1999 рік

The synthesis and polymerization of 2,3-diazanorborn-5-enes are reported. Reaction of 2,3-diazanorborn-5-ene derivatives 6–9 with molybdenum based initiators 1–3 leads, in most cases to ring opening metathesis polymerization. By contrast, monomers 6–9 are not polymerized by ruthenium based initiator 4. The polymers were characterized by NMR, GPC and MALDI-TOF mass spectrometry and were found to have narrow molecular weight distributions and to show a resonance attributable to a propagating alkylidene in the 1H NMR spectrum.

Детальніше...  

Poly(2-hydroxyethyl methacrylate-b-styrene-b-2-hydroxyethyl methacrylate) triblock copolymer was synthesized by hydrolysis of poly(2[(trimethylsilyl)oxy]ethyl methacrylate-b-styrene-b-2[(trimethylsilyl)oxy]ethyl methacrylate) triblock copolymer polymerised with ol-methylstyrene tetramer disodium as an initiator by an anionic living polymerisation technique. Micelle formation of the poly(2-hydroxyethyl methacrylate-b-styrene-b-2-hydroxyethyl methacrylate) triblock copolymer was investigated in dimethylformamide (DMF), toluene, cyclohexane and a tolueneln-hexane mixture by dynamic light scattering (DLS) method. Monodispersed polymer micelle with a poly(2-hydroxyethyl methacrylate) core and polystyrene shell chains was formed with the triblock copolymer in a toluene and n-hexane mixture when n-hexane was 62 ~01%. The poly(2-hydroxylethyl methacrylate) core was crosslinked with 1,6_hexanediisocyanate. Crosslinked products were spherical and their diameters were measured in the toluene by the DLS method. Additionally, microgel with free end shell chains was synthesized as the reference of the microgels with loop shell chains by scission of the loop shell chains at their centre by exposure to ultraviolet light.

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Peroxide-induced crosslinking of isotactic (iPP) and syndiotactic (sPP) polypropylene was investigated experimentally. It was found that the two polymers showed similar crosslinking behavior, with iPP having a higher crosslinking efficiency. Our electron spin resonance measurements elucidated that the attack of peroxide radicals on iPP and sPP chains took place at the same position, giving rise to similar spectral hyperfine structures. However, a lower radical concentration was observed in sPP than in iPP with the same peroxide concentration and temperature. This might be caused by steric hindrance to the hydrogen abstraction of peroxide radicals. This low concentration of radicals is believed to be responsible for the lower crosslinking efficiency found with the sPP system. The crosslinking of both polymers was determined by temperature and peroxide type and concentration. The crosslinking kinetics showed that the initial gelation rate increased linearly with the peroxide concentration and temperature. It was also observed that, at high peroxide concentration levels, significant bscission and other side reactions occurred simultaneously, introducing carbonyl and unsaturated groups to the structure of the polymeric networks.

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The development of bicontinuous morphologies in 10 wt% polysulfone (PSu)–epoxy (DGEBA)/anhydride (MTHPA) blends, was followed by optical and scanning electron microscopy. Blends cured at 808C revealed the formation of large epoxy-rich domains surrounded by a PSu-rich matrix, soon after the cloud point. Advancing the cure led to an increase in the volume fraction and the coalescence of epoxyrich domains. A bicontinuous primary morphology was thus generated. A secondary phase separation was observed in both primary phases from the very beginning of the phase-separation process. While spinodal demixing was clearly the mechanism by which the primary morphology was generated, nucleation-growth could be responsible of the secondary phase separation. Postcure steps produced a change in the composition of phases as revealed by DMA, and in the secondary morphology as observed by SEM. A postcure at 1208C led to a single Tg at 1158C with a small shoulder at higher temperatures. A postcure at 2008C led to a Tg at 1088C for the epoxy-rich phase and a Tg at 1378C for the PSu-rich phase. The partial purification of the thermoplastic phase produced a significant enhancement of toughness. KIC was increased from 0.65 MPa m 1/2 for the neat thermoset to 1.10 MPa m 1/2 for the blend postcured at 2008C.

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A method for the determination and quantification of higher-functional epoxides in liquid diglycidylether of bisphenol-A (DGEBA)-based epoxy resins, and for the determination and quantification of the amount of branching in solid DGEBA-based epoxy resins by 13C n.m.r. is described. Model compounds were synthesised and their chemical shifts assigned. Examples for both types of resin are given, and the usefulness of the method is shown by the example of a production campaign of a solid resin. The increase of branching and the build up of molecular weight are compared and discussed.

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Determination of average molecular weight between cross-links (M c ) of itaconic acid-containing poly(vinyl pyrrolidone) and maleic acidcontaining poly(acrylamide) hydrogels sensitive to pH changes of the swelling medium was investigated. Poly-electrolyte hydrogels with varying compositions were prepared in the form of rods from ternary mixtures of N-vinyl 2-pyrrolidone/itaconic acid/water and acrylamide/ maleic acid/water. Gelation was achieved by irradiating the system with g rays at ambient temperature. The equation modified by the authors recently for the determination of M c is observed to describe very well the swelling behaviour of charged polymeric networks. In addition to the evaluation of M c from swelling data, the same equation also provides the simultaneous measurement of polymer–solvent interaction parameter of the systems investigated.

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The effect of the evaporation step on the occurrence of particles in poly(ethylene-co-vinyl alcohol) (EVAL) membranes cast from DMSO solutions via the dry/wet process were studied. The structure of the EVAL membranes can be changed from an asymmetrical structure consisting of a dense skin layer and finger-like macrovoids in the sublayer to a skinless and symmetric structure by constituent particles bonded to each other. From the analysis of the membrane formation mechanism, the competition of different phase separation process during membrane formation is most important. The onset of the phase separation may be either liquid–liquid demixing or solid–liquid demixing, which determines the resulting membrane properties. Directly immersing the casting solution into a water bath, liquid–liquid demixing is of considerable importance during the phase separation of the solutions. As a result of the lower activation energy for nucleation, liquid–liquid demixing can precede solid–liquid demixing even in cases where solid–liquid demixing is favored thermodynamically. By using the evaporation process, the phase separation proceeds slowly via solid–liquid demixing and thus leads to a particulate morphology in the membrane. This suggests that the evaporation step cause crystallization of EVAL molecules from the casting solution to inhibit the macrovoid formation. In addition, the duration of the evaporation step is shown to have a strong influence on the disappearance of particles. The results presented here offer a qualitative basis for the development of membranes with a particulate morphology.

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Five samples of isotactic polypropylene, iPP, and two copolymers of iPP with I-hexene, synthesized with different catalyst systems, both heterogeneous and homogeneous, have been studied in order to analyse the effect of the catalyst system, the presence of comonomer units and the crystallization conditions on the phase structure of iPP and, in particular, on the amount of y modification obtained. Minor amounts, if any, of y modification are present in iPP samples synthesized with highly isospecific Ziegler-Natta catalysts and crystallized from the melt at different cooling rates, ranging from 100 to 3Wmin. On the contrary, considerable amounts of the y form have been obtained both in samples prepared with catalysts of very low isospecificity or in those prepared with homogeneous metallocene catalysts. It has been shown that the relative proportion of the cxand y modifications can be controlled just by changing the crystallization conditions. A clear influence of the presence of comonomer units on favouring the formation of the y phase has not been ascertained. Moreover, attempts to deduce the y content from the d.s.c. melting patterns have been unsuccessful, since the two modifications exhibit very similar melting temperatures.

Детальніше...  

The structural changes of hydrophilic polymer networks were studied by SANS. The size of heterogeneities in the network was measured for gels swollen in different amount of water. The type of cross-linking agent, degree of cross-linking and degree of ionisation were varied. The dependence of the size of heterogeneities on the degree of swelling shows a great difference for gels which exhibit strong polyelectrolyte properties (can swell hundreds of times) comparing with the ones which can swell less than 100 g/g. The interpretation of the observed phenomenon is based on the appearance of strong electrostatic interactions which are dominating in the gel-swelling mechanism at high degree of swelling.

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The influence of electron donors on the synthesis of polypropene-block-poly(ethene-co-propene) (PP-b-(PE-co-PP)) using a modified stopped-flow polymerization method was examined in terms of correlation of the microstructure of the polymer with its crystalline morphology. The results of propene homopolymerization and ethene–propene copolymerization indicated that the application of suitable internal and external donors induced an improvement in the stereoregularity of polypropene (PP) without a decrease in the initial activity and a significant change in the monomer composition and monomer sequence distribution of the poly(ethene-co-propene) (PE-co-PP). The TiCl4/ ethylbenzoate/MgCl2 – cyclohexylmethyldimethoxysilane catalyst system was found to produce PP-b-(PE-co-PP) having highly isotactic PP segments. The resulting block copolymers were investigated by cross-fractionation chromatography and differential scannning calorimetry, suggesting that the improved stereoregularity of the PP part in the PP-b-(PE-co-PP) had an effect only on the crystallinity and crystallinity distribution but had no influence on the crystallizability and lamellar thickness of the block copolymer.

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