1993 рік

The influence of chemical ageing and microcrack damage on the mechanical behaviour of polymer materials is studied. The theoretical model is derived from irreversible thermodynamics, chemical reaction kinetics and a crack extending model. Ageing tests, swelling tests, acoustic emission tests and relaxation tests have been done to determine the material constants of the constitutive equations. The consistency between the theory and the experiments is quite good when the model is applied to HTPB composite solid propellant materials.

Детальніше...  

The development of orientation has been studied in samples of poly(aryl ether ether ketone) (PEEK) drawn at 150°C from initially amorphous material of thickness 7 gm at an initial strain rate of 40% min-1 to draw ratios 2 in the range 1-2.92. A sample of increased crystallinity and orientation was produced by annealing a sample with 2 = 2.92 at 300°C. Polarized spectra were obtained in the range 640-2000 cm-1. By suitably combining parallel and perpendicular spectra it was possible to predict the spectra of randomly oriented samples with different crystallinities. The conclusion of Nguyen and Ishida that the spectrum of PEEK may be regarded as the sum of two overlapping spectra, due to molecules in amorphous and crystalline conformations, was then confirmed both by spectral subtraction and by curve fitting and shown to account for the apparent shift of peaks with change of polarization direction for oriented samples. The areas under the characteristic peaks have been used to show that the content of crystalline conformers exceeds the degree of crystallinity except for the most highly oriented sample and the annealed sample, and an explanation has been found for the observation of Chalmers et al. that there is a linear relationship between the ratio of the absorbances at 1306 and 1280 cm- 1 and the degree of crystallinity. The C--~O stretching mode has been used to derive the orientation average (P2(cos 0))c for the molecules in the crystalline conformation and this is found to be very much lower than that of the chains in the crystallites, determined in part 1. The infra-red dipole orientation with respect to the chain axis has been derived for several modes due to molecules in either the crystalline or the amorphous conformations. Combining the results with those of part 1 allows the conclusion that the development of molecular orientation in the present samples fits well into the general picture built up by other workers using a variety of somewhat different techniques for producing orientation.

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Glycidyl methacrylate polymers and their random or block copolymers with methyl methacrylate have been synthesized anionically with a lithium organic initiator in tetrahydrofuran solution, at low temperature, with lithium chloride added to the reaction medium. In all cases, initiation and propagation involved exclusively methacrylic unsaturation, without touching the oxirane group of glycidyl methacrylate. The macromolecular species formed were characterized by titration of the epoxide groups, 1H n.m.r., i.r. spectroscopy, s.e.c, and light scattering measurements. The determination of compositions, average molecular weight and polydispersity indices of the various samples were in agreement with the values expected.

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Thermal and micromechanical properties of amorphous and semicrystalline poly(ethylene terephthalate) are found to be dependent on ageing effects occurring by thermal treatments below T~. These effects are studied using two complementary techniques: differential scanning calorimetry and dynamic mechanical spectroscopy at low frequencies (1 10 -4 Hz). Experimental results can be described through a physical model assuming diffusion and annihilation of 'quasi-point defects'. The distribution in the mobility of these defects is able to take into account all the phenomena observed after physical ageing, in particular the effects of low-temperature ageing. The ageing effects in the semicrystalline material cannot be deduced from those observed for the amorphous one by a single two-phase rule. Changes in the correlation factor of molecular movements and changes in the distribution of defects are necessary to describe all the observed effects, putting in evidence the role of the crystallites to reduce the segmental mobility

Детальніше...  

The development of orientation has been studied in samples of poly(aryl ether ether ketone) (PEEK) drawn at 150°C from initially amorphous material of thickness 7 gm at an initial strain rate of 40% min-1 to draw ratios 2 in the range 1-2.92. A sample of increased crystallinity and orientation was produced by annealing a sample with 2 = 2.92 at 300°C. Polarized spectra were obtained in the range 640-2000 cm-1. By suitably combining parallel and perpendicular spectra it was possible to predict the spectra of randomly oriented samples with different crystallinities. The conclusion of Nguyen and Ishida that the spectrum of PEEK may be regarded as the sum of two overlapping spectra, due to molecules in amorphous and crystalline conformations, was then confirmed both by spectral subtraction and by curve fitting and shown to account for the apparent shift of peaks with change of polarization direction for oriented samples. The areas under the characteristic peaks have been used to show that the content of crystalline conformers exceeds the degree of crystallinity except for the most highly oriented sample and the annealed sample, and an explanation has been found for the observation of Chalmers et al. that there is a linear relationship between the ratio of the absorbances at 1306 and 1280 cm- 1 and the degree of crystallinity. The C--~O stretching mode has been used to derive the orientation average (P2(cos 0))c for the molecules in the crystalline conformation and this is found to be very much lower than that of the chains in the crystallites, determined in part 1. The infra-red dipole orientation with respect to the chain axis has been derived for several modes due to molecules in either the crystalline or the amorphous conformations. Combining the results with those of part 1 allows the conclusion that the development of molecular orientation in the present samples fits well into the general picture built up by other workers using a variety of somewhat different techniques for producing orientation.

Детальніше...  

A macromolecular-pendent-group N,N-dibromosulfonamide ([P]-SO2NBr2) having a macroporous styrene-divinylbenzene matrix structure has been obtained by three methods: by the action of (a) bromine in aqueous medium and (b) aqueous metal hypobromites in acidic medium, on the appropriate macromolecular-pendent-group sulfonamide, and by (c) the action of an aqueous metal bromide on the appropriate macromolecular N,N-dichlorosulfonamide. In the best case the product was obtained with nearly 100% functional yield and it contained 2.4 mmol g- 1 -SO2NBr 2 groups, i.e. ca. 0.38 g g- ~ of active bromine. The stability of the resin was investigated in the range 20-100°C in the dry state as well as in aqueous media. Thermoanalysis (thermogravimetric, differential thermogravimetric and differential thermal analysis curves) was carried out up to 1000°C and its course was interpreted. The resin has brominating, oxidative and microbiocidal properties. It reacts in a reversible way: after loss of the active bromine, it can be reactivated again.

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Volume dilatometry was used to explore the origin of the fl transition (Tt0 and its influence on physical ageing. The results show that volume recovery does occur even at temperatures below T~, a phenomenon indicating the existence of molecular rearrangements and cooperative motions below this point. Lowering the ageing temperatures through T~ progressively retards short-term molecular rearrangements and cooperative motions. Nonetheless, the Tp quench does not incur any excess free-volume trapping, and its associated change in specific volume follows a linear relationship with respect to temperature. The existence of T/~ is time-dependent, which leads to the conclusion that the formation of Tp has a kinetic origin.

Детальніше...  

A composite of poly(p-phenylenevinylene) (PPV) with V205 is prepared through sol-gel processing and compared with the corresponding composite with silica. The effective conjugation length as determined by the band gap remains the same when V205 is chosen as a dopant, whereas the effective conjugation length is reduced in the PPV/silica composite. These composites exhibit good optical quality and show promise for optical waveguiding. The waveguide Raman spectra of these composites can be easily obtained. The refractive index of the PPV/V205 composite is higher than that of the PPV/silica composite. Furthermore, the Raman spectra do not indicate any existence of chemical bonds between the vinyl group of PPV and the glassy matrix (silica oxide or V205). The sol-gel processed PPV/V205 composite also exhibits a larger g t3) value than that of the PPV/silica composite.

Детальніше...  

The synthesis of a soluble poly(para-phenylene) (PPP) derivative with two p-carboxyphenoxymethylene substituents in the 2,5-positions of every alternate phenylene moiety is described, transforming a bromomethylene-substituted precursor PPP in a two-step reaction sequence. Both of these steps, namely a Williamson reaction and an ester cleavage using anhydrous potassium hydroxide, occur selectively and almost quantitatively (>98%, n.m.r.). Structurally homogeneous, rigid-rod polyelectrolytes with 80-130 phenylene rings attached to each other may be obtained by this method.

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The melting and crystallization behaviour of poly(vinylidene fluoride) (PVF2) blends with each of poly(phenyl acrylate) (PPA), poly(methyl acrylate) (PMA), poly(vinyl benzoate) (PVBz), poly(vinyl acetate) (PVAc) and poly(aryl ether ether ketone) (PEEK) has been studied. Two PVF2 samples with different amounts of head-to-head defect structure were used. The compatibility of PVF z with each of PPA, PVBz and PEEK has been tested by glass transition temperature measurements. The results indicate immiscibility in each case, which is attributed to steric hindrance of the phenyl group. The melting-point depression of PVF 2 in incompatible pairs of the above is less than in compatible pairs. Also, the initial slopes of the melting-point depression curves for lower-defect-content blends (compatible or incompatible) are lower compared with those of higher-defect-content samples. The crystallization kinetics of pure PVF2, PVF2 in incompatible blends and PVF2 in compatible blends at weight fraction WpVF, =0.8 were measured at 147°C. The rate of crystallization of PVFz varies as: pure PVF 2 >incompatible blend>compatible blend. Analysis of the Avrami exponent indicates that the nucleation mechanism is the same for all cases, though the growth becomes dltlUslon-controlled lor higher-defect-content PVF 2 samples and for its blends. Comparison of crystallization rate between lower- and higher-defect-content PVF 2 blends again indicates greater miscibility for higher-defect-content samples. The results have been explained qualitatively by homopolymer~opolymer miscibility theory.

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