Acta Polymerica最新文献

Water-soluble diblock copolymers (DBCPs) with a polyelectrolyte block are accessible by sequential anionic polymerization of tert-butylmethacrylate (TBMA) and ethylene oxide (EO), followed by polymer analogous conversion of the PTBMA into poly(methyacrylic acid) (PMAA) blocks. These materials are highly efficient dispersants for oxide ceramic powders in aqueous media. A series of block copolymer samples with M_n ranging from 1300 to 38 900, and (EO:MAA) block length ratios from 0.5 to 11.7 were prepared with polydispersities close to 1.2. The influence of overall molecular weight and block length ratio, pH and ionic strength on the stability of aqueous α-Al₂O₃ suspensions was investigated by sedimentation and adsorption experiments, surface plasmon resonance (SPR) and electrokinetic measurements. The copolymers are capable of stabilizing alumina suspensions by a combination of specific adsorption of one block on the particle surface and a shielding effect provided by the nonadsorbing block. In addition, the adsorption of the negatively charged PMAA block on the oppositely charged alumina surface reverses the electrophoretic potential of the oxide particles, a process which is strongly pH dependent. With respect to the powder dispersing efficiency, an optimum was found when the DBCP consisted of a short PMAA anchoring and an approximately tenfold longer PEO stabilizer block with an overall molecular weight of about 5000. Alumina suspensions with the DBCP added were highly dispersed and rather stable against salt addition in a much wider pH window than block-copolymer-free suspensions.

An overview of the synthesis of optically active polymers due to main-chain chirality based on the configurational arrangement in cyclopolymerization and cyclocopolymerization is presented. The template having chiral twist transmitted its chirality to the main chain in the intramolecular cyclization of divinyl monomers to form the enantiomeric racemo cyclic units. The asymmetric cyclocopolymerization produced the polymers which were optically active even after cleaving the template. The synthesis of optically active polymers from achiral monomers using a slight amount of chiral initiator was achieved by the enantioselective cyclopolymerization. The cyclopolymerization capable of controlling stereochemistry of the cyclic constitutional units was one of useful means for asymmetric polymerization.

Crosslinked poly(styrene)-poly(dimethylsiloxane)-cyclodextrin systems are a new class of multicomponent polymer networks. They are prepared by swelling a polystyrene gel (PS) with the components of the poly(dimethylsiloxane)-cyclodextrin network (PDMS(CDX)) and subsequent crosslinking. Electron spectroscopic imaging and elemental mapping are appropriate methods for the investigation of these polymer systems, which contain heteroatoms in one of their components. It is shown that PDMS(CDX) is uniformly distributed within PS. Interpenetration with PS is responsible for the stable anchoring of PDMS(CDX) and prevents phase separation.

A number of OH-terminated three-arm star molecules were prepared by anionic polymerization with a specially functionalized initiator. The stars were crosslinked over the OH groups with varying amounts of toluene-2,4-diisocyanate. The samples were fractionated by size exclusion chromatography (SEC) on-line with a low angle laser light scattering (LALLS) and a viscosity (VISC) detector. The Kuhn—Mark—Houwink—Sakurada (KMHS) relationships from the various samples resulted in a common relationship. Shrinking parameters g′_j≡[η]_j,b/[η]_j,lin were determined for each slice and could satisfactorily be interpreted on the basis of the Zimm—Stockmayer theory in connection with a suggestion by Kurata et al. The Kurata suggestion was checked with data from star molecules. The examination revealed that randomly branched and star-branched macromolecules exhibit significantly different hydrodynamic behavior. The ratio of the Fox—Flory coefficients Φ_b/Φ_lin in the relationship [η] = Φ(R/M) increases in both cases with branching. This is a consequence of the enhanced segment density and the resulting increase in the hydrodynamic interaction.

Polydipentylsilylene and -siloxane copolymers, containing bis[(S)-(+)-2-methylbutyl]silane units have been synthesized. Thermal analysis demonstrated for both types of copolymers that crystallization was hindered on incorporation of the 2-methylbutyl groups. Optical rotation measurements in various solvents yielded a much stronger optical activity of the polysilylene copolymers, compared to the polysiloxanes. This effect has been explained by the formation of a chiral supramolecular structure in the case of the polysilylenes. The measurements have indicated that the solvent quality can be employed to enhance the formation of this structure at relatively low 2-methylbutyl contents.

The solvatochromic properties of 3-hexyl-substituted polythiophenes functionalized with an ester, ether or hydroxy group at the end of the alkyl side chain are reported. An extraordinarily large shift, Δ_max = 99 nm, and a dramatic change of the UV/ vis spectral profile are found when the carboxylate is replaced with the hydroxy group by functionalization exchange. The solvatochromic behavior of each polymer is believed to be strictly related to the type of functional group owing to different physical interactions (solvation) between groups different in polarity and solvent molecules. A unique and common chromophoric unit for the final conformation of the 3-hexyl-substituted polythiophenes, induced by the solvatochromic effect, is also suggested.

The amorphous polyester 1a prepared by melt condensation of 1,5-bis-(9-hydroxy-1,4,7-trioxanonyl)naphthalene (2a) and adipoylchloride (3a), which contains bis-1,4,7-trioxanonyl (triethyleneglycol) segments in its main chain, represents a matrix for ion-conducting materials. Despite the fact that only triethyleneglycol segments are present, which are shorter than the minimum required for full solvation of Li⁺ cations, ion-conductivities of σ = 3.3 times; 10^—5 S cm^—1 at 368 K are found for Li⁺/1a 0.25 (cations per polymer repeat unit). Hence, more than one bis-1,4,7-trioxanonyl segment (either intra- or interchain) has to be involved in Li⁺ complexation. The σ value compares favorably with previous data for more complex polymer matrices.

Block copolymers consisting of polystyrene and a block containing long fluorinated side chains are made by two very effective polymer analogous reactions from commercially available polystyrene-b-polybutadiene block copolymers. Since these polymers consist of two mutually immiscible blocks with hydrophobic and very hydrophobic character they form micelles in a wide range of media down to very hydrophobic solvents, e. g. toluene, tetrahydrofurane (THF), perfluorohexane, perfluoro(methylcyclohexane), hexafluorobenzene and 1,1,2-trichlorotrifluoroethane (Freon 113). First indications that our polymers are useful as effective steric stabilizers in such solvents are given by characterization of some polymer dispersions made in the solvents via precipitation polymerization.

In the solid state, mesophase formation typical for block copolymers is observed and characterized by quantitative small angle X-ray scattering (SAXS). The surface of such films is controlled by the fluorinated block and exhibits an ultra-low surface energy of the order of γ_SV = 15 mN/m, below that of polytetrafluoroethylene (PTFE) or polydimethylsiloxanes (PDMS). Unlike PTFE, the blocky nature of the presented polymer enables thermoplastic processing and dissolution in standard solvents, which is of practical relevance. Gas-permeability measurements on membranes coated with these copolymers exhibit some interesting selectivities.

We have measured the dielectric relaxation of the polymeric glass-former 1,4-polybutadiene in the glassy and supercooled liquid state. The loss spectra ϵ″ of the β-relaxation are measured between the lower detection limit at T = 125 K and T = 225 K > T_β, with T_β ≈ 210 K being the α-β-merging temperature. The related storage data ϵ′(ω) indicates that the β-process is a signature of supressing the higher frequency polarization with increasing temperature, rather than giving rise to additional polarizability at frequencies below the average relaxation frequency of the β-process. This observation, as confirmed by data obtained for o-terphenyl and salol, contradicts the previous understandings of the mechanism underlying the Johari—Goldstein-type β-process in glasses. A tentative link between the β-process and high frequency vibrational modes is established.