1993 рік

Poly(L-glutamic acid) (PLG) membranes were prepared by crosslinking poly[~-(2-chloroethyl) L-glutamate] with diethylenetriamine and subsequently hydrolysing the side-chain esters. Selective amino acid transport across the membrane was examined using L-phenylglycine, L-phenylalanine and L-tryptophan. Transport selectivity increased with increasing pH of the solute solution, i.e. from 2.0 to 5.7. When pH was raised to 8.0 or when CaCI2 was added, transport selectivity was lost. Conformational changes of PLG and electrostatic interactions between the solute and the membrane are suggested to cause the observed permselectivity behaviour.

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The influence of processing, crystallization conditions and molecular characteristics on the fracture behaviour of isotactic polypropylene obtained by a new catalyst system was investigated. The sample specimens were prepared by using two extreme crystallization conditions -- quenching or slow crystallization -- in order to obtain crystals and amorphous phases with different structure. Interesting correlations between fracture parameters and some morphological parameters such as long spacing and lamellar thickness of the samples were found. The SEM fractographic analysis provides useful information on the influence of factors such as molecular weight, spherulite size and undercooling. Nucleating agents were used to tailor the dimensions of spherulites.

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The polar substituted polypyrrole, poly(5-acetamido-4,5,6,7-tetrahydro-2H-benzo[c]pyrrole), can be oxidized up to a degree of oxidation of 0.8 charges per pyrrole ring without substantial degradation. Its cyclic voltammogram (CV) shows two redox peaks. Variation of the sweep rate of the CVs and in situ gravimetric experiments show that the second anodic peak is associated with a swelling of the polymer. In situ u.v.-vis and e.p.r, spectra are in accordance with existing theories on the electronic properties of polypyrrole. While the neutral polymer is an insulator, the electrical conductivity increases to a maximum of 4 x 10- 3 S cm- 1 when the polymer is 'halt" oxidized in a linear potential sweep, and then decreases to less than 10 4 S cm 1 when fully oxidized. The potential dependence of the conductivity is completely reversible. The maximum of the conductivity occurs when polarons and bipolarons have the same concentration. This supports a hopping model of the conductivity in which charge transfer occurs between polarons and bipolarons.

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This paper discusses the compatibility of aqueous hydroxypropyl methylcellulose (HPMC)/poly(vinyl alcohol) (PVA) blended systems. Dynamic mechanical analysis and differential scanning calorimetry have provided clear evidence of two phases. Each phase precluded almost completely the other component. The HPMC-rich phase was amorphous with a constant Tg, while the PVA-rich phase showed a limited level of crystallinity and a constant T s for the amorphous part. Fourier-transform infra-red studies indicated hydrogen-bonding interactions involving the carbonyl and hydroxyl groups of like molecules but no detectable interactions between HPMC and PVA. A new hydrogen-bonding environment of the hydroxyls was observed in the blended system but was not capable of inducing any compatibility. The gross incompatibility was successfully predicted by the total solubility parameters of the two polymers and attributed to significant differences in the polar and hydrogen-bonding contributions. Surface energy analysis corroborated the inability of the two chains to participate in acid-base interactions between unlike molecules due to significant discrepancies in their Lewis acid-base characteristics. Finally, determination of the free energy of polymer-polymer interaction in water allowed approximate calculation of the Flory-Huggins parameters of aqueous PVA and HPMC systems

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In part 1 of this series, the concept of a critical material thickness was introduced and demonstrated experimentally using polystyrene (PS) as a test material. Below the critical thickness, brittle polymers become ductile. The value of the critical thickness is material-dependent and related to the entanglement density. The dependence of the critical thickness on the entanglement density was investigated using the miscible system polystyrene-poly(2,6-dimethyl-l,4-phenylene ether) (PS-PPE). PS possesses a low entanglement density and PPE a high entanglement density, and the system's entanglement density can be varied depending on the relative ratio of PS and PPE in the mixture. Equivalent to the experimental procedure developed in our previous paper, the thickness was set by either changing the PS-PPE layer thickness in stratified PS-PPE/PE tapes (polyethylene (PE) is present to separate the PS-PPE layers) or by adjusting the volume fraction of non-adhering core-shell rubbery particles in the PS-PPE blend, i.e. the ligament thickness. The experimentally determined critical thickness (IDc) proved to increase continuously from 0.06 pm for PS-PPE 80-20 to 0.18 #m for PS-PPE 40-60 blends. This compares well with the value of 0.05/~m found for pure PS. Under the (moderate) testing conditions used, the PS-PPE 20-80 blend was always tough. The maximum macroscopic strain to break (2 .... ) of the PS-PPE blends correlated with the theoretical value (J'max) based on stretching the entanglement network to its full extension. The transition from a macroscopic, brittle-to-ductile deformation behaviour is associated with a change in type of deformation mechanism from void formation (e.g. crazing) to shearing, except for the PS-PPE 20--80 blend, which always deforms by shear deformation. A simple model based on an energy criterion could explain the occurrence of a critical material ligament thickness as well as its dependence on the entanglement density

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The living character of the anionic polymerization of t-butyl methacrylate (BMA) using 2,3- diphenylbutadienemagnesium (DPB-Mg) as a bifunctional initiator was investigated by varying the polymerization conditions. Anionic polymerization of BMA at both - 78 and - 45°C involved no transfer reaction and the molecular weight distribution ofpoly(t-butyl methacrylate) was very narrow (Mw/M . = 1.20). However, the polymerization at 0°C involved a small amount of side reaction, i.e. attack of the living ends of the ester molecules. The structure of the DPB-Mg complex was characterized by spectroscopic analysis and quantitative titration of Mg metal.

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The physical properties and phase structure of blends of the multiblock poly (ether-ester) elastomer (PEE) and the multiblock poly((ether-carbonate)-urethane) elastomer (PUC) have been investigated using the methods of differential scanning calorimetry and dynamic mechanical analysis. The blends were prepared in the melt. The system is characterized by one glass transition temperature (Tg) for the soft phase, which points to the occurrence of partial miscibility. The blends appear to possess two hard phases, a polyester phase and a polyurethane phase, with glass transition temperatures and melting points close to the temperatures of the initial copolymers. Suggestions have been made regarding the possibility of occurrence of an intermediate phase conspicuous by extra glass transition and crystallization temperatures

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The glass transition temperature (Tg) of poly(cyclohexyl acrylate) (PCA) has been obtained from measurements of the oxygen permeability (P=0.60 barrers, Tg=23.5+2.0°C) using a potentiostatic electrochemical sensor, and from the oxygen diffusion coefficient (D = 0.12 x 10- s cm 2 s - 1, Tg = 24.0 + 1.8°C) by the time-lag method. A Tg of 25+1°C was found by differential scanning calorimetry and a T~ of 25.0_ 3.5°C was obtained from the specific volume by dilatometry. The fractional free volume (vf) at Tg is 0.020 + 0.002, which is slightly below (~ 20%) most glassy polymers. A linear correlation has been observed between In D and the reciprocal of vf, which suggests that the polymer morphology is not influenced by the presence of oxygen. The diffusion coefficient of oxygen through PCA membranes follows Arrhenius behaviour and has an activation energy of 26_2 kcal mol- 1, a surprisingly high value considering that the range of activation energies observed in most systems is 1-20 kcal mol- 1.

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The kinetics of the radical-initiated solution homopolymerization of a styrene-terminated poly(vinyl alcohol) macromonomer with a degree of polymerization (DP) of 7-9 (A) and copolymerization with styrene (S) have been investigated at 60°C in deuterated dimethyl sulfoxide solution using azobisisobutyronitrile as initiator and 1H n.m.r, as the analytical technique. It was found that the macromonomer exhibits a lower reactivity than styrene and a model monomer with DP = 0, which was confirmed by copolymerization with styrene (rA =0.9 + 0.1 and rs= 1.3___0.1). The hydrophilic macromonomer was then copolymerized onto seed polystyrene latex particles by a shot growth process, a more efficient surface yield being ensured by using polar comonomers like methyl methacrylate and acrylonitrile

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Ellipsometry was employed to estimate the thickness of the interface 2 between two bulk layers of dissimilar polymers. The polymer/polymer combinations investigated were poly(methyl methacrylate) (PMMA)/ poly(styrene-co-acrylonitrile) (SAN) (AN content < 10 and > 33 wt%), PMMA/poly(methyl methacrylateco- styrene) (MS) and polystyrene/MS. They provide a series of immiscible combinations with various values of the Flory interaction parameter X ranging from 1.72 x 10-3 to 8.71 x 10-2. The bilayer specimen prepared by the spin-coating technique was annealed at 130°C (> T~) and the time variation of 2 was observed during annealing. The observed 2 attained a constant value after annealing for ~ 1 h and then remained constant. The constant value was assumed to be the equilibrium thickness. The thickness ranged from ~ 5 to 40 nm, depending on the magnitude of X. The thin interface (< ~ 10 nm) was nicely explained by the recent theory by Broseta et al., which deals with the effect of molecular weight on 2 within the strong segregation limit.

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