1994 рік

Ethylene-ct-methylstyrene (EMS) copolymers were prepared by anionic polymerization using butyllithium and N,N,N',N'-tetramethylethylenediamine (TMEDA) as an initiator. The 13C n.m.r, spectra of EMS copolymers are observed and assigned mainly using two kinds of n.m.r, techniques: 13C distortionless enhancement by polarization transfer (d.e.p.t.) and the two-dimensional incredible natural abundance double quantum transfer experiment (2D-INADEQUATE). The monomer composition, the sequence distribution in the triad level and the number-average sequence length are determined on the basis of the 13 C n.m.r, results

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The polymerization of L-lactide with triethylaluminium and alkylaluminium alkoxides was studied in detail. It was found that commercial triethylaluminium is often contaminated with aluminium ethoxide groups resulting from the rapid oxidation of triethylaluminium. The ethoxide group is a far more reactive initiator than the aluminium-carbon bond. Therefore reactive initiators can be prepared in situ by the reaction of pure triethylaluminium with vitamins, hormones or drugs containing hydroxyl groups. Polymerization of L-lactide with these in situ prepared initiators yields oligo- or poly(L-lactide) with covalently bound vitamins, hormones or drugs. The bioactive co-initiators used in this study were geraniol, stigmasterol, tocopherol, testosterone, pregnenolone, ergocalciferol, cortisone and quinine. It is demonstrated by 13 C n.m.r, spectroscopy that the keto groups of steroids do not undergo redox reactions during the polymerization process.

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Forward recoil spectrometry was used to obtain the tracer (D*) and mutual (/)) diffusion coefficients in a miscible polymer blend of poly(methyl methacrylate) (PM MA, T~ = 136°C) and poly(styrene-co-acrylonitrile) with ~ 23 wt% acrylonitrile content (SAN, T~= 112°C). For blends with SAN weight fraction w of 0.2 and 0.5, the temperature dependence of D* for both species was nearly identical. Tracer diffusion coefficients DpMMA and D~'AN were determined for matrices consisting of 176 000 molecular weight SAN (w = 0.5) and various molecular weights of PMMA ranging from 27000 to 840000. The results were consistent with those expected from the mechanisms of reptation and constraint release. Analysis of the tracer diffusion coefficients D* showed that SAN has a monomer friction coefficient, (O.PMMA, about five times smaller than that of PMMA ((o.PuM^) in a matrix of pure PMMA, but the difference decreased monotonically as w increased, so that the (o values were nearly equal when w = 1. Corresponding to this relative change in (o, the glass transition process is broad for PMMA-rich blends and narrow for SAN-rich ones, raising the possibility that the difference in (o in the PMMA-rich blends is due to the existence of two local glass transitions, one for each species in the blend. For blends at the composition of w = 0.5, the/) was measured as a function of PMMA molecular weight. The data followed closely the predictions of the fast theory, the result expected if /) is ultimately controlled by the diffusion of the faster moving species. From the measurements of/) and D* at 187°C, the composition dependence of the Flory interaction parameter was also obtained, which showed good agreement with the recent small angle neutron scattering results by Hahn et al.

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Nuclear magnetic resonance (n.m.r.) spectroscopy and Fourier transform infra-red (FTi.r.) spectroscopy are used to characterize the trans-gauche isomerization of the methylene groups in a new'class of main-chain liquid crystal polymers. This new series of liquid crystalline copolymers is based on the 1-(4-hydroxyphenyl)- 2-(2-R-4-hydroxyphenyl)ethane mesogen, where R is F, C1 or CH 3 and flexible spacers containing an odd number of menthylene units. These copolymers are particularly interesting because they show two nematic states. Trans-oauche isomerization is characterized by FTi.r. spectroscopy through measurements of the absorbances of characteristically trans bands, and characteristically gauche bands. Trans-gauche isomerization is characterized by 1 aC n.m.r, spectroscopy through measurements of the ~ aC chemical shifts in the solid state. FTi.r. spectroscopy shows that an increase in temperature results in an increase in the percentage of gauche isomers for the methylene units in both the spacer and the mesogen. N.m.r. spectroscopy shows 13% more 9auche isomers in the high-temperature nematic state, relative to the low-temperature nematic state, for rotation about the C2s-C29 bond axis in the spacer.

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An ~t,og-(3-methacryloyloxypropyl dimethyl siloxane) macromonomer (I) with a molecular weight of 3700 g mol- 1 was copolymerized with styrene in bulk at 60°C using azobisisobutyronitrile as the initiator. Phase separation during polymerization was monitored macroscopically by clouding and microscopically by domain formation. The copolymer was separated into hexane soluble and insoluble fractions. From u.v., 1H n.m.r, and g.p.c, analysis of these fractions, the composition of the copolymers was determined as a function of feed composition (up to 50wt% macromonomer) and reaction time (up to 6 h). Analysis of the hexane insoluble fraction showed that the macromonomer rapidly formed a crosslinked network copolymer with styrene, showing little residual functionality. The hexane soluble fraction contained a copolymer with blocks of dimers and trimers of I.

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The reduced viscosity, qr, of poly(vinyl alcohol)-Congo red (PVA-CR) complexes in aqueous solution has some unique features, such as (1) an appearance of a maximum at a given PVA concentration (CpvA) and (2) a time evolution ofqa, but only in the regime where the maximum appears. These characteristic features of the viscosity behaviour are simulated by using the theory of Rice and Kirkwood which takes into account the electrostatic interaction in the reduced viscosity.

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Measurements of molecular orientation in stretched films of polydimethylsiloxane using infra-red linear dichroism have been improved by the introduction of polarization modulation of the incident electromagnetic field. This technique is able to measure small dichroic effects with high sensitivity, even for very small sample draw ratios.

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An ~t,to-(3-methacryloyloxypropyl dimethyl siloxane) macromonomer (I) with a molecular weight of 21 300 g mol- 1 was copolymerized with styrene in bulk at 60°C using azobisisobutyronitrile as the initiator. Loadings from 10 to 50 wt% of the macromonomer were used. Cloud point and optical microscopy studies showed mixtures containing > 10 wt% macromonomer were phase separated before polymerization, whereas mixtures containing 10wt% macromonomer became phase separated after 18min reaction time. The siloxane-polystyrene copolymers formed have a graft structure containing up to three macromonomers in a polystyrene backbone with no crosslinking. ~H n.m.r, analysis showed one unreacted methacryloyloxypropyl group remained per macromonomer. Compositional and molecular weight studies showed that the macromonomer did not homopolymerize under these conditions. The differences between this behaviour and that of a lower molecular weight macromonomer (3700gmo1-1) which formed both siloxane homopolymer and a crosslinked network is believed to be related to shielding of the methacryloyloxypropyl reactive groups.

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The effects of ionic interaction on the uniaxial extensional flow of polymer melts were investigated using poly(ethylene-co-methacrylic acid) and its metal (Zn, Na, Mg) salts. The presence of ionic bonding enhanced the elongational viscosity in the linear region and the degree of non-linearity. Both the linear and non-linear elongational viscosities were increased more by zinc ions than by sodium or magnesium ions. The order of elongational viscosities was in agreement with the order of activation energies obtained from dynamic shear measurements. It was concluded that the increase in both linear elongational viscosity and non-linearity is determined by the degree of ionic interaction. The different flow properties between zinc and magnesium ionomers indicated that the ionic cluster formation weakens the ionic interaction. A Bernstein-Kearsley-Zapas constitutive analysis revealed that the effect of ionic interaction under elongational deformation in the linear region is similar to that under dynamic shear deformation

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The morphologies of blends of polyethylene (PE) with a range of copolymers with different basic structure (diblock, triblock, starblock or statistical) have been compared in order to assess the relative importance and interrelation of rheological and molecular parameters. The development of well defined dispersed phases in extruded-blown PE films results from two successive steps. The first is the dispersion step, dominated by shear force fields and shear viscosity in the extruder barrel and die. The second step involves drawing of the melt and freezing of the structures. If the dispersed phase is characterized by a high value of the shear viscosity ratio r/d/r/m and hence by a low deformability, elongation can be performed in neither step. This was observed for diblock styrene-butadiene (SB) in a low viscosity PE matrix, for SBSr2 and for StatSB which all give spheres or ellipsoids. The dimension of the spheres is governed by the viscosity of the matrix and by the value of r/d/r/m according to the Wu equation. If the dispersed phase is highly deformable (~/d/~/m< 1), it is elongated in one or two dimensions at the exit of the die. For a Newtonian melt, these structures break during the elongation time preceding freezing of the structures if the breaking time given by Tomotika's theory is shorter than the elongation time. Broken filaments have indeed been observed for SBS at 240°C and also for styrene-isoprene-styrene (SIS) at 270°C. For a non-Newtonian dispersed phase, stabilization of the structures occurs by different mechanisms involving the elastic contribution of the elongational viscosity. It is related to entanglements and increases with molecular weight. Yielding or tension thickening behaviour can be involved. This is illustrated by the formation of co-continuous elongated structures when the high-molecular-weight diblock SB and triblock SIS are blended with PE, respectively at 240 and 220°C. Two starblock copolymers, SBSrl and SBSr3, although non-Newtonian and characterized by a rather low value of r/d/~/m, do not give continuous structures. This is attributed to the low molecular weight of the branches which does not allow the formation of a sufficient number of entanglements.

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