1999 рік

Vapor–liquid equilibria for binary solutions of polyisobutylene in C6 through C9 n-alkanes were obtained at 658C. Using a classic gravimetric-sorption method, the amount of solvent absorbed by polyisobutylene was measured as a function of solvent vapor pressure. The data were interpreted by the Flory–Huggins theory and the Prigogine–Flory–Patterson theory.

Детальніше...  

Hyperbranched analogues of the Tomalia PAMAM dendrimer system have been prepared from a series of AB 2 aminoacrylate hydrochloride monomers using Michael addition chemistry, where A represents a Michael acceptor and B 2 a primary aliphatic amine. This stepgrowth polymerization process has led to the synthesis of poly(amidoamine) hyperbranched materials, examples of which exhibit a branching factor close to one. This degree of branching, calculated from quantitative 15N NMR spectroscopy data, is consistent with other spectroscopic characterization undertaken and the assignment of the 15N NMR shifts observed correlates well with the available literature data. The use of MALDI-TOF mass spectrometry as a tool to evaluate the possibility of intramolecular cyclization reactions between the focus and terminal groups is discussed along with the determination of molecular weights and molecular weight distributions for these amidoamine hyperbranched polymers. The synthesis of pseudo-dendrimers through 'one-pot’ AB 2/B n copolymerizations, where control over both molecular weight and extent of branching through the B n unit is illustrated. A poly(amidoamine) hyperbranched AB 2/ B 6 copolymer is shown to display a viscosity/molecular weight profile similar to that exhibited by dendrimers. The effect of varying the internal spacer unit length between the branch points A and B is discussed, with reference to both the standard AB 2 polymerizations and the core terminated AB 2/B n copolymerizations. Preliminary physical characterisation (T g, DSC and solution viscosity) of these novel materials is reported.

Детальніше...  

The effect of shear on the alignment of the smectic A phase formed by a polyacrylate side chain liquid crystal polymer (SCLCP) has been investigated using small-angle X-ray scattering (SAXS) with simultaneous rheology. Large amplitude oscillatory shear led to a highly oriented state with the layer normal parallel to the neutral direction. The sample mosaic width obtained after large amplitude shear (⬃10°, Gaussian HWHM) was comparable to that obtained by magnetic alignment of the mesogens. The smectic phase was found to exhibit shear thinning, with a sharp reduction in both dynamic shear moduli upon application of shear. Cessation of shear led to a rapid recovery in the moduli to their initial values, although the anisotropy of the SAXS peaks relaxed more slowly. Nevertheless, a high degree of alignment was retained even after the relaxation period.

Детальніше...  

Zinc acetate was used as catalyst in the synthesis of a phenolic resol resin. The evolution of the addition and condensation reactions was quantitatively followed by high performance liquid chromatography. The addition of formaldehyde to phenol was almost exclusively ortho directed. Very low advancement in polymerization was observed, and diaryl compounds were only slightly formed. 13C NMR studies did not show para, para methylene bridges as a result of the poor addition of hydroxymethyl groups onto the para phenolic sites. The change in concentration of free phenol as a parameter of the evolution of the resin during storage was followed by gas chromatography. Disappearance of free phenol did not stop at any of the analysed storing temperatures (room temperature, 48C, and 2208C).

Детальніше...  

A simple model for aggregate formation in cellulose solutions is presented. Owing to a strong tendency of hydrogen-bond formation among the cellulose molecules, aggregation does not lead to a completely random arrangement of the molecules. In a core, parts of the chain molecules are laterally aligned. This core is surrounded by disordered regions that give rise to the formation of coronas under the action of solvent molecules while the core is completely immiscible with the solvent. Therefore, the aggregates can be seen as fringed micelles. The equilibrium structure of these micelles, number of aggregated chains and size of the coronas, is discussed as a function of the interfacial tension between core and solvent. It turns out that both number of aggregated chains and thickness of the coronas increase with increasing interfacial tension. In perfect solutions of the micelles, these quantities also increase with cellulose concentration. If one admits attractions between coronas of different micelles, as a small perturbation, clustering of micelles might be induced. This may cause phase instability of the particle phase which results in the coexistence of a diluted and a more concentrated solution.

Детальніше...  

This paper describes the results of positron annihilation lifetime measurements on a series of poly(ether imide)s for which permeabilities and permselectivities for gas separation have been determined previously. Positron lifetimes and intensities vary systematically with changes in polymer structure and correlate with the permeabilities in separation of carbon dioxide and methane. Some apparent scatter in the data might, in part, be associated with differences in the interactions of ortho-positronium with molecular cavities as polymer structures are varied.

Детальніше...  

We report the micellar behavior of polystyrene/poly(2-vinyl pyridine) heteroarm star copolymers (PSnP2VPn) in toluene which is a selective solvent of polystyrene. A series of PSnP2VPn were synthesized by anionic polymerization and their micellar characteristics such as critical micelle concentrations (cmc), aggregation numbers, hydrodynamic dimensions and core–corona sizes were determined by the means of static and dynamic light scattering, viscometry and transmission electron microscopy. All these parameters were studied as a function of the insoluble arm length and the corresponding scaling relations were established. Our system seems to belong to the class of amphiphile micelles characterized by strong dependencies of the aggregation number and the microdomain sizes on the insoluble arm length. This behavior is also characteristic of strongly segregated diblock and triblock copolymer systems.

Детальніше...  

To disperse RTV-silicone elastomer as fine particles in a bisphenol-A-type epoxy resin, silicone-methylmethacrylate graft copolymer was added as a compatibilizer. The molecular weight of the silicone segment and the MMA segment between silicone branches in the graft copolymer strongly affected the effectiveness of compatibilizer. It was revealed that the graft copolymer which acts as a more effective compatibilizer was more highly concentrated at the interface between the silicone-dispersed phase and the epoxy matrix. The interfacial tension around the silicone-dispersed phase decreased with the formation of the interphase which is mainly composed of the graft copolymers. The diameter of the dispersed phase decreased with a decrease in the interfacial tension. The fracture toughness of the modified resins increased with a decrease in the diameter of the silicone phase.

Детальніше...  

New chelating resins containing N-substituted diamides of malonic acid have been contacted with various Cu(II) solutions and the structural parameters of complexes formed have been investigated using the EPR method. Correlation between EPR spectra obtained and Cu(II) uptake shows that two main factors are influencing the ability of the ligand to coordinate metal ion. The first is the length of the pendant group (distance from the backbone chain) facilitating ligand accessibility and the second is the ligand potential to create a chelate ring with Cu(II). This effect is dominant in resins with N-substituted diamides of malonic acid, in which neighbouring ligands can cooperate efficiently. At higher pH, unusually stable Cu(II) complexes are formed by resins with 2-aminoethyl and 2-aminomethylpyridine substituted diamides of malonic acid due to the involvement of deprotonated amide nitrogen and formation of tetradentate ligands.

Детальніше...  

Linear poly(ethylene terephthalate) (LPET) was blended with branched poly(ethylene terephthalate) (BPET) by varying the branching comonomer and blend proportion of BPET. The crystallinity and melting temperatures of LPET in blends with BPET was measured using differential scanning calorimetry (DSC). Thermal analysis revealed that the crystallinity and crystallisation/melting temperatures of LPET– BPET blends were increased compared with LPET. The rheological measurements of the blends were conducted using a controlled stress rheometer, with dynamic oscillatory shear, as well as steady shear measurements. The acceleration of the LPET crystallisation was dependent on the blend proportion and on the branching comonomer. The rheological measurements indicated a 50% increase of melt viscosity of LPET when blended with 30 mass% of BPET. This was attributed to increasing chain entanglements with the proportion of branching comonomer of the BPETs. Also, blends of LPET with BPET of different branching comonomer were investigated, and the rheology was found to be highly sensitive to composition. Dynamic rheological measurements of the blends were made, and compared with the data for LPET, to evaluate the effect of BPET on the viscoelasticity.

Детальніше...