1994 рік

Ab initio molecular orbital calculations have been performed on the reactions of unsaturated monomers with both hydrogen and methyl radicals. These calculations have utilized several basis sets, namely STO-3G, SV3-21G, SV Dunning and triple zeta valence. The correlation between the experimental Q and the energy of formation of the product radical was examined, whilst the electronegativity of the monomer was correlated with the e parameter. In addition, a Q-e-e*-type scheme was analysed to correct for the simplification in the standard Q-e scheme that the monomer and the radical are identical with respect to their charge distributions. In this instance experimental e parameters were correlated with the average electronegativity of monomer and radical. This work shows that correlations exist between these parameters as other simpler studies (based on Huckel calculations) have reported. However, while the correlations with both e and the e-e* average parameter were predictive, the relationship between Q and a general reactivity of the radical proved only qualitative. A statistical analysis of the data revealed a significant polar influence on this relationship; by taking this into account we were able to improve the correlation. We also rescaled the Q-e scheme as suggested by other authors and correlated the results of our calculations with the new numbers. This work emphasizes the difficulties in correlation analysis, which requires the separation and parameterization of polar, steric and general reactivity effects. It also stresses the requirement for accurate experimental data.

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1H nuclear magnetic resonance (n.m.r.) studies of two examples of poly(vinyl alcohol/vinyl acetate) copolymers, of differing residual acetate content and hydroxyl block size distribution, but similar acetate block size distribution, are reported. The studies were undertaken over the temperature range of 2 to 30°C, at 2°C intervals, in D20 solution. The data appear to show correlation with previously determined conformational changes in the polymer as a function of temperature; additionally the data suggest that differences in the composition of the copolymers are reflected in differences in their n.m.r, spectra.

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Copolymers of N-isopropyl acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and an acrylic ester of a nitroxide radical have been synthesized by radical polymerization. The products are polymers where the nitroxide side group is reversibly protonated by the sulfonic acid groups. The lower critical solution temperature (LCS1) and the amount and size of the aggregates formed in aqueous solutions above the LCST are dependent not only on the amount of sulfonic acid groups in the polymer, but also on the pH and ionic strength of the solvent. Electron paramagnetic resonance spectra of the labelled polymers are sensitive to the rate and amplitude of the segmental motion of the polymer chain, and also to the rate of proton exchange between the sulfonic acid and the nitroxide

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This paper presents the synthesis of copolymers of vinyl p-t-butylbenzoate (PTBBA) with 3-methacryloxypropyltris(trimethylsiloxy)silane (Si-1) and their permselective properties for oxygen and nitrogen. Analysis of the glass transition temperature profile by the Kwei equation implied the existence of some interaction between PTBBA and Si-1 moieties. The oxygen permeability of 1.6 x 10- 8 cma(STP) cm cm - 2 s - 1 cmHg- 1 for the Si-I homopolymer decreased with decreasing Si-1 content to arrive at 9.1 ? 10-10 cm 3(STP) cm cm- 2 s- l cmHg- 1 for the PTBBA homopolymer. Oxygen and nitrogen permeabilities were investigated in connection with Si-I content and the glass transition temperature in the copolymer. Furthermore, the effect of the condition of preparation on gas permeability was studied; that is, we prepared the copolymers of PTBBA and Si-1 with various polymerization times, and investigated their permselective properties for oxygen and nitrogen.

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The local motions of aryl-aliphatic polyesters based on ethylene glycol and hydroxybenzoic acid or isophthalic acid were investigated in chloroform and trifluoroacetic acid/dichloromethane solutions using 13C n.m.r. Measurements of the 13C spin-lattice relaxation times have indicated the existence of hydrogen-bonding interactions between the carboxyl groups of the polymers and the solvent in the trifluoroacetic acid/dichloromethane mixture. Interpretation of the ~3C spin-lattice relaxation times in terms of segmental main-chain motions was carried out using the Dejean-Laupr6tre-Monnerie orientation autocorrelation function. The internal motions of the aromatic rings have been described in terms of jumps between two equilibrium positions for the hydroxybenzoic rings and by stochastic jump processes for the isophthalic rings. The energy barriers determined for these motions have been compared with those measured in related systems or estimated from conformational energy calculations

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A series of phenylethynyl-terminated imide oligomers were prepared by the reaction of aromatic dianhydride(s) with a stoichiometric excess of aromatic diamine(s) at calculated number average molecular weights of 1500-9000gmo1-1 and end-capped with phenylethynylphthalic anhydrides in N-methyl-2- pyrrolidinone. Unoriented thin films cured in flowing air to 350°C exhibited tensile strengths and moduli of 105.5-139.3 MPa and 2.8-3.2GPa at 23°C, respectively, with good retention of properties at 177°C. Stressed film specimens exhibited excellent resistance to a variety of solvents after a 2 week exposure period at ambient temperature. One phenylethynyl-terminated imide oligomer was selected for more extensive evaluation and gave high fracture toughness, adhesive and composite properties. The chemistry, physical and mechanical properties of these materials are discussed.

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Poly(ethylene terephthalate) (PET) copolymers were prepared by melt polycondensation of dimethyl terephthalate and excess ethylene glycol with 5-30 mol% (in feed) of 1,3-propanediol and 2,2-dialkyl-l,3- propanediols, where the dialkyl groups are dimethyl, diethyl and butyl-ethyl. No significant depression of reduced specific viscosity was observed. The comonomer contents in the copolymers are considerably smaller than those in the feed. The crystallinities and densities of heat-treated films decreased with increasing content of comonomer and length of alkyl side chain in the comonomer. The glass transition temperature (Tg) and melting temperature (Tin) were decreased by the copolymerization, while an increase in the length of the alkyl side chain hardly affected the T m depression of heat-treated films. Alkali resistance, dyeability and thermal shrinkage were remarkably increased by the incorporation of comonomer having an alkyl side chain.

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Uniaxially oriented poly(vinyl chloride) films drawn at 90, 100 and ll0°C to draw ratios in the range 1.5 3.5 have been investigated using i.r. spectroscopy and refractive index measurements. Their 10 s isochronal extensional creep moduli and dynamic tensile moduli have also been measured. One-way drawn films with draw ratios in the range 1.56 x 1-2.56 x 1 and simultaneously and sequentially two-way drawn films with draw ratios in the range 1.46 x 1.46-1.79 x 1.79 have been investigated using refractive index measurements and i.r. spectroscopy. These films were drawn at 85 and 95°C. Thermal analysis of the drawn samples showed that the uniaxially drawn films were drawn at temperatures well above their final glass transition temperatures, whereas the one-way and two-way drawn films were drawn close to their glass transition temperatures. The results show that the crystallites are more highly oriented than the amorphous chains and that one-way drawn samples depart slightly from uniaxiality. No significant difference in orientation was found between simultaneously and sequentially two-way drawn samples. It was shown that the i.r. peak at 613 cm 1, attributed to short syndiotactic segments, could be used to characterize the orientation of the amorphous chains. The elastic moduli of the uniaxially drawn samples were found to depend on orientation in a similar way to that previously reported

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A new series of phenylethynyl end-capped imide oligomers was synthesized and analysed for thermal stability, thermo-oxidative stability, glass transition temperature (Tg), cure temperatures and adhesive strength. Thus, 4-(phenylethynyl)phthalic anhydride (PEPA) and four substituted PEPAs containing electron-withdrawing groups were synthesized and used as end-capping agents for oligomers of 1,4-diaminobenzene and 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. The oligomers underwent an exothermic reaction above 350°C to afford an insoluble network. The cure reaction was apparently promoted by electron-withdrawing groups as the oligomers end-capped with the substituted PEPAs started to cure at lower temperatures. Their rate of cure was also faster, as evidenced by the faster rise in their Tgs. The cured resins had T~s as high as 405°C and displayed good thermo-oxidative stability at 371 °C. A cured sample ofa PEPA end-capped oligomer with a calculated molecular weight of 4200 g mol- 1 displayed moderate high-temperature adhesive strength.

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The chemorheology of phenol-formaldehyde (PF) resin, PF mixed with ammonium salt lignosulfonate (ALS) filler, and PF mixed with methylolated ALS (MALS) filler was studied in order to characterize their isothermal cure behaviours. The results show that both ALS and MALS have a lower reactivity than PF and act as diluents in the resin. The cure rate of the resin decreases when increasing the ALS or MALS content in the PF. In addition the cure rate constant increases when PF is mixed with ALS and decreases when PF is mixed with MALS. These results are in agreement with those obtained from differential scanning calorimetry (d.s.c.) studies. The viscosity can also be expressed as a function of the cure degree, which is calculated from the kinetic parameters of the d.s.c, measurements by using the Williams-Landel-Ferry (WLF) equation. This expression gives a better calculated profile than does the dual Arrhenius model, in particular for PF mixed with filler, above the gel point.

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