How to manipulate the upper critical solution temperature (UCST)?

In this mini-review, we discuss multi-stimuli-responsive polymers, which exhibit upper critical solution temperature (UCST) behavior mainly in aqueous solutions. Firstly, we discuss both the lower and upper critical solution temperature behavior of thermoresponsive polymers, to understand the differences between the two. This will be followed by examples of polymers that undergo a UCST phase transition. Secondly, we show how the solution properties of multi stimuli-responsive polymers can be influenced by several factors in addition to temperature, such as counter ions, electricity, light, or pH. Common to all stimuli are their capabilities to induce changes in the conformations and interactions of the polymers. With UCST polymers, the thermoresponsiveness is predominantly dependent upon the presence of strong supramolecular interactions between the polymer side groups. These are known to be affected by the molecular weight of the polymer, the solution concentration, and the presence of salts. With all these different ways to affect the cloud point of the polymers, we have systems that are readily tunable to many applications.Peer reviewe

Soft Poly(N-vinylcaprolactam) Based Aqueous Particles

Soft nanoparticles are an important class of material with potential to be used as carriers of active compounds. Swollen, penetrable particles can act as a host for the active ingredients and provide stability, stimuli-responsiveness and recyclability for the guest. Thermoresponsive colloidal gel particles are especially attractive for such applications due to the extremely soft structure, size and responsiveness. Poly(N-vinylcaprolactam) (PNVCL) is a much studied, popular thermoresponsive polymer. The polymer has low toxicity and the phase transition temperature is close to body temperature. During the phase transition, the polymer becomes less soluble, the particle expels a large part of water and the particle collapses to a more compact form. The diffusion of material in and from the particles is largely affected by this transition.  As the solubility of the polymer changes, so do the interactions with the loaded compound.  This feature article focuses on the synthetic methods, properties and applications of soft PNVCL particles.Peer reviewe

Bicatalytic poly(N-acryloyl glycinamide) microgels

In both chemo- and biocatalysis the immobilization of catalysts to carriers is often beneficial in terms of catalytic activity and ease of operation. In the present study we encapsulated an enzyme, beta-D-glucosidase, inside thermosensitive poly(N-acryloyl glycinamide) microgels by radical polymerization of N-acryloyl glycinamide in the presence of the enzymes. Properties of these hybrid microgels were studied varying the enzyme-monomer ratio and the degree of crosslinking. The enzymatic activities of the microgels were assessed using a model reaction, enzymatic cleavage of p-nitrophenyl-beta-D-glucopyranoside under different conditions. The microgel encapsulated enzymes showed enhanced activity at high pH compared to the native enzymes. Once the enzymatic activity of the microgels was ascertained, introduction of silver nanoparticles inside the enzyme carrying microgels was made to develop bicatalytic systems. The bicatalytic microgels were shown to be capable of carrying out a cascade reaction combining enzymatic catalysis and reduction of the reaction product 4-nitrophenol to 4-aminophenol.Peer reviewe

Influence of photo-isomerisation on host-guest interactions in poly(azocalix[4]arene)s

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Upper or lower critical solution temperature, or both? : Studies on cationic copolymers of N-isopropylacrylamide

The solution properties of statistical copolymers of N-isopropyl acrylamide (NIPAm) and cationic (3-acrylamidopropyl) trimethylammonium chloride (AMPTMA) have been studied. The phase behavior of the copolymers in aqueous solutions is strongly affected by the addition of lithium bis(trifluoromethane)sulfonimide (LiNTf2), NaCl, or both. Hydrophobic NTf2 counter ions bind to the AMPTMA repeating units. By adjusting the balance between hydrophobic and electrostatic interactions the transition temperature of the copolymers may be tuned over a wide temperature range. It was observed that a homopolymer PAMPTMA undergoes an UCST-type phase separation in an aqueous solution in the presence of both NaCl and LiNTf2. When AMPTMA and NIPAm are present in the copolymer in nearly equal amounts both LCST and UCST can coexist. It was observed that the effect of LiNTf2 is similar to that of the salts in the kosmotropic end of the Hofmeister series for PNIPAm.Peer reviewe

Visualization data on the freezing process of micrometer-scaled aqueous citric acid drops

Abstract The visualization data (8 movies) presented in this article are related to the research article entitled “Freezing and glass transitions upon cooling and warming and ice/freeze-concentration-solution morphology of emulsified aqueous citric acid” (A. Bogdan, M.J. Molina, H. Tenhu, 2016) [1]. The movies recorded in-situ with optical cryo-miscroscopy (OC-M) demonstrate for the first time freezing processes that occur during the cooling and subsequent warming of emulsified micrometer-scaled aqueous citric acid (CA) drops. The movies are made publicly available to enable critical or extended analyzes.Peer reviewe

Tough Materials Through Ionic Interactions

This article introduces butyl acrylate-based materials that are toughened with dynamic crosslinkers. These dynamic crosslinkers are salts where both the anion and cation polymerize. The ion pairs between the polymerized anions and cations form dynamic crosslinks that break and reform under deformation. Chemical crosslinker was used to bring shape stability. The extent of dynamic and chemical crosslinking was related to the mechanical and thermal properties of the materials. Furthermore, the dependence of the material properties on different dynamic crosslinkers-tributyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C4ASA) and trihexyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C6ASA)-was studied. The materials' mechanical and thermal properties were characterized by means of tensile tests, dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. The dynamic crosslinks strengthened the materials considerably. Chemical crosslinks decreased the elasticity of the materials but did not significantly affect their strength. Comparison of the two ionic crosslinkers revealed that changing the crosslinker from C4ASA to C6ASA results in more elastic, but slightly weaker materials. In conclusion, dynamic crosslinks provide substantial enhancement of mechanical properties of the materials. This is a unique approach that is utilizable for a wide variety of polymer materials.Peer reviewe

Phase Transition Behavior and Catalytic Activity of Poly(N-acryloylglycinamide-co-methacrylic acid) Microgels

Poly(N-acryloyl glycinamide) is a well-known thermoresponsive polymer possessing an upper critical solution temperature (UCST) in water. By copolymerizing N-acryloyl glycinamide (NAGA) with methacrylic acid (MAA) in the presence of a crosslinker, poly(N-acryloyl glycinamide-co-methacrylic acid) [P(NAGA-MAA)] copolymer microgels with an MAA molar fraction of 10-70 mol % were obtained. The polymerization kinetics suggests that the copolymer microgels have a random structure. The size of the microgels was between 60 and 120 nm in the non-aggregated swollen state in aqueous medium and depending on the solvent conditions, they show reversible swelling and shrinking upon temperature change. Their phase transition behavior was studied by a combination of methods to understand the process of the UCST-type behavior and interactions between NAGA and MAA. P(NAGA-MAA) microgels were loaded with silver nanoparticles (AgNPs) by the reduction of AgNO3 under UV light. Compared with the chemical reduction of AgNO3, the photoreduction results in smaller AgNPs and the amount and size of the AgNPs are dependent on the comonomer ratio. The catalytic activity of the AgNP-loaded microgels in 4-nitrophenol reduction was tested.Peer reviewe

Phase Separation of Aqueous Poly(diisopropylaminoethyl methacrylate) upon Heating

Poly(diisopropylaminoethyl methacrylate) (PDPA) is a pH- and thermally responsive water-soluble polymer. This study deepens the understanding of its phase separation behavior upon heating. Phase separation upon heating was investigated in salt solutions of varying pH and ionic strength. The effect of the counterion on the phase transition upon heating is clearly demonstrated for chloride-, phosphate-, and citrate-anions. Phase separation did not occur in pure water. The buffer solutions exhibited similar cloud points, but phase separation occurred in different pH ranges and with different mechanisms. The solution behavior of a block copolymer comprising poly(dimethylaminoethyl methacrylate) (PDMAEMA) and PDPA was investigated. Since the PDMAEMA and PDPA blocks phase separate within different pH- and temperature ranges, the block copolymer forms micelle-like structures at high temperature or pH.Poly(diisopropylaminoethyl methacrylate) (PDPA) is a pH-and thermally responsive water-soluble polymer. This study deepens theunderstanding of its phase separation behavior upon heating. Phase separationupon heating was investigated in salt solutions of varying pH and ionicstrength. The effect of the counterion on the phase transition upon heating isclearly demonstrated for chloride-, phosphate-, and citrate-anions. Phaseseparation did not occur in pure water. The buffer solutions exhibited similarcloud points, but phase separation occurred in different pH ranges and withdifferent mechanisms. The solution behavior of a block copolymer comprisingpoly(dimethylaminoethyl methacrylate) (PDMAEMA) and PDPA wasinvestigated. Since the PDMAEMA and PDPA blocks phase separate withindifferent pH- and temperature ranges, the block copolymer forms micelle-likestructures at high temperature or pHPeer reviewe

Well-dispersed clay in photopolymerized poly(ionic liquid) matrix

This contribution presents a methodology for combining the solvating power of ionic liquids with polymer composite synthesis. A polymerizable ionic liquid was used as solvent to disperse clay, after which the mixture was polymerized into a solid polymer-clay composite. Polymer-clay composites were prepared with filler load-ings up to 10 wt%. The addition of clay as filler enhanced mechanical properties; tensile strength and stiffness of the materials exhibited appreciable improvements. The glass transition temperature of the materials shifted to slightly higher temperatures due to the hindered segmental motions of the polymer chains. The improvements were the highest at approximately 5 wt% filler content. When the filler content was increased further, excessive aggregate formation impaired the material properties.Peer reviewe