1993 рік

Determining qsp/C as a function of time has been used to investigate rates of dissolution and stabilities of solutions of liquid-crystalline polyesters (LCPs). The dissolution behaviour of commercial LCPs and prepolymers, post-condensates and pelletized LCPs prepared in-house has been studied in 3,5-bis(trifluoromethyl)phenol (BTFMP) and pentafluorophenol (PFPh). All of the investigated polyesters dissolved in both solvents. Optimum temperatures for dissolution appeared to be 25°C for BTFMP and 40°C for PFPh. Dissolution times ranged from 10 to 40 h. The solutions remained stable for at least 80h. Dissolution of post-condensates is very difficult. Fortunately, by reducing the crystallinity, they dissolved more readily in both solvents without modifications of the molecular structure. Therefore, determining reliable molecular parameters of LCPs seems to be possible in these solvents.

Детальніше...  

The stability of neck propagation and its oscillatory mode have been studied for the cold drawing of poly(ethylene terephthalate) films. On the basis of Barenblatt's model considering a temperature rise at the neck, the stability has been analysed for neck propagation at constant speed and at constant load. It is shown that the stability is directly connected to the sign of the slope of the stress-drawing rate plot; unstable neck propagation should be in the region of negative slope. It is argued that the unstable mode changes to an oscillatory neck propagation for drawing at constant speed, while the mode in drawing at constant load is transformed to the other stable region. Experimental study has confirmed the unstable drawing at constant load and the transition of neck propagation rate. Oscillatory neck propagation has also been examined by a numerical calculation of non-linear differential equations based on Barenblatt's model. The limits of Barenblatt's model are also discussed.

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Conformational energy surfaces are determined as functions of torsional angles of bonds between rings for an ct-~t pyrrole trimer by using semiempirical quantum-mechanical calculations (AM1). Isomeric states for torsional rotations are identified as positions of minimum energy. Statistical weights determined on this basis are used in calculating the average dimensions of polypyrrole chains by the rotational isomeric state (RIS) model and the matrix multiplication scheme. Results of calculations indicate that the chains have unusually large characteristic ratios and persistence lengths with very strong temperature dependence. Characteristic ratios obtained by the RIS model show perfect agreement, in a wide temperature range, with the predictions of the worm-like chain model.

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A series of poly(ether ketone) copolymers were synthesized by the Friedel-Crafts reaction of diphenyl ether and a mixed acid chloride comprising isophthaloyl chloride and terephthaloyl chloride. The permeabilities of H2, 02, CO and Nz through the as-cast films at 30°C were influenced by the catenation effect, that is, increase of m-catenation ratio relative to p-catenation induces denser packing of the polymer chains and reduces gas diffusion. The difference of crystallinity among the polymers was too small to exhibit a clear influence on the gas permeability. Changes of permeability through the copolymer films by thermal treatment were interpreted as being due to the decrease of the amorphous region caused by thermal crystallization. Densification of the polymer matrix in the amorphous region was not observed.

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The strength of impact-modified polystyrene is related to the volume of craze matter generated in fracture. In the Izod test, crazes tend to initiate and propagate between interacting neighbours. The efficiency of craze termination is also critical to toughness, and depends upon the particle morphology. Using computer simulations of crazes propagating through an array of model particles, it has been possible to quantitatively model the crazing process and predict strength. The model is applicable to materials having a wide range of particle sizes, rubber phase volumes, and either solid or occluded particles. Predictions are in good agreement with data obtained for experimental blends as well as commercial HIPS.

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Sequential interpenetrating polymer networks (SIPNs) composed of poly(2-hydroxyethyl methacrylate) (PHEMA) and polystyrene (PS) were synthesized by photopolymerization at room temperature. A common solvent, N,N-dimethylformamide, was used to impregnate the styrene monomer into the PHEMA network. The IPNs had a microphase-separated structure of the order of 100 nm. It was found that the gradient composition in the surface region (to a depth of 0.1 mm) was obtained by polymerization inhibition with oxygen which had diffused from the surfaces of the swollen PHEMA gel. X-ray photoelectron spectroscopy measurements revealed that the PS component was enriched in the surface of the IPN. Thermodynamic effects on the composition of the outermost surface layer are discussed

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Biocompatible copolymers of N-(4-methacryloyloxyphenyl)-2-(4-methoxyphenyl)acetamide (OM) and N-I-4-(4-methoxyphenylacetyloxy)phenyl]methacrylamide (MA) with 2-hydroxyethyl methacrylate (HEMA) were prepared by free-radical polymerization. The kinetic behaviour of these systems was studied gravimetrically, and the results obtained are discussed in the light of kinetic models. The reactivity ratios were determined by the application of linearization methods to the general copolymerization equation, as well as by non-linear least-squares analysis, the most correct values being roM = 1.49, rnEMA = 0.61 and rMA = 0.29, r,EMA = 1.47. The microstructure of copolymer chains was determined statistically from the reactivity ratios, considering the terminal model for both copolymerization processes.

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In size exclusion chromatography (s.e.c.) analysis both the universal calibration curve (UCC) and the secondary calibration curve (molecular weight calibration) can be constructed using the Mark-Houwink-Sakurada (MHS) relation, [7] = KM". Over a very wide range of molecular weights neither K nor a is constant; therefore, the interpretation of results using one set of K and a values may not be correct (i.e. in the case of commercial resins). An alternative model which relates molecular weight (MW) to It/] over a very wide range of molecular weights is [q]=KoMrl/2+K'Mw where K o and K' do not change. The intrinsic viscosities of polystyrene standards with MWs of 10 3 to 2 X 106 were measured and the results were used to establish a UCC for a multi-detector s.e.c, system. From the UCC two calibration curves were established using the MHS relation and the alternative model. Commercial polystyrene and polyethylene resins were analysed using the same analytical system. At the higher molecular weight end one observes a deviation between the results from absolute MWdetectors and the calibration curve based on a particular set of K and a values. On the other hand, MWs obtained from the calibration curve using the alternative model were consistent with results from absolute molecular weight detectors. This indicates that the use of the alternative model will provide a more accurate calibration for s.e.c, analysis when the molecular weight distribution is relatively wide.

Детальніше...  

Taking into account the possible advantages of the introduction of thioether bonds into the polymerization product, the polymerization of formulations based on 1,4-butanediol diacrylate or 1,6-hexanediol diacrylate and containing additives of various linear sulfides or cyclic dithioacetals was investigated to discover the influence of sulfides on the polymerization rate and yield. A commercial photofragmentating initiator was applied. It was found that the observed effect is the result of accelerating and retarding processes occurring simultaneously and their relationship and the extent of the effect depended on the reaction conditions and the sulfide structure. A schematic representation of the most important reactions is given.

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Partial and saturation swelling experiments in latex particles of styrene methyl methacrylate copolymers, polybutadiene and composite particles containing polybutadiene with styrene and methyl methacrylate grafted were performed with the centrifugation method. Very good agreement is achieved between the experimental results of partial swelling and a simplified theoretical model. The behaviour of monomer partitioning when no monomer droplets are present is independent of polymer type, polymer crosslinking and latex particle diameter. The saturation monomer partitioning experiments also showed good agreement with a simplified thermodynamic theory. The polymer type does not influence the ratio of two monomers (styrene and methyl methacrylate) in the particle and droplet phases. Also, there is no preferential solvation of styrene in polybutadiene. This is an important result for the graft polymerization of styrene and methyl methacrylate. Differences observed in copolymer composition between graft and free copolymers thus cannot be ascribed to monomer partitioning.

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