1993 рік

Mass- and energy-balance equations that describe the hydrolytic polymerization of nylon-6 in an autothermal industrial reactor are written. These account for both axial and radial variations of the temperature, as well as of the concentrations of the various molecular species. The set of coupled, non-linear partial differential equations are transformed into a set of ordinary differential equations (ODEs) using the finite-difference technique in the radial direction. The resulting stiff ODEs are then solved using Gear's algorithm. The method of successive substitutions is used to obtain convergence to the final results. Sharp radial gradients in temperature and average molecular weights are observed because of near-equilibrium conversions at the reactor walls and the low thermal conductivity of the reaction mass. It is demonstrated that these need to be accounted for in the proper design and analysis of reactors having this configuration. The effects of various operating conditions and parameters are studied. It is found that the hot-spot is parametrically sensitive to the feed-water concentration and to the presence of monofunctional acid in the feed. In addition, product having the same molecular weight can be obtained using two different feed-water concentrations. The results are found to be sensitive to the values of the heat transfer coefficients, and so good estimates of this parameter are required for simulation purposes

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Poly (methacrylic acid) (PMAA) copolymers with uniform polystyrene (PS) side-chains were synthesized through the macromer technique. The graft copolymers (PMAA-g-PS) were purified by extraction with water and hot cyclohexane, and were then characterized by gel permeation chromatography, infra-red spectrophotometry and 1H nuclear magnetic resonance spectroscopy. The average grafting number was determined to be 3-7, and decreases with increasing conversion. The graft copolymers showed good emulsifying properties and high water absorbency, and can be used as a good compatibilizer for blending polystyrene with epichlorohydrin rubber (CHR). Only 2-3 wt% of the graft copolymer based on the blend was needed to enhance the mechanical properties of the blends, and the blends with weight ratio of PS/CHR = 3/7 to 5/5 behave as good thermoplastic elastomers. When PMAA-g-PS was mixed with polyoxyethylene (PEO) grafted with PS in molar ratio of PEO/PMAA = 1, a thermoplastic intermolecular complex membrane was formed. This behaved as a chemical valve, as its permeability can be controlled reversibly by changing the pH value.

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Dielectric experiments are carried out on poly(4-phenoxyphenyl acrylate) (PPOA) and 4-phenoxyphenyl- 2,4-dimethylglutarate (PPODG), a low-molecular-weight analogue of PPOA. The intramolecular dipolar correlation coefficient gintra for the polymer changes from 0.821 at 30°C to 0.835 at 60°C. This coefficient is similar to that of PPODG (0.817 at 30°C), suggesting that the asymptotic value of gintra is reached for rather low molecular weights. The dipolar correlation coefficient g that accounts for both the intramolecular and intermolecular dipolar interactions is smaller than gintra for both the polymer and the low-molecularweight compound. The dielectric spectrum of PPOA in the bulk presents a prominent glass-rubber relaxation followed by a subglass absorption that seems to be the combination of a process in the high-temperature zone (named fl) that overlaps with the glass-rubber relaxation and a well developed absorption (named y) in the low-temperature region. This latter process can be explained in terms of motions of the dipole associated with the C"'-O-C ar bonds whereas motions about the C=-C*O * bonds may also intervene in the fl process. The low-molecular-weight compound only exhibits a prominent glass-liquid absorption followed by a diffuse and weak subglass relaxation; this behaviour cannot be explained in terms of only intramolecular interactions, and therefore intermolecular interactions must play an important role in this process. The mechanical and the dielectric glass-rubber relaxations are interpreted in terms of the coupling scheme. This study suggests that the complexities of both processes are similar, in contrast with what occurs in many other systems.

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It is evident from Monsanto rheometry, solvent swelling and infra-red spectroscopic studies that miscible blends of poly(vinyl chloride) (PVC) and epoxidized natural rubber (ENR) can be crosslinked during high-temperature moulding for prolonged periods. The crosslinking has no effect on blend miscibility. The physical properties of the blends depend on the blend composition. As the PVC content increases the blend behaviour changes from elastomeric to glassy.

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Impact polypropylene copolymers may be produced in a two-reactor system to yield a blend of homopolymer with an ethylene-propylene rubber (EPR). The polypropylene homopolymer, which is itself brittle and has low impact strength, is markedly toughened by the presence of the EPR. The rubber-like EPR exists as a phase-segregated discrete particle in a continuous matrix of the hard phase. The molecular structure analysis of the resulting complex mixture is a formidable task. The purpose of this paper is to describe a preparative and an analytical temperature-rising elution fractionation (t.r.e.f.) technique as the primary tools to separate and characterize a commercial impact copolymer. These techniques permit the isolation and subsequent characterization of the components of the impact copolymer by ancillary techniques, primarily 13C nuclear magnetic resonance and differential scanning calorimetry. These techniques were applied to the structural analysis of a commercial impact-grade polypropylene with a melt flow rate of 6. It was found that this impact copolymer was composed of about 75wt% of isotactic polypropylene, about 17wt% of a highly non-crystalline EPR and about 8 wt% of semicrystalline ethylene-propylene copolymers. A major component of the semicrystalline ethylene-propylene copolymers was an ethylene-rich copolymer containing between 0 and 8 wt% of propylene comonomer. The separate characterization of the components of the impact copolymer gives insight into the chemical synthesis process used to produce these copolymers. Further, it permits one to gain a better understanding of the location and function of each of the components in the complex mixture

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A number of novel triaromatic diisocyanates based on para-linked ester mesogens (1,4-phenylene dibenzoate, diphenyl terephthalate, diphenyl trans-1,4-cyclohexanedicarboxylate and trans-dicyclohexyl trans-l,4-cyclohexanedicarboxylate) are described. One series was synthesized by reaction of 4-trimethylsiloxyphenyl or cyclohexyl isocyanate with terephthaloyl chloride or trans-l,4- cyclohexanedicarbonyl chloride, and the other from a 4-isocyanatobenzoyl chloride with a 1,4-bis (trimethylsiloxy) benzene (trimethylsilyl-protected hydroquinone). Acidic and basic catalysts were used in this reaction. The influence of substituents in various positions of the mesogenic cores on the phase transitions was also investigated. All liquid-crystalline diisocyanates have a nematic mesophase

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A differential scanning calorimeter has been used to investigate the crystallization and melting behaviour of two segmented nematic polyesters based on 4,4'-(alkanedioyldioxy)dibenzoyl dichlorides and hydroquinones. Isothermal crystallizations from the nematic melt have been carried out on both samples. The results are discussed in terms of variation of the successive melting profiles as a function of various parameters, such as the crystallization temperature and time and, in more detail, the conditions of the preliminary treatment in the nematic state. The complex melting behaviour evidenced by the polyesters can be justified on the basis of a new scheme, introduced in our previous paper, which accounts for the slow evolution of the nematic melt towards its thermodynamic equilibrium. Above the crystal-nematic transition a certain registry of neighbouring chains persists in the melt, which becomes poorer and poorer upon increasing temperature and time and eventually disappears when equilibrium is attained. These results raise some fundamental questions on the ability of thermotropic polymers to crystallize from the nematic melt and underline that a careful adjustment of the process parameters must be achieved to obtain reproducible results and to control the final properties of the material

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The effect of thermal treatment on the thermodynamic properties and morphology of two nematic, thermotropic main-chain polymers with mesogenic groups containing 3,3'-biphenylene units and flexible spacers of different lengths has been studied by differential scanning calorimetry and X-ray scattering. An appreciable difference in the thermal behaviour of polymer samples with hexamethylene (BF6) and octamethylene (BF8) spacers was observed, despite identical thermal treatments. We found that BF6 samples had a crystalline structure after slow cooling from the isotropic state, whereas BF8 samples did not crystallize even at very slow cooling rates and possessed a glassy, nematic structure at room temperature. The enthalpy of the nematic-isotropic transition and the increase in the specific heat capacity at the glass transition for BF8 samples were greatly influenced by the rate used to cool the isotropic melt. This result is tentatively explained in terms of an improvement of the nematic ordering during cooling

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Anisotropic networks were obtained by photoinduced cationic polymerization of liquid-crystalline (LC) divinyl ethers. For this purpose two new LC divinyl ethers were synthesized. Transition temperatures, phase behaviour and the temperature dependence of the order parameter for the two systems were determined before polymerization. Subsequently the polymerization of the system was induced upon obtaining long-range uniaxial orientation of the molecules in the nematic state. In this way highly oriented networks were obtained. These anisotropic networks showed temperature-stable optical properties. Thermomechanical properties were found also to be anisotropic and highly dependent on relaxation mechanisms effective within the networks.

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Aldol group transfer polymerization of t-butyldimethylsilyl vinyl ether initiated by an unsaturated compound, p-formylstyrene, and catalysed by zinc bromide allowed synthesis of styryl-silylated poly (vinyl alcohol ) macromonomers bearing an aldehyde end-function. Various macromonomers with average degrees of polymerization up to 28 were prepared and characterized according to different analytical methods, such as 1H and 13C nuclear magnetic resonance and ultra-violet spectroscopy and gel permeation chromatography. In order to improve the stability of the end-group, the aldehyde function was reacted with a silylketene acetal compound, 1-methoxy-2-methyl-l-trimethylsiloxypropene, which leads to a fl-hydroxy ester end-group, much more stable towards side-reactions. Afterwards, the silyl ether side-groups were hydrolysed with tetrabutylammonium fluoride in order to obtain hydrophilic styryl-poly(vinyl alcohol) macromonomers

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