Double stimuli-responsive behavior of linear and star-shaped poly(N,N-diethylaminoethyl methacrylate) in aqueous solution

We report on the synthesis and characterization of linear and star-shaped poly(N,N-diethylaminoethyl methacrylate) (PDEA). The synthesis was accomplished by Atom Transfer Radical Polymerization (ATRP) via a core-first approach using sugar-based multifunctional initiators. The investigation of the solution properties in water shows that PDEA is both pH- and temperature-responsive, analogous to the behavior of poly(N,N-dimethylaminoethyl methacrylate) (PDMA). In literature, PDEA is frequently referred to as being only pH-sensitive. The critical pH values for the aggregation are close to the apparent pK_a values in all cases, i.e. a high charge density is necessary to keep the polymers soluble. The cloud points show a strong dependence on the pH value of the solution but no dependence on either molecular weight or architecture. Thus, the two polymers differ only quantitatively, as PDEA has cloud points about 40 K lower than PDMA and critical pH values which are 1.5-2 units lower than PDMA.     

Blends of poly(2,5-dimethoxy aniline) and fluoropolymers as protective coatings

This work presents the study of blends formed by a substituted polyaniline (PAni), poly(2,5-dimethoxy aniline) (PDMA), and the fluoropolymers poly(vinylidene fluoride) (PVDF) and poly(tetrafluoroethylene-co-vinylidene fluoride-co-propylene) (co-PTFE). PDMA was electrochemically synthesized from monomer 2,5-dimethoxy aniline (DMA) onto stainless steel surfaces. The polymers and the ternary blends were dissolved in N,N-dimethyl formamide and were deposited onto polished stainless steel plates. Thermal analysis (differential scanning calorimetry (DSC) and thermogravimetric analysis (TG)) and atomic force microscopy (AFM) were used in order to evaluate the properties of the materials. Ternary blends with good adhesion properties and thermal stability (with degradation over 400 °C) were obtained. Porous morphology with indication of PVDF enrichment on surface was observed. Preliminary open circuit potential (OCP) test showed an increase in this potential in the blends, compared with uncoated stainless steel. Therefore, blends with phase separation and very promising new protective properties were produced. Co-PTFE guarantees good adhesion, PDMA the special corrosion protection, and both fluoropolymers contribute to the large thermal stability.     

pH/Temperature control of interpolymer complexation between poly(acrylic acid) and weak polybases in aqueous solutions

The formation of interpolymer complexes (IPC) between poly(acrylic acid) (PAA) and poly(acrylamide) (PAM), poly(N,N-dimethylacrylamide) (PDMA), and statistical copolymers of acrylamide (AM) and N,N-dimethylacrylamide (DMA) has been studied as a function of pH, salt concentration and temperature (0–70 °C). The cloud points of dilute solutions were measured by turbidimetry and phase diagrams were determined as a function of temperature and pH in pure water and as a function of pH and salt concentration at room temperature. For each temperature and salt concentration a critical pH (pH_crit) below which IPC are observed was defined. In the case of PAA/PAM, pH_crit continuously decreased with increasing temperature, from pH 3.5 at 0 °C to pH 1.9 at 60 °C (UCST-type). In the case of PAA/PDMA, pH_crit, increased with temperature. The LCST-type behavior of the hydrogen-bonding complex formed between PAA and PDMA was attributed to the dimethyl substitution of amide groups that puts in hydrophobic interactions at high temperature. PAA and statistical copolymers P(AM-co-DMA) showed an intermediate behavior between PAA/PAM and PAA/PDMA with a continuous shift from UCST-type to LCST-type with increasing amount of DMA. This behavior can be attributed to changes in configurational entropy due to the IPC formation and (for PDMA) to the release of water molecules initially confined in hydrophobic hydration cages around DMA units. While at low salt concentration, the stability of PAA/PAM and PAA/PDMA complexes only slightly increases with the screening of ionized acrylic units, there is a sharp increase of pH_crit at high salt concentration in relation with the weakening of the solvent quality. In this regime, the complex formation of PAA/PDMA is greatly enhanced compared to PAA/PAM due to the interference of hydrophobic interactions.     

Suppression of inhomogeneities in hydrogels formed by free-radical crosslinking copolymerization

Network microstructures of poly(acrylamide) (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) hydrogels were compared by static light scattering and elasticity measurements. The hydrogels were prepared by free-radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N-dimethylacrylamide (DMA) with N,N′-methylenebis(acrylamide) as a crosslinker. During the formation of PAAm gels, the reaction time dependence of the scattered light intensity exhibits a maximum at a critical reaction time, while in case of PDMA gels, both a maximum and a minimum were observed, corresponding to the chain overlap threshold and the gel point, respectively. This difference in the time-course between the two gelling systems is due to the late onset of gelation in the DMA system with respect to the critical overlap concentration. Compared to the AAm system, no significant scattered light intensity rise was observed during the crosslinking polymerization of DMA. It was shown that, regardless of the crosslinker ratio and of the initial monomer concentration, PDMA gel is much more homogeneous than the corresponding PAAm gel due to the shift of the gelation threshold to the semidilute regime of the reaction system. The results suggest that the spatial gel inhomogeneity can be controlled by varying the gel point with respect to the critical overlap concentration during the preparation of gels by free-radical mechanism.     

Electrocatalytic oxidation of ascorbic acid by [Fe(CN)6] redox couple electrostatically trapped in cationic N,N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN)₆]⁴⁻), was electrostatically trapped in the cationic polymer film of N,N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN)₆]^3−/4−|PDMA|BDD) electrode. This [Fe(CN)₆]^3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ_Fe, within the polymer film. Diffusion coefficient (D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10⁻⁶ cm² s⁻¹. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10⁴ M⁻¹ s⁻¹. In the hydrodynamic amperometry using the [Fe(CN)₆]^3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm⁻² μM⁻¹.     

Reentrant conformation transition in poly(N,N-dimethylacrylamide) hydrogels in water-organic solvent mixtures

Conformational changes in poly(N,N-dimethylacrylamide) (PDMA) networks swollen in aqueous solutions of organic solvents are studied both experimentally and theoretically. PDMA hydrogels of various charge densities were prepared by free-radical crosslinking copolymerization. Swelling behavior of the hydrogels was investigated in aqueous organic solvent mixtures as functions of solvent species and the concentration. With increasing volume fraction ? of acetone, tetrahydrofuran, or 1,4-dioxane in the aqueous solution, PDMA hydrogels exhibit reentrant conformation transition. During this transition, the gel first deswells in the range of ? between 0.4 and 0.9, and then rapidly reswells if ? is monotonically increased. The reswelling of the collapsed PDMA gel occurs in a narrow of ? above ?=0.97. It was shown that the reentrant transition in PDMA gels requires moderate hydrogen bonding organic solvents, so that the hydrophobic interactions between PDMA and the organic solvent dominate the swelling process. The results were interpreted using the theory of equilibrium swelling. The interaction parameters in the gel system as well as the partition parameter of the organic solvent between the gel and the solution phases were calculated.     

New derivatives of indazole as electronically active materials

Synthesis and thermal, optical, electrochemical and photoelectrical properties of new indazole-based electroactive materials are reported. 1-Phenyl-5(6)-[N,N-(bisphenyl)]aminoindazoles and their methoxy-substituted analogues exhibit high thermal stabilities with the onset temperatures of thermal degradation ranging from 352 to 424 °C. The synthesized indazole derivatives form glasses with glass transition temperatures ranging from 35 to 39 °C. The synthesized compounds are electrochemically stable: their cyclic voltammograms show one reversible oxidation couple and no reduction waves. The ionization potentials of the solid samples of the synthesized materials are in the range of 5.3-5.9 eV. Methoxy-substituted derivatives show lower ionization potentials. Time-of-flight hole drift mobilities of 50% solid solution of 1-(4-methoxyphenyl)-5-{N,N-[bis(4-methoxyphenyl)]}aminoindazole in bisphenol Z polycarbonate reach 10⁻⁵ cm²/V s at high electric fields.     

Thermal degradation in bulk and thin films of 2-, 4-, and 6-arm polystyrene stars with a C60 core

The thermal stability (TS) of hexa-, tetra-, and di-arm polystyrene (PS) stars with a C₆₀ core was studied by thermal gravimetric analysis and mass-spectrometry. The quantitative production of volatile products, their composition and their formation kinetics during heating of (PS_xC₆₀) are reported. A bimodal release of styrene is observed. The first release takes place about 100 °C before the depolymerization temperature of styrene and all the C₆₀ comes out at this lower temperature. That results from a complete breaking of the weak PS-C₆₀ bonds followed by a partial depolymerization of the PS arms initiated by the so formed radicals. The amount of PS ‘surviving’ this first depolymerization step increases with the length of the arms and its TS is close to that of pure PS. The thermal stability of the PS_xC₆₀ stars decreases if the number of arms increases and, from the activation energy of the release of styrene and C₆₀, it was possible to estimate the PS-C₆₀ bond strength for these three adducts.     

RAFT cryopolymerizations of acrylamides and acrylates in dioxane at −5 °C

Reversible addition-fragmentation chain transfer (RAFT) cryopolymerizations of acrylamides and acrylates were successfully carried out at −5 °C with cumene hydroperoxide/ascorbic acid as redox initiation couple and 2-dodecylsulfanyl- thiocarbonylsulfanyl-2-methylpropinoic acid as chain transfer agent. The cryopolymerization features of N,N-dimethylacrylamide (DMA) and tert-butyl acrylate (tBA) were investigated in view of kinetics, molecular weight and its distribution by proton nuclear magnetic resonance analysis and gel permeation chromatography. Furthermore, sequential block cryopolymerizations of N-isopropylacrylamide were performed with the obtained trithiocarbonate- functionalized PDMA or PtBA as macro-CTA and the corresponding block polymers were obtained. All the results demonstrated that these cryopolymerizations bear all the characteristics of controlled/living radical polymerizations.     

b-direction anomalous thermal expansion behavior in SmCrO3

The ceramic composites of Sm_1-xBi_xCrO₃ (x = 0, 0.1, 0.2, 0.3) have been prepared by solid state reaction. X-ray diffraction patterns demonstrate that Bi-doped polycrystalline samples match with standard pattern of SmCrO₃. After Bi doping, the lattice constants in three directions increase slightly. The thermal expansion properties were studied by X-ray powder diffraction over the temperature range from 100 to 400 K. The results showed that the compound SmCrO₃ exhibits positive thermal expansion behavior in the a and c directions in the whole measurement temperature range. The anomaly is that in the b direction the lattice exhibits positive thermal expansion behavior at low temperatures, while above 200 K it exhibits near zero thermal expansion behavior. The thermal expansion behaviors of all Bi-doped SmCrO₃ are similar to that of SmCrO₃, but unexpectedly, negative thermal expansion behaviors were observed in the b direction above 200 K in the highly Bi doped samples. The dielectric measurement shows that there are two thermal activation processes in all Sm_1-xBi_xCrO₃ (x = 0, 0.1, 0.2, 0.3) respectively, with the low-temperature activation corresponding to the grain itself, while the high-temperature activation comes from the grain boundary. After low-temperature thermal activation, with increasing temperature, the enhanced vibration of the corner oxygen atoms of the CrO₆ octahedron is mainly perpendicular to the b axis, which leads to the near zero thermal expansion behavior in the b direction. Moreover, after high-temperature thermal activation, as the temperature increases, the enhanced interaction between Bi³⁺ and the free oxygen atom (space oxygen atom) at the grain boundary will further lead to the negative thermal expansion behavior in the b-direction of the high concentration doped samples.     

Novel soluble polyimides derived from 2,2′-bis[4-(5-amino-2-pyridinoxy)phenyl]hexafluoropropane: Preparation,characterization, and optical,dielectric properties

Two novel aromatic diamine monomer, 2,2′-bis[4-(5-amino-2-pyridinoxy)phenyl] hexafluoropropane (6FBAPDP) and 2,2′-bis[4-(5-amino-2-pyridinoxy)phenyl] propane (BAPDP), were successfully synthesized. Aimed at clarifying the structure-property relationships of pyridine-containing high-performance polymers, a series of novel fluorinated polyimides PI-(1–4) were prepared from 6FBAPDP with various commercially aromatic anhydrides, and polyimide (PI-5) was synthesized derived from BAPDP and 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) via a two-stage process with heating imidization method. The fluorinated polyimides PI-(1–4) exhibited good solubility in strong polar solvents, such as N, N-dimethylacetamide, N, N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, m-cresol, and could afford flexible, tough and transparent films with an UV–visible absorption cut-off wavelength at 342–393 nm. The glass transition temperatures of these polyimides were recorded between 239 and 306 °C by differential scanning calorimetry, and the 5% weight loss occurred at temperatures above 498 and 490 °C, in nitrogen and air, respectively. The polyimide films had the in-plane coefficients of thermal expansion (CTE) that ranged from 54 to 74 ppm °C⁻¹. Moreover, the fluorinated polyimide films showed low moisture absorptions of 0.51–0.82% and outstanding mechanical properties with the tensile strengths of 75–100 MPa, tensile moduli of 3.2–4.0 GPa and elongation at break of 5.5–10.3%, good dielectric properties with low dielectric constants of 2.71–2.92 at 1 MHz.     

Fabrication of thermo-responsive hydrogels from star-shaped copolymer with a biocompatible β-cyclodextrin core

A thermally reversible hydrogel composed of a three-arm star copolymer with a specific host β-cyclodextrin (β-CD) center has been developed. The synthesis of this star copolymer initiates with β-CD core, from which sequential polymerization of a temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) block and a hydrophilic poly(N,N-dimethylacrylamide) (PDMA) block as asymmetric arms (named β-CD-g-(PNIPAM-b-PDMA)₃) is performed via RAFT protocol. Below the lower critical solution temperature (LCST) of PNIPAM segment, the polymer is of good water-solubility and exhibits a sol state. Upon thermal stimulus, free-standing hydrogels can be formed rapidly at sufficiently high concentrations. By comparing the sol–gel transition of the star polymer with that of its linear counterpart without this feature, we concluded that the special star-shape topology and the thermal-collapsed PNIPAM chains were responsible for this gelation behavior. The rheology measurements indicate the mechanical properties of the polymer hydrogels and the thermal reversibility of the sol–gel transition. Using Rhodamine B as a molecule to model a typical drug, we realize the favorable encapsulation and releasing process from the hydrogel, demonstrating that this star polymer has the potential to function as an injectable hydrogel for drug delivery and gene transport.     

Electromagnetic shielding and thermal conductive properties of SiC nanowires reinforced C/(PyC-SiC)n composites

SiC was introduced as nanowires and multilayered structure matrix to modify C/C composites, then SiC nanowires reinforced C/(PyC-SiC)_n (SM-CS) composites were prepared. The electromagnetic shielding and thermal conductive properties were investigated and the further relationship between these properties and the number of cycles of preparation (N) was also studied. The results showed that total shielding effectiveness (SET) values of modified composites were all higher than 30 dB which meant more than 99.9% electromagnetic wave was shielded. And the SET values increased with the rising of N (SM-CS4--47.6 dB > SM-CS3--42.7 dB > SM-CS2--37.1 dB). With the rising of N, not only the conductivity of SM-CSN composite increased, but also the interfaces inside the matrix increased, leading to a continuous increase in reflection and absorption of electromagnetic waves. Meanwhile, the thermal diffusivities and conductivities of the SM-CS composites in the temperature range from 25 to 1500 °C were all higher than those of pure C/C composites, and they were also almost increased by N. That was because the improvement of SiC nanowires in heat transport was very large, and even exceeded the reducing of multilayered structure. Due to these good functional properties, the modified composites would exhibit excellent potential in aerospace field.     

Quantitative microstructural analysis and piezoelectricity of highly dense,submicron-structured NaNbO3 ceramics from mechanically activated precursors

We study in this work the processing of NaNbO₃ ceramics prepared in a single thermal treatment of highly reactive precursors obtained by mechanical activation of different reagents, aiming to determine optimum conditions for piezoelectric ceramics production. Pressure-less sintering at 1200 °C leads to dense ceramics (<5% porosity) with poor mechanical stability, unsuitable for practical uses. Dense hot-pressed ceramics were also obtained at lower temperatures (900–1100 °C), all of them in the submicron range of average grain sizes (<400 nm). Their microstructure was quantitatively characterized and their elastic and electromechanical properties determined by an automatic iterative method from impedance measurements at resonance. A noticeable ensemble of piezoelectric and elastic properties (d₃₃ = 38 pC N^?1 and N_p = 3252 kHz mm) was measured for hot-pressed ceramics, from precursors obtained by a combined route of wet-chemistry and mechanical activation, with a microstructure characterized by 0.4% residual porosity and a bimodal lognormal distribution of grain size.     

Synergistic effect of binary fluoride sintering additives on the properties of silicon nitride ceramics

A variety of combinations of YF₃ and MgF₂ were used as sintering aids in the fabrication of Si₃N₄ ceramics via gas pressure sintering (GPS). The synergistic effects of YF₃ and MgF₂ on the liquid viscosity, mechanical properties, thermal conductivities, and grain growth kinetics of the Si₃N₄ ceramics were investigated. The results showed that appropriately adjusting the YF₃/MgF₂ ratio could decrease liquid viscosity, reducing the diffusion energy barrier of the solute atom and promoting mass transfer. Meanwhile, the chemical bonding strength in the grain boundary complexions formed by the metal cations also influenced grain boundary migration. Samples doped with 4 mol% YF₃ and 2 mol% MgF₂ achieved the lowest grain growth exponent (n = 2.9) and growth activation energy (Q = 616.7 ± 16.5 kJ mol^?1) as well as the highest thermal conductivity (83 W m^?1 K^?1) and fracture toughness (8.82 ± 0.13 MPa m^1/2).     

Effect of in situ synthesized and additives on the thermal performance of mullite thermal storage ceramics

High-performance thermal storage ceramics can enable utilization of solar thermal power generation plants. In this work, in situ synthesis was used to prepare mullite thermal storage ceramics. Calcined bauxite, talc, and kaolin were used as raw materials. The effects of additives (e.g., SiC, Si₃N₄, TiC, and ZrB₂) on the density, mechanical durability, phase components, microstructure, and thermal performance of the mullite ceramics were studied. The results showed that the thermal expansion coefficient, thermal conductivity, and heat storage density of the mullite ceramics were affected by their phase components. SiC and Si₃N₄ did not decompose during the in situ syntheses, but TiC and ZrB₂ decomposed. With the addition of 10 wt% SiC, the thermal conductivity improved to 2.72 W (m K)^?1 (298 K). The heat storage density of this material was 688 kJ kg^?1 (273–1073 K). Consequently, the in situ synthesized mullite thermal storage ceramic with added SiC could be a promising candidate material for a compound latent-sensible heat storage system.     

Effects of oxygen on the thermal stability of silicon nitride fibers

Two types of Si₃N₄ fibers with different oxygen contents were annealed in a nitrogen atmosphere at 1500 °C for 1 h. After annealing, the fiber (SN-L) containing 0.5 wt% oxygen crystallized to α-Si₃N₄ but lost its strength, whereas the fiber (SN-H) containing 4.2 wt% oxygen was amorphous and retained 63.6% of its strength. The phase transition in these fibers was mainly influenced by the oxygen level. The lower oxygen content in the SN-L favored the precipitation of an almost stoichiometric composition of α-Si₃N₄ initially at ~1450 °C with an activation energy (Ea) of 663.357 kJ/mol. Nanopores existing naturally in the fiber promoted crystallization via heterogeneous nucleation. SN-H precipitated as an amorphous SiN_xO_y metaphase preferentially at ~1400 °C with an Ea of 440.434 kJ/mol, owing to the higher oxygen content approaching that of Si₂N₂O. SiN_xO_y inhibited the crystallization of α-Si₃N₄, making SN-H more thermally stable than SN-L at temperatures above 1500 °C.     

Study on the influence of metal residue on thermal degradation of poly(cyclohexene carbonate)

Poly(cyclohexene carbonate) (PCHC) is an alternative copolymer of carbon dioxide and cyclohexene oxide. Severe thermal decomposition occurs during melt processing of PCHC, thus, it is important to find the reason for thermal decomposition and raise its thermal stability. In this report, as-polymerized PCHC was obtained under Y(CCl₃COO)₃-ZnEt₂-glycerin rare earth coordination ternary catalyst. The pyrolysis gas chromatography with mass spectrometry analysis disclosed that chain unzipping depolymerization dominated the thermal decomposition process. The residue metal (mainly ZnO in this catalyst system) was important in PCHC degradation, and the onset decomposition temperature of PCHC containing 5 ppm zinc was 56 °C higher than that of PCHC containing 4,400 ppm Zn, corresponding to apparent activation energy change from 190 KJ/mol to 146 KJ/mol. When PCHC was subjected to melt processing, its average molecular weight decreased more rapidly, as it contained more catalyst residue, indicating metal residue in PCHC could accelerate the decomposition process, and removing the metal residue should be effective in raising its thermal stability.     

Synthesis,characterisation and solution thermal behaviour of a family of poly (N-isopropyl acrylamide-co-N-hydroxymethyl acrylamide) copolymers

Thermo-responsive copolymers of poly (N-isopropyl acrylamide-co-N-hydroxymethyl acrylamide)p(NIPAAm-co-HMAAm) with a range of content of hydroxy groups have been synthesised by free radical polymerisation. The polymers were characterised by NMR, FTIR and GPC. A detailed study of the solution thermal properties showed that polymers with up to about 50 wt.% hydroxy monomer show lower critical solution temperature (LCST) properties in water. Polymers with higher hydroxy monomer content are fully soluble at all temperatures up to 100 °C. The effect of pH, salts and solvent additives on the solution thermal behaviour of the copolymers was investigated, showing that “salting-out” salts lowered the LCST and “salting-in” salts caused an initial increase in LCST at low concentrations, but reduced LCST at higher concentrations, in line with the Hoffmeister series. The LCST of any copolymer composition from this family in pure water can be predicted from the empirical equation; LCST = 0.015x² + 0.25x + 31.76, where x is the fraction of the hydroxy monomer. Due to differences in polarity and the length of the carbon chain, methanol and ethanol altered LCST of p(NIPAAm) and its hydroxyl copolymers in different manners, showing a transition from cononsolvency to cosolvency as the hydroxyl content of the copolymer increased. A more complex polyhydroxy compound, sucrose, had very little effect on LCST for either p(NIPAAm) or its hydroxyl copolymers.