1994 рік

The dilute solution properties of poly(p-t-butylstyrene) (PtBS) in cyclohexane, a good solvent, and 2-octanol, a theta solvent, are reported. The polymers are narrow-molecular-weight-distribution samples produced by an anionic polymerization process and span more than two orders of magnitude of molecular weight. The techniques used to study solutions of PtBS include static and dynamic light scattering, solution viscometry, and size exclusion chromatography. The results are compared with those obtained from other polymer/solvent systems and with predictions from various theories

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It is well known that there is an enormous disparity between the tensile elastic modulus E of a rubber and its bulk modulus K. For a typical rubber, E is of order 105 N m-2 and is primarily due to entropic effects. By contrast, K is of order 109 N m-2. This paper suggests that K arises from van der Waals interactions between groups in one chain and groups in neighbouring chains. Using a Lennard-Jones type of potential, U = -c/rr+ D/r 12, and assuming pairwise addition, the free surface energy ~ of rubber may be calculated and compared with typical experimental values (~ = 35 mJ m-2). From this, a typical value is obtained for C of order 60 x 10-79j m s (corresponding to a Hamaker constant ~9.3 x 10-20 j). This value may be used to calculate the bulk modulus, assuming a regular arrangement of interacting groups. The result agrees with the experimental value of K to better than an order of magnitude.

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Polypyrrole (PPy) dispersions are prepared at room temperature by oxidative polymerization using FeC13 oxidant in the presence of poly(vinyl methyl ether) (PVME) as stabilizer, and ethanol or aqueous ethanol as the dispersion medium. With water as dispersion medium, lower temperatures are required when the solvent property of water towards the PVME stabilizer becomes good enough to effect steric stabilization. PPy prepared in 50% ethanol or in water exhibits high specific conductivity, about 10 S cm- 1, while PPy prepared in ethanol under similar conditions has a conductivity two to four decades lower, depending on the FeCI3 concentration and temperature used in the preparation. Transmission electron micrographs of PPy reveal that the particles are spherical but polydisperse.

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A model for the heterogeneous graft polymerization of vinyl and/or acrylic monomers on cellulose initiated by ceric ions is developed. The model can be easily extended to other (poly)hydroxylic substrates, other one-electron oxidants and homogeneous systems. Three component mechanisms are suggested to be relevant in a radical graft polymerization: from, onto and from-to. Several competitive reactions appear in the polymerization scheme that lead to related mechanistic cases. The electronic nature of the monomer and the number of polymerization sites are taken into account. Expressions for reaction parameters as a function of reaction variables are deduced. Some structural knowledge of the copolymer is derived from the model.

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Poly(hydroxy ether of bisphenol-A) (phenoxy resin) and diglycidyl ether of bisphenol-A type epoxy resin (DGEBA) are found to be excellent compatibilizers for polyamide 6 (nylon 6)/methyl methacrylate-styrenebutadiene (MBS) graft polymer blends. When the blends are modified by phenoxy of DGEBA, phase dispersion, mechanical properties and rheological properties are significantly improved. The compatibility between phenoxy and MBS is based on thermodynamic miscibility between phenoxy and poly(methyl methacrylate) in MBS. The compatibility between nylon 6 and phenoxy seems to originate from the reactivity between functional groups of the two polymers. The compatibility between DGEBA and MBS is also based on thermodynamic miscibility between the chain component of the epoxy and poly(methyl methacrylate) in MBS. The compatibility between nylon 6 and the epoxy resin originates from reactivity between the functional groups of the two polymers. The role of phenoxy or DGEBA as compatibilizers in nylon 6/MBS blends is demonstrated and the two compatibilizers are compared.

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The graft copolymerization of methyl methacrylate (MMA) on jute was studied using a combination of NalO4 and CuSO4 as the initiator in an aqueous medium. Concentrations of IO~ = 0.005 mol dm- 3 and Cu 2 + =0.001 mol dm-3 gave optimum grafting conditions and percentage conversion at 50°C. 104 and Cu 2 + when used separately failed to induce either homopolymerization or graft copolymerization of MMA in the presence of jute. The effects of variation of time, temperature, monomer concentration and pH of the medium on grafting parameters and percentage conversion were also studied and reported. Progressive removal oflignin and hemicellulose from jute separately produced significant improvements in the percentage grafting and the percentage conversion without greatly affecting the grafting efficiency. A mechanism for the graft copolymerization reaction has been proposed. Grafting of poly(methyl methacrylate) (PMMA) to the extent of 20-25% was found to improve certain properties, including fibre strength and modulus, moisture regain, rot resistance, whiteness index, light-fastness and resistance to thermal degradation. I.r. spectra and X-ray crystallinity of the fibre samples with different degrees of grafting have been compared. Surface morphology, as studied using scanning electron microscopy, indicates that PMMA grafting occurs on surfaces and in the intercellular regions of the multicellular jute fibre.

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The ring-opening metathesis polymerization (ROMP) of enantiomerically pure 2-substituted norbornenes carried out with Mo(CH-t-BuXNArXO-t-Bu)2 in chlorobenzene or with K2[RuC15(H20)] in aqueous solvents leads to polymers that show optical activity. The increasing specific rotations of the polymers from acetate to butyrate to benzoate with both catalysts is explained by increasing substituent group size.

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The influence of testing speed and temperature on the brittle-to-tough transition of non-adhering core-shell rubber-modified polystyrene-poly(2,6-dimethyl-l,4-phenylene ether) (PS-PPE) blends was studied. The validity of the concept of a network density dependent, critical matrix ligament thickness (ID?, as introduced in this series and verified mainly by slow-speed uniaxial tensile testing) is demonstrated for notched high-speed (1 m s- 1) tensile testing at different temperatures. The influence of testing speed and temperature on the absolute value of ID? can be quantitatively understood in terms of a strain rate and temperature dependence of the yield stress. The simple model introduced in part 2 of this series proves to be valid under all testing conditions studied varying from temperatures of 50 to I50°C below the glass transition temperature of the PS-PPE blends. The absolute value of the tensile toughness, on the contrary, is a not yet quantified function of the test geometry applied and, consequently, cannot be directly derived from a simple strain-to-break argument.

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Six copolymers with short alternating conjugated and non-conjugated blocks were synthesized using the Wittig reaction. The conjugated block consists of two and one-half units of p-phenylene vinylene with methoxy groups substituted at the 3 and 5 positions of the flanking phenyl rings. These rigid blocks are linked to flexible linear alkyl chain spacers by ester groups. The polymers, characterized by 13C n.m.r., FTi.r., elemental analysis, g.p.c, and d.s.c, are soluble and fusible. Free-standing films were cast from chloroform solutions and showed electrical conductivities ranging from 10- 3 to 10- 2 S cm- 1 upon doping by iodine vapour at room temperature.

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Four different liquid crystalline diepoxides were synthesized. Replacement of a methylene group by an oxygen atom in the spacer segment of the molecule decreased the thermal stability of the mesophase. In order to produce highly anisotropic networks, these compounds were cationically photopolymerized in the nematic phase. Differential scanning calorimetry and Fourier transform infra-red were used to monitor the polymerization. The degrees of conversion calculated from both techniques were in good agreement with each other. The replacement of a methylene group by an oxygen atom in the spacer part of the molecule also increased the rate of the polymerization reaction. Post-curing was sometimes necessary to produce highly stable networks. Furthermore, some results suggest that liquid crystalline nematic ordering increases the polymerization rate.

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