1993 рік

1,2-Polybutadiene of moderate molecular weight was grafted onto porous silica by hydrosilylation reaction of a part of its vinyl units, and coupling of the silane with the silanol groups of the silica. Two ways of grafting are discussed: 'grafting from' and 'grafting onto'; the latter gave better results in terms of capacity of residual double bonds grafted onto silica and available for further reactions. However, the capacity was limited to less than 1 meq g-1

Детальніше...  

Two particulate-filled grades of poly(butylene terephthalate) (PBT) containing about 10 and 30 wt % mineral fillers, one particulate-filled polyester blend containing about 65 wt% mineral filler, and two glass-filled grades of PBT containing about 15 and 30 wt% glass fibres have been studied to assess the effects of fillers on the strength of vibration welded butt joints. The weldability of these materials has been characterized at weld frequencies of 120 and 250 Hz and weld pressures of 0.9 and 3.45 MPa. Each of these five materials is shown to exhibit the four phases of vibration welding that are common in unfilled resins. Welds of these five materials are shown to achieve strengths on the order of 89, 58, 85, 69 and 60% of the strengths of the base (filled) resins.

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A standard laboratory ultrasonic bath can be used to greatly enhance polymerization of acrylate monomers in the presence of a thermolabile initiator (AIBN) and at a much lower temperature than is otherwise practicable. Initiation appears to result from the action of ultrasonic irradiation on the initiator. Water has also been shown to produce radicals under similar conditions.

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A new method of synthesizing cyclic polyesters is described, using polymer-supported reactions. This method involves the attachment of co-bromocarboxylate anions to the surface of an ion-exchange resin, followed by polymerization and cyclization by inter- and intramolecular alkylation reactions, respectively. The polymerization reaction results in the formation of oligomeric chain polyester molecules that remain bound to the resin by the carboxylate anion end group. The cyclization reaction results in the detachment of the cyclic product from the resin support. The cyclic polyester [-(CH2)loCO.O]x obtained was divided into a series of sharp fractions using preparative g.p.c, and each fraction was examined by analytical g.p.c. The cyclic nature of the polyester product was established by n.m.r, spectroscopy and by fast atom bombardment (f.a.b.) mass spectrometry. The f.a.b, mass spectrum of a fraction showed a series of spectral lines corresponding to [(CH2)10CO.O]x with x=2-7. The cyclic products were directly compared with analogous linear polymers.

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The development of microstructure during the heat treatment of poly(ethylene terephthalate) fibres has been investigated. Specimens representing different durations (down to a few milliseconds) of isothermal treatment were prepared through interrupted annealing and rapid quenching. Characterization of the structure and properties of such specimens demonstrated that the transformation occurred in two stages. In the first stage, microfibrillar crystals develop. The shrinkage behaviour of the material is set at this stage. In a second stage, the morphology transforms from fibrillar to lamellar. During this process, crystals become increasingly perfect and the crystallinity increases. The stiffness of the fibre increases continuously through these stages.

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Notched impact strength (N1S) has been determined (before and after thermal ageing at 150°C) as a measure of toughness for a wide range of poly(aryl ether sulphone)s and the results correlated with the structure of the polymers' repeat units. It appears that the most important structural factor determining toughness is the symmetry of the extended polymer chains; the incorporation of repeat units that disrupt symmetry leads to large reductions in NIS, while the introduction of structures that increase symmetry provides a significant improvement

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A topological trapping method was employed to confirm the cyclic nature of the polyester, which was prepared in part 1 using a polymer-supported reagent. It was found that approximately equal amounts of cyclic polyesters and cyclic poly(dimethylsiloxane)s (PDMS) (with comparable molar masses) could be entrapped in PDMS networks. By contrast, under similar conditions, far smaller quantities of a linear polyester were entrapped. The 'sizes' and 'shapes' of cyclic polyesters were compared with those of cyclic PDMS using a rotational isomeric state model (RISM). Molecular modelling using Quanta and CHARMm programs showed that cyclic polyesters adopted approximately circular conformations when they were subject to energy minimization.

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A study on the toughening of polyamide-6 (PA-6) with a new elastomeric modifier, brominated poly(isobutylene-co-p-methylstyrene) (BrXP-50), was conducted. Morphological analyses and mechanical tests were combined to evaluate the toughening mechanisms of modified PA-6 and the fracture mechanism of unmodified PA-6. We conclude that the fracture mechanism of unmodified PA-6 in the impact test is crazing-cracking under the dilatational stress created by the plastic constraint, while that of the BrXP-50 elastomer modified PA-6 is plastic tearing limited by the ultimate tensile strain. Cavitation of BrXP-50 elastomer particles occurred in the impact test, and is proposed as a controlling step in toughening: it relieves the plastic constraint so that the cracking process is postponed and additional plastic deformation can occur, leading to higher impact toughness.

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The use of confocal laser scanning microscopy (CLSM) as a new technique for the determination of the morphology of polymer blends is described. With this technique it is possible to obtain three-dimensional images of the morphology of the blends. An advantage of CLSM, compared to optical and electron microscopy techniques, is that the samples do not have to be prepared. As an example, the morphology of blends consisting of a styrene(ethylene/butylene)styrene( SEBS) block copolymer and a poly(ether ester) as obtained with CLSM is shown to compare well with that obtained by scanning electron microscopy

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Polymerization of methyl methacrylate has been studied in ternary o/w microemulsions that were stabilized by stearyltrimethylammonium chloride. The latices produced still remain transparent and stable about one year after preparation. It is calculated that each small latex particle of hydrodynamic radius (Rh) ranging from 20 to 24 nm contained two or three polymer chains of high molecular weight of 5 to 8 million. The rate of polymerization shows only two intervals for most of the systems. This corresponds to the rates of nucleation and growth of particles. But a constant rate of polymerization is also noted for the first time for microemulsions containing a lower concentration of initiator and/or at a lower temperature (T< 55°C). A possible mechanism related to rates of free-radical generation and polymerization is proposed to explain this constant rate of polymerization in a microemulsion.

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