RAFT polymerization of temperature- and salt-responsive block copolymers as reversible hydrogels

Reversible-addition fragmentation chain transfer (RAFT) polymerization enabled the synthesis of novel, stimuli-responsive, AB and ABA block copolymers. The B block contained oligo(ethylene glycol) methyl ether methacrylate (OEG) and was permanently hydrophilic in the conditions examined. The A block consisted of diethylene glycol methyl ether methacrylate (DEG) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (TMA). The A block displayed both salt- and temperature-response with lower critical solution temperatures (LCSTs) dependent on the molar content of TMA and the presence of salt. Higher TMA content in the AB diblock copolymers increased the critical micelle temperatures (CMT) in HPLC-grade water due to an increased hydrophilicity of the A block. Upon addition of 0.9 wt% NaCl, the CMTs of poly(OEG-b-DEG₉₅TMA₅) decreased from 50 °C to 36 °C due to screening of electrostatic repulsion between the TMA units. ABA triblock copolymers displayed excellent hydrogel properties with salt- and temperature-dependent gel points. TMA incorporation in the A block increased the gel points for all triblock copolymers, and salt-response increased with higher TMA composition in the A block. For example, poly(DEG₉₈TMA₂-b-OEG-b-DEG₉₈TMA₂) formed a hydrogel at 40 °C in HPLC-grade water and 26 °C in 0.9 wt% NaCl aqueous solution. These salt- and temperature-responsive AB diblock and ABA triblock copolymers find applications as drug delivery vehicles, adhesives, and hydrogels.     

Synthesis and characterisation of stimuli-responsive poly(N,N′-diethylacrylamide) hydrogels

Stimuli-responsive poly(N,N′-diethylacrylamide) gels were prepared by free radical polymerisation in aqueous solution, using N,N-methylenebisacrylamide as crosslinking agent. The gels were compared with the corresponding poly(N-isopropylacrylamide)-based gels. In particular, the swelling ratio of both gel types including the effect of the crosslinker content, their swelling and deswelling kinetics, their permeability and finally their drug (insulin) storage and controlled release ability were compared. In spite of the similarity in the monomer/crosslinker ratio, the deswelling kinetics and the critical temperatures (ca. 30-32 °C in pure water), some differences could be observed. Compared to poly(N-isopropylacrylamide)-based gels, poly(N,N′-diethylacrylamide)-based gels show a broader phase transition temperature interval, a more pronounced dependency of the swelling ratio on the crosslinker content, slower reswelling kinetics, a higher ingress percentage for dextran standards ranging from 5 to 70 kD, but lower ingress percentages for proteins (BSA, insulin) and much faster drug (insulin) release kinetics. While a non-linear release kinetic was observed in the case of the poly(N-isopropylacraylamide)-based gels both in water and in PBS (phosphate buffered saline), this was not the case for the poly(N,N′-diethylacrylamide)-based gels.     

Synthesis and ion responsiveness of optically active polyacetylenes containing salicylidene Schiff-base moieties

Acetylenic monomers containing salicylidene Schiff-base groups (1a and 1b) as well as Schiff-base and hydroxy groups (1c) were synthesized and polymerized with [(nbd)RhCl]₂/Et₃N catalyst to afford the corresponding polymers 2a–c with high molecular weights (M_n = 2.6–7.2 × 10⁵) in high yields (75–97%). Polarimetric, circular dichroism (CD), and UV–vis spectroscopic analyses indicated that the polymers formed helical structures with a predominantly one-handed screw sense. The addition of metal ions to salicylidene Schiff-base-containing polymers 2a and 2b produced insoluble polymer/metal complexes through ionic cross-linking as a result of salicylaldimine–metal ion complexation. Polymers 2b and 2c underwent a helix–coil transition upon the addition of HSO₄⁻, whereas these polymers did not exhibit responsiveness to other anions, such as F⁻, Cl⁻, and Br⁻.     

Tailored dispersion of carbon nanotubes in water with pH-responsive polymers

In an effort to control the level of carbon nanotube exfoliation in water, pH-responsive polymers (i.e., weak polyelectrolytes) have been used as stabilizers in water. This noncovalent functionalization of single-walled carbon nanotubes (SWNTs) results in suspensions whose dispersion state can be altered by simply changing pH. In this study poly(acrylic acid), poly(methacrylic acid), poly(allylamine) and polyethyleneimine were used to stabilize aqueous SWNT suspensions. The results indicate that SWNTs stabilized with these polymers show a pH tailorable exfoliation and bundling in water, as evidenced by cryo-TEM images and shifts in suspension viscosity. Composite films prepared by drying these aqueous suspensions suggest that nanotube microstructure in the liquid state is largely preserved in the solid composites, with more bundled/networked structures showing higher electrical conductivity. A stabilization mechanism based upon the results obtained is proposed to explain the exfoliation and aggregation behavior of SWNTs. This method of controlling the microstructure of SWNTs in liquid state with pH could have a significant impact on the ability to tailor the microstructure and properties of composites.     

Comprehensive Alcohol-/Ion-Responsive Properties of Poly(N-Isopropylacrylamide-co-Benzo-18-Crown-6-Acrylamide) Copolymers

abstract In this paper, we report on the comprehensive alcohol-/ion-responsive properties of a smart copolymer poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (P(NIPAM-co-BCAm)). The orthogo...     

Temperature and light sensitive copolymers containing azobenzene moieties prepared via a polymer analogous reaction

Four different series of polyacrylamides containing different amounts of azobenzene moieties have been synthesized via a polymer analogous reaction of poly(pentafluorophenylacrylate) (PPFPA). All copolymers were designed to exhibit a lower critical solution temperature (LCST) in aqueous solution, which was dependent on (i) the amount of incorporated chromophoric azobenzene groups and (ii) the isomerization state of the respective azobenzene group. Higher LCST values were measured for UV-irradiated solutions of the copolymers in comparison to the non-irradiated copolymer solutions. A maximum difference in the LCST of up to 7 °C was found for the copolymer poly(N,N-dimethylacrylamide) containing 8.5 mol% of azobenzene groups. Within this temperature range, a reversible solubility change of the copolymer could be induced by irradiation with light.     

Optimal dispatch for reversible solid oxide cell-based hydrogen/electric vehicle aggregator via stimuli-responsive charging decision estimation

The ongoing growth of green vehicles had led to an increase in demand of cost-effective and driver-satisfactory hydrogen/electric vehicle aggregators (HEVAs). However, existing approaches for cost minimization of HEVA can lead to poor performance due to the inaccurate modelling of power–gas exchange system and neglection of schedulable characteristics of loads. Furthermore, the behaviour of drivers was rarely considered from a psychological perspective. To resolve these limitations, the optimal dispatch scheme of HEVA, equipped with reversible solid oxide cell (rSOC), is investigated by quantifying drivers’ charging decision response toward pricing stimuli. As the core of the bi-directional energy conversion, rSOC is modelled by considering the climbing power constraints and time-dependent restart-up cost. At the driver side, EVs are aggregated as clusters for efficient computation. Two charging modes are designed for drivers with incentive discounts. To measure the relationship between external factors and charging decision response, the stimuli-responsive charging decision estimation is proposed by introducing Weber–Fechner law (W–F Law). To minimum operation cost, a mixed integer nonlinear programming (MINP) method is presented. The results validate that the operation cost of HEVA can be decreased by 19.37%, and the maximum utilization of energy is realised in the proposed scheme. Additionally, the impacts of sizes of power–gas exchange devices are investigated for practical reference. Under a given charging demand, the proposed dispatch scheme can realise installation of smaller devices, and thereby, resulting in lower construction cost.     

Synthesis, characterization, and stimuli-sensitive properties of novel polycarbobetaines

Novel monomers bearing propyl and cyclohexyl groups on the side chain were synthesized from the acetoacetic ester and propylamine and cyclohexylamine. Michael addition reaction with acrylic (methacrylic) acid followed by radical polymerization resulted in preparation of both linear and crosslinked betaine type polyampholytes. The polycarbobetaines were characterized by potentiometric titration, viscometry, GPC, FTIR, NMR, and UV-Vis. The ionization constants of acid and base groups were calculated from the Henderson-Hasselbalch equation. Considerable influence of water content on the yield of linear polymers and swelling degree of hydrogels was established. The dependence of swelling degree on the chain length and concentration of crosslinking agents was found. The stimuli-responsive properties of hydrogels were studied in water-organic solvents mixtures and as a function of pH and ionic strength. The adsorption-desorption recycling stages of hydrogels with respect to copper(II) ions were demonstrated.     

Laws of 4D Printing

Three-dimensional (3D) printing is an additive manufacturing process. Accordingly, four-dimensional (4D) printing is a manufacturing process that involves multiple research fields. 4D printing conserves the general attributes of 3D printing (such as material waste reduction, and elimination of molds, dies, and machining) and further enables the fourth dimension of products to provide intelligent behavior over time. This intelligent behavior is encoded (usually by an inverse mathematical problem) into stimuli-responsive multi-materials during printing, and is enabled by stimuli after printing. The main difference between 3D- and 4D-printed structures is the presence of one additional dimension, which provides for smart evolution over time. However, currently there is no general formula for modeling and predicting this additional dimension. Herein, by starting from fundamentals, we derive and validate a general bi-exponential formula with a particular format that can model the time-dependent behavior of nearly all 4D (hydro-, photochemical-, photothermal-, solvent-, pH-, moisture-, electrochemical-, electrothermal-, ultrasound-, etc. responsive) structures. We show that two types of time constants are needed to capture the correct time-dependent behavior of 4D multi-materials. We introduce the concept of mismatch-driven stress at the interface of active and passive materials in 4D multi-material structures, leading to one of the two time constants. We develop and extract the other time constant from our unified model of time-dependent behavior of nearly all stimuli-responsive materials. Our results starting from the most fundamental concepts and ending with governing equations can serve as general design principles for future research in the field of 4D printing, where time-dependent behaviors should be properly understood, modeled, and predicted.     

Dual responsive block copolymer coated hollow mesoporous silica nanoparticles for glucose-mediated transcutaneous drug delivery

A self-regulated anti-diabetic drug release device mimicking pancreatic cells is highly desirable for the therapy of diabetes. Herein, a glucose-mediated dual-responsive drug delivery system, which combines pH- and H₂O₂-responsive block copolymer grafted hollow mesoporous silica nanoparticles (HMSNs) with microneedle (MN) array patch, has been developed to achieve self-regulated administration. The poly[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl acrylate]-b-poly[2-(dimethylamino)ethyl methacrylate] (PPBEM-b-PDM) polymer serves as gate keeper to prevent drug release from the cavity of HMSNs at normoglycemic level. In contrast, the drug release rate is significantly enhanced upon H₂O₂ and pH stimuli due to the chemical change of H₂O₂ sensitive PPBEM block and acid responsive PDM block. Therefore, incorporation of anti-diabetic drug and glucose oxidase (GOx, which can oxidize glucose to gluconic acid and in-situ produce H₂O₂) into stimulus polymer coated HMSNs results in a glucose-mediated MN device after depositing the drug-loaded nanoparticles into MN array patch. Both in vitro and in vivo results show this MN device presents a glucose mediated self-regulated drug release characteristic, which possesses a rapid drug release at hyperglycemic level but retarded drug release at normoglycemic level. The result indicates that the fabricated smart drug delivery system is a good candidate for the therapy of diabetes.