, with a Zimm plot indicating an absolute Mw of approximately 2.5 × 106 g mol-1 when targeting a PNAM DP of 6000. Finally, it is emphasized that targeting such high DPs leads to a sulfur content for this latter formulation of just 23 ppm, which minimizes the cost, color, and malodor associated with the organosulfur RAFT agent.

Synthesis and Characterization of All-Acrylic Tetrablock Copolymer Nanoparticles: Waterborne Thermoplastic Elastomers via One-Pot RAFT Aqueous Emulsion Polymerization

Reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization is used to prepare well-defined ABCB tetrablock copolymer nanoparticles via sequential monomer addition at 30 °C. The A block comprises water-soluble poly(2-(N-acryloyloxy)ethyl pyrrolidone) (PNAEP), while the B and C blocks comprise poly(t-butyl acrylate) (PtBA) and poly(n-butyl acrylate) (PnBA), respectively. High conversions are achieved at each stage, and the final sterically stabilized spherical nanoparticles can be obtained at 20% w/w solids at pH 3 and at up to 40% w/w solids at pH 7. A relatively long PnBA block is targeted to ensure that the final tetrablock copolymer nanoparticles form highly transparent films on drying such aqueous dispersions at ambient temperature. The kinetics of polymerization and particle growth are studied using 1H nuclear magnetic resonance spectroscopy, dynamic light scattering, and transmission electron microscopy, while gel permeation chromatography analysis confirmed a high blocking efficiency for each stage of the polymerization. Differential scanning calorimetry and small-angle X-ray scattering studies confirm microphase separation between the hard PtBA and soft PnBA blocks, and preliminary mechanical property measurements indicate that such tetrablock copolymer films exhibit promising thermoplastic elastomeric behavior. Finally, it is emphasized that targeting an overall degree of polymerization of more than 1000 for such tetrablock copolymers mitigates the cost, color, and malodor conferred by the RAFT agent.

Continuous-Flow Laboratory SAXS for In Situ Determination of the Impact of Hydrophilic Block Length on Spherical Nano-Object Formation during Polymerization-Induced Self-Assembly

In situ small-angle X-ray scattering (SAXS) is a powerful technique for characterizing block-copolymer nano-object formation during polymerization-induced self-assembly. To work effectively in situ, it requires high intensity X-rays which enable the short acquisition times required for real-time measurements. However, routine access to synchrotron X-ray sources is expensive and highly competitive. Flow reactors provide an opportunity to obtain temporal resolution by operating at a consistent flow rate. Here, we equip a flow-reactor with an X-ray transparent flow-cell at the outlet which facilitates the use of a low-flux laboratory SAXS instrument for in situ monitoring. The formation and morphological evolution of spherical block copolymer nano-objects was characterized during reversible addition fragmentation chain transfer polymerization of diacetone acrylamide in the presence of a series of poly(dimethylacrylamide) (PDMAm) macromolecular chain transfer agents with varying degrees of polymerization. SAXS analysis indicated that during the polymerization, highly solvated, loosely defined aggregates form after approximately 100 s, followed by expulsion of solvent to form well-defined spherical particles with PDAAm cores and PDMAm stabilizer chains, which then grow as the polymerization proceeds. Analysis also indicates that the aggregation number (Nagg) increases during the reaction, likely due to collisions between swollen, growing nanoparticles. In situ SAXS conducted on PISA syntheses using different PDMAm DPs indicated a varying conformation of the chains in the particle cores, from collapsed chains for PDMAm47 to extended chains for PDMAm143. At high conversion, the final Nagg decreased as a function of increasing PDMAm DP, indicating increased steric stabilization afforded by the longer chains which is reflected by a decrease in both core diameter (from SAXS) and hydrodynamic diameter (from DLS) for a constant core DP of 400.

Histidine-Functionalized Diblock Copolymer Nanoparticles Exhibit Enhanced Adsorption onto Planar Stainless Steel

RAFT aqueous emulsion polymerization of isopropylideneglycerol monomethacrylate (IPGMA) is used to prepare a series of PGEO5MA46 -PIPGMAy nanoparticles, where PGEO5MA is a hydrophilic methacrylic steric stabilizer block bearing pendent cis-diol groups. TEM studies confirm a spherical morphology while dynamic light scattering (DLS) analysis indicated that the z-average particle diameter can be adjusted by varying the target degree of polymerization for the core-forming PIPGMA block. Periodate oxidation is used to convert the cis-diol groups on PGEO5MA46 -PIPGMA500 and PGEO5MA46 -PIPGMA1000 nanoparticles into the analogous aldehyde-functionalized nanoparticles, which are then reacted with histidine via reductive amination. In each case, the extent of functionalization is more than 99% as determined by 1 H NMR spectroscopy. Aqueous electrophoresis studies indicate that such derivatization converts initially neutral nanoparticles into zwitterionic nanoparticles with an isoelectric point at pH 7. DLS studies confirm that such histidine-derivatized nanoparticles remain colloidally stable over a wide pH range. A quartz crystal microbalance is employed at 25°C to assess the adsorption of both the cis-diol- and histidine-functionalized nanoparticles onto planar stainless steel at pH 6. The histidine-bearing nanoparticles adsorb much more strongly than their cis-diol counterparts. For the highest adsorbed amount of 70.5 mg m-2 , SEM indicates a fractional surface coverage of 0.23 for the adsorbed nanoparticles.

Covalent Adaptable Polymethacrylate Networks by Hydrazide Crosslinking Via Isosorbide Levulinate Side Groups

Reversible crosslinking offers an attractive strategy to modify and improve the properties of polymer materials while concurrently enabling a pathway for chemical recycling. This can, for example, be achieved by incorporating a ketone functionality into the polymer structure to enable post-polymerization crosslinking with dihydrazides. The resulting covalent adaptable network contains acylhydrazone bonds cleavable under acidic conditions, thereby providing reversibility. In the present work, we regioselectively prepare a novel isosorbide monomethacrylate with a pendant levulinoyl group via a two-step biocatalytic synthesis. Subsequently, a series of copolymers with different contents of the levulinic isosorbide monomer and methyl methacrylate are prepared by radical polymerization. Using dihydrazides, these linear copolymers are then crosslinked via reaction with the ketone groups in the levulinic side chains. Compared to the linear prepolymers, the crosslinked networks exhibit enhanced glass transition temperatures and thermal stability, up to 170 and 286 °C, respectively. Moreover, the dynamic covalent acylhydrazone bonds are efficiently and selectively cleaved under acidic conditions to retrieve the linear polymethacrylates. We next show that recovered polymers can again be crosslinked with adipic dihydrazide, thus demonstrating the circularity of the materials. Consequently, we envision that these novel levulinic isosorbide-based dynamic polymethacrylate networks have great potential in the field of recyclable and reusable biobased thermoset polymers.

Multifunctional Layer-by-Layer Coating Based on a New Amphiphilic Block Copolymer via RAFT-Mediated Polymerization-Induced Self-Assembly Process

In this hi-tech world, the "smart coatings" have sparked significant attention among materials scientists because of their versatile applications. Various strategies have been developed to generate smart coatings in the past 2 decades. The layer-by-layer (LbL) technique is the most commonly employed strategy to produce a smart coating for suitable applications. Here, we present a smart coating with healing, antifogging, and fluorescence properties fabricated by the LbL assembly of an anionic amphiphilic block copolymer latex and cationic inorganic POSS (polyhedral-oligomeric-silsesquioxane) nanoparticles. In this case, a new anionic block copolymer (BCP), {poly(sodium styrene sulfonate)-block-poly[2-(acetoacetoxy)ethyl methacrylate]}, (PSS-b-PAAEMA) was synthesized via surfactant-free RAFT-mediated emulsion polymerization using the PISA technique. The PSS-b-PAAEMA was characterized by ¹H NMR, dynamic light scattering, scanning electron microscopy, and transmission electron microscopy analyses as well as by UV-vis and photoluminescence spectroscopy. For LbL coating fabrication, an amine-modified glass was successively dipped in the anionic latex and cationic POSS solution. The transparent coating exhibited good fluorescence properties under UV light (blue color). The antifogging performance of the coating was also investigated using both cold-warm and hot-vapor techniques. Additionally, the coating surface showed a significant healing activity with a healing efficiency of >75% through ionic interaction. Thus, this finding provides a simple low volatile organic compound (VOC) water-based LbL coating with multifunctional properties that can be a potential material for versatile applications.

Polymerization-Induced Self-Assembly for Efficient Fabrication of Biomedical Nanoplatforms

Amphiphilic copolymers can self-assemble into nano-objects in aqueous solution. However, the self-assembly process is usually performed in a diluted solution (<1 wt%), which greatly limits scale-up production and further biomedical applications. With recent development of controlled polymerization techniques, polymerization-induced self-assembly (PISA) has emerged as an efficient approach for facile fabrication of nano-sized structures with a high concentration as high as 50 wt%. In this review, after the introduction, various polymerization method-mediated PISAs that include nitroxide-mediated polymerization-mediated PISA (NMP-PISA), reversible addition-fragmentation chain transfer polymerization-mediated PISA (RAFT-PISA), atom transfer radical polymerization-mediated PISA (ATRP-PISA), and ring-opening polymerization-mediated PISA (ROP-PISA) are discussed carefully. Afterward, recent biomedical applications of PISA are illustrated from the following aspects, i.e., bioimaging, disease treatment, biocatalysis, and antimicrobial. In the end, current achievements and future perspectives of PISA are given. It is envisioned that PISA strategy can bring great chance for future design and construction of functional nano-vehicles.

Shape-Shifting Thermoresponsive Block Copolymer Nano-Objects

In this Feature Article, we review our recent progress in the design of shape-shifting thermoresponsive diblock copolymer nano-objects, which are prepared using various hydroxyl-functional (meth)acrylic monomers (e.g. 2‑hydroxypropyl methacrylate, 4‑hydroxybutyl acrylate or hydroxybutyl methacrylate) to generate the thermoresponsive block. Unlike traditional thermoresponsive polymers such as poly(N-isopropylacrylamide), there is no transition between soluble and insoluble polymer chains in aqueous solution. Instead, thermally driven transitions between a series of copolymer morphologies (e.g. spheres, worms, vesicles or lamellae) occur on adjusting the aqueous solution temperature owing to a subtle change in the partial degree of hydration of the permanently insoluble thermoresponsive block. Such remarkable self-assembly behavior is unprecedented in colloid science: no other amphiphilic diblock copolymer or surfactant system undergoes such behavior at a fixed chemical composition and concentration. Such shape-shifting nano-objects are characterized by transmission electron microscopy, dynamic light scattering, small-angle X-ray scattering, rheology and variable temperature ¹H NMR spectroscopy. Potential applications for this fascinating new class of amphiphiles are briefly considered.

Tunable Polymer Nanoreactors from RAFT Polymerization-Induced Self-Assembly: Fabrication of Nanostructured Carbon-Coated Anatase as Battery Anode Materials with Variable Morphology and Porosity

We demonstrate a modular synthesis approach to yield mesoporous carbon-coated anatase (denoted as TiO₂/C) nanostructures. Combining polymerization-induced self-assembly (PISA) and reversible addition-fragmentation chain-transfer (RAFT) dispersion polymerization enabled the fabrication of uniform core-shell polymeric nanoreactors with tunable morphologies. The nanoreactors comprised of a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell and a poly(benzyl methacrylate) (PBzMA) core. We selected worm-like and vesicular morphologies to guide the nanostructuring of a TiO₂ precursor, namely, titanium(IV) bis(ammonium lactato)dihydroxide (TALH). Subsequent carbonization yielded nanocrystalline anatase and simultaneously introduced a porous carbon framework, which also suppressed the crystal growth (∼5 nm crystallites). The as-prepared TiO₂/C materials comprised of a porous structure, with large specific surface areas (>85 m²/g) and various carbon contents (20-30 wt %). As anode components in lithium-ion batteries, our TiO₂/C nanomaterials improved the cycling stability, facilitated high overall capacities, and minimized the capacity loss compared to both their sans carbon and commercial anatase analogues.

Unimer Exchange Is not Necessary for Morphological Transitions in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that-unexpectedly-unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.

Polymer nanoparticles pass the plant interface

As agriculture strives to feed an ever-increasing number of people, it must also adapt to increasing exposure to minute plastic particles. To learn about the accumulation of nanoplastics by plants, we prepared well-defined block copolymer nanoparticles by aqueous dispersion polymerisation. A fluorophore was incorporated via hydrazone formation and uptake into roots and protoplasts of Arabidopsis thaliana was investigated using confocal microscopy. Here we show that uptake is inversely proportional to nanoparticle size. Positively charged particles accumulate around root surfaces and are not taken up by roots or protoplasts, whereas negatively charged nanoparticles accumulate slowly and become prominent over time in the xylem of intact roots. Neutral nanoparticles penetrate rapidly into intact cells at the surfaces of plant roots and into protoplasts, but xylem loading is lower than for negative nanoparticles. These behaviours differ from those of animal cells and our results show that despite the protection of rigid cell walls, plants are accessible to nanoplastics in soil and water.

Synthesis of High Molecular Weight Water-Soluble Polymers as Low-Viscosity Latex Particles by RAFT Aqueous Dispersion Polymerization in Highly Salty Media

We report the synthesis of sterically-stabilized diblock copolymer particles at 20% w/w solids via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of N,N′-dimethylacrylamide (DMAC) in highly salty media (2.0 M (NH₄)₂SO₄). This is achieved by selecting a well-known zwitterionic water-soluble polymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), to act as the salt-tolerant soluble precursor block. A relatively high degree of polymerization (DP) can be targeted for the salt-insoluble PDMAC block, which leads to the formation of a turbid free-flowing dispersion of PDMAC-core particles by a steric stabilization mechanism. ¹H NMR spectroscopy studies indicate that relatively high DMAC conversions (>99%) can be achieved within a few hours at 30 °C. Aqueous GPC analysis indicates high blocking efficiencies and unimodal molecular weight distributions, although dispersities increase monotonically as higher degrees of polymerization (DPs) are targeted for the PDMAC block. Particle characterization techniques include dynamic light scattering (DLS) and electrophoretic light scattering (ELS) using a state-of-the-art instrument that enables accurate ζ potential measurements in a concentrated salt solution. ¹H NMR spectroscopy studies confirm that dilution of the as-synthesized dispersions using deionized water lowers the background salt concentration and hence causes in situ molecular dissolution of the salt-intolerant PDMAC chains, which leads to a substantial thickening effect and the formation of transparent gels. Thus, this new polymerization-induced self-assembly (PISA) formulation enables high molecular weight water-soluble polymers to be prepared in a highly convenient, low-viscosity form. In principle, such aqueous PISA formulations are highly attractive: there are various commercial applications for high molecular weight water-soluble polymers, while the well-known negative aspects of using a RAFT agent (i.e., its cost, color, and malodor) are minimized when targeting such high DPs.

Recent insights into polysaccharide-based hydrogels and their potential applications in food sector: A review

Hydrogels are ideal for various food applications because of their softness, elasticity, absorbent nature, flexibility, and hygroscopic nature. Polysaccharide hydrogels are particularly suitable because of the hydrophilic nature, their food compatibility, and their non-immunogenic character. Such hydrogels offer a wide range of successful applications such as food preservation, pharmaceuticals, agriculture, and food packaging. Additionally, polysaccharide hydrogels have proven to play a significant role in the formulation of food flavor carrier systems, thus diversifying the horizons of newer developments in food processing sector. Polysaccharide hydrogels are comprised of natural polymers such as alginate, chitosan, starch, pectin and hyaluronic acid when crosslinked physically or chemically. Hydrogels with interchangeable, antimicrobial and barrier properties are referred to as smart hydrogels. This review brings together the recent and relevant polysaccharide research in these polysaccharide hydrogel applications areas and seeks to point the way forward for future research and interventions. Applications in carrying out the process of flavor carrier system directly through their incorporation in food matrices, broadening the domain for food application innovations. The classification and important features of polysaccharide-based hydrogels in food processing are the topics of the current review study.

Oxygen-Tolerant RAFT Polymerization Initiated by Living Bacteria

Living organisms can synthesize a wide range of macromolecules from a small set of natural building blocks, yet there is potential for even greater materials diversity by exploiting biochemical processes to convert unnatural feedstocks into new abiotic polymers. Ultimately, the synthesis of these polymers in situ might aid the coupling of organisms with synthetic matrices, and the generation of biohybrids or engineered living materials. The key step in biohybrid materials preparation is to harness the relevant biological pathways to produce synthetic polymers with predictable molar masses and defined architectures under ambient conditions. Accordingly, we report an aqueous, oxygen-tolerant RAFT polymerization platform based on a modified Fenton reaction, which is initiated by Cupriavidus metallidurans CH34, a bacterial species with iron-reducing capabilities. We show the synthesis of a range of water-soluble polymers under normoxic conditions, with control over the molar mass distribution, and also the production of block copolymer nanoparticles via polymerization-induced self-assembly. Finally, we highlight the benefits of using a bacterial initiation system by recycling the cells for multiple polymerizations. Overall, our method represents a highly versatile approach to producing well-defined polymeric materials within a hybrid natural-synthetic polymerization platform and in engineered living materials with properties beyond those of biotic macromolecules.

Long-Term Stability of Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles: Effect of Nanoparticle Core Crosslinking, Oil Type, and the Role Played by Excess Copolymers

A poly(N,N'-dimethylacrylamide) (PDMAC) precursor is chain-extended via reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of diacetone acrylamide (PDAAM) to produce PDMAC₇₇-PDAAM₄₀ spherical nanoparticles. Post-polymerization core-crosslinking of such nanoparticles was performed at 20 °C, and the resulting covalently stabilized nanoparticles survive exposure to methanol. The linear and core-crosslinked nanoparticles were subjected to high-shear homogenization in turn in the presence of n-dodecane to form macroemulsions. Subsequent processing of these macroemulsions via high-pressure microfluidization produced nanoemulsions. When using the core crosslinked nanoparticles, the droplet diameter was strongly dependent on the copolymer concentration. This indicates that such nanoparticles remain intact under the processing conditions, leading to formation of genuine Pickering nanoemulsions with a z-average diameter of 244 ± 60 nm. In contrast, the linear nanoparticles undergo disassembly to afford molecularly dissolved diblock copolymer chains, which stabilize oil droplets of 170 ± 59 nm diameter. The long-term stability of these two types of n-dodecane-in-water nanoemulsions with respect to Ostwald ripening was examined using analytical centrifugation. When prepared at the same copolymer concentration, Pickering nanoemulsions stabilized by core-crosslinked nanoparticles proved to be significantly more stable than the nanoemulsion stabilized by the amphiphilic PDMAC₇₇-PDAAM₄₀ chains. Moreover, higher copolymer concentrations led to a significantly faster rate of droplet growth. This is attributed to excess copolymer facilitating the diffusion of n-dodecane through the aqueous phase. Finally, analytical centrifugation is used to assess the long-term stability of the analogous squalane-in-water nanoemulsions. These systems are much more stable than the corresponding n-dodecane-in-water nanoemulsions, regardless of whether the copolymer is adsorbed as sterically stabilized nanoparticles or surface-active chains.

(Bio)degradable and Biocompatible Nano-Objects from Polymerization-Induced and Crystallization-Driven Self-Assembly

Polymerization-induced self-assembly (PISA) and crystallization-driven self-assembly (CDSA) techniques have emerged as powerful approaches to produce a broad range of advanced synthetic nano-objects with high potential in biomedical applications. PISA produces nano-objects of different morphologies (e.g., spheres, vesicles and worms), with high solids content (∼10-50 wt %) and without additional surfactant. CDSA can finely control the self-assembly of block copolymers and readily forms nonspherical crystalline nano-objects and more complex, hierarchical assemblies, with spatial and dimensional control over particle length or surface area, which is typically difficult to achieve by PISA. Considering the importance of these two assembly techniques in the current scientific landscape of block copolymer self-assembly and the craze for their use in the biomedical field, this review will focus on the advances in PISA and CDSA to produce nano-objects suitable for biomedical applications in terms of (bio)degradability and biocompatibility. This review will therefore discuss these two aspects in order to guide the future design of block copolymer nanoparticles for future translation toward clinical applications.

Stimuli-Responsive Liquid Crystal Printheads for Spatial and Temporal Control of Polymerization

Polymerization reactions triggered by stimuli play a pivotal role in materials science, with applications ranging from lithography to biomedicine to adaptive materials. However, the development of chemically triggered, stimuli-responsive systems that can confer spatial and temporal control on polymerization remains a challenge. Herein, chemical-stimuli-induced polymerization based on a liquid crystal (LC) printhead is presented. The LC responds to a local chemical stimulus at its aqueous interface, resulting in the ejection of initiator into the solution to trigger polymerization. Various LC printhead geometries are designed, allowing programming of: i) bulk solution polymerization, ii) synthesis of a thin surface-confined polymeric coating, iii) polymerization-induced self-assembly of block copolymers to form various nanostructures (sphere, worm-like, and vesicles), and iv) 3D polymeric structures printed according to local solution conditions. The approach is demonstrated using amphiphiles, multivalent ions, and biomolecules as stimuli.

Morphological transitions of cationic PISA particles by salt, triflate ions and temperature; comparison of three polycations

Three strong polycation stabilizers, poly((vinylbenzyl) trimethylammonium chloride), PVBTMAC, poly((2-(methacryloyloxy)ethyl)trimethylammonium chloride), PMOTAC, and poly((3-acrylamidopropyl) trimethylammonium chloride), PAMPTMAC have been synthesized with reversible addition-fragmentation chain transfer, RAFT, reactions. Solubilities of the polycations...

Reverse sequence polymerization-induced self-assembly in aqueous media: a counter-intuitive approach to sterically-stabilized diblock copolymer nano-objects

A 500 nm charge-stabilized latex is converted into 40 nm sterically-stabilized nanoparticles via reverse sequence polymerization-induced self-assembly (PISA).

Silica nanoparticle-loaded thermoresponsive block copolymer vesicles: a new post-polymerization encapsulation strategy and thermally triggered release

A thermoresponsive amphiphilic diblock copolymer that can form spheres, worms or vesicles in aqueous media at neutral pH by simply raising the dispersion temperature from 1 °C (spheres) to 25 °C (worms) to 50 °C (vesicles) is prepared via polymerization-induced self-assembly (PISA). Heating such an aqueous copolymer dispersion from 1 °C up to 50 °C in the presence of 19 nm glycerol-functionalized silica nanoparticles enables this remarkable ‘shape-shifting’ behavior to be exploited as a new post-polymerization encapsulation strategy. The silica-loaded vesicles formed at 50 °C are then crosslinked using a disulfide-based dihydrazide reagent. Such covalent stabilization enables the dispersion to be cooled to room temperature without loss of the vesicle morphology, thus aiding characterization and enabling the loading efficiency to be determined as a function of both copolymer and silica concentration. Small-angle X-ray scattering (SAXS) analysis indicated a mean vesicle membrane thickness of approximately 20 ± 2 nm for the linear vesicles and TEM studies confirmed encapsulation of the silica nanoparticles within these nano-objects. After removal of the non-encapsulated silica nanoparticles via multiple centrifugation–redispersion cycles, thermogravimetric analysis indicated that vesicle loading efficiencies of up to 86% can be achieved under optimized conditions. Thermally-triggered release of the silica nanoparticles is achieved by cleaving the disulfide bonds at 50 °C using tris(2-carboxyethyl)phosphine (TCEP), followed by cooling to 20 °C to induce vesicle dissociation. SAXS is also used to confirm the release of silica nanoparticles by monitoring the disappearance of the structure factor peak arising from silica–silica interactions.

A loading efficiency of up to 86% is achieved for silica nanoparticles encapsulated within crosslinkable redox-sensitive thermoresponsive diblock copolymer vesicles in water at 50 °C; triggered release is also demonstrated for this system.

Aldehyde-Functional Diblock Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization

We report the rational design of aldehyde-functional sterically stabilized diblock copolymer nano-objects in aqueous solution via polymerization-induced self-assembly. More specifically, reversible addition-fragmentation chain transfer aqueous dispersion polymerization of 2-hydroxypropyl methacrylate is conducted using a water-soluble precursor block in which every methacrylic repeat unit contains a pendent oligo(ethylene glycol) side chain capped with a cis-diol unit. Systematic variation of the reaction conditions enables the construction of a pseudo-phase diagram, which ensures the reproducible targeting of pure spheres, worms, or vesicles. Selective oxidation of the pendent cis-diol groups using aqueous sodium periodate under mild conditions introduces geminal diols (i.e., the hydrated form of an aldehyde obtained in the presence of water) into the steric stabilizer chains without loss of colloidal stability. In the case of diblock copolymer vesicles, such derivatization leads to the formation of a worm population, indicating partial loss of the original morphology. However, this problem can be circumvented by cross-linking the membrane-forming block prior to periodate oxidation. Moreover, such covalently stabilized aldehyde-functionalized vesicles can be subsequently reacted with either glycine or histidine in aqueous solution, followed by reductive amination to prevent hydrolysis of the labile imine bond. ζ potential measurements confirm that this derivatization significantly affects the electrophoretic behavior of these vesicles. Similarly, the membrane-crosslinked aldehyde-functionalized vesicles can be reacted with a model globular protein, bovine serum albumin, to produce "stealthy" protein-decorated vesicles.

Pickering Emulsions Stabilized by Diblock Copolymer Worms Prepared via Reversible Addition-Fragmentation Chain Transfer Aqueous Dispersion Polymerization: How Does the Stimulus Sensitivity Affect the Rate of Demulsification?

Responsive Pickering emulsions exhibit promising application in industry owing to the integration of the high storage stability with on-demand demulsification. In this study, stimuli-responsive Pickering emulsions stabilized by poly[oligo(ethylene glycol) methyl ether methacrylate]₁₅-b-poly(diacetone acrylamide)₁₂₀ (E₁₅D₁₂₀) worms were indicated, in which E₁₅D₁₂₀ worms were prepared via reversible addition-fragmentation chain transfer-based aqueous dispersion polymerization using thermo-sensitive POEGMA₁₅ as both the stabilizer block and macro-chain transfer agent. The factors influencing the morphologies of copolymers during polymerization-induced self assembly have been investigated. A series of different morphological polymer nanoparticles including spheres, worms, and vesicles could be produced through rational synthesis. E₁₅D₁₂₀ worms demonstrated excellent emulsifying performances and could be used as emulsifiers to form n-dodecane-in-water Pickering emulsions at a low content. The formed n-dodecane-in-water Pickering emulsions revealed a slow demulsification at pH 10 or 70 °C or pH 10/70 °C combinations, and several hours were needed for the demulsification of Pickering emulsions. However, n-dodecane-in-water Pickering emulsions displayed a rapid demulsification (∼10 min) at an elevated temperature, such as 90 °C. The different demulsification rates were attributed to different sensitivities of E₁₅D₁₂₀ worms to external stimuli. Pickering emulsions integrating a rapid responsive demulsification with a slow one would be well satisfactory on different occasions.

Surface Grafting "Band-Aid" for "Everyone": Filter Paper-Assisted Surface-Initiated Polymerization in the Presence of Air

We report herein a facile and generalized approach to the modification of solid surfaces with polymer brushes under ambient conditions: filter paper-assisted surface-initiated Cu0 -mediated controlled radical polymerization (PSI-CuCRP). The polymerization solution wetted filter paper is sandwiched between a copper plate and an initiator-modified substrate, which allows the creation of a surface-initiated polymerization (SIP) "band-aid" so that everyone can perform the surface grafting selectively with good control over the quality of the polymer brushes employing low concentration and microliter amounts of the monomer solution. The versatility of this method is demonstrated by grafting different homo-, block-, and multicomponent polymer brushes by using the same activation system and reaction conditions, the polymerization process can be precisely controlled to yield uniform polymers and show high chain-end functionality which is exemplified by in situ tetra-copolymerization. The combination of photolithography and paper cutting enables to prepare arbitrary three-dimensional patterned polymer brushes on the surface.

Polymer Self-Assembly Induced Enhancement of Ice Recrystallization Inhibition

Ice binding proteins modulate ice nucleation/growth and have huge (bio)technological potential. There are few synthetic materials that reproduce their function, and rational design is challenging due to the outstanding questions about the mechanisms of ice binding, including whether ice binding is essential to reproduce all their macroscopic properties. Here we report that nanoparticles obtained by polymerization-induced self-assembly (PISA) inhibit ice recrystallization (IRI) despite their constituent polymers having no apparent activity. Poly(ethylene glycol), poly(dimethylacrylamide), and poly(vinylpyrrolidone) coronas were all IRI-active when assembled into nanoparticles. Different core-forming blocks were also screened, revealing the core chemistry had no effect. These observations show ice binding domains are not essential for macroscopic IRI activity and suggest that the size, and crowding, of polymers may increase the IRI activity of "non-active" polymers. It was also discovered that poly(vinylpyrrolidone) particles had ice crystal shaping activity, indicating this polymer can engage ice crystal surfaces, even though on its own it does not show any appreciable ice recrystallization inhibition. Larger (vesicle) nanoparticles are shown to have higher ice recrystallization inhibition activity compared to smaller (sphere) particles, whereas ice nucleation activity was not found for any material. This shows that assembly into larger structures can increase IRI activity and that increasing the "size" of an IRI does not always lead to ice nucleation. This nanoparticle approach offers a platform toward ice-controlling soft materials and insight into how IRI activity scales with molecular size of additives.

Pathway-Dependent Co-Assembly of Elastin-Like Polypeptides

In natural systems, temperature-induced assembly of biomolecules can lead to the formation of distinct assembly states, created out of the same set of starting compounds, based on the heating trajectory followed. Until now it has been difficult to achieve similar behavior in synthetic polymer mixtures. Here, a novel pathway-dependent assembly based on stimulus-responsive polymers is shown. When a mixture of mono- and diblock copolymers, based on elastin-like polypeptides, is heated with a critical heating rate co-assembled particles are created that are monodisperse, stable, and have tunable hydrodynamic radii between 20 and 120 nm. Below this critical heating rate, the constituents separately form polymer assemblies. This process is kinetically driven and reversible in thermodynamically closed systems. Using the co-assembly pathway, fluorescent proteins and bioluminescent enzymes are encapsulated with high efficiency. Encapsulated cargo shows unperturbed function even after delivery into cells. The pathway-dependent co-assembly of elastin-like polypeptides is not only of fundamental interest from a materials science perspective, allowing the formation of multiple distinct assemblies from the same starting compounds, which can be interconverted by going back to the molecularly dissolved states. It also enables a versatile way for constructing highly effective vehicles for the cellular delivery of biomolecular cargo.

Constructing Cylindrical Nanostructures Via Directional Morphology Evolution Induced by Seeded Polymerization

Herein, spindle-shaped block copolymer (BCP) nanoparticles are used in seeded polymerization of methyl methacrylate as a novel approach to generating cylindrical nanostructures. The chain-extension of BCP seeds by an amorphous coil-type polymer within the seed core composed of semifluorinated liquid-crystalline blocks triggers the deforming, stretching, and directional growth of the seeds along the long axis, eventually leads to nanorods.

Directional effect on the fusion of ellipsoidal morphologies into nanorods and nanotubes

Particle fusion is well-recognized as an important spontaneous process to produce higher-order nanostructures during morphology transition in polymerization-induced self-assembly (PISA). However, to our knowledge, the directional contact, adhesion, and fusion of adjacent nanoparticles have been rarely elucidated in PISA. Herein, a directional fusion of ellipsoidal morphologies was demonstrated during PISA of semi-fluorinated liquid-crystalline (SFLC) block copolymers. The ellipsoidal nanostructures, including micelles and vesicles, preferred to undergo a directional fusion in a head-to-head model, leading to the formation of nanorods and nanotubes, respectively. We believe the directional fusion will offer insightful guidance in PISA to the preparation of complicated functional nanostructures.

Tuning the vesicle-to-worm transition for thermoresponsive block copolymer vesicles prepared via polymerisation-induced self-assembly

Does statistical copolymerization of n-butyl methacrylate with benzyl methacrylate lower the critical temperature required for vesicle-to-worm and worm-to-sphere transitions for diblock copolymer nano-objects in mineral oil?

In situ cross-linking polymerization-induced self-assembly not only generates cross-linked structures but also promotes morphology transition by the cross-linker

Not only cross-linked structures but also a promoting effect on morphology transition has been observed during the in situ cross-linking PISA by RAFT dispersion copolymerization of 2-(diisopropylamino)ethyl methacrylate and cystaminebismethacrylamide.

Synthesis of diblock copolymer spheres, worms and vesicles via RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate

RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate using a poly(glycerol monomethacrylate) precursor leads to diblock copolymer spheres, worms or vesicles. A pseudo-phase diagram is constructed and the vesicles are briefly evaluated as a Pickering emulsifier.

Synthesis of well-defined diblock copolymer nano-objects by RAFT non-aqueous emulsion polymerization of N-(2-acryloyloxy)ethyl pyrrolidone in non-polar media

RAFT non-aqueous emulsion polymerization of N-(2-acryloyloxy)ethyl pyrrolidone in n-dodecane using a poly(stearyl methacrylate) precursor is used to prepare sterically-stabilized nanoparticles, which are evaluated as a putative Pickering emulsifier.

Forced gradient copolymerisation: a simplified approach for polymerisation-induced self-assembly

In this work, a novel and versatile gradient copolymerisation approach to simplify polymeric nanoparticle synthesis through polymerisation-induced self-assembly (PISA) is reported.

Pickering Emulsifiers Based on Block Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly

Block copolymer nanoparticles prepared via polymerization-induced self-assembly (PISA) represent an emerging class of organic Pickering emulsifiers. Such nanoparticles are readily prepared by chain-extending a soluble homopolymer precursor using a carefully selected second monomer that forms an insoluble block in the chosen solvent. As the second block grows, it undergoes phase separation that drives in situ self-assembly to form sterically stabilized nanoparticles. Conducting such PISA syntheses in aqueous solution leads to hydrophilic nanoparticles that enable the formation of oil-in-water emulsions. Alternatively, hydrophobic nanoparticles can be prepared in non-polar media (e.g., n-alkanes), which enables water-in-oil emulsions to be produced. In this review, the specific advantages of using PISA to prepare such bespoke Pickering emulsifiers are highlighted, which include fine control over particle size, copolymer morphology, and surface wettability. This has enabled various fundamental scientific questions regarding Pickering emulsions to be addressed. Moreover, block copolymer nanoparticles can be used to prepare Pickering emulsions over various length scales, with mean droplet diameters ranging from millimeters to less than 200 nm.

One polymer composition, various morphologies: the decisive influence of conditions on the polymerization-induced self-assembly (PISA) of N-acryloyl thiomorpholine

Polymerization-induced self-assembly (PISA) represents a powerful technique for the preparation of nanostructures comprising various morphologies. Herein, we demonstrate that the recently introduced monomer N-acryloylthiomorpholine (NAT) features a unique self-assembly behaviour during an aqueous PISA. The one-pot, aqueous RAFT dispersion polymerization starting from short poly(N-acryloylmorpholine) (PNAM) enables access to all common solution morphologies including spheres, worms, vesicles and lamellae, at very low molar masses (< 8 kDa). Moreover, all these structures can be obtained for the same polymer composition and size by the variation of the polymerization temperature and concentration of the monomer. This exceptional self-assembly behavior is associated with the combination of a high glass transition temperature, excellent water solubility of the monomer, and the early onset of aggregation during the polymerization, which stabilizes the morphology at different stages. This PISA system opens up new opportunities to reproducibly create versatile, functional nanostructures and enables an independent evaluation of morphology-property relationships, as it is exemplarily shown for the oxidative degradation of spherical and wormlike micelles, as well as vesicles.