1992 рік

For a particular crosslinking polymer, it is often considered that the final state of crosslinking may be reached by post-curing at temperatures above its ultimate glass transition temperature, Tg,,, regardless of previous curing histories. Although this appears to be true over a certain range o'f variation in curing history, the Tg,u and the network structure depend strongly on the curing history over a wide range of conditions for the homopolymerization of diglycidyl ether of bisphenol-A (DGEBA) catalysed by 2-ethyl-4-methyl imidazole. As degrees of cure and crosslink increase, the glass transition temperature, Tg, of a sample may increase from the monomeric DGEBA value of -22°C to the highest value near 180°C. Depending on the curing thermal history, some samples may only attain a Tg,u of ~ 100°C, even after post-curing at 200°C for 16 h. Although the influence of thermal history on Tg may rank second al~er the influence of the degree of cure, it is the most important factor on Tg.u for a fixed resin and catalyst composition. The reversible physical ageing process appears to be the least influential on Tg. As all crosslinking reactions involve competing reactions with different kinetic parameters, we believe that these phenomena are universally observable to a greater or lesser degree in all crosslinking reactions.

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The microstructure of copolymers can be characterized in terms of triad fractions and tacticity parameters (i.e. intramolecular structure), and in terms of a three-dimensional molar-mass chemical-composition distribution (MMCCD) (i.e. intermolecular structure). The microstructure obtained is controlled by the copolymerization conditions, for instance by the choice of the reaction system (homogeneous solution or heterogeneous emulsion), the degree of conversion and the choice of reactants. Computer simulations of emulsion copolymerization (SIEMCO), accounting for the main chemical and physical processes occurring, provide MMCCD predictions of emulsion copolymers. These MMCCDs are calculated by considering the conversion heterogeneity (composition drift) as well as the instantaneous (statistical) composition distribution of the copolymers formed. Cross-fractionation (two-dimensional chromatography) was used to verify the predicted MMCCDs of the copolymer products. The copolymers are separated according to molar mass by means of size exclusion chromatography (s.e.c.), and each s.e.c, fraction is subsequently analysed according to chemical composition by means of gradient elution quantitative thin-!ayer chromatography/flame ionization detection (t.l.c./f.i.d.). The difference in water solubility of the two monomers (styrene (S) and methyl acrylate (MA)) appears to be one of the major factors determining the microstructure of the copolymers. Depending on conversion, monomer ratio and monomer-to-water ratio, the model predicts either single- or double-peaked MMCCDs, in full agreement with the experimentally obtained distributions.

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Determination of the microstructure of N-octylacrylamide/sodium acrylate copolymers has been performed by 13C n.m.r. The 13C n.m.r, spectra were obtained in a CDaOD/D20 mixture in order to minimize the hydrophobic aggregation. These hydrophobically modified water-soluble polymers exhibit a random distribution of the hydrophobic groups along the polymer chain.

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The theoretical treatment proposed by Couchman to describe the compositional dependence of T, for polymer-diluent systems has been applied, in its several forms, to selected pairs for which heat capacity data are found in the literature. The most accurate equation gives the least satisfactory description of the experimental T, data. This result seems to be accounted for by the assumption, first suggested by Couchman, that in some cases the entropy of mixing is not continuous at T,.

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Conditions are described for the preparation of eellulose/poly(2-hydroxyethyl methacrylate) (CELL/ PHEMA) composites from solutions of cellulose in dimethylacetamide-lithium chloride (DMAc LiCl) by coagulation with 2-hydroxyethyl methacrylate monomer and subsequent bulk polymerization of the coagulated films. The CELL/PHEMA composites obtained over a wide composition range were characterized mainly through measurements of the temperature dependence of the dynamic storage modulus (E') and loss modulus (E"). For compositions rich in PHEMA (PHEMA > 80 wt%), it was assumed that an original structure of 'as-coagulated' cellulose gels was successfully perpetuated into the PHEMA matrix and, concurrently, an 'IPN'-type organization resulted. These samples gave a Tg value higher than that of PHEMA ( ~ 75°C), and the degree of lowering of their modulus E' in the glass transition region became extremely small with increasing CELL content. The E" versus temperature curves for cellulose-rich compositions (PHEMA < 30 wt%) yielded a single, broad dispersion peak, the maximum position of which was located above 150°C and shifted to the side of higher temperature along with the decrease in PHEMA component. This dispersion was interpreted as due to a molecular relaxation in the abundant amorphous regions of cellulose into which PHEMA constituent was intimately incorporated. Other samples with intermediate CELL/PHEMA compositions showed a dual transition behaviour, due to phase separation into two distinct amorphous mixtures of both polymers.

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Low molecular weight samples of poly(ethylene terephthalate), obtained by direct bulk polyesterification of terephthalic acid and ethylene glycol, were studied by 1H and 13C n.m.r. It was possible to determine degrees of conversions of acid and alcohol groups, distributions of free, monoesterified and diesterified terephthalic acid and ethylene glycol and to identify the dimer, trimer and tetramer. Side reactions were also studied.

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The present paper refers to the well known fact that the glass transition-composition curves of miscible binary systems (e.g. polymer--diluent and polymer-polymer mixtures) often exhibit a singular point when the Tg values of the two components are far apart. Two different equations apparently apply above and below the temperature of this singularity. Starting from Couchman's entropic treatment of the glass transition of mixtures, and from Angell's suggestion for the calculation of To ('ideal glass transition temperature' of the component with higher Tg), it is shown that below To an equation should be applicable to binary mixtures which differs considerably from the equations normally used in the literature. The new equation gives a satisfactory description of selected experimental data in the proximity of the component with lower T s.

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Nitration of 2,5-bis ( p-chlorobenzylidene )cyclopentanone (I) afforded the corresponding 2,5-bis ( p-chlorom- nitrobenzylidene)cyclopentanone (II). Polymerization of II with Na2 S in N-methylpyrrolidone gave poly [2,5-bis(3-nitrobenzylidene)cyclopentanone sulphide]. The monomer was characterized by infra-red and 1H nuclear magnetic resonance spectroscopy and elemental analysis. The polymer formed was confirmed by elemental analysis, infra-red spectroscopy, viscometry, differential thermal analysis measurements, thermogravimetry and X-ray analysis. An isothermal electrical conductivity test of the new polysulphide polymer indicates that it has a value of about 3.2 x 10- 3 ohm- 1 cm- 1 at 390 K. Using the Pariser-Parr- Pople method the C=O oxygen atom in compound II has a partially positive charge in the normal state, reduced and oxidized forms. The ionization potential and the electron affinity are not altered by replacing the sulphur atom, i.e. 9.55 (9.56) and 7.94 (7.77).

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Based on analysis of the photochemistry of a model compound, N-phenylphthalimide, the photolysis of a polyimide derived from 4,4'-oxydianiline (ODA) and a diarylanhydride with a hexafluorinated isopropylidine bridging group (6F) is proposed to proceed via formation of a phthalic anhydride type photoproduct. The quantum yield for product formation of N-phenylphthalimide is quite low ( < 10- 3) indicating a rather inefficient first step in the overall oxidative degradation process. However, based on the efficiency of the photo-oxidation of the primary photoproducts, a process leading to the ultimate failure and complete ablation of the 6F-ODA films in air upon exposure to the unfiltered output of a mercury lamp is postulated.

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Pressure-volume temperature measurements have been performed for a series of polymethacrylates. Samples of poly (ethyl methacrylate) that have been pressed and quenched show densification between 100 and 150c'C which disappears up to 200°C. Samples which have been heated above 200°C appear not to show this effect on reheating. Densifications similar to the above are also observed for the propyl and butyl polymethacrylates. Densification is not seen for poly(methyl methacrylate) nor for polyacrylates. Several other initial observations were made using other techniques. D.s.c. peaks are seen in this region which depend on the thermal history. The rheology is very dependent on thermal history. Some small changes in X-ray patterns are produced by thermal treatments. These observations could be interpreted as suggesting that polymethacrylates can be partially crystalline.

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