Photoisomerizable and thermoresponsive N-isopropylacrylamide-surfmer copolymer hydrogels prepared upon electrostatic self-assembly of an azobenzene bolaamphiphile.

Photoreactive and thermoresponsive N-isopropylacrylamide (NIPAM)-surfmer copolymer hydrogels containing 4,4'-di(6-sulfato-hexyloxy)azobenzene (DSHA) dianions are described. The functional hydrogels are obtained in a two steps. First a micellar aqueous solution of (11-(acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB) and NIPAM is exposed to (60) Co-gamma irradiation, and a thermoresponsive copolymer gel is obtained. Second, DSHA is included by shrinking the gel at 50 °C and subsequent reswelling in an aqueous solution of DSHA disodium salt at 20 °C. Reswelling is accompanied by electrostatic adsorption of DSHA dianions at the positively charged AUTMAB headgroups replacing the bromide ions. Gels containing trans-DSHA are transparent yellow at room temperature (λmax = 370 nm), while gels containing cis-rich DSHA are orange (λmax = 460 and 330 nm). Energy dispersive X-ray measurements indicate that 41% of the bromide ions are exchanged if trans-DSHA is used for adsorption, and only 7.5% if cis-DSHA is used. The incorporation of DSHA lowers the lower critical solution temperature (LCST) from 34 to 32 °C. Below the LCST, DSHA can be switched from the trans- to the cis-rich state and vice versa upon irradiation with UV (λ = 366 nm) or visible light (λ ≥ 450 nm). Above the LCST no photoreaction takes place.

Copolymer hydrogels of acrylic acid and a nonionic surfmer: pH-induced switching of transparency and volume and improved mechanical stability.

Copolymer hydrogels were prepared from an aqueous micellar solution of the nonionic surfactant monomer (surfmer) ω-methoxy poly(ethylene oxide)40undecyl-α-methacrylate (PEO-R-MA-40) and acrylic acid (AA) in a one-step reaction using γ-irradiation. The hydrogels were transparent if the polymerization was carried out at pH ≥ 4, whereas turbid gels were obtained if the polymerization was carried out at lower pH. Exposure of the turbid gels to an aqueous solution of pH 11 led to swelling and clearing, whereas subsequent exposure to pH 1 had the reverse effect. Clear gels prepared at pH 4 became turbid, if exposed to an aqueous solution of lower pH and became clear again if reswollen at higher pH. The pH at which clouding set in increased with the amount of surfmer copolymerized in the gel. Pure poly(acrylic acid) (P-AA) hydrogels did not show any changes in transparency if the pH was varied. The presence of surfmer led to more pronounced shrinking and swelling, especially if the gels were prepared at pH 4. The mechanical stability of P-AA and copolymer hydrogels was studied using elongational flow measurements. The presence of surfmer led to increased mechanical stability of the hydrogels. The increase originates from copolymerized micellar aggregates acting as additional, stable cross-linking units in the gel. The true stress at break of copolymer hydrogels prepared at pH 2.4 (or 4) was 5.5 (or 3.4) times larger than for surfmer-free P-AA gels. Possible origins for the higher stability such as complex formation between P-AA and oxyethylene segments of copolymerized PEO-R-MA-40 are discussed.

Conjugated microporous networks on the basis of 2,3,5,6-tetraarylated diketopyrrolo[3,4-c]pyrrole.

π-Conjugated microporous networks have been prepared from the tetraarylated diketopyrrolo[3,4-c]pyrrole unit as a tetrafunctional building block. The reactions are carried out using microwave-assisted Yamamoto or Sonogashira cross-coupling. Red insoluble powders are obtained, showing intense fluorescence. The polymer networks exhibit a high gas storage capability, with BET surface areas up to about 500 m(2)·g(-1).

Composite hydrogels with temperature sensitive heterogeneities: influence of gel matrix on the volume phase transition of embedded poly-(N-isopropylacrylamide) microgels.

The thermo-responsive behaviour of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in covalently cross-linked non-temperature-sensitive polyacrylamide (PAam) hydrogel matrixes with different compositions was investigated by using small angle neutron scattering (SANS). The composition of the composite hydrogel was varied by (a) increasing the cross-linker and acrylamide concentration leading to strong hydrogel matrixes and (b) by increasing the microgel concentration to obtain composite gels with an internal structure. Additionally we synthesized composite hydrogels by using γ-irradiation as initiation for the polymerisation. This leads to the formation of chemical bonds between the PNiPAM microgels and the surrounding polyacrylamide matrix. Thus it is possible to synthesize hydrogels without an additional cross-linker, as well as pure particle networks. Some samples were prepared at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or totally collapsed microgels during the incorporation step. The volume phase transition of microgels is not influenced by a hydrogel matrix with high acrylamide concentration independent of the preparation temperature. However, an increased cross-linker concentration leads to a corset like constraint on microgel swelling. Microgels, which are embedded in the collapsed state (at 50 °C), are not able to swell upon cooling, whereas microgels embedded in the swollen state can collapse upon heating. For samples with an increased microgel concentration, the close microgel packing was disturbed by the formation of the polyacrylamide matrix. The hydrogel matrix squeezes the microgels together and leads to partial aggregation. The experiments demonstrate how composite hydrogels with stimuli-sensitive heterogeneities can be prepared such that the full responsiveness of the embedded microgels is retained while the macroscopic dimensions of the gel are not affected by the volume phase transition of the microgels.

Conjugated polymers containing diketopyrrolopyrrole units in the main chain.

Research activities in the field of diketopyrrolopyrrole (DPP)-based polymers are reviewed. Synthetic pathways to monomers and polymers, and the characteristic properties of the polymers are described. Potential applications in the field of organic electronic materials such as light emitting diodes, organic solar cells and organic field effect transistors are discussed.

Facile fabrication of a Prussian Blue film by direct aerosol deposition on a Pt electrode.

A facile aerosol deposition approach, which was simulated as feasible by density functional theory (DFT), was applied to synthesize a Prussian Blue (PB) film directly on a Pt electrode surface.

Electrochemical Polymerisation of N-Arylated and N-Alkylated EDOT-Substituted Pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) Derivatives: Influence of Substitution Pattern on Optical and Electronic Properties.

New pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) derivatives carrying 3,4-ethylenedioxy-thiophenylphenyl (EDOT-phenyl) substituent groups in the 3- and 6-position, or in the 2- and 5-position of the DPP chromophore were synthesised and electrochemically polymerised. The properties of the polymers were investigated using cyclic voltammetry and UV/Vis absorption spectroscopy. It was found that the optical and electronic properties differ greatly between the two polymers. Materials with EDOT-phenyl groups in the 3- and 6-positions represent conjugated polymers with a low oxidation potential and reversible electrochromic properties, whereas the polymer with EDOT-phenyl groups in the 2- and 5-positions is non-conjugated and possesses a high oxidation potential and irreversible redox behaviour.

Layer-by-layer assembled membranes of protonated 18-azacrown-6 and polyvinylsulfate and their application for highly efficient anion separation.

Ultrathin separation membranes were prepared upon alternating electrostatic adsorption of 1,4,7,10,13,16-hexaazacyclooctadecane (18-azacrown-6, hexacyclen, aza6) and polyvinylsulfate on porous polyacrylonitrile/polyethylene terephthalate (PAN/PET) substrates. The resulting composite membranes were highly permeable for electrolyte solutions, but the selectivity in ion transport was poor (alpha(Cl-/SO42-) = 2). However, after a treatment of the membrane with 0.1 M aqueous copper(II) acetate solution, the rejection of divalent anions was strongly enhanced. Separation factors alpha(Cl-/SO42-) = 110 and alpha(Cl-/SO32-) = 1420 were found. Spectroscopic studies indicate a highly specific complex formation of aza6 and copper acetate in the membrane. Treatment of the membranes with cobalt(II) and nickel(II) acetate also led to an enhancement of the separation factors, but the effect was much smaller. Detailed studies of the influence of the pH of the dipping solutions and number of deposited bilayers are also reported. A model is presented describing the origin of the high transport selectivity.

Electrostatic layer-by-layer assembly of ultrathin films containing hexacyclen and p-sulfonatocalix[n]arene macrocycles.

Formation and characteristic properties of new layer-by-layer (LbL) assembled ultrathin films based on 1,4,7,10,13,16-hexaazacyclooctadecane (aza6) and p-sulfonatocalix[n]arenes with n=6 (calix6) and n=8 (calix8) are described. In particular, multilayered films of aza6 and polystyrenesulfonate (PSS), aza6 and calix8 or calix6, aza6 and hexacyanoferrate(II) (HCFII), calix8 and lanthanum(III), and calix8, 18-crown-6 and lanthanum(III) were investigated. The films were prepared upon alternating electrostatic adsorption of the cationic and anionic compounds from aqueous solution at charged substrates. Since the protonation of aza6 proceeds over a wide pH range, the film formation is strongly pH dependent. Linear growth of aza6/PSS films is found at pH 1.7 and 6.0, and superlinear growth at pH 0.9, 2.7, 3.7, and 5.0. The high affinity of aza6 towards inorganic ions favours the formation of LbL-assemblies of aza6 and HCFII counterions. Exposure of these films to Fe(III) leads to formation of Prussian Blue at the surface. The high affinity of p-sulfonato-calix[n]arenes towards La(III) favours the formation of LbL-assemblies of calix6 or calix8 and lanthanum. If additional 18-crown-6 is present in the La(III)-containing dipping solution, multilayered films of calix8, 18-crown-6 and La(III) are obtained.

Selective transport of ions and molecules across layer-by-layer assembled membranes of polyelectrolytes, p-sulfonato-calix[n]arenes and Prussian Blue-type complex salts.

Our recent studies in the field of ultrathin membranes prepared upon layer-by-layer assembly of various polyionic compounds such as polyelectrolytes, calixarenes and polyelectrolytes, and metal hexacyanoferrate salts such as Prussian Blue are reviewed. It is demonstrated that polyelectrolyte multilayers can be used (a) as nanofiltration and reverse osmosis membranes suitable for water softening and seawater desalination and (b) as molecular sieves and ion sieves for size-selective separation of neutral and charged aromatic compounds. Furthermore, hybrid membranes of p-sulfonato-calixarenes and cationic polyelectrolytes showing specific host-guest interactions with permeating ions are described. The membranes exhibit high selectivities for distinct metal ions. Finally, it is demonstrated that purely inorganic membranes of Prussian Blue (PB) and analogues can be prepared upon multiple sequential adsorption of transition metal cations and hexacyanoferrate anions. Due to the porous lattice of PB, the membranes are useful as ion filters able to separate cesium from sodium ions, for example.

Coordinative layer-by-layer assembly of electrochromic thin films based on metal ion complexes of terpyridine-substituted polyaniline derivatives.

Preparation, metal ion complexation, and coordinative assembly into organized electro-chromic films of a polyaniline derivative P1 substituted with tert-butyloxycarbonyl (boc) and terpyridine (tpy) substituent groups in alternating sequence are described. Cleavage of the boc groups after processing into thin films is also described. P1 is prepared upon Pd-catalyzed polycondensation of N-tert-butyloxycarbonyl-4,4'-dibromodiphenylamine and 4'(4-aminophenyl)-2,2':6,2''-terpyridine. The molecular weight is in the range of oligomers, the tetramer and pentamer being the most abundant species. P1 is soluble in common organic solvents. Solutions are pale yellow with blue or green fluorescence depending on the solvent. Fluorescence quantum yields up to 68% are found. P1 is able to complex divalent metal salts such as zinc(II) chloride, for example. Titration experiments indicate the formation of 2:1 tpy:metal ion complexes. Layer-by-layer (LbL) assembled films of metal ion complexes of P1 can be prepared, if negatively charged substrates are alternately dipped into solutions of metal(II) hexafluorophosphates and P1. Films of the zinc and nickel ion complex of P1 are lemon yellow in the neutral state and change color into greenish gray upon anodic oxidation, while Co-containing films are purple and change color into grayish blue upon oxidation. All color changes are reversible under ambient conditions. ATR-IR studies indicate that thermal treatment of the films at 180 °C, or acid treatment, e.g. with 5% aqueous trifluoroacetic acid solution, results in cleavage of the boc groups. P1 is transformed into the polyaniline derivative P2 with tpy substituent groups at every second N atom in the backbone. Films of metal ion complexes of P2 are also electrochromic albeit the colors differ slightly from those of P1. For example, the absorption maximum of the Zn-P2 film is at 456 nm, whereas it is at 446 nm for the corresponding Zn-P1 film. Prior to cleavage of the boc group, electrochromic switching times are 1.1 to 2.0 s for 30 to 40 nm thick films, while after the cleavage 0.5 to 1.2 s are found. The contrast is 13 to 19%, and not affected by the cleavage. Because of high stability, fast switching, and high contrast, the films might be useful as active materials in electrochromic devices.

Double-electrochromic coordination polymer network films.

Formation and characteristic properties of organized double-electrochromic films consisting of electrochromic poly(4-(2,2':6,2″-terpyridyl)phenyliminofluorene) (P-1)-zinc ion complexes and electrochromic anions are reported. The anions are 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonate) (ABTS) and poly((4-sulfonatophenyl)iminofluorene) (P-2). The films were prepared upon multiple sequential adsorption of P-1 and the zinc salts of ABTS and P-2 on solid supports using coordinative interactions between the Zn ions and the terpyridine (tpy) ligands. The ABTS and P-2 ions are incorporated in the films via electrostatic forces neutralizing the charge of the complexed divalent zinc (Zn(2+)) ions. The optical, electrochemical, and electrochromic properties of the films are described. Films consisting of the Zn ion complex of P-1 and ABTS are yellow in the neutral state and change their color to brownish gray and finally blue, if anodically oxidized at ∼640 mV vs FOC. Films containing the Zn ion complex of P-1, with P-2 as a counterion, are yellow in the neutral state and change color to dark red and finally blue, if anodically oxidized at ∼450 mV vs FOC. Compared with previously reported films of the Zn ion complex of P-1 with nonelectroactive hexafluorophosphate as the counterion, the new films exhibit faster response times, as well as higher contrast, and the colors in the oxidized state are modified. The films are stable under ambient conditions and might be useful as active layers in electrochromic devices.

Thermoresponsive copolymer hydrogels based on N-isopropylacrylamide and cationic surfactant monomers prepared from micellar solution and microemulsion in a one-step reaction.

Thermoresponsive hydrogels were prepared upon radiation-induced copolymerization of aqueous micellar solutions containing N-isopropylacrylamide (NiPAAm) and a cationic surfactant monomer (surfmer), and of microemulsions containing NiPAAm, surfmer, and styrene. Three surfmer compounds were used: (11-(acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB), (11-(methacryloyloxy)undecyl)trimethylammonium bromide (MUTMAB), and (2-(methacryloyloxy)ethyl)dodecyldimethylammonium bromide (MEDDAB). Comonomer solutions were studied on their phase behavior and structure using small-angle neutron scattering (SANS). The presence of surfmers increased the solubility of NiPAAm in the aqueous phase. SANS studies indicate that the surfmers form spherical micelles, which in the presence of styrene are increased and in the presence of NiPAAm are decreased in size. Styrene is incorporated in the core, and NiPAAm is incorporated in the shell of the micelles. If styrene and NiPAAm are present, the effects of both compensate each other, the micelle size remains unchanged, and only small amounts of styrene are solubilized. Evaluation of scattering curves indicated remarkable changes in headgroup dissociation of surfmers in the presence of NiPAAm in the micellar solutions. If exposed to (60)Co-gamma irradiation (dose: 80 kGy), stable, transparent, and thermoresponsive hydrogels were directly obtained. The lower critical solution temperature (LCST) of gels containing surfmer in low concentration was higher than that for pure NiPAAm gels, whereas in gels with high surfmer concentration it was lower. The lowest LCST was observed if MEDDAB was present in the gel. 1 % (w/w) was already sufficient to lower the LCST from 33.2 to 28.5 degrees C. Gels with low surfmer concentration (< or = 1 wt %) exhibited a strong, rapid swelling in water at 20 degrees C and a rapid and reversible shrinking at 50 degrees C. For a gel containing 1% AUTMAB, the swelling ratio was 2.4 times higher (MUTMAB, 2.8; MEDDAB, 1.5) than that for a pure NiPAAm gel. Copolymer gels containing more than 1 wt % surfmer exhibited a strong and rapid swelling below and above the LCST, because the copolymerized ionic surfmer induced an osmotic pressure in the gel. The effects of a variation of NiPAAm and surfmer concentration were studied, and the origins of the thermoresponsive properties are discussed.

Supramolecular sequential assembly of polymer thin films based on dimeric, dendrimeric, and polymeric Schiff-base ligands and metal ions.

New metal-Schiff-base coordination polymer films were prepared using multiple sequential adsorption of metal ions and salen-based ligand molecules. As the ligands, bis-bidentate 5,5'-methylene-bis(N-methylsalicylidenamine) (MBSA), tetra-bidentate N,N',N' ',N' ''-tetrasalicylidene-polyamidoamine (TSPA), and multi-bidentate poly(N-salicylidenevinylamine) (PSVA) were used. The metal ions were Cu(II), Zn(II), Fe(II), Fe(III), and Ce(IV). The resulting films are deeply colored due to the formation of coordinative bonds between the metal ions and the salen groups. Our study indicates that film formation becomes progressively easier, if the number of salen groups per ligand molecule increases. While Cu(II), Ni(II), Fe, and Ce(IV) are well suited for complex formation, Zn(II) is less suited. Possible structures of the polymers are discussed. Cyclic voltammetric studies of the films are also presented.