1994 рік

A kinetic model is presented for the post-gelation period of free-radical monovinyl-divinyl monomer copolymerization reactions. The model involves the moment equations of both the primary and the branched molecules in the sol, and predicts the vinyl-group conversions, the number of crosslinks and the chain-length averages as a function of the reaction time. Formulae for the weight fraction and cycle-rank density of the gel and its equilibrium degree of swelling are derived. The predictions are found to be in good agreement with experimental data on the copolymerization of styrene (S) with m-divinylbenzene (DVB) obtained by Hild and Okasha. The reaction time for incipient phase separation during S-DVB copolymerization in the presence of a solvent and the threshold concentration of the DVB for the formation of heterogeneous structures are also calculated

Детальніше...  

The phase behaviour of a series of uncrosslinked and crosslinked side-chain liquid-crystal copolymers is reported. These materials show a reversible nematic-isotropic transition. The two parent homopolymers prepared from the constituent monomers, which differ only in the length of the coupling chain, exhibit, in one case, a preferential alignment of the mesogenic units parallel to the polymer backbone (Nm) and, in the second case, an alignment of the mesogenic units perpendicular to the backbone (NI). It is shown that it is possible to prepare a random copolymer, in which the competing influences of these two opposing couplings lead to materials that exhibit no preferential alignment of the mesogens with respect to the polymer chain. Such materials exhibit almost zero coupling between the mesogens and the polymer backbone (No). At this 'null' composition for the elastomer, it is found that the application of a mechanical field can lead to a transition between an Nm nematic phase and an N~ nematic phase. The coupling between the mesogenic side groups and the polymer backbone can be resolved into the influence of the nematic field and a hinge effect arising from the detail of the chemical architecture of the coupling chain. Using these observations and the results of a mean-field model of the coupling, we show that the 'hinge' effect is some 2.0 to 1.5 times as effective as the 'nematic'-like interaction between the mesogens and the polymer chain in determining the nature of the coupling

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The molecular mobility of two semicrystalline polymers with flexible chains--chemically crosslinked low density polyethylene (XLPE) and poly(vinylidene fluoride) (PVDF)---has been investigated in the temperature range between room temperature (above their glass transition temperature) and their melting point. D.s.c. was used to characterize the conformational evolutions related to crystalline domains. Thermostimulated currents and creep were used to study the dynamics of these movements. For both polymers, the ~-relaxation/retardation mode is located in the temperature range of a small endothermic phenomenon. These low temperature d.s.c, peaks are not related to any melting process but rather to conformational disorder in the crystal-amorphous interphase. The fractional polarization/stress technique was applied to obtain the distribution of relaxation/retardation times. A compensation phenomenon was found above the •-relaxation mode of/~-PVDF and XLPE. It has been ascribed to conformational movements associated with an order-disorder phase transition in the vicinity of the melting point. These high temperature phases would correspond to the paraelectric phase of fl-PVDF and to the hexagonal phase of XLPE. Insofar as the ~-mode is liberating molecular mobility at the crystal-amorphous interphase, it appears as a precursor of the order~lisorder transition.

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A full, two-level factorial experimental design with temperature and concentrations of zirconocene dichloride and methylaluminoxane as variables was employed to study the polymerization of ethylene. Rate of polymerization and molecular weight data were used to develop a kinetic model and estimate the kinetic parameters. The polymerization rate was continuously recorded from a semi-batch reactor and molecular weights were measured at the end of each experimental run. The analysis of the data suggests the presence of two kinds of active species. One kind of species is produced from the other via a pseudo-first-order reaction.

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This work deals with the crystallization of polymer chains within silica-poly(dimethylsiloxane) mixtures, irrespective of the type of crystalline structure that is actually present in the system. An analysis of the transverse magnetization relaxation function is proposed as a method for melting point determination as well as for determination of the extent of crystallization as a function of time during an isothermal crystallization. We first pay attention to pure poly(dimethylsiloxane) (PDMS), for which we have measured the enthalpy of fusion per mole of structural units and defined the isothermal crystallization kinetics. The results obtained for the mixtures are compared to those obtained for pure PDMS. It turns out that the presence of silica particles does not affect the dimensionality of the crystalline growth. However, it reduces the extent of crystallization and has two antagonistic effects on the overall rate of crystallization. The first is a nucleation effect which is dominant at low silica volume fractions, and the second effect is a topological constraint which becomes dominant and hinders the crystalline growth at high silica volume fractions. As a result of the competition between these two effects, the overall rate of crystallization goes through a maximum with increasing silica volume fraction.

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This paper presents a quick, useful method for obtaining an equilibrium phase diagram (including the melting point depression curve) for a semicrystalline polymer--diluent mixture from cloud point data. Good agreement was obtained between the theoretically predicted and experimentally generated equilibrium crystallization curves in spite of simplifying assumptions. An accurate extrapolation of the coexistence curve to regions in the phase diagram where experimental generation of cloud point data is not possible was also obtained. Flory's original polymer swelling theory was used to determine the complete phase diagram. In the system studied (isotactic polypropylene in diphenyl ether), the interaction parameter was assumed to be a function of temperature only. Cloud points were generated experimentally, and an interaction parameter was determined at each temperature. A melting point depression curve was calculated from the temperature dependent interaction parameter and compared with experimentally determined equilibrium melting points. The resulting phase diagram was used to interpret the morphology of microporous structures prepared by the thermally induced phase separation mechanism

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A survey of the data reported in the literature on the dilute solution characterization of polyphosphazenes is presented. There are relatively few studies dealing with this kind of characterization, at least in comparison with the enormous amount of work devoted to the synthesis, solid-state properties and possible practical applications of these materials. Furthermore the analyses published to date give surprising and often contradictory results which indicate that the solution properties of these polymers are difficult to measure and not yet well understood. High polydispersity of the samples, formation of intermolecular aggregations and branching are the problems more frequently invoked to explain the experimental results. Some conclusions are presented that could facilitate future characterizations of these polymers

Детальніше...  

The direction of the averaged molecular orientation as well as the order parameter in injection-moulded flat plates of liquid-crystalline polymer (Vectra A950, A530 and A130) were investigated by infra-red dichroism. The measurements were performed with a low-cost Fourier-transform infra-red reflectance microscope. Lateral and depth profiles, inclusive of three-dimensional imaging, of the molecular orientation were measured as a function of thickness of the fiat plate and injection speed

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Polyethylene/n-pentane/carbon dioxide ternary systems have been modelled using the Sanchez-Lacombe lattice-fluid model. Phase diagrams have been generated at pressures up to 300 MPa and temperatures up to 460 K. The results show that the system may display two-phase or three-phase equilibrium depending upon the pressure. At a given temperature, the three-phase regions disappear with increasing pressure. Depending upon the pressure, calculations also predict the experimentally observed shifts from lower critical solution temperature to upper critical solution temperature type behaviours, which are illustrated in the ternary diagrams as shifts in the phase boundaries with temper~ature. It is further shown that for high-molecular-weight polymer samples the ternary calculations can be simplified by assuming that the polymer-lean phase is essentially free of polymer.

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Several trifunctional, hydroxy-telechelic polyester and poly(ester-carbonate) homopolymers and copolymers were synthesized by the triol-initiated, ring-opening bulk polymerization of D,e-lactide, glycolide, e-caprolactone, and/or trimethylene carbonate. The molar compositions of the copolymers were determined from 13C n.m.r, spectra; hydroxy equivalent weights were determined by acetylation titration, and the glass transition temperatures (Tg) and melting points were determined by differential scanning calorimetry (d.s.c.). Crosslinked polyurethane networks were prepared by reacting the hydroxy-telechelic prepolymers with tolylene diisocyanate. Network characterization included determination of sol content by solvent extraction, Tg by d.s.c., tensile properties by stress-strain measurements, and degradation properties by hydrolytic degradation studies. Equilibrium swelling results indicated that the per cent weight gains correlated well with the prepolymer hydroxy equivalent weights, with the exception of the poly(glycolide-co-trimethylene carbonate) network. The poly(D,L-lactide) and poly(o,L-lactide-co-trimethylene carbonate) (PLTMC) networks had the highest tensile strengths of 49.60 and 41.27 MPa, respectively, and glass transition temperatures of 51.3 and 2!.3°C, respectively. All other networks were highly flexible with tensile strengths of 12 MPa or less. Hydrolytic degradation studies were conducted by placing network samples in a solution of phosphate-buffered saline (pH = 7.4) at 37°C. Weight uptake of buffer and weight loss of the networks were monitored gravimetrically over time. Tensile properties, monitored as a function of degradation time, indicated that the poly(e-caprolactone-co-D,L-lactide) and PLTMC networks displayed a linear loss of strength with respect to weight during the first 30 days of degradation; the other networks degraded either too slowly or too quickly to establish such a linear relationship.

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