Nonequilibrium Processes in Polymer Membrane Formation: Theory and Experiment.

Porous polymer and copolymer membranes are useful for ultrafiltration of functional macromolecules, colloids, and water purification. In particular, block copolymer membranes offer a bottom-up approach to form isoporous membranes. To optimize permeability, selectivity, longevity, and cost, and to rationally design fabrication processes, direct insights into the spatiotemporal structure evolution are necessary. Because of a multitude of nonequilibrium processes in polymer membrane formation, theoretical predictions via continuum models and particle simulations remain a challenge. We compiled experimental observations and theoretical approaches for homo- and block copolymer membranes prepared by nonsolvent-induced phase separation and highlight the interplay of multiple nonequilibrium processes─evaporation, solvent-nonsolvent exchange, diffusion, hydrodynamic flow, viscoelasticity, macro- and microphase separation, and dynamic arrest─that dictates the complex structure of the membrane on different scales.

Double thermoresponsive graft copolymers with different chain ends: feasible precursors for covalently crosslinked hydrogels.

The tailored synthesis of graft copolymers from acrylic and methacrylic monomers can be accomplished solely through photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization. Samples with poly[oligo(ethylene glycol) methacrylate] (POEGMA) backbones synthesized under green light irradiation and poly(N-isopropylacrylamide) (PNIPAM) side chains growing under blue light irradiation are presented. As monitored by temperature-dependent dynamic light scattering (DLS) measurements and temperature-variable nuclear magnetic resonance (NMR) spectroscopy, the architecture of the graft copolymers allows unique two-step lower critical solution temperature (LCST) transitions in aqueous solutions. Meanwhile, different end-groups introduced by the corresponding RAFT agents affect the detailed thermoresponsive behavior remarkably. This RAFT strategy shows more advantages when the multiple trithiocarbonate groups are converted into thiol reactive pyridyl disulfide (PDS) groups via a facile post-polymerization modification. The PDS-terminated graft copolymer can then be regarded as a usable precursor for various applications, such as thermoresponsive hydrogels.

Improved alkali metal ion capturing utilizing crown ether-based diblock copolymers in a sandwich-type complexation.

The compexation behavior of metals with free crown ethers (CE) and diblock copolymer-based CE is investigated. The latter shows at least 10 000 times stronger complexation than free CEs. On this basis, a highly stable CE complex within the polymer for efficient extraction of metal ions from low concentrations, e.g. lithium in seawater, is presented.

Thermodynamic study of crown ether-lithium/magnesium complexes based on benz-1,4-dioxane and its homologues.

The synthesis and characterization of benz-1,4-dioxane crown ethers (CEs) and some of its homologues are described and analyzed. The effect of added C-atom within the CE ring (increasing the hydrophobicity of the CE ring by increasing the number of CH2-units) on the Li+ and Mg2+ complexation within a liquid-liquid extraction (LLE) is investigated and thermodynamically analyzed. The complex stability constant K, the change of entropy ΔS and enthalpy ΔH, and the Gibbs energy ΔG are determined. The enhanced hydrophobicity of the CE ring results in stronger complexation stability of the Mg2+ complex, while the Li+ complexes are less favored. This effect mainly occurs due to the increased entropy term with improved hydrophobicity of the CE. These results indicate a stronger extraction of Li+ in Mg2+-containing aqueous resources if more hydrophilic CEs are used.

Nonionic UCST-LCST Diblock Copolymers with Tunable Thermoresponsiveness Synthesized via PhotoRAFT Polymerization.

Nonionic double thermoresponsive diblock copolymers with both upper critical solution temperature (UCST) and lower critical solution temperature (LCST) phase transitions are synthesized via eco-friendly photoiniferter reversible addition-fragmentation chain transfer polymerization. While the biocompatible random copolymer of di(ethylene glycol) methyl ether methacrylate and oligo(ethylene glycol) methacrylate accounts for the LCST transition, the block of polymethacrylamide from an easily accessible monomer with low health hazard is responsible for the UCST transition. Temperature-dependent dynamic light scattering measurements confirm the formation of micellar aggregates in water at the temperatures below UCST- and above LCST-type cloud points. Additionally, the temperature interval between UCST and LCST, where both blocks are dissolved, can be tailored by varying the comonomer ratio in the random copolymer block. With these unique advantages, the presented work introduces a new polymer system for the design of schizophrenic polymers.

A Highly Selective Polymer Material using Benzo-9-Crown-3 for the Extraction of Lithium in Presence of Other Interfering Alkali Metal Ions.

The recovery of lithium from global water resources continues to be challenging due to interfering metal ions with similar solution properties. Hence, a lithium-selective diblock copolymer system containing crown ethers (CEs) is developed. A polystyrene-block-poly(methacrylic acid) diblock copolymer is synthesized first via a one-pot solution-emulsion reversible addition-fragmentation chain transfer polymerization. A subsequent Steglich esterification yields the CE functionalized polymer. The complexation properties with different alkali metals are first investigated by liquid-liquid extraction (LLE) in dichloromethane (DCM) - water systems using free benzo-9-crown (B9C3), benzo-12-crown-4 (B12C4), and benzo-15-crown-5 (B15C5) CEs as reference components, followed by the correspondingly CE-functionalized polymers. Extraction complexation constants in the aqueous phase are determined and the impact of the complexation constants on the extractability is estimated. The B9C3 CE is especially appealing since it has the smallest cavity size among all CEs. It is too small to complex sodium or potassium ions; however, it forms sandwich complexes with a lithium-ion resulting in extraordinary complexation constants in polymer systems avoiding other interfering alkali metal ions. On this basis, a new material for the efficient extraction of lithium ion traces in global water resources is established.

Tailoring Crosslinked Polyether Networks for Separation of CO2 from Light Gases.

Crosslinked poly(ethylene oxide) or poly(ethylene glycol) (PEG) is an ideal membrane material for separation of CO2 from light gases (e.g., H2 , N2 , O2 , CH4 etc). In these membranes, crosslinking is used as a tool to suppress crystallinity of the PEG segments. In spite of the extensive effort to develop crosslinked PEG membranes in the last two decades, it remains a challenge to establish the structure-property relationships. This paper points out the fundamental limitations to correlate the chain topology of a network with the gas permeation mechanism. While a quantitative comparison of the molecular weight between crosslinks of networks and gas permeation mechanism reported by different research groups is challenging, effort is made to draw a qualitative picture. In this review, a focus is also put on the progress of utilization of dangling chain fractions to tailor the gas permeation behavior of PEG networks.

An eco-friendly pathway to thermosensitive micellar nanoobjects via photoRAFT PISA: the full guide to poly(N-acryloylpyrrolidin)-block-polystyrene diblock copolymers.

Spherical macromolecular assemblies, so-called latexes, consisting of polystyrene (PS) resemble a relevant class of synthetic polymers used for a plethora of applications ranging from coatings or lubricants to biomedical applications. Their synthesis is usually tailored to the respective application where emulsifiers, radical initiators, or other additives still play a major role in achieving the desired properties. Herein, we demonstrate an alternative based on the photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization-induced self-assembly (PISA) of Poly(N-acryloylpyrrolidin)-block-polystyrene (PAPy-b-PS). This approach yields monodisperse nanospheres with tunable sizes based on an aqueous formulation with only two ingredients. These nanospheres are additionally thermosensitive, meaning that they change their hydrodynamic diameter linearly with the temperature in a broad range between 10 °C and 70 °C. Combined with the eco-friendly synthesis in pure water at 40 °C, the herein presented route constitutes an unprecedented pathway to thermosensitive diblock copolymer aggregates in short reaction times without any additives.

Solubility behaviour of random and gradient copolymers of di- and oligo(ethylene oxide) methacrylate in water: effect of various additives.

Poly[oligo(ethylene oxide)] based gradient and random copolymers with different compositions are synthesized via Cu-based atom transfer radical polymerization. The solubility behavior of these copolymers in pure water and in the presence of different salts, surfactants and ethanol is investigated. According to dynamic light scattering results, the lower critical solution temperature (LCST) depends on the structure of the copolymer and changes slightly in the presence of additives. Good cosolvents like ethanol can increase the LCST through dissolving the collapsed copolymer chains to some extent. The same effect is observed for surfactants that make the copolymer solution more stable by preventing aggregation. Above a certain concentration of surfactant, depending on the copolymer structure, the solution is stable at all temperatures (no LCST). The effect of salts on the solubility of the copolymers follows the Hofmeister series and it is related linearly to the salt concentration. Based on their affinity to the copolymer, the salts can increase or decrease the LCST. There is a considerable difference in phase transition changes for gradient or random copolymers after salt addition. While both copolymers show a two-step phase transition in the presence of different salts, the changes in the hydrodynamic radius and normalized scattering intensity are rather broad for random compared to gradient copolymers. Contrary to what was expected, varying the cations has no distinguishable effect on the LCST for both copolymers. All chlorides decrease the LCST. This decrease is almost the same for gradient copolymers and fluctuates for random copolymers.

Acid-Mediated Autocatalysis in Vinylogous Urethane Vitrimers.

Vitrimers are a class of polymeric materials with outstanding properties. Intramolecular substitution reactions lead to a dynamic exchange within the polymer network which enables thermoreversible stress relaxation in yet permanently crosslinked materials. In this paper, the acid-mediated autocatalysis is explored as a rearrangement pathway for vinylogous urethane vitrimers. The autocatalysis enables transimination reactions, resulting in a dynamic exchange among the enamine-one species, without an excess of free amines. Therefore, the enamine-ones are protonated by a Brønsted acid and turn into electrophilic iminium-ones, thus enabling fast backward and substitution reactions with water and free amines. This work provides an in-depth investigation of the mechanism by kinetic studies of selected compounds. In addition, novel elastomeric and thermosetting poly(vinylogous urethane) networks with and without free amine groups and additional para-toluene sulfonic acid as a Brønsted catalyst are prepared by bulk polymerization of hexane-1,6-diylbis(3-oxobutanoate) and tris(2-aminoethyl)amine. The underlying exchange mechanisms are determined by stress-relaxation experiments with stress relaxation times of 0.3-54 000 s at 110 °C.

Continuous Kinetic Sampling of Flow Polymerizations via Inline UV-Vis Spectroscopy.

Gapless monitoring of polymerization reactions is of paramount interest for academia and the polymer industry, allowing for efficient reaction screening and precise tailoring of the polymeric products. Herein, UV-visible spectroscopy (UV-vis) is employed as an operando measurement technique in continuous flow polymerization with ex situ calibration, to calculate monomer conversions with unprecedented resolution of only 10 s. A mathematical model based on volume contraction is provided for the first time, which yields monomer conversions from the absorption in the visible region for theoretically any homopolymerization. This model is validated for different monomers, solvents, and concentrations in a photoiniferter reversible addition-fragmentation chain transfer polymerization, proving the versatility of the presented setup. Notably, an ultralow measurement volume of merely a few hundred nanoliters is enough to ensure high accuracy.

Quaternization of a Polystyrene-block-poly(4-vinylpyridine) Isoporous Membrane: An Approach to Tune the Pore Size and the Charge Density.

Isoporous integral asymmetric membranes derived from the self-assembly of block copolymers combined with the non-solvent-induced phase separation (SNIPS) have gained great attention. To extend their utility, good control over pore size and surface functionality in a facile manner is highly desirable. Here, an approach is proposed to achieve this by quaternization of the poly(4-vinylpyridine) moiety of a polystyrene-block-poly(4-vinylpyridine) SNIPS membrane using alkyl iodides via a scalable gas-solid heterogeneous reaction. By changing the size of the alkyl groups of the quaternization agent and the degree of quaternization, the effective pore size of the membrane is tailored in a wide range from the ultrafiltration to the nanofiltration regime. A quaternization of approximately half of the 4VP repeating units of the membranes with methyl iodide, ethyl iodide, or 1-propyl iodide leads to a retention of methylene blue from a 10 mg L-1 aqueous solution of 96%, 87%, and 83%, respectively.

Hydrogen bonding and thermoplastic elastomers - a nice couple with temperature-adjustable mechanical properties.

Styrene-butadiene copolymers are modified with varying fractions of benzoic acid moieties being able to perform hydrogen bonding. This is done by using a simple synthetic approach which utilizes click chemistry. Temperature-dependent dynamic mechanical properties are studied, and it turns out that even the apparently rather simple hydrogen bonding motif has a marked impact on the material properties due to the fact that it facilitates the formation of a supramolecular polymer network. Besides a glass transition, the investigated functionalized copolymers exhibit a second endothermic transition, known as a quasi-melting. This is related to the opening of the hydrogen bonding complexes. Additionally to dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), temperature-dependent infrared (IR) spectroscopy and small angle X-ray scattering (SAXS) are used to understand the structure-property relationships.

Hydroperoxide Traces in Common Cyclic Ethers as Initiators for Controlled RAFT Polymerizations.

Herein, a reversible addition-fragmentation chain transfer (RAFT) polymerization is introduced for reactive monomers like N-acryloylpyrrolidine or N,N-dimethylacrylamide working without a conventional radical initiator. As a very straightforward proof of principle, the method takes advantage of the usually inconvenient radical-generating hydroperoxide contaminations in cyclic ethers like tetrahydrofuran or 1,4-dioxane, which are very common solvents in polymer sciences. The polymerizations are surprisingly well controlled and the polymers can be extended with a second block, indicating their high livingness. "Solvent-initiated" RAFT polymerizations hence prove to be a feasible access to tailored materials with minimal experimental effort and standard laboratory equipment, only requiring the following ingredients: hydroperoxide-contaminated solvent, monomer, and RAFT agent. In other respects, however, the potential coinitiating ability of the used solvent is to be considered when investigating the kinetics of RAFT polymerizations or aiming for the synthesis of high-livingness polymers, e.g., multiblock copolymers.

Novel Post-Treatment Approaches to Tailor the Pore Size of PS-b-PHEMA Isoporous Membranes.

Using the example of an integral-asymmetric isoporous membrane prepared from polystyrene-block-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA), physical and chemical ways of post-treatment are introduced with the aim to tailor the pore size. These post-treatments are i) thermal annealing and ii) urethane chemistry of ethyl isocyanate (EI) in the presence of perfluoro(methyl cyclohexane). Via these approaches, the pore size of PS-b-PHEMA membranes is successfully tailored in the range of 10-20 nm with narrow pore size distribution by controlling the duration of thermal annealing and chemical reaction, respectively. The excellent hydrophilicity of the PS-b-PHEMA membrane is not changed by thermal annealing. The chemical postmodification using EI is associated with a loss of hydrophilicity with increasing conversion.

Ultrafast PhotoRAFT Block Copolymerization of Isoprene and Styrene Facilitated through Continuous-Flow Operation.

Polymers made from isoprene and styrene resemble an important class of synthetic macromolecules found in a wide range of everyday commodity products. Their synthesis is usually limited to radical emulsion or anionic polymerization. Herein, we report on ultrafast photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization of isoprene and styrene in a continuous-flow microreactor. The cooperative action of a high photoinitiation efficiency and use of elevated temperatures considerably reduces the reaction times to less than half an hour to give high monomer conversions, allowing for the first time polyisoprene to be yielded from controlled radical polymerization in high definition and reasonable reaction times. High chain-end fidelities are maintained and block copolymers were prepared including a polystyrene-block-polyisoprene-block-polystyrene (PS-b-PI-b-PS) triblock copolymer.

Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties.

It is commonly accepted that the combination of the anisotropic shape and nanoscale dimensions of the mineral constituents of natural biological composites underlies their superior mechanical properties when compared to those of their rather weak mineral and organic constituents. Here, we show that the self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid molecules leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomechanical model, we determined that these exceptional mechanical properties are dominated by the covalent backbone of the linked organic molecules. Because oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.

Thermodynamic analysis of alkali metal complex formation of polymer-bonded crown ether.

The complex formation of two crown ethers with colored alkali metal salts was investigated by UV/Vis spectroscopy. Complexation was accomplished with free benzo-15-crown-5 (B15C5) and 15-crown-5 bonded to a diblock copolymer (Poly15C5). The diblock copolymer was synthesized by two controlled polymerization techniques and copper(i)-catalyzed azide-alkyne cycloaddition. Depending on the inserted cation, 1 : 1- or 1 : 2-complexes are formed. A significant difference of the stability constants was determined by concentration dependence solvent extraction with sodium or potassium salt. For Poly15C5 the stability constants increase for both salts compared to the stability constants of B15C5, which suggests a more effective complexation. Evaluation of the thermodynamics (ΔH, ΔS, ΔG) of cation complexation was achieved by temperature dependence phase extraction on the basis of established thermodynamic equations. Remarkably, in all cases the entropic gain seems to be the major propulsion facilitating the complexation between alkali metal salts and crown ethers. Indeed, by using Poly15C5 a more pronounced dependency of enthalpy and entropy on the complex formation is calculated.

A Facile Method to Prepare Double-Layer Isoporous Hollow Fiber Membrane by In Situ Hydrogen Bond Formation in the Spinning Line.

A double-layer hollow fiber is fabricated where an isoporous surface of polystyrene-block-poly(4-vinylpyridine) is fixed on a support layer by co-extrusion. Due to the sulfonation of the support layer material, delamination of the two layers is suppressed without increasing the number of subsequent processing steps for isoporous composite membrane formation. Electron microscope-energy-dispersive X-ray spectroscopy images unveil the existence of a high sulfur concentration in the interfacial region by which in-process H-bond formation between the layers is evidenced. For the very first time, our study reports a facile method to fabricate a sturdy isoporous double-layer hollow fiber.

Isoporous block copolymer membranes.

The developments in membranes based on tailored block copolymers are reported with an emphasis on isoporous membranes. These membranes can be prepared in different geometries, namely flat sheets and hollow fibers. They display narrow pore size distributions due to their formation by self-assembly. The preparation of these membranes and possibilities to further functionalize such membranes will be discussed. Different ways to control the pore size will be addressed, and the potential of block copolymer blends to fabricate membranes with tailored pore sizes will be shown.