Critical Solution Temperature Behavior Research Articles

Multifunctional Polymeric Platform of Magnetic Ferrite Colloidal Superparticles for Luminescence, Imaging, and Hyperthermia Applications.

Adequately designed multiresponsive water-soluble graft copolymers were used to serve as a multifunctional polymeric platform for the encapsulation and transfer in aqueous media of hydrophobic magnetic nanoparticles (MNPs). The backbone of the graft copolymers was composed of hydrophilic sodium methacrylate units, hydrophobic dodecyl methacrylate units, and luminescent quinoline-based units, while either the homopolymer poly(N-isopropylacrylamide) or a poly(N,N-dimethylacrylamide-co-N-isopropylacrylamide) copolymer was used as thermosensitive pendent side chains. The polymeric platform forms micellar-type assemblies in aqueous solution, and exhibits pH-responsive luminescent properties and a lower critical solution temperature behavior in water. Depending on the design of the side chains, the cloud point temperatures were determined at 38 and 42 °C, close or slightly above body temperature (37 °C). Above the critical micelle concentration (CMC), both graft copolymers can effectively stabilize in aqueous media as magnetic colloidal superparticles (MSPs), oleylamine-coated MnFe2O4 MNPs, as well as 1:1 mixture of oleylamine-coated MnFe2O4 and CoFe2O4 MNPs. When CoFe2O4 particles were mixed with MnFeO4 in equal amounts, the specific loss power increased significantly, while an opposite trend was observed in the magnetic resonance imaging (MRI) studies, probably due to the anisotropy of cobalt. As a consequence, fine-tuning of the chemical structure of the copolymers and the composition of the MSPs can lead to materials that are able to act simultaneously as luminescent, hyperthermia, and contrast MRI agents.

High‐Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi‐Length Scale Morphology and Device Performance

Organic solar cells (OSCs) made of donor/acceptor bulk‐heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X‐ray scattering) of the record‐efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%‐efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in‐depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.

Reversibly Tunable Lower Critical Solution Temperature Behavior Induced by H-Bonded Aromatic Amide Macrocycle and Imidazolium Host-Guest Complexation.

A new supramolecular host-guest motif comprising an H-bonded aromatic amide macrocycle and imidazolium cation based ionic liquids was developed, which allows tunable binding affinity via altering N-substitution or counterions. This host-guest system exhibits lower critical solution temperature behavior that can be precisely controlled by adjusting concentration, competitive guest, and acid/base. The demonstrated separation of two organic dyes with the supramolecular complex holds promising applications in separation science.

RAFT preparation and self-assembly behavior of thermosensitive triblock PNIPAAm-b-PODA-b-PNIPAAm copolymers

A series of well-defined thermoresponsive ABA-type triblock copolymers poly(N-isopropylacrylamide)-b-poly(n-octadecylacrylate)-b-poly(N-isopropylacrylamide) (PNIPAAm-b-PODA-b-PNIPAAm) consisting of two blocks of PNIPAAm and one block of PODA in different ratio are prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The structures of the copolymers are characterized by 1HNMR. All block copolymers show lower critical micelle concentration (CMC) (<10 mg/L) in water, indicating the strong tendency of the triblock copolymers to self-assemble into highly stable micelles. It is interesting that the resulting micelles demonstrate double lower critical solution temperature (LCST) behavior. Dynamic light-scattering (DLS) experiments show that the micelles are well dispersed at nanoscale and have a narrow size distribution. Moreover, the drug-loading behavior of micelles is further investigated. The high drug encapsulation efficiency indicates the potential of micelles formed from PNIPAAm-b-PODA-b-PNIPAAm as drug carriers.

Multifunctional thermoresponsive designer peptide hydrogels.

In general, it has been frequently observed that chemical biofunctionalization of peptide hydrogels adversely affects peptide assembly, hydrogel formation or mechanical properties, which severely compromises their application. A functionalization protocol that allows to generate peptide hydrogels that display significantly improved mechanical properties over their unfunctionalized counterparts is reported in this work. These peptides also showed thermoresponsive viscoelastic characteristics, including an example of a peptide hydrogel that displays lower critical solution temperature behaviour.

New stimuli-responsive polyampholyte: Effect of chemical structure and composition on solution properties and swelling mechanism

Abstract New polyampholyte linear polymers and hydrogels based on N -acryloyl- N ′-ethyl piperazine (AcrNEP) and maleic acid (MaA) were prepared by free-radical solution polymerization. The microstructure of copolymers was statistically estimated by the terminal copolymerization model. A maximum tendency (80%) of alternating sequence of monomers units occurred at about 85 mol % of AcrNEP in the reaction feed. The copolymers at definite monomer compositions (40 mol % of AcrNEP and 59.6 mol % of MaA) exhibited lower critical solution temperature (LCST) behavior in water. The LCST was dependent on monomer composition, pH of external solution, type and concentration of simple salts. The isoelectric points (IEP) of copolymers were found to be influenced by the overall composition of ionic monomer units. The influence of high maleic acid content (14 mol % and 59 mol %) and the type of chemical crosslinker on pH-responsive swelling behavior, water diffusion and dye sorption properties of the hydrogels were studied in detail. The gels exhibited interesting swelling behavior as function of pH (ionic strength = 0.01 M) due to a combination of factors such as pK a of the monomers, charge density, and type of crosslinker. The IEP of the gels were also determined from swelling experiments. The swelling ratio of the gels increased at the IEP with increase in ionic strength of the solution due to anti-polyelectrolyte effect. However below the IEP, the swelling ratio decreased considerably. Dynamic swelling properties of the amphoteric gels in water without added salt were measured. The swelling behavior was found to depend on the ionization state of the monomers, and the swelling followed non-Fickian (anomalous) mechanism. The swelling process of the hydrogels in water followed second-order kinetics. The amount of bound and unbound water in swollen gels was quantified using differential scanning calorimetry. Dye adsorption capacity of the new hydrogels was investigated using a Congo red as a model dye and was in the range 8.37–11.45 mg g −1 . This corresponds to an absorption efficiency of range 68–93%.

Photoactivation of Noncovalently Assembled Peptide Ligands on Carbon Nanotubes Enables the Dynamic Regulation of Stem Cell Differentiation.

Stimuli-responsive hybrid materials that combine the dynamic nature self-assembled organic nanostructures, unique photophysical properties of inorganic materials, and molecular recognition capability of biopolymers can provide sophisticated nanoarchitectures with unprecedented functions. In this report, infrared (IR)-responsive self-assembled peptide-carbon nanotube (CNT) hybrids that enable the spatiotemporal control of bioactive ligand multivalency and subsequent human neural stem cell (hNSC) differentiation are reported. The switching between the ligand presented and hidden states was controlled via IR-induced photothermal heating of CNTs, followed by the shrinkage of the thermoresponsive dendrimers that exhibited lower critical solution temperature (LCST) behavior. The control of the ligand spacing via molecular coassembly and IR-triggered ligand presentation promoted the sequential events of integrin receptor clustering and the differentiation of hNSCs into electrophysiologically functional neurons. Therefore, the combination of our nanohybrid with biomaterial scaffolds may be able to further improve effectiveness, durability, and functionality of the nanohybrid systems for spatiotemporal control of stem cell differentiation. Moreover, these responsive hybrids with remote-controllable functions can be developed as therapeutics for the treatment of neuronal disorders and as materials for the smart control of cell function.

LCST Behavior of Symmetrical PNiPAm-b-PEtOx-b-PNiPAm Triblock Copolymers

Poly(N-isopropylacrylamide) (PNiPAm) and poly(2-ethyl-2-oxazoline) (PEtOx) represent two polymer types that are well-known for their lower critical solution temperature (LCST) behavior in aqueous media. To synthesize triblock copolymers containing both polymers, a crossover of two different polymerization methods was applied using a bifunctional initiator for the living cationic ring-opening polymerization (CROP) of EtOx. Quantitative end-functionalization with a trithiocarbonate resulted in a bifunctional PEtOx macro chain transfer agent (CTA). A series of well-defined PNiPAm-b-PEtOx-b-PNiPAm triblock copolymers were obtained by subsequent reversible addition–fragmentation chain transfer (RAFT) polymerization of NiPAm. The influence of the PNiPAm to PEtOx ratio on the thermoresponsive properties was intensively investigated via turbidimetry, dynamic light scattering, cryo transmission electron microscopy, and 1H NMR studies, revealing hydrogen bonds between both copolymer segments that strongly lower the...

Morphological change of thermosensitive imidazolium‐based poly(ionic liquid)/poly(phenylethylmethacrylate) composite particles

Composite particles comprising poly(2‐phenylethyl methacrylate) (PPhEMA) and imidazolium‐based poly(ionic liquid)s were prepared by suspension polymerization of 1‐vinyl‐3‐ethylimidazolium bis(trifluoromethanesulfonyl)amide as an ionic liquid monomer with dissolved PPhEMA. Not only PPhEMA exhibits lower critical solution temperature (LCST) behavior in 1‐vinyl‐3‐ethylimidazolium bis(trifluoromethanesulfonyl)amide but also the polymer blend in the bulk state exhibited LCST behavior. However, the composite polymer particles obtained after polymerization at 70°C maintained a homogeneous inner structure after heat treatment as the polymerization temperature was greater than the LCST in this system due to the formation of a cross‐linked structure during polymerization. When the composite particles were prepared by suspension polymerization at 30°C, their inner morphology changed from homogeneous to phase separated during the subsequent heat treatment. Moreover, the morphology transformation of the composite particles was dependent on the PPhEMA molecular weight. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Thermal- and pH-Dependent Size Variable Radical Nanoparticles and Its Water Proton Relaxivity for Metal-Free MRI Functional Contrast Agents.

For development of the metal-free MRI contrast agents, we prepared the supra-molecular organic radical, TEMPO-UBD, carrying TEMPO radical, as well as the urea, alkyl group, and phenyl ring, which demonstrate self-assembly behaviors using noncovalent bonds in an aqueous solution. In addition, TEMPO-UBD has the tertiary amine and the oligoethylene glycol chains (OEGs) for the function of pH and thermal responsiveness. By dynamic light scattering and transmission electron microscopy imaging, the resulting self-assembly was seen to form the spherical nanoparticles 10-150 nm in size. On heating, interestingly, the nanoparticles showed a lower critical solution temperature (LCST) behavior having two-step variation. This double-LCST behavior is the first such example among the supra-molecules. To evaluate of the ability as MRI contrast agents, the values of proton ((1)H) longitudinal relaxivity (r1) were determined using MRI apparatus. In conditions below and above CAC at pH 7.0, the distinguishable r1 values were estimated to be 0.17 and 0.21 mM(-1) s(1), indicating the suppression of fast tumbling motion of TEMPO moiety in a nanoparticle. Furthermore, r1 values became larger in the order of pH 7.0 > 9.0 > 5.0. Those thermal and pH dependencies indicated the possibility of metal-fee MRI functional contrast agents in the future.

Aggregation Behavior of Doubly Thermoresponsive Polysulfobetaine-b-poly(N-isopropylacrylamide) Diblock Copolymers

A 2-fold thermoresponsive diblock copolymer PSPP430-b-PNIPAM200 consisting of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block is prepared by successive RAFT polymerizations. In aqueous solution, the corresponding homopolymers PSPP and PNIPAM feature both upper and lower critical solution temperature (UCST and LCST) behavior, respectively. The diblock copolymer exhibits thermally induced “schizophrenic” aggregation behavior in aqueous solutions. Moreover, the ion sensitivity of the cloud point of the zwitterionic PSPP block to both the ionic strength and the nature of the salt offers the possibility to create switchable systems which respond sensitively to changes of the temperature and of the electrolyte type and concentration. The diblock copolymer solutions in D2O are investigated by means of turbidimetry and small-angle neutron scattering (SANS) with respect to the phase behavior and the self-assembled structures in dependence on temperature and el...

Cation-Induced Hydration Effects Cause Lower Critical Solution Temperature Behavior in Protein Solutions.

The phase behavior of protein solutions is important for numerous phenomena in biology and soft matter. We report a lower critical solution temperature (LCST) phase behavior of aqueous solutions of a globular protein induced by multivalent metal ions around physiological temperatures. The LCST behavior manifests itself via a liquid-liquid phase separation of the protein-salt solution upon heating. Isothermal titration calorimetry and zeta-potential measurements indicate that here cation-protein binding is an endothermic, entropy-driven process. We offer a mechanistic explanation of the LCST. First, cations bind to protein surface groups driven by entropy changes of hydration water. Second, the bound cations bridge to other protein molecules, inducing an entropy-driven attraction causing the LCST. Our findings have general implications for condensation, LCST, and hydration behavior of (bio)polymer solutions as well as the understanding of biological effects of (heavy) metal ions and their hydration.

Xanthate-Functional Temperature-Responsive Polymers: Effect on Lower Critical Solution Temperature Behavior and Affinity toward Sulfide Surfaces.

Xanthate-functional polymers represent an exciting opportunity to provide temperature-responsive materials with the ability to selectively attach to specific metals, while also modifying the lower critical solution temperature (LCST) behavior. To investigate this, random copolymers of poly(N-isopropylacrylamide) (PNIPAM) with xanthate incorporations ranging from 2 to 32% were prepared via free radical polymerization. Functionalization with 2% xanthate increased the LCST by 5 °C relative to the same polymer without xanthate. With increasing xanthate composition, the transition temperature increased and the transition range broadened until a critical composition of the hydrophilic xanthate groups (≥18%) where the transition disappeared completely. The adsorption of the polymers at room temperature onto chalcopyrite (CuFeS2) surfaces increased with xanthate composition, while adsorption onto quartz (SiO2) was negligible. These findings demonstrate the affinity of these functional smart polymers toward copper iron sulfide relative to quartz surfaces, presumably due to the interactions between xanthate and specific metal centers.

Molecular characterization of thermoreversibility and temperature dependent physical properties of cellulose solution in N,N-dimethylacetamide and lithium chloride

The effects of temperature on the physical properties of the cellulose solutions in N,N-dimethylacetamide (DMAc) containing 9 (solvent-9) or 6 wt% (solvent-6) lithium chloride (LiCl) were investigated over the temperature range of 30 to 80 °C. The cellulose solution exhibited a lower critical solution temperature (LCST) behavior over the temperature range observed. The content of LiCl affected the thermoreversible LCST behavior of cellulose solutions, which was almost thermoreversible over the temperature range of 30 to 80 °C for solvent-9 and 30 to 50 °C for solvent-6. The partial thermoreversibility of cellulose chain between 60 and 80 °C in solvent-6 could be explained by increased intramolecular interactions between cellulose molecules with increasing temperature. The thermoreversible LCST behavior of cellulose solution for solvent-9 was further supported by dynamic light scattering measurement which also verified the larger decrease of cellulose chain dimensions in solvent-6 between 60 and 80 °C. The cellulose solutions in DMAc/LiCl exhibited little thermal degradation in the short-term aging between 30 and 80 °C. However, they produced a little thermal degradation in the long-term aging between 80 and 100 °C.

Ionoprinted Multi-Responsive Hydrogel Actuators.

We report multi-responsive and double-folding bilayer hydrogel sheet actuators, whose directional bending response is tuned by modulating the solvent quality and temperature and where locally crosslinked regions, induced by ionoprinting, enable the actuators to invert their bending axis. The sheets are made multi-responsive by combining two stimuli responsive gels that incur opposing and complementary swelling and shrinking responses to the same stimulus. The lower critical solution temperature (LCST) can be tuned to specific temperatures depending on the EtOH concentration, enabling the actuators to change direction isothermally. Higher EtOH concentrations cause upper critical solution temperature (UCST) behavior in the poly(N-isopropylacrylamide) (pNIPAAm) gel networks, which can induce an amplifying effect during bilayer bending. External ionoprints reliably and repeatedly invert the gel bilayer bending axis between water and EtOH. Placing the ionoprint at the gel/gel interface can lead to opposite shape conformations, but with no clear trend in the bending behavior. We hypothesize that this is due to the ionoprint passing through the neutral axis of the bilayer during shrinking in hot water. Finally, we demonstrate the ability of the actuators to achieve shapes unique to the specific external conditions towards developing more responsive and adaptive soft actuator devices.

Synthesis, Thermal Properties, and Thermoresponsive Behaviors of Cyclic Poly(2-(dimethylamino)ethyl Methacrylate)s.

This study aims at physicochemical properties of thermo- and pH/CO2 -responsive cyclic homopolymers. Three examples of cyclic poly(2-(dimethylamino)ethyl methacrylate)s (PDMAs) are synthesized by combining the reversible addition-fragmentation chain transfer process and the Diels-Alder ring-closure reaction. After cyclization, the glass transition temperature significantly increases (ΔTg = 51.8-59.7 °C) due to the different configurational entropy and end groups, and the maximum decomposition temperature to lose the pendent groups is drastically decreased from 309 to 278 °C. Effects of polymerization degree, polymer concentration, additive of NaCl, and pH/CO2 on lower critical solution temperature behaviors of PDMA aqueous solutions are investigated. The cloud points (Tc ) of ring PDMAs are usually higher than their linear precursors, and the ΔTc values obtained under a fixed condition can reach up to 20.7 °C, revealing the crucial role of the topology effect. This study paves the way for unique properties and applications of smart cyclic polymers and their derivatives.

Molecular Characterization on the Anomalous Viscosity Behavior of Cellulose Solutions in N,N-Dimethyl Acetamide and Lithium Chloride

The physical properties of dilute cellulose solutions in N,N-dimethyl acetamide (DMAc) including 9 wt% lithium chloride (LiCl) were investigated in terms of concentration, temperature and molecular weight of cellulose. Over the concentration range of 0.01 to 2.5 g/dL, the viscosity of the cellulose solutions exhibited a lower critical solution temperature (LCST) behavior which proved thermoreversible between 30 and 60 °C. The LCST behavior was further supported by dynamic light scattering measurement. In the extremely dilute concentration range, 0.01 to 0.08 g/dL, the reduced viscosity (η red ) of cellulose solutions was increased with decreasing concentration. The anomalous coil expansion with decreasing concentration could be explained by the increase of the conductivity of cellulose solutions with decreasing concentration, which was also verified by dynamic light scattering experiment. In the concentration range of 0.1 and 2.5 g/dL, both cellulose solutions gave a drastic increase of η red in the vicinity of the critical concentration (C*), 0.9 g/dL. The slope of the curve of η red vs. concentration was higher for the cellulose of higher molecular weight, but it did not change with temperature between 30 and 60 °C.

Construction of Smart Supramolecular Polymeric Hydrogels Cross-linked by Discrete Organoplatinum(II) Metallacycles via Post-Assembly Polymerization.

Postassembly modification strategy has been successfully employed in the construction of discrete metallosupramolecular assemblies. However, the most known reports have been limited to the simple structural conversion through the easy covalent reactions, thus hindering the development of organometallic functional materials. In this study, we first combined coordination-driven self-assembly and postassembly reversible addition-fragmentation chain-transfer (RAFT) polymerization to produce a new family of star supramolecular polymers containing well-defined metallacycles as cores, which featured typical lower critical solution temperature (LCST) behavior in water because of the existence of poly(N-isopropylacrylamide) (PNIPAAM) moieties. Moreover, the obtained star polymers could further form supramolecular hydrogels cross-linked by discrete hexagonal metallacycles at room temperature without heating-cooling process. Interestingly, the resultant polymeric hydrogels exhibited stimuli-responsive behavior toward temperature and bromide anion as well as self-healing property. We demonstrated that the dynamic nature of Pt-N bonds in the hexagonal metallacycles played an important role in determining the stimuli-responsive and self-healing property of the final soft matters. Thus, merging coordination-driven self-assembly and postassembly polymerization provided a new avenue to the preparation of functional materials containing well-defined, discrete metal-organic assemblies as main scaffolds.

Reactive polycarbonate/diallyl phthalate blends with high optical transparency, good flowability and high mechanical properties

Bisphenol-A polycarbonate (BAPC) is an important engineering plastic with superior optical and mechanical properties, but it is difficult to be processed due to the high melt viscosity. In this work, the blends of BAPC and reactive plasticizer of diallyl phthalate (DAP) were studied before and after polymerization of DAP using dicumyl peroxide (DCP) as thermal initiator. The morphology evolution and phase transitions of the blends during heating were investigated by polarized optical microscopy equipped with hot stage and differential scanning calorimetry. With raising temperature, the apparent phase transitions, i.e., thermally induced partial phase separation with upper critical solution temperature (UCST) behavior, BAPC cold crystallization, DAP polymerization and BAPC crystal melting, occurred in sequence. Compared to pure BAPC, the BAPC/poly(DAP) blends with 10–15 wt.% of poly(DAP) had good performances, including similar glass transition temperature, tensile strength and ∼80% of optical transmittance in the wavelength range of 600–800 nm, as well as an increase of 64–68% for moduli and 100–135% for melt flow index, respectively. These good performances were attributed to the bicontinuous structure of the blends. This study provides a facile strategy to realize the easy processing for intractable polymers and to maintain, even to enhance the high performances of the original polymers.

Tuning the wetting–dewetting and dispersion–aggregation transitions in polymer nanocomposites using composition of graft and matrix polymers

Recent simulation and experimental work on polymer nanocomposites composed of polymer grafted particles and free matrix polymers, where the graft and matrix homopolymers are chemically dissimilar and exhibit lower critical solution temperature behavior with temperature, has shown that wetting to dewetting is a gradual and distinct transition from the sharp particle dispersion–aggregation transition. In this study, using coarse-grained molecular simulations, we demonstrate that the extent of wetting of the grafted polymer layer and the particle dispersion–aggregation transition are tuned using the composition of graft and matrix polymers. Specifically, we study composites where the graft and matrix chains are random copolymers composed of attractive and athermal monomers. We maintain a dense grafting density on the spherical particles of diameter five times the monomer diameter and study matrix lengths five times that of the graft chain length or equal graft and matrix chain lengths. We vary the fraction of attractive monomers in the graft and matrix chains, graft–matrix chain composition ratio and the graft–matrix interaction strength, as characterized by the Flory–Huggins interaction parameter between graft and matrix attractive monomers: When is negative, decreasing and/or decreases the extent of grafted layer wetting by matrix chains because the enthalpic driving force for wetting is reduced. As the increases and becomes positive, the extent of wetting decreases gradually till it reaches the wetting of analogous athermal composites. That value of where the extent of wetting is the same as that of an analogous athermal polymer nanocomposite marks the onset of dispersion–aggregation transition. For symmetric graft and matrix chain compositions , the magnitude of and tunes the overall extent of wetting of the grafted particles in the dispersed state but not the dispersion–aggregation transition. Varying the asymmetry of the graft and matrix chain composition (i.e. fG/fM) tunes both the extent of wetting of the grafted layer and the dispersion–aggregation transition.