Hans-Jürgen P. Adler

Temperature and pH dependent solubility of novel poly(N-isopropylacrylamide)-copolymers

Temperature and pH sensitive polymers were prepared by the copolymerization of N-isopropylacrylamide (NIPAAm) with varying amounts of acrylamide derivatives bearing carboxylic groups attached to spacers with different chain length (C n AAm). Aqueous solutions of the copolymers show lower critical solution temperature behaviour (LCST). The LCSTs of the aqueous solutions of these copolymers decrease with increasing comonomer content and spacer chain length. Through the use of suitable amounts of the monomers it was possible to accurately predict the phase transition temperature and values were obtained between 10 and 32°C. The LCSTs also strongly depend on the pH-value of the solution. An increase of the pH-value leads to a significant increase in LCST due to the formation of a more hydrophilic copolymer. Through altering the pH it is possible to obtain phase transition temperatures between 10 to 50°C. The LCST behaviour was investigated by means of DSC as a standard method which allows the measurement of the phase transition over a wide range of temperatures and pH-values. Comparison of these results with those from other methods (mainly turbidimetric and viscosimetric measurements) shows a two step mechanism for the phase separation. Dependence of the phase transition temperature of aqueous P(NIPAAm co C 2 AAm) copolymer solutions on the pH-value (lines were drawn to make the diagram clearer).

Polyurethane macromers—new building blocks for acrylic hybrid emulsions with outstanding performance

Abstract Emulsion polymerization is an efficient process to produce waterborne binders. It has been observed that films prepared from traditional latices do not show high performance especially concerning humidity and stone chip resistance. Polymeric emulsifiers are promising alternatives to low molecular weight surfactants. Polyurethanes are an interesting polymer class. The structure of the polymer backbone can be varied in a wide range by using different building blocks. The synthesis of copolymerizable polyurethanes, which have an amphiphilic structure similar to traditional surfactants, and their use in aqueous polymerization is discussed in detail. This process leads to polymer emulsions with structured particle morphology. The resulting films show outstanding properties. The polyurethane–polyacrylic dispersions can be used as binders for high performance waterborne coating formulations.

Interpenetrating polymer network (IPN) nanogels based on gelatin and poly(acrylic acid) by inverse miniemulsion technique: synthesis and characterization.

Novel interpenetrating polymer network (IPN) nanogels composed of poly(acrylic acid) and gelatin were synthesised by one pot inverse miniemulsion (IME) technique. This is based on the concept of nanoreactor and cross-checked from template polymerization technique. Acrylic acid (AA) monomer stabilized around the gelatin macromolecules in each droplet was polymerized using ammonium persulfate (APS) and tetramethyl ethylene diamine (TEMED) in 1:5 molar ratio and cross-linked with N,N-methylene bisacrylamide (BIS) to form semi-IPN (sIPN) nanogels, which were sequentially cross-linked using glutaraldehyde (Glu) to form IPNs. Span 20, an FDA approved surfactant was employed for the formation of homopolymer, sIPN and IPN nanogels. Formation of stable gelatin-AA droplets were observed at 2% surfactant concentration. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) studies of purified nanogels showed small, spherical IPN nanogels with an average diameter of 255 nm. In contrast, sIPN prepared using the same method gave nanogels of larger size. Fourier-transform infrared (FT-IR) spectroscopy, SEM, DLS, X-ray photoelectron spectroscopy (XPS) and zeta potential studies confirm the interpenetration of the two networks. Leaching of free PAA chains in sIPN upon dialysis against distilled water leads to porous nanogels. The non-uniform surface of IPN nanogels seen in transmission electron microscopy (TEM) images suggests the phase separation of two polymer networks. An increase of N/C ratio from 0.07 to 0.17 (from PAA gel to IPN) and O/C ratio from 0.22 to 0.37 (from gelatin gel to IPN) of the nanogels by XPS measurements showed that both polymer components at the nanogel surface are interpenetrated. These nanogels have tailoring properties in order to use them as high potential drug delivery vehicles for cancer targeting.

Evaluation of surface modification by electrokinetic measurements

Abstract For a long time, measurements of the electrokinetic potential have been used in colloid chemistry mainly for the calculation of electrostatic interaction forces in colloid systems. Moreover, electrokinetic measurements are increasingly successful in characterising solid surfaces. Studies of the effects of pH, surface-active agents, etc. on the electrokinetic potential give valuable information about the nature of solid surfaces. According to Jacobasch [Prog. Org. Coat. 17 (1989) 115], it is possible to distinguish between dissociation and adsorption processes. We are also able to recognise chemical reactions on the surface during measurement. Electrokinetic properties of different polyformamidines should give us an idea about the chances of this method. While electrokinetic properties of colloidal particles have been widely investigated by means of electrophoresis, the electrokinetic characterisation of the surface of macroscopic specimens is still rather uncommon. Indeed, the use of a commercial electrokinetic analyser (EKA) streaming potential measurement apparatus (Anton Paar KG, Graz, Austria) allows the rapid and reproducible measurement of streaming potential, including those of plates and sheets. By means of these investigations, we are able to tackle problems of adhesion of coatings on polymers. This was already demonstrated at the influence of different finishing methods like oxygen plasma treatment or flame treatment on the surface of polypropylene rubber blends [Prog. Org. Coat. 26 (1995) 131; J. Adhes. Sci. Technol. 9 (1995) 327]. By creating special conditions, we succeeded in measuring streaming potentials on metal surfaces, as shown by the example of Al-sheets. With the help of these measurements, different surface treatments were determined.

Photochemical switching of hydrogel film properties

Abstract A copolymer of N -isopropyl acrylamide and acrylic acid was modified in a polymer analogous reaction by an acridizinium chromophore capable of reversible photochemical dimerization. The acridizinium chromophore allows a faster and more complete photocrosslinking and a better photoreversibility as compared to stilbene and stilbazolium chromophores. Due to its hydrophilic nature the chromophore only moderately affected cloud points T c of the copolymer and lower critical solution temperatures (LCST) of photocrosslinked films of the copolymer, even at high chromophore content (3.5% w/w). Such films can be swollen in water to form hydrogels. LCST and T c depend on the content and on the counterion of the chromophore, on the solvent from which films are prepared, and on the degree of crosslinking. The latter dependence allows the exploitation for isothermal switching of film properties by exposure to light.

Photocrosslinking of thin films of temperature‐sensitive polymers

The use of polymeric materials with temperature-dependent degrees of swelling (especially polymers that exhibit lower critical solution temperature (LCST) behaviour in aqueous solutions) in microsystems requires the preparation and patterning of layers in the µm range. Copolymers based on N-isopropylacrylamide were modi-fied with a stilbazolium salt chromophore to yield photocrosslinkable temperature-sensitive polymers. The chromophore and the polymers were characterized by UV, IR, 1H-NMR (nuclear magnetic resonance) and 13C-NMR spectra. The resulting polymers showed LCST behaviour, which was measured by differential scanning calorimetry. The photocrosslinking properties were studied by UV irradiation of the thin films and measurement of the changes in the UV absorption spectra. By irradiation of thin films through a mask it was possible to obtain patterned networks in the µm range (20 µm space width and ≥50 µm line width). The resulting patterned networks showed temperature-dependent swelling properties in aqueous media. Copyright

Polyperoxidic Surfactants for Interface Modification and Compatibilization of Polymer Colloidal Systems. II. Design of Compatibilizing Layers

The polymer peroxide surfactants obtained by copolymerization of a perox- ide monomer with maleic anhydride were either physically or chemically sorbed on the dispersed-phase surfaces, for example, on mineral fillers and latex particles. Subse- quent initiation of graft copolymerization from the surface resulted in the formation of interfacial compatibilizing polymer layers in water emulsions and dispersed-filled polyethylene. The morphology of the resulting filled polymer was characterized by scanning electron microscopy.

Kinetics of Aqueous Heterophase Polymerization of Styrene with Polyurethane Emulsifier

This paper describes the synthesis of an amphiphilic polyester polyurethane and its use as a stabilizer for the aqueous heterophase polymerization of styrene. The polymerization process was investigated using dilatometry and reaction calorimetry methods. The polyurethane proved to be an efficient stabilizer in the heterophase polymerization of styrene.

pH- and Metal Ion- Sensitive Hydrogels based on N-[2-(dimethylaminoethyl)acrylamide]

Smart hydrogels are promising materials for actuators and sensors, as they can respond to small changes in their environment with a large property change. Hydrogels can respond to a variety of stimuli, for example temperature, pH, metal ions, etc. In this article, the synthesis and characterization of polyampholyte hydrogels based on open chain ligands showing pH and metal ion sensitivity are described. Copolymer and terpolymer gels using different mixtures of monomers i.e., N-[2-(dimethylaminoethyl)acrylamide] (DMAEAAm), N,N-dimethylacrylamide (DMAAm), acrylic acid (AA) and 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPS), have been synthesized. The effect of copolymer composition, i.e., the ratio and amount of ionic monomers and the degree of crosslinking on the swelling characteristics, was evaluated as a function of pH. On this basis, metal ion sensitivity measurements were performed at selected pH values. The metal ion sensitivity was measured by varying the concentration of Cu2+, Zn2+ and Ag+ ions under acidic pH conditions.

Preparation and properties of thin films of photocrosslinkable hydrophilic polymers

New hydrophilic photocrosslinkable polymers were prepared from water-soluble copolymers and photodimerizable chromophores. Water-soluble prepolymers were obtained by copolymerization of N-isopropylacrylamide (NIPAAm) or N,N-dimethylacrylamide (DMAAm) with acrylic acid (AAc). The polymers were then modified with hydrophobic or hydrophilic photoreactive groups, trans-4-hydroxystilbene and styrylpyridinium (stilbazolium) salts, in polymer analogous reactions. Thereby water-soluble photoreactive polymers with or without temperature-sensitive solubility were obtained. The glass transition temperatures of the polymers and the cloud points of their solutions in water were determined and found to depend on the content of the photoreactive groups. Thin films of the materials on quartz plates were prepared from solutions in different solvents such as butanone, chloroform, ethanol, tetrahydrofuran, and water, and the photocrosslinking properties of these films were studied. The nature of the chromophores as well as the solvents used for the film preparation were found to influence the photocrosslinking behaviour.