1999 рік

The transport of a functionalised dye solute has been studied in various amorphous polymers above the glass transition temperature. An azo dye containing two hydroxyalkyl groups was transferred from a constant dye-donor polymer matrix to a variety of dye-acceptor polymer matrices using dye diffusion thermal transfer printing. Using the solution–diffusion model of permeability as the starting point, equations were developed to correlate solute transport with dye and polymer structural characteristics. Diffusivity is strongly influenced by polymer Tg whereas solubility is controlled by the balance of physical forces and specific interactions between dye and dye-acceptor polymer. An excellent correlation was established for permeability as a function of polymer Tg, dye–polymer solubility parameter difference and dye– polymer specific interaction as characterised by the infrared shift of solute O–H vibration. This predicted that maximum permeability would be obtained via a combination of low Tg, matched dye–polymer solubility parameters and strong specific acid–base dye–polymer interactions.

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Blends of a 100 penetration grade bitumen with four different polyethylenes having up to 40 pph or 29% of polymer by weight were prepared using a Z-blade mixer at more than 1608C. The blends were studied by fluorescence optical microscopy, differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). The optical measurements indicated the presence of bitumen-rich and polymer-rich phases. The DSC showed that the melting point of the crystallites was lowered and within the polymer-rich phases, the extent of crystallinity reduced by the presence of the bitumen. Annealing made significant changes to crystallite size, suggested by the raising of the melting point of the polymer crystallites. The DMTA measurements show that these polymers lower the temperature at which the glass transition softens the material, but raises the temperature at which the blend undergoes viscous flow. When the polymer fraction is over 20% to 28% the flow starts only when the crystallites melt, so the polymer then provides an associating junction network.

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A new method for increasing fracture toughness of brittle thermoplastic-modified thermosets by using triblock copolymers has been successfully investigated. The selected systems were polyphenylene ether (PPE)- and polyetherimide (PEI)-modified epoxy networks. Our choice was restricted to available commercial copolymers possibly with some chemical modifications. PPE presents the substantial advantage of having a negative enthalpy of mixing with polystyrene. The maleic anhydride-modified poly(styrene-b-ethylene-co-butene-b-styrene) triblock copolymer, containing an immiscible elastomer central block, was then selected. The reactivity of succinic anhydride functions towards primary amines was used to graft on ethylene-co-butene blocks, chains which are miscible or able to react with the growing epoxy network. The two problems encountered with PEI is that PEI is not miscible with any other polymer and that a commercial triblock with a PEI block does not actually exist. The only copolymer commercially available is a poly(etherimide-b-dimethylsiloxane) segmented copolymer, with elastomer segments which are known to be strongly immiscible with any components. In order to obtain the characteristics of the required compatibilizer, the poly(caprolactone-b-dimethylsiloxane-b-caprolactone) triblock copolymer was associated since (a) the polydimethylsiloxane elastomer central block is chemically identical to the elastomer segment of the previous copolymer, and (b) the polycaprolactone blocks are totally miscible with epoxy. For both thermoplastic-modified epoxy networks, spectacular mechanical reinforcements were measured with only 10%b.w. thermoplastic as a result of interfacial activities of selected compatiblizing systems, with a relative enhancement of fracture toughness close to 50% with around 1% of copolymer. The positive effects on mechanical properties always result from the same causes: large decrease of the particle size (submicron size) and formation of a copolymer-rich interphase characterized by a micromechanical transition in mechanical spectroscopy.

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In a certain unfavourable environment, a single DNA molecule undergoes a conformational transition from an expanded coil state to a collapsed form. Here, this transition is induced by poly(ethylene glycol) (PEG) in a solvent mixture composed of an aqueous salt buffer and methanol. A theoretical description is presented in terms of the free energy of mixing DNA, PEG, and solvent, the elastic part of the free energy for DNA chains, and the translational entropy of the low molecular ions. Further effects taken into account are DNA–counterion binding and solvent quality. The theoretical predictions are: (1) The transition between the coil and the collapsed state is discontinuous. There exists a coexistence region where both states coexist side by side, but its width is very small. (2) The collapse takes place at a certain critical PEG concentration cPEG;c . The value of this PEG-concentration depends on the degree of PEG polymerisation, Pw;PEG , the molar fraction, xmethanol , of methanol, and on the concentration, csalt , of the added salt. For given values of xmethanol and csalt , cPEG;c decreases with increasing Pw;PEG . That is, it is easier to induce the DNA collapse with PEG of high molar mass than with PEG of low molar mass. If both Pw;PEG and csalt are constant, cPEG;c increases as the methanol concentration decreases. This means that addition of methanol promotes DNA condensation. If finally Pw;PEG and xmethanol are chosen constant, cPEG;c decreases as csalt increases. Thus we can say, the collapsed DNA state is the more stabile the higher are Pw;PEG , xmethanol as well as csalt . That is, these three parameters act synergistically. (3) Theory gives some information about the DNA-molecule size. While in the coil state the expansion factor, a, is of the order of 1.8–2.4, it is of the order of 0.3 in the compact state. Results of measurements presented in the companion paper affirm these results.

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A metallocene catalyst system, primarily the bis(h 5-cyclopentadienyl)cobalt (Cp2Co) combined with n-butyllithium (n-BuLi), was found to be active for the hydrogenation of polystyrene-b-polybutadiene-b-polystyrene (SBS) block copolymer. Data from the UV spectroscopy of the catalyst system indicated that one of the Cp ligand in the original Cp2Co was substituted by the butyl group forming a CpCoBu as the major catalytic species. The kinetics of the hydrogenation reaction were studied by monitoring the extent of saturation of double bonds on the polybutadiene segment of the copolymer using FTIR spectroscopy. The temperature dependence of the hydrogenation rate was analyzed and the activation energy was determined as 14.11 kcal/mol of the double bond.

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One of the drawbacks of phenol–formaldehyde (PF) resol resin is its slow cure which requires longer hot-pressing time for the manufacture of wood composite products, especially for thick fibreboard products. In this study, PF resol resin was modified with three carbonates (i.e. propylene carbonate, sodium carbonate and potassium carbonate) to accelerate the cure of the resin for the manufacture of medium density fibreboard. The thermal behaviour of carbonate-modified PF resol resins was characterized with differential scanning calorimetry (d.s.c.) using three different thermal scanning methods (single-heating rate, multi-heating rate and isothermal method). The results of the single-heating rate method were not consistent when compared to the other two methods. The rate constants were calculated with the activation energy and pre-exponential factor obtained by the multi-heating method. The calculated rate constant increased with increasing carbonate level and the temperature selected. The isothermal method revealed that the curing of propylene carbonate-modified resins follows autocatalytic kinetics while the other two additives showed nth-order kinetics with a partial autocatalytic effect.

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Viscometry, UV/Vis–centrifugation and dynamic light scattering (DLS) were used to monitor the coil–globule transition of calf-thymus DNA by poly(ethylene glycol) (PEG) in aqueous/methanolic NaCl solutions. All three methods confirm that methanol and PEG promote the transition synergistically. The PEG concentration at which the DNA collapses decreases as the methanol fraction of the solvent is increased. The values found for the critical PEG and methanol concentrations agree quite well with those predicted by the modified Flory–Huggins theory. This is rather surprising, because effects such as selective solvent adsorption or intramolecular charge repulsion are neglected. The most informative experimental technique for the present investigations is DLS. The DNA molecules are not affected by outside forces and DLS allows the measurement of molecular size as well as the distribution of the conformational states. It was observed that there are no intermediate conformational states. A DNA molecule is either in the expanded coil state or in the collapsed state.

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We proposed a new approach to characterize the reverse osmosis (RO) permeability in conjunction with the macromolecular structures and inherent polymer properties for crosslinked and linear model aromatic polyamides. Aromatic polyamides were synthesized via interfacial reaction of phenylene diamines (p- and m-phenylene diamines) with either tri-functional (trimesoyl chloride) or di-functional (terephthaloyl and isophthaloyl chlorides) acyl halides.The relaxation properties obtained by cross polarization/magic angle spinning (CP/MAS) 13C NMR spectroscopy, in conjunction with the chemical structures, built a bridge between the specific polymer properties and the RO performance of the aromatic polyamides. The spin-lattice relaxation time in the rotating frame, T1r and hence chain mobility seemed to be an important parameter to control the RO membrane permeability. The longer T1r resulted from the crosslinked aromatic polyamides, in which presence of the crosslinking retarded local polymer motion, and a lower water flux resulted. In contrast, the shorter T1r for the linear crosslinked polyamides played a significant role for the higher water flux.

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The specific temperature effects in hydrodynamic properties (intrinsic viscosity, sedimentation velocity and translational diffusion) of side-on comb-like polymers with molecular weights from 5.70 х 10 5 to 3.02 х 10 6 were observed in dilute solution in two different solvents, benzene and chloroform, over a temperature range from 2 to 73°C. The viscosity temperature coefficient dln[h]/dT was found to be negative in sign in chloroform ( ¹ 0.007 K ¹1) and positive in benzene ( þ 0.005 K ¹1). Increase in temperature leads to intensification of the excluded volume effects in real benzene solution of the polymer and to weakening of those in chloroform. Simultaneously, increase in temperature leads to decreasing unperturbed dimensions of the macromolecule chain in both the solvents. This effect is more pronounced in benzene (dlnhh 2i/dT ¼ ¹ 0.005 K ¹1, where hh 2i is the mean-square end-to-end distance) than in chloroform ( ¹ 0.001 K ¹1). Origins of the temperature effects are discussed with relation to the lateral structure of the macromolecule where every monomer unit contains the threecyclic mesogenic moiety which is laterally (with the middle part) attached to a side chain consisting of 11 carbon atoms.

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The conducting interpenetrating polymer network (IPN) is prepared by sequential crosslinking of tetrethyl orthosilicate (TEOS) with acrylic acid grafted styrene–isoprene–styrene triblock copolymer (SIS) and polypyrrole doped with dodecylbenzenesulfonic acid (DBSA). The resulting IPN not only has good conductivity, but also exhibits far superior thermal stability. Various factors affecting the properties of conductive IPNs are investigated. The amounts of water and TEOS used have a profound effect on the conductivity. Significantly, under appropriate experimental conditions, a conductive IPN with conductivity as high as that of pure PPyDBSA has been prepared. Heating the conductive IPNs to 140°C has generally caused the conductivity to decrease. However, samples with conductivity relatively unaffected by the heat treatment can be prepared with proper selections of H2O/TEOS ratio. The effect of heating on the conductivity is discussed. All the IPNs have excellent thermal stability as clearly shown by the results of thermal gravitational analysis (TGA). The morphologies of the IPN films are investigated using a scanning electron microscope (SEM).

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