1993 рік

Isothermally crystallized and annealed blends of poly(D-(-)-3-hydroxybutyrate) (PHB) and atactic poly(epichlorohydrin) (aPECH) were examined by wide-angle X-ray diffraction and small-angle X-ray scattering (WAXD and SAXS). The lateral crystal dimensions of PHB increased with annealing treatment and with increasing aPECH component in the blends, while the long-period distances of the blends were slightly smaller than for pure PHB. The scattering observed for the blends resulted from the superposition of the scattering arising from the crystalline regions (made up by alternate stacking of lamellae and thin amorphous layers) and from the aPECH inhomogeneity present outside them. The structural information was obtained, from the SAXS curves, on the basis of Glatter and Debye-Bueche approaches. The results suggest that, during melt-crystallization of PHB, segregation ofaPECH occurs within the growing spherulite with its dispersion in the interfibrillar zones that are larger than the interlamellar regions but smaller than the overall spherulite. The aPECH molecules are dispersed at the molecular level in the interfibrillar zones where they can assume a random-coil conformation. The detection of a glass transition temperature (Tg) close to the aPECH Tg value indicates low interactions at the segmental level between the two polymers. The annealing treatment produced a general perfectioning and rearrangement of the sample morphology, enhancing the crystallinity and the crystal dimensions of PHB in the pure state and in the blends, and probably favouring the trend of the aPECH molecules to assume a globular conformation.

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Oriented liquid crystal copolyesters are mechanically non-linear at surprisingly low levels of strain when stretched in the orientation direction. As the static strain increases the axial Young's modulus also increases in an asymptotic fashion, the effect being most marked above the glass transition. Several models, each with its own distinct philosophy, have been advanced to account for this behaviour. In order to evaluate critically the various models we have studied the non-linear behaviour of a highly oriented random copolyester film over a wide temperature range. We have also measured the shear modulus of the film and similar fibres (where a wider temperature range can be covered) since this is a key parameter in the various theories. On the basis of comparisons between measured and predicted shear moduli we conclude that the models proposed by Allen and Roche, and by Zhang, Davies and Ward describe the non-linearity best, especially at high temperatures where the units of structure are free to rotate, while the model of Northolt and van der Hout is unsatisfactory. None of the models is applicable well below the glass transition. We also note that all the models are exponential in character, whereas better fits to the data are obtained with other empirical asymptotic expressions.

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The structures of polyaniline salts chemically synthesized in HC104 of varying concentrations have been examined by means of X-ray photoelectron spectroscopy. The results indicate that, although the imine units of the as-synthesized salt are preferentially protonated, a large proportion of the amine units are also protonated in the presence of a large excess of HC104 on the surface of the salt particles. Protonated amine units are also observed when emeraldine base films cast from N-methylpyrrolidinone solutions are reprotonated by HC104. The deprotonation behaviours of the salt synthesized in HC104, the reprotonated film and polyaniline from the simultaneous oxidation and polymerization of aniline by Cu(CIOJ2.6H20 in acetonitrile are compared. It is found that the protonated amine units undergo deprotonation in aqueous media more readily than the protonated imine units and that both types of structures are stable when heated to 150°C in air.

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Simple, analytical expressions are derived for the surface composition and surface excess (adsorbed amount) of the lower surface energy component of a miscible polymer blend at equilibrium. In practical cases, the total surface excess is predicted to depend more strongly on the value of the Flory-Huggins interaction parameter between the segments of the two polymers than on the difference in surface energies between the components of the blend.

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The mobility of ethyl acetate (EAc) as a function of crystallinity in poly(vinylidene fluoride) (PVF2) has been quantified utilizing methodology based on the recently developed rheo-photoacoustic Fourier transform infrared spectroscopy (RPA FTi.r.). The diffusion measurements obtained with the photoacoustic cell, combined with the independent sorption and X-ray measurements, indicate that the crystalline phase of the polymer is impermeable to EAc. Spectroscopic RPA FTi.r. measurements agree with the independently determined diffusion coefficients and the theoretical predictions relating diffusion to the per cent crystalline phase in PVF2. This study indicates that RPA FTi.r. methodology is highly sensitive to the gas phase analysis and, with the use of proper calibration procedures, can be applied to a quantitative analysis of diffusion in polymers.

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A high-molecular-weight polypropylene (Mw= 83 000, Mw/M.= 3.0) has been crystallized isothermally as a function of pressure at a constant supercooling of 50°C. Wide-angle X-ray diffraction (WAXD) shows that the X-ray diffraction spectrum of the 7 phase is identical to that reported for low-molecular-weight polymers, with the exception that the peaks are broader. Calculations of the y content using WAXD show the nearly 100% 7 phase is formed at 2 kbar, but a mixture of 7 and ~ crystals are formed at lower crystallization pressures. Small-angle X-ray scattering (SAXS) studies show that 7 lamellae are approximately one-half the thickness of ~ lamellae produced at an equivalent supercooling. Morphological studies conducted using transmission electron microscopy of carbon replicas of permanganically etched surfaces confirm the thin nature of the lamellae and demonstrate their inherent waviness. Unlike ~ crystals, 7 lamellae do not form cross-hatched textures. A model is proposed, involving 7/ct lamellae and epitaxy during the growth process, to account for all the observable morphological data.

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Electron energy-loss spectroscopy is shown to provide information on the chemical state of carbon atoms in the poly(ether ether ketone) molecule, through comparison with X-ray photoelectron spectroscopy. A careful study of beam damage effects on the energy-loss spectra is presented as a function of electron dose. This ensures that the final results relate to the undamaged polymer, and permits a discussion of the limits of spatial resolution with which the energy-loss technique can be applied to phase-separated systems incorporating an aromatic polymer. The conclusion reached is that energy-loss spectroscopy can provide chemical state information for aromatic polymer systems from regions as small as 140 x 140 nm 2.

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Kinetics of emulsion polymerization of vinyl acetate have been studied in the absence of emulsifiers using potassium persulphate as the radical generator. Uniformity of particle sizes, in batch operation, indicated that nucleation of stable particles occurs early in the polymerization. The polymerization rate was proportional to the 0.69 power of the initial monomer concentration. Particle sizes were consistent with the suggestion that particles are stabilized by initiator fragments. When polymerization was carried out in the presence of a dialysed seed latex, with no added emulsifier, the polymerization rate was found to depend on the initiator concentration raised to the power 0.60_+0.05 but was independent of the monomer-to-polymer ratio. New stable particles could be produced when the monomer-to-polymer ratio was high. The production of these new particles did not affect the polymerization rate significantly. Polymerization rate was insensitive to the stirrer speed unless the speed was very high. At high speed the polymerization rate was reduced.

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Conformational analysis was performed on some aromatic copolyesters in solution by means of ~H-~H nuclear Overhauser effect (NOE) measurements, using the cyclic dimer of isophthalic acid and ethylene glycol (DIMIS) as the model compound. From the observed NOE, a folding of the ethylene glycol towards the aromatic ring can be assumed for all the polyesters containing an isophthalic ring. In addition, the spatial interaction between adjacent aromatic rings, put forward in order to explain the chemical shift distribution observed in the 1H n.m.r, spectra of these copolymers, has been demonstrated with steady-state NOE experiments. The average distance between protons on adjacent aromatic rings has been calculated by means of dynamic NOE measurements. The most probable conformation, giving rise to these NOE effects, has been postulated on the basis of a formalism introduced in a previous work

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The aim of this work is to investigate the complex spherulitic structure of bulk polypropylene samples from direct scanning electron microscopy (SEM) observations of etched surfaces. Thick plates of isotactic polypropylene were moulded by intrusion. Preliminary characterization, involving X-ray diffraction, microdensitometry and differential scanning calorimetry (d.s.c.), showed that the slow solidification process develops a variable proportion of the monoclinic (~) and hexagonal (/~) phases, ranging from 0% of/3-crystals at the surface to 60 vol% of this phase at the core. In addition, samples were cut across the thickness of the plates, finely polished and then etched with an appropriate reagent which preferentially attacks the amorphous fraction of the polymer. SEM examination of such samples revealed two populations of spherulites with quite different contrasts, which were unambiguously associated with the two crystalline structures. The ~-spherulites have a dark aspect while the/~-ones are very bright. These contrast effects are discussed in terms of the topology of the etched surface. For the ~-spherulites, whose lamellae are straight and finely interlocked along the radial and tangential directions, the etched sections are very smooth and consequently the lateral diffusion of normal incident electrons is weak. In contrast, the fl-spherulites are characterized by curved lamellae and sheaf-like structures, thus making the surface rougher after etching, and which contribute to the emission of more secondary electrons to the detector. This interpretation is confirmed by the corresponding contrast observed in metallographic microscopy using low-angle illumination.

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