Frank Böhme

Ionic Modification Turns Commercial Rubber into a Self-Healing Material

Invented by Charles Goodyear, chemical cross-linking of rubbers by sulfur vulcanization is the only method by which modern automobile tires are manufactured. The formation of these cross-linked network structures leads to highly elastic properties, which substantially reduces the viscous properties of these materials. Here, we describe a simple approach to converting commercially available and widely used bromobutyl rubber (BIIR) into a highly elastic material with extraordinary self-healing properties without using conventional cross-linking or vulcanising agents. Transformation of the bromine functionalities of BIIR into ionic imidazolium bromide groups results in the formation of reversible ionic associates that exhibit physical cross-linking ability. The reversibility of the ionic association facilitates the healing processes by temperature- or stress-induced rearrangements, thereby enabling a fully cut sample to retain its original properties after application of the self-healing process. Other mechanical properties, such as the elastic modulus, tensile strength, ductility, and hysteresis loss, were found to be superior to those of conventionally sulfur-cured BIIR. This simple and easy approach to preparing a commercial rubber with self-healing properties offers unique development opportunities in the field of highly engineered materials, such as tires, for which safety, performance, and longer fatigue life are crucial factors.

Synthesis and properties of polyamidines

Polymers with amidine groups NHCRN in the main chain were prepared by acid catalysed melt polycondensation of 4,4′-diaminodiphenyl methane (DAPM) with various orthoesters. The resulting polyamidines were characterized by 1H-, 13C- and 15N-NMR spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TGA) and wide angle x-ray scattering (WAXS) measurements. The properties of the polyamidines are strongly determined by the substituent R on the amidine group. Some peculiarities of the polyformamidine (RH) could be attributed to the conformation of the amidine group. Glass transition temperatures varied from 106 to 161°C depending on the substituent R. All polyamidines possessed good thermal stability up to 370°C and, with exception of the polyformamidine, good solubility in common solvents.

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.

New hyperbranched poly(ether amide)s via nucleophilic ring opening of 2-oxazoline-containing monomers

An AB 2 monomer containing one oxazoline and two phenolic units was synthesized. The polymerization of the monomer 2-(3,5-dihydroxyphenyl)-1,3-oxazoline in N-methylcaprolactam resulted in a well-defined and fully soluble, high molar mass, hyperbranched polymer with etheramide structure. The hydrolysis of some oxazoline groups as side reaction limits the achieved molar mass when the polymerization is carried out in tetramethylene sulfone or in bulk. The polymers were characterized by one (1D) and two (2D) dimensional 1 H and 13 C NMR spectroscopy which allowed to determine the degree of branching to be 50% as expected from statistics. DSC and TGA measurements revealed a glass transition temperature of 176°C and a decomposition onset of 330°C. The thermal ring-opening reaction was studied in situ by DSC measurements.

Surface properties of polyamidines investigated by inverse gas chromatography

Abstract Polymers containing amidine groups –NH–CRN– (R: –H, –CH 3 , –C 6 H 5 ) in the main chain were characterised by inverse gas chromatography. In dependence on the residue R the dispersive contributions of the surface free energy were determined. From interactions with polar probe molecules the Lewis acid–base properties of the polymer surfaces were concluded. Semiquantitative K A - and K B -parameters describing the possibility of the stationary phase to act as electron donor or acceptor showed that polyamidines are Lewis bases. The results were discussed with respect to the conformation of the amidine group and interactions between the polymer chains.

LC multiblock copolymers containing polysulfone segments. I. Synthesis and morphology

Multiblock copolymers offer the possibility to combine the properties of different polymers. Thus, new materials with tailor-made unique properties are available by coupling of different suitable polymeric segments. The goal of the work discussed in this paper was to combine advantageous properties of liquid-crystalline polymers (LCP) with those of polysulfone (PSU). Therefore, liquid crystalline poly(ethylene terephthalate-co-1,4-oxybenzoates) were connected with PSU oligomers. Chemically homogeneous multiblock copolymers with high molecular weight were obtained by a melt transesterification procedure. It was demonstrated by wide angle x-ray scattering (WAXS), polarizing microscopy, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) that the properties of the multiblock copolymers (solid phase structure, phase behavior, morphology, glass transition, and melting behavior) can be balanced by the segment length of the incorporated blocks. The investigations clearly reveal the existence of a two-phase structure. However, a change of properties compared to the corresponding homopolymers refers to certain interactions between the phase due to the chemical connection of the LCP and PSU segments.

Synthesis and properties of aliphatic polyacetamidines

A series of aliphatic polyacetamidines (1) with the common structure [-CH 2 ) m -N=C(CH 3 )-NH-] n was synthesized by the phenol-catalyzed melt polycondensation reaction of various aliphatic diamines with triethyl ortho acetate. The resulting polyacetamidines were characterized by 1 H and 13 C NMR spectroscopy. It could be shown that the NMR spectra are strongly influenced by the solvents used. In pyridine and CD 3 OD, prototropic tautomerism of the amidine group was observed. In pyridine this process is mainly caused by intermolecular proton exchange between two amidine groups whereas in CD 3 OD, proton exchange mainly proceeds via the solvent. The methyl residue on the amidine group revealed pronounced CH-acidity, which is apparent in a proton-deuteron exchange when CD 3 OD was used as a solvent. In CF 3 COOD, the respective amidinium salts were formed. It was found the thermal stability of the polymers increased with increasing chain length of the aliphatic diamines. The shorter aliphatic diamines formed cyclic products with a defined structure.

Synthesis and thermal polymerization of aromatic 2‐oxazolines containing carboxylic groups

2-(4-Carboxyphenyl)-, 2-(3-carboxyphenyl)- and 2-(6-carboxynaphtyl-2-yl)-1,3-oxazoline were synthesized by reaction of monomethyl ester chlorides of aromatic dicarboxylic acids with 2-chloroethylamine hydrochloride in the presence of triethylamine followed by cyclization with methanolic KOH. Thermal bulk polymzerization within a few minutes at 200-220°C resulted in new linear poly(ester amide)s without significant side reactions. The polymerization occurred in the melt phase or in the solid state and the resulting polymers are amorphous or semicrystalline.

2‐Oxazoline‐Terminated Polystyrene by Atom‐Transfer Radical Polymerization

Macromonomers of polystyrene containing one functional 1,3-oxazoline end group were obtained by atom-transfer radical polymerization (ATRP) using oxazoline-containing initiators. 2-(1-Bromoethyl)-1,3-oxazoline (2b) and 2-(4-(1-bromoethyl)phenyl)-1,-oxazoline (6) were prepared for this purpose. 2-(1-Bromoethyl)-1,3-oxazoline was prepared by the bromination of 2-ethyl-1,3-oxazoline with N-bromosuccinimide (NBS). 2-(4-(1-Bromoethyl)phenyl)-1,3-oxazoline was obtained by the treatment of 4-(1-bromoethyl) benzoic acid with ethanolamine and subsequent cyclization with SOC1 2 . Both initiators proved to be suitable for the ATRP of styrene. Oxazoline mono-terminated polystyrenes with Mn between 2 000 and 3 500 G . mol -1 were obtained. The oxazoline groups were observed in the NMR spectra

Hyperbranched Poly(Ether Amide)s via Nucleophilic Ring Opening Reaction of Oxazolines

The nucleophilic ring opening addition reaction of phenol groups towards oxazoline units has been used for the preparation of hyperbranched poly(ether amide)s. For this the AB2 monomer 2-(3,5-bishydroxyphenyl)-1,3-oxazoline was synthesized and converted in a highly branched polymer in bulk or solution at temperatures above 190°C. The resulting hyperbranched polymers exhibit a degree of branching of 50%, as verified by high-resolution NMR spectroscopy, and are highly soluble in polar organic solvents, for example DMF and DMAc, with low solution viscosity. Their glass transition temperatures are in the region of 170°C and degradation does not start below 300°C. Melt rheology measurements revealed a predominantly elastic behaviour with a relatively high viscosity at low frequency. A lower melt viscosity was achieved by end group modification. Hyperbranched poly(ether amide)s with phenolic and acetate end groups are fully miscible with linear polyamide 6. When a small amount of the hyperbranched material is added to the PA6 matrix a significant reduction of the melt viscosity could be achieved without changing the mechanical properties of the matrix polymer.