Acta Polymerica最新文献

Poly(methacrylic acid) (PMA) and poly(acrylamide) (PAAm) with a small mole fraction of naphthol (NpOH) residues were prepared by polymerization of methacrylic acid or acrylamide with three position isomers, methacryloylaminonaphthol. The substituent effect on the apparent pK values (pK_obsd), determined by spectroscopic pH titrations in aqueous solution, was observed for the NpOH-labeled PAAm. The electrostatic surface potential (ψ) for PMA was calculated from the difference in the pK_obsd values for the labeled PMA and PAAm. The obtained ψ value decreased as the spacing between the OH moiety and the polymer main chain increased. This clearly indicates that the ψ value is a sensitive function of the distance from the polyion surface.

Due to the low oxidation potential of poly-(2,5-thienylenevinylene) (PTV) an ohmic contact for hole injection is established at an indium tin oxide (ITO)/PTV interface. It gives rise to space-charge-limited conduction in an ITO/PTV/Al diode. The ohmic nature of the ITO/PTV contact can be exploited to increase hole injection into multilayer organic light emitting diodes in which internal charge accumulation would otherwise lower majority carrier injection by virtue of electric field screening at the anode.

Direct depth profiling techniques to date have largely lacked the necessary depth resolution to investigate interfacial phenomena of the order of the bulk correlation length (5—10 nm for a wide range of systems). Here we investigate the optimal spatial resolution and depth of probe that may be attained for composition — depth profiling of polymeric samples via nuclear reaction analysis (NRA) using the ²H(³He, ¹H)⁴He reaction. We find that the spatial resolution can be greatly improved by using a grazing incidence geometry of the incident ³He beam on the sample, and analyzing the emitted protons in a backwards direction. This results in spatial resolutions down to about 3 nm at the sample surface, compared to a value of some 7 nm or more previously reported in earlier studies when emitted α-particles were detected in the forward direction. At the same time the depth to which samples can be profiled via the backwards emitted protons may be considerably extended relative to the α-particle detection mode, when the ³He beam impinges on the sample surface at normal incidence (up to about 4 μm into the sample for incident energies of 1.2 MeV in the proton-detection mode compared to only 1 μm for the equivalent α-particle detection mode).

Crazes were produced and analyzed in unnotched tensile bars of poly(styrene-co-acrylonitrile) (SAN) and polycarbonate (PC) in creep experiments above T_VF (the Vogel temperature). The craze microstructure was investigated as a function of temperature (T) and load (σ) by means of small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Contrary to expectation, the scattering vector of maximum intensity (s_max), which is inversely proportional to the distance between the fibrils of crazes, was not linearly dependent on σ at constant temperature. At the highest stresses (regime III), s_max was independent of stress, and the average distance between the fibrils reaches a minimum value. At intermediate stresses (regime II), a strong increase of fibrillation energy Γ was detected, as the temperature was reduced. In the vicinity of T_VF, Γ reached values of the order of the polymer chain fracture energy. At the lowest stresses (regime I), the energy of fibril formation was independent of temperature and corresponded to the van der Waals surface energy. The molecular motions during fibril formation may be linked to local stress-induced flow processes of polymer chains (regime I) and α-relaxations (regime II). Increasing stress restricts the range of mobility of macromolecules to shorter and shorter units and a transition from the formation of fibrillated crazes to homogeneous crazes or shear deformation processes occurs at the highest stresses (regime III). A pressure—temperature diagram was constructed from the transition between the regimes, particularly at negative pressure.

The efficiency of rubber toughening of PSAN depends on the size of the rubber particles, their agglomeration, the deformation rate, the temperature, and the orientation of the polymer molecules. Large particles are more effective than small particles. By a suitable choice of processing, however, small particles can agglomerate, forming large soft units and improving in this way impact toughness. At high deformation rates crazing or/and stretching of the matrix wall between the rubber particles must be activated for ductility, otherwise the material is brittle. The temperature at impact must be above the glass transition temperature of the rubbery phase for toughening. Increasing the orientation of the material decreases the tendency for craze formation with the consequence of embrittlement, if stretching is not activated.

The steady-state fluorescence (SSF) technique was used to study interdiffusion of polymer chains across the particle–particle junction during film formation from hard latex particles. The latex films were prepared from pyrene (P) and naphthalene (N) labeled poly(methyl methacrylate) (PMMA) particles and annealed for long time intervals above the glass transition temperature (T_g) between 170 and 220°C. A novel correction method was suggested and employed to eliminate changing of the optical density of latex films after such annealing processes. P and N fluorescence intensities solely from the energy-transfer processes were monitored versus annealing time and were used to measure the polymer chain diffusion coefficients D_p and D_N, which were found to be between 4.34 × 10^–14 and 7.68 × 10^–13 cm²/s for an annealing temperature between 170 and 220°C. Using these diffusion coefficients with the Arrhenius relation, the diffusion activation energy for the polymer backbone was found to be 28 kcal/mol.

Poly(dipentylsilylene) copolymers containing 4,7,10-trioxadodecylmethylsilylene units as comonomer were synthesized via a Wurtz-type coupling reaction from the corresponding dichlorosilanes. The copolymers were characterized by GPC and NMR spectroscopy. Solvent- and temperature-dependent UV absorption was observed. The bulk polymers underwent broad endothermic transitions below room temperature, whose temperature and heat effect depended on the incorporation of comonomer. This suggested that the high-temperature state can be described as a columnar mesophase corresponding to the well established case of the poly(dipentylsilylene). Correlated to this transition, the copolymers also showed thermochromic behavior at low temperatures. The materials could be oriented by shearing in the mesophase, which was demonstrated by polarized optical microscopy, polarized IR spectroscopy and UV spectroscopy.

The order and molecular dynamics of wholly aromatic thermotropic copolyesters prepared from hydroxybenzoic acid (HBA), hydroquinone (HQ) and phenoxyterephthalic acid (Ph-O-TPA) have been studied by X-ray analysis and deuteron NMR spectroscopy of specimens containing either deuterated hydroquinone or deuterated phenoxy side-chain units. X-ray analysis shows the coexistence of nematic and three-dimensionally ordered regions that are characterized by registry of adjacent chains with interleaved side groups. The molecular motions in these nematic/three-dimensionally ordered polymers are compared to those of a highly crystalline copolyester prepared from deuterated TPA and HQ. In the fast motional limit the motion in the crystalline system is characterized by 180° flips about the TPA axis, whereas in the nematic/three-dimensionally ordered copolyester the same type of axially symmetric spectrum is observed for both the nematic and the three-dimensionally ordered regions. The motion of the HQ units is characterized by rotational diffusion about the C₁C₄ axis, augmented by fast rotational fluctuation of this axis about the main-chain direction. A model of free rotation about the axis of the phenyl group augmented by librations of restricted amplitude about the PhO-TPA bond for the ordered regions superimposed by slow axial motions about the main-chain axis in the nematic phase is consistent with both the NMR data and X-ray analysis.

The preparation of hollow-fiber membranes with high asymmetric morphology by a wet spinning process by phase inversion of polymer solutions with a nonsolvent medium is normally connected with the formation of a relatively dense unwelcome skin. One possibility to eliminate this disadvantage is a phase inversion process with gel/sol/gel-treatment, whose physicochemical peculiarities are descibed in this paper. In the gel/sol/gel-treatment an initially wet hollow-fiber membrane is contacted with a solvent and subsequently with a nonsolvent again. Such a technology causes a second phase inversion at the outer surface of the hollow-fiber membrane. Results about parameters influencing the gel/sol/gel-process are presented as an illustration for the treatment of polyacrylonitrile hollow-fiber membranes with a special structure. In conclusion, first ideas of the mechanism of this process are formulated.

Molecular motions in low molar mass (46200 g mol^—1) poly(diethylsiloxane) (PDES) are studied by ²H NMR. From DSC measurements the existence of the crystalline phases α₁, β₁ and α₂, β₂, as well as a pronounced mesogenic phase α_m, is clearly established. Based upon calculations of averaged field gradient tensors based on cone models a detailed motional analysis in the different phases has been conducted. At low temperatures the only motion present besides methyl rotation are librations of the ethyl groups with amplitudes below 25°. The motion in the conformational disordered phase is characterized by conformational changes within the side chains, accompanied by occasional rotations through angles larger than 70° within the backbone. The flexibility of the ethyl groups, however, is limited by steric hindrance, prohibiting a full rotation. This situation is observed also in the meso phase where there is an additional rotational degree of freedom around the main chain axis.